Self-aligned bipolar transistor with inverted polycide base contact

Abstract

An inverted polycide extrinsic base contact serves as a diffusion source, yet still has low resistivity and is readily etchable down to silicon by techniques useful in manufacturing integrated circuits. The extrinsic base contact layer is made up of a metal silicide (e.g. WSi.sub.2) with an overlying doped polysilicon layer with coextensive apertures through doped polysilicon and metal silicide layers defining the emitter and intrinsic base region. The extrinsic base region is formed by diffusing boron impurities from the p.sup.+ polysilicon layer through the silicide layer. The silicide layer is of a metal silicide such as tungsten silicide (WSi.sub.2). The polysilicon layer acts as a diffusion source, since appropriate dopants (e.g., boron) diffuse rapidly through the metal silicide. Both the top surface of the p.sup.+ polysilicon layer and the sidewall edges of the polysilicon and silicide layers are covered by an insulating layer (e.g. SiO.sub.2) which also separates the emitter contact from the base contact layers.