NULLING OPTICAL BRIDGE FOR NONINTRUSIVE TEMPERATURE-MEASUREMENTS

A null-point optical bridge for determining the relative power of two optical beams, derived from a common source, within the shot noise limit of the detector, independent of power fluctuations of the source has been developed. The optical bridge method can be used to measure very small changes in sample reflectivity caused by changes in sample temperature and can, therefore, be used as a nonintrusive optical thermometer for semiconductor substrates during processing. The range of application extends from very low temperatures to very high temperature where, for example, the sample may melt. For silicon and under ideal laboratory conditions, a temperature resolution of 0.2-degrees-C has been demonstrated.