13,935 research outputs found
Evaluation of a Direct Detection Selenium-CMOS 8×8 Passive Pixel Sensor Array for Digital X-Ray Imaging Applications
Digital imaging systems for medical applications use amorphous silicon thin-film transistor (TFT) technology due to its ability to be manufactured over large areas, making it useful for X-ray imaging, which requires imagers to be the size of the subject, unlike optical imaging. TFT technology is used to make imaging arrays coated with an X-ray detector called amorphous selenium (a-Se), which can be grown easily over large areas by being evaporated on a substrate. However, TFT technology is far inferior to crystalline silicon CMOS technology in terms of the speed, stability, noise susceptibility, and feature size. Where CMOS technology falls short is its inability to be manufactured in large wafers at a competitive cost, allowing TFT technology to continue to be dominant in the medical imaging field, unlike the optical imaging industry.
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 it is integrated with the amorphous selenium detector. Results 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
a-Si:H-Silicon Hybrid Low Energy X-ray Detector
Low energy X-ray (< 20 keV) detection is a key technological requirement in applications such as protein crystallography or diffraction imaging. Silicon based optical cameras based on CCDs or CMOS imaging chips coupled to X-ray conversion scintillators have become a mainstay in the field. They are attractive because of fast readout capability and ease of integrated circuit implementation due to modern semiconductor fabrication technology. More recently, hydrogenated amorphous silicon (a-Si:H) thin film technology, that had enabled a huge influx of large area display products into the commercial display market, has been introduced to digital imaging in the form of active matrix flat panel imagers (AMFPIs). Although thin film technology can enable large area X-ray imaging at a potentially lower cost, the existing technology lacks spatial resolution requirements for higher performance crystallography and diffraction imaging applications.
This work introduces a high resolution direct conversion silicon X-ray detector integrated with large area thin film silicon technology for sub-20 keV photon X-ray imagers. A prototype pixel was fabricated in-house using a fabrication facility (G2N) utilizing plasma enhanced chemical vapor deposition (PECVD), reactive ion etching (RIE), photo-lithography, and metal sputtering technologies. Unlike most active matrix display products, top-gate staggered a-Si:H thin film transistor (TFT) were implemented to take advantage of a novel thin film silicon pixel amplification device architecture.
The detector performance was evaluated with an iron 55 isotope gamma ray source to mimic low energy X-ray exposure. I-V and C-V measurement techniques indicate that the hybrid pixel functions as expected and is promising for low cost, high resolution, large area X-ray imaging (< 20 keV) applications. We also performed a noise spectrum investigation to estimate the lowest detection signal level limit and proposed a model rooted in device physics for the pixel output and gain
Performance of a novel wafer scale CMOS active pixel sensor for bio-medical imaging
Recently CMOS Active Pixels Sensors (APSs) have become a valuable alternative to amorphous Silicon and Selenium Flat Panel Imagers (FPIs) in bio-medical imaging applications. CMOS APSs can now be scaled up to the standard 20 cm diameter wafer size by means of a reticle stitching block process. However despite wafer scale CMOS APS being monolithic, sources of non-uniformity of response and regional variations can persist representing a significant challenge for wafer scale sensor response. Non-uniformity of stitched sensors can arise from a number of factors related to the manufacturing process, including variation of amplification, variation between readout components, wafer defects and process variations across the wafer due to manufacturing processes. This paper reports on an investigation into the spatial non-uniformity and regional variations of a wafer scale stitched CMOS APS. For the first time a per-pixel analysis of the electro-optical performance of a wafer CMOS APS is presented, to address inhomogeneity issues arising from the stitching techniques used to manufacture wafer scale sensors. A complete model of the signal generation in the pixel array has been provided and proved capable of accounting for noise and gain variations across the pixel array. This novel analysis leads to readout noise and conversion gain being evaluated at pixel level, stitching block level and in regions of interest, resulting in a coefficient of variation ≤ 1.9%. The uniformity of the image quality performance has been further investigated in a typical X-ray application, i.e. mammography, showing a uniformity in terms of CNR among the highest when compared with mammography detectors commonly used in clinical practise. Finally, in order to compare the detection capability of this novel APS with the currently used technology (i.e. FPIs), theoretical evaluation of the Detection Quantum Efficiency (DQE) at zero-frequency has been performed, resulting in a higher DQE for this detector compared to FPIs. Optical characterization, X-ray contrast measurements and theoretical DQE evaluation suggest that a trade off can be found between the need of a large imaging area and the requirement of a uniform imaging performance, making the DynAMITe large area CMOS APS suitable for a range of bio-medical applications
Optical and mechanical properties of amorphous Mg-Si-O-N thin films deposited by reactive magnetron sputtering
In this work, amorphous thin films in Mg-Si-O-N system were prepared in order
to investigate the dependence of optical and mechanical properties on Mg
composition. Reactive RF magnetron co-sputtering from magnesium and silicon
targets were used for the deposition of Mg-Si-O-N thin films. Films were
deposited on float glass, silica wafers and sapphire substrates in an Ar, N2
and O2 gas mixture. X-ray photoelectron spectroscopy, atomic force microscopy,
scanning electron microscopy, spectroscopic ellipsometry, and nanoindentation
were employed to characterize the composition, surface morphology, and
properties of the films
On evolution of CMOS image sensors
CMOS Image Sensors have become the principal technology in majority of digital cameras. They started replacing the film and Charge Coupled Devices in the last decade with the promise of lower cost, lower power requirement, higher integration and the potential of focal plane processing. However, the principal factor behind their success has been the ability to utilise the shrinkage in CMOS technology to make smaller pixels, and thereby have more resolution without increasing the cost. With the market of image sensors exploding courtesy their inte- gration with communication and computation devices, technology developers improved the CMOS processes to have better optical performance. Nevertheless, the promises of focal plane processing as well as on-chip integration have not been fulfilled. The market is still being pushed by the desire of having higher number of pixels and better image quality, however, differentiation is being difficult for any image sensor manufacturer. In the paper, we will explore potential disruptive growth directions for CMOS Image sensors and ways to achieve the same
Design and Characterization of a High-resolution Cardiovascular Imager
Fluoroscopic imaging devices for interventional radiology and cardiovascular applications have traditionally used image-intensifiers optically coupled to either charge-coupled devices (CCDs) or video pick-up tubes. While such devices provide image quality sufficient for most clinical applications, there are several limitations, such as loss of resolution in the fringes of the image-intensifier, veiling glare and associated contrast loss, distortion, size, and degradation with time. This work is aimed at overcoming these limitations posed by image-intensifiers, while improving on the image quality. System design parameters related to the development of a high-resolution CCD-based imager are presented. The proposed system uses four 8 x 8-cm three-side buttable CCDs tiled in a seamless fashion to achieve a field of view (FOV) of 16 x 16-cm. Larger FOVs can be achieved by tiling more CCDs in a similar manner. The system employs a thallium-doped cesium iodide (CsI:Tl) scintillator coupled to the CCDs by straight (non-tapering) fiberoptics and can be operated in 78, 156 or 234-microns pixel pitch modes. Design parameters such as quantum efficiency and scintillation yield of CsI:Tl, optical coupling efficiency and estimation of the thickness of fiberoptics to provide reasonable protection to the CCD, linearity, sensitivity, dynamic range, noise characteristics of the CCD, techniques for tiling the CCDs in a seamless fashion, and extending the field of view are addressed. The signal and noise propagation in the imager was modeled as a cascade of linear-systems and used to predict objective image quality parameters such as the spatial frequency-dependent modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE). The theoretical predictions were compared with experimental measurements of the MTF, NPS and DQE of a single 8 x 8-cm module coupled to a 450-microns thick CsI:Tl at x-ray beam quality appropriate for cardiovascular fluoroscopy. The measured limiting spatial resolution (10% MTF) was 3.9 cy/mm and 3.6 cy/mm along the two orthogonal axes. The measured DQE(0) was ~0.62 and showed no dependence with incident exposure rate over the range of measurement. The experimental DQE measurements demonstrated good agreement with the theoretical estimate obtained using the parallel-cascaded linear-systems model. The temporal imaging properties were characterized in terms of image lag and showed a first frame image lag of 0.9%. The imager demonstrated the ability to provide images of high and uniform spatial resolution, while preserving and potentially improving on DQE performance at dose levels lower than that currently used in clinical practice. These results provide strong support for potential adaptation of this type of imager for cardiovascular and pediatric angiography
Determination of the detective quantum efficiency of a prototype, megavoltage indirect detection, active matrix flatâ panel imager
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134773/1/mp3516.pd
Substrate and Passivation Techniques for Flexible Amorphous Silicon-Based X-ray Detectors
abstract: Flexible active matrix display technology has been adapted to create new flexible photo-sensing electronic devices, including flexible X-ray detectors. Monolithic integration of amorphous silicon (a-Si) PIN photodiodes on a flexible substrate poses significant challenges associated with the intrinsic film stress of amorphous silicon. This paper examines how altering device structuring and diode passivation layers can greatly improve the electrical performance and the mechanical reliability of the device, thereby eliminating one of the major weaknesses of a-Si PIN diodes in comparison to alternative photodetector technology, such as organic bulk heterojunction photodiodes and amorphous selenium. A dark current of 0.5 pA/mm[superscript 2] and photodiode quantum efficiency of 74% are possible with a pixelated diode structure with a silicon nitride/SU-8 bilayer passivation structure on a 20 µm-thick polyimide substrate
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