| 摘要 |
PURPOSE:To detect the gap between a wafer surface and a reduction lens accurately in a circumferential portion as well as in a central portion of the wafer by providing gap detectors individually around the optical axis of exposure. CONSTITUTION:Air microflanges 17 are provided with four blow-off outlets around the optical axis at their lower parts. The gas of constant pressure PS is supplied to each outlet individually and blown against a surface of the wafer. In this constitution, the back pressure PB is inputted the corresponding differential pressure converters 18A-18D to draw out the differential pressure signals a-d corresponding to the difference from the reference pressure PR made by pressure reduction of the constant pressure PS. The differential pressure signals a- d are supplied to a sensor selecting circuit 20 through output-adjusting circuits 19A-19D for making the signals to be the same level signals for the same gap value. The sensor selecting circuit 20 selects the most suitable differential pressure signals a-d for detection of the gap in the circumferential portion of the wafer. When the gap in the central portion of the wafer is detected, the sensor selecting circuit 20 outputs all of the differential pressure signals a-d and supplies them to an equalizing circuit 22.
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