Patent ID: 7136163

Claim:
A method of detecting a signal indicative of a property of a structure in a semiconductor wafer, the method comprising: receiving a semiconductor wafer that comprises a test structure, wherein the test structure comprises a first and second region that differ from each other by at least one property and wherein the first region comprises an interface in the wafer; illuminating the first region with one of S and P polarized portions of a beam of coherent electromagnetic radiation and illuminating the second region with the other of the S and P polarized portions, wherein the beam has an absorption length less than a thickness of the wafer but greater than a depth of an interface in the wafer and wherein intensity of the S and P polarized portions is modulated over time; using a photodetector to measure intensity of a portion of the beam reflected from the wafer; and synchronously detecting, at a frequency of modulation of intensity of the S and P polarized portions, an amplitude of an electrical signal generated by the photodetector during measurement by the photodetector.