Optical models for cavity profiles in high-dose helium-implanted and annealed silicon measured by ellipsometry

Cavities created by He implantation with a dose of 5×10¹⁶ cm^-2 and energy of 40 keV into single-crystalline silicon and annealing at 650–1000 °C for 15–60 min were characterized by multiple angles of incidence spectroscopic ellipsometry. Optical models of increasing complexity were developed assuming the cavity layer either to be homogeneous, or to have a Gaussian profile, or sublayers with independently fitted cavity ratios. Cavity profiles of different annealing conditions were compared and cross-checked by transmission electron microscopy. A strategy for the ellipsometric evaluation was proposed to reduce the computation time and the probability of getting in local minima using complex models with numerous parameters. High sensitivity on the angle of incidence was found, and the choice and the determination of the angle of incidence were discussed.