Abstract

The position detector has a sensitivity characteristic graded along a direction transverse to the surface, so that the output of the position detector is used to determine a height of the surface. A surface height detection and positioning device for use in a surface inspection system. An incident beam of light impinges obliquely upon the surface, and a position detector is disposed to receive specularly reflected light, producing a plurality of electrical signals, with a mechanical window, defining an aperture, placed in front of the detector. The aperture's width, along the scan direction, is of sufficient size so as to create a train of signals from each of the plurality of electrical signals, having a frequency equal to the scan frequency. These electrical signals carry information responsive to both the position of reflected beam impinging on the detector and the beam's intensity and are, in turn, related to a height of the surface. To abrogate information responsive to intensity variations at the position sensitive detector, an electronic circuit is employed which determines the sum and the difference of the plurality of signals relating to wafer height and beam intensity, producing a summed signal and a difference signal respectively. The difference signal is then divided by the summed signal, thereby producing a normalized signal which represents the height of the wafer surface without regards to these reflected beam intensity variations. These signals are synchronized to the scan frequency which facilitates removing unwanted signals resulting from thermal drifts and ambient light.