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23 September 2009 Enhanced laser-writing techniques for bimetallic grayscale photomasks
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Under laser exposure, bimetallic thin films of Bi/In and Sn/In oxidize becoming transparent. By controlling the power, direct-write binary and grayscale photomasks have been produced with the mask's transparency (optical density, OD), ranging between ~3.0 (unexposed) to <0.22 (fully exposed). Precise 3D micro-optics require both high vertical accuracy, gray levels over large OD changes, and precise lateral pattern creation. To achieve this result, an OD measurement system has been developed that provides real-time measurements while the masks are being written. Using stationary exposures of Bi/In and Sn/In films with varying laser powers, the reduction in OD of the films is measured with respect to time. Using 1-minute exposures, the films reach a 'saturated' level within a second at 180 mW while at 50 mW their OD gradually reduces. The influence of film's thickness is examined with thicker films requiring a longer exposure time in order to reach a similar OD level. For mask-writing, the optimal line spacing is dependent upon the laser beam's power distribution profile. Using a line-spacing 3-5 times smaller than the effective spot-size, variations in the patterned mask caused by a Gaussian-distributed beam can be minimized at the cost of increasing the writing time of the mask by the same factor. The Gaussian-distributed beam at different laser powers is also found to create shifts in the OD measurements that are problematic for a closed-loop mask-writing system. The influence of the beam's power distribution is discussed along with solutions to eliminate the problems.
© (2009) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
James M. Dykes and Glenn H. Chapman "Enhanced laser-writing techniques for bimetallic grayscale photomasks", Proc. SPIE 7488, Photomask Technology 2009, 74880O (23 September 2009);

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