Tungsten Nucleation on Thermal Oxide during LPCVD of Tungsten by the Hydrogen Reduction of Tungsten Hexafluoride

Abstract

The rate of tungsten nucleation and island growth is characterized on thermal silicon dioxide in the absence of exposed silicon surfaces in a stainless steel cold wall high vacuum differential reactor. Tungsten hexafluoride (0.05–2.50 torr) was found to react with undoped thermal silicon dioxide in the presence of adjacent tungsten areas for temperatures ranging from 240° to 380°C and reaction times from 1.10 to 2h. The silicon dioxide film did not change thickness, but electron spectroscopy for chemical analysis (ESCA) revealed a surface covering of tungsten, oxygen, fluorine, and silicon. Tungsten nucleates rapidly on silicon dioxide in the presence of reactive surfaces and the hydrogen reduction reaction of tungsten hexafluoride. The observed nucleation is autocatalytic and initiated by an intermediate diffusing from areas of tungsten deposition. For a gas flow rate of 160 sccm, tungsten hexafluoride pressure of 0.05 torr, a hydrogen pressure of 0.70 torr, and temperatures ranging from 268° to 348°C, the activation energy for initiation of nucleation is 23–25 kcal mol−1.