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The Process

The photoresist process used in photomask fabrication involves process steps that are similar to photoresist processes used to make silicon devices. The mask blank is a 6 in. x 6 in. quartz blank with a thickness of 0.25 in. The blank is coated with a film stack that is a non-homogeneous layer of chromium oxide (200 Å) on top of a thin layer of pure chromium (800 Å). The chromium layer provides an opaque and mechanically strong layer to prevent light penetration through the mask. The chromium oxide layer is grown on the chromium layer to minimize the reflections off of the mask.

The photomask is coated with photoresist and then baked (post-application) to remove solvent from the resist. A laser pattern generator is then used to expose the individual features of the circuit pattern on the resist-coated mask. The exposure time takes hours, which is the rate limiting step in the photomask fabrication process.

The exposed photoresist is then developed with an aqueous base developer (commonly tetramethyl ammonium hydroxide-TMAH). The features are then etched into the chromium layer and the remaining photoresist is stripped off the photomask to complete the pattern transfer process into the chromium layer.

 

 


Last updated Monday, February 14, 2000
© 2000 Photomask Lithography, Willson Research Group,
University of Texas at Austin
Phone: 512.471.6364
Email:
rathsack@mail.utexas.edu
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