The effects of a protective capping layer on the microstructure of ultrathin separation by implantation of oxygen (SIMOX) materials formed by ion implantation were studied using transmission electron microscopy. A set of SIMOX wafers were implanted at 65 keV in a dose range of 1.5 to 7.0 x 10(17) O+/cm(2) followed by a high temperature (1350 degreesC) annealing with and without a protective cap. The lowest dose to form a continuous buried oxide (BOX) layer without Si islands at 65 keV is 2.0 x 10(17) O+/cm(2) without a protective cap and 2.5 x 10(17) O+/cm(2) with a protective cap. Above 2.5 x 10(17) O+/cm(2) under both annealing conditions, the BOX layer formed continuously but with Si islands present. The uncapped samples show slightly lower density of Si islands.,Oxygen from the annealing ambient can diffuse in the uncapped samples through the thin top Si layer, which helps the BOX layer grow laterally and lower the Si island density. The density of defects in the top Si layer is also slightly lower in the uncapped samples because of the ability of Si interstitials to incorporate into the surface and the effective annihilation of extended defects during the final annealing step. The top Si layers of the uncapped samples are thinner than those of the capped samples due to surface thermal oxidation. (C) 2001 The Electrochemical Society. All rights reserved.