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

Abstract

ABSTRACT We have developed a high resolution amorphous selenium (a-Se) direct detection imager using a large-areacompatible 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 initiatesnucleation of crystalline selenium and results in high detector dark currents. A CMOS-compatible polyimidebu er layer was used to planarize the backplane and provide a low stress and thermally stable surface for a-Se.The bu er layer inhibits crystallization and provides detector stability that is not only a performance factor butalso critical for favorable long term cost-bene t considerations in the application of CMOS digital x-ray imagersin medical practice. The detector structure is comprised of a polyimide (PI) bu er layer, the a-Se layer, and agold (Au) top electrode. The PI layer is applied by spin-coating and is patterned using dry etching to open thebackplane bond pads for wire bonding. Thermal evaporation is used to deposit the a-Se and Au layers, and thedetector 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  m pixel pitch and have a pre-samplingmodulation transfer function (MTF) that is signi cantly limited by the pixel aperture. Our results con rm that,for a densely integrated 25  m 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. Preliminaryimages demonstrating high spatial resolution have been obtained from a rst prototype imager.Keywords: x-ray imaging, high spatial resolution, amorphous selenium, CMOS, CMOS-selenium, array pro-cessing, direct conversion, at panel imaging