Comparative Study of the Low Pressure Chemical Vapor Deposition Processes of W and Mo

Abstract

We present a comparative study of low pressure chemical vapor deposition (LPCVD) of the refractory metals W and Mo on silicon substrates by the silicon reduction of their fluorides and . Our results contribute a new insight into the LPCVD process, explaining both the microstructure of the films and the kinetics of their deposition. The processes of W and Mo deposition follow a very similar course as indicated by the similarity of the deposits; both films are porous, and their interfaces with silicon have identical textures. Moreover, from experiments with excess , even the intermediate reactions contributing into the deposition process follow similar course. Despite the similarity of W and Mo deposition the two processes exhibit a puzzling difference. Mo films grow linearly with time, whereas tungsten deposition is inhibited at an early stage by a self‐limiting effect. We propose a mechanism in which the nonvolatile lower fluorides of W interrupt further deposition of metal tungsten. Our model is supported by SIMS studies. In terms of our model, it becomes clear why the self‐limiting effect never occurs during Mo deposition: since all Mo subfluorides are volatile, they cannot act as a blocking agent and interrupt the process. The difference in volatility between Mo‐F and W‐F compounds may also explain why the process of Mo deposition via hydrogen reduction never loses its absolute selectivity, whereas the analogous tungsten process is plagued by the selectivity loss.