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 fi lms on heavily phosphorus doped polycrystalline silicon films. The ons et of morphological instability is indicated by an increase of layer r esistivity. Cross-section transmission electron micrographs of high re sistance films, which were formed by rapid thermal annealing at 700-de grees-C for 30 s, show a series of disconnected ''islands'' of CoSi2 w ith or without a highly disordered (amorphous) Si-rich top surface lay er. A continuous band of voids, attributed to the Kirkendall effect an d to phase transformation induced volume changes, initially appear at the CoSi/CoSi2 interface. In the highly agglomerated films, these void s separate the buried COSi2 islands and buried polycrystalline Si from the disordered surface layer. The sequence of events is analyzed in t erms of grain boundary diffusion, grain boundary grooving, and the imp act of phosphorus concentration (approximately 10(20) cm-3) on Si diff usivity.