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14 February 2011 Bimetallic grayscale photomasks written using optical density feedback control
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When bimetallic thin films of Bi/In and Sn/In are laser exposed, they oxidize and become variably transparent. By controlling the writing laser power, binary and grayscale photomasks can be produced with the mask's transparency (optical density, OD), ranging between ~3.0 (unexposed) to <0.22 OD (fully exposed). Targeting the production of grayscale masks with 256 levels, the mask-writing system when combined with photodiode sensors obtains real-time OD and laser power measurements and uses them to adjust the laser's writing power during the patterning process. For a single-line stepped pattern, laser writing without OD feedback control demonstrates an average absolute error of 4.2 gray levels, while with OD feedback control and the appropriate parameters, the same pattern is produced with an average absolute error of 0.3 gray levels. The control parameters are shown to influence the characteristics of the resulting mask pattern, particularly the overshoot and rise-time of the pixel transitions. With multi-line mask patterns being rasterscanned written, the overlap of the lines combined with the laser's Gaussian profile creates variations in the mask, and measurement problems for the OD feedback control. An interlaced raster-scan approach is proposed where a first pass patterns non-overlapping lines using an ideal set of control parameters. A second and third pass then patterns the lines inbetween and at the pixel boundaries using another set parameters designed to account for the overlap. The technique allows feedback to be used for the entire mask writing process.
© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
James M. Dykes, Reza Qarehbaghi, and Glenn H. Chapman "Bimetallic grayscale photomasks written using optical density feedback control", Proc. SPIE 7927, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics IV, 792719 (14 February 2011);

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