Method of making rf transistor employing dual metallization with

Abstract

An improved dual metallization process in which self-aligned tungsten contacts are formed to closely-spaced emitter or source sites in RF power silicon devices. Low-resistivity ohmic contacts are made by selectively depositing tungsten on the exposed silicon surfaces as a first metal layer without a photomasking process and after a dielectric layer deposition and via opening process. The metallization process is completed by depositing a second metal or polysilicon layer on the dielectric layer and through vias to selected tungsten contacts. The tungsten combines with doped silicon in the emitter or source regions to form the low-resistivity ohmic contacts without the requirement of a platinum or palladium deposition and siliciding step as in prior art. The tungsten is preferably chemical-vapor-deposited in a two-temperature step when a first few hundred Angstroms of tungsten are grown at a low temperature on the order of 250.degree. C.-350.degree. C. to avoid metal encroaching into the side of the shallow emitter or source diffusions and shorting the device. After a few hundred Angstroms of slow tungsten growth, the temperature may be raised up to the order of 400.degree.C.-650.degree. C. to increase the deposition rate of the tungsten.