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  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 ,
iii) During HF cleaning, a significant amount of fluorine (F) atoms can diffuse toward the polycrystalline silicon substrate and segregates near the surface, 
iv) Presence of F at the surface retards interaction between Co and Si, and thus non-uniform cobalt silicide is formed. 
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 ,
ii) Co/refractory bilayers ,
iii) In situ low energy sputter cleaning prior to PVD Co deposition , 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 .
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