The vacuum is maintained by controlled operation of a vacuum pump 24 and of inlet gas sources 25 that may include, for example, an optional inert gas source 27 for a gas such as argon (Ar) or helium (He) and one or more reducing gas sources 26 of silane (SiH4) and, for example, hydrogen H2) for use in carrying out a tantalum pentafluoride or pentachloride reduction reaction to deposit TaSiy.