Surface and interface morphology of CoSi2 films formed by multilayer solid-state reaction

Materials Characterization 48 (2002) 229 – 235

Surface and interface morphology of CoSi2 films formed by multilayer solid-state reaction Guo-Ping Rua,*, Bing-Zong Lia, Guo-Bao Jianga, Xin-Ping Qua, Jing Liua, R.L. Van Meirhaegheb, F. Cardonb a

Department of Microelectronics, Asic & System State Key Lab., Fudan University, Shanghai 200433, China b Department of Solid State Sciences, University of Gent, Krijgslaan 281/S1, B-9000 Gent, Belgium Received 1 June 2001; accepted 17 October 2001

Abstract CoSi2 is a promising material for self-aligned silicide (salicide) applications in sub-0.25 mm complementary metal – oxide – semiconductor (CMOS) technology. In conventional salicide technology, silicides are formed by a solid-state reaction (SSR) after source/drain formation. With the continued scaling down of junction depths, surface and interface roughness of silicides is a growing concern. In this work, a comparative study has been made to investigate the morphology and thermal stability of CoSi2 formed by SSR of different structures, i.e. Co/Si, TiN/Co/Si, Ti/Co/Si, Co/Ti/Si and Ti/Co/Ti/Si. Atomic force microscopy and other techniques were used to characterize the morphology and thermal stability. Compared with the Co/Si reaction, TiN or Ti capping reduces the roughness and improves the thermal stability. The reaction with a Ti interfacial layer shows epitaxial growth of CoSi2 on Si (100). The morphology and thermal stability of epitaxial CoSi2 were significantly improved. The epitaxial CoSi2 may be useful for contact in deep submicrometer CMOS devices. D 2002 Elsevier Science Inc. All rights reserved.

1. Introduction TiSi2 is widely used in submicrometer complementary metal – oxide – semiconductor (CMOS) circuit fabrication. However, its applicability to sub-0.25 mm technologies has been questioned due to the difficulty of maintaining low sheet resistance on small geometry [1,2]. CoSi2 is regarded as a good choice to replace TiSi2 in 0.18 mm and beyond technologies, because CoSi2 has some properties that are superior to those of TiSi2, such as no line-width effect on sheet resistance and no strong interaction with dopants. For CoSi2, the concerns are shifted from issues * Corresponding author. E-mail address: [email protected] (G.-P. Ru).

related to lateral scaling to issues related to vertical scaling of junction depth, such as silicided diode leakage [3]. In the self-aligned silicide (salicide) process, silicide film is formed by a solid-state reaction (SSR) of deposited metals with substrate silicon, which results in some consumption of silicon. As the device size is scaled down, the silicide/Si interface approaches more to a pn junction interface. As a consequence, the roughness of silicide/Si interface becomes a critical concern, because rough interfaces or spikes may degrade or even destroy the pn junction [4]. On the other hand, the silicide film may agglomerate after high-temperature annealing. Thus, how to form a thermally stable silicide film with a very smooth interface to Si becomes a main issue in CoSi2 salicide technology.

1044-5803/02/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved. PII: S 1 0 4 4 - 5 8 0 3 ( 0 2 ) 0 0 2 5 2 - 8

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The atomic force microscope (AFM) is a new powerful instrument for studying surface morphologies. Compared to the scanning electron microscope (SEM), the AFM has higher resolution and is capable of three-dimensional probing. Compared to the transmission electron microscope (TEM), the AFM needs much less sample preparation. In a previous paper, we studied the surface morphology of CoSi2 films by AFM [5]. In this paper, we describe the use of the AFM, together with other techniques, in studies of the surface and interface morphology of CoSi2 formed by SSR of several different multilayer structures, i.e. Co/Si, TiN/Co/Si, Ti/Co/Si, Co/Ti/Si, and Ti/Co/Ti/Si. 2. Experimental details N-type Si (100) wafers with a resistivity of 5 – 8 V cm were used as substrates. After a standard RCA cleaning followed by a diluted HF dip, the substrates were immediately loaded into a multifunctional sputtering system. Different multilayer structures (Co/Si, TiN/Co/Si, Ti/Co/Si, Co/Ti/Si and Ti/Co/Ti/Si) were deposited by either ion-beam sputtering or magnetron dc sputtering techniques. The base pressure of the sputtering system was below 1
10  6 Torr. The silicide was then formed after the as-deposited structure was ex situ annealed in a N2 ambient in a rapid thermal annealing (RTA) system.

All the AFM measurements were performed in air using a commercially available AFM/STM (Topometrix 2000) with a 70-mm range tube scanner driving the sample. Vibration isolation was achieved by spring suspension. AFM detection was done by optical beam deflection and involved a position-sensitive detector and laser beam alignment screws. Contact-mode AFM measurements were performed using commercially available Si3N4 cantilevers with a 50-nm radius tip. Force calibration for individual cantilevers has been obtained by measuring force – distance curves. The quality of tips was characterized by measuring the pull-out force. The surface morphology of the silicide film was directly characterized by AFM. The CoSi2/Si interface morphology was characterized by AFM after the silicide layer was chemically removed away. Besides the AFM, several other instruments, such as a fourpoint probe (FPP) and an X-ray diffractometer (XRD), were used to characterize silicide film agglomeration and silicide epitaxial quality.

3. Results and discussion 3.1. Polycrystalline CoSi2 film Polycrystalline CoSi2 film can be formed by SSR of different multilayer structures. The simplest one is Co/Si. Fig. 1 shows a typical AFM image of the

Fig. 1. AFM image of the surface of CoSi2 film formed by two-step annealing (650 C/60 s + S.E. + 800C/30 s) of a Co(10 nm)/ Si(100) structure.

G.-P. Ru et al. / Materials Characterization 48 (2002) 229–235

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Fig. 2. Normalized roughness f ( = Ra/L) as a function of grooving angle q, calculated by one- and two-dimensional grooving models. The inset shows schematic diagram of thermal grooving for a polycrystalline silicide thin film on a Si substrate.

surface of CoSi2 film formed by the Co/Si reaction. The surface shows a hillock structure, with a surface roughness Ra of 4.08 nm. The roughness of polycrystalline CoSi2 films typically ranges from 3 to 6 nm, depending on the annealing temperature and annealing time. It has been found that the roughness increases systematically as the annealing time is increased [5]. This phenomenon can be explained by the thermal grooving mechanism. In a polycrys-

talline thin film/Si system, the atoms in the thin film may diffuse across surface, interface and grain boundaries during thermal processes [6]. The inset of Fig. 2 shows a schematic diagram of a polycrystalline silicide thin film on a Si substrate. From the thermodynamic point of view, the grooving angles of surface and interface (qs, qi) are fixed for a given system. They are related to the silicide surface energy, gs, the silicide/Si interface energy, gi, and

Fig. 3. Sheet resistance of CoSi2 films as a function of annealing temperature. The annealing time is fixed at 60 s.

232 G.-P. Ru et al. / Materials Characterization 48 (2002) 229–235 Fig. 4. (a) Surface and (b) interface topography images of CoSi2 film formed by two-step annealing (600 C/60 s + S.E. + 800 C/30 s) of a Ti(8 nm)/Co(15 nm)/Si(100) structure; (c) surface and (d) interface topography images of CoSi2 film formed by two-step annealing (650 C/60 s + S.E. + 800 C/30 s) of a Ti(8 nm)/Co(15 nm)/Ti(4 nm)/Si(100) structure.

G.-P. Ru et al. / Materials Characterization 48 (2002) 229–235

the silicide grain boundary energy, gb. The relations can be expressed as follows gb ¼ 2gs sinqs ;

ð1Þ

gb ¼ 2gi sinqi :

ð2Þ

As the grain grows, both the surface and interface roughnesses increase. To a certain extent, the film will agglomerate, i.e. hole formation or islanding occurs. Because the morphology of the film changes after a thermal process, the sheet resistance, somewhat dependent on the morphology, also changes. Some models predict that as the thermal grooving process develops, the sheet resistance increases. When hole formation or islanding occurs, the sheet resistance will increase significantly. Fig. 3 depicts the variation of sheet resistance (Rsh) of CoSi2 film with annealing temperature, with the annealing time fixed at 60 s. It is apparent from the figure that the thicker the CoSi2 film is, the higher is the temperature at which the sheet resistance begins to increase. This is in good agreement with the predictions of the thermal grooving and agglomeration models [7,8]. To prevent the silicide film from thermal agglomeration, some people have suggested the use of TiNor Ti-capped structures, i.e. TiN/Co/Si or Ti/Co/Si, when forming CoSi2 film [9 – 11]. The role of the capping layer in preventing thermal agglomeration has two aspects. First, the capping layer can prevent the ambient contamination of the silicide surface and even reduce the Si surface contamination. Second, the capping layer slows down surface diffusion [12], the main cause of thermal grooving in the CoSi2/Si system [6]. Fig. 4(a) and (b) shows typical surface and interface topography images of CoSi 2 film formed from a Ti-capped structure, i.e. Ti(8 nm)/ Co(15 nm)/Si. The surface roughness Ras and the interface roughness Rai are 2.53 and 5.35 nm, respectively. As either the annealing temperature or the annealing time increases, both the surface rough-

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ness and interface roughness increase. However, the interface roughness is always larger than the surface roughness, and the ratio of interface roughness to surface roughness is located in the range of 2 – 3. The roughness results, including the ratio where applicable, are listed in Table 1. It is of interest to investigate the roughness ratio by the grooving models. From Eqs. (1) and (2), and the inset of Fig. 2, one can see that the roughness ratio of interface to surface can be calculated if one knows the grooving angles or the ratio of interface energy to surface energy. Xiao et al. [7] performed an analysis on a one-dimensional grooving process and Nolan et al. [8] did the same on a two-dimensional grooving process. We calculated the roughness of a grooving surface as a function of grooving angles for both cases. A normalized roughness can be defined as follows: f ¼ Ra=L;

ð3Þ

where L is the grain size. Fig. 2 shows the normalized roughness f as a function of grooving angle q. For a rough estimate, the silicide grain boundary and interface energies are comparable (gb  gi) and are about one third of the surface energy, gs, as would be the case for a pure metal [8]. So the surface and interface grooving angles are then calculated to be about 10 and 30, respectively. In fact, the surface and interface grooving angles for TiSi2 are 14 and 32, respectively, as determined by TEM [8]. If we assume the surface and interface grooving angles for CoSi2 are the same as for a pure metal (10/30), the interface – surface roughness ratio is calculated to be 3.02 and 3.06 by one- and two-dimensional grooving profiles, respectively. If we take the same grooving angles of TiSi2 (14/32), for CoSi2 the interface – surface roughness ratio is calculated to be 2.30 and 2.33 by one- and two-dimensional grooving profiles, respectively. So our experimental results of roughness ratio (Table 1) are in good agreement with the theoretical calculation.

Table 1

Roughness results of CoSi2 formed from different multilayer structures Poly

Epi

Initial structure

Silicidation process

Co(5)/Si Co(10)/Si Co(10)/Si TiN(10)/Co(10)/Si Ti(8)/Co(15)/Si Ti(8)/Co(15)/Ti(4)/Si Ti(8)/Co(15)/Ti(2)/Si Co(5)/Ti(3)/Si

950 600 600 800 600 650 650 950

C C C C C C C C

(60 s) (30 s) + 800 (30 s) + 800 (30 s) (60 s) + 800 (60 s) + 800 (60 s) + 950 (300 s)

Ras (nm) C (30 s) C (30 s) + 950 C (300 s) C (30 s) C (30 s) C (30 s)

3.33 4.08 5.37 3.11 2.53 0.62 0.71 0.73

Rai (nm)

Rai/Ras

5.35 1.28 1.97

2.11 2.06 2.77

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Fig. 5. XRD spectrum of CoSi2 film formed by annealing a Ti(8 nm)/Co/(15 nm)/Ti(2 nm) /Si(100) structure at 850 C for 30 s.

3.2. Epitaxial CoSi2 film To meet the scaling-down requirements, epitaxial silicide may be a good candidate for future ULSI contact technology, because it has smooth surface/ interface and very good thermal stability. Molecular beam epitaxy and other related techniques can grow high epitaxial quality CoSi2 film on Si substrate, but they cannot form a salicide structures [13 – 16]. This makes them incompatible with the ULSI fabrication process. Since the early 1990s, epitaxial growth of CoSi2 on Si by multilayer SSR has been studied intensively. Interlayers such as Ti, Ta, Zr, Ti/a-Si and oxide have been shown to be able to promote epitaxial growth of CoSi2 during SSR [17 – 21]. These techniques can be classified as interlayer mediate solid phase epitaxy (IMSPE). In principle, salicide structures can be formed by the IMSPE technique, thereby making this technique attractive for future ULSI technology. Fig. 5 shows an XRD spectrum of IMSPE-grown CoSi2, formed by annealing a Ti(8 nm)/Co/(15 nm)/ Ti(2 nm)/Si(100) structure at 850 C for 30 s. The strong (400) series peaks and the almost invisible diffraction peaks from other orientations demonstrate the epitaxial growth of CoSi2 film on Si(100). Fig. 4(c) and (d) shows surface and interface topography images of an epitaxial CoSi2 film formed by two-step annealing (650 C/60 s + S.E. + 800 C/30 s) of a Ti(8 nm)/Co(15 nm)/Ti(4 nm)/Si(100) structure. The surface roughness and interface roughness are 0.61 and 1.28 nm, respectively, which are about one third of those of polycrystalline CoSi2. As the annealing temperature increases, for example, to 950 C, the roughness also increases, but only to a very limited

extent, as shown in Table 1. Interestingly, the interface – surface roughness ratio of the epitaxial CoSi2 is also located in the range of 2 – 3. Without a Ti capping layer, the roughness also decreases from Co/Si SSR to Co/Ti/Si SSR. As shown in Table 1, the roughness of IMSPE CoSi2 from Co(5 nm)/Ti(3 nm)/Si(100) remained low after annealing at 950 C for 300 s, but the roughness of the polycrystalline CoSi2 from Co/Si increased substantially after 950 C annealing. So the Ti interlayer plays a crucial role in roughness reduction of CoSi2 film formed by multilayer SSR. In the meantime, the sheet resistance of epitaxial CoSi2 remained extremely stable, even after 1000 C annealing, as shown in Fig. 3. Such superior thermal stability results from the epitaxial nature of CoSi2 [22]. In an epitaxial film, thermal grooving or islanding should be less energy favorable.

4. Conclusions The surface and interface morphologies of various CoSi2 films formed by different multilayer SSR have been studied using the AFM technique. Thin CoSi2 film tended to agglomerate with a moderate thermal treatment. Ti capping and TiN capping are good approaches to prevent the film from thermal agglomeration. The roughness of the capped polycrystalline CoSi2 was reduced compared to that of the simple Co/Si reaction. The interface roughness was larger than the surface roughness. The interface – surface roughness ratio was found to be located in the range of 2 – 3, which is in good agreement with the predictions of one- and two-dimensional models. The IMSPE-grown CoSi2 film showed a very smooth

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surface and interface. The surface and interface roughnesses were about one third of those of a polycrystalline CoSi2 film. The sheet resistance was thermally stable up to 1000 C. These superior properties of IMSPE-grown CoSi2 film make it attractive for future ULSI fabrication.

[7]

Acknowledgements

[9]

This work was supported by Natural Science Foundation of China NSFC-60106002 and bilateral cooperation project Bil 96/74/B017 from the Flemish Minister of Science and Technology and the State Science and Technology Commission of China and Shuguang project from Shanghai Municipal Education Commission and Shanghai Education Development Foundation. We appreciate the successful cooperation with colleagues in University of Gent and IMEC.

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