REACTION PATHWAYS FOR ARF EXCIMER-LASER ASSISTED TUNGSTEN CHEMICAL-VAPOR-DEPOSITION FROM A WF6-H2 GAS-MIXTURE

Photolytic tungsten chemical vapor deposition by an ArF excimer laser from a WF6/H2/Ar gas mixture is a very complicated process with reacti ons occurring both in series and in parallel In this article different reaction pathways were modeled and compared with experimental deposit ion rate data. The absorption cross section for WF6 was measured to 1. 7 x 10(-18) cm2 at the ArF excimer laser wavelength of 193 nm. From a rate equation model, it was concluded that the direct photolytic contr ibution to the deposition rate was negligible. Moreover, reduction of tungsten subfluorides by hydrogen atoms, formed in reactions between p hotolytically released fluorine atoms and molecular hydrogen, was also found to be negligible. However, photolysis in combination with vario us radical reactions generate a relatively high concentration of tungs ten subfluorides (WF3, WF4, and WF5). Thermochemical calculations indi cate that H2 reduction of tungsten subfluorides to solid tungsten, for ming clusters in the vapor, is highly probable. Among the different cl uster nucleation mechanisms polymerization of tungsten subfluorides se ems to be an important step. This also explains the influence of WF6 p artial pressure on the deposition rate.