MORPHOLOGICAL INSTABILITY AND SI DIFFUSION IN NANOSCALE COBALT SILICIDE FILMS FORMED ON HEAVILY PHOSPHORUS-DOPED POLYCRYSTALLINE SILICON

In this letter we examine the interdiffusion and reaction of deposited cobalt layers during the formation of nanoscale (< 35 nm) silicide films on heavily phosphorus doped polycrystalline silicon films. The onset of morphological instability is indicated by an increase of layer resistivity. Cross-section transmission electron micrographs of high resistance films, which were formed by rapid thermal annealing at 700-degrees-C for 30 s, show a series of disconnected ''islands'' of CoSi2 with or without a highly disordered (amorphous) Si-rich top surface layer. A continuous band of voids, attributed to the Kirkendall effect and to phase transformation induced volume changes, initially appear at the CoSi/CoSi2 interface. In the highly agglomerated films, these voids separate the buried COSi2 islands and buried polycrystalline Si from the disordered surface layer. The sequence of events is analyzed in terms of grain boundary diffusion, grain boundary grooving, and the impact of phosphorus concentration (approximately 10(20) cm-3) on Si diffusivity.