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

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.