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17 February 2012 Bimetallic grayscale photomasks written using flat-top beam vs. Gaussian beam
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Abstract
Grayscale photomasks are bi-layer metallic films consist of two thin layers of Bi-on-Indium or Tin-on-Indium. These films become controllably transparent by accurately varying laser power such that the optical density changes almost linearly from ~3 OD (unexposed) to <0.22OD (fully exposed). Previously, a direct-write raster-scan photomask system with a multi-line CW Argon-ion laser was used with feedback-controlled Gaussian beam to achieve 256-level grayscale masks. With the Gaussian laser spot, the feedback system was effective such that the average gray-level error reduced from ±4.2 gray-levels in an open-loop approach to ±0.3 gray-levels in a closed-loop approach. As most of the gray-level errors are due to the Gaussian beam profile making variations on the mask, a beam shaper was used to change the laser spot to a flat-top beam. Raster-scanning the mask using the flat-top beam helps further reduce the gray-level errors. Preliminary results show that the flat-top beam reduces gray-level fluctuations, and lines can be written with less overlapped area helping to have higher resolution masks. Having lines closer with smaller overlap suggests that accurately controlled laser power results in an accurate OD profile on the mask even with an open-loop approach. The accuracy of the laser power is also a reason for variations as it has only 1% accuracy. Some test patterns are written on the mask using open-loop and closed-loop approaches to demonstrate how accurate the gray-levels of the bimetallic thinfilms are using a flat-top laser beam.
© (2012) 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 "Bimetallic grayscale photomasks written using flat-top beam vs. Gaussian beam", Proc. SPIE 8243, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XVII, 824305 (17 February 2012); https://doi.org/10.1117/12.909543
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