INTERFACE EVOLUTION AND EPITAXIAL REALIGNMENT IN POLYCRYSTAL SINGLE-CRYSTAL SI STRUCTURES

Our recent work on the poly/single crystal Si interface evolution and on the expitaxial realignment under high temperature (approximately 1000-degrees-C) rapid thermal annealing is reviewed. The roles of the microcrystalline morphology, of the interfacial native oxide film, of the doping level and of the processing temperature on the realignment kinetics are addressed. Two different realignment modes are observed. For undoped layers a quasi planar interface motion occurs, while in highly doped polycrystalline layers the realignment proceeds via the formation of epitaxial columns and their lateral growth. These two different modes arise from the doping enhancement of the interface kinetics. It is shown that these processes can be interpreted within an Avrami-Mehl-Johnson nucleation and growth scheme. In the early stages of interface realignment the process is controlled by oxide clustering, arising from the evolution of the native oxide layer, and by the density of grain boundaries intersecting the interface with the single crystal. Once nucleated at specific sites the epitaxial front can then proceed, the kinetics and mode of realignment depending on the density of nucleation sites and on the growth velocity. It is also demonstrated that when narrow (approximately 0.25 mum) polycrystalline strips defined by oxide layers (as in real devices) are used in place of infinite planar layers the realignment mode and kinetics change due to the presence of size effects. These data are presented and their implications for applications to bipolar devices are discussed.