Epitaxial regrowth of n+ and p+ polycrystalline silicon layers given single and double diffusions

Abstract

This article investigates the epitaxial regrowth of n-type and p-type polycrystalline silicon (polysilicon) layers deposited on silicon, which are subjected to either a single emitter diffusion or consecutive base and emitter diffusions. A wide range of diffusion conditions is considered, covering both rapid thermal and furnace diffusion in the temperature range 950–1200 °C. The sheet resistances of single-diffused n-type polysilicon layers are significantly higher than those of double diffused layers for rapid-thermal emitter diffusions in the temperature range 1025–1125 °C. This is explained by the epitaxial regrowth of the polysilicon during the emitter diffusion, caused by the partial break-up of the interfacial oxide during the base diffusion. In contrast the sheet resistances of single- and double-diffused p-type polysilicon layers are found to be similar. Rutherford backscattering spectra are presented which show that the structures of the single- and double-diffused polysilicon layers are similar. This is explained by the effect which fluorine, incorporated into the polysilicon during the BF2 emitter implant, has in accelerating the break-up of the interfacial oxide during the early part of the emitter diffusion. Estimates are made of the time to break up the interfacial oxide layer and the time to vertically epitaxially align the polysilicon at different temperatures, and activation energies of 4.9 and 4.7 eV, respectively obtained. In n-type polysilicon, the epitaxial regrowth is dominated by the time to break up the interfacial oxide layer, whereas in BF2 implanted p-type polysilicon it is dominated by the time to vertically epitaxially align the polysilicon. A vertical epitaxial alignment rate of 1000 Å/s is obtained for n-type polysilicon at 1050 °C, compared with 240 Å/s for p-type polysilicon at 1100 °C.