Failure of Cobalt Silicidation in ICs
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Since the 0.25 µm generation in VLSI (very-large-scale integration), cobalt silicide (CoSi2) has been preferred for its lower resistivity and greater thermal stability [1] than titanium silicide (TiSi2). However, CoSi2 process can still have various uncertainties:
         i) Void formation,
         ii) Low tolerance to surface oxygen contamination, which results in non-uniform CoSi2/Si interface and formation of Co spikes [2],
         iii) During HF cleaning, a significant amount of fluorine (F) atoms can diffuse toward the polycrystalline silicon substrate and segregates near the surface, [6]
         iv) Presence of F at the surface retards interaction between Co and Si, and thus non-uniform cobalt silicide is formed. [6]

Therefore, a well-controlled pre-cleaning process prior to Co deposition is needed. The main pre-cleaning processes of Si (silicon) wafer for CoSi2 formation are:
         i) HF wet cleaning [3],
         ii) Co/refractory bilayers [4],
         iii) In situ low energy sputter cleaning prior to PVD Co deposition [5], which is most widely used due to its manufactuability.

Some treatments can improve the IC properties, for instance, the pre-existing F atoms block the diffusion of Co and stabilize the gate oxide [6].






[1] S.P. Muraka, Silicide for VLSI Applications, Academic Press, New York, 1983.
[2] K. Maex, A. Lauwers, P. Besser, E. Kondoh, M. de Potter, A. Steegen, IEEE. Trans. Elect. Dev. 46 (7) (1999) 1545.
[3] S.M. Sze, VLSI Technology, McGraw-Hill Publishing, 1988.
[4] M.L.A. Dass, D.B. Fraser, C.-S. Wei, Appl. Phys. Lett. 58 (1991) 1308.
[5] J.A. Kittl, W.-T. Shiau, D. Miles, K.E. Violette, J.C. Hu, Q.-Z. Hong, Solid State Tech. June 42 (1999) 81.
[6] Sang-Yun Lee, Wanqing Cao, Eric Lee, Patrick Lo, S.K. Lee, The effect of HF vapor pre-treatment on the cobalt salicide process, Thin Solid Films 405 (2002) 73–76.



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