Phase transformation of Ni/Si thin films induced by nanoindentation and annealing

Appl Phys A (2010) 100: 1089–1096 DOI 10.1007/s00339-010-5706-0

Phase transformation of Ni/Si thin films induced

by nanoindentation and annealing

Woei-Shyan Lee· Tao-Hsing Chen · Chi-Feng Lin · Jyun-Ming Chen

Received: 28 September 2009 / Accepted: 12 April 2010 / Published online: 6 May 2010 © Springer-Verlag 2010

Abstract Thin Ni/Si films are prepared by depositing a Ni layer with a thickness of 100 nm on a Si (100) substrate. The as-deposited thin-film specimens are indented to a max-imum depth of 500 nm using a nanoindentation technique and are then annealed at temperatures of 200°C, 300°C, 500°C and 800°C for 2 min. The microstructural changes and phases induced in the various specimens are observed using transmission electron microscopy (TEM) and micro-Raman scattering spectroscopy (RSS). Based on the load-displacement data obtained in the nanoindentation tests, the hardness and Young’s modulus of the as-deposited speci-mens are found to be 13 GPa and 177 GPa, respectively. The microstructural observations reveal that the nanoinden-tation process prompts the transformation of the indennanoinden-tation- indentation-affected zone of the silicon substrate from a diamond cubic structure to a mixed structure comprising amorphous phase and metastable Si III and Si XII phases. Following annealing at temperatures of 200∼500°C, the indented zone contains either a mixture of amorphous phase and Si III and Si XII phases, or Si III and Si XII phases only, depending on the annealing temperature. In addition, the annealing process prompts the formation of nickel silicide phases at the Ni/Si interface or within the indentation zone. The composition of

W.-S. Lee (



Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan

e-mail:[email protected] Fax: +886-6-2352973

T.-H. Chen

Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan

C.-F. Lin

National Center for High-Performance Computing, Hsin-Shi, Tainan County 744, Taiwan

these phases depends on the annealing temperature. Specif-ically, Ni2Si is formed at a temperature of 200°C, NiSi is formed at a temperature of 300°C and 500°C, and NiSi2is formed at 800°C.

1 Introduction

Thfilm materials are widely employed in many fields, in-cluding microelectronics, microelectro-mechanical systems, optical devices, magnetic storage equipment, and so on [1–4]. However, to reduce the characteristic size of these systems and devices, and to improve their integration, thin-ner films and heterostructures are required [5]. Accordingly, various physical and chemical methods have been proposed for fabricating and modifying thin-film systems [6]. Due to the different nature of the materials used to fabricate thin film structures, the mechanical behaviour of such structures is quite different from that of the respective bulk materi-als. In addition, the properties of thin films depend fun-damentally upon the fabrication process, the film thick-ness, the substrate effect, and so on [7–9]. The mechan-ical properties of thin films are generally evaluated using some form of nanoindentation technique [10]. However, it has been reported that the nanoindentation process prompts a phase transformation within the indentation-affected area, which has a significant effect upon the load-displacement response [11,12]. Furthermore, for thin films deposited on a silicon substrate, various types of silicide are commonly formed at the film/substrate interface during subsequent an-nealing [13]. Therefore, in developing thin-film systems for device applications, a thorough understanding of the nanoin-dentation behaviour and microstructural evolution of thin film structures during nanoindentation and annealing is re-quired.

1090 W.-S. Lee et al. Nickel has emerged as a promising candidate for

thin-film systems due to its excellent mechanical and physical properties and its improved interconnect performance com-pared to that of traditional thin-film materials such as Au or Cu [14,15]. As a result, the interfacial properties of Ni/Si systems and the nanoindentation response and microstruc-tural evolution of Ni/Si thin films have attracted increas-ing attention in recent years. Many researchers have demon-strated the use of isothermal furnace annealing or rapid ther-mal annealing (RTA) techniques in forming high quality Ni silicides at the interface between Ni films and single-crystal Si substrates by means of a solid-state reaction [16–18]. The formation of these Ni silicides is the consequence of a reduc-tion in the free energy during the reacreduc-tion process, and the final form of the silicide phase (e.g. Ni2Si, NiSi or NiSi2) is critically dependent upon the temperature at which the reaction takes place [19]. Although the formation and char-acteristics of Ni silicides for complementary metal-oxide-semiconductor (CMOS) applications have been extensively reported [20–22], the combined effects of nanoindentation deformation and annealing on the microstructural evolution and phase of Ni/Si thin films are not yet fully understood. Accordingly, this study commences by investigating the me-chanical properties of as-deposited Ni/Si thin films using a nanoindentation technique. The indented specimens are then annealed at temperatures of 200°C, 300°C, 500°C or 800°C for 2 min. The microstructural characteristics of the as-deposited and annealed specimens are then examined us-ing transmission electron microscopy (TEM). Finally, the nature of the Ni silicide phases formed at the Ni/Si interface under each annealing temperature is identified using micro-Raman scattering spectroscopy (RSS).

2 Experimental procedure

The Ni/Si thin-film specimens used in this study were pre-pared by depositing a Ni film with a thickness of approx-imately 100 nm on a silicon (100) substrate using a ther-mal evaporation technique under high-vacuum conditions (10 s−5 to 10 s−7torr). During the deposition process, the substrate was maintained at a temperature of 150°C to en-hance the evaporation process and to improve the unifor-mity of the thin film. The thickness of the Ni film was mon-itored continuously throughout the deposition process using a quartz-crystal microbalance and was verified via X-ray re-flectometry once the fabrication 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 all 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) at a con-stant rate of 0.2 mN/s, (2) holding in this position for 10 s, and (3) smoothly unloading over a period of 30 s. The hard-ness and Young’s modulus of the as-deposited Ni/Si thin films were then calculated from the load-displacement data using the Oliver and Pharr method [23].

The indented specimens were annealed for 2 min at tem-peratures of 200°C, 300°C, 500°C or 800°C using a Heat-pulse 610i RTA system with a temperature accuracy of ±5°C. The annealing process was performed in a nitrogen environment (Ni purity: 99.999%, Ni flow rate: 3 L min−1) using a heating rate of 200°C s−1and a cooling rate of ap-proximately 5°C s−1. Thin foil specimens for TEM inspec-tion were prepared from the as-deposited and annealed sam-ples using an FEI Nova 200 focused ion beam (FIB) system with an operating voltage of 30 keV. During the thin foil preparation process, the FIB chamber was maintained at a constant pressure of 10−6∼10−7torr using a hybrid pump-ing system comprispump-ing a mechanical pump and an oil diffu-sion pump. The TEM foils were milled from the thin-film specimens using a Ga+ion beam and were extracted in such a way that they contained the centre of the indentation zone. Note that before the foils were removed, a thin film (1 µm) of Pt was deposited on the specimen surface to protect the indentation region from accidental damage by the ion beam. The cross-sectional microstructures of the as-deposited and annealed indented specimens were observed using a Phillips Tecnai F30 Field Emission gun transmission microscope with a scanning voltage of 300 keV. In addition, the nickel silicides formed in the annealed indented specimens were analysed using both TEM and micro-Raman scattering spec-troscopy (RSS). The RSS procedure was performed at room temperature using a 513 nm argon laser beam with a focused spot diameter of around 1 µm.

3 Results and discussion 3.1 Loading-unloading curve

Figure1(a) presents the loading-unloading curve of the as-deposited Ni/Si thin film. Both the loading and the unload-ing parts of the curve are smooth and continuous. For in-dentation depths of less than 50 nm, the indenter remains fully within the Ni layer and the load is very small. How-ever, as the indentation distance increases, the indenter pene-trates through the Ni layer into the underlying, harder Si sub-strate, and hence the load increases rapidly toward a max-imum value of 53 mN at the maxmax-imum indentation depth of 500 mN. The unloading portion of the load-displacement curve in Fig.1(a) has a smooth, step gradient, which implies that no de-bonding or cracking occurs during the unloading stage of the indentation process. However, an elbow feature

Phase transformation of Ni/Si thin films induced by nanoindentation and annealing 1095

Fig. 8 Bright field TEM micrograph of indented specimen annealed at 800°C for 2 min (diffraction patterns of zones A and B shown in

insets)

and/or layer inversion. Note that the formation of NiSi2 sili-cide phase on the silicon substrate is to be expected here since NiSi2nucleates in an abrupt and almost instantaneous manner at 750°C on Si (100) substrates [38].

The microstructural observations discussed above show that in the present Ni/Si system, metal-rich Ni2Si forms at an annealing temperature of 200°C. In addition, monosili-cide (NiSi) forms at annealing temperatures of 300°C and 500°C, while disilicide (NiSi2)forms at around 800°C. The present experimental evidence suggests that the Ni atoms are the major moving species during the growth of Ni2Si, NiSi and NiSi2. Furthermore, the results show that the anneal-ing temperature plays a key role in determinanneal-ing the nature of the nickel silicides formed at the Ni/Si interface. Finally, the present results suggest that nanoindentation to a depth of 500 nm followed by annealing at a temperature of either 300°C or 500°C represents the optimum process for the for-mation of the NiSi phase, desirable in IC device applica-tions.

4 Conclusions

The contact-induced structural deformation behaviour of Ni/Si thin films has been investigated by performing nanoin-dentation tests to a maximum depth of 500 nm. The mi-crostructures of the an-deposited indented specimens and indented specimens annealed at temperatures in the range 200∼800°C have been examined via transmission electron microscopy (TEM) and micro-Raman spectroscopy. The

nanoidentation results have shown that the loading and un-loading profiles of the Ni/Si thin films have a smooth and continuous characteristic. For both the as-deposited speci-mens and the specispeci-mens annealed at a temperature of 200°C, the nanoindentation process prompts a transformation of the microstructure within the indentation-affected zone from a pure diamond cubic phase to a mixture of amorphous phase and Si III and Si XII phases. Furthermore, the annealing process results in the formation of Ni2Si phase at the Ni/Si interface as the result of an atomic diffusion mechanism. In the specimens annealed at 300°C, the microstructure within the indentation zone is characterised by a mixture of Si III and Si XII metastable phases and NiSi phases at the Ni/Si interface. When the annealing temperature is increased to 500°C, the microstructure within the indentation zone con-tains a mixture of Si III and Si XII phases and a NiSi layer at the Ni/Si interface. Finally, at an annealing temperature of 800°C, the Si III and Si XII phases disappear and the microstructure within the indentation zone is dominated by NiSi2phase only. Overall, the results presented in this study provide a useful insight into the combined effects of nanoin-dentation deformation and annealing on the microstructural evolution of thin Ni/Si films and confirm that the annealing temperature plays a key role in determining the nature of the nickel silicides formed at the Ni/Si interface.

Acknowledgements The authors gratefully acknowledge the finan-cial support provided to this study by the National Science Council (NSC) of Taiwan under contract no. NSC97-2221-E-006-047.

References

1. C.T. Seo, J.H. Lee, J.R. Lee, Y.H. Bae, Int. J. Mod. Phys. B 17, 2001 (2003)

2. A. Inberg, Y. Shacham-Diamand, E. Rabinovich, G. Golanb, N. Croitoru, Thin Solid Films 389, 213 (2001)

3. D.M. Cao, T. Wang, B. Feng, W.J. Meng, K.W. Kelly, Thin Solid Films 398, 553 (2001)

4. F. Abdelmalek, Thin Solid Films 389, 296 (2001)

5. G. Patriarche, E. Le Bourhis, Appl. Surf. Sci. 178, 134 (2001) 6. H.S. Nalwa, Handbook of Nanostructured Materials and

Technol-ogy (Elsevier, Amsterdam, 1999)

7. M. Gadelkak, The MEMS Handbook (CRC Press, New York, 2002)

8. R. Saha, W.D. Nix, Acta Mater. 50, 23 (2002)

9. Y. Cao, S. Allanmeh, D. Nankivil, S. Sethiaraj, T. Otiti, W. Soboyejo, Mater. Sci. Eng. A 427, 232 (2006)

10. G.M. Pharr, Mater. Sci. Eng. A 253, 151 (1998)

11. J. Yang, H. Takahashi, X. Gai, H. Harada, J. Tamaki, T. Kuriya-gawa, Mater. Sci. Eng. A 423, 19 (2006)

12. W.S. Lee, F.J. Fong, Mater. Trans. 48, 2650 (2007) 13. W.S. Lee, T.Y. Liu, Nanotechnology 18, 335701 (2007)

14. P.S. Lee, K.L. Pey, D. Mangelinck, J. Ding, D.Z. Chi, J.Y. Dai, L. Chan, J. Electrochem. Soc. 149, G331 (2002)

15. F.F. Zhao, J.Z. Zheng, Z.X. Shen, T. Osipowicz, W.Z. Gao, L.H. Chan, Microelectron. Eng. 71, 104 (2004)

16. B.A. Julies, D. Knoesen, R. Pretorius, D. Adams, Thin Solid Films 347, 201 (1999)

1096 W.-S. Lee et al. 17. J. Foggiato, W.S. Yoo, M. Ouaknine, T. Murakami, T. Fukada,

Mater. Sci. Eng. B 114–115, 56 (2004)

18. Y.J. Chang, J.L. Erskine, Phys. Rev. B 28, 5766 (1983)

19. J.E.E. Baglin, H.A. Atwater, D. Gupta, F.M. d’Heurle, Thin Solid Films 93, 255 (1982)

20. M.A. Pawlak, J.A. Kittl, O. Chamirian, A. Veloso, A. Lauwers, T. Schram, K. Maex, A. Vantomme, Microelectron. Eng. 76, 349 (2004)

21. H. Iwai, T. Ohguro, S.I. Ohmi, Microelectron. Eng. 60, 157 (2002)

22. F.F. Zhao, J.Z. Zheng, Z.X. Shen, T. Osipowicz, W.Z. Gao, L.H. Chan, Microelectron. Eng. 71, 104 (2004)

23. J.W.C. Oliver, G.M. Pharr, J. Mater. Res. 7, 1564 (1992) 24. V. Domnich, Y. Gogotsi, S. Dub, Appl. Phys. Lett. 76, 2214

(2000)

25. G.M. Pharr, W.C. Oliver, D.S. Harding, J. Mater. Res. 6, 1129 (1991)

26. X.J. Zheng, Y.C. Zhou, J.Y. Li, Acta Mater. 51, 3985 (2003) 27. A.J. Leistner, A.C. Fischer-Cripps, J.M. Bennett, in Proc.

Interna-tional Society of Optical Engineering (SPIE), ed. by W.A.

Good-man (Bellingham, 2003), pp. 215–222

28. R. Rao, J.E. Bradby, S. Ruffell, J.S. Williams, Microelectron. J. 38, 722 (2007)

29. A. Kailer, Y.G. Gogotsi, K.G. Nickel, J. Appl. Phys. 81, 3057 (1997)

30. R. Saha, Z. Xue, Y. Huang, W.D. Nix, J. Mech. Phys. Solids 49, 1997 (2001)

31. A.J. Leistner, A.C. Fischer-Cripps, J.M. Bennett, in Proc.

Interna-tional Society of Optical Engineering (SPIE), ed. by W.A.

Good-man (Bellingham, 2003), pp. 215–222

32. H. Gao, C.H. Chiu, J. Lee, Int. J. Solid Struct. 29, 2471 (1992) 33. M. Wittling, A. Bendavid, P.J. Martin, M.V. Swain, Thin Solid

Films 270, 283 (1995)

34. A. Kailer, K.G. Nickel, Y.G. Gogotsi, J. Raman Spectrosc. 30, 939 (1999)

35. P.S. Lee, D. Mangelinck, K.L. Pey, Z.X. Shen, J. Ding, T. Osipow-icz, A.K. See, Electrochem. Solid-State Lett. 3, 153 (2000) 36. Y.L. Jiang, A. Agarwal, G.P. Ru, G. Cai, B.Z. Li, Nucl. Instrum.

Methods B 237, 160 (2005)

37. P.S. Lee, K.L. Pey, D. Mangelinck, J. Ding, D.Z. Chi, J.Y. Dai, L. Chan, J. Electrochem. Soc. 149, G331 (2002)

38. S. Nygren, D. Caffin, M. Ostling, F.M. d’Henrle, Appl. Surf. Sci. 53, 87 (1991)