ANALYSIS OF LATERAL UNIFORMITY OF ULTRASHALLOW JUNCTIONS IN POLYCRYSTALLINE SILICON-ON-SINGLE CRYSTAL SILICON SYSTEMS

The lateral uniformity of n+-p junctions formed by indiffusion of As or P from an amorphous or polycrystalline silicon thin-film source into the underlying silicon substrate has been investigated using a concentration-dependent etch and transmission electron microscopy. Grain boundaries act as fast diffusant pipelines and also possibly inject point defects into the substrate, thereby enhancing bulk diffusivities locally in the substrate. Delineated ultrashallow junctions show significant doping lateral inhomogeneities in the substrate for as-deposited amorphous silicon diffusion sources but not for as-deposited polycrystalline silicon diffusion sources because of a larger final grain microstructure after annealing in the former case. However, the doping inhomogeneities are gradually smeared out as impurities diffuse deeper into the substrate.