Ru nanodot synthesis using CO2 supercritical fluid deposition

Journal of Physics and Chemistry of Solids 74 (2013) 664–667

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Ru nanodot synthesis using CO2 supercritical fluid deposition Doyoung Kim a,n, Han-Bo-Ram Lee b, Jaehong Yoon c, Hyungjun Kim c,nn a b c

School of Electrical and Electronic Engineering, Ulsan College, Ulsan 680-749, Republic of Korea Chemical Engineering, Stanford University, CA 94305, USA School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 October 2012 Received in revised form 17 December 2012 Accepted 26 December 2012 Available online 5 January 2013

CO2-based supercritical fluid deposition (SCFD) was utilized for the synthesis of ruthenium nanostructures on Si substrates and hafnium oxide (HfO2) films. Ru nanodots were formed by the reduction of a Ru precursor under a hydrogen environment. Ruthenium carbonyl was dissolved in supercritical CO2 in a pre-mix vessel, and this mixture was pumped into a reaction vessel which allowed it to undergo rapid expansion. The formation of Ru nanodots was studied as a function of key process parameters such as the weight of precursor used, the reaction temperature and the H2 reactant pressure. Utilizing the Ru nanodots on a HfO2 control oxide, a flash memory device containing nanodots was fabricated and its memory characteristics were demonstrated. & 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Metals A. Nanostructures B. Chemical synthesis C. X-ray diffraction D. Electrical properties

1. Introduction Nanoscale materials have received a great deal of attention because of their novel electronic and optical properties that arise from the quantum confinement effect [1] and because of their unique structural characteristics [2], which make them promising candidates for use in nanowire-based field effect transistors [3], light-emitting diodes [4], and nonvolatile nanocrystal memory [5]. In particular, Ru and Ru-based materials are important materials in many nanoscale devices and other emerging electronics applications. For example, Ru and RuO2 are promising candidates for use in electrodes in nanoscale memory devices [6], and Ru is a good seed layer/adhesion promoter for copper interconnections. Additionally, a supercapacitor fabricated using Ru dioxide nanoparticles [7] was reported to have a high energy density, a high power density, and a long lifetime at a low operation voltage. Ru deposition has been achieved through various techniques such as atomic layer deposition (ALD) [8], chemical vapor deposition (CVD) [9], and physical vapor deposition (PVD) [10]. Recently, supercritical fluid deposition (SCFD) has emerged as a potentially useful thin film deposition technique due to its excellent gap fill properties [11–13]. Since supercritical fluids (SCF) have a high diffusivity and zero surface tension, they can penetrate into extremely narrow structures, thus giving them an

ultra-high gap fill property when they are used as solvents for thin film deposition. In particular, carbon dioxide (CO2) SCF is non-flammable, non-toxic, and environmentally friendly and has a low critical temperature and pressure of 31 1C and 71 bar. For example, Blackburn et al. [14] filled copper and nickel film into nanoscale trenches using a CO2-based SCFD. Long et al. [15] reported the deposition of various metals, including Pt, Pd, Au, and Rh, on polymer substrates in CO2 at 60 1C. In addition to the preparation of thin film materials, SCFD can be utilized for the synthesis of nanomaterials such as nanoparticles, nanowires, and nanotubes. For example, ordered germanium nanowires were synthesized within mesoporous silica using SCFD [16]. Ye et al. fabricated metallic nanowires by depositing metal into carbon nanotubes [17]. In addition, the synthesis of gold nanopraticles supported on silica and Ge nanomaterials using CO2 SCFD was also recently reported [18,19]. In this study, we synthesized Ru nanodots without the use of catalysts or templates using CO2 SCFD. Ru nanodots with different densities and size distributions were synthesized by controlling the process parameters such as the precursor weight, substrate temperature, and reactant (H2) pressure. These nanodots formed on ALD highk thin films were used to fabricate non-volatile memory devices.

2. Experimental details n

Corresponding author. Tel.: þ82-1043107163. Corresponding author. School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea. E-mail addresses: [email protected] (D. Kim), [email protected] (H. Kim). nn

0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.12.022

We used p-type Si(100) and thermally oxidized silicon (SiO2) wafers as substrates. Square 1 cm  1 cm pieces were cleaved from the wafer and cleaned according to a standard cleaning

D. Kim et al. / Journal of Physics and Chemistry of Solids 74 (2013) 664–667

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process using piranha solution [20] at 110 1C for 30 min. The substrate was then loaded into a reaction cell in a glovebox under nitrogen. All chemicals utilized in the reaction were anhydrous and were stored under nitrogen upon receipt. The experimental setup for SCF deposition was described in a previous report [21]. The phase of the CO2 was changed from a liquid to a supercritical fluid in a condenser chamber used as a surge tank. In the pre-mix chamber, a ruthenium carbonyl (Ru3(CO)12) precursor was dissolved at 90 1C and 230 bar in supercritical CO2 for 1 h. In order to control the Ru nanodot size, the weight of the Ru3(CO)12 precursor and the reaction temperature were varied. Thereafter, the CO2 solution was pumped into a deposition chamber where the H2 reaction gas was supplied. Bare Si and SiO2 on Si substrate were mounted on a substrate holder, which was heated by a cartridge ceramic heater. In contrast to the more conventional hot-wall-type heating configuration, [22] we used a cold-walltype reaction in order to improve temperature control. The reactions were carried out under a pressure of 10 bar and a substrate temperature of 220 1C. The pressure of hydrogen (H2), which is a reactive gas, was also controlled. After the process, the reactor was depressurized. The morphology and size of the samples were analyzed by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) using synchro˚ at tron radiation X-ray diffraction (SRXRD) (l ¼ 1.54 A˚ and 1.24 A) the Pohang Accelerator Laboratory (PAL). Nanodot memory was fabricated using Ru nanodots formed on HfO2, which was deposited by atomic layer deposition (ALD). The detailed HfO2 ALD process was described in a previous report [23]. The capacitance– voltage (C–V) characteristics were determined by using a Keithley 4200 semiconductor parameter analyzer with an HP4284 LCR meter. The applied voltage was swept from the inversion voltage of þ2.5 V to the accumulation voltage of 2.5 V and back again to determine the amount of C–V hysteresis.

3. Results and discussion In order to confirm the gap fill performance of Ru SFCD, we filled a SiO2 trench pattern with a height of 500 nm and a diameter of 100 nm with Ru metal, as shown in Fig. 1(a). After etching the SiO2 layer using a hydrogen fluoride solution, a Ru nanorod array was fabricated. The results demonstrated the excellent gap fill properties of the SCFD process, as shown in Fig. 1(b). For the synthesis of Ru nanodots using SCFD, the effects of the amount of Ru precursor used were investigated. Ru SCFD was carried out at a substrate temperature of 220 1C, a CO2 pressure of 10 bar, and a H2 reactant pressure of 20 bar in a reaction vessel with different amounts of dissolved Ru precursor. At Ru precursor weight of 350 mg, a continuous Ru metal film was deposited, as shown in Fig. 2(a). However, a discontinuous film was deposited with decreasing amounts of Ru precursor. For example, at a Ru precursor weight of 140 mg, irregular Ru agglomerates were observed, as shown in Fig. 2(b). These results indicate that continuous metal film formation requires a minimum level of dissolved metal precursor in the pre-mix chamber. Ru nanodot formation is clearly observed at a Ru precursor weight of 35 mg. As shown in Fig. 2(c), Ru nanodots with a relatively uniform diameter was formed over the entire substrate area. The diameter of the Ru nanodots ranged from 10 to 28 nm with an average diameter of 15.0474.0 nm as based on the measurement of more than 100 nanodots. All of the nanodots were verified to be crystalline Ru, as shown in the SRXRD curve in Fig. 2(d). One diffraction peak was only observed in XRD analysis. It means that the orientation of (101) plane is dominant in the possible diffraction peaks of randomly oriented hexagonal Ru microstructure

Fig. 1. FE-SEM images of (a) the SiO2 hole pattern formed on a Si substrate and (b) Ru metal after removal of the SiO2 hole pattern.

since the Ru nanodots are small, even the dominant XRD peak intensity is low and other peaks, such as the orientation of (100) and (002) which are typically observed in Ru thin films, are not observed. Fig. 3 shows FE-SEM images of Ru nanodots prepared at temperatures of 250 1C and 280 1C. From these images, it can be seen that the size and density of the Ru nanodots were sensitive to the growth temperature. The diameter distribution and density of the Ru nanodots as functions of growth temperature are shown in Fig. 4. The average diameter (15.5–41.9 nm) increased by a factor of three as the growth temperature increased from 220 1C to 280 1C. In addition, the number density of nanodots decreased from 1.4  1013 to 1.0  1012/cm2 with increasing substrate temperature, as shown in Fig. 4(b). It can be inferred that a high temperature in the reaction chamber accelerates the reduction of the ruthenium precursor due to the availability of a sufficient supply of energy for nucleation, resulting in a large enhancement in nanodot size. In order to evaluate the charge-trapping properties of the Runanodot-embedded dielectric layer (HfO2), we fabricated metal insulator semiconductor (MIS) structures with and without Ru nanodots. The thickness of the HfO2 film grown by ALD on a Si (001) wafer as a tunneling oxide was 5 nm. Ru nanodots were then formed at 220 1C using the SCFD process. A control oxide, which was 40-nm-thick HfO2, was deposited on a Ru nanodot

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D. Kim et al. / Journal of Physics and Chemistry of Solids 74 (2013) 664–667

Fig. 2. FE-SEM images of Ru morphology at precursor weights of (a) 350 mg, (b) 140 mg, (c) and 35 mg as well as the (d) SR-XRD curve. All samples were formed at a temperature of 220 1C.

Fig. 3. FE-SEM images of Ru nandots formed at substrate temperatures of (a) 250 1C and (b) 280 1C at a precursor weight of 35 mg in CO2 SCF.

structure by an ALD technique. Lastly, a Ru metal electrode was deposited on the control oxide by a sputtering method. The MIS structure is schematically shown in the inset of Fig. 5. From this MIS structure, we measured the capacitance–voltage properties, as shown in Fig. 5. In the case of MIS without Ru nanodots, a hysteresis window, which is the flat band voltage (VFB) difference between the forward bias and reverse bias, was not observed. However, the MIS device containing Ru nanodots exhibited a hysteresis of about 0.5 V with a VFB shift. This result demonstrates the existence of asymmetric charging and discharging phenomenon caused by Ru metal nanodots acting as charge trapping sites [24]. The current SCFD-based Ru nanodots synthesis process

could have significant benefits for emerging three-dimensional device fabrication [25,26].

4. Conclusion We demonstrated that Ru nanodots can be synthesized on Si substrates and HfO2 films using CO2 SCF deposition. A hole pattern was filled with Ru film with good conformality by a SCF technique. The diameter, density, and size distribution of the Ru nanodots varied depending on the deposition parameters such as the weight of precursor and the substrate temperature. Pure Ru

D. Kim et al. / Journal of Physics and Chemistry of Solids 74 (2013) 664–667

References

50

o

SCFD Ru/Si(100)

T s = 220 C o

T s = 250 C

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40

o

T s = 280 C 30

20

10

0

0

20

40

60

80

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Diameter (nm) 20

60 SCFD Ru/Si(100)

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20 5

Density (x1010/cm2)

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Diameter Density

10 0

220

230

240

250 Ts(oC)

260

270

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0

Fig. 4. (a) The distribution of their diameters of Ru nanodots (b) average diameter and density of Ru nanodots as a function of substrate temperature (Ts).

w/o Ru nanodots with Ru nanodots

0.8 0.6 C/Co

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0.4 0.2 0

-2

-1

0 Va(V)

1

2

Fig. 5. Normalized capacitance–voltage curves with and without Ru nanodots. The inset shows the structure of the MIS capacitor including an HfO2 insulator produced via ALD.

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