STUDIES OF THE POSSIBLE REACTION OF WF6 WITH SIO2 AND SI3N4 AT SEVERAL TEMPERATURES

Chemical vapor deposition (CVD) of tungsten by the hydrogen reduction of WF6 is increasingly important in very large scale integration technology. There is, however, a concern about possible interference of this CVD process by the reaction of WF6 with the SiO2 mask material. Because of disagreement between thermodynamic calculations and some experimental evidence concerning this reaction, the reaction was examined by several techniques not previously employed. Oxide coated silicon wafers were exposed to the hexafluoride in argon, with and without hydrogen present, at several temperatures from room temperature to 400-degrees-C. The oxide surface was examined using ellipsometry and surface analytical techniques such as secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). No thickness change in the oxide after extensive exposure to WF6 at 400-degrees-C was observed, and it was found that the surf ace concentration of tungsten after exposure to WF6 at temperatures up to 400-degrees-C is only a fraction of a monolayer which decreases with increasing temperature. This behavior can be attributed entirely to adsorption of WF6 on the SiO2 surface. No evidence of any interfacial reaction between the SiO2 surf ace and the WF6 gas, or SiO2 and deposited tungsten in the hydrogen reduction process, was obtained via any of the analytical techniques employed. A new thermodynamic analysis of the reaction was conducted to examine the effects of errors in the enthalpies of formation of critical species. It was found, for example, that if DELTA-H(f)0(25-degrees-C) SiF4 was made more positive by only 15%, the WF6-SiO2 reaction becomes unfavorable. Such a mechanism could be responsible for most of the confusion that exists.