Initial steps toward the realization of large area arrays of single photon counting pixels based on polycrystalline silicon TFTs

Abstract

The thin-film semiconductor processing methods that enabled creation of inexpensive liquid crystal displays based on amorphous silicon transistors for cell phones and televisions, as well as desktop, laptop and mobile computers, also facilitated the development of devices that have become ubiquitous in medical x-ray imaging environments. These devices, called active matrix flat-panel imagers (AMFPIs), measure the integrated signal generated by incident X rays and offer detection areas as large as ~43×43 cm 2 . In recent years, there has been growing interest in medical x-ray imagers that record information from X ray photons on an individual basis. However, such photon counting devices have generally been based on crystalline silicon, a material not inherently suited to the cost-effective manufacture of monolithic devices of a size comparable to that of AMFPIs. Motivated by these considerations, we have developed an initial set of small area prototype arrays using thin-film processing methods and polycrystalline silicon transistors. These prototypes were developed in the spirit of exploring the possibility of creating large area arrays offering single photon counting capabilities and, to our knowledge, are the first photon counting arrays fabricated using thin film techniques. In this paper, the architecture of the prototype pixels is presented and considerations that influenced the design of the pixel circuits, including amplifier noise, TFT performance variations, and minimum feature size, are discussed.

Performance of in-pixel circuits for photon counting arrays (PCAs) based on polycrystalline silicon TFTs

“…Each of the 11 SPC1 array designs incorporate a pixel type corresponding to some combination of designs for these four main components. (Liang et al , 2014) The first component, the amplifier, magnifies an input signal, after which the comparator circuit outputs a digital “high” if the input exceeds a user-defined threshold level. If the comparator output is high, the signal is converted by the clock generator into a two-phase clock (i.e., two separate signals with non-overlapping pulses), which is then used to increment the counter.…”

“…Recently, the first prototype PCAs based on poly-Si TFTs, which include 11 separate pixel designs collectively referred to as SPC1, have been developed toward the long-term goal of creating large-area, monolithic devices (Liang et al , 2014). In parallel, simulation frameworks and mathematical models have been developed to provide insight into the signal and noise behavior of individual poly-Si TFTs within the photon counting circuits, as well as the impact of variations in TFT performance on overall photon counting performance — information that is not obtainable empirically from prototypes.…”

Theoretical investigation of the count rate capabilities of in‐pixel amplifiers for photon counting arrays based on polycrystalline silicon TFTs

“…Circuit simulations were performed to examine the count rate performance of the amplifier circuit designs incorporated in the pixels of the previously mentioned prototype poly-Si photon counting arrays. 40 In addition, hypothetical variations of those prototype amplifier designs that provide higher count rates, while maintaining or improving signal gain, linearity of signal response and energy resolution, were identified and investigated.…”

“…Designs SPC1-amp1 and SPC1-amp2 are first-order and second-order bandpass circuit designs that correspond to the prototype amplifier designs 40 and are illustrated in Figs. 3(a) and 3(b), respectively.…”

“…For these reasons, our group has been examining the possibility of photon counting arrays (PCAs) based on poly‐Si TFTs. In the spirit of exploring the potential development of poly‐Si photon counting arrays for use at fluoroscopic and radiographic x‐ray energies, a number of early prototype poly‐Si PCAs have been designed and fabricated, and are currently undergoing characterization. A pixel pitch of 1 mm was chosen to reduce the number of challenges in the design of these initial prototype arrays .…”