Gold in Silicon: Effect on Resistivity and Diffusion in Heavily-Doped Layers

Resistivity changes caused by gold diffusion into silicon were studied in de-tail. The results did not agree with previous theoretical calculations. Radio-tracer studies also showed that previously diffused n + (phosphorus) layers greatly retard the diffusion of gold into silicon and increase its solubility, p+ (boron) diffused layers do not retard gold diffusion but cause a more ideal type of diffusion. Autoradiographs showed lateral nonuniformity of gold con-centration unless oxide formation is prevented during pretreatment, plating, and diffusion. The diffusion of gold into silicon and its effects on the properties of silicon are of great fundamental and practical importance. Collins (1) found that gold has two ionization energy levels in silicon, a donor level at 0.35 ev and an acceptor level at 0.62 ev from the valence band. Because of this gold can compensate both p-type and n-type silicon. Since the levels are deep-lying, however, the compensation is not, in genera1, one for one. In addition, gold lowers the mobil ity of current carriers in silicon, which also tends to increase the resistivity. Boltaks (2) made theoretical calculations of resistivity vs. gold concentration for various tart-ing resistivities. He assumed a constant value for mo-bilities and made approximate calculations for the fraction of gold ionized. Unfortunately, little experi-mental data are available with which to test his con-clusions. Consequently, an extensive set of experi-ments were required to determine resistivity changes for a number of useful concentrat ion-temperature combinations. Because of device applications the characteristics of the diffusion of gold into silicon are of considerable importance and interest. This diffusion has been found to be complex and sensitive to the perfection of the silicon (3). A second purpose of the present paper was to investigate the effect of heavy boron or phosphorus doping on gold diffusion. Effects of various other dif-fusion parameters on the diffusion were also studied