Conversion of Poly-Silicon into

The substitution of (undoped) poly crystalline silicon by tungsten using WF6 has been studied by Kobayashi et al.191. It was found that after a proper pretreatment (=oxidation in H202/NH40H) of the poly-Si very thick (up to 1.5 iim) tungsten could be obtained by reaction with WF6 at 300°C. The properties of the tungsten films obtained were  

This material is without any doubt very acceptable for plug applications. The process flow for a contact to Si fill is then as follows (see also figure 8.8)  

1) After contact etching 200 nm TiN is reactively sputtered onto the surface. 

2) Poly-Si is deposited at 625°C, such that the contacts are completely filled. 

3) Back etching of the poly-Si layer is done using RIE in SF6. 

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