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.
Stabilized amorphous selenium (a-Se) is widely used for Digital Imaging Systems for medical applications
due to its ability to be manufactured over large areas and because of its capability of relatively high charge
collection efficiency. In this work we have shown the behavior of a gated selenium detector. A three-terminal
photodetector is presented where the photoconductive layer is deposited on two coplanar electrodes on a
corning glass substrate. These two coplanar electrodes act as the source and the drain. A third electrode is
deposited on top of the photoconductor layer, which functions as the gate terminal of the device. This work
investigates the feasibility of controlling the charge collection of a selenium-based photo-detector by applying
voltage at the gate terminal. Three different types of structures of the three-terminal device are presented. In
order to investigate the charge collection behavior both the thickness of the bulk and the distance between two
electrodes have been varied. 20, 70 and 200 - micron device thicknesses were chosen while the electrode
spacing was varied from 6 to 10 micron. In order to compare between the different types of devices a fixed
level of electric field at 2.5 V/micron was applied across the bulk of the device. Result shows that the gate
voltage can control the current-voltage transfer characteristics and is dependent on the incident radiation.
One of the two main advantages of the gated photodetectors is that they can be manufactured with a simple
fabrication process. The other advantage is that the gate voltage can control charge collection, which has the
potential for eliminating the thin film transistor (TFT) switches in large-area direct or indirect X-ray imaging.
Amorphous selenium photodetectors, in both lateral and vertical structures, have been investigated for indirect
conversion medical imaging applications. The low dark current, high responsivity for blue light, and possibility for gain
are all among the attractive features of these devices. Typically these devices are deposited on a solid glass substrate and
are read out using thin-film-transistors (TFTs). With the advent of flexible TFTs, flexible electronics have become a
viable technology. This technology may be leveraged for flexible imaging, however the detection and conversion
materials that serve to convert the incident x-rays to collectable charge will also need to be flexible. In this paper, we
investigate the use of amorphous selenium photodetectors on flexible substrates and compare their performance with
amorphous selenium photodetectors deposited on glass.
An indirect digital x-ray detector is designed, fabricated, and tested. The detector integrates a high speed, low noise
CMOS substrate with two types of amorphous semiconductors on the circuit surface. Using a laterally oriented layout
a-Si:H or a-Se can be used to coat the CMOS circuit and provide high speed photoresponse to complement the high speed
circuits possible on CMOS technology. The circuit also aims to reduce the effect of slow carriers by integrated a Frisch
style grid on the photoconductive layer to screen for the slow carriers. Simulations show a uniform photoresponse for
photons absorbed on the top layer and an enhanced response when using a Frisch grid. EQE and noise results are
presented. Finally, possible applications and improvements to the area of indirect x-ray imaging that are capable of easily
being implemented on the substrate are suggested.
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.
Digital imaging systems for medical applications use amorphous silicon thin-film transistor (TFT) technology due to its
ability to be manufactured over large areas. However, TFT technology is far inferior to crystalline silicon CMOS
technology in terms of the speed, stability, noise susceptibility, and feature size. This work investigates the feasibility of
integrating an imaging array fabricated in CMOS technology with an a-Se detector. The design of a CMOS passive pixel
sensor (PPS) array is presented, in addition to how an 8×8 PPS array is integrated with the 75 micron thick stabilized
amorphous selenium detector. A non-linear increase in the dark current of 200 pA, 500 pA and 2 nA is observed with
0.27, 0.67 and 1.33 V/micron electric field respectively, which shows a successful integration of selenium layer with the
CMOS array. Results also show that the integrated Selenium-CMOS PPS array has good responsivity to optical light and
X-rays, leaving the door open for further research on implementing CMOS imaging architectures going forward.
Demonstrating that the PPS chips using CMOS technology can use a-Se as a detector is thus the first step in a promising
path of research, which should yield substantial and exciting results for the field. Though area may still prove
challenging, larger CMOS wafers can be manufactured and tiled to allow for a large enough size for certain diagnostic
imaging applications and potentially even large area applications like digital mammography.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.