The nature of the self-limiting effect in the low pressure chemical vapor deposition of tungsten.

We propose a mechanism for the self-limiting effect which occurs in the LPCVD of tungsten. When W is deposited by silicon reduction of tungsten hexafluoride WF sub 6, the deposition suddenly ceases at an early stage. No such effect is observed in the deposition of Mo using analogous chemistry. We believe the self-limiting effect is due to the formation of nonvolatile lower fluorides of tungsten. Our hypothesis is supported by SIMS studies which indicate the presence of fluorine (~3%) in W films, whereas in Mo films the concentration of fluorine is an order of magnitude lower.