CASCADE MIXING LIMITATIONS IN SPUTTER PROFILING

The most fundamental physical processes which limit the depth resolution achieved in depth profiling by sputtering and surface analysis techniques are surface microroughening and atomic mixing in the collisional cascade. A survey of theoretical modeling of cascade mixing and its effect on the shape of measured profiles is presented. In particular, diffusion type models show reasonable agreement with recent experimental results of secondary ion mass spectroscopy and Auger electron spectroscopy profiles on delta function marker layers and on multilayer sandwich structures. The mixing range is related to the range of the primary ions and determines the depth resolution. Minimum values obtained under optimized sputtering conditions are of the order of 1 nm. Depending on the gradient of the true in-depth distribution of composition, deconvolution procedures may become necessary to disclose the latter. Because sputtering is based on cascade mixing, the ultimate achievable resolution in depth profiling is limited by this effect.