Ultra-thin Silicon-On-Insulator (SOI) materials are becoming increasingly attractive for fabrication of ultra-large scale integrated (ULSI) devices due to their performance, speed, and power reduction advantages [1]. In SIMOX (Separation by IMplanted OXygen) materials, extensive damage can be repaired by high-temperature annealing in an argon ambient. A small percentage of oxygen is added to give a sufficient oxygen partial pressure to prevent formation of silicon monoxide lumps and subsequent pits [2]. Capping the sample surface to prevent external thermal oxidation during annealing is important to preserve the thickness of the top silicon layer. This is particularly critical for the ultra-thin SIMOX produced by the low-dose low-energy process. However, the surface capping also affects the formation of the buried oxide (BOX). To study this effect, two sets of samples implanted at the same doses of 1.5, 2.0, 2.5, and 3.0x 1017O+ /cm2 at 65keV were annealed with or without protective cap on the sample surface.