Direct bonding and mechanical thinning of pre-etched silicon wafers have been studied for the fabrication of silicon-on-insulator (SOI) wafers with buried cavities. The thin Si diaphragm over the cavity is deflected downward during the grinding and polishing, as the thinning is carried out without supporting the diaphragm. The deflection causes thickness variation for the Si diaphragm that can also be observed as a hill on the wafer surface after thinning. The results show that the thickness variation of the Si diaphragm increases with increasing cavity size and with decreasing SOI layer thickness. After grinding the measured hill height was about 1.5 mu m for a 20-mu m-thick Si diaphragm over a 1x1 mm cavity. The hill height was reduced to less than 0.5 mu m when a small supporting column was placed under the diaphragm. With polishing the hill height was further reduced to < 0.1 mu m. It appears that mechanical thinning of the bonded wafers with pre-etched cavities is a viable method for various applications.