Observations on selectivity loss during tungsten CVD

Possible mechanisms for the loss of selectivity during tungsten CVD (i.e., growth on SiO/sub 2/) were investigated using a dual vacuum system composed of a growth chamber and a UHV analysis chamber equipped with Auger and mass spectroscopy. An oxide-covered silicon sample could be transferred between the two chambers while under vacuum to maintain surface composition integrity. With this experimental system, the authors have found evidence for a selectivity loss mechanism which involves desorption of tungsten subfluorides from tungsten surfaces. The disproportionation of the subfluorides then produces a reactive state of tungsten on the SiO/sub 2/ surfaces that leads directly to selectivity loss