Patent ID: 7808653

Claim:
A method of characterizing a surface defect on a substrate comprising; providing a coherent radiation beam; providing a specular, non-flat substrate; splitting the radiation beam into a probe beam and a reference beam; intercepting and reflecting the probe beam with a first reflective element to perpendicularly irradiate a local surface of the substrate comprising a surface defect wherein the local surface is tilted relative to a longitudinal axis of the probe beam; intercepting and reflecting the reference beam with a second reflective element; collecting the probe beam reflected from the local surface and the reference beam reflected from the second reflective element; combining the reference beam reflected from the second reflective element and the probe beam reflected from the local surface to produce a first set of interference fringes resulting from the local surface tilt and a second set of interference fringes resulting from the surface defect; detecting the first and second sets of interference fringes; steering the probe beam with the first reflective element to minimize the number of interference fringes resulting from the local surface tilt; and using the detected interference fringes resulting from the surface defect to characterize the defect.