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8 November 2012 Bimetallic grayscale photomasks for micro-optics fabrication using dual wavelength laser writing techniques
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Microfabrication of high-resolution micro-optic devices requires <1/8λ (~60nm) precision both vertically and horizontally. More critical is the creation of 256-level grayscale masks to create sufficient vertical precision in the photoresist. Grayscale bimetallic photomasks are bi-layer thermal resists of Bismuth-on-Indium or Tin-on-Indium become controllably transparent by varying laser power thermally producing alloy oxide ranging ~3OD (unexposed) to <0.22OD (fully exposed). Previously, a direct-write multi-line CW Argon-ion laser writing system with feedbackcontrolled Gaussian beam achieved 256-level grayscale masks. The feedback system effectively reduced the average gray-level error from ±4.2 gray-levels in an open-loop approach to ±0.3 gray-levels in a closed-loop approach. Remaining gray-level errors were due to the Gaussian beam profile creating variations in gray-levels. Preliminary results show that a beam shaper creating a flattop beam helps reducing gray-level fluctuations. The multi-line Argon laser enables having multiple single beams separated from a single stabilized laser source. The single 514.5nm line used for writing gives better control of beam shape in the modulated laser beam. At the same time a lower power 457.9nm line introduced in the beam path to characterize the grayscale pattern both during and after the writing process. Filtering the writing laser line, sensor below the mask plate measures only the 457.9nm line enabling the high accuracy transparency measurements of the written mask near G-line (435.8nm). One target application is the creation of micro-lens arrays, which are lenses whose optical shape varies from lenslet to lenslet across the entire patterned surface of cm size. Laser direct-written grayscale masks enable relatively low cost, rapid turnaround mask production needed for creating such structures with microfabrication processes.
© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Reza Qarehbaghi and Glenn H. Chapman "Bimetallic grayscale photomasks for micro-optics fabrication using dual wavelength laser writing techniques", Proc. SPIE 8522, Photomask Technology 2012, 85222K (8 November 2012);

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