Position measurement unit, measurement system and lithographic apparatus comprising such position measurement unit
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United States of America Patent
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Apr 22, 2008
Grant Date -
Mar 15, 2007
app pub date -
Sep 15, 2005
filing date -
Sep 15, 2005
priority date (Note) -
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Abstract
A measurement unit to determine a position in a first and second dimension includes a diffraction type encoder and an interferometer. The diffraction type encoder determines by means of a diffraction on a first and a second diffraction grating the position in the first dimension of the second grating with respect to the first grating, The interferometer determines the position in the second dimension of a mirror. The measurement unit includes a combined optical unit to transfer an encoder measurement beam as well as an interferometer measurement beam. Further, the measurement unit may include a combined light source for the encoder as well as the interferometer. One of the first and second diffraction gratings may further show some degree of zero order reflection to provide the mirror of the interferometer.
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