Patent ID: 6856385

Claim:
An apparatus for evaluating the characteristics of a sample wherein the sample includes a measurement area defined by a region having characteristics different from the surrounding region, comprising: a laser for generating a probe beam of radiation which is directed to illuminate a spot on the surface of the sample wherein the size of the probe beam spot is similar to the size of the measurement area; an optical detector for monitoring the probe beam after reflection from the sample; a polarizer positioned in the probe beam path between the laser and the detector; an analyzer positioned in the probe beam path between the laser and the detector; a movable stage for supporting the sample in a manner to translate the sample with respect to the probe beam such that the probe beam spot strikes the sample at a location substantially in the center of the measurement area and at locations on either side thereof allowing optical measurements with the optical detector to be made at a plurality of the locations; and a processor for determining the change in polarization state of the probe beam induced by interaction with the sample and computing the characteristics of the sample for each of the measurement locations and for determining an extremum in the computed characteristics to identify the measurement location best corresponding to the center of the measurement area.