Experimental Evaluation of High Energy Ion Implantation Gradients for Possible Fabrication of a Transistor Pedestal Collector

Abstract

The use of ion accelerators to implant impurities in crystals has become the subject of widespread research. Such studies have been limited mainly to low energies with acceleration voltages of 50 to 500 kilovolts. In this energy range, impurities are implanted into the upper micron or less of the surface.The present work describes certain characteristics of high energy ion implantation. The ions used were boron and phosphorus. They were implanted into silicon with energies of 2 to 4 megavolts. In this energy range, the impurities have a useful positive impurit,r concentration gradient from the surface. The surface concentration is about 4 X 10 16 em-3, and the peak concentration exceeds 10 1 ern-3 depending on dosage. The peak concentration occurs about 2.5 /Lm deep. After annealing the radiation damage introduced into the semiconductor, it was determined that the surface silicon recovered over 90% of its expected maximum conductivity and mobility.A discussion is given of the concentration gradients required to fabricate a collector pedestal for a high-speed switching transistor, and it is shown that such gradients can be obtained by using high energy ion-implantation. The pedestal may be implanted after the base and emitter diffusions, and annealed at a low temperature, thus keeping a sharp impurity gradient. Also, since it is put into the final epitaxial layer, its vertical position relative to the emitter-base junction will be independent of epitaxial undulations. Figure 1 Cross section of a pedestal-collector transistor. The first epitaxial layer contains the pedestal diffusion and a reachthrough diffusion to the buried collector. After the second epitaxial layer is grown, the base and emitter are diffused into the top surface.