Evaluation of the x-ray response of amorphous selenium coated 100-μm pitch a-Si active pixel sensors for tomosynthesis applications

Abstract

We detail the integration of hydrogenated amorphous silicon (a-Si:H) active pixel sensor (APS) test arrays with an overlying amorphous selenium (a-Se) x-ray photoconductor, and report on results of their x-ray response and imaging properties. The a-Se/a-Si:H APS arrays incorporate a two-transistor (2T) gate-switched pixel amplifier architecture designed to provide high detector array resolution, as well as a controllable on-pixel gain. The direct x-ray detectors consist of in-house fabricated, dual mode active and passive sensor arrays with detector element (del) pitches of 100 μm and 200 μm, coated with an 80 μm thick stabilized amorphous selenium layer. These selenium layers were selected for preliminary work and represent a quantum efficiency (QE) of 69% for x-ray spectra (tungsten target, 0.44 mm Al filtration) of 35 kV. Detector response was evaluated for a-Se bias fields of both 5 V/μm and 10 V/μm. A detector dark current of 110 pA/cm 2 (0.01 pA/100 μm del) at 10V/µm electric field, a controllable detector conversion gain up to 15.3 nA/mR at 30 kVp were measured. Active pixel gains of 6.7 and 9.6 were measured for 100 μm and 200 μm pitch detectors respectively. The amplified readout exhibits a better detection limit (by one order of magnitude) compared to the passive readout implemented on the same pixel. Capabilities of amplified pixels such as nondestructive readout, as well as programmable pixel conversion gain, and dynamic range control are demonstrated. In light of their adaptable gain and dynamic range, these detectors represent a promising technology for high-resolution high gain x-ray digital imaging, particularly in mammography tomosynthesis.