The dependence of the modulation transfer function on the blocking layer thickness in amorphous selenium x‐ray detectors

Abstract

Blocking layers are used to reduce leakage current in amorphous selenium detectors. The effect of the thickness of the blocking layer on the presampling modulation transfer function (MTF) and on dark current was experimentally determined in prototype single-line CCD-based amorphous selenium (a-Se) x-ray detectors. The sampling pitch of the detectors evaluated was 25 microm and the blocking layer thicknesses varied from 1 to 51 microm. The blocking layers resided on the signal collection electrodes which, in this configuration, were used to collect electrons. The combined thickness of the blocking layer and a-Se bulk in each detector was approximately 200 microm. As expected, the dark current increased monotonically as the thickness of the blocking layer was decreased. It was found that if the blocking layer thickness was small compared to the sampling pitch, it caused a negligible reduction in MTF. However, the MTF was observed to decrease dramatically at spatial frequencies near the Nyquist frequency as the blocking layer thickness approached or exceeded the electrode sampling pitch. This observed reduction in MTF is shown to be consistent with predictions of an electrostatic model wherein the image charge from the a-Se is trapped at a characteristic depth within the blocking layer, generally near the interface between the blocking layer and the a-Se bulk.

Reduction of the dark current in stabilized a-Se based X-ray detectors

“…An n-layer (d) produced from the same alloy (Se:0.5%As without Cl) by cold deposition in which T substrate = 25°C. An n-i structure (Ç) produced as a sister sample for one of the X-ray detectors described in [10]. Both layers are produced from Se:0.2%As without Cl.…”

“…4. This particular detector structure was recently used in prototype detectors for high-resolution mammography based on the slot scanning imaging technique [10]. Both layers in the latter structure were produced from Se:0.5%As alloy.…”

Dependence of the electrical properties of stabilized a-Se on the preparation conditions and the development of a double layer X-ray detector structure

“…6 is a photograph of the complete detector along with X-ray images of a resolution phantom obtained with this detector. Modulation transfer functions, MTFs, obtained on our experimental X-ray detectors [13] show that the spatial frequency f 0.5 (f at which MTF = 0.5) is about 11-14 lp mm À1 when d 6 15 lm. It was found that f 0.5 falls as d becomes thicker.…”

“…Differently scaled X-ray images of a resolution bar phantom that show the a-Se CCD X-ray detector can resolve bar patterns with resolution better than 16 lp mm À1 . detectors for high-resolution mammography based on a CCD readout circuit [12]. The complete detector was fabricated using a CCD chip, with a pixel size 20 lm  20 lm, coated with an i-n double layer a-Se with a Pt negative electrode.…”

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

“…This process is called "cold deposition" since the substrate temperature is well below the glass transition temperature of a-Se. Whereas the n-like blocking layer is small compared to the sampling pitch, the reduction of the MTF due to this layer will be small [17]. The fabrication of the n-like layers was completed by annealing the devices at 51 °C for 1 hour in the vacuum chamber.…”