Alteration for a diffusion barrier design concept in future high-density dynamic and ferroelectric random access memory devices

Progress in Materials Science 48 (2003) 275–371 www.elsevier.com/locate/pmatsci Alteration for a diﬀusion barrier design concept in future high-densi...

Download PDF

4MB Sizes 0 Downloads 23 Views

Report

Full Text

Progress in Materials Science 48 (2003) 275–371 www.elsevier.com/locate/pmatsci

Alteration for a diﬀusion barrier design concept in future high-density dynamic and ferroelectric random access memory devices Dong-Soo Yoona,*, Jae Sung Roha, Sung-Man Leeb, Hong Koo Baikc a

Advanced Process—Capacitor, Memory Research & Development Division, Hynix Semiconductor Inc., Kyoungki-do 467-701, Republic of Korea b Department of Advanced Materials Science and Engineering, Kangwon National University, Chuncheon, Kangwon-Do 200-701, Republic of Korea c Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Korea Received 1 March 2002; accepted 1 May 2002

Abstract The barrier properties and failure mechanisms for many diﬀusion barriers in high-density volatile and non-volatile capacitors were reviewed. Based on failure mechanisms of these barriers reported by others, we suggested the new design concept for a diﬀusion barrier and developed the new Ta+CeO2 and Ta+RuO2 barriers. Although both barriers were shown to exhibit good diﬀusion barrier properties, however, oxide-incorporated barriers result in the surface oxidation of the under-layer during deposition and/or post-thermal budgets, resulting in the degradation of capacitor performance. The design concept for a diﬀusion barrier should be changed to sacriﬁcial oxygen diﬀusion barrier concept, and both the RuTiN and the RuTiO ﬁlms, as new sacriﬁcial oxygen diﬀusion barriers, were proposed. New RuTiN and RuTiO barriers showed the higher oxidation resistance and cell capacitance and the lower contact resistance up to high temperatures. Therefore, the design concept of a sacriﬁcial diﬀusion barrier should be emphasized to achieve high-density dynamic and ferroelectric random access memory devices. # 2003 Elsevier Science Ltd. All rights reserved.

* Corresponding author. E-mail address: [email protected] (D.-S. Yoon). 0079-6425/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0079-6425(02)00012-9

276

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Contents 1. Introduction ....................................................................................................................276 2. Barrier properties and failure mechanisms of various barriers in high-density DRAM/ FRAM capacitor bottom electrodes ............................................................................... 282 3. Suggestion of new design concept for diﬀusion barrier of high-density memory capacitors ........................................................................................................................286 3.1. Barrier properties of Ta ﬁlm prepared with/without non-conductive CeO2 added 287 3.1.1. Thermal stability of diﬀusion barrier and barrier/Si interface ...................287 3.1.2. Barrier properties of Ta barrier prepared with/without non-conductive CeO2 added ................................................................................................ 294 3.1.3. Summaries ..................................................................................................301 3.2. Barrier properties of Ta-conductive RuO2 diﬀusion barrier ..................................301 3.2.1. Oxidation resistance of the Ta–RuO2 diﬀusion barrier..............................301 3.2.2. Electrical properties of the Ta-conductive RuO2 diﬀusion barrier.............310 3.2.3. Electrical properties of the Ta-conductive RuO2 diﬀusion barrier on n++-poly-Si substrate ................................................................................ 325 4. Change of a design concept for a diﬀusion barrier in high-density capacitor.................341 4.1. Oxidation resistance and thermal stability of a new diﬀusion barrier....................343 4.2. Contact resistance characteristics for a new barrier ...............................................347 4.3. Capacitance and leakage current behavior using a simple stack structure inserted glue layer ................................................................................................................355 5. Future direction for a diﬀusion barrier to achieve high-density capacitors ....................367 6. Conclusions .....................................................................................................................368 References ............................................................................................................................369

1. Introduction Since the invention of memory device, a new generation of DRAM has been developed approximately every 3 years. Although all parts of the device require continual improvement, the capacitor scaling has become progressively harder because the in-plane area available to the capacitor is shrinking with the cell area, yet the required capacitance decrease little. The capacitance (C) of a parallel plate capacitor is given by [1]: C¼

"0 "A t

ð1Þ

where "0 is the permittivity of free space, " is the relative permittivity of the dielectric material, A is the area of the capacitor, t is the thickness of the dielectric.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

277

It can be seen from Eq. (1) that in order to increase the capacitance, one must increase the reduce the thickness of the storage capacitor, capacitor area, or choose a dielectric with high permittivity. In the past few years, the continual improvement in capacitance per cell area using SiO2 or SiO2/Si3N4 (ON) dielectric has been occurred by thinning the dielectric and the three-dimensional shapes. Another means to increase cell capacitance is to change the capacitor dielectric material to one with a higher dielectric constant than SiO2 (3.9) or Si3N4 (9). Several materials are currently being considered. They include medium-permittivity dielectrics such as Ta2O5, Al2O3 and TiO2, as well as high-permittivity dielectrics such as (Ba,Sr)TiO3 (BST) and SrTiO3 (STO) [2–5]. The relative dielectric constant of the proposed medium permittivity materials is in the range of 25–100. In comparison, the relative dielectric constant of the proposed high-permittivity materials is in the range of 200– 1400. The roadmap for a DRAM technology shows to extend Ta2O5 dielectric using metal–insulator–semiconductor (MIS) to less than 0.1 mm generation (Fig. 1). For dynamic random access memory with 0.1 mm technology or below, metalinsulator–semiconductor (MIS) capacitor structure can not be used due to limitation of cell capacitance. Instead, a metal–insulator–metal (MIM) capacitor structure, using Ta2O5 or (Ba,Sr)TiO3 (BST) with medium or high permittivity, as a dielectric ﬁlm, has been extensively considered for obtaining the required cell capacitance. Various capacitor structures have been proposed for high-permittivity dielectircs such as either stack or concave type but currently, a simple concave capacitor structure, as seen in Fig. 2, with the capacitor above the bit line, has been focused on. Many eﬀorts have been done in order to obtain electrical properties, such as capacitance and leakage current per cell, for the storage node height and critical dimension (CD) given by the design rule, using MIM capacitor structure such as BST/Ru, and Ta2O5/Ru, but various issues have come out during integration. They

Fig. 1. The roadmap of capacitor with dynamic random access memory technology.

278

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 2. (a) Eﬀective oxide thickness versus capacitor height with dielectric and electrode material; (b) concavetype capacitor structure in volatile memory cell; (c) stack-type capacitor structure in non-volatile memory cell.

have been acted as limitation factors to achieve the high-density memory capacitors. The most critical issues up to these days are as follows: dielectic ﬁlms themselves (Fig. 3), bottom electrode (Fig. 4), and diﬀusion barrier (Fig. 5) [6–10]. In ferroelectric random access memory device with 0.25 mm technology or under, ferroelectric cell capacitors, using PZT, SBT, and BLT, have been normally fabricated by using either noble metals or their oxide electrodes [11]. Currently, the capacitor structures using multilayers with noble metals and their oxide electrodes, such multiplayer structures, for example, a Pt/IrOx/Ir/TiN/plugs, have been focused on in order to improve leakage current, ferroelectric properties, fatigue, and retention (Fig. 6). For the next generation of high-density FRAM with design rule below 0.25 mm, however, as shown in Fig. 6(b), the cell structure should be changed to a stack type or concave type . The same critical integration issues as those of volatile memories will emerge. Among these issues, the development of a diﬀusion barrier is one of the most important parameters. The high permittivity materials either need to be deposited

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

279

Fig. 3. (a) Post-annealing dependence of electrical properties of dielectric material; (b) composition nonuniformity and step coverage dependence with design rule.

rather high temperature of above 400 C in oxidizing ambient, or require high postthermal budget to improve their chemical and electrical properties. Such high temperature process, oxidizing ambient, in the ranges of 450–800 C severely inﬂuences the bottom electrode structure. The diﬀusion barrier for high-density capacitors must prevent the oxidation of a bottom electrode structure involving TiSi2, ohmic contact layer, poly-Si or TiN plug, and the reactions or inter-diﬀusion between the electrode and the plug. Especially, the electrical properties of diﬀusion barrier must remain intact before and after the fabrication processes of a capacitor. Up to now, many researches have been extensively conducted in the use of an electrode material, oxygen and Si diﬀusion barrier for capacitor bottom electrodes, including polycrystalline nitrides and amorphous ternary compound barrier developed for Al or Cu metallization [12–22]. Polycrystalline metals, nitrides, and amorphous binary and ternary diﬀusion barrier [23–29], which have been widely used as diﬀusion barriers for capacitor bottom electrodes, are susceptible to the reaction, and inter-diffusion between electrode metal, Si, and oxygen through the diﬀusion barrier because they show either polycrystalline microstructure that possesses the grain boundaries, or the binding force formed between elements of diﬀusion barrier is weak. The oxidized

280

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 4. Bottom electrode material properties; (a) oxygen impurity; (b) porous structure; (c) agglomeration.

Fig. 5. Oxidation of diﬀusion barrier material; (a) concave-type structure; (b) stack-type structure.

layer increases the contact resistance of bottom electrode layer structure. It signiﬁcantly increases the roughness of the bottom electrode and decreases the capacitance property of the deposited dielectrics. In this review jourmal, therefore, we critically investigated the barrier properties and failure mechanisms of many diﬀusion barriers, and concentrated on the development of new diﬀusion barrier. This review article is divided into six sections. Section 2 investigates barrier properties of various diﬀusion barriers adopted to achieve the capacitors, emphasizing failure mechanism for many diﬀusion barrier groups.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

281

Fig. 6. (a) current FRAM structure; (b) and (c) stack-type and concave-type structure using ferroelectric material.

In Section 3, we suggest a new design concept of diﬀusion barrier for high-density memory devices, according to the failure mechanisms of various barriers considered in Section 2. Based on the new design concept, Ta+CeO2 and Ta+RuO2 are developed as new diﬀusion barrier materials. The barrier properties for the Ta+CeO2 and Ta+RuO2 diﬀusion barriers are characterized as a function of the amount of CeO2 and RuO2 added, and the annealing condition. The diﬀusion barrier mechanism for these barriers is also evaluated. The bottom electrode, Pt and the developed Ta+RuO2 diﬀusion barrier layer on the real capacitor structures, TiSi2/poly-Si/ SiO2/Si and n++-poly-Si/SiO2/Si substrates are deposited by using a metal mask,

282

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

respectively and then the electrical properties, such as total resistance and ohmic behavior, are investigated. Section 4 is presented the alteration of a new design concept based on the diﬀusion barrier mechanism for both the Ta+CeO2 and the Ta+RuO2 barriers. We proposed both RuTiN and RuTiO, as sacriﬁcial diﬀusion barriers for oxygen and investigated the electrical properties of the newly suggested RuTiN and RuTiO barriers, providing some solutions for oxygen diﬀusion barriers. To evaluate the feasibility of the newly developed RuTiN or RuTiO diﬀusion barriers on real memory devices, we measured the contact resistance, and the capacitance and leakage current properties, the most important electrical parameter of diﬀusion barrier requirements because the lower structure of the capacitor is connected to the transfer device through a conductive plug, and therefore the investigation of electrical characteristics for the developed barrier under capacitor fabrication processes was inevitable. Finally, future direction for a diﬀusion barrier to advance high-density memory capacitors is described in Section 5.

2. Barrier properties and failure mechanisms of various barriers in high-density DRAM/FRAM capacitor bottom electrodes For 0.1 mm technology and beyond, as emphasized in Section 1, the most critical issues for integration in MIM capacitor were found out such as the electrode material, dielectrics themselves, and diﬀusion barrier. Most integration Schemes for medium or high dielectric materials use noble metal or noble metal oxides in combination with a deposited diﬀusion barrier material at the electrode/plug interface. In this section, we brieﬂy survey electrode materials and dielectrics, and critically focus on barrier properties and failure mechanisms of various diﬀusion barriers under consideration for use with Ta2O5 or BST dielectrics. As seen in Figs. 2 and 6, the dielectric and bottom electrode material signiﬁcantly inﬂuences the barrier properties of diﬀusion barrier because the dielectric processes require the high temperature and oxygen ambient, and the direct contact between the bottom electrode and barrier. First, Pt, Ru, Ir, RuO2, and IrO2 materials potentially suit for use as electrodes with Ta2O5 or BST dielectrics have been suggested by considering work function, electrical resistivity, etchability, diﬀusion barrier for oxygen, and CVD possibility [30]. In viewpoint of integration, metal organic chemical vapor deposition (MOCVD) and/or atomic layer deposition (ALD) process should be required due to step coverage issue. Among these electrodes, the MOCVD-Ru ﬁlm has been widely investigated using Ru(EtCp)2, C14H18Ru(bis(Z5-ethylcyclopentadienyl)ruthenium) and Ru(od)3,C24 H39O6Ru [Tris(octane-2,4-dionato)ruthenium] chemical sources. The Ru ﬁlm is deposited by a reaction between chemical source and oxygen as a reaction gas and volatile organic compounds (CxHyOz) is produced in the deposition temperature range of 250–270 C [31–33]. This process using two sources can be provided many advantages for the low temperature deposition and the high deposition rate, but it should be overcome many drawbacks such as surface morphology, agglomeration,

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

283

step coverage, and decrease of impurities, especially, oxygen (Fig. 4). In viewpoint of diﬀusion barrier to oxygen attack, when Pt is used as a bottom electrode, oxygen freely diﬀuses along with Pt grain boundaries and oxidizes the barrier and plug material due to no interaction with oxygen. It would require a more oxidation resistance barrier than other electrodes. Oxide electrode (RuO2 or IrO2) is less oxygen permeable electrode than Pt, but oxygen incorporating process at a given deposition temperature would oxidize the surface of diﬀusion barrier. The Ru and Ir electrode moderate for oxygen diﬀusion compared to Pt and oxide electrode. It would require a less oxidation resistance than other electrodes. Second, the CVD–Ta2O5 or (Ba,Sr)TiO3 ﬁlm, as dielectric material, has been currently adopted on real memory devices. In the case of BST ﬁlm, the BST ﬁlm have been deposited by metal organic chemical vapor deposition (MOCVD) using Ba(thd)2 -pmdt (Ba(2,2,6,6-tetramethylheptane-3,5-dionate)2-1,1,4,7,7-pentamethyliethylenettria-mine), Sr(thd)2-pmdt (Sr(2,2,6,6-tetramethylheptane3,5-dionate)2-1,1, 4,7,7-entamethyldiethyle-netriamine) and Ti(O-i-Pr)2(thd)2 (Ti(iso-propoxide)2-(2,2, 6,6-tetramethylheptane-3,5-dionate)2 and Ba(methd)2 (Ba(C12H25O4)2, methoxyethoxytetramethylheptanedionato barium), Sr(methd)2 ((Sr(C12H25O4)2, methoxyethoxytetramethyl metalheptanedionato strontium)), and Ti(mpd)(thd)2 ((Ti(C6H12 O2)(C11H19O2)2, methylpentanedioxy tetramethylheptanedionato titanium)) sources at substrate temperature of 420 C. Argon gas was used as a carrier gas to deliver the evaporated source to the process chamber and O2/N2O gas was employed as an oxidant gas. This also emerges out vaious problems including stochiometry of dielectric ﬁlm, step coverage, and high temperature process (deposition and/or postannealing) (Fig. 5) [34–37]. Among three critical issues, both the CVD-electrode and the dielectric ﬁlms related problems, such as surface morphology, impurity, step coverage, and non-stoichiometry, could be suﬃciently overcome by process optimization, chemical suorce and technology development. However, the high crystallization temperature of dielectric materials with high permittivity can not be basically solved because of a material characteristic. The high dielctric ﬁlms deposited at room temperature have amorphous microstructure. To change from amorphous structure to crystalline structure, the high permittivity thin ﬁlms usually require a high temperature more than 500 C in an oxygen plasma or ambient. This should be done for improving chemical and electrical properties, such as stoichiometry and leakage current. Therefore, such high temperature processes in oxygen ambient for a capacitor fabrication are inevitable. At present, the research direction for all capacitor processes have been focused to minimize the damage of a bottom electrode structure as well as to improve the capacitor performance. From these considerations, two viewpoints, that is, either the development of low temperature capacitor process in minimum oxygen concentration or that of new diﬀusion barrier material can be focused to advance high-density capacitors. In this review jourmal, we critically investigated the barrier properties and failure mechanisms of many diﬀusion barriers and concentrated on the development of new diﬀusion barrier. Most barrier materials are introduced to the structure as deposited layers. Barriers used with doped polycrystalline Si plugs typically include a metal silicide layer at the

284

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

barrier/plug interface to lower contact resistance. Many diﬀusion barriers incorporated into the device structure in the non-recessed geometry have been typically evaluated because CVD process makes very slow progress. Here, the capacitor bottom electrode and barrier layer are patterned before dielectric ﬁlm deposition. This provides a lateral path for oxidation of the plug, diﬀusion barrier and ohmic contact layer due to directly exposure during the deposition and/or post-annealing of the high dielectric oxide ﬁlms. They require high oxidation resistance during in-situ deposition or post-annealing in an oxygen plasma or ambient. In the case of TiN barrier, it have the recessed geometry that are typically patterned by planarization and self-aligned to recessed plugs due to the possibility of CVD process, as already seen Figs. 2(a) and 6(b). So far, the various diﬀusion barriers for Al and Cu metallization have been studied, and these can be classiﬁed in to six groups: (1) polycrystalline transition metal barriers (2) polycrystalline or amorphous transition metal alloy barriers (3) polycrystalline or amorphous transition metal–silicon (including silicide) barriers (4) polycrystalline or amorphous transition metal–nitrogen (including nitride), –oxygen, and –boron (borides) barriers (5) amorphous ternary barriers and (6) amorphous carbon barriers. Currently, the diﬀusion barriers developed for Al or Cu metallization have been applied as a diﬀusion barrier for the high-density DRAM/FRAM capacitor bottom electrode, their properties and failure mechanism can be described as follows: First, single transition metal ﬁlms usually have a polycrystalline structure. When Pt is used as a bottom electrode, the transition metal barriers, including Cr [38], Ta [39], W [40] and Ti [41], between thin Pt ﬁlms and TiSi2 ohmic layer, poly-Si plug is oxidized after annealing at less than 600 C in oxygen ambient. In addition, Pt can react with near-noble metals such as Cr [38], Co [38] and with refractory metals such as Ti, Mo [39], Ta, and W in the temperature range of 600– 800 C and form the compounds such as Pt–Ta, Pt–Ti [42–45]. It should also be noted that the metals from ﬁrst group mentioned here (near noble metal) react with poly-Si and TiSi2 ohimc layer to form silicide at lower temperature (100–450 C) than those in the second group (525–650 C) [46]. Therefore, it is not surprising that the metals from the ﬁrst and second group fail as diﬀusion barriers at relatively low temperatures. The failure mechanism is usually due to the high reactivity and diﬀusivity between Pt and barrier metal, TiSi2, Si or between oxygen and barrier metal, TiSi2, Si, followed by various compounds such as electrode material–barrier metal, silicides and oxides for the metals at relatively low temperatures in the ﬁrst and second group. In general, transition metals are not stable diﬀusion barriers for capacitor bottom electrodes. Second, transition metal alloy barriers [47] studied usually consist of near noble metal and on refractory metal element. They can be deposited in amorphous states [48,49], hence free of grain boundaries. However, the crystallization temperatures of these amorphous alloy ﬁlms are not as high as those of some other binary or ternary amorphous systems, which consist of at least one nonmetal element. Grain boundaries form in these ﬁlms at elevated temperatures. Nevertheless, a stable temperature of transition metal alloy diﬀusion barriers, which means that the temperature can prevent the reaction between electrode material and underlying layers at a given

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

285

annealing temperatures and times, are higher than those of polycrystalline transition metal diﬀusion barriers. The metal alloy–electrode material, ohmic layer and Si reaction is suppressed due to the presence of an amorphous microstructure. However, the diﬀusion of oxygen through metal alloy barrier, and reaction with oxygen cannot prevent. Failure occurs as a result of electrode material, oxygen and Si–metal alloy reaction or oxidation of underlying layers due to oxygen diﬀusion through grain boundaries of the polycrystalline metal alloy ﬁlms at elevated temperatures. Third, for Cu metallization processes, the eﬀectiveness of silicides or transition metal–Si systems as diﬀusion barriers has been reported that a trend similar to that of their corresponding transition metal diﬀusion barriers, but the silicides are better by 100–200 C. The diﬀusion barriers formed by refractory metal (Ta and W)–Si [50] are more stable (stable temperature ranges from 450 to 700 C) than near-noble metal (Cr and Co)–Si [38] barriers (stable temperature is about 300 C). Adding Si to refractory metals (Ta, Mo and W) to form an amorphous refractory metal–Si diﬀusion barrier also improves barrier performance (stable temperature=450– 700 C) [51] as compared to a polycrystalline W barrier (stable temperature=200– 450 C). The barrier fail by the reaction of Cu with the silicide barrier to form Cu silicides, by Cu diﬀusion through polycrystalline transition metal silicides, or by Cuinduced premature crystallization of the amorphous metal–Si barrier ﬁlms. On the other hand, silicides or transition metal–Si systems as diﬀusion barriers for highdensity capacitor bottom electrode would be expected that their properties are similar to that of polycrystalline metals. In addition, the reaction between oxygen and elements such as Si, Mo, Ta, W, Co and Cr consisting of silicides occurs at the relatively low temperature because the chemical aﬃnity between oxygen and elements of silicides is strong, and the binding force between Si-metals is weak [43]. Fourth, TiN [52], HfN [53], TaN [54] and WN [55] are fourth transition metal– nitrogen systems studied for diﬀusion barrier applications. The high stability of these barriers is due to the non-reactivity of Cu with N, Ta and W. Barrier failure is caused by diﬀusion of Cu along grain boundaries or through defects generated at elevated temperature in the barrier ﬁlms, which are relatively intact, or by the reactions between barrier ﬁlms and Si to form metal-rich silicides. For the application of the high-density memory devices, the barrier properties of fourth transition metal– nitrogen systems, that is, the inter-diﬀusion of electrode material and Si through diﬀusion barrier layer, are better than that of single metal, alloy and silicide barriers. This is because the binding force of nitride barriers is stronger than that of them. However, the oxidation properties of nitride barriers are similar to those of them during the thermal anneals at the temperature range of 500–600 C in oxygen atmosphere. Therefore, the failure mechanism of these barriers is the transformation from nitrides to oxides such as TiO2, Ta2O5, HfO2 and WO2 by reaction between nitrides and external oxygen. Finally, some amorphous ternary systems consisting of either one or two nonmetal components such as Ta–Si–N and W–Si–N [56] exhibit highly stable barrier properties due to their high crystallization temperatures. Although their crystallization process is somewhat accelerated by the presence of a Cu over-layer, their stable temperatures are still among the highest. When these barriers are used as a

286

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

diﬀusion barrier for capacitor bottom electrode, the barrier properties with respect to the reaction between electrode material and underlying layers are superior to others barriers including metal, alloy, silicide and nitride barriers. It has been reported that the surface of the Ta–Si–N, amorphous ternary compound barrier is oxidized [57]. It indicates that the amorphous barrier could not be used as an oxygen diﬀusion barrier for the lower electrode because the formation of thin oxidized layer, such as Ta2O5 and SiO2 at the surface of the Ta–Si–N ﬁlm, terminates the electrical contact. Therefore, they are oxidized under capacitor integration process and thus, could not to be used as an oxygen diﬀusion barrier. The failure mechanism of amorphous ternary barriers is the surface oxidation of barriers themselves by reaction between barrier ﬁlms and in-diﬀused oxygen because the binding force formed between each element consisting of barrier ﬁlms is weak to prevent the oxidation.

3. Suggestion of new design concept for diﬀusion barrier of high-density memory capacitors The high-density DRAM cell structures have to stack a capacitor on a poly-Si plug to realize 1transistor/1capacitor cells. Based on Section 2, as shown in Fig. 7, the failure mechanism of various diﬀusion barriers is divided into the two modes; inter-diﬀusion or reaction involving electrode, barrier metal, silicide, oxygen, and Si, and oxidation of diﬀusion barrier. The polycrystalline transition metal barriers, polycrystalline or amorphous transition metal alloy barriers, and polycrystalline or amorphous transition metal–silicon (including silicide) barriers comply with the ﬁrst failure mechanism. The polycrystalline or amorphous transition metal–nitrogen (including nitride), –oxygen, and –boron (borides) barriers and amorphous ternary barriers conform to the second failure mechanism. A diﬀusion barrier for high-density

Fig. 7. Failure mechanism of diﬀusion barrier developed up to these days in memory devices.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

287

DRAM/FRAM capacitor bottom electrodes should not only prevent the inter-diffusion of electrode metal, Si and oxygen but also require the high oxidation resistance at high temperatures. The previously developed barriers such as TiN [45], TaN [58], amorphous-WN [59] and Ta–Si–N [57], for Al or Cu metallization are easily susceptible to in-diﬀusion as well as a reaction with the oxygen during capacitor fabrication processes because of high reactivity and fast diﬀusivity of oxygen. To improve the oxidation resistance and prevent the in-diﬀusion of oxygen at high temperatures, a design concept in order to develop a new diﬀusion barrier for high-density DRAM capacitor bottom electrode should be diﬀerent from that for Al or Cu metallization. The diﬀusion barrier for Al metallization was designed as a concept for preventing the reaction between Al metal and barrier layer because of good reactivity of Al metal [60–63]. Refractory metals, refractory metal incorporated oxide and nitrides have been extensively studied as a diﬀusion barrier for Cu metallization because of fast diﬀusivity of Cu metal, which do not react with Cu because of a thermodynamically stable interface in barrier metals/Cu contact system [64–67]. The oxygen in the capacitor bottom electrode has both drawbacks of Al and Cu because oxygen not only reacts easily to oxidize the diﬀusion barrier, but also diﬀuses rapidly through the diﬀusion barrier and then oxidizes the underlying layer. In order to prevent the fast diﬀusion of oxygen at high temperatures, the microstructure of the diﬀusion barrier itself should be an amorphous structure with no grain boundaries. To prevent the good reactivity of oxygen, the binding force formed between the matrix metal and the added material should be chemically strong in the as-deposited state. To satisfy these requirements, the diﬀusion barrier needs the added material into the matrix metal. The added material should be conductive oxide because the electrical properties should remain after annealing in an oxygen atmosphere. In this work, cerium dioxide (CeO2) and ruthenium dioxide (RuO2) are chosen, as an added material into the Ta matrix metal. The reasons are as follows: ﬁrst, they are electrically conductive (RuO2) and non-conductive (CeO2) oxide. The incorporation of CeO2 and RuO2 into a Ta barrier layer will amorphize the microstructure of barrier itself. Finally, they will form the strong bonds with Ta–O or Ta–Ce (Ru)–O. 3.1. Barrier properties of Ta ﬁlm prepared with/without non-conductive CeO2 added 3.1.1. Thermal stability of diﬀusion barrier and barrier/Si interface First, the eﬀect of CeO2 addition on the microstructural modiﬁcation of diﬀusion barrier, the thermal stability of Ta+CeO2 diﬀusion barrier and Ta+CeO2/Si interface, and the change of binding state with Ta–Ce–O system depending on the amount of CeO2 and vacuum annealing temperature was investigated. The Ta ﬁlms were deposited with CeO2 addition, depending on rf power. The thickness and composition of the deposited Ta+CeO2 barrier ﬁlms were analyzed by the RBS rump simulation and the results are shown in Fig. 8. Oxygen resonance (3.06 MeV, H++ ion) was used to obtain the amount of oxygen in the Ta+CeO2 ﬁlm. As shown from Fig. 8, the amount of CeO2 in the Ta+CeO2 ﬁlm increased with increasing rf power.

288

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

To examine the variation in sheet resistance according to the amount of CeO2 added for Ta+CeO2 (100 nm)/Si structure, rf power of Ta target was ﬁxed at 200 W, and that of the CeO2 was varied from 130 to 170 W. The results were shown in Fig. 9 and show the various annealing temperatures for 30 min in vacuum. The change in electrical resistance for as-deposited samples is attributed to the amount of CeO2 with the variation of rf power. In the case of 130 and 150 W rf power, the resistance gradually increased with the annealing temperature. When the Ta+CeO2 barrier was deposited by 170 W rf power, however, the behavior of sheet resistance is similar to the data of 130 and 150 W rf power up to 700 C, and it drastically increases thereafter. These results imply that the change of microstructure of diﬀusion barrier or reactions involving Ta, Ce, O and Si take place with an increase in the annealing temperature. From the sheet resistance data, the variation of sheet resistance for Ta+CeO2 barrier itself is measured at a range of 55–728 ohm/sq according to the amount of CeO2 added and the vacuum annealing temperature. Although the electrical resistance of Ta+CeO2 barrier is higher than that of Ta barrier layer prepared without CeO2 added, it is thought that the Ta+CeO2 ﬁlm, as a diﬀusion barrier for the high-density DRAM/FRAM capacitor bottom electrodes, can be applied to Si process technology. To determine the crystal structure of diﬀusion barrier itself and the thermal stability of barrier/Si interface, glancing angle XRD analysis was performed for Ta+CeO2/Si contact system depending on the amount of CeO2 and annealing temperature, and the results were shown in Fig. 10, respectively. For Ta/Si systems prepared by CeO2 addition, XRD peaks corresponding to Ta were considerably broadened for the as-deposited samples. This implies that the CeO2 addition during the deposition of the Ta layer led to the microstructural modiﬁcation of diﬀusion barrier. The as-deposited samples of Ta+CeO2 diﬀusion barrier clearly showed an amorphous-like microstructure without any distinct peaks as shown in Fig. 10. When the CeO2 addition into a Ta layer was prepared at 130 and 150 W rf power,

Fig. 8. RBS spectra for Ta+CeO2 ﬁlm with rf power.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

289

Fig. 9. The variation of sheet resistance according to the amount of CeO2 added and various annealing temperature for Ta+CeO2 (100 nm)/Si structure.

Fig. 10. XRD patterns according to the amount of CeO2 added and various annealing temperature for Ta+CeO2 (100 nm)/Si structure.

respectively, the amorphous microstructure of the as-deposited Ta+CeO2 diﬀusion barrier was sustained up to 750 C, and then it crystallized after annealing at 800 C, as shown in Fig. 10(a) and (b), respectively. However, the amorphous microstructure in which the CeO2 addition in Ta layer was done by 170 W rf power, was sustained stable up to 800 C. In addition, after annealing at 750 C, the peak separation occurred and more clearly observed at 800 C, as shown in Fig. 10(c).

290

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

The broad peak near 28 corresponds to CeO2 and the resistance sharply increased after annealing at 750 C, as shown in Fig. 10(c). These indicate that the crystallization temperature of amorphous Ta+CeO2 diﬀusion barrier depends on the amount of CeO2 added. In the experimental results, the thermal stability between Ta+CeO2 barrier and Si substrate was sustained up to 800? irrespective of the amount of CeO2 added. This implies that the Ta+CeO2/Si interface was not broken and did not form Ta-silicides. The silicidation temperature for the Ta+CeO2/Si system was signiﬁcantly enhanced compared to that for the Ta/Si system, where Ta formed a silicide at 650 C [46]. Since fast diﬀusion along grain boundaries in diffusion barrier and free Si due to Ta-silicide formation result in the degradation of diﬀusion barrier at low temperature, the amorphous microstructure and suppression of Ta-silicide formation shows the desirable properties of Ta+CeO2 diﬀusion barrier for preventing the in-diﬀusion of oxygen, electrode material and out-diﬀusion of Si at high temperatures. In order to investigate the binding state for the Ta–Ce–O system, XPS was examined for the samples annealed at elevated temperature, and the results were shown in Fig. 11. After loading the Ta+CeO2 samples into the chamber, in–situ ion beam sputter etching was performed to remove the surface oxide and carbon for 10 min. Fig. 11 showed the Ta4f spectra for the Ta+CeO2 (130, 150, 170 W) ﬁlms with annealing temperature, and was compared with the Ta4f spectrum from pure Ta, respectively. In comparison to the pure Ta spectrum, the Ta4f spectrum of Ta–Ce–O (130 W) ﬁlm showed two deconvolution at high binding energy for as-deposited sample, corresponding to the binding state between Ta and O, as shown in Fig. 11(b). This implies that Ta is bound to oxygen during deposition at room temperature for the Ta+CeO2/Si system. After annealing at 700 C, the two deconvoluted peaks changed, and the number of chemical bonds of Ta–O or Ta—Ce–O were increased, and became clearer at 800 C, as seen in Fig. 11(b). In the case of the Ta+CeO2 (150 W) ﬁlms, the results are similar to the data of the Ta+CeO2 (150 W) ﬁlms, as shown in Fig. 11(c). Fig. 11(d) showed the Ta4f spectrum of Ta–Ce–O (170 W) ﬁlm, which exhibited the higher sheet resistance and larger amount of CeO2 added compared to the Ta+CeO2 (130, 150 W) ﬁlms. The number of chemical bonds of Ta–O increased with increasing annealing temperature. When the Ta4f spectra of Ta–Ce–O (170 W) ﬁlms were compared with that of Ta–Ce–O (130, 150 W) ﬁlms, the number of bonds of Ta–O more clearly increased with increasing annealing temperature because of large amount of CeO2. These results suggested that the electron transfer from Ta to O took place during annealing, and that the oxygen atoms formed strong covalent or ionic bonds with Ta atoms. Figs. 10 and 11 showed the microstructural and binding state change of diﬀusion barrier itself depending on the amount of CeO2 added and the annealing temperature. The microstructure of the Ta+CeO2 ﬁlm showed an amorphous structure in the as-deposited state and the interface stability of Ta+CeO2/Si systems is a stable to 800 C, irrespective of amount of CeO2 added in the Ta ﬁlm. In addition, the Ta– O bonds with the Ta–Ce–O system are formed in the as-deposited state and increased with increasing annealing temperature and amount of CeO2 added. The chemically strong bonds of Ta or Ta–Ce–O result in an amorphous microstructure

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

291

Fig. 11. Ta 4f XPS peak of Ta+CeO2 ﬁlm with amount of CeO2 added and various annealing temperatures.

292

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

of Ta+CeO2 diﬀusion barrier in the as-deposited state because of the surface mobility limitation of adatoms during the deposition at room temperature, as already seen in Fig. 10. The higher thermal stability at the Ta+CeO2/Si interface than at the Ta/Si interface is attributed to the chemically strong bonds of Ta or Ta– Ce–O and an amorphous microstructure with Ta–Ce–O system. This is supported by the results that the interfacial stability of the nitride ﬁlms, which have the strong bonds between metals and nitrogen, are higher than that of polycrystalline metal ﬁlms. In addition, the crystallization temperature of an amorphous structure in the Ta+CeO2 (130, 150 W) ﬁlms is lower than that in the Ta+CeO2 (170 W) ﬁlm, as shown in Fig. 10. This is because the amount of CeO2 added and the number of Ta– O bonds in the Ta+CeO2 ﬁlm is small. The number of the metallic state Ta, Ta–Ta bonds, that is, is much larger than that of the Ta–O bonds. In order to elucidate the eﬀects of CeO2 incorporation into Ta ﬁlm, the microstructure of Ta+CeO2 (130, 170 W) layer were investigated by plan-view TEM and compared with those of Ta layer. Fig. 12 showed the plan-view TEM micrographs of Ta- and CeO2-incorporated Ta layers in the as-deposited state, respectively. The low magniﬁcation image of the Ta layer shows many black spots, which are thought to be crystallites, as shown in Fig. 12(a). This implies that an as-sputtered Ta ﬁlm has a polycrystalline microstructure. Many rings in the selected area diﬀraction

Fig. 12. Plan-view TEM images of Ta (a), (b) and CeO2-incorporated Ta ﬁlm (c), (d).

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

293

pattern also indicate that Ta ﬁlm is a polycrystalline ﬁlm whose grain size is very small. In order to clarify the microstructure of Ta ﬁlm, high resolution (HR) TEM was performed. As shown in Fig. 12(b), the HR-TEM micrograph of Ta layer shows many regions indicating a regular arrangement of atoms. Meanwhile, for the planview TEM micrograph of CeO2-incorporated Ta layer, no black spot was detected in the low magniﬁcation image, and an diﬀused ring was observed in the selected area diﬀraction pattern, as shown in Fig. 12(c). This indicates that Ta+CeO2 ﬁlm is an amorphous-like ﬁlm. In the HR-TEM micrograph of Ta+CeO2 layer, no indication of a regular arrangement of atoms was detected, as shown in Fig. 12(d). From the plan-view TEM work, it is demonstrated that Ta ﬁlm is a polycrystalline ﬁlm and that CeO2-incorporated Ta (130, 170 W) ﬁlm has an amorphous-like microstructure. In order to clarify the microstructure of Ta+CeO2 diﬀusion barrier, plain-view TEM analysis with the selected area diﬀraction pattern was performed depending on the amount of CeO2 added and the annealing temperature and the results were shown in Fig. 13. When the CeO2 addition in the Ta ﬁlm was deposited at 130 W rf power, the as-deposited micrograph of Fig. 13(a) clearly showed that the microstructure of diﬀusion barrier appeared to be an amorphous structure and it was partially crystallized after annealing at 800 C. When the CeO2 addition in the Ta ﬁlm was deposited at 170 W rf power, however, the microstructure of the as-deposited state was similar to the result of 130 W and its microstructure was sustained stable up to 800 C, as seen in Fig. 13(b). These were consistent with the glancingXRD results shown in Fig. 10. The ideal diﬀusion barrier between electrode materials and TiSi2 or poly-Si for the high-density DRAM capacitor bottom electrode need to be chemically stable (low reactivity between electrode materials and TiSi2 or poly-Si), free of defects such as grain boundaries (amorphous structure) up to elevated temperatures, and resistant to high solubilities and high diﬀusivities of oxygen, electrode materials, and Si. It has been suggested that the formation of refractory metal silicide is initiated by grain boundary diﬀusion of Si into the metal over-layer, inhomogeneously, followed by silicide formation at grain surface [68]. This implies that the grain boundaries play a role in supplying a signiﬁcant number of Si atoms into the Ta matrix with relatively low activation energy, followed by the initiation of a reaction between Ta and Si. For the Ta/Si system, the Ta-silicides were formed at 650 C, whereas for Ta+CeO2/Si system, the interfacial stability remained stable even after annealing at 800 C. These dissimilar results may be interpreted as due to the role of CeO2 in the Ta layer. From the GXRD, XPS, and the plain-view TEM results, it should be noted that the role of CeO2 in the Ta layer is as follows: the CeO2 addition during the deposition of the Ta layer leads to the formation of an amorphous microstructure and the number of chemically strong bonds increases with Ta–O or Ta– Ce–O. In addition, the cerium dioxide can not easily diﬀuse underneath layer because of its larger atomic size and heavier atomic weight compared to stuﬀed-O2 or N2. Therefore, when Ta ﬁlm was deposited with CeO2 added, the amorphous microstructure as well as the number of chemical bonds with Ta–Ce–O resulted in a higher thermal stability than that of the Ta/Si system. Therefore, these might be expected that the inter-diﬀusion of Pt, electrode element and Si of TiSi2 or poly-Si

294

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 13. Plan-view TEM images of Ta +CeO2(130 W)(a) and Ta +CeO2(170 W)(b) with various annealing temperatures.

through diﬀusion barrier would prevent up to high temperatures. When the Ta ﬁlm was deposited with CeO2 addition, in particular, 170 W rf power, the Ta+CeO2 diﬀusion barrier could have eﬀectively retarded the oxygen in-diﬀusion due to an amorphous microstructure with large amount of CeO2 added and the number of Ta–O bonds. 3.1.2. Barrier properties of Ta barrier prepared with/without non-conductive CeO2 added The variation in sheet resistance for Pt/Ta/TiSi2/poly-Si/SiO2/Si structure with and without CeO2 addition versus post-annealing temperature was shown in Fig. 14. Note that for the Pt(50 nm)/Ta(10, 50 nm)/TiSi2/poly-Si/SiO2/Si structure after annealing at 650 C, the sheet resistance can not be measured because of reactions involving barrier layer (Ta), bottom electrode (Pt), ohmic contact layer (TiSi2), polyplug (poly-Si) and ambient oxygen as shown in Fig. 15. However, for Pt/Ta+CeO2/

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

295

Fig. 14. Variation of sheet resistance for for both the Pt/Ta/TiSi2/poly-Si/SiO2/Si and the Pt/Ta+CeO2/ TiSi2/poly-Si/SiO2/Si structure before and after annealing for 30 min in air.

Fig. 15. X-ray diﬀraction results of the Pt/Ta/TiSi2/poly-Si/SiO2/Si system at which the Ta diﬀusion barrier was deposited without and with CeO2 added and annealed at various temperatures for 30 min in air.

296

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

TiSi2/poly-Si/SiO2/Si structures with diﬀerent thickness of barrier ﬁlm and amount of CeO2 added, it appeared that the sheet resistance remained constant up to 800 C for all systems and the color of sample surface sustained the bright Pt color even after annealing at 800 C. Since the sharp change in the resistance–temperature trace indicates a reaction involving the O, Pt, Ta+CeO2 and Si, it implies that the in-diffusion of Pt through the diﬀusion barrier to form the Pt-silicides at the Ta+CeO2/ TiSi2 interface does not occur and the inter-diﬀusion at the interface between Pt and Ta+CeO2 does not take place. The high permittivity metal oxides usually require high-temperature processing (deposition and post annealing) in oxygen ambient. Since the high dielectric thin ﬁlms deposited at room temperature are amorphous, the formation of the perovskite phase generally demands a high temperature of above 600 C. For these reasons, the ﬁrst annealing temperature was carried out at 650 C in air atmosphere. The highest temperature at which a barrier layer prevents interactions involving barrier layer (Ta), bottom electrode (Pt), ohmic contact layer(TiSi2), poly-plug (poly-Si) and ambient oxygen during the subsequent annealing can be used as a measure of diﬀusion barrier eﬀectiveness. In this respect, the incorporation of CeO2 in the Ta layer signiﬁcantly enhances the barrier property of Ta in the Pt/Ta/TiSi2/poly-Si/SiO2/Si system as shown in Fig. 14. The thickness of bottom electrode (Pt) for all systems was 50 nm. In particular, for the barrier layer (Ta+CeO2) with 10 nm thickness, the sheet resistance with increasing annealing temperature was similar to the data of 50 nm thickness. These results implied that for a Ta+CeO2 layer of 10 nm thickness, the barrier property was as much as that for a Ta+CeO2 layer of 50 nm thickness. It should be note that the Ta barrier properties with CeO2 addition did not depend on the barrier thickness, as a result of presented data. But, since the CeO2 is dielectric material, the electrical resistance of Ta+CeO2 diﬀusion barrier ﬁlm is higher than that of Ta barrier ﬁlm. The X-ray diﬀraction results of the Pt(50 nm)/Ta(50 nm)/TiSi2/poly-Si/SiO2/Si system at which the Ta diﬀusion barrier without and with CeO2 added was deposited after annealing at various temperatures for 30 min in air atmosphere were shown in Fig. 15. It clearly showed that the Pt-silicide, Pt–Ta compound and Ta2O5 after annealing at 650 C, which can not be measured in the sheet resistance, were formed by reactions involving barrier layer (Ta), bottom electrode (Pt), ohmic contact layer (TiSi2), poly-plug (poly-Si) and ambient oxygen. The interaction between Pt and ohmic contact layer (TiSi2) and poly-plug (poly-Si) led to the formation of Ptsilicide. This is supported by the fact that the Pt-silicide is formed at relatively low temperature ranging from 300 to 400 C and Pt is moving species in the Pt-silicide (Ptrich or Si-rich) formation in Pt/Si system [46]. Also, the interdiﬀusion of Pt and Ta led to the formation of Pt–Ta compounds [69]. The ambient oxygen can freely diﬀuse through the grains and grain boundaries of bottom electrode because the Pt does not form a stable oxide, and thus the oxygen can form the tantalum oxide (Ta2O5) by reaction with Ta. These results are similar to that of the Pt/Ti/SiO2/Si system, which is the diﬀusion of both Ti and O through the Pt layer, resulting in the formation of TiO2 and Pt-Ti compounds [45]. The X-ray diﬀraction results of the Pt/Ta+CeO2/ TiSi2/poly-Si/SiO2/Si systems annealed at various temperatures were shown in

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

297

Fig. 15(b). The rf power of CeO2 target was 170 W, and the thickness of diﬀusion barrier layer was 10 nm, respectively. The CeO2 peaks were not detected in XRD patterns for all systems. The two possibilities for this result may be predicted. It ﬁrst noted that the amount of CeO2 added was very small. The peak positions of CeO2 can be overlapped that of the other elements (Si, Ta, Pt) and compound (TiSi2). However, since the CeO2 peak position with maximum intensity ratio was near 28 , it did not overlap with the other elements and compounds. Therefore, it is thought that the CeO2 peaks did not detected in XRD patterns because of the small amount of CeO2 added. As shown in Fig. 15(b), in addition, an amorphous Ta+CeO2 diffusion barrier has lower intensity than other elements (Si) and compound (TiSi2), and thus it was not appeared in XRD pattern in the as-deposited state. It showed no indication of reaction products even after annealing at 800 C. These results indicated no reacted precipitates involving ambient oxygen, platinum and Si, and consisted with the sheet resistance measurements shown in Fig. 14. This behavior of Ta diﬀusion barrier with CeO2 addition was contrasted with that of no CeO2 addition, which the inter-diﬀusion of O, Ta, Pt and Si through diﬀusion barrier to form the Ta2O5, Pt–Ta compounds and Pt-silicides after annealing at 650 C occurred as already shown in Fig. 15(a). Therefore, the thermal stability of Ta diﬀusion barrier with CeO2 addition was better than that of no CeO2 addition. Fig. 16(a) showed optical micrographs of the Pt surface in Pt(50 nm)/Ta(50 nm)/ TiSi2/poly-Si/SiO2/Si systems annealed at various temperatures. The reaction products were observed after annealing at 650 C, and the density and size of reacted products increased with annealing temperature. This behavior was consistent with the XRD results shown in Fig. 15(a). From the above results, the Ta diﬀusion barrier without CeO2 addition at 650 C was completely degraded by the reactions involving oxygen, Ta, Pt and Si. Therefore, it can be expected that the 50 nm-Ta diﬀusion barrier was failed at less than 650 C. The polycrystalline Ta ﬁlm has been widely studied as a diﬀusion barrier for Cu metallization material. Ta is nonreactive and immicible with Cu, and it reacts with Si to form TaSi2 around 650 C. Therefore, Ta is a good diﬀusion barrier candidate compared to other polycrystalline transition metal ﬁlms (Cr, Ni, Co), which usually react, with both Cu and Si at relatively low temperatures. However, Ta fails to prevent from diﬀusing through its grain boundaries to react with Si at 500 C in the Cu/Ta/Si contact structure. Ta barrier is completely vulnerable to the inter-diﬀusion of Pt, Si and O through diﬀusion barrier as well as the reaction between Pt, Si and oxygen, and Ta barrier ﬁlm after annealing at 650 C, irrespective of thickness of the Ta barrier ﬁlm, as shown in Fig. 16(a). These are attributed to the polycrystalline structure of the Ta ﬁlm, which has grain boundaries as can act the fast diﬀusion paths and reaction initiation at the early stage of the failure. Fig. 16(b) showed optical micrographs of the Pt surface in Pt/Ta+CeO2 (10 nm, 170 W)/TiSi2/poly-Si/SiO2/Si systems after annealing. It demonstrated a clean surface even after annealing at 800 C. This optical microscopy(OM) images are distinct from that for the Pt(50 nm)/Ta(10 nm, 50 nm)/TiSi2/ poly-Si/SiO2/Si system after annealing at 650 C in which the reacted precipitates involving oxygen, Ta, Pt and Si appear as shown in Fig. 16(b). This fact indicates that the inter-diﬀusion of O, Pt and Si through diﬀusion barrier is prevented up to

298

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 16. Optical micrographs of the Pt surface for both the Pt/Ta/TiSi2/poly-Si/SiO2/Si and the Pt/ Ta+CeO2(170 W)/TiSi2/poly-Si/SiO2/Si system annealed at various temperatures.

high temperatures. Generally, it has been reported that the failure temperature increased with increasing the thickness of the barrier layer, as expected from the fact that for a given contact system, the diﬀusion distance of O, Pt and Si will increase with the thickness of the diﬀusion barrier layer. In the present results, the thermal stability to the in-diﬀusion of oxygen, Pt and out-diﬀusion of Si for the Pt/ Ta+CeO2(10 nm)/TiSi2/poly-Si/SiO2/Si system was higher than that the for Pt(50 nm)/Ta(50 nm)/TiSi2/poly-Si/SiO2/Si system, although thickness of barrier layer with Ta+CeO2 ﬁlm was very thin. To investigate the reaction products for the Pt(50 nm)/Ta+CeO2(10 nm, 170 W)/ TiSi2/poly-Si/SiO2/Si systems as a function of annealing temperature, the cross-section TEM micrographs were shown in Fig. 17. The thin amorphous layer before heat treatment was observed at the interface between barrier layer (Ta+CeO2) and ohmic contact layer (TiSi2). The three possible explanation for the formation of amorphous layer is as follows: First, for the formation of ohmic contact layer (TiSi2) case, both TiSi2 and TiNx were simultaneously grown by rapid thermal annealing of Ti/poly-Si/SiO2/Si sample in a nitrogen ambient and then amorphous TiNx layer was etched by chemical solution. The residues of amorphous TiNx layer could be remained after chemical etching. However, the color of sample surface showed the

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

299

Fig. 17. TEM images of the Pt/Ta+CeO2(170 W)/TiSi2/poly-Si/SiO2/Si system annealed at various temperatures and EDS spectrum.

bright TiSi2 gray color. If the residues of amorphous TiNx layer on the TiSi2 surface remained, the brightness of TiSi2 surface would be lost. Prior to the deposition of diﬀusion barrier layer, the TiSi2 layer was sputter etched (dc bias 300 V for 10 min) to remove surface impurities or native oxide and thus the amorphous layer could be formed by Ar+ ion bombardment under dc ﬁeld. However, Ar+ ion energy by dc bias 300 V is very low to amorphize the crystalline TiSi2. Finally, the surface of TiSi2 ohmic contact layer could be thinly oxidized by oxygen ions in the plasma during the deposition of the Ta+CeO2 ﬁlm. Therefore, it is thought that the thin amorphous layer observed at the interface between Ta+CeO2 barrier ﬁlm and TiSi2 ohmic in the as-deposited state is attributed to the oxidized layer formed by oxygen ions. After annealing at 800 C for 30 min, the reacted products were observed at the ohmic contact (TiSi2)/poly-plug(poly-Si) interface. The reaction products using energy dispersive spectroscopy (EDS) were analyzed. The result was shown in Fig. 17(c). The reacted precipitates had the composition of 81.36 at.% Si and 18.36 at.% Ti, implying that the Ti-silicide was formed. The agglomeration of the TiSi2

300

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

ﬁlm with increasing annealing temperature was observed, resulting in the roughening at both TiSi2/poly-Si and Ta+CeO2/TiSi2 interface. This behavior with annealing temperature was consistent with the reported by others [70,71]. Since the structural uniformity of ohmic contact layer (TiSi2) inﬂuences the reliability of capacitor, the improvement of the thermal stability of TiSi2 layer should be required. But, even after annealing at 800 C, it showed that no reaction products by the reactions involving O, Pt and Si such as Ta2O5, Ta–Pt compounds, Pt-silicide were observed at Pt/ Ta+CeO2 and/or Ta+CeO2/TiSi2 and/or TiSi2/poly-Si interface. Currently, many diﬀusion barriers including Ti, Ta, O2 or N2-stuﬀed polycrystalline nitrides, amorphous WN and Ta–Si–N barriers [42–45,52–57] have been extensively examined for DRAM capacitor bottom electrodes. Ti and Ta are inserted as an adhesion layer and diﬀusion barrier for both Pt/Ti and Pt/Ta contact systems. After annealing at air or oxygen ambient, the reaction between Pt and Ti (or Ta) as well as the diﬀusion of oxygen through Pt grain boundaries occurs. This results in the formation of compounds such as Pt–Ti and Pt–Ta, and oxides such as TiO2 and Ta2O5. Therefore, the failure mechanism is usually due to the high reactivity and diﬀusivity between Pt and barrier metal, TiSi2, Si or between oxygen and barrier metal, TiSi2, Si, followed by various compounds such as electrode material–barrier metal, silicides and oxides for the metals at relatively low temperatures. As shown in Figs. 15(a) and 16(a), this is consistent with the results for the Pt/Ta/TiSi2/poly-Si/ SiO2/Si system. The polycrystalline and amorphous nitride diﬀusion barriers such as TiN, amorphous WN and Ta–Si–N can oxidize during capacitor fabrication such as high temperature deposition or post-annealing in oxygen ambient. For instance, TiN diﬀusion barrier is oxidized from TiN to TiO2 for the Pt/TiN/Ti system after annealing in oxygen ambient [72]. In addition, the O2 or N2 in the O2 or N2-stuﬀed polycrystalline nitride diﬀusion barriers can easily diﬀuse at relatively low temperatures because the binding force between the stuﬀed O2 or N2 and matrix is weak, and N and O have small atomic size and are light elements. Thus the inter-diﬀusion of O, Pt and Si through grain boundaries in barrier ﬁlm is susceptible to form SiO2 or Pt-silicide during deposition or post-annealing, resulting in the failure of diﬀusion barrier. In this section, when the Ta ﬁlm was deposited with CeO2 addition, the barrier properties showed the similar results irrespective of amount of CeO2 and barrier thickness. The barrier properties of the Pt/Ta+CeO2/TiSi2/poly-Si/SiO2/Si contact system were better than that of the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system, which observed the various reaction phases by reactions involving Pt, Ta, O, Ti and Si, such as Ta2O5, TiO2, SiO2, Pt-silicide and Pt–Ta compound. For the Pt/Ta+ CeO2/TiSi2/poly-Si/SiO2/Si contact system, the addition of cerium dioxide (CeO2) during Ta deposition leads to an amorphous microstructure. It has no grain boundaries that can act as fast diﬀusion paths for Pt, Si and oxygen. Also, it can not easily diﬀuse into underneath layer during annealing because CeO2 is much heavier atomic weight and have larger atomic size than O2 or N2 in the O2 or N2-stuﬀed polycrystalline nitride diﬀusion barriers and is strongly bounded to Ta matrix. These are supported by the previous results that CeO2-incorporated Ta barrier prevented the reaction between Al metal and Ta+CeO2 ﬁlm as well as the Cu in-diﬀusion

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

301

through the Ta+CeO2 layer up to high temperatures because of amorphous microstructure and chemically strong Ta–O or Ta–Ce–O bonds. So the inter-diﬀusion of O, Pt and Si through the diﬀusion barrier for the Pt/Ta+CeO2/TiSi2/poly-Si/SiO2/ Si system was prevented up to high temperatures compared to that for the Pt/Ta/ TiSi2/poly-Si/SiO2/Si system. In this respect, it can be concluded that the Ta barrier properties with CeO2 addition are quite superior to that without CeO2 incorporation as well as any diﬀusion barriers reported by others. 3.1.3. Summaries In Section 3.1, the eﬀects of CeO2 addition on the microstructural change of diﬀusion barrier itself and the barrier properties of a Ta diﬀusion barrier were investigated. The amorphous microstructure of Ta+CeO2 diﬀusion barrier was sustained up to high temperatures and the thermal stability of the Ta+CeO2/Si interface was higher than that of the Ta/Si interface. The Pt/Ta+CeO2/TiSi2/poly-Si/SiO2/Si systems retained their structures up to 800 C without an increase in resistivity, while the Pt/Ta/TiSi2/ poly-Si/SiO2/Si system completely degraded after annealing at 650 C. In the former case, CeO2 in the Ta layer plays a role to amorphize the microstructure of the Ta+CeO2 barrier ﬁlm and to strongly bind the Ta–O and/or Ta–Ce–O system during the deposition of the Ta layer. This prevented the inter-diﬀusion of O, Pt and Si through grain boundaries, which can act as fast diﬀusion paths to high temperatures. It had also much larger atomic size and heavier atomic weight than any stuﬀed elements. Therefore, the Ta ﬁlm prepared by CeO2 addition eﬀectively prevented the inter-diﬀusion of Pt, Si and oxygen through the diﬀusion barrier up to 800 C. 3.2. Barrier properties of the Ta-conductive RuO2 diﬀusion barrier 3.2.1. Oxidation resistance of the Ta–RuO2 diﬀusion barrier The sheet resistance for Ta/Si systems prepared with and without RuO2 addition was measured as a function of the amount of RuO2 added and post-annealing temperatures in air atmosphere for 30 min, respectively. The results are shown in Fig. 18. When the Ta layer was deposited without RuO2 addition, the sheet resistance of Ta the barrier ﬁlm could not be measured after annealing at 450 C. However, when the Ta +RuO2 ﬁlms were prepared with 100 and 150 W rf power, respectively, the sheet resistance decreased initially and thereafter, was sustained even after annealing at 800 C. In the case of 170 W rf power for the Ta+RuO2/Si system, the sheet resistance of the sample annealed at 550 C showed the range-error. The Ta ﬁlms were deposited with RuO2 addition, depending on rf power. The thickness and composition of the deposited Ta+RuO2 barrier ﬁlms were analyzed by the RBS rump simulation and the results are shown in Fig. 19. Oxygen resonance (3.06 MeV, H++ ion) was used to obtain the amount of oxygen in the Ta+RuO2 ﬁlm. As shown from Fig. 18, the amount of RuO2 in the Ta+RuO2 ﬁlm increased with decreasing rf power. It is thought that the deposition rate of Ta is relatively more decreased than that of RuO2 with decreasing the rf power. To investigate the structural change of the diﬀusion barrier itself and the reaction products of a Ta diﬀusion barrier deposited with and without RuO2 addition, glancing

302

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 18. Variation of sheet resistance for Ta+RuO2(100 nm)/Si system before and after annealing at diﬀerent temperatures in air and amount of RuO2 added.

angle XRD analysis (CuKa radiation=1.542 A˚) was performed for the Ta+RuO2/ Si contact systems and the results are shown in Fig. 20. The amount of RuO2 added and annealing temperature in air ambient was changed, respectively. In the case of the Ta layer prepared without RuO2 addition, the as-deposited Ta ﬁlm showed a polycrystalline microstructure and partially oxidized after annealing at 300 C. The Ta barrier layer was completely oxidized and the tantalum oxide (Ta2O5) was formed after annealing at 450 C. The tantalum oxide has a dielectric property and the sheet resistance could not be measured. For the Ta/Si system prepared with RuO2 addition, however, the XRD peak was considerably broadened for the asdeposited state. This implies that the RuO2 addition during the deposition of the Ta layer leads to the microstructural change of the diﬀusion barrier. The as-deposited sample of a Ta+RuO2 diﬀusion barrier clearly showed an amorphous microstructure without distinct peaks, as shown in Fig. 20(a) and (b). In the case of 100 W rf power, the RuO2 peaks in the Ta+RuO2 ﬁlm annealed at 650 C were observed and the peak intensity increased with increasing the annealing temperature. No

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

303

Fig. 19. RBS spectra for Ta+RuO2 ﬁlm with amount of RuO2 added.

other phases were observed except the formation of RuO2 phase. The lower sheet resistance of Ta+RuO2 sample annealed at 650 C compared to that of the asdeposited state is attributed to the formation of conductive oxide (RuO2) phase. The Ta2O5 formation in the Ta+RuO2 ﬁlm after annealing results in an abrupt increase of the sheet resistance because of a dielectric material. Therefore, the tantalum oxides did not form even after annealing at 800 C, as shown in Fig. 20(a). When the Ta +RuO2 layer was deposited with 150 W rf power, the glancing angle XRD result was similar to the data of 100 W rf power up to 750 C. The RuO2 and Ta2O5 peaks were simultaneously observed after annealing at 800 C, as shown in Fig. 20(b). However, although the dielectric oxide(Ta2O5) was formed, the sheet resistance was sustained up to 800 C, as shown in Fig. 18. It implies that the Ta+RuO2 sample deposited with 150 W rf power was not fully oxidized, but the surface was partially oxidized after annealing at 800 C. When the Ta+RuO2 layer was deposited with

304

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 20. X-ray diﬀraction results of Ta+RuO2/Si system at which the Ta diﬀusion barrier was deposited with RuO2 added and annealed at various temperatures for 30 min in air.

170 W rf power, the Ta peak disappeared and the broadened Ta2O5 peak was detected after annealing at 550 C, as shown in Fig. 20(c). These results are consistent with the sheet resistance results shown in Fig. 18. From the sheet resistance and glancing angle XRD results, the oxidation resistance of the Ta+RuO2/Si system was better than that of the Ta/Si system prepared without RuO2 addition. Titanium nitride has been investigated extensively for high dielectric applications. TiN barrier for oxygen diﬀusion was reported to peel oﬀ at 650 C because of volume expansion by oxidation of the TiN barrier [72]. Amorphous ternary compound barriers, which showed good barrier properties for Cu metallization, have been reported showing that the barrier surface is oxidized and

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

305

the oxygen penetrated into the barrier layer [57]. It indicates that the amorphous barrier could not be used as an oxygen diﬀusion barrier for the lower electrode. The reasons are as follows: ﬁrst, the electrical property of high dielectric ﬁlm degrades because a double capacitor is formed by surface oxidation of the diﬀusion barrier. The electron ﬂow from transistor to bottom electrode is screened because of the oxidized layer of the diﬀusion barrier. The electrical resistivity signiﬁcantly increases because of the amount of in-diﬀused oxygen in the barrier layer. Since the polycrystalline metal and nitride barriers, and amorphous ternary compound barriers developed for Al or Cu metallization, as an oxygen diﬀusion barrier for capacitor bottom electrodes, are oxidized under high temperature and oxygen ambient processes, it is expected that they could not be applied for high density thin ﬁlm capacitors. In the present study, the Ta+RuO2 diﬀusion barriers prepared with 100 and 150 W rf power prevented the surface oxidation up to 800 and 750 C, respectively, and the sheet resistance was sustained even after annealing at 800 C. In this point, the oxidation resistance of Ta–RuO2 diﬀusion barrier is superior to that of the polycrystalline nitrides (TiN, TaN) and amorphous ternary barriers reported by others. XPS was used to examine the binding state for the Ta+RuO2 samples annealed at elevated temperatures in an air atmosphere, and the results are shown in Fig. 21. After loading the Ta+RuO2 samples into the chamber, insitu ion beam sputter etching was performed to remove the surface oxide and carbon for 10 min. Fig. 21 shows the Ta4f and Ru3d spectra for the Ta+RuO2 (100 W) ﬁlms with diﬀerent annealing temperatures, respectively. From comparison with the Ta4f of pure Ta, the Ta4f spectrum showed two deconvoluted peaks at high binding energy in the

Fig. 21. Ta 4f and Ru 3d XPS peak of Ta+RuO2 (100 W) system annealed at various temperatures for 30 min in air.

306

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

as-deposited sample and they correspond to the binding state between Ta and O, as shown in Fig. 21(a). This indicates that for the Ta+RuO2/Si system, Ta is bound to oxygen during deposition at room temperature. In contrast, the Ru3d spectrum shows three deconvoluted peaks at low binding energy in the as-deposited sample, corresponding to the Ru metal and Ru–O binding state, as shown in Fig. 21(b). The chemically strong bonds of Ta–O or Ta–Ru–O result in an amorphous microstructure of the Ta+RuO2 diﬀusion barrier in the as-deposited state because of the surface mobility limitation of adatoms during the deposition at room temperature, as already seen in Fig. 20. After annealing at 700 C, the chemical bonds of Ta–O or Ta–Ru–O were slightly increased, and for the sample annealed at 800 C, it was similar to the result at 700 C, as shown in Fig. 21(a). In addition, the Ta metal states for the annealed samples at 700 and 800 C were changed compared to that in the as-deposited state. It is thought that the oxygen of Ta–O or Ru–O bonds is preferentially sputtered by Ar+ ions during insitu ion beam etching. As shown in Fig. 21(b), the Ru–O binding state was only observed after annealing at 700 C and showed a little change up to 800 C. For Ta+RuO2 layers deposited with 150 W rf power, the binding states of the Ta+RuO2 ﬁlm annealed at various temperatures in an air atmosphere are shown in Fig. 22. The Ta4f and Ru3d spectra were similar to the data of the Ta+RuO2 (100 W) ﬁlm in the as-deposited state. However, the amount of Ta–O bonds for the Ta+RuO2 (150 W) ﬁlm was smaller than that of the Ta+RuO2 (100 W) ﬁlm

Fig. 22. Ta 4f and Ru 3d XPS peak of Ta+RuO2 (150 W) system annealed at various temperatures for 30 min in air.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

307

because of the small amount of RuO2 in the Ta+RuO2 (150 W) ﬁlm. Comparing Fig. 21 with Fig. 22, the change of both Ta and Ru binding state with increasing annealing temperature showed the same behavior. Fig. 23 shows the Ta4f binding state of the Ta+RuO2 ﬁlm deposited with 170 W rf power in the as-deposited state. The Ta4f binding state in the as-deposited state is in contrast with that of the Ta+RuO2 (100, 150 W) ﬁlm. As shown in Fig. 23, the Ta+RuO2 (170 W) ﬁlm had much more Ta–Ta bonds than Ta–O bond. The amount of Ta–O bonds in the Ta+RuO2 (170 W) ﬁlm was much smaller than that in the Ta+RuO2 (100, 150 W) ﬁlm. In the present study, Ta+RuO2 diﬀusion barriers deposited with 100 and 150 W rf power showed that Ta is signiﬁcantly bound to oxygen in the as-deposited state, but RuO2 is deconvoluted into Ru metallic and Ru–O binding states. With increasing annealing temperature, the Ta–O bonds slightly increased compared to the asdeposited state, whereas the Ru–O bonds increased much more. When the Ta ﬁlm was deposited with 170 W rf power, however, the amount of metallic Ta–Ta bonds is larger than that of Ta–O bonds. With increasing annealing temperature, Ta,

Fig. 23. Ta 4f XPS peak of Ta+RuO2 (170 W) system in as-deposited state.

308

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

which is not bound to oxygen, changed into the tantalum oxide by reaction with the in-diﬀused oxygen from an air atmosphere. This is supported by the results of sheet resistance measurement and glancing angle XRD analysis. Correspondingly, the Ta+RuO2 diﬀusion barrier involving a large amount of RuO2 eﬀectively prevented the in-diﬀusion of oxygen and increased its oxidation resistance up to 800 C. In order to clarify the microstructure of the Ta+RuO2 (100 W) diﬀusion barrier, planar TEM analysis with a selected area diﬀraction pattern was performed for the samples annealed at various temperatures in an air atmosphere and the results are shown in Fig. 24. The micrograph for the as-deposited ﬁlm, Fig. 24(a), clearly demonstrates that the microstructure of diﬀusion barrier showed a nanocrystalline embedded-amorphous structure. From the selected area diﬀraction pattern of the as-deposited ﬁlm, the amorphous and the nanocrystalline phases were found to be Ta and RuOx, respectively. Here, the RuO2 in the Ta+RuO2 ﬁlm deposited from the Ta+RuO2 target is named as RuOx because it is neither Ru nor RuO2 from the diﬀracted pattern. As already seen in Fig. 20(a), the RuO2 phase in the as-deposited Ta+RuO2 (100 W) ﬁlm was not detected in glancing angle XRD pattern because of the small crystal size of RuOx phase. After annealing at 800 C, the nanocrystalline RuOx phase changed to RuO2 crystalline by reaction between RuOx and in-diﬀused oxygen from atmosphere. In addition, the Ta amorphous microstructure remained stable up to 800 C.

Fig. 24. Plan-view TEM images of Ta+RuO2 (100 W) ﬁlm with diﬀerent annealing temperatures in air for 30 min.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

309

In the previous section, based on the new design concept of diﬀusion barrier, Ta and CeO2 is chosen as a matrix metal and an added oxide, respectively. First of all, the microstructure of Ta+CeO2 (170 W) diﬀusion barrier is found to be an amorphous structure and its structure was sustained up to 800 C. This is attributed to the chemically strong bonds of the Ta–O and/or Ta–Ce–O system in the as-deposited state. Pt/Ta+CeO2 bilayer is deposited on the real capacitor structure, TiSi2/ poly-Si/SiO2/Si substrate and then the barrier properties such as the formation of Pt-silicides, Pt–Ta compound and oxides (Ta2O5, TiO2, SiO2) with respect to material reaction aspect were characterized. For the Pt/Ta+CeO2/TiSi2/poly-Si/SiO2/Si contact system, no various reaction phases were observed even after annealing at 800 C. However, oxidation resistance of Ta+CeO2 diﬀusion barrier itself did not characterize according to various annealing temperatures in air atmosphere. From comparison for the barrier properties of both Ta+CeO2 and Ta+RuO2 diﬀusion barriers according to XRD, XPS and plain-view TEM data, oxidation resistance of Ta+CeO2 diﬀusion barrier itself can be discussed. When the rf power of CeO2 target in the Ta+CeO2 ﬁlms was varied from 130 to 170 W, the amount of Ta–O bonds in the as-deposited state increased with increasing the rf power of CeO2 target, that is, increasing the amount of CeO2, as already shown in Fig. 11. In addition, the microstructure of the Ta+CeO2 diﬀusion barrier in the as-deposited state showed an amorphous structure, irrespective of the amount of CeO2, as already seen in Fig. 10. Therefore, both Ta+RuO2 and Ta+CeO2 barriers exhibited an amorphous microstructure in the as-deposited state. The large diﬀerence between Ta+RuO2 and Ta+CeO2 diﬀusion barriers, however, is the amount of Ta–O bond in the as-deposited state due to the amount of CeO2 and RuO2 added in the Ta ﬁlm, respectively. When the Ta+RuO2 ﬁlms were deposited with the rf power of 100 and 150 W, respectively, Ta is signiﬁcantly bound to oxygen in the as-deposited state. Whereas, the amount of Ta–O bonds with the Ta+CeO2 ﬁlm deposited by the rf power of 170 W is much smaller than that with the Ta+RuO2 ﬁlm prepared by the rf power of 100 and 150 W, from comparing Figs. 21 and 22 with Fig. 11. In the case of the Ta+CeO2 ﬁlm deposited by the rf power of 170 W, the amount of Ta–Ta bonds is larger than that of Ta–O bonds. This implies that the Ta–Ta bonds can easily react with oxygen in-diﬀused during annealing in air atmosphere. Therefore, it may be expected that the oxidation resistance of the Ta+CeO2 diﬀusion barrier is lower than that of the Ta+RuO2 (100, 150 W) diﬀusion barrier. If the amount of CeO2 added in the Ta ﬁlm is suﬃciently large (more than 50%), almost all Ta with Ta–Ce–O system is bound to oxygen in the as-deposited state, both barrier properties and oxidation resistance of Ta+CeO2 diﬀusion barrier would be clearly improved. However, the Ta+CeO2 diﬀusion barrier could not be used as a diﬀusion barrier for high-density DRAM/FRAM capacitor bottom electrodes because the electrical resistance of the Ta+CeO2 diﬀusion barrier itself would be very high due to electrical non-conducting oxide, CeO2. Moreover, this indicates that in order to simultaneously improve both barrier properties and oxidation resistance, the added oxide requires a large amount of electrically conducting material. From glancing angle XRD, XPS and TEM results, the Ta2O5 phase was not formed in the Ta+RuO2 (100 W) barrier ﬁlms after annealing in an air, but the

310

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

RuO2 phase was formed. This phenomenon can not be explained from the viewpoint of thermodynamics because the heat of formation of Ta2O5 is more negative than that of RuO2. This indicates that the in-diﬀused oxygen from an air atmosphere can react more easily with Ta than RuO2. However, Ta with Ta–Ru–O system does not exist in a pure metal state, but Ta is signiﬁcantly bound to oxygen in the as-deposited state. In addition, RuO2 exists as non-stoichiometry state because of the Ta–O binding state. Therefore, the Ta+RuO2 barrier ﬁlm consisted of Ta–O and RuOx rather than pure Ta and RuO2. Here, the driving force for oxidation may be diﬀerent from that required for the formation of Ta2O5 and RuO2 phase from pure Ta and Ru, respectively. From this viewpoint, it may be assumed that the driving force for Ta2O5 formation from Ta(O) by reaction with oxygen is comparable with that of RuO2 formation from RuOx. Moreover, the microstructure of the Ta+RuO2 diﬀusion barrier showed a Ta–O amorphous structure and an embedded RuOx nanocrystalline phase. That is, both Ta and O atoms should be rearranged to form the Ta2O5 phase, but the RuOx nanocrystalline phase by supplying external oxygen, may be more easily changed into the RuO2 crystalline phase. Therefore, the formation of the RuO2 phase by the reaction between the in-diﬀused oxygen from an air atmosphere and the RuOx nanocrystallites is kinetically more favorable than that of Ta2O5 phase. From the above discussion, the proposed tantalum–ruthenium dioxide diﬀusion barrier for high density thin ﬁlm capacitor bottom electrodes prevented both oxygen in-diﬀusion and its oxidation resistance even after annealing at 800 C because of the large amount of conductive RuO2 and chemically strong Ta–O bonds. In summary, the properties of both oxygen in-diﬀusion and oxidation resistance in a Ta+RuO2 layer for high-density memory devices were investigated. The Ta+RuO2/Si structure sustained up to 800 C without an increase in resistivity. The Ta+RuO2 diﬀusion barrier showed a Ta amorphous microstructure and an embedded RuOx nanocrystalline structure in the as-deposited state. The Ta+RuO2 ﬁlm showed the formation of RuO2 phase by reaction with the in-diﬀused oxygen from atmosphere after annealing in an air ambient. Ta is signiﬁcantly bound to oxygen in the as-deposited state, but RuO2 is divided into Ru and Ru–O binding state. The Ta–O bonds showed a little change compared to the as-deposited state with increasing annealing temperature, whereas Ru–O bonds increased much more. Correspondingly, RuO2 addition into the Ta layer improved the barrier properties for oxygen in-diﬀusion and its oxidation resistance up to 800 C. 3.2.2. Electrical properties of the Ta-conductive RuO2 diﬀusion barrier 3.2.2.1. Current-voltage characteristics of the Ta-conductive RuO2 diffusion barrier. The variation of the total resistance for the Ta ﬁlms prepared with and without RuO2 added on the TiSi2/poly-Si/SiO2/Si substrate was measured at V=0 as a function of post-annealing temperatures in an air atmosphere for 30 min, respectively. The results were shown in Fig. 25. When the Ta ﬁlm was deposited without a RuO2 addition, the total resistance for the Ta/TiSi2/poly-Si/SiO2/Si contact system was measured few tens of Mega- in value after annealing at 650 C and signiﬁcantly increased with increasing annealing temperature. In the case of the Ta+RuO2 ﬁlms fabricated with 100 W rf power, however, the total resistance for

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

311

the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system showed the higher value in the as-deposited state compared to that in the annealed samples, as shown in Fig. 25. After annealing at 650 C, the total resistance decreased initially and was sustained up to 800 C. Since good electrical conductivity between bottom electrode, diﬀusion barrier, poly-Si plug and transistor is responsible for the capacitor performance, it is thought that the lower total resistance of the annealed Ta+RuO2/TiSi2/poly-Si/ SiO2/Si sample is desirable feature for DRAM/FRAM capacitor bottom electrode. In order to investigate the ohmic characteristics, I–V measurement between 4 and 4 V was carried out for both Ta+RuO2 (100 W)/TiSi2/poly-Si/SiO2/Si and Ta/ TiSi2/poly-Si/SiO2/Si contact systems before and after annealing at various annealing temperatures. The results were shown in Fig. 26. In the case of the Ta ﬁlm prepared without RuO2 addition, as shown in Fig. 26(a), the I–V curves for the Ta/ TiSi2/poly-Si/SiO2/Si contact system showed ohmic characteristics in the as-deposited state and schottky behavior with increasing annealing temperature. For the Ta layer prepared with RuO2 addition, as shown in Fig. 26(b), I–V curve for the

Fig. 25. Variation of the total resistance for the Ta(100 nm)/TiSix/n++-poly-Si/SiO2/Si contact systems prepared with and without RuO2 addition.

312

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 26. I–V curves for both the Ta/TiSix/n++-poly-Si/SiO2/Si and the Ta+RuO2/TiSix/n++-poly-Si/ SiO2/Si contact systems before and after annealing at various temperatures.

Ta+RuO2(100 W)/TiSi2/poly-Si/SiO2/Si contact system showed the non-ohmic behavior in the as-deposited state. This result should be thought for Ta+RuO2/ TiSi2 and TiSi2/poly-Si interface, respectively. First, the formation of ohmic contact layer (TiSi2) was grown by rapid thermal annealing of Ti/poly-Si/SiO2/Si sample in nitrogen ambient and thus the TiSi2/poly-Si interface does not aﬀect the non-ohmic behavior. The surface of TiSi2 ohmic contact layer could be thinly oxidized by oxygen ions in the plasma during the deposition of the Ta+RuO2 ﬁlm. It is thought that the non-ohmic behavior in the as-deposited state is attributed to the thin oxidized layer formed at the interface between Ta+RuO2 barrier ﬁlm and TiSi2 ohmic layer. In the case of the Ta ﬁlm prepared without RuO2 addition, I–V curve for the Ta/TiSi2/poly-Si/SiO2/Si contact system showed the linear characteristics in the asdeposited state. This result is an evidence for showing the non-ohmic behavior of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system in the as-deposited state due to the formation of thin oxidized layer at the TiSi2 surface. In addition, the thin oxidized layer at the Ta+CeO2/TiSi2 interface for the Pt/Ta+CeO2/TiSi2/poly-Si/SiO2/Si contact system before annealing was observed by cross-sectional TEM. The Ta+ RuO2/TiSi2/poly-Si/SiO2/Si contact system showed the perfect ohmic behavior with increasing annealing temperature. Although the total resistance slightly increased, in particular, I–V curve showed the linear characteristics even after annealing at 800 C, as shown in Fig. 26(b). These results imply that the microstructural change of the diﬀusion barrier or reactions involving Ta, Ru and O take place during annealing at various temperatures in air. Fig. 27 showed the X-ray diﬀraction results for both Ta+RuO2 (100 W)/TiSi2/polySi/SiO2/Si and Ta/TiSi2/poly-Si/SiO2/Si contact systems according to the various

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

313

Fig. 27. X-ray diﬀraction results for both the Ta/TiSix/n++-poly-Si/SiO2/Si and the Ta+RuO2/TiSix/ n++-poly-Si/SiO2/Si contact systems before and after annealing at various temperatures.

annealing temperatures in air. In the case of the Ta ﬁlm deposited without RuO2 addition, both Ta barrier and TiSi2 ohmic layer was oxidized and then formed the various oxides (Ta2O5, TiO2, SiO2) by reactions involving Ta, Ti, Si and O after annealing at 650 C, as shown in Fig. 27(a). The oxidized layers formed in both Ta barrier ﬁlm and TiSi2 ohmic layers have a dielectric property. This increased the total resistance and resulted in the deviation from linearity of I–V curve, as shown in Figs. 25 and 26(a). When the Ta layer was deposited with RuO2 addition, the RuO2 peaks in the Ta+RuO2 ﬁlms were not detected in XRD patterns in the as-deposited state and three possibilities for this result can be thought. First, the amount of RuO2 added in the Ta ﬁlm was very small. From RBS data, the amount of RuO2 added in the Ta ﬁlm (Ta:RuO2=50:50) is suﬃcient for detecting by XRD. The peak positions of RuO2 can be overlapped with other elements (Si, Ta) and compound (TiSi2). However, since the RuO2 peak position with maximum intensity was near 28 , it did not overlap with other elements and compounds. Finally, the crystallite size of RuO2 is very small. Therefore, it is thought that RuO2 peaks were not detected in XRD patterns in the as-deposited state because of the small crystal size of RuO2. Ta peaks also did not appear in the as-deposited state. The Ta ﬁlm is amorphized by RuO2 incorporation. It has lower intensity than other elements (Si) and compound (TiSi2), and thus it was not appeared in XRD patterns. Therefore, the microstructure of Ta+RuO2 diﬀusion barrier was found to be a Ta amorphous structure and an embedded nanocrystalline RuOx in the as-deposited state. For the Ta+RuO2/TiSi2/

314

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

poly-Si/SiO2/Si contact system, the RuO2 peaks in the Ta+RuO2 ﬁlms annealed at 650 C were observed and no other phases were observed except for the formation of RuO2 phase with increasing the annealing temperature. In particular, Ta2O5 was not observed even after annealing at 800 C, as shown in Fig. 27(b). Both the formation of Ta2O5 in the Ta+RuO2 ﬁlm and oxidation of TiSi2 layer by reaction with external oxygen from air during annealing would have increased the total resistance and not appeared the ohmic behavior because the oxidized layer has a dielectric property. This indicates that the Ta+RuO2/TiSi2/poly-Si/SiO2/Si layer structure was not oxidized by the reaction with the in-diﬀused oxygen from air atmosphere. In the present study, the lower total resistance and linear I–V curve of the annealed Ta+RuO2 ﬁlms compared to that of as-deposited ﬁlm is attributed to the formation of conductive RuO2 phase in the barrier ﬁlm by reaction with oxygen after annealing in air. From the total resistance, I–V curve and glancing angle XRD results, both barrier properties and oxidation resistance for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system was better than that for the Ta/TiSi2/poly-Si/SiO2/Si contact system as well as that for various diﬀusion barriers previously reported by others. For instance, an amorphous W80N20 ﬁlm was oxidized by dry and wet oxidizing ambient in the temperature range of 450–575 C. In the case of TiN ﬁlm, although Pt bottom electrode is deposited and annealing duration is short, the electrical resistivity between Pt, TiN and poly-Si substrate at 650 C is too high. It has been reported that the surface of Ta–Si–N amorphous ternary barrier is oxidized. The formation of thin oxidized layer, such as Ta2O5 and SiO2 at the surface of the Ta–Si–N ﬁlm terminates the electrical contact from bottom electrode to transistor. Therefore, since the various barriers developed for metallization, as an oxygen diﬀusion barrier for DRAM capacitor bottom electrode, are oxidized under high dielectric thin ﬁlm integration processes, it is expected that they could not to be applied for high-density memory devices. In the present study, the Ta+RuO2 diﬀusion barrier prepared with 100 W rf power showed the lower total resistance and ohmic characteristics and its the surface oxidation was prevented even after annealing at 800 C. In order to investigate the diﬀusion behavior of the oxygen and other elements, AES depth proﬁle was carried out for the Ta+RuO2 (100 W)/TiSi2/poly-Si/SiO2/Si sample at no SiO2 capping layer before and after annealing at various temperatures and the results were shown in Fig. 28. We ﬁrst noted that the sputtering depth by Ar+ ion was performed to the surface of poly-Si for the Ta+RuO2/TiSi2/poly-Si/ SiO2/Si sample. The layer structure of the Ta+RuO2/TiSi2/poly-Si sample was well deﬁned in the as-deposited state. After annealing at 700 C, the surface of TiSi2 ohmic contact layer was oxidized by reaction with the in-diﬀused oxygen from air atmosphere. However, the oxidized thickness of TiSi2 layer was a little changed with the increasing annealing temperature. As shown in Fig. 28, the Si atoms of TiSi2 ohmic layer were oxidized by external oxygen. This is supported by the fact that the Si atom is dominant moving species during the silicide formation and oxidation. However, the oxygen did not diﬀuse to TiSi2/poly-Si interface even after annealing at 800 C. Note that the amount of Ru atoms in the Ta+RuO2 ﬁlm was gradually decreased with increasing annealing temperature.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

315

Fig. 29 showed the depth proﬁling of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si sample at the SiO2 capped layer after annealing at 700 C. This result is similar to the data of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si sample at no SiO2 capped layer after annealing at 700 C, as shown in Fig. 29. This implies that the SiO2 capped layer does not prevent the in-diﬀusion of oxygen during annealing in air.

Fig. 28. AES depth proﬁle for the Ta+RuO2 (100W)/TiSi2/poly-Si/SiO2/Si sample at no SiO2 capping layer before and after annealing at various temperatures.

316

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Figs. 28 and 29 showed the diﬀusion behavior of each element for the Ta+RuO2/ TiSi2/poly-Si/SiO2/Si contact system with and without the SiO2 capped layer part depending on the annealing temperature in air. For the Ta+RuO2/TiSi2/poly-Si/ SiO2/Si sample, it was observed the thin oxidized layer at the Ta+RuO2/TiSi2 interface irrespective of no SiO2 and SiO2 deposited layer. This indicates that the electrical contact between Ta+RuO2 barrier layer and TiSi2 ohmic layer is broken. Therefore, the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system should show the higher total resistance and non-ohmic behavior with increasing the annealing temperature. As shown in Figs. 25 and 26(b), however, the lower total resistance and ohmic characteristics for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system is appeared after annealing at various temperatures in air. To ﬁnd out the mechanism of ohmic behavior, the Ta+RuO2/TiSi2/poly-Si/SiO2/ Si contact systems with diﬀerent annealing temperatures were analyzed by scanning auger microscopy (SAM) micrographs and AES spectra, and the results were shown

Fig. 29. AES depth proﬁle for the Ta+RuO2 (100W)/TiSi2/poly-Si/SiO2/Si sample at SiO2 capping layer at 700 C for 30 min in air.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

317

in Fig. 30. In SAM images of Fig. 30, the depth proﬁles for the annealed Ta+RuO2/ TiSi2/poly-Si/SiO2/Si samples were obtained from area 1, the crystallites-free zone in Fig. 28. In addition, AES spectra in Fig. 30 were obtained by auger electron point analysis from area 2, the crystallites. A few numbers of the crystallites were distributed in the as-received state of as-deposited ﬁlm and remained after depth proﬁle. However, a large number of the crystallites in the annealed samples at 700 and 800 C compared to that in the as-deposited ﬁlm was observed even after depth proﬁle. This indicates that the crystallites exist at the surface of poly-Si. However, it is thought that the crystallites do not diﬀuse to the surface of poly-Si during the annealing because they are observed in the as-deposited state. Therefore, it would be taken place the diﬀerence of sputtering rate by Ar+ ion between crystallites and crystallite-free zone during insitu ion beam etching. The surface element constituents of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si samples annealed at 700 and 800 C were characterized by AES. The spectra taken from the crystallites consisted of Ru and O containing a small amount of Ta, but only Si peaks were detected for the crystallitesfree zone. Although the surface of TiSi2 ohmic contact layer was oxidized, the ohmic behavior for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system is attributed to an embedded RuO2 crystallites in the thin oxidized TiSi2 surface layer. To ﬁnd out the reaction products for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si sample after annealing at 800 C, cross-sectional TEM analysis was carried out and the results were shown in Fig. 31. Fig. 31(a) showed the full layer structure at low magniﬁcation. The thin oxidized layer at the Ta+RuO2/TiSi2 interface was

Fig. 30. SAM images and AES spectrum for the Ta+RuO2 (100 W)/TiSi2/poly-Si/SiO2/Si sample before and after annealing at various temperatures.

318

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 31. TEM images for the Ta+RuO2 (100 W)/TiSi2/poly-Si/SiO2/Si sample annealed at 800 C for 30 min in air.

observed. This result was consistent with AES depth proﬁle data shown in Fig. 28. In order to clearly investigate the Ta+RuO2/oxidized layer/TiSi2 interfaces, high resolution TEM was performed and the result was shown in Fig. 31(b). The invaded crystalline phases were observed in the thin oxidized layer. This was found to be RuO2 containing a small amount of Ta phase by d-spacing in value. From the I–V curve, AES and TEM results, although the surface of TiSi2 layer is oxidized by the reaction through in-diﬀused oxygen from air, the Ta+RuO2/TiSi2/ poly-Si/SiO2/Si contact system showed the linear I–V curve. The reason why ohmic behavior is exhibited for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system is attributed to an embedded RuO2 crystalline phase involving a small amount of Ta as shown by SAM, AES point analysis and TEM data. If the invaded RuO2 crystalline phase is the ohmic mechanism for Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system after annealing at diﬀerent temperatures, I–V curve of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si sample prepared without the SiO2 protection layer should show the linear behavior. That is, if the TiSi2 channel layer was not fully oxidized by the direct exposure in air and the TiSi2 layer on both sides of the Ta+RuO2/TiSi2 interface was only partially oxidized by the lateral diﬀusion of oxygen, and if the invaded RuO2 crystalline phase, which is the current path, did exist, then the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system should show an ohmic behavior after annealing in air. Fig. 32 showed the I–V curve of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si sample prepared without the SiO2 capping layer after annealing at diﬀerent temperatures in air. This result is similar to the data of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si sample prepared with the SiO2 capping layer shown in Fig. 26(b). Therefore, it suggests that the ohmic mechanism for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system is an embedded RuO2 crystalline phase in a Ta amorphous structure. 3.2.2.2. Barrier properties of the Pt/Ta-conductive RuO2 diffusion barrier. In the Section 3.2.2.1, the electrical properties for the Ta+RuO2 ﬁlm itself deposited on

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

319

Fig. 32. I–V curves for the Ta+RuO2/TiSix/n++-poly-Si/SiO2/Si contact systems without SiO2 capping layer before and after annealing at various temperatures.

the TiSi2/poly-Si/SiO2/Si substrate were characterized and ohmic mechanism for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system was suggested. In this section, when the full layer structure of bottom electrode was fabricated by Pt bottom electrode layer deposited on the Ta+RuO2 barrier ﬁlm, the barrier properties of the Pt/ Ta+RuO2 double layer were investigated and compared with those of Pt/Ta ﬁlms prepared without RuO2 addition. Fig. 33 showed the variation of the total resistance for both Pt/Ta/TiSi2/poly-Si/ SiO2/Si and Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact systems measured at V=0 as a function of post-annealing temperatures in an air atmosphere for 30 min, respectively. For the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system, the total resistance was measured 1.23105 in value after annealing at 650 C and further annealing temperature, dramatically increased. Whereas, for the Pt/Ta+RuO2/TiSi2/poly-Si/ SiO2/Si contact system, the total resistance showed the higher value in the asdeposited state compared to that in the annealed samples, as shown in Fig. 33. After

320

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 33. Variation of the total resistance for the Pt(100 nm)/Ta(100 nm)/TiSix/n++-poly-Si/SiO2/Si contact systems prepared with and without RuO2 addition.

annealing at 650 C, the total resistance decreased initially and was a little changed even after annealing at 800 C. Fig. 34 showed I–V curves for both Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si and Pt/ Ta/TiSi2/poly-Si/SiO2/Si contact systems measured at 4 V before and after annealing at various temperatures. As shown in Fig. 34(a), the I–V curves for the Pt/ Ta/TiSi2/poly-Si/SiO2/Si contact system showed ohmic characteristics in the asdeposited state and schottky behavior with increasing the annealing temperature. After annealing at 650 and 700 C, respectively, I–V curves for the Pt/Ta/TiSi2/polySi/SiO2/Si contact system clearly did not show the schottky behavior. This is because the amount of current ﬂow is smaller than in the as-deposited state. For the Ta layer prepared with RuO2 addition, as shown in Fig. 34(b), I–V curve for the Pt/ Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system exhibited the non-ohmic behavior

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

321

Fig. 34. I–V curves for both the Pt/Ta/TiSix/n++-poly-Si/SiO2/Si and the Pt/Ta+RuO2/TiSix/n++-polySi/SiO2/Si contact systems before and after annealing at various temperatures.

in the as-deposited state. This result should be thought for Pt/Ta+RuO2, Ta+ RuO2/TiSi2 and TiSi2/poly-Si interface, respectively. First, the Pt ﬁlm on the Ta+RuO2 layer was deposited without breaking vacuum. This implies that the interface between Pt metal ﬁlm and Ta+RuO2 layer is not a reason for the nonohmic characteristics in the as-deposited state. Second, TiSi2 was grown by rapid thermal annealing of Ti/poly-Si/SiO2/Si sample in the nitrogen ambient. The silicidation, that is, the TiSi2/poly-Si interface do not aﬀect the non-ohmic behavior. Finally, the surface of TiSi2 ohmic contact layer can thinly oxidize by oxygen ions in the plasma during the deposition of the Ta+RuO2 ﬁlm. The formation of thin oxidized layer at the interface between Ta+RuO2 barrier ﬁlm and TiSi2 ohmic layer can prevent the current ﬂow from the barrier ﬁlm to the TiSi2 layer. Therefore, the non-ohmic behavior in the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system is attributed to the thin oxidized layer at the Ta+RuO2/TiSi2 interface. This is also supported by the result that the I–V curve for the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system showed the linear characteristics in the as-deposited state, as shown in Fig. 34(a). The Pt/ Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system showed the linearity with increasing the annealing temperature. In particular, the I–V curve showed the linear characteristics even after annealing at 800 C, as shown in Fig. 34(b). These results imply that the microstructural change of the diﬀusion barrier or reactions involving Pt, Ta, Ru, Ti, Si and O take place during the annealing at various temperatures in air. When the Ta ﬁlm was deposited without RuO2 addition, the X-ray diﬀraction (CuKa radiation) result for the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system annealed at various temperatures for 30 min in air atmosphere was shown in Fig. 35(a). It clearly shows that the Pt-silicide, Pt–Ta compound and Ta2O5 were formed by

322

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 35. X-ray diﬀraction results for both the Pt/Ta/TiSix/n++-poly-Si/SiO2/Si and the Pt/Ta+RuO2/ TiSix/n++-poly-Si/SiO2/Si contact systems before and after annealing at various temperatures.

reactions involving Ta, Pt, poly-Si and external oxygen after annealing at 650 C, compared to the data of the as-deposited state. The interaction between Pt and TiSi2, poly-Si plug led to the formation of Pt-silicide. The external oxygen from air can freely diﬀuse through the grain boundaries of Pt bottom electrode because Pt does not form a stable oxide, and thus the oxygen can form the tantalum oxide (Ta2O5) by reaction with Ta. These results are similar to that of the Pt/Ti/SiO2/Si system, which is the diﬀusion of both Ti and O through the Pt layer, resulting in the formation of the TiO2 and Pt–Ti compounds. Fig. 35(b) showed the X-ray diﬀraction for the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system with annealing temperature. Comparing Fig. 27(b) with Fig. 35(b), all peak positions for each element and compound, such as Si, TiSi2 and RuO2, are similar according to annealing temperature in air, except for Pt peaks. Like the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system, no other phases were observed except for the formation of RuO2 phase. Therefore, the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system showed no reaction products, such as Ta2O5, TiO2, Pt-silicide and Pt–Ta compound except for the formation of conductive RuO2 phase. The XRD result for the Pt/Ta+RuO2/ TiSi2/poly-Si/SiO2/Si contact system was contrasted with that for the Pt/Ta/TiSi2/ poly-Si/SiO2/Si contact system shown in Fig. 35(a). Comparing Fig. 35(a) with Fig. 35(b), the thermal stability of the Ta ﬁlm deposited with RuO2 addition is higher than that of no RuO2 addition.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

323

From the total resistance, I–V curve and XRD results, both barrier properties and oxidation resistance for the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system was better than that for the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system. The formation of various reaction phases by reactions involving Pt, Ta, O, Ti and Si, such as Ta2O5, TiO2, SiO2, Pt-silicide and Pt–Ta compound occurs a serious problems at the bottom electrode structure of the DRAM/FRAM capacitor. From the structural aspect in the bottom electrode, the interaction between electrode metal, barrier metal, ohmic layer and poly-Si after annealing in air can destroy the bottom electrode layer structure. The reaction between oxygen and each layers consisting of the bottom electrode structure resulted in the formation of various oxides. The transformation from the metal to the oxide arises from a large volume expansion and then the bottom electrode layer structure can modify. These imply that the capacitor operation is oﬀ. In the electrical property respect, both the destruction of bottom electrode structure and oxidation of each layer increase the leakage current of the deposited dielectric ﬁlm and PZT ﬁlm as well as contact resistance. In these points, the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system showed the various reaction products after annealing at diﬀerent temperatures, as already seen in Fig. 35(a). This resulted in an increase of the total resistance and the deviation of ohmic behavior for the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system, as shown in Figs. 33 and 34(a). However, no reaction phases for the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system were observed, the only conductive RuO2 phase in the barrier ﬁlm was formed by reaction between RuOx and external oxygen from air after annealing, as shown in Fig. 35(b). This led to the lower total resistance and perfect ohmic characteristics for the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system, as shown in Figs. 33 and 34(b). To prevent the oxidation of barrier, alternative bottom electrode structures have been proposed in which the diﬀusion barrier is not directly exposed to an oxygen containing ambient during BST deposition. The TiO2 or SiO2 sidewall structure was used to prevent the oxidation on both sides of TiN diﬀusion barrier. However, even when the TiN is protected from direct oxidation using an appropriate capacitor structure, the diﬀusion of oxygen through Pt bottom electrode still causes a problem [1]. In the case of the amorphous barriers including WN and TaSiN, the oxidation of barrier itself occurs under oxidizing ambient at more than 600 C. For the bottom electrode structure, the oxidation of barrier surface is screened the electron ﬂow from bottom electrode to transistor. In the present study, the Ta+RuO2 diﬀusion barrier exhibited the lower total resistance and ohmic characteristics, and the surface oxidation was also prevented up to 800 C. This is attributed to the formation of conductive RuO2 phase in the barrier ﬁlm by reaction involving the indiﬀused oxygen and RuOx after annealing. In this point, the barrier properties of the Ta–RuO2 diﬀusion barrier are superior to that of the TiN, WN, TaN and Ta–Si–N barrier [42–45,52–57]. Fig. 36 showed optical micrographs of the Pt surface in the Ta layer prepared with and without RuO2 addition after annealing at various temperatures for 30 min. In the case of the Ta ﬁlm prepared without RuO2 addition, the reaction products were observed, as shown in Fig. 36(a), and it was corresponded to the various phases, as

324

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 36. Optical micrographs of the Pt surface for both the Pt/Ta/TiSi2/poly-Si/SiO2/Si and the Pt/ Ta+RuO2(100 W)/TiSi2/poly-Si/SiO2/Si system annealed at various temperatures.

the XRD result shown in Fig. 35(a). From the total resistance, I–V curve, XRD, and OM results, the Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system was completely degraded by the reactions involving oxygen, Ta, Pt and Si at 650 C. On the other hand, when the Ta ﬁlm was deposited with RuO2 addition, it showed a clean surface even after annealing at 800 C, as shown in Fig. 36(b). These optical microscopy (OM) images are distinct from that for the Pt/Ta/TiSi2/poly-Si/SiO2/Si system in which the reacted precipitates involving oxygen, Ta, Pt and Si appear, as already shown in Fig. 36. This fact indicates that the inter-diﬀusion of O, Pt and Si through diﬀusion barrier was prevented up to 800 C. From comparison with the barrier properties for both Ta+RuO2 itself and Pt/ Ta+RuO2 diﬀusion barrier, as already described in Sections 3.2.2.1 and 3.2.2.2, respectively, the total resistance and I–V data, which is the most of important data in the DRAM/FRAM capacitor bottom electrode showed the similar results for both systems after annealing at various temperatures in air atmosphere. In addition, from XRD data, all peaks such as TiSi2, poly-Si and RuO2 were detected for both systems before and after annealing, except for only Pt peaks, as shown in Figs. 27(b) and 34(b). Based on the above results, although AES depth and TEM analysis do not perform for the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system, respectively, it can be thought that the ohmic mechanism for the Pt/Ta+RuO2/TiSi2/polySi/SiO2/Si contact system is identiﬁed that for the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system. In summary, the barrier and electrical properties of the developed Ta+RuO2 layer as a diﬀusion barrier for Pt, Si and oxygen in the DRAM/FRAM capacitor bottom electrode is characterized by using TiSi2/poly-Si/SiO2/Si substrate in this chapter. Both Ta+RuO2/TiSi2/poly-Si/SiO2/Si and Pt/Ta+RuO2/TiSi2/poly-Si/

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

325

SiO2/Si contact systems deposited with the SiO2 capping layer showed the lower total resistance and ohmic characteristics up to 800 C. For both systems, no other phases were observed except for the formation of conductive RuO2 phase in the barrier ﬁlm by reaction with the in-diﬀused oxygen after annealing in air, but in the case of the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system, the thin oxidized layer at the Ta+RuO2/TiSi2 interface was formed by external oxygen. For the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system, however, a large number of the crystallites in the annealed samples compared to that in the as-deposited ﬁlm were observed even after depth proﬁle. The crystallites consisted of Ru and O containing a small amount of Ta. In addition, an embedded RuO2 crystalline phase was observed in the thin oxidized TiSi2 surface layer. Correspondingly, it suggests that the ohmic mechanism for both systems is an embedded RuO2 crystalline phase involving a small amount of Ta in a Ta amorphous structure. 3.2.3. Electrical properties of the Ta-conductive RuO2 diﬀusion barrier on n++-polySi substrate 3.2.3.1. Ohmic behavior of the Ta-conductive RuO2 diffusion barrier on n++-poly-Si substrate. The variation of the total resistance for the Ta/n++-poly-Si/SiO2/Si contact systems prepared with and without RuO2 addition was measured at V=0 as a function of post-annealing temperatures in an air atmosphere for 30 min, respectively. The results were shown in Fig. 37. Here, the results for both total resistance and I–V curve for the Ta/n++-poly-Si/SiO2/Si contact systems deposited with/ without RuO2 added were obtained by using the SiO2 capping layer. When the Ta ﬁlm was deposited without the RuO2 addition, the total resistance for the Ta/n++poly-Si/SiO2/Si contact system was measured few tens of Mega- in value after annealing at 650 C and much more increased with increasing annealing temperature. When the composition ratio of Ta:RuO2 was 1:1, however, the total resistance showed the higher value in the as-deposited state compared to that in the annealed samples, as shown in Fig. 37. After annealing at 650 C, the total resistance decreased initially and was sustained even after annealing at 800 C. In order to investigate the ohmic characteristics, I–V measurement between 4 and 4 V was carried out for both Ta+RuO2/n++-poly-Si/SiO2/Si and Ta/n++poly-Si/SiO2/Si contact systems before and after annealing at various temperatures. The results were shown in Fig. 38. In the case of the Ta ﬁlm prepared without RuO2 addition, as shown in Fig. 38(a), the I–V curves for the Ta/n++-poly-Si/SiO2/Si contact system exhibited the linearity in the as-deposited state and the non-linear behavior with increasing annealing temperature. For the Ta layer prepared with RuO2 addition, as shown in Fig. 38(b), the I–V curve for the Ta+RuO2/n++-polySi/SiO2/Si contact system showed the non-ohmic behavior in the as-deposited state. This result can be interpreted like the Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system, as already described in Section 3.2.2.1. The Ta+RuO2/n++-poly-Si/SiO2/Si contact system showed the perfect ohmic behavior with increasing annealing temperature. Although the total resistance slightly increased, in particular, the I–V curve showed the linear characteristics even after annealing at 800 C, as shown in Fig. 38(b).

326

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 37. Variation of the total resistance for the Ta(100 nm)/n++-poly-Si/SiO2/Si contact systems prepared with and without RuO2 addition.

In order to investigate the reaction for both Ta+RuO2/n++-poly-Si/SiO2/Si and Ta/n++-poly-Si/SiO2/Si contact systems with annealing temperature, the X-ray diffraction analysis was performed and the results were shown in Fig. 39. In the case of the Ta ﬁlm deposited without RuO2 addition, as shown in Fig. 39(a), both Ta barrier layer and n++-poly-Si plug were oxidized by reactions involving Ta, Si and O after annealing at 650 C. The oxidized layers formed from both Ta barrier ﬁlm and n++-poly-Si layer have a dielectric property, resulting in the increase of the total resistance and schottky characteristic, as shown in Figs. 37 and 38(a). When the Ta layer was deposited with RuO2 addition, both Ta and RuO2 peaks in the Ta+RuO2 ﬁlms were not detected in XRD patterns in the as-deposited state. This result can be thought that the crystallites size of RuO2 is very small and the Ta+RuO2 diﬀusion barrier is amorphized by RuO2 addition. Therefore, it is thought that RuO2 peaks were not detected in XRD patterns in the as-deposited state because of the small crystal size of RuO2. Amorphous Ta has lower intensity than Si, and thus it was also not appeared in XRD patterns. This is supported by the previous data that observed

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

327

Fig. 38. I–V curves for both the Ta/n++-poly-Si/SiO2/Si and the Ta+RuO2/n++-poly-Si/SiO2/Si contact systems before and after annealing at various temperatures.

Fig. 39. X-ray diﬀraction results for both the Ta/n++-poly-Si/SiO2/Si and the Ta+RuO2/n++-poly Si/ SiO2/Si contact systems before and after annealing at various temperatures.

328

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

a Ta amorphous structure and an embedded nanocrystalline RuOx in the as-deposited state as shown by planar TEM result. For the Ta+RuO2/n++-poly-Si/SiO2/Si contact system, the RuO2 peaks in the Ta+RuO2 ﬁlm annealed at 650 C were observed and more clearly showed with increasing the annealing temperature. No other phases were observed except for the formation of RuO2 phase. In particular, Ta2O5 was not observed even after annealing at 800 C, as shown in Fig. 39(b). Both the formation of Ta2O5 in the Ta+RuO2 ﬁlm and oxidation of n++-poly-Si plug layer would have increased the total resistance and not shown the ohmic behavior because the oxidized layer has a dielectric. This indicates that the Ta+RuO2/n++poly-Si/SiO2/Si layer structure was not oxidized by the reaction with in-diﬀused oxygen from air atmosphere. In the present study, the lower total resistance and linear I–V curve for the Ta+RuO2 ﬁlms after annealing compared to that of asdeposited ﬁlm is attributed to the formation of conductive RuO2 phase in the barrier ﬁlm by reaction between the in-diﬀused oxygen and RuOx. To exactly characterize the electrical properties of the developed Ta+RuO2 diffusion barrier, I–V curve for the Ta+RuO2/n++-poly-Si/SiO2/Si sample prepared without the SiO2 protection layer should measure and show an ohmic behavior. In other words, the n++-poly-Si channel layer is not fully oxidized by the direct exposure in air and the n++-poly-Si layer on both sides of the Ta+RuO2/n++-poly-Si interface is only partially oxidized by the lateral diﬀusion of oxygen, that is, and if the current path in both Ta+RuO2 ﬁlm and Ta+RuO2/n++-poly-Si interface exists, then the Ta+RuO2/n++-poly-Si/SiO2/Si contact system should show the linear behavior after annealing in air. Fig. 40 showed the I–V curve for the Ta+RuO2/n++-poly-Si/SiO2/Si sample prepared without the SiO2 capping layer after annealing at diﬀerent temperatures in air. This result is similar to the data for the Ta+RuO2/n++-poly-Si/SiO2/Si sample prepared with the SiO2 capping layer shown in Fig. 38(b). From the total resistance, I–V curve and glancing angle XRD results, both barrier properties and oxidation resistance for the Ta+RuO2/n++-poly-Si/SiO2/Si contact system were better than that for the Ta/n++-poly-Si/SiO2/Si contact system. Kusida-Abdelghafar et al. [72] reported that for the Pt/TiN/n++-poly-Si structure annealed at 650 C/2 min in an oxygen ambient, the electrical resistivity between Pt and n++-poly-Si was measured more than 106 in value and the oxidation portion of TiN ﬁlm was about 70%, when Pt thickness is less than 100 nm. So far, many diﬀusion barriers, such as Ti, Ta, TiN, TaN, WN, TaSiN, TiSiN, MoSiN and WSiN, have been developed for Al or Cu metallization. Much research has been conducted in the use of an oxygen diﬀusion barrier for the high density DRAM/ FRAM capacitor bottom electrode, including the various barriers developed for Al or Cu metallization. The polycrystalline barrier group (Ti, Ta, TiN, TaN, and WN) is susceptible to the oxygen diﬀusion through grain boundaries of barrier ﬁlm as well as the reaction between oxygen and barrier ﬁlm. Amorphous barrier group (TiN, TaN, WN, TaSiN, TiSiN, MoSiN and WSiN) is vulnerable to the reaction between oxygen and barrier ﬁlm rather than the oxygen diﬀusion through barrier ﬁlm. In the case of amorphous barrier, since the chemical aﬃnity for oxygen of each element consisting of barrier ﬁlm is very large and the binding force formed between each

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

329

Fig. 40. I–V curves for the Ta+RuO2/n++-poly-Si/SiO2/Si contact systems without SiO2 capping layer before and after annealing at various temperatures.

element consisting of barrier ﬁlm is weak, the reaction between barrier ﬁlm and oxygen results in the oxidation of barrier itself. In addition, the polycrystalline barrier possesses the grain boundaries, which can act as fast diﬀusion paths as well as nucleation site to form oxide. These imply that the diﬀusion for and reaction with oxygen can not prevent at more than 600 C. Therefore, it may be expected that they could not to be applied for advancing memories. In the present study, the lower total resistance and ohmic characteristics of the Ta+RuO2 (1:1) diﬀusion was sustained and the surface oxidation was prevented even after annealing at 800 C. This is attributed to the formation of conductive RuO2 phase in the barrier ﬁlm by reaction involving indiﬀused oxygen and RuOx after annealing. In this point, the electrical properties of Ta–RuO2 diﬀusion barrier are superior to that of the polycrystalline and amorphous barriers. When the Pt bottom electrode layer was deposited on the Ta+RuO2 ﬁlm, the electrical properties for the Pt/Ta+RuO2/n++-poly-Si/SiO2/Si contact system were investigated and compared with those for the Pt/Ta/n++-poly-Si/SiO2/Si contact

330

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 41. Variation of the total resistance for the Pt(100 nm)/Ta(100 nm)/n++-poly-Si/SiO2/Si contact systems prepared with and without RuO2 addition.

system. Here, for both Pt/Ta+RuO2/n++-poly-Si/SiO2/Si and Pt/Ta/n++-poly-Si/ SiO2/Si contact systems, the data of both total resistance and I–V curve were obtained without using the SiO2 protection layer. Fig. 41 showed the total resistance measured at V=0 for both systems before and after annealing in air. The total resistance for the Pt/Ta/n++-poly-Si/SiO2/Si contact system sharply increased with increasing annealing temperature, whereas that for the Pt/Ta+RuO2/n++-poly-Si/ SiO2/Si contact system showed a little change up to 800 C. The I–V curve for both Pt/Ta+RuO2/n++-poly-Si/SiO2/Si and Pt/Ta/n++-polySi/SiO2/Si contact systems measured between 4 and 4 V showed before and after annealing at various temperatures. The results were shown in Fig. 42. When the Ta ﬁlm was prepared without RuO2 addition, as shown in Fig. 42(a), the I–V curves for the Pt/Ta/n++-poly-Si/SiO2/Si contact system exhibited ohmic characteristics in the as-deposited state and Schottky behavior with increasing annealing temperature. In the case of Ta layer prepared with RuO2 addition, as shown in Fig. 42(b), The I–V curve for the Pt/Ta+RuO2/n++-poly-Si/SiO2/Si contact system showed the nonohmic behavior in the as-deposited state. This can be interpreted like the result for the Ta+RuO2/n++-poly-Si/SiO2/Si contact system in the as-deposited state. The

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

331

Fig. 42. I–V curves for both the Pt/Ta/n++-poly-Si/SiO2/Si and the Pt/Ta+RuO2/n++-poly-Si/SiO2/Si contact systems before and after annealing at various temperatures.

Pt/Ta+RuO2/n++-poly-Si/SiO2/Si contact system appeared the perfect ohmic behavior with increasing annealing temperature. Although the total resistance slightly increased, in particular, the I–V curve showed the linear characteristics even after annealing at 800 C, as shown in Fig. 42(b). Fig. 43 showed the X-ray diﬀraction results for both Pt/Ta+RuO2/n++-poly-Si/ SiO2/Si and Pt/Ta/n++-poly-Si/SiO2/Si contact systems with annealing temperature. As shown in Fig. 43(a), Ta2O5 in the Pt/Ta/n++-poly-Si/SiO2/Si contact system detected after annealing at 650 C. This resulted in the increase of the total resistance and deviation from linearity of the I–V curve due to the formation of Ta2O5, which have a dielectric property, as shown in Figs. 41 and 42(a). Comparing Fig. 39(b) with Fig. 43(b) with annealing temperature, the XRD pattern for the Pt/ Ta+RuO2/n++-poly-Si/SiO2/Si system is similar to that for the Ta+RuO2/n++poly-Si/SiO2/Si system, except for Pt peaks. Therefore, the lower total resistance and linear I–V curve for the annealed Ta+RuO2 ﬁlms compared to that for the asdeposited ﬁlm is attributed to the formation of conductive RuO2 phase in the barrier ﬁlm by reaction between the in-diﬀused oxygen and RuOx. This is also the reason exhibited the higher thermal stability than in the Pt/Ta/n++-poly-Si/SiO2/Si system. When the Ta+RuO2 ﬁlms were deposited on the TiSi2/n++-poly-Si/SiO2/Si and ++ n -poly-Si/SiO2/Si substrate, respectively, the electrical properties showed the similar results, as already seen in Figs. 25 and 38. In the case of the TiSi2/poly-Si/ SiO2/Si substrate, the ohmic mechanism suggested an embedded RuO2 crystalline phase involving a small amount of Ta in a Ta amorphous structure. This implies that the large amount of an embedded RuO2 crystalline phase in a Ta amorphous structure can be existed by the large amount of RuO2 added. To examine the eﬀect of the amount of RuO2 added in the Ta ﬁlm on the electrical properties, the

332

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 43. X-ray diﬀraction results for both the Pt/Ta/n++-poly-Si/SiO2/Si and the Pt/Ta+RuO2/n++poly Si/SiO2/Si contact systems before and after annealing at various temperatures.

composition ratio of Ta:RuO2 varied from 50:50 to 30:70. Fig. 44 showed the I–V curves of the Ta+RuO2 (3:7)/n++-poly-Si/SiO2/Si sample prepared without the SiO2 capping layer after annealing at diﬀerent temperatures in air. The I–V curve for the Ta+ RuO2 (3:7)/n++-poly-Si/SiO2/Si system is similar to the result for the Ta+RuO2 (1:1)/n++-poly-Si/SiO2/Si system. Fig. 45 showed the X-ray diﬀraction result for the Ta+RuO2 (3:7)/n++-poly-Si/ SiO2/Si contact system with annealing temperature. As shown in Fig. 45, the RuO2 peaks in the Ta+RuO2 ﬁlm annealed at 650 C were observed and more clearly showed with increasing the annealing temperature. No other phases were observed except for the formation of RuO2 phase. In particular, Ta2O5 was not observed even after annealing at 800 C, as shown in Fig. 45. This is similar to the results obtained by using the composition ratio (Ta:RuO2) of 1:1. Fig. 46 showed AES depth proﬁle for the Ta+RuO2 (3:7)/n++-poly-Si/SiO2/Si sample before and after annealing at various temperatures. The layer structure of the Ta+RuO2/n++-poly-Si was well deﬁned in the as-deposited state. After annealing at 700 C, the interface between Ta+RuO2 and n++-poly-Si showed little change from comparison of that in the as-deposited state. The Ta+RuO2 (3:7)/ n++-poly-Si/SiO2/Si sample annealed at 800 C showed that the surface of n++poly-Si was oxidized by reaction with the in-diﬀused oxygen from air atmosphere. However, although the total resistance slightly increased, the Ta+RuO2 (3:7)/n++poly-Si/SiO2/Si contact system showed the ohmic behavior even after annealing at

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

333

Fig. 44. I–V curves for the Ta+RuO2 (3:7)/n++-poly-Si/SiO2/Si contact systems without SiO2 capping layer before and after annealing at various temperatures.

800 C, as seen in Fig. 44. A little increase of the total resistance after annealing at 800 C is attributed to the thin oxidized layer formed between Ta+RuO2 and n++poly-Si. To ﬁnd out the mechanism of ohmic behavior, the Ta+RuO2/n++-poly-Si/SiO2/ Si contact system with diﬀerent annealing temperatures are analyzed by scanning auger microscopy (SAM) micrographs and AES spectra, and the results were shown in Fig. 47. In SAM images of Fig. 47, the depth proﬁles for the annealed samples were obtained from area 1, the crystallites-free zone in Fig. 46. In addition, AES spectra in Fig. 47 were obtained by auger electron point analysis from area 2, the crystallites. A large numbers of the crystallites were distributed in the as-received state of as-deposited ﬁlm and remained after depth proﬁle. After annealing at 700 and 800 C, a large number of the crystallites were also observed even after depth proﬁle. This indicates that the crystallites exist at the surface of n++-poly-Si. It would be taken place the diﬀerence of sputtering rate by Ar+ ion between crystallites and crystallite-free zone during insitu ion beam etching. The surface element constituents of the Ta+RuO2/n++-poly-Si/SiO2/Si samples annealed at 700 and 800 C were characterized by AES. The spectra taken from the crystallites consisted

334

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 45. X-ray diﬀraction result for the Ta+RuO2 (3:7)/n++-poly-Si/SiO2/Si contact systems without SiO2 capping layer before and after annealing at various temperatures.

of Ru and O containing a small amount of Ta, but only Si peaks were detected for the crystallites-free zone. Although the surface of n++-poly-Si layer was oxidized, the ohmic behavior of the Ta+RuO2/n++-poly-Si/SiO2/Si contact system is attributed to an embedded RuO2 crystallites in the thin oxidized n++-poly-Si surface layer. From total resistance, I–V curve and glancing angle XRD results, only RuO2 phase was formed in the Ta+RuO2 barrier ﬁlms for both Ta+RuO2/n++-poly-Si/ SiO2/Si and Pt/Ta+RuO2/n++-poly-Si/SiO2/Si contact systems after annealing in air irrespective of the composition ratio of Ta:RuO2. In addition, the ohmic mechanism for both contact systems is attributed to an embedded RuO2 crystalline phase involving a small amount of Ta in a Ta amorphous structure. This implies that the Ta2O5 phase was not formed in the Ta+RuO2 barrier ﬁlm. For the Ta+RuO2 system, Ta2O5 phase should form after annealing in air because the heat of formation of Ta2O5 is more negative than that of RuO2. This indicates that the in-diﬀused oxygen from air atmosphere can react more easily with Ta than RuO2. As a result, RuO2 phase was formed in the Ta+RuO2 barrier ﬁlm after annealing in

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

335

Fig. 46. AES depth proﬁle for the Ta+RuO2 (3:7)/n++-poly-Si/SiO2/Si contact systems without SiO2 capping layer before and after annealing at various temperatures.

air, as already shown in Figs. 39 and 43. Also, this indicates that an amorphous phase, Ta remained stable even after annealing in air at 800 C. This can not explain from thermodynamic aspect. This phenomenon may provide an interesting aspect for the material reaction in the thin ﬁlm interactions. In the competing growth between two phases, the change of free energy is rate dependent. Time-dependent, kinetic term rather than time-independent, ﬁnal state free energy changes are considered. The free energy change in a given time t0 is [73] ð t0 dG G ¼ dt dt 0 and the rate of energy change is

336

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 47. SAM images and AES spectrum for the Ta+RuO2 (3:7)/TiSi2/poly-Si/SiO2/Si sample before and after annealing at various temperatures.

dG dG dx ¼ ¼F dt dx dt

where F=dG/dx is the driving force of the reaction or chemical aﬃnity, that is, thermodynamic driving force, and v=dx/dt is its growth or reaction rate, that is, kinetic term. The Ta+RuO2 system can be considered after annealing in air in the above equation. First, the driving force required for the formation of Ta2O5 and RuO2 phase from pure Ta and Ru is proportional to the heat of formation (H) for each element, respectively. Here, the kinetic term for both phases may be similar from the results that both Ta2O5 and RuO2 phases were formed after annealing in air at more than 600 C. Therefore, the thermodynamic driving force required for the formation of Ta2O5 and RuO2 phase from pure Ta and Ru state is dominate, compared to the kinetic term for that. dG 292 kJ=mol of atoms dx Ta ! Ta2 O5 dG 102 kJ=mol of atoms dx Ru ! RuO2 That is when the thermodynamic driving forces to form the crystalline phases, Ta2O5 and RuO2 phase, are comparable, however, the kinetic term can quickly lead to a large rate of free energy change in the initial period if it can nucleate and grow fast. So

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

337

after nucleation the fast growth of the RuO2 phase is favored. Ta with Ta–Ru–O system does not exist in a pure metal state, but Ta is signiﬁcantly bound to oxygen in the as-deposited state. In addition, RuO2 exists as non-stoichiometry state because of the Ta–O binding state. Therefore, the Ta+RuO2 barrier ﬁlm consisted of Ta(O) and RuOx rather than pure Ta and RuO2. Here, the thermodynamic driving force for oxidation in Ta(O) and RuOx may be diﬀerent from that required for the formation of Ta2O5 and RuO2 phase from pure Ta and Ru, respectively. From this viewpoint, it may be assumed that the driving force for Ta2O5 formation from Ta(O) by reaction with oxygen is comparable with that of RuO2 formation from RuOx. dG dG dx a-TaðOÞ ! Ta2 O5 dx RuOx ! RuO2

dx dt

dx 4 dt monocrystalline a-TaðOÞ ! Ta2 O5

RuOx ! RuO2

Moreover, the microstructure of the Ta+RuO2 diﬀusion barrier showed a Ta(O) amorphous structure and an embedded RuOx nanocrystalline phase. In order to form the Ta2O5 phase, it should nucleate by the rearrangement of both Ta(O) and O atoms and then grow. However, the RuOx nanocrystalline phase by supplying external oxygen, may be more easily changed into the RuO2 crystalline phase. It implies that nanocrystalline RuOx phase is already nucleated to form the RuO2 phase and then easily can grow by reaction between nanocrystalline RuOx phase and in-diﬀused oxygen during annealing in air. That is, the kinetic barrier for nucleation in the formation of crystalline Ta2O5 phase from an amorphous Ta(O) phase is much higher than that of crystalline RuO2 phase from nanocrystalline RuOx phase. Therefore, the formation of the RuO2 phase by the reaction between the in-diﬀused oxygen from an air atmosphere and the RuOx nanocrystallites is kinetically more favorable than that of Ta2O5 phase. This is supported by the results as follows [74]; in view of the known phase diagrams, which usually indicate which stable phases can be expected to occur for certain alloy systems, such an observation is unexpected as the amorphous phase counterpart, which is formed by solid-state reaction, of a crystalline solid solution has a larger (less negative) Gibbs free energy of formation. In particular, the occurrence of solid-state amorphization (SSA) at the metal/Si interface prior to the nucleation of a crystalline phase has been recently reported in many metal–Si systems. The formation of an amorphous phase is considered to be due to a negative heat of mixing of two elements and also due to the fast diﬀusion of species. That is, although the thermodynamic driving force required for the formation of an amorphous phase is smaller than that for a crystalline phase, the amorphous phase prior to nucleation of a crystalline phase can be formed because the kinetic barrier for nucleation in the formation of amorphous phase by solid-state reaction is much lower than that of crystalline phase. From the above discussion, the proposed Ta+RuO2 diﬀusion barrier for high density thin ﬁlm capacitor bottom electrodes prevented both oxygen in-diﬀusion and oxidation

338

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

resistance even after annealing at 800 C. In addition, both Ta+RuO2/n++-poly-Si/ SiO2/Si and Pt/Ta+RuO2/n++-poly-Si/SiO2/Si contact systems showed the lower total resistance and ohmic characteristics as well as their layer structures were sustained up to 800 C. These are attributed to the formation of the conductive RuO2 phase in the Ta+RuO2 barrier ﬁlm because of the large amount of conductive RuO2 and chemically strong Ta–O bonds. 3.2.3.2. Size effect. To investigate the eﬀect the contact size on the electrical properties, the size of metal mask was varied from 22 mm to 500 mm dot. To examine the eﬀect of the amount of RuO2 added in the Ta ﬁlm on the electrical properties at a given 500 mm contact size, in addition, the composition ratio of Ta:RuO2 varied from 50:50 to 30:70. Fig. 48 showed the I–V curves for the Ta+RuO2 (5:5, 4:6, 3:7)/n++poly-Si/SiO2/Si samples prepared without the SiO2 capping layer after annealing at

Fig. 48. I–V curves of the Ta+RuO2 (5:5, 4:6, 3:7, 50 nm)/n++-poly-Si/SiO2/Si sample prepared without the SiO2 capping layer after annealing at diﬀerent temperatures for 30 min in air.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

339

diﬀerent temperatures in air. As seen in Fig.48, I–V curves for the Ta+RuO2/n++poly-Si/SiO2/Si contact system showed the non-ohmic behavior in the as-deposited state irrespective of the composition ratio of Ta : RuO2. The Ta+RuO2/n++-polySi/SiO2/Si contact system for all compositions showed the perfect ohmic behavior with increasing annealing temperature. Although the total resistance slightly increased, in particular, I–V curves for all compositions showed the linear characteristics even after annealing at 800 C, as shown in Fig. 48. In addition, the total resistance for the composition ratio of 5:5 is larger than that for the composition ratio of 4:6 and 3:7. This is because the amount of RuO2 added in the Ta ﬁlm is small. When the Ta+RuO2 ﬁlms were prepared by using 500 mm contact size, the Ta+RuO2/n++-poly-Si/SiO2/Si contact systems showed the ohmic behavior for all compositions. This implies that the ohmic mechanism, which is an embedded RuO2 crystalline phase involving a small amount of Ta in a Ta amorphous structure, of the Ta+RuO2/n++-poly-Si/SiO2/Si contact systems for all compositions is identiﬁed. When the Pt bottom electrode layer was deposited on the Ta+RuO2 ﬁlm, I–V curves for the Pt/Ta+RuO2 (5:5, 3:7)/n++-poly-Si/SiO2/Si contact systems were shown in Fig. 49. As seen in Fig. 49, I–V curves for the Pt/Ta+RuO2/n++-poly-Si/ SiO2/Si contact systems exhibited the ohmic behavior with increasing annealing temperature irrespective of the composition ratio of Ta:RuO2. Figs. 48 and 49 showed the I–V characteristics for both Ta+RuO2/n++-poly-Si/ SiO2/Si and Pt/Ta+RuO2/n++-poly-Si/SiO2/Si contact systems fabricated by using 500 mm contact size as a function of composition ratio of Ta:RuO2 and various annealing temperatures. The I–V curves for both systems exhibited the linear behavior for all compositions and annealing temperatures. Therefore, the electrical properties of the Ta+RuO2 ﬁlm deposited on the n++-poly-Si substrate do not aﬀect the contact size up to 500 m contact size.

Fig. 49. I–V curves of the Pt/Ta+RuO2 (5:5, 3:7, 50 nm)/n++-poly-Si/SiO2/Si sample prepared without the SiO2 capping layer after annealing at diﬀerent temperatures for 30 min in air.

340

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Currently, many diﬀusion barriers including TiN, WN, TaSiN, TiAlN and TiSiN have been extensively examined for the high-density DRAM/FRAM capacitor bottom electrode. TiN is oxidized after annealing in oxygen ambient and thus, the bottom electrode layer structure takes place lift-oﬀ because of volume expansion by oxidation. In order to improve the oxidation resistance of TiN ﬁlm, the eﬀorts to develop ternary compound of (Ti,X)N have been demanded [75]. Here, X includes Al, Cr, V and Zr. (Ti,X)N compounds were basically developed to improve the mechanical properties such as wear resistance, hardness and adhesion. Among the various compound ﬁlms, in particular, TiAlN ﬁlm showed the high anti-oxidation characteristics up to 700 C. The roles of Al were summarized as follows: At the initial stage of oxidation, Al ions diﬀused to the surface, and then formed the AlxTiyOz phase which acted as a passive layer against further oxidation. Because the oxidation was preceded by continual oxygen diﬀusion through oxidized surface layer, the passive layer of the AlxTiyOz formed on TiAlN ﬁlm played an eﬀective role as a diﬀusion barrier for oxygen. However, to be used the TiAlN diﬀusion barrier for capacitor bottom electrode, eﬀect of the surface oxidized layer on the electrical properties should be investigated by using the thin thickness of TiAlN ﬁlm. Amorphous-WN, -TiSiN and -TaSiN layers are oxidized under oxidizing ambient at more than 600 C. Ta, Ti, Si and W elements consisting of their barriers have a strong chemical aﬃnity for oxygen. For example, the Gibbs free energy change at 298 K with respect with Ta2O5, TiO2, SiO2 and WO2 is 273, 296, 285 and 178 kJ/mol of atoms, respectively. It indicates that the amorphous barriers could not be used as an oxygen diﬀusion barrier for the lower electrode because the formation of thin oxidized layer, such as TiO2, WO2, Ta2O5 and SiO2, terminates the electrical contact. Therefore, they are oxidized under capacitor integration process and thus, could not to be used as an oxygen diﬀusion barrier. In the present study, the I–V characteristics for both Ta+RuO2/n++-poly-Si/SiO2/Si and Pt/Ta+RuO2/ n++-poly-Si/SiO2/Si contact systems fabricated by using 500 mm contact size exhibited the ohmic behavior for all compositions and annealing temperatures. In addition, the surface oxidation of the Ta+RuO2 diﬀusion was prevented even after annealing at 800 C. This is attributed to the formation of conductive RuO2 phase in the barrier ﬁlm by reaction involving the in-diﬀused oxygen and RuOx after annealing. Based on the these results, the barrier properties of Ta–RuO2 diﬀusion barrier are superior to those of the TiN, WN, TaN, TiAlN, TiSiN and TaSiN barriers. In particular, the electrical properties of the Ta+RuO2 ﬁlm deposited on the n++-poly-Si substrate do not depend on the contact size up to 500 mm contact size. In summary, the electrical properties of the deposited Ta–RuO2 diﬀusion barrier were characterized by using n++-poly-Si substrate. To examine the eﬀect of the amount of RuO2 in the Ta ﬁlm on the electrical properties, the composition ratio of Ta : RuO2 varied from 50:50 to 30:70. In addition, perimeter eﬀect, that is, the eﬀect of contact size on the electrical properties was investigated. Both Ta+RuO2(1:1, 3:7)/n++-poly-Si/SiO2/Si and Pt/Ta+RuO2(1:1)/n++-poly-Si/SiO2/Si contact systems showed the lower total resistance and ohmic characteristics as well as their layer structures were sustained up to 800 C. However, both Ta/n++-poly-Si/SiO2/ Si and Pt/Ta/n++-poly-Si/SiO2/Si contact systems showed not only the higher total

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

341

resistance and non-linear I–V curve, but also was completely destroyed after annealing at 650 C, attributing to the formation of Ta2O5 phase with a dielectric property. For both the former contact systems, no other phases were observed except for the formation of conductive RuO2 phase in the barrier ﬁlm by reaction with the in-diﬀused oxygen after annealing in air, but the thin oxidized layer at the Ta+RuO2/n++-poly-Si interface was formed by external oxygen after annealing at 800 C. However, a large number of the crystallites were observed even after depth proﬁle. The crystallites consisted of Ru and O containing a small amount of Ta, followed by the ohmic mechanism for the n++-poly-Si/SiO2/Si substrates.

4. Change of a design concept for a diﬀusion barrier in high-density capacitor For improvement of the oxidation resistance and prevention of the in-diﬀusion of oxygen at high temperatures, the design concept for the development of diﬀusion barriers for capacitor bottom electrodes should be diﬀerent from that for Al or Cu metallization. When Al reacts with a polycrystalline barrier, reaction of the barrier with Al starts at the grain boundaries of the barrier and propagates toward the intragranular regions. The reaction of Al at the grain boundaries of the barrier reduces the free energy of system because the interfacial free energy of two interfaces between the reaction products and the Al over-layer is much less than that of the original grain boundaries [68]. Thus, the grain boundaries play a role of providing a chemical driving force for the occurrence of the initial stage of interfacial reaction. Therefore, the diﬀusion barrier design concept for Al metallization was to prevent reaction between the Al metal and the barrier layer because of the good reactivity of Al metal. Meanwhile, refractory metals, refractory metal incorporated oxide and nitrides do not react with Cu because of a thermodynamically stable interface in barrier metals/Cu contact system [76]. This implies that the Cu diﬀusion through barrier layer, in particular, grain boundaries and other microstructural defects, which can act as fast diﬀusion paths, occurs to form Cu-silicides at the barrier/Si interface, resulting in the failure of diﬀusion barrier. So the diﬀusion barrier design concept for Cu metallization was to control the in-diﬀusion through grain boundaries of the diﬀusion barrier because of the fast diﬀusivity of the Cu metal. The oxygen in the capacitor bottom electrode has both the drawbacks of Al and Cu because oxygen not only reacts easily to oxidize the diﬀusion barrier, but also diffuses rapidly through diﬀusion barrier to oxidize the underlying layer. To prevent the fast diﬀusion of oxygen at high temperatures, the microstructure of the diﬀusion barrier itself should be amorphous with no grain boundaries. Such fact is supported by the previously reported experimental results that show that barrier properties of amorphous ternary barriers were better than that of polycrystalline barriers [76]. To suppress the strong reactivity of oxygen, the binding force formed between the matrix metal and the added material should be chemically strong in as-deposited state. With a Pt/Ti/Si sequence, extensive out-diﬀusion Ti and Si takes place during SrTiO3 capacitor ﬁlm growth. Replacing Ti with Ta reduces diﬀusion, but, leads to a very high contact resistance, presumably due to the oxidation of Ta to Ta2O5. In the

342

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Pt/TiN/Si contact system, (La,Sr)CoO3/PZT/(La,Sr)CoO3 stacks are stable up to 550 C [77]. This implies that the oxidation resistance of barriers with strong chemical bonds between the matrix metal and the added material is better that that of pure metals, leading to a diﬀerence in activation energy for oxidation between the two types of barriers. Here, however, between strong reactivity and fast diﬀusivity of oxygen, the more important parameter for the diﬀusion barrier in high-density capacitors should be determined. The mixed linear-parabolic relationship for oxidation can be given [78]: x2 þ Ax ¼ Bð þ tÞ

ð1Þ

where, x=oxide thickness, =a shift in the time coordinate to account for the presence of the initial oxide layer, t=oxidation time. In the reaction-controlled region from Eq. (1), ð2Þ x ¼ B=Að þ tÞ; B=A ¼ s C =N1 where, s=the rate constant of chemical surface reaction for oxidation, C*=equilibrium concentration in the oxide, N1=the number of oxidant molecules incorporated in a unit volume of oxide layer. In the diﬀusion-controlled region from Eq. (1), x2 ¼ Bð þ tÞ;

B ¼ Bo expðEa =RTÞ

ð3Þ

where, Ea=activation energy for the oxidation process. The binary nitride barriers (TiN, TaN, WN) are oxidized at temperatures less than 500 C and the oxidation of ternary barriers (TiSiN, TiAlN, TaSiN) are observed to be at temperatures 50 C higher than that of binary nitride barriers. In Eqs. (2) and (3), the most important factors, that is the control parameters are the rate constant of the chemical surface reaction for oxidation (s) and the activation energy for the oxidation process (Ea), respectively. At a given temperature and time, the rate constant of chemical surface reaction for oxidation (s) depends strongly on the bond strength of the barrier metal and the activation energy for the oxidation process (Ea) is closely related to the oxide thickness formed. As known from Table 1 and the experimental results reported by others, from the viewpoint of diﬀusion barrier, many barriers developed Table 1 Bond strengths in diatomic molecules for nitrides and heat of formation for oxides [79] Contents Materials

Bond strengths (kJ/mol)

Heat of formation (kJ/mol of atoms)/oxides

Electrical resistivity (ohms. cm)/oxides

Ti–N Al–N Si–N Ta–N

476.1 297 470 611

315/TiO2 335/Al2O3 304/SiO2 292/Ta2O5

3107/TiO2 11016/Al2O3 11014/SiO2 1105/Ta2O5

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

343

up to now are placed in the diﬀusion-controlled region under the thermal budget of capacitor processes. That is, the chemical bonds of barriers are already completely broken by oxygen, followed by the formation of various non-conducting oxides by reactions involving oxygen and the elements composing barriers. This implies that the diﬀusion barriers are completely failed and that they can not be used for the bottom electrode of capacitors. Therefore, the reaction between oxygen and the barrier is inevitable under capacitor fabrication processes involving high temperature, an oxidizing ambient. This implies that strong reactivity is more important parameter than fast diﬀusivity. From these considerations, the diﬀusion barrier against oxygen for high-density capacitors should be designed with the concept of a sacriﬁcial barrier. That is, the oxide layer formed by a reaction between oxygen and the elements composing the barrier play the role of a diﬀusion barrier preventing the additional oxygen diﬀusion through oxide layer. Not only that, the oxide layer formed should be a conductor. From this point of view, the matrix element has to be Ru, Ir, Rh, Re, and Os. Ti, Ta, and W can be chosen as an added element by considering their bond strengths, solubility in the matrix and their utility. In this work, both Ru and Ti were selected as a matrix and an incorporated material. 4.1. Oxidation resistance and thermal stability of new diﬀusion barrier To examine the ﬁlm resistance of the newly designed RuTiN and RuTiO barrier after oxidation, ﬁrst of all, the variation of sheet resistance with gas ratio of N2/ Ar+N2 and O2/Ar+O2 was measured for both the RuTiN (100 nm)/SiO2/Si and the RuTiO (100 nm)/SiO2/Si structures before and after annealing at various temperatures for 5 min in pure oxygen ambient, and the results are shown in Fig. 50. For both structures, the sheet resistance increases in the as-deposited state, as the

Fig. 50. Variation of sheet resistance of RuTiN, RuTiO, and TiN barrier itself with oxidation temperature.

344

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

gas ratio of N2/Ar+N2 and O2/Ar+O2 increases, respectively. Such increase in the sheet resistance is attributed to the increase of amount of either nitrogen or oxygen in the ﬁlms with increasing gas ratio. Here, the electrical resistance with oxidation temperature is compared TiN, which has been widely used as a diﬀusion barrier, with each RuTiN and RuTiO barrier. In the case of TiN ﬁlm, after annealing at 500 C, the sheet resistance was measured to be below 30 ohm/sq, but the gold-like color of TiN ﬁlm on the surface was completely lost, indicating that the oxidation of TiN barrier began at 500 C. At 600 C, the sheet resistance measurement showed range-error, implying that the TiN ﬁlm was completely oxidized. When the RuTi ﬁlm was incorporated by either nitrogen or oxygen, however, the sheet resistance was below 200 ohm/sq even after annealing at 800 C, irrespective of oxygen or nitrogen content. Therefore, the RuTiN and RuTiO ﬁlms can be used as sacriﬁcial oxygen diﬀusion barriers due to their conducting properties even after oxidation, as required by the new design concept. The compositions of the deposited RuTiN and RuTiO barrier ﬁlms with gas ratio of N2/Ar+N2 and O2/Ar+O2 were analyzed by the RBS rump simulation. Oxygen resonance (3.06 MeV, H++ ion) was used to obtain the amount of oxygen in the RuTiO ﬁlm. As shown in Fig. 51, the nitrogen content in the RTN ﬁlm increases, as the gas ratio of N2/Ar+N2 is varied from 5 to 20% N. In the case of RuTiO ﬁlm prepared with 5% gas ratio, the composition of RTO ﬁlm was Ru0.38Ti0.24O0.37 C0.1. To examine the surface morphology after oxidation, SEM analysis was carried out and the resultant images are shown in Figs. 52 and 53. As the gas ratio of N2/ Ar+N2 and O2/Ar+O2 is varied from 5 to 50% N and 10% O, the RuTiN and RuTiO itself showed a smooth surface morphology up to 600 C. In the case of the RuTiO ﬁlm, as shown in Fig. 53, it has a very rough surface morphology in the asdeposited state and no change with increasing oxidation temperature. For the RuTiN ﬁlm, the interface between bottom electrode and barrier occurred lifting at the wafer edge after deposition above N2/Ar+N2=60% N. In this work, the gas ratio of more than 60% N and 15% O was excepted from the investigation of barrier properties. Fig. 54 shows the microstructure both the RuTiN and the RuTiO barrier ﬁlms in the as-deposited state. From the selected area diﬀraction pattern, the microstructures of both diﬀusion barriers were amorphous-like structures. Such microstructures are formed because the adatom mobility for ﬁnding original positions on the surface during the deposition is limited due to Ti–N, Ru–O, and Ti–O bonds. To investigate the thermal stability of new barriers, the PVD-Pt (250 nm)/PVDBST (50 nm)/PVD-Pt (350 nm)/RuTiN (50 nm) or RuTiO (50 nm)/RuTiN (50 nm) layer structures was sequentially deposited using SiO2/Si and TiSix/poly-Si/SiO2/Si substrate, respectively. Notice that PVD-BST ﬁlm was prepared with Ar and O2 (10%) gas mixture at Tsub=450 C. The post-annealing for both the Pt/BST/Pt/ barrier/SiO2/Si and the Pt/BST/Pt/barrier/TiSix/poly-Si/SiO2/Si layer structures was carried out at a temperature range of 500–650 C for 30 min in N2+O2 gas ambient and the resultant cross-sectional SEM images are shown in Figs 55 and 56. After annealing, each interface of Pt bottom electrode/barrier for both structures is stable up to 650 C. Based on these results, although a higher annealing temperature than 650 C was not done because of a furnace contamination by particle formation and

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

345

Fig. 51. RBS spectra of both the RTN and RTO ﬁlms as a function of reactive gas ratio. (a) N2/Ar+N2 =5% (b) N2/Ar+N2 =10% (c) N2/Ar+N2 =20% (d) O2/Ar+O2=5%.

lifting of wafer edge, it can be expected that both the RuTiN and RuTiO/RuTiN barriers would be stable up to much higher temperature than 650 C done in this work. To investigate the diﬀusion behavior of oxygen as a function of post-annealing temperature, auger depth proﬁling was performed for both the Pt/BST/Pt/RuTiN/ SiO2/Si and the Pt/BST/Pt/RuTiO/RuTiN/SiO2/Si systems and the results are shown in Fig. 57. For both structures, as shown in Fig. 57, oxygen is accumulated at the surface of diﬀusion barrier, but evidence for barrier oxidation does not observe up to 650 C. This is supported by cross-sectional TEM results shown in Figs. 58 and 59. It demonstrated that the Pt–RuTiN interface is clean after post-annealing at 650 C. To examine the barrier oxidation after annealing at 650 C in more severe oxidation ambient, in addition, SEM analysis was done and the resultant images are shown in Fig. 60. The Pt–barrier interface is smooth, enhancing the barrier properties in real thermal budgets of capacitor fabrication processes involving soft oxidation ambient.

346

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 52. Cross-sectional SEM images; (a) RuTiN/SiO2/Si and (b) RuTiO/SiO2/Si structure with various oxidation temperatures for 5 min in pure O2 ambient.

To exactly examine the thermal stability of the RuTiN and RuTiO barriers, both the CVD-BST(30 nm)/RuTiN(50 nm)/TiOx(adhesion layer, 20 nm)/SiO2/Si and the CVD-BST(30 nm)/RuTiO(50 nm)TiOx/SiO2/Si structures was annealed at 700 C for 180 s in N2 and at 350 C for 180 s in N2O plasma treatment. After that, the Pt(350 nm) ﬁlm was deposited by sputtering system. Both structures was carried out in the annealing temperature range of 550–650 C for 30 min in N2+O2 ambient. Fig. 61 shows the cross-sectional TEM images for the Pt (350 nm)/CVD-BST (30 nm)/RuTiN(50 nm)/TiOx(adhesion layer, 20 nm)/SiO2/Si structure depending on various post-annealing temperature. The interface between the RuTiN ﬁlm and the BST layer is stable up to 650 C. Fig. 62 exhibits the auger depth proﬁles for the Pt (350 nm)/CVD-BST (30 nm)/RuTiN(50 nm)/TiOx(adhesion layer, 20 nm)/SiO2/Si structure with diﬀerent post-annealing temperatures. The oxygen proﬁle is a little change from 600 C, but the RuTiN barrier is not oxidized, as shown in Fig. 61. Fig. 63 the cross-sectional TEM images for the Pt (350 nm)/CVD-BST (30 nm)/

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

347

Fig. 53. Cross-sectional SEM images for the RuTiO(15% O)/SiO2/Si structure with various oxidation temperatures for 5 min in pure O2 ambient.

RuTiO(50 nm)/TiOx(adhesion layer, 20 nm)/SiO2/Si structure after annealing various temperatures. This resultant images are similar to the images for the Pt/CVDBST/RuTiN/TiOx/SiO2/Si structure of Fig. 61. These results support that the RuTiN and RuTiO barriers would be stable up to more than 650 C. 4.2. Contact resistance characteristics for new barrier As shown in Fig. 64, to measure the contact resistance, on contact plugs with the 0.220.22 mm2 in contact hole, the ohmic contact layer, Ti-silicide, was selectively formed and the 5 nm-TiN glue layer was deposited to prevent lifting in un-patterned areas. RuTiN and RuTiO ﬁlms were deposited on the TiSix/poly-plug substrates in a direct-current (dc) sputtering system in a mixture of Ar and N2 or O2. The BST(30 nm)/Pt(350 nm) ﬁlms on the barrier/TiSix/poly-plug substrates were sequentially deposited by sputter and metal organic chemical vapor deposition (MOCVD) using

348

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 54. Cross-sectional TEM images and selected diﬀraction patterns for (a) RuTiN and (b) RuTiO diﬀusion barrier in the as-deposited state.

Fig. 55. Cross-sectional SEM images; (a) Pt/PVD-BST/Pt/RuTiN/SiO2/Si and (b) Pt/PVD-BST/Pt/RuTiO/ RuTiN/SiO2/Si structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

Ba(thd)2 -pmdt (Ba(2,2,6,6-tetramethylheptane-3,5-dionate)2-1,1,4,7,7-pentamethyldiethylenettriamine), Sr(thd)2-mdt (Sr(2,2,6,6-tetramethylheptane3,5-dionate)2-1,1,4, 7,7-entamethyldiethylenetriamine) and Ti(O-i-Pr)2(thd)2 (Ti(iso-propoxide)2-(2,2,6, 6-tetramethylheptane-3,5-dionate)2 metal sources at substrate temperature of 400 C, and the total working pressure during deposition was 2 Torr. Argon gas was used as

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

349

Fig. 56. Cross-sectional SEM images for the Pt/PVD-BST/Pt/RuTiN/TiSix/poly-Si/SiO2/Si structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

a carrier gas to deliver the evaporated source to the process chamber and O2/N2O gas was employed as an oxidant gas. The ﬂow rate of Ar gas was 400 sccm and that of O2/N2O gas was 100/100 sccm. After deposition of the BST ﬁlm, to suﬃciently diﬀuse oxygen ions, atoms and molecules toward the diﬀusion barrier, the whole structure was N2O plasma-treated at 350 C for 180 s and then rapid thermal annealing was proceeded at 650 C for 180 s in nitrogen ambient.. After deposition of the BST ﬁlm, to suﬃciently diﬀuse oxygen ions, atoms and molecules toward the diﬀusion barrier, the whole structure was N2O plasma-treated at 350 C for 180 s and then rapid thermal annealing was proceeded at 650 C for 180 s in nitrogen ambient. After removal of the BST ﬁlm, post-annealing was carried out at a temperature range of 550–750 C for 30 min in N2+O2. Patterns for measuring the current–voltage (I–V) characteristics were deﬁned by photo and etch processes. To evaluate the feasibility for the newly developed RuTiN and RuTiO diﬀusion barrier on real memory capacitor applications, the contact resistance for diﬀusion barrier with annealing temperature is the most important electrical parameter in a capacitor bottom electrode structure because the lower structure of the capacitor including barrier is connected to the transfer device through a conductive plug. Fig. 65 show the variation of contact resistance for the new RuTiO and RuTiN barrier with various annealing temperatures. As seen in Fig. 65, the I–V curves for both the Pt(350 nm)/RuTiN(50 nm)/TiSix/n++-p-Si plug/n+ Si channel layer/Si and Pt(350 nm)/RuTiO (25 nm)/RuTiN (25 nm)/TiSix/n++-p-Si plug/n+ Si channel layer/Si contact structures showed non-linear ohmic behavior, irrespective of annealing temperature and type of diﬀusion barrier. For both contact structures, the contact resistance measured from the 0.220.22 mm2 in contact hole was as low as 5 kohm at 3 V up to more than 700 C. Because the sharp change in the resistance– temperature trend indicates a reaction involving the oxygen, the barrier, the electrode and Si, the result indicates that the in-diﬀusion of oxygen through the Pt bottom electrode and the resulting formation of oxide at the Pt–RuTiN or –RuTiO

350

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 57. Auger depth proﬁles; (a) Pt/PVD-BST/Pt/RuTiN/SiO2/Si and (b) Pt/PVD-BST/Pt/RuTiO/ RuTiN/SiO2/Si structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

interface occur and the inter-diﬀusion of Pt and Si elements takes place through the RuTiN or RuTiO barrier. The highest temperature at which the barrier layer prevents interactions involving the barrier layer (RuTiN, RuTiO), bottom electrode (Pt), Si, and the ambient oxygen during annealing can be used as a measure of diffusion-barrier failure. In this respect, the RuTiN and RuTiO barriers are thermally stable up to 750 C done in this work. Since good electrical conductivity between bottom electrode, diﬀusion barrier, poly-Si plug and transistor is required for the capacitor performance, it is thought that the lower contact resistance of the

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

351

Fig. 58. Cross-sectional TEM images for the Pt/PVD-BST/Pt/RuTiN/SiO2/Si structure with various postannealing temperatures for 30 min in N2+O2 ambient.

annealed sample is desirable feature for the capacitor bottom electrode. To investigate the reaction products formed after annealing, the cross-sectional TEM analysis for the two contact structures annealed at various temperatures was performed and the resultant images are shown in Fig. 66. As seen in Fig. 66, there is neither the oxidation of the new barrier nor the inter-diﬀusion of the Pt and the Si element through the barrier up to 750 C. The RuTiN barrier in the un-patterned area is also

352

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 59. Cross-sectional TEM images for the Pt/PVD-BST/Pt/RTN/TiSix/poly-Si/SiO2/Si structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

stable up to 750 C, as seen in Fig. 67. Such images are consistent with the contact resistant results shown in Fig. 65. From the new design concept, the sheet resistance data, and the I–V data, the new RuTiN and RuTiO oxygen diﬀusion barriers for high-density capacitors were proposed using the concept of a sacriﬁcial diﬀusion barrier and the electrical properties were investigated. The oxidation resistance of both the RuTiN and the RuTiO diffusion barriers themselves was superior to that of nitride barriers (TiN, WN, TaN, TiAlN, TiSiN, TaSiN) reported by others [52–57]. In the reaction-controlled region, that is, under the thermal budgets of the capacitor fabrication processes, previously

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

353

Fig. 60. Cross-sectional SEM images for the Pt/PVD-BST/Pt/RuTiO/RuTiN/SiO2/Si structure at 650 C for 10 min in air ambient.

reported barriers lead to by-products on the surface of barrier, which is either the various oxides or complexes bound with oxygen. Formed by the reaction with oxygen, the by-products such as Ti-, Ta-, Al-, Si-oxides, and various complexes, have dielectric characteristics, those of insulators. One of the most important requirements for the capacitor barrier is that it should not be readily oxidized because oxidation leads to the termination of electron transport through a conductive plug with the connecting transistor. Also, such oxidation increases the contact resistance, followed by a failure in high-speed operations of memory devices. From this point of view, the RuTiN and the RuTiO diﬀusion barrier have unique advantages because of their conductive properties after oxidation, contrary to many other barriers. To use as a sacriﬁcial barrier layer the oxide layer formed, its thickness should be uniform. This implies that the reaction between the barrier and the oxygen has to occur homogeneously on the surface. The microstructure of the barrier must be amorphous in

354

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 61. Cross-sectional TEM images for the Pt/CVD-BST/RuTiN/SiO2/Si structure with various postannealing temperatures for 30 min in N2+O2 ambient.

order to form a uniform oxide layer because amorphous structures have no grain boundaries, which can act as heterogeneous reaction sites with oxygen or have much smaller grain size. The RuTiN and the RuTiO barriers comply with this requirement because of their amorphous-like structures. The oxygen in-diﬀusion at low temperatures can also be prevented because of their lack of fast diﬀusion paths. Although non-linear ohmic behavior stems from the native oxide formed on the channel surface, the contact resistance for the Pt/barriers/TiSix/n++poly-plug/n+ channel layer/Si substrate structures was shown to be below 5 kohm up to 750 C. The variation of contact resistance with annealing temperature gives information for either the oxidation of the barrier ﬁlm or/and the inter-diﬀusion of each element through the barrier layer. After annealing, the total contact resistance is determined by 2(RPt/barrier+Rbarrier/TiSix) because of either the oxidation of the barrier or TiSix or/and of some reaction between two interfaces. The thermal stability of both the RuTiN and the RuTiO diﬀusion barriers was better than that of other barriers reported by others, because even after annealing up to 750 C, as already seen in Figs. 65–67, the contact resistance was still low.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

355

Fig. 62. Auger depth proﬁles for the Pt/CVD-BST/RuTiN/SiO2/Si structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

4.3. Capacitance and leakage current behavior using a simple stack structure inserted glue layer Fig. 68 compares the stability of the CVD-BST stack-type structure adopting RuTiN glue layer with that TiN glue layer as a function of the annealing temperature. Fig. 68(a) shows the schematic diagrams for the fabrication of Pt/BST/Pt capacitors. On the recessed TiN plugs, the 20 nm-RuTiN and the 10 nm-TiN glue layer was sputter deposited, respectively, and the 350 nm-Pt bottom electrode and the 60 nm-TiN hard mask was deposited on the glue layers/TiN/TiSix/plug structures. The Pt and glue layers were patterned by photo and etch processes. After formation of the bottom electrode structure, The 30 nm-thick BST ﬁlms were grown on the Pt/glue layers/TiN//TiSix/poly-plug substrates by MOCVD. After deposition

356

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 63. Cross-sectional TEM images for the Pt/CVD-BST/RuTiO/SiO2/Si structure with various postannealing temperatures for 30 min in N2+O2 ambient.

Fig. 64. Schematic diagram for current–voltage measurement using two contact chain structure with 0.220.22 mm2 in contact hole size and for the formation of a simple stack-type structure.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

357

Fig. 65. Current–voltage curves and contact resistance for the Pt/barriers/TiSix/n++ploy plug/n+ conducting channel layer/Si structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

of the BST ﬁlm, to crystallize of and supply oxygen into the BST ﬁlm, the CVDBST/Pt/glue layers/TiN//TiSix/poly-plug structures were N2O plasma-treated at 350 C for 180 s and then rapid thermal annealing was proceeded at 650 C for 180 s in nitrogen ambient. The 200 nm-Pt top electrode was deposited and after formation of the top electrode structure by mask and etch processes, post-annealing was carried out at a temperature range of 450–650 C for 30 min in N2+O2 ambient. Finally, pads for measuring the capacitance and leakage current characteristics were deﬁned by photo and etch processes. Notice that electrical properties with diﬀerent annealing temperatures can not be measured due to loss of the conducting channel layer in pad areas by etch. Here, the structural stability of the simple stack-type structure adopted RuTiN and TiN glue layer was only compared with various annealing temperatures, respectively. In the case of TiN glue layer, after annealing

358

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 66. Cross-sectional TEM images for the Pt/barriers/TiSix/n++ploy plug/n+conducting channel layer/Si structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

Fig. 67. Cross-sectional TEM images for the Pt/RuTiN/TiSix/n++ploy plug/n+conducting channel layer/Si structure at un-pattern area with various post-annealing temperatures for 30 min in N2+O2 ambient.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

359

Fig. 68. Cross-sectional TEM images for; (a) Pt/CVD-BST/Pt/TiN glue layer/TiN recessed plug structure and (b) Pt/CVD-BST/Pt/RuTiN glue layer/TiN recessed plug structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

at 550 C for 10 min, both the TiN glue ﬁlm and the recessed TiN layers were oxidized. Meanwhile, the RTN glue layer was still stable even after annealing at 650 C for 30 min in N2+O2 ambient. This implies that the electrical properties of high dielectric materials can be more improved. The structural stability of CVD-BST simple stack adopted RuTiN glue layer was much better than that TiN glue layer and other barriers reported by others. After the formation of storage contact holes with 0.220.22 mm2 in size, as seen in Fig. 69, heavily doped poly-Si was ﬁlled into the contact hole. The 20 nm-RuTiN layer was sputter deposited on the recessed TiN/TiSix/poly-plug substrates. The 350 nm-Pt bottom electrode and the 60 nm-TiN hard mask were deposited on the glue layers/TiN/TiSix/plug structures. The Pt and glue layers were patterned by photo and etch processes. After formation of the bottom electrode structure, The 60 nmthick sputtered BST ﬁlms were grown on the Pt/glue layers/TiN//TiSix/poly-plug substrates in a mixture of Ar and 10%O2 and at Tsub=450 C. After BST deposition, the crystallization annealing is done at 650 C for 180 s in N2+O2 ambient. The 200 nm-Pt top electrode was deposited and after formation of the top electrode structure by mask and etch processes, post-annealing was carried out at a temperature range of 450–750 C for 30 min in N2+O2. Pads for measuring the capacitance and leakage current characteristics were deﬁned by photo and etch processes. Notice that the capacitance and leakage current was measured from the 256,000 cell array

360

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 69. Schematic diagram for capacitance and leakage current measurement using a simple stack-type structure.

with the 0.260.62 mm2 cell area. Cells are connected in serial on the recessed TiN plugs, but cell-to-cell is linked in parallel. Finally, the capacitance and leakage current are measured between the top electrode pad to the bottom pad of the n++ channel layer on Si substrate. Fig. 70 shows the variation of capacitance for the Pt/PVD-BST/Pt simple stack capacitor adopted RuTiN, RuTiO/RuTiN, and TiN glue layer with various postannealing temperatures for 30 min in N2+O2 ambient. Here, the capacitance was measured from the 256 k cell array with the 0.220.48 mm2 projected area and the thickness of glue layer of RuTiN (or RuTiO/RuTiN) and TiN ﬁlm was 20 and 10 nm, respectively. In the case of TiN glue layer, after annealing at 450 C, the capacitance was more than 20 fF/cell, but at 500 C, initially decreased and thereafter, completely degraded. However, when the new RuTiN and RuTiO/RuTiN layer was used as a glue layer, as seen in Fig. 70(b) and (c), capacitance increases with increasing annealing temperature. To investigate the reaction products formed after annealing, especially, oxidation of the glue layer and the recessed TiN ﬁlm, the cross-sectional TEM analysis for the capacitor structures with three types of glue layers annealed at various temperatures was carried out and the resultant images are shown in Fig. 71. When the TiN ﬁlm was adopted as a glue layer, as seen in Fig. 68(b), the oxidation at the TiN glue layer as well as the recessed TiN ﬁlm occurred after annealing at 550 C. In the case of both the RuTiN and the RuTiO/ RuTiN glue layers, as shown in Fig. 71, the interface between the Pt bottom electrode and the glue layers was clean up to 550 C. Such TEM results were consistent with the variation of capacitance results shown in Fig. 70. To examine reproducibility of the diﬀusion barrier performance for the new oxygen barrier, the capacitance and leakage current characteristics as a function of barrier metal and post-annealing temperature for 30 min in N2+O2 ambient are measured and the resultant plots are shown in Fig. 72. Here, the electrical properties with post-annealing temperature is compared between TiN, which has been widely used as a diﬀusion barrier, and new RuTiN barrier. When the Pt/PVD-BST/Pt simple stack-type structure using the TiN ﬁlm and RuTiN layer is fabricated, the largest diﬀerence for capacitor fabrication processes is the deposition temperature of BST ﬁlm. The BST ﬁlm for the formation of simple structure using TiN barrier is deposited at temperature 50 C lower than that using RuTiN barrier. The reason for

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

361

Fig. 70. The capacitance characteristics as a function of post-annealing temperatures for 30 min in N2+O2 ambient: (a) TiN, (b) RuTiN, (c) RuTiN/RuTiO.

this is to suppress the oxidation of TiN barrier. In the case of TiN barrier, the capacitance is very low ( 6 fF/cell) in the as-deposited state and decreases, thereafter, but leakage current improves with increasing annealing temperature. For the RuTiN barrier, capacitance is to be above 30 fF/cell up to 600 C, and after that, decreases to about zero. Leakage current shows leaky characteristics up to 650 C, but above 700 C, is very low ( 1016 to 1017 A/cell). To understand the poor leakage current characteristic up to 600 C for the simple stack structure using RuTiN barrier, the MOS-capacitor area pattern with an area of 200200 mm2 was fabricated for high frequency capacitance (C)–voltage (V)

362

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 71. Cross-sectional TEM images for the Pt/PVD-BST/Pt/ RuTiN/recessed TiN plug structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

measurement. The C–V curves were obtained at 1 MHz as a function of thermal oxide thickness. The ﬂat band voltages (VFB) were found from the C–V curves by interpolation, for diﬀerent thermal oxide thickness. Assuming that the case of thermal SiO2, oxide charge involving mobile ionic charge, and oxide trapped charge can be ignored, unlike ﬁxed oxide charge. Under these assumptions, the ﬂat-band voltage (VFB) with variation of gate oxide thickness can be given as: VFB ¼ Fms =q Qf Tox ="ox

ð4Þ

where, Fms =work function, q=electron charge, Qf =ﬁxed oxide charge (C/cm2), "ox =oxide permittivity (F/cm), Tox =oxide thickness. In general, the ﬁxed oxide charge of gate oxides grown by the same condition is nearly identiﬁed at near gate oxide interface, irrespective of oxide thickness. In Eq. (4), the ﬁxed oxide charge is a constant. Therefore, Eq. (1) exhibits a linear graph with gate oxide thickness versus VFB, where, the y-axis intercept and the slope is ms =q and Qf ="ox , respectively. Therefore, the work function of barriers is calculated by Eqs.(5) and (6), Fms ¼ Fm Fs

ð5Þ

Fs ¼ XSi þ Eg =2 þ ðkTÞ lnðNa =Ni Þ 4:93 eV

ð6Þ

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

363

Fig. 72. The capacitance and leakage current characteristics as a function of post-annealing temperatures for 30 min in N2+O2 ambient; (a) TiN, (b) RuTiN.

Fig. 73 shows the VFB versus thermal oxide thickness with the TiN and the RuTiN barrier. As results, the work function for each RuTiN and PVD–TiN ﬁlm is 4.43 and 4.70 eV, respectively. Based on these work functions, as shown in Fig. 74, the energy band diagram before and after contact for Pt–BST–RuTiN contact system can be drawn. As shown in Fig. 74, the barrier height of the RuTiN–BST contact is much lower that that of the Pt–BST contact, leading to a large leakage current path. Fig. 75 shows the TEM images for the simple stack structure using, TiN barrier, for diﬀerent post-annealing temperatures. The Pt–TiN and BST–TiN interfaces are oxidized in the as-deposited state, although the BST ﬁlm is deposited at 400 C. Such oxidation results from the degradation of capacitance in the as-deposited state due to the serial capacitor formation consisting of TiOx and BST. At 550 C, the TiN layer is nearly oxidized, followed by more decrease of capacitance. On the contrary, the oxidation of barrier metal leads to the improvement of leakage current characteristics because of the oxidation increased resistance for a simple stack structure. Although the work function between two barriers is a similar, for the

364

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

Fig. 73. The ﬂat band voltage versus thermal oxide thickness plot depending on TiN and RuTiN barrier.

Fig. 74. Schematic of energy band diagram; (a) before and (b) after contact.

structure using TiN barrier, the reason for the better leakage current properties than that using RuTiN barrier at lower post-annealing temperature is the oxidation of TiN barrier metal. Fig. 76 shows the TEM photographs for the Pt/BST/Pt simple stack structure with inserted RuTiN barrier layer for diﬀerent post-annealing temperatures. Up to 600 C, both the Pt–RuTiN and the BST–RuTiN interfaces are clean. This is supported by the capacitance results shown in Fig. 72(b). Here, notice that the bottom electrode structure and the poly-plug is not properly aligned. Such

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

365

Fig. 75. Cross-sectional TEM images for the Pt/PVD-BST/Pt/TiN plug structure with various postannealing temperatures for 30 min in N2+O2 ambient.

Fig. 76. Cross-sectional TEM images for the Pt/PVD-BST/Pt/ RuTiN/recessed TiN plug structure with various post-annealing temperatures for 30 min in N2+O2 ambient.

366

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

weak parts provide the oxygen diﬀusion paths, leading to the oxidation of diﬀusion barrier. Although the misalignment at RuTiN barrier-recessed TiN plug structure occurs, the RuTiN and recessed TiN barrier is not oxidized up to 600 C. At 650 C, the oxidation of the recessed TiN barrier is started by oxygen diﬀusion through the weak part formed by misalignment, and at higher annealing temperature, the TiN barrier is signiﬁcantly oxidized. Such results are consistent with the capacitance and leakage current behavior shown in Fig. 72. If the alignment between the bottom electrode structure and TiN recessed plug structure is very well, the RuTiN glue layer can be endured above 600 C. Such prediction is supported by the TEM results of Figs. 61 and 77. From the capacitance and the leakage current data, and the TEM data, the new RuTiN ﬁlm was applied as diﬀusion barrier on real capacitor structure, and the electrical properties were investigated. In case of the simple stack-type structure adopted TiN glue layer using PVD–BST dielectric ﬁlm, in the as-deposited state, the capacitance and leakage current is shown to be 7 fF/cell and 1013 A/cell. Such results come from the oxidation of the TiN diﬀusion barrier. Although for the Pt/ BST/Pt simple stack using RuTiN barrier, the BST ﬁlm is deposited at higher BST

Fig. 77. Cross-sectional TEM images for the Pt/PVD-BST/Pt/ RuTiN/recessed TiN plug structure in un-patterned area with various post-annealing temperatures for 30 min in N2+O2 ambient.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

367

deposition temperature than that using TiN barrier, capacitance was shown to be above 30 fF/cell up to 600 C. This indicates that the RuTiN/recessed TiN barrier layer was not oxidized up to 600 C. If the misalignment does not occur, it can be expected that the RuTiN barrier could be thermally stable up to more than 650 C. In the present data, the leakage current property for the simple stack structure using RuTiN barrier was unsatisfactory. If the RuTiN diﬀusion barrier can be ﬁlled into plug like CVD–TiN barrier, however, the leakage current problem can be resolved. Based on these new results, the RuTiN layer, as a diﬀusion barrier for oxygen, is a highly potential material for high-density volatile and non-volatile memory devices. In summaries, we proposed both the RuTiN and the RuTiO materials, as sacriﬁcial oxygen diﬀusion barriers, using the new design concept for high-density memory capacitors. The newly developed RuTiN and RuTiO barriers showed much lower sheet resistance than various barriers reported by others up to 800 C. Contact resistance, the most important electrical parameter for the diﬀusion barrier in a capacitor structure, was shown to be below 5 kohm even after annealing up to 750 C. The thermal stability of the CVD–BST simple stack-type structure adopted RuTiN glue layer was observed to be 150 C higher than that TiN glue layer. Moreover, the capacitance of the PVD–BST simple stack-type structure adopted TiN glue layer initially degraded after annealing at 500 C, and thereafter, completely failed. However, in the case of RuTiN and RuTiO/RuTiN glue layer, capacitance was shown to be 30 fF/cell up to 600 C. Therefore, new RuTiN and RuTiO ﬁlms, as diﬀusion barriers for oxygen, are very promising material to achieve the high-density capacitors.

5. Future direction for a diﬀusion barrier to achieve high-density capacitors In ferroelectric random access memory (FRAM) device with 0.25 mm technology, ferroelectric cell capacitors using PZT, SBT, and BLT have been normally fabricated by utilizing either noble metals or their oxide electrodes. Currently, the capacitor structures using mutilayers with noble metals and their oxide electrodes, such as Pt/IrOx/Ir/TiN/plug, have been focused to improve the leakage current, ferroelectric properties, fatigue, and retention. For the next generation of high-density FRAM with below 0.25 mm design rule, however, as shown in Fig. 6(c), the cell structure should be changed to a stack type or concave type structure. As mentioned in the Section 1, the same critical integration issues, such as dielctric ﬁlms, bottom electrode, and diﬀusion barrier, will be emerged out. Among three problems, the diﬀusion barrier will suﬀer from a severe thermal cycling. By current ferroelectric capacitor fabrication processes, the nucleation and cystallization of ferroelectric ﬁlm after deposition of ferroelectric material should be annealed at above 650 C for 60 min in pure oxygen ambient, respectively. After capacitor patterning and capacitor contact etch, the recovery annealing of two times has to perform for relieving the etch damage and supplying oxygen into the ferroelectric ﬁlm at above 650 C for 30 min in pure oxygen ambient. For below 0.25 mm technology, the thickness of each layer consisting of top electrode, ferroelectric material, and bottom electrode will be

368

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

also decreased. Under these hard thermal budgets, the diﬀusion barrier must be intact. The previously barriers (TiN, WN, TaN, TiAlN, TiSiN, TaSiN) reported by others under the hard thermal budgets of the ferroelectric capacitor fabrication processes lead to by-products on the surface of barrier, which is either the various oxides or complexes bound with oxygen. Formed by the reaction with oxygen, the by-products such as TiO2, Ta2O5, Al2O3, and SiO2, and various complexes (TiOx, TaOx, AlOx, SiOx) have dielectric characteristics, those of insulators. One of the most important requirements for the capacitor barrier is violated as well as the ferroelectric capacitor is not operated. To overcome this issue, as already required in Section 4, the concept of a sacriﬁcial diﬀusion barrier, which retains conductive properties after oxidation, should be introduced to achieve the high-density ferroelectric memory device. In the dynamic random access memory (DRAM) with 0.1 mm technology, the MIM capacitor structure using Ta2O5 dielectric ﬁlm has been concentrated to obtain the leakage current and capacitance required. On the recessed TiN/poly plug, the 1.5 k-thick oxide was deposited and the concave structure was made by photo and etching processes. After that, the bottom electrode (30 nm-thick CVD–Ru), the dielectric ﬁlm (15 nm-thick Ta2O5), and the top electrode (100 nm-thick CVD–TiN) were sequentially grown and the MIM concave type structure was completed by photo and etching processes, followed by the backend processes including metal 1, 2, and passivation. According to the current MIM capacitor processes using Ta2O5 dielectric ﬁlm, the oxygen plasma treatment to supply the oxygen into its ﬁlm after Ta2O5 deposition was done at above 400 C and the post-annealing was carried out at above 500 C for 1 h in nitrogen ambient. After that, the post-annealing of twice was performed at above 600 C for 30 min in nitrogen ambient after top electrode and etching. The reason for low temperature processes, especially, the oxygen plasma treatment and post-annealing in nitrogen ambient after Ta2O5 deposition, is attributed to minimize the damage of TiN diﬀusion barrier for oxygen, although the CVD-Ru ﬁlm have a large amount of oxygen. At present, the electrical properties are unsatisfactory under these low thermal budgets in nitrogen ambient, acting as a limitation factor to advance the high-density capacitor. To improve the electrical properties using MIM capacitor adopted Ta2O5 dielectric ﬁlm, therefore, the high temperature processes in oxygen ambient are essential. When the high temperature processes in oxygen ambient is introduced to the MIM capacitor using Ta2O5 dielectric ﬁlm, the most important critical issue is the oxidation of diﬀusion barrier. Under these thermal cycling in oxygen ambient, the TiN barrier as well as other barriers are oxidized and can not used as a diﬀusion barrier for high-density capacitor bottom electrode. Based on considerations of the DRAM/FRAM capacitor processes, the introduction of a sacriﬁcial diﬀusion barrier concept should be more emphasized to improve the electrical properties and achieve the high-density capacitor.

6. Conclusions We reviewed the barrier properties and failure mechanisms for various diﬀusion barriers in high-density DRAM/FRAM capacitors. A diﬀusion barrier for the high-density

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

369

DRAM and FRAM capacitor bottom electrodes should not only prevent the interdiﬀusion of oxygen, electrode metal and Si but also require the high oxidation resistance at high temperatures. The previously many barriers reported by others are susceptible to the reaction and inter-diﬀusion between electrode metal, Si and oxygen through diﬀusion barrier because these show either polycrystalline microstructure, which possesses the grain boundaries, or the binding force formed between elements of diﬀusion barrier is weak. In this article, based on these failure mechanisms, we suggested the new design concept to improve many problems of previous barriers, and both the Ta+CeO2 and the Ta+RuO2 materials were developed as new diﬀusion barrier materials for the high-density capacitors. The Ta+CeO2 diﬀusion barrier showed the good barrier properties, attributing to the strong chemical bonding of Ta–Ce–O or Ta–O and the amorphous microstructure. However, it exhibited the poor oxidation resistance due to a small amount and nonconducting properties of CeO2 added. The Ta+RuO2 diﬀusion barrier appeared both the good electrical properties and the oxidation resistance, originating in strong chemical bonding of Ta–Ru–O or Ta–O and the amorphous microstructure as well as an embedded nano-crystalline RuOx phase in Ta–O amorphous structure. Although both barriers were shown to exhibit good diﬀusion barrier properties, however, oxide-incorporated barriers result in the surface oxidation of the underlayer during deposition and/or post-thermal budgets, followed by the degradation of capacitor performance. After all, the design concept for a diﬀusion barrier should be changed to sacriﬁcial oxygen diﬀusion barrier concept, and both the RuTiN and the RuTiO ﬁlms, as new sacriﬁcial oxygen diﬀusion barriers, were proposed. New RuTiN and RuTiO barriers showed the higher oxidation resistance and cell capacitance and the lower contact resistance up to high temperatures. Therefore, the introduction of design concept for a diﬀusion barrier in high-density DRAM/ FRAM capacitors should be essential.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

Ramesh R. Thin ﬁlm ferroelectric materials and devices. Boston: Kluwer Academic; 1997. p. 2. Kingon AI, Maria J-P, Streiﬀer K. Nature 2000;406:1032. Chaneliere C, Devine RAB, Balland B. Mater Sci Eng R, R22 1998:269. Ramesh R, Aggarand S, Auciello O. Mater Sci Eng R 2001;32:191. Scott JF. Annu Rev Mater Sci 1998;28:79. Nakamura Y, et al. 2001. Symposium of VLSI Technology, 2001. p. 39–40. Kotecki DE, et al. IBM J Res Devlop 1999;43(3):367. Ono K, et al. lEDM Tech Digest Int 1998:803–6. Hieda K, et al. IEDM Tech Digest Int 1999:789–92. Lee BT, et al. IEDM Tech Digest Int 1998:815–8. Dongjin J, et al. IEDM Tech Digest Int 2000:801–8804. Ono H, Nakano T, Ohta T. Appl Phys Lett 1994;64:1511. Olowolafe JO, Li J, Mayer JW. J Appl Phys 1990;68:6207. Yoon D-S, Baik HK, Kang B-S, Lee S-M. Appl J Phys 1996;80(11):6550. Mattoso FN, Achete C, Freire Jr FL. Thin Solid Films 1992;220:184. Holloway K, Fryer PM, Cabral Jr C, Harper JME, Bailey PJ, Kelleher KH. J Appl Phys 1992; 71:5433.

370

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

[17] Clevenger LA, Bojarczuk NA, Holloway K, Harper JME, Cabral Jr C, Schad RG, et al. J Appl Phys 1993;73:300. [18] Kolawa E, Chen JS, Reid JS, Pokela PJ, Nicolet MA. J Appl Phys 1991;70:1369. [19] Wang SQ, Raaijmakers I, Burrow BJ, Suther S, Redkar S, Kim KB. J Appl, Phys 1990;68:5176. [20] Wang SQ, Suthar S, Hoeﬂich C, Burrow BJ. J Appl Phys 1993;73:2301. [21] Pokela PJ, Kolawa E, Ruiz RP, Nicolet MA. Thin Solid Films 1991;203:259. [22] Choi CS, Ruggles GA, Shah AS, Xing GC, Osburn CM, Hunn JD. J Electrochem Soc 1991; 138:3062. [23] Al-Shareef HN, Chen X, Lichtenwalner DJ, Kingon AI. Thin Solid Films 1996;280:256. [24] Milosev I, Strehblow H-H, Navinsek B. Surf Interf Analys 1998;26:242. [25] McIntyre PC, Summerfelt SR, Maggiore CJ. Appl Phys Lett 1997;70:711. [26] McIntyre PC, Summerfelt SR. J Appl Phys 1997;82:4577. [27] Grill A, Cabral Jr C. J Mater Res 1999;14:1581. [28] Takemura K, Yamamichi S, Lesaicherre P-Y, Tokashiki K, Miyamoto H, Ono H, et al. Jpn J Appl Phys 1995;34:5224. [29] Panjan P, Navinsek B, Cvelbar AC, Zalar A, Milosev I. Thin Solid Films 1996;281–82:298. [30] Tsai M-S, Sun S-C, Tseng T-Y. J Am Ceram Soc 1999;82(2):351–8. [31] Nabatame T, Hiratani M, Kadoshima M, Shima Y, Matsui Y, Ohji Y, et al. Jpn J Appl Phys 2000; 39:L1188. [32] Aoyama T, Eguchi K. Jpn J Appl Phys 1999;38:L1134. [33] Aoyama T, Kiyotoshi M, Yamazaki S, Eguchi K. Jpn J Appl Phys 1999;38:2194. [34] Kimura T, Yamauchi H, Machida H, Kokubun H, Yamada M. Jpn J Appl Phys 1994;33:5119. [35] Takeshima Y, Shiratsuyu K, Takagi H, Sakabe Y. Jpn J Appl Phys 1997;36:5870. [36] Tsuyama T, Inaishi Y, Sawada Y, Kobayashi I. Jpn J Appl Phys 1996;35:4875. [37] Horikawa T, Tanimura J, Kawahara T, Yamamuka M, Tarutani M, Ono K. IEICE Trans Electron 1999;E81-C:497. [38] Olowolafe JO, Li J, Mayer JW. J Appl Phys 1990;68:6207. [39] Kim JH, Yoon D-S, Kwak JS, Lee S-M, Baik HK. J Electron Mater 1999;28:6. [40] Chang CA. J Appl Phys 1990;67(10):6184. [41] Kim S-T, Kim H-H, Lee M-Y, Lee W-J. Jpn J Appl Phys 1997;36:294. [42] Ono H, Nakano T, Ohta T. Appl Phys Lett 1994;64:1511. [43] deBoer FR, Boom R, Mattens WCM, Miedema AC, Niessem K. Cohesion in metals—transition metal alloys, vol. 1. The Netherlands: North-Holland Physics; 1989. [44] Grill A, Kane W, Viggiano J, Brady M, Laibowitz R. J Mater Res 1992;7:3260. [45] Olowalafe JO, Jones Jr RE, Campbell AC, Megde RI, Mogab CJ, Grgory RB. J Appl Phys 1993; 73(4):1764. [46] Maex K, Rossum MV. Properties of metal silicides. EMIS Datareviews series no.14. London: INSPEC; 1995. [47] Wang SQ, Suthar S, Hoeﬂich C, Burrow BJ. J Appl J Phys 1993;73:2301. [48] Suni I, Nicolet MA, Pai CS, Lau SS. Thin Solid Films 1983;107:73. [49] Thomas RE, Kuo K-J, Aaron DB, Dobisz EA, Perepezko JH, Wiley JD. Thin Solid Films 1987; 150:245. [50] Holloway K, Fryer PM, Cabral Jr C, Harper JME, Bailey PJ, Kelleher KH. J Appl Phys 1992; 71:5433. [51] Thomas RE, Perepezko JH, Wiley JD. Appl Surf Sci 1987;26:1454. [52] Olowolafe JO, Li J, Mayer JW, Colgan EG. Appl Phys Lett 1991;58:469. [53] Suni I, Maenpaa M, Nicolet MA, Luomajavi M. J Electrochem Soc: Solid-State Sci Technol 1983; 130:1215. [54] Sun B, Chiang T, Ding P, Chin B. Advanced metallization and interconnect system for ULSI application. 1997. p. 137. [55] Charai A, Hornstorm HE, Thopmas O, Fryer PM, Harper JME. J Vac Sci Technol 1989;A7:784. [56] Reid JS, Kolawa E, Garland CM, Nicolet MA, Cardone F, Gupta D, et al. J Appl Phys 1995; 79(2):1109.

D.-S. Yoon et al. / Progress in Materials Science 48 (2003) 275–371

371

[57] Hara T, Kitamura T, Tanaka M, Kobayashi T, Sakiyama K, Onishi S, et al. J Electrochem Soc 1996; 143(11):L264. [58] Hara T, Tanaka M, Sakiyama K, Onishi S, Ishihara K, Kudo J. Jpn J Appl Phys 1997;36:L893. [59] Vu QT, Pokela PJ, Garden CL, Kolawa E, Raud S, Nicolet MA. J Appl Phys 1991;68:6420. [60] Wittmer M, Vac J. Sci Technol 1984;A2(2):273. [61] Colgan EG, Mayer JW. J Mater Res 1989;4(4):815. [62] Mayer JW, Lau SS, editors. Electronic materials science for integrated circuits in Si and GaAs, New York: Macmillan; 1990. p. 313. [63] Kim J, Yoon D-S, Kwak JS, Baik HK, Lee S-M. J Electron Mater 1999;28:6. [64] Yoon D-S, Lee SM, Baik HK. J Appl Phys 1998;83:1333. [65] Yoon D-S, Kang BS, Lee SM, Baik HK. J Appl Phys 1996;80(10):6550. [66] Yoon D-S, Lee SM, Baik HK. J Appl Phys 1998;83:8074. [67] Yoon D-S, Lee SM, Baik HK. J Vac Sci Tech 1999;17:174. [68] Wolf D, Yip S, editors. Materials interfaces, London: Chapman and Hall; 1992 [chapter 20]. [69] Massalski TB. Binary alloy phase diagram. 2nd ed. Metals Park: ASM/NIST; 1986. [70] Hsia SL, Tan TY, Smith P, McGuire GE. J Appl Phys 1992;71(5):1864. [71] Wong CY, Wang LK, McFarland PA, Ting CY. J Appl Phys 1986;60(1):243. [72] Kushida-Abdelghafar K, Hiratani M, Torii K, Fujusaki Y. Integrated Ferroelectrics 1996;13:113. [73] Tu KN, Mayer JW, Feldman LC. Electronic thin ﬁlm science for electrical engineers and materials sciensts. New York: Macmillan; 1992. p. 254. [74] Benedictus R, Han K, Traeholt C, Bottger A, Mittemeijer EJ. Acta Mater 1998;46(15):5491. [75] Kim CW, Kim KH. Thin Solid Films 1997;307:113. [76] Lin C-L, Ku S-R, Chen M-C. Jpn J Appl Phys, 40 N 2001;6A:4181. [77] Maeder T, Muralt P, Sagalowiez L, Setter N. J Electrochem Soc 1999;146(9):3393. [78] Sze SM, editor. VLSI technology. New York: McGraw-Hill; 1988 [chapter 3]. [79] Lide DR, editor. CRC handbook of chemical and physics. Boca Raton: CRC Press; 1913–1995. p. 9–51.

Alteration for a diffusion barrier design concept in future high-density dynamic and ferroelectric random access memory devices

Alteration for a diffusion barrier design concept in future high-density dynamic and ferroelectric random access memory devices

Recommend Documents