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Method and apparatus for maintaining desired exposure levels

  • US 4,987,044 A
  • Filed: 05/31/1989
  • Issued: 01/22/1991
  • Est. Priority Date: 05/31/1989
  • Status: Expired due to Term
First Claim
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1. A method for subjecting predetermined portions of a surface to desired exposure levels with a beam of radiation, comprising the steps of:

  • deflecting said beam towards said surface through a set of a first mirror and a second mirror, the first mirror having a first rotational axis, the second mirror having a second rotational axis;

    oscillating said first mirror about said first axis at a desired frequency and effective amplitude with a substantially sinusoidal motion, and oscillating said second mirror about said second axis with a substantially sinusoidal motion at substantially said desired frequency and effective amplitude as employed for said first mirror, so that both mirrors are characterized by a common frequency value and a common effective amplitude value;

    maintaining a substantially 90 degree phase difference between the sinusoidal motion of the first mirror and the sinusoidal motion of the second mirror;

    tracing said surface with said beam in a spiral path by changing gradually and simultaneously the common value of the effective amplitude and the common value of the frequency inversely, so that the product of the two values is controlled so as to yield a desired exposure level on the traced surface when the beam is on; and

    turning the beam off and on at predetermined angular positions in order to subject said predetermined portions of said surface located within the prearranged positions to a desired exposure level.

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