The formation of helium induced cavities in silicon during short-time annealing is analyzed by spectroscopic ellipsometry. Specimens implanted with 40 keV He+ ions to a dose of 5×1016 cm-2 are heat treated at 800 °C for times of 1–1200 s by rapid thermal annealing. Spectroscopic ellipsometry is employed to obtain quantitative information on the cavity volume depth profiles. A newly developed formula is used to model the optical multilayer depth profiles. The cavity volume is found to increase during annealing for about 300 s and to decrease for longer annealing times. Over this characteristic time a marked change in the He loss occurs, which has been reported only recently. Swelling of the helium implanted and annealed silicon is analyzed using an atomic force microscope. Step heights are consistent with the cavity volume per unit area obtained from spectroscopic ellipsometry data analysis. The number density of cavities after annealing for 600 s is calculated to be ≈1.16±0.27×1017 cm-3 and is found to be largely independent of depth in the central part of the cavity layer. © 1999 American Institute of Physics.