Mechanisms of selectivity loss

Several reasons are known to be responsible for selectivity loss. Although much effort has been put forth to unravel the mechanism, we are still far from a complete understanding of the phenomenon. In the following we will discuss some facts about selectivity loss. 

Surface contamination Surface contamination can be a major reason for premature loss of selectivity. Such contamination can be, for instance, particles from a prior wet clean step. Another high risk is if a salicide process is done before the tungsten deposition. After the silicidation of the metal the excess metal needs to be removed. This is normally done in a wet metal strip step. One can understand that if metal residues are left at the dielectric layer, these can act as a catalyst for tungsten deposition. Especially with a Pt-salicide process this is known to be a severe problem. 

With regards to this it is clear that the wafer pretreatment is of key importance for obtaining good selectivity. Such pretreatments can vary from wet clean steps to in situ dry clean steps. In situ clean steps have the advantage that they can be done in vacuo in an integrated (cluster) tool. Unfortunately not much has been published in the literature about the in situ pretreatments. In one study a NF3 plasma is reported to be able to remove native oxide from silicon [Kajiyana et al.84]. 

Creighton128,129, found the relatively stable WF4 compound as a possible by-product which could, after adsorption on the oxide, initiate tungsten growth on oxide according to  

Once the tungsten is formed at the oxide surface it will catalyze further tungsten growth. 

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