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

The mechanism of tungsten chemical vapor deposition (CVD) using hydrogen reduction of tungsten hexafluoride is reviewed, with emphasis on the relevant fundamental surface chemistry and kinetics. We also briefly review the kinetics and mechanism of selectivity loss involving tungsten subfluorides. Most kinetic studies of the H-2 + WF6 reaction for typical 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 to yield the observed rate law are inconsistent with the known (or estimated) surface chemical properties of H-2 and WF6 on tungsten. There are also many conditions where the accepted rate law is not valid. For instance, as the H-2 pressure is lowered and becomes comparable with the WF6 pressure, the deposition rate drops to zero. Under these conditions hydrogen chemisorption is apparently completely quenched and the surface is saturated with adsorbed fluorine. Just above the H-2 Pressure 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 also strongly negative order rather than zeroth order. All the deviations from the normal rate law can be qualitatively explained using a Langmuir-Hinshelwood reaction mechanism with competitive adsorption. An important intrinsic mechanism of selectivity loss that occurs during tungsten CVD involves tungsten transport by the formation and disproportionation of volatile tungsten subfluorides. We present some recent measurements of the tungsten subfluoride formation rate using the microbalance technique.