Nanoindentation Behaviour and Annealed Microstructural Evolution
of Ni/Si Thin Film
Woei-Shyan Lee
1;*, Tao-Hsing Chen
2, Chi-Feng Lin
3and Jyun-Ming Chen
11Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R. O. China
2Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, R. O. China 3National Center for High-Performance Computing, Hsin-Shi Tainan County 744, Taiwan, R. O. China
The nano-mechanical properties of as-deposited Ni/Si thin films indented to a maximum depth of 800 nm are measured using a nanoindentation technique. The microstructural evolutions of the indented as-deposited specimens and indented specimens annealed at 200C,
300C, 500C and 800C for 2 min, respectively, are examined via transmission electron microscopy (TEM) and micro-Raman scattering
spectroscopy (RSS). The loading curve for the as-deposited Ni/Si thin film is found to be continuous. However, the unloading curve has a prominent pop-out feature. The hardness and Young’s modulus of the Ni/Si thin film are found to vary with the nanoindentation depth, and have values of 13 GPa and 177 GPa, respectively, at the maximum depth of 800 nm. The deformation induced in the nanoindentation process causes the microstructure of the indented zone in the as-deposited thin film to transform from a diamond cubic structure to a mixed structure comprising both amorphous phase and metastable Si III and Si XII phases. However, after annealing at temperatures of 200C500C and 800C, the
microstructure within the indented zone contains only Si III and Si XII phases and epitaxial NiSi2phase, respectively. The annealing process
prompts the formation of nickel silicides at the Ni/Si interface. The silicides have the form of Ni2Si in the samples annealed at 200C, but
transform to low-resistivity NiSi at annealing temperatures of 300C or 500C. At the highest annealing temperature of 800C, the NiSi phases
are replaced by high-resistivity NiSi2phases. [doi:10.2320/matertrans.M2010323]
(Received September 21, 2010; Accepted April 13, 2011; Published May 25, 2011)
Keywords: nickel/silicon thin films, nickel silicides, nanoindentation, pop-out feature, phase transformation
1. Introduction
The mechanical properties and microstructural evolution of thin film materials have attracted significant attention in recent years due to their many applications in the MEMS and IC fields. The mechanical properties and microstructures of such materials not only govern the mechanical performance of the thin film system, but also determine the overall electrical and optical performance of the device itself. Moreover, the properties and microstructures of thin film materials generally differ from those of the corresponding bulk materials, and typically vary with the fabrication conditions, the film thickness, the substrate effect, the indentation maximum load, the loading rate, and so on.1–5) Various methods have been proposed for determining the mechanical properties of thin film systems and observing their microstructural evolution.6–9) Previous studies have reported that nanoindentation prompts a phase transforma-tion within the indented zone, and therefore has a significant effect upon the load-displacement response of the film.10,11)
Furthermore, for thin films deposited on a silicon substrate, various forms of silicide phase are formed at the film/ substrate interface during substrate annealing.12)Therefore,
in developing optimum thin film systems for MEMS and IC applications, it is essential that the nanoindentation behaviour and microstructural evolution of the thin film structure during nanoindentation and annealing are properly understood.
The formation of high-quality metallic silicides at low temperature is of significant benefit in developing miniatur-ised Si devices. For example, NiSi is a promising candidate for shallow junction formation due to its low formation energy, thermal stability and low Si consumption. High
quality Ni silicides are formed at the Ni/Si interface of thin film systems by means of a solid-state reaction induced by isothermal furnace annealing or rapid thermal annealing (RTA) techniques.13–15) Nickel silicide has many phases,16)
thereby compounding the complexity of its formation. However, previous research17) has shown that the
predom-inant phases are Ni2Si, NiSi and NiSi2, respectively, where
the formation of each phase is dependent upon the annealing temperature at which the reaction takes place.18)Although the
formation and characteristics of Ni silicides for complemen-tary metal-oxide-semiconductor (CMOS) applications have been extensively reported,19–21) the combined effects of
nanoindentation deformation and annealing on the micro-structural evolution and phase of Ni/Si thin films are not yet fully understood.
Accordingly, this study indents as-deposited Ni/Si thin films to a maximum depth of 800 nm, and then anneals the indented specimens at temperatures of 200C, 300C, 500C
and 800C for 2 min, respectively. The hardness and Young’s
modulus of the as-deposited Ni/Si film are determined from the loading–unloading curve, and the microstructural char-acteristics of the as-deposited and annealed specimens are observed using transmission electron microscopy (TEM). Finally, the effect of the annealing temperature on the composition of the Ni silicides formed at the Ni/Si interface is clarified via micro-Raman scattering spectroscopy (RSS). 2. Experimental Procedure
The Ni/Si thin film specimens were prepared by deposit-ing a Ni film with a thickness of approximately 100 nm on a silicon(100) substrate using a thermal evaporation technique in an inert nitrogen gas environment. The thickness of the Ni film was monitored continuously throughout the deposition
*Corresponding author, E-mail: [email protected] Materials Transactions, Vol. 52, No. 7 (2011) pp. 1374 to 1380 #2011 The Japan Institute of Metals
process using a quartz-crystal microbalance and was verified via X-ray reflectometry once the deposition process was complete. The nanoindentation tests were performed at room temperature using an MTS Nano Indenter-XP system with a Berkovich diamond pyramid tip. The specimens were indented to a maximum depth of 800 nm using the indenter system set in a depth-control mode. The indentation procedure involved the following steps: (1) loading to the position of maximum load (corresponding to the maximum indentation depth), (2) holding in this position for 10 s, and (3) smoothly unloading over a period of 30 s. The hardness and Young’s modulus of the Ni/Si thin film were then calculated from the load-displacement data using the Oliver and Pharr method.22)
Following the nanoindentation tests, the indented speci-mens were annealed for 2 min at temperatures of 200C,
300C, 500C or 800C using a Heatpulse 610i RTA system
with a temperature accuracy of 5C. The annealing process
was performed in a nitrogen environment (Ni purity: 99.999%, Ni flow rate: 3 L min1) with a heating rate of
200C s1and a cooling rate of approximately 5C s1. Thin
foil specimens for TEM inspection were prepared from the as-deposited and annealed samples using an FEI Nova 200 focused ion beam (FIB) system with an operating voltage of 30 kV. During the preparation process, the FIB chamber was maintained at a constant pressure of 106107torr using a hybrid pumping system comprising a mechanical pump and an oil diffusion pump. The TEM foils were milled from the thin film specimens using a Gaþion beam and were extracted
in such a way that they included the centre of the indented zone. Note that before the foils were removed, a thin film (1 mm) of Pt was deposited on the specimen surface to protect the indentation region from accidental damage during the milling process. The cross-sectional microstructures of the as-deposited and annealed indented specimens were ob-served using a Philips Tecnai F30 Field Emission gun transmission microscope with a scanning voltage of 300 kV. In addition, the nickel silicides formed in the annealed indented specimens were analysed using TEM and micro-Raman scattering spectroscopy (RSS). The RSS procedure was performed at room temperature using a 513 nm Argon laser beam with a focused spot diameter of around 1 mm. 3. Results and Discussion
3.1 Loading–unloading curve
Figure 1 presents the loading–unloading curve of the as-deposited Ni/Si thin film indented to a depth of 800 nm. For indentation depths of less than 80 nm, the indenter remains fully within the soft Ni layer, and thus the load acting on the indenter has a low and approximately constant value. However, as the indentation depth increases, the tip pene-trates the underlying substrate, and thus the load increases rapidly. From inspection, the maximum load occurs at an indentation depth of 800 nm and has a value of 122 mN.
The loading curve is smooth and continuous. However, the unloading curve contains a prominent pop-out feature. The pop-out event occurs at a load of around 30 mN, i.e. a similar load to that reported by Yan et al.10)for nanoindented
single-crystal silicon. The pop-out feature observed in
nanoinden-tation tests has been variously attributed to an undensification of Si,23) residual deformation of the thin film,24) phase
transformation,25) and the indentation rate.26) Domnich et al.27)discussed the effects of phase transformation on the shape of the unloading curve of nanoindented silicon, and suggested that the pop-out feature was the result of the formation of Si III and Si XII phases. The TEM and micro-RSS results obtained in the present study suggest that the pop-out feature observed in the Ni/Si unloading curve shown in Fig. 1 is similarly the result of phase transformation within the indented zone.
3.2 Young’s modulus and hardness of Ni/Si thin film Figure 2(a) shows the variation of the Young’s modulus of the Ni/Si thin film with the nanoindentation depth. For indentation depths of less than 10 nm, the contact force is very low and the area between the indenter and the thin film is very small. As a result, the Young’s modulus has a relatively high value of 120 GPa since under low load conditions, the elastic modulus of a material varies inversely with the contact area. However, as the indenter penetrates more deeply into the Ni layer, the Young’s modulus reduces to a value of approximately 30 GPa at an indentation depth of approx-imately 25 nm, and then increases rapidly to a maximum value of around 188 GPa at a depth of 100 nm as a result of strain gradient hardening effects.28)At indentation depths
of 100200 nm, the indenter tip penetrates the Si substrate, and thus the Young’s modulus falls slightly to a value of approximately 180 GPa. For indentation depths of 200 800 nm (the maximum indentation depth), the indenter tip is embedded entirely within the Si substrate, and the Young’s modulus has a relatively constant value of 180 GPa, which is close to that of the Si(100) substrate (178 GPa).26)
Figure 2(b) shows the variation of the Ni/Si thin film hardness with the nanoindentation depth. At very low indentation depths (i.e. <10 nm), the thin film has a high hardness of around 8 GPa due to the small contact area between the film and the indenter. However, as the indentation depth increases, the hardness drops rapidly to a minimum value of approximately 3.5 GPa at an indentation
Fig. 1 Loading–unloading curve for as-deposited Ni/Si thin film indented to maximum depth of 800 nm.
III and Si XII phases. For the indented specimens annealed at 200C, the microstructure of the indented zone comprises
a mixture of Si III and Si XII phases. Furthermore, the annealing temperature results in the formation of Ni2Si phase
at the Ni/Si interface as the result of a diffusion of the Ni atoms. In the specimens annealed at 300C and 500C, the
microstructure within the indented zone is characterised by a mixture of Si III and Si XII metastable phases and NiSi phases at the Ni/Si interface. Finally, at the highest temper-ature of 800C, the Si III and Si XII phases disappear and the
microstructure of both the indented zone and at the Ni/Si interface contains NiSi2 phase only.
Acknowledgement
The authors gratefully acknowledge the financial support provided to this study by the National Science Council (NSC) of Taiwan under contract no. NSC97-2221-E-006-047. REFERENCES
1) M. Gadelkak: The MEMS Handbook, (CRC press, New York, 2002). 2) R. Saha and W. D. Nix: Acta Mater. 50 (2002) 23–38.
3) Y. Cao, S. Allanmeh, D. Nankivil, S. Sethiaraj, T. Otiti and W. Soboyejo: Mater. Sci. Eng. A 427 (2006) 232–240.
4) I. Zarudi and L. C. Zhang: Tribol. Int. 32 (1999) 701–712.
5) J. Jang, M. J. Lance, S. Wen, T. Y. Tsui and G. M. Pharr: Acta Mater. 53(2005) 1759–1770.
6) W. C. Oliver and G. M. Pharr: J. Mater. Res. 19 (2004) 3–20. 7) H. Pelletier, J. Krier and P. Mille: Mech. Mater. 38 (2006) 1182–1198. 8) Y. L. Jiang, A. Agarwal, G. P. Ru, G. Cai and B. Z. Li: Nucl. Instrum.
Meth. B 237 (2005) 160–166.
9) J. E. Bradby, J. S. Williams and M. V. Swain: Phys. Rev. B 67 (2003) 085205.
10) J. Yang, H. Takahashi, X. Gai, H. Harada, J. Tamaki and T. Kuriyagawa: Mater. Sci. Eng. A 423 (2006) 19–23.
11) W. S. Lee and F. J. Fong: Mater. Trans. 48 (2007) 2650–2658. 12) W. S. Lee and T. Y. Liu: Nanotechnology 18 (2007) 335701. 13) P. S. Lee, K. L. Pey, D. Mangelinck, J. Ding, D. Z. Chi, J. Y. Dai and L.
Chan: J. Electrochem. Soc. 149 (2002) G331.
14) F. F. Zhao, J. Z. Zheng, Z. X. Shen, T. Osipowicz, W. Z. Gao and L. H. Chan: Microelectron. Eng. 71 (2004) 104–111.
15) B. A. Julies, D. Knoesen, R. Pretorius and D. Adams: Thin Solid Films
347(1999) 201–207.
16) T. Morimoto, H. S. Momose, T. Iinuma, I. Kunishima, K. Suguro, H. Okano, I. Katakabe, H. Nakajima, M. Tsuchiaki, M. Ono, Y. Katsumata and H. Iwai: Tech. Dig. Int. Electron Devices Meet., IEDM ’91, (1991) pp. 653–656.
17) F. M. d’Heurle: J. Mater. Res. 3 (1988) 167–195.
18) J. Foggiato, W. S. Yoo, M. Ouaknine, T. Murakami and T. Fukada: Mater. Sci. Eng. B 56 (2004) 114–115.
19) Y. J. Chang and J. L. Erskine: Phys. Rev. B 28 (1983) 5766–5773. 20) J. E. E. Baglin, H. A. Atwater, D. Gupta and F. M. d’Heurle: Thin Solid
Films 93 (1982) 255–264.
21) M. A. Pawlak, J. A. Kittl, O. Chamirian, A. Veloso, A. Lauwers, T. Schram, K. Maex and A. Vantomme: Microelectron. Eng. 76 (2004) 349–353.
22) J. W. C. Oliver and G. M. Pharr: J. Mater. Res. 7 (1992) 1564–1583. 23) G. M. Pharr, W. C. Oliver and D. S. Harding: J. Mater. Res. 6 (1991)
1129–1130.
24) X. J. Zheng, Y. C. Zhou and J. Y. Li: Acta Mater. 51 (2003) 3985– 3997.
25) A. J. Leistner, A. C. Fischer-Cripps and J. M. Bennett: Proc. Int. Soc. Optical Engineering (SPIE), ed. by W. A. Goodman, (Bellingham, 2003) pp. 215–222.
26) R. Rao, J. E. Bradby, S. Ruffell and J. S. Williams: Microelectron. J. 38 (2007) 722–726.
27) V. Domnich, Y. Gogotsi and S. Dub: Appl. Phys. Lett. 76 (2000) 2214– 2216.
28) R. Saha, Z. Xue, Y. Huang and W. D. Nix: J. Mech. Phys. Solids 49 (2001) 1997–2014.
29) D. Ge, V. Domnich and Y. Gogotsi: J. Appl. Phys. 93 (2003) 2418– 2423.
30) A. Kailer, K. G. Nickel and Y. G. Gogotsi: J. Raman Spectrosc. 30 (1999) 939–946.
31) Y. Song, A. L. Schmitt and S. Jin: Nano Lett. 7 (2007) 965–969. 32) P. S. Lee, D. Mangelinck, K. L. Pey, Z. X. Shen, J. Ding, T. Osipowicz
and A. K. See: Electrochem. Solid-State Lett. 3 (2000) 153–155. 33) Y. L. Jiang, A. Agarwal, G. P. Ru, G. Cai and B. Z. Li: Nucl. Instr.
Meth. B 237 (2005) 160–166.
34) N. Oyama, Y. Ogura, Y. Mitake, Y. Tomita, H. Sakamoto, S. Nagase, M. Watanabe, N. Fujiwara, S. Ohshima and F. Hirose: Jpn. J. Appl. Phys. 46 (2007) L506–L508.
35) J. A. Kittl et al.: Microelectron. Eng. 83 (2006) 2117–2121. 36) F. F. Zhao, J. Z. Zheng, Z. X. Shen, T. Osipowicz, W. Z. Gao and L. H.
Chan: Microelectron. Eng. 71 (2004) 104–111.
37) I. Zarudi, L. C. Zhang, W. C. D. Cheong and T. X. Yu: Acta Mater. 53 (2005) 4795–4800.
38) S. Nygren, D. Caffin, M. Ostling and F. M. d’Henrle: Appl. Surf. Sci. 53 (1991) 87–91.
