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Compensation for errors in off-axis interferometric measurements

  • US 7,262,860 B2
  • Filed: 11/03/2004
  • Issued: 08/28/2007
  • Est. Priority Date: 07/29/2002
  • Status: Expired due to Fees
First Claim
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1. A method for determining the location of an alignment mark on a stage, the method comprising:

  • directing a measurement beam along a path between an interferometer and a mirror, wherein at least the interferometer or the mirror is mounted on the stage;

    combining the measurement beam with another beam to produce an output beam comprising information about the location of the stage;

    measuring from the output beam a location, x1, of the stage along a first measurement axis;

    measuring a location, x2, of the stage along a second measurement axis substantially parallel to the first measurement axis;

    calculating a correction term, ψ

    3, from predetermined information characterizing surface variations of the mirror for different spatial frequencies, wherein contributions to the correction term from different spatial frequencies are weighted differently;

    determining a location of the alignment mark along a third axis parallel to the first measurement axis based on x1, x2, and the correction term; and

    outputting a signal from an electronic controller based on the determined location of the alignment mark.

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