Importance

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Abstract

An improved gas bearing X-Y-.theta. stage assembly is disclosed which has a base, a stage element with three retroreflectors fixedly mounted thereto, and a control system for controlling the movement of the stage element. The control system is made up of three interferometer measuring systems for measuring parameters associated with changes in position of the three retroreflectors as the stage element moves. Also, the control system includes a transport system for moving the three interferometers to maintain optical coupling between the interferometers and retroreflectors. In addition, the control system includes control circuitry for causing the stage element to move to a preselected X-Y-.theta. position in response to signals from the interferometer measuring systems. In one embodiment, a first interferometer of a first one of the three interferometer measuring systems is moved linearly by the transport system on a gas bearing slide in a direction parallel to the Y-axis of the system, while second and third interferometers of the other two interferometer measuring systems are moved together on a second gas bearing slide parallel to the X-axis of the system.

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First Claim