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Proceedings Paper

Amorphous selenium direct detection CMOS digital x-ray imager with 25 micron pixel pitch
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Paper Abstract

We have developed a high resolution amorphous selenium (a-Se) direct detection imager using a large-area compatible back-end fabrication process on top of a CMOS active pixel sensor having 25 micron pixel pitch. Integration of a-Se with CMOS technology requires overcoming CMOS/a-Se interfacial strain, which initiates nucleation of crystalline selenium and results in high detector dark currents. A CMOS-compatible polyimide buffer layer was used to planarize the backplane and provide a low stress and thermally stable surface for a-Se. The buffer layer inhibits crystallization and provides detector stability that is not only a performance factor but also critical for favorable long term cost-benefit considerations in the application of CMOS digital x-ray imagers in medical practice. The detector structure is comprised of a polyimide (PI) buffer layer, the a-Se layer, and a gold (Au) top electrode. The PI layer is applied by spin-coating and is patterned using dry etching to open the backplane bond pads for wire bonding. Thermal evaporation is used to deposit the a-Se and Au layers, and the detector is operated in hole collection mode (i.e. a positive bias on the Au top electrode). High resolution a-Se diagnostic systems typically use 70 to 100 μm pixel pitch and have a pre-sampling modulation transfer function (MTF) that is significantly limited by the pixel aperture. Our results confirm that, for a densely integrated 25 μm pixel pitch CMOS array, the MTF approaches the fundamental material limit, i.e. where the MTF begins to be limited by the a-Se material properties and not the pixel aperture. Preliminary images demonstrating high spatial resolution have been obtained from a frst prototype imager.

Paper Details

Date Published: 19 March 2014
PDF: 7 pages
Proc. SPIE 9033, Medical Imaging 2014: Physics of Medical Imaging, 90331G (19 March 2014); doi: 10.1117/12.2043770
Show Author Affiliations
Christopher C. Scott, Univ. of Waterloo (Canada)
Shiva Abbaszadeh, Univ. of Waterloo (Canada)
Sina Ghanbarzadeh, Univ. of Waterloo (Canada)
Gary Allan, Teledyne DALSA Inc. (Canada)
Michael Farrier, Teledyne DALSA Inc. (Canada)
Ian A. Cunningham, Robarts Research Institute (Canada)
Lawson Health Research Institute (Canada)
Univ. of Western Ontario (Canada)
Karim S. Karim, Univ. of Waterloo (Canada)

Published in SPIE Proceedings Vol. 9033:
Medical Imaging 2014: Physics of Medical Imaging
Bruce R. Whiting; Christoph Hoeschen, Editor(s)

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