Reduction of Excess Phosphorus and Elimination of Defects in Phosphorus Emitter Diffusions

Abstract

The successful production of LSI bipolar circuits requires that a high level of transistor junction yield be maintained. One of the chief causes of junction degradation is the defects induced by the high concentration of phosphorus normally used in the emitter diffusion of bipolar transistors. This diffusion results in a total phosphorus concentration that is greater than the electrically active phosphorus; precipitates and other defects result from this excess phosphorus. These defects can cause crystal damage which in turn can degrade junction integrity, and in the worst case lead to emitter‐collector shorts. The purpose of this work was to find processing parameters which may be used to minimize the excess phosphorus in an emitter diffusion. We have studied the effect of bubble rate, the amount of preheat time, thickness of preformed barrier oxide, percent oxygen in the gas stream, and percent time that the bubbler is on for a 1000°C phosphorus diffusion using. The diffusions were characterized by sheet resistance, junction depth, total surface concentration (as measured by an electron microprobe), and Sirtl etching. It is shown that the proper choice of diffusion processing parameters minimizes the excess phosphorus, eliminates etching defects, improves transistor junction yield, and results in only a small increase in final sheet resistance. This occurs when the total phosphorus concentration, averaged over 0.34 μm from the surface, is kept below .