The effects of making oxide on the diffusion of boron into silicon

An investigation into the effects of masking oxide on the diffusion of boron into silicon has been made. It has been observed that in the case of deposition from a solid planar boron source there is a discrepancy between the values of sheet resistance produced on a large boron diffused check slice and the sheet resistance determined from actual fabricated resistors. It is concluded that the amount of boron diffused into the windows opened in oxide is always less than the amount of boron diffused in the check slices. The actual discrepancy depends upon the size of the window and the surrounding masking oxide width. The discrepancy is found to occur at the deposition stage and reduces with an increase in the total amount of doping. Spreading resistance measurements made on a large diffused sample with respect to an oxide edge show a variation of resistivity extending for several hundred microns from the edge.The use of silicon nitride as the mask shows none of the effects observed with oxide masking. A careful investigation using multilayered structures of nitride and oxide leads to the development of a theoretical model. This model is based on the 'surface diffusion' of boron over silicon assuming a high solubility of boron in oxide. The numerical calculations made demonstrate the capability of the proposed theory to explain many of the experimental observations. It has also been demonstrated, both experimentally and theoretically, that the spatial resolution of the spreading resistance method of profiling is poor in the case of shallow doped layers. This poor resolution influences the measurements made on a bevelled sample, as for example, in the case of profiling a doped layer. A computer program has been developed to correct the data for the limited spatial resolution. The calculations show that the experimentally observed effects of bevelling on the spreading resistance measurements near the surface of the sample may be explained on the basis of this lack of resolution.