SOLID SOURCE DIFFUSION FROM AGGLOMERATING SILICIDE SOURCES .1. MEASUREMENT AND MODELING

An optimized secondary ion mass spectrometry (SIMS) technique was used to characterize the solid source diffusion of dopants, i.e., arsenic and boron, from agglomerating cobalt disilicide sources. It was found that interface roughness plays a dominant role in determining the dopa nt profiles observed by SIMS. Double convolution of a step function wi th an exponential function, which represents the SIMS cascade mixing, and then again with a Gaussian function, which represents the silicide interface roughening, were performed to fit the SIMS silicon signal i n the transition region of the silicide/silicon interface. Experimenta l profiles were then deconvoluted with the same functions to derive th e ''true'' dopant diffusion. This convolution methodology eliminates t he broadening factors provided by the measuring instrument and by the sample itself, and therefore results in a more physically meaningful d iffusion profile. The broadening of SIMS silicon signal also provides a very precise characterization of the thermal stability of the thin p olycrystalline silicide film. An analytical model was established to r elate the SIMS silicon broadening and the sheet resistance of the aggl omerated silicide film. This model predicts that while the amplitude o f the root-mean-square roughening goes as the square root of time, the increase in sheet resistance varies linearly with time, provided that the amplitude of roughening is considerably smaller than the thicknes s of the silicide film.