Patent ID: 7092082

Claim:
An apparatus for inspecting a semiconductor wafer, said apparatus comprising: (a) a first light source for producing a first beam of light, said first beam of light illuminating an area on the semiconductor wafer, (b) a main imaging camera disposed above the semiconductor wafer for detecting light scattered from the area illuminated by the first beam of light, and (c) a main imaging lens for imaging the area on the semiconductor wafer illuminated by the first beam of light onto said main imaging camera, and (d) an auto-focus system to compensate for vertical deviations in the topology of said semiconductor wafer, said auto-focus system ensuring that said main imaging lens images the area on said semiconductor wafer onto said main imaging camera in focus, said auto-focus system comprising, (i) a second light source with associated optics for producing a second beam of light, said second beam of light reflecting off the area on the semiconductor wafer, (ii) a sensor with associated optics for detecting light reflected from the semiconductor wafer by the second beam of light, wherein said sensor is a linear position sensor; and (iii) circuitry for converting the light detected by said linear position sensor into a voltage which is proportional to the relative vertical position of the area of the semiconductor wafer.