Process for making a self-aligned bipolar sinker structure

Abstract

Bipolar devices on a common substrate are formed in tubs defined by a sinker that is self-aligning with the isolating trench and provides a relatively low vertical resistance contact from a surface contact to the underlying buried layer. In a first embodiment, the isolating trench initially is defined only down to the top surface of the buried layer. The trench walls are then doped, and the dopant allowed to diffuse laterally through the trench sidewalls. The resultant sinker is self-aligned with the isolating trench. The trench depth is then further increased to a desired depth preferably penetrating into the substrate. Thereafter, a bipolar device is formed in the tub, preferably with the device collector adjacent the trench and thus aligned with the sinker. In a second embodiment, an opening is formed in the surface oxide defining the area where the trench (when formed) will be cut. The opening is then used to diffuse dopant into the sinker, the dopant diffusing downward through the epitaxial layer to the buried layer, and also diffusing somewhat laterally. The trench is then cut in a single step through the epitaxial layer and the buried layer and into the underlying substrate, cutting directly through the region in which the sinker has been previously diffused. However, because of the lateral sinker diffusion, a portion of the heavily doped region (e.g., the sinker) remains, and is self-aligned with the edges of the tub defining trench.