A Mechanism for Selectivity Loss during Tungsten CVD

We have investigated possible mechanisms for the loss of selectivity ( i.e. , deposition on silicon dioxide) during tungsten CVD by reduction of tungsten hexafluoride and found strong evidence that selectivity loss is initiated by desorption of tungsten subfluorides formed by the reaction of with metallic tungsten surfaces. Adsorption and disproportionation of the tungsten subfluorides on the silicon dioxide surface produces a reactive state of tungsten that can lead directly to selectivity loss. The key feature of the experimental setup is the ability to independently heat a tungsten foil and a nearby oxide‐covered silicon sample in the presence of tungsten hexafluoride. With the tungsten foil at 600°C and the sample at ∼30°C under a ambient, a tungsten subfluoride was found to deposit on the surface. Auger electron spectroscopy was used to measure a F/W ratio of . Heating this tungsten subfluoride overlayer resulted in disproportionation to yield gas‐phase and metallic tungsten which remained on the surface. With the tungsten foil at 600°C and the sample at 300°C in the presence of , metallic tungsten deposited directly on the without stopping at the subfluoride adsorption step. The net effect of this tungsten subfluoride desorption‐disproportionation mechanism is the transport of tungsten from tungsten surfaces to silicon dioxide surfaces as well as other regions in the deposition chamber. Extrapolated rates for this process are high enough to explain the magnitude of the selectivity loss seen at normal CVD temperatures.