Low coherence optical interferometry  has been proven to be an effective tool for characterization of thin and ultra-thin semiconductor Si wafers . Purpose of this paper is to present an extension of this method to characterization of ultra-thin compound wide bandgap wafers mounted on insulating carriers.
The metrology of thin and ultra-thin compound materials wafers has been identified very early as one of the technology gaps of the industry. Most commonly employed metrologies include capacitance and air pressure techniques. These two competing technologies have been proven reliable and quite accurate methods of measurement of thickness, bow, warp and related parameters for relatively thick and well conducting materials. Capacitance method however is not suitable for measuring thickness of semi-insulating and insulating materials such as sapphire, or SiC, and very thin wafers (thinner than 100 um). Air pressure based sensors are able to measure insulating materials, however they cannot be directly applied to wafers mounted on carriers. Both these competing technologies require access from both top and bottom sides of the wafer. Very small standoff distance makes difficult to integrate these metrologies in in-situ or ultra-clean environments.
In this paper we present alternative technique, which does not suffer from above discussed limitations, and can be easily integrated in the production tools, and provide accurate measurement of samples while they are being processed.
 D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, J. G. Fujimoto, Science 254, 1178-1181 (1991).
 W.J. Walecki, R. Lu, J. Lee, M. Watman, S.H. Lau, A. Koo, 3 rd International Workshop on Thin Semiconductor Devices - Manufacturing and Applications November 25, 2002, Munich, Germany