Protein crystallography is a key method for protein structure investigation in modern medicine and
X-ray diffraction detectors are key to performance. We introduced a silicon detector, based on an
active-pixel readout of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) for
protein crystallography. In this work, we present the fabrication process of the detector array,
performance of the first fabricated TFT arrays, and the performance of the TFTs in terms of fieldeffect
mobility, gate material quality, and stability under long stress using a Fe-55 (50 μCi)gamma
ray source (6 to 10 keV photon energies). Device fabrication was performed in an in-house facility,
Giga-to-Nano microfabrication facility, at the University of Waterloo, and involved plasma
enhanced chemical vapor deposition (PECVD) and wet and dry etch techniques with a simple two
mask process. The TFT test results promise higher effective field effect mobility of 16.49 cm2/V·s
due to the presence of silicon substrate contacting the a-Si:H channel layer along with a compromise
in leakage current, yielding a 104 ON/OFF ratio. Meanwhile, the threshold voltage shift is
manageable by applying a negative voltage of a duration less than 1/10 of the duty cycle. From the
detector leakage test, the leakage current through the TFT gate was acceptable range while the
photo-generated current needs to be suppressed with positive voltage bias at the gate electrode. Thus,
minimizing the negative gate bias in readout operation is crucial. Finally, TFT readout current under
the same Fe-55 X-ray source shows that optimal operation range can be determined when bulk bias
is higher than TFT operation bias.
An interface has been developed to capture frames taken by X-ray array imagers up to 64×64 pixels. An application
specific integrated circuit (ASIC) designed solely for X-ray flat panel imaging readout circuitry, manufactured by
FLIR® called Indigo (also known as ISC9717), was used as part of charge-amplifier block. An Altera Cyclone II FPGA
is used to serve three purposes: Create pulses required for gate-driver block, Receive fast-stream data coming from the
Indigo chip, and Send data through RS-232 protocol over a serial cable to a personal computer.
Initial results for a 32×32 passive pixel sensor (PPS) with lateral amorphous Selenium metal-semiconductor-metal
(MSM) photodetector were presented in [1]. This work focuses more on methods used to improve the images obtained
from the array. Sharper images produced in sync with the light source are presented. In addition, insight into array
readout circuitry and capturing a frame from an array is discussed.
Crystalline Silicon Drift Detectors (SDDs) have been used for spectroscopy and particle physics applications since they
were first reported in the 1980s[1][2]. However, spatial resolution and a complex fabrication process are two major
hurdles that prevent SDDs from seeing widespread use in diffraction imaging and protein crystallography. To overcome
the issues with SDDs, we proposed a new type of silicon radiation detector integrated with a hydrogenated amorphous
silicon (a-Si:H) thin film transistor (TFT) readout technology intended for low X-ray energy detection with high spatial
resolution [3]. This work aims to extend our previous effort by examining in detail the operation of the proposed
integrated silicon TFT detector and to evaluate its X-ray sensitive performance via numerical analysis. In this research
we simulate a 250-μm-thick slightly doped p-type silicon substrate that also functions as an X-ray detector integrated
with a TFT readout having a channel length of 15 μm and a source/drain width of 15 μm. The simulations performed
focus on the potential distribution and band structure at the heterostructure interface between the TFT and the silicon
detector, and also on the current-voltage characteristics of the TFT due to X-ray exposure. Based on simulation results,
the expected lower and upper limits of performance will be presented. In particular, the feasibility of a single 6 keV
photon detection (arguably the minimum signal for a crystallography application) with such a device will be examined.
Previously, we reported on a single-pixel detector based on a lateral a-Se metal-semiconductor-metal structure, intended
for indirect conversion X-ray imaging. This work is the continuous effort leading to the first prototype of an indirect
conversion X-ray imaging sensor array utilizing lateral amorphous selenium. To replace a structurally-sophisticated
vertical multilayer amorphous silicon photodiode, a lateral a-Se MSM photodetector is employed which can be easily
integrated with an amorphous silicon thin film transistor passive pixel sensor array. In this work, both 2×2 macro-pixel
and 32×32 micro-pixel arrays were fabricated and tested along with discussion of the results.
Cadmium Zinc Telluride (CdZnTe or CZT) is a polycrystalline radiation detector that has been investigated over the
years for a variety of applications including Constellation X-ray space mission [1] and direct-conversion medical
imaging such as digital mammography [2]. Due to its high conversion gain and low electron-hole pair creation energy (~4.43 eV) [3], it has found use in high end, photon counting medical imaging applications including positron emission tomography (PET), computed tomography (CT) and single photon emission computed tomography (SPECT). However, its potential in low photon energy applications has not been fully explored. In this work, we explore the capacity of the
CZT material to count low photon energies (6 keV - 20 keV). These energies are of direct relevance to applications in
gamma ray breast brachytheraphy and mammography, X-ray protein crystallography, X-ray mammography and
mammography tomosynthesis. We also present a design that integrates the CZT direct conversion detector with an inhouse
fabricated amorphous silicon (a-Si:H) thin film transistor (TFT) passive pixel sensor (PPS) array. A CZT photoconductor (2 cm x 2 cm size, 5-mm-thick) prepared by the traveling heat method (THM) from RedlenTM is characterized. The current-voltage characteristics reveal a resistivity of 3.3 x 1011 Ω•cm and a steady state dark current in the range of nA. Photocurrent transients under different biases and illumination pulses are studied to investigate photogeneration and the charge trapping process. It is found that charge trapping plays a more significant role in transient behavior at low biases and low frequency.
A single photon counting Voltage Controlled Oscillator (VCO) based pixel architecture in amorphous silicon (a-Si)
technology is reported for large area digital medical imaging. The VCO converts X-ray generated input charge into an
output oscillating frequency signal. Experimental results for an in-house fabricated VCO circuit in a-Si technology are
presented and external readout circuits to extract the image information from the VCO's frequency output are discussed.
These readout circuits can be optimized to reduce the fixed pattern noise and fringing effects in an imaging array
containing many such VCO pixels. Noise estimations, stability simulations and measurements for the fabricated VCO
are presented. The reported architecture is particularly promising for large area photon counting applications (e.g. low
dose fluoroscopy, dental computed tomography (CT)) due to its very low input referred electronic noise, high sensitivity
and ease of fabrication in low cost a-Si technology.
Thick amorphous selenium (a-Se) as an excellent photoconductor has been used in direct conversion X-ray imaging modalities such as mammography. However, due to substantial charge trapping, such detectors experience a long X-ray response time and as a result, suffer from a slow speed of operation. Therefore, its deployment to speed-required applications such as real-time fluoroscopy remains a challenge. In this work, we aim to investigate a lateral a-Se MSM
photodetector as an indirect conversion X-ray imager and its utilization in high speed, high energy medical applications.
The dark current density of the newly-fabricated detector is below 20 pA/mm2 for a 200 μm×50 μm pixel pitch at electric
field strengths ranging from 6 to 12 V/μm. The photoresponsivity reaches up to 2.3A/W towards blue wavelength of 468
nm at an electric field strength of 20 V/μm. Furthermore, the photocurrent has a fast speed of photoresponse, demonstrating rise time, fall time and time constant of 50 μs, 60 μs and 30 μs, respectively. Given that low dark current and high photoresponsivity this detector holds, coupled with fast photoresponse, it is believed that lateral a-Se MSM photodetector is promising for indirect conversion X-ray imager integrated with either CMOS or TFT arrays.
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