Abstract
A focus tracking system for direct wafer steppers has been tested and its operation simulated on computer to account for transducer fabrication limitations. The focus condition of the projection lens was determined by sensing the contrast level in the backprojected image of a test grating object mounted in a plane conjugate to the reticle plane. To permit continuous focus tracking, the test grating was illuminated with red light from a solid-state light emitting diode to prevent exposure of UV-sensitive photoresists. The contrast sensor consisted of a pair of large area silicon photodiodes with grating windows arranged so that the diodes viewed complementary space and bar projected fields of the test grating. The projection lens used in these tests was a 40× microscope objective having a numerical aperture of 0.95. The focus sensing range was found to be better than 1.6 μm, and the focus sensitivity was <0.1 μm.
© 1986 Optical Society of America
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