THE SURFACE-CHEMISTRY AND KINETICS OF TUNGSTEN CHEMICAL-VAPOR-DEPOSITION AND SELECTIVITY LOSS

The mechanism of tungsten chemical vapor deposition (CVD) using hydrog en reduction of tungsten hexafluoride is reviewed, with emphasis on th e relevant fundamental surface chemistry and kinetics. We also briefly review the kinetics and mechanism of selectivity loss involving tungs ten subfluorides. Most kinetic studies of the H-2 + WF6 reaction for t ypical low pressure CVD conditions report a phenomenological rate law for deposition that is zeroth order in WF6 pressure and 1/2-order in H -2 pressure. Unfortunately, most of the reaction mechanisms reported t o yield the observed rate law are inconsistent with the known (or esti mated) surface chemical properties of H-2 and WF6 on tungsten. There a re also many conditions where the accepted rate law is not valid. For instance, as the H-2 pressure is lowered and becomes comparable with t he WF6 pressure, the deposition rate drops to zero. Under these condit ions hydrogen chemisorption is apparently completely quenched and the surface is saturated with adsorbed fluorine. Just above the H-2 Pressu re threshold the deposition rate is first order rather than 1/2-order with respect to H-2. In this regime the WF6 pressure dependence is als o strongly negative order rather than zeroth order. All the deviations from the normal rate law can be qualitatively explained using a Langm uir-Hinshelwood reaction mechanism with competitive adsorption. An imp ortant intrinsic mechanism of selectivity loss that occurs during tung sten CVD involves tungsten transport by the formation and disproportio nation of volatile tungsten subfluorides. We present some recent measu rements of the tungsten subfluoride formation rate using the microbala nce technique.