Epitaxial alignment of polycrystalline silicon and its implications for analogue bipolar circuits

Williams, John Dilwyn (1992) Epitaxial alignment of polycrystalline silicon and its implications for analogue bipolar circuits. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This thesis investigates the conditions which lead to the epitaxial alignment of n-type and p-type polycrystalline silicon layers deposited on silicon, which are subjected to either a single emitter diffusion (single diffused) or consecutive base and emitter diffusions (double diffused). A wide range of diffusion conditions is considered, covering both rapid thermal and furnace diffusion in the temperature range 950-1200^oC. The sheet resistances of double diffused polysilicon layers are significantly lower than those of single diffused layers for rapid-thermal emitter diffusions in the temperature range 1025-1125^oC. This is explained by the epitaxial alignment of the polysilicon in the double diffused sample during the emitter diffusion, which follows either partial or complete break up of the interfacial oxide during the earlier base diffusion. In contrast the sheet resistances of single and double diffused p-type polysilicon layers are found to be similar within this temperature range. Rutherford backscattering spectra and cross-sectional TEM micrographs show that the single and double diffused samples have a similar polysilicon morphology. 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 and the time to vertically epitaxially align n-type polysilicon at different temperatures, and activation energies of 4.9eV and 4.7EV respectively obtained. In n-type polysilicon, the epitaxial regrowth is dominated by the time to break the interfacial oxide layer, whereas in BF₂⁺ implanted polysilicon it is dominated by the time to vertically epitaxially align the polysilicon. A theoretical model is proposed for the process of epitaxial alignment, which accounts for the observed linear epitaxial alignment rates and the measured activation energy of 4.7eV. In addition, the author proposes an alternative to the standard theoretical models for the break up of the interfacial oxide. It is proposed that both oxide break up and vertical epitaxial alignment are dominated by the self-diffusion of silicon, which has a theoretical activation energy of 4.8eV.

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