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13 October 2011 Improving the accuracy of the bimetallic grayscale photomasks using a feedback controlled flat-top beam
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Bimetallic grayscale photomasks contain two thin layers of metals (Bismuth/Indium or Tin/Indium), 15-300nm thick on transparent substrates. Laser exposure converts the films by thermal reaction into transparent alloy oxides. The Optical Density changes from ~3.0OD (unexposed) to <0.22OD (fully exposed), creating grayscale photomasks. With using a open loop direct-write raster-scan writing system with a CW Argon-ion laser, grayscale masks with 6-bit accuracy was achieved. To reduce mask nonuniformity, sensors added to the beam path before and after the mask turning the system to create a real-time OD and beam measurement feedback system. This feedback compensates for changes in film parameters: film thickness reducing the mask error to ±0.3 gray-levels, yielding to a potential 256 gray-level mask. This leaves nonuniformities due to the Gaussian shape still limits the accuracy. To remove these, a refractive field-mapping shaper was used to create a nearly flat-top power laser spot power distribution. The OD system also allows us to profile the grayscale levels in more detail after writing. A full 8-bit picture was written on a test mask with the feedback system showed significant improvement in the number of gray-levels. Some fluctuations were observed at the final pattern due to spaces between lines and nonuniformity of table speed. Some 3D test structures are created on the photoresist with the masks written with the feedback system to demonstrate the accuracy and grayscale abilities of the bimetallic thin-films
© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Reza Qarehbaghi, Glenn H. Chapman, and Waris Boonyasiriwat "Improving the accuracy of the bimetallic grayscale photomasks using a feedback controlled flat-top beam", Proc. SPIE 8166, Photomask Technology 2011, 816641 (13 October 2011);

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