Importance

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Abstract

By providing redundant interferometer laser-metering devices, either one of which is capable by itself of providing measurement data of the angular value .theta. of rotation of the X-Y movable stage of a wafer stepper about a vertical Z axis for use by a computer-controlled servo devices that controls the operation of the X-Y movable stage, the servo devices, in a calibration mode, may receive data of specific measurements defining the respective values of undesired departures from flatness or straightness of the nominally-plane mirror surfaces of the the X-Y movable stage. These specific measurements, which may be made quickly at any time without disturbing the stage's mirrors, will also include those departures induced only at the point of use of the mirror and permit the stored control data used by the computer-controlled servo devices to be modified in order to compensate for these undesired departures from flatness or straightness of the nominally-plane mirror surfaces of the X-Y movable stage.

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First Claim