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.
Previously a-Si:H metal-semiconductor-metal (MSM) lateral detectors for indirect medical imaging applications had been proposed by our research group. These lateral detectors are attractive due to their ease of fabrication primarily because there is no p+ doped semiconductor layer, thus making it compatible with industry standard amorphous silicon thin film transistor electronics processing. However the earlier devices exhibited high dark current which is problematic for integration mode imaging. In the other words, they were limited in term of dynamic range. In this study, we demonstrate an a-Si:H MSM lateral structure with low dark current, high dynamic range and comparable sensitivity and quantum efficiency to conventional p-i-n photodiodes. These improvements are achieved by the introduction of a thin polymer layer as a blocking contact. The fabricated amorphous silicon based MSM detector exhibits a photo-response of more than 3 orders of magnitude to a green light source (λ = 525nm). In comparison to vertical p-i-n structures, the reported MSM lateral devices show gains in terms of dynamic range, ease of fabrication (no p+ layer), faster speed at the cost of a slightly reduced quantum efficiency. The experimental results of dark and photocurrent measurements as well as the responsivity for two in-house fabricated MSM structures at different bias voltages and light intensity are presented. This results are promising and encourage the development of a-Si:H lateral MSM devices for indirect conversion large area medical imaging applications and especially low cost flat panel computed tomography.