An SOI wafer has a device layer of initial thickness that is formed into a set of mesas in the interval between which a temporary layer of polysilicon is deposited to a precisely controlled thickness. This polysilicon is entirely converted in a self-limiting process to an oxide etch stop having a thickness much smaller than the initial thickness. The mesas are thinned by a chemical mechanical polishing technique until the mesa is the same level as the top surface of the new oxide. The etch stop layer of oxide is not removed but serves both as an isolating layer to provide dielectric isolation between mesas in the final circuit and also as a visual gauge to determine the time when the polishing process should stop.