Migration on fine molten wires in thin silicon wafers

Abstract

A series of molten Al-rich wires between 10 and 500 μ wide were translated through thin (100) and (111) silicon wafers in a thermal gradient to produce vertical p-n junctions, which are sufficiently narrow to be considered for some integrated-circuit applications. Thermal gradients were generated by heating the silicon wafers with either a scanning electron beam in vacuum or a hot tungsten filament in various atmospheres. The migration rate rapidly increased with the wafer temperature in agreement with calculations of the temperature gradient through the wafer. A reduction in the magnitude of the gradient near the colder wafer surface was explained by radiative-heat transport through the semitransparent silicon wafer. The width of the resulting doped region was finer in (100) wafers than in (111) wafers, because the shape of the liquid wire was narrower for the (100) orientation. A relationship between the linewidth, wafer temperature, and Al film geometry was derived to predict the linewidths from 30 to 140 μ in (100) wafers.