An investigation of signal performance enhancements achieved through innovative pixel design across several generations of indirect detection, active matrix, flat‐panel arrays

Abstract

Active matrix flat-panel imager (AMFPI) technology is being employed for an increasing variety of imaging applications. An important element in the adoption of this technology has been significant ongoing improvements in optical signal collection achieved through innovations in indirect detection array pixel design. Such improvements have a particularly beneficial effect on performance in applications involving low exposures and/or high spatial frequencies, where detective quantum efficiency is strongly reduced due to the relatively high level of additive electronic noise compared to signal levels of AMFPI devices. In this article, an examination of various signal properties, as determined through measurements and calculations related to novel array designs, is reported in the context of the evolution of AMFPI pixel design. For these studies, dark, optical, and radiation signal measurements were performed on prototype imagers incorporating a variety of increasingly sophisticated array designs, with pixel pitches ranging from 75 to 127 microm. For each design, detailed measurements of fundamental pixel-level properties conducted under radiographic and fluoroscopic operating conditions are reported and the results are compared. A series of 127 microm pitch arrays employing discrete photodiodes culminated in a novel design providing an optical fill factor of approximately 80% (thereby assuring improved x-ray sensitivity), and demonstrating low dark current, very low charge trapping and charge release, and a large range of linear signal response. In two of the designs having 75 and 90 microm pitches, a novel continuous photodiode structure was found to provide fill factors that approach the theoretical maximum of 100%. Both sets of novel designs achieved large fill factors by employing architectures in which some, or all of the photodiode structure was elevated above the plane of the pixel addressing transistor. Generally, enhancement of the fill factor in either discrete or continuous photodiode arrays was observed to result in no degradation in MTF due to charge sharing between pixels. While the continuous designs exhibited relatively high levels of charge trapping and release, as well as shorter ranges of linearity, it is possible that these behaviors can be addressed through further refinements to pixel design. Both the continuous and the most recent discrete photodiode designs accommodate more sophisticated pixel circuitry than is present on conventional AMFPIs--such as a pixel clamp circuit, which is demonstrated to limit signal saturation under conditions corresponding to high exposures. It is anticipated that photodiode structures such as the ones reported in this study will enable the development of even more complex pixel circuitry, such as pixel-level amplifiers, that will lead to further significant improvements in imager performance.

Active pixel imagers incorporating pixel‐level amplifiers based on polycrystalline‐silicon thin‐film transistors

“…Compared to the discrete photodiodes commonly employed in conventional AMFPIs, implementation of out-of-plane photodiode structures provides larger optical fill factors ͑defined as the fraction of the pixel surface that is optically sensitive to incident light͒ that are close to unity. 3,14 Such out-of-plane photodiode structures circumvent the competition for pixel area that would otherwise occur between the pixel TFTs of an amplification circuit and a conventional discrete photodiode.…”

“…The array has a pixel format of 384 ϫ 256 pixels and a pitch of 90 m, giving an active area of ϳ3.45ϫ 2.30 cm 2 . PSI-1 ͑which has been previously characterized͒ 14 provides a reference for the performance of an array based on poly-Si TFTs, without in-pixel amplification. PSI-2 incorporates a single-stage, in-pixel amplifier with a pixel format of 384ϫ 256 pixels and a pitch of 90 m, resulting in the same active area as for PSI-1.…”

“…The prototypes, referred to as polycrystalline-silicon imaging ͑PSI͒ arrays, employ various pixel circuits and incorporate a continuous photodiode structure. 14,25,26 Two designs incorporate an amplification circuit in each pixel, which we will refer to as an "active pixel" ͑AP͒ architecture. The performance of these prototype arrays is characterized in terms of basic pixel signal and noise properties, as well as the spatial-frequency-dependent metrics, modulation transfer function ͑MTF͒, noise power spectrum ͑NPS͒, and DQE.…”

“…13 In the case of indirect detection AMFPIs, one approach involves increasing the fraction of the area in each pixel that is optically sensitive through the use of a continuous photodiode design to achieve near-unity fill factor. 3,14 Another approach involves the introduction of a thin a-Se avalanche multiplication layer between the scintillator and the active matrix array to provide amplification of the light signal emerging from the scintillator. [15][16][17] An approach that can be applied to both direct and indirect detection AMFPIs involves the incorporation of an amplification circuit within each pixel, enhancing the pixel signal prior to readout.…”

“…The photodiodes are largely fabricated above the plane of the TFTs and consist of a patterned a-Si n-doped layer, as well as continuous intrinsic and p-doped a-Si layers. 3,14 The discrete n-doped layer connects to an underlying collection electrode, which defines the geometric fill factor of the pixel. Compared to the discrete photodiodes commonly employed in conventional AMFPIs, implementation of out-of-plane photodiode structures provides larger optical fill factors ͑defined as the fraction of the pixel surface that is optically sensitive to incident light͒ that are close to unity.…”