Large-area X-ray Imaging Research Articles

Comparison of Semiconductor Radiation Detectors for Large Area X-Ray Imaging

We have developed a large area, flat panel detector for general applications to digital radiology. This paper presents the x-ray detection characteristics with various semiconductor radiation detectors (HgI2, PbI2, PbO, CdTe) derived by a novel wet coating process for large area deposition. The wet coating process could easily be made from large area films with printing paste mixed with semiconductor and binder material at room temperature. X-ray performance data such as dark current, sensitivity and signal to noise ratio (SNR) were evaluated. The HgI2 semiconductor was shown in much lower dark current than the others, and also has the best sensitivity. In this paper, reactivity and combination characters of semiconductor and binder material that affect electrical and x-ray detection properties would be verified through our experimental results.

Nanowire-based active matrix backplanes for the control of large area X-ray imagers

In digital X-ray sensors, pixel complexity is limited by the instabilities of amorphous silicon transistors or by the lack of homogeneity of their polycrystalline silicon counterparts. Here, we present a novel approach to the fabrication of thin-film transistors, based on the use of silicon nanowires grown in porous alumina templates. Transistors made from silicon nanowires are essentially studied for the post-MOS era and they exhibit excellent transport characteristics. The technology we propose is simple, as it only employs chemical vapour deposition processes for transistor fabrication. Also, as far as active matrix fabrication is concerned, a small number of masks is needed and we present a 5 mask process for the fabrication of an X-ray panel with pixel amplification.

Results on a prototype of a large-area X-ray imaging device using CMOS hybrid detectors

This paper presents the first results on a prototype of a large-area X-ray imaging device made out of hybrid CMOS pixel detectors. The challenges of manufacturing and implementing imaging devices on an area larger than the single component size, with a seamless sensitive area, are addressed via a preliminary evaluation of the images. A sensitive area of approximately 6×3 cm 2 was built with eight single ASIC chips performing photon counting and bump-bonded to two high-resistivity p–n silicon sensors working in a reverse bias mode. Each chip consists of 256×256 identical square pixels of 55 μm side. The image delivered is a 1024×512-pixel matrix. Dedicated read-out electronics, software and mechanical supports have been developed. We report and discuss the challenges of the system in terms of the resulting quality of static images obtained with a 70 kV X-ray tube.

Large area x-ray and neutron imaging using three-dimensional arrays of microlenses

Linear arrays of biconcave microlenses have been shown to be capable of imaging small objects using either x rays or neutrons. Because these lenses have small apertures and finite lengths, they are limited in their field of view (FOV). To increase the FOV, we propose that two sets of three-dimensional arrays of these microlenses be used. The spacing of the microlenses is calculated to achieve a complete image with uniform brightness. General design criteria are discussed in situations where either a one-to-one image or a magnified image is required.

High voltage amorphous silicon TFT for use in large area applications

Previous work in high voltage amorphous silicon (a-Si) TFTs (HVTFTs) using an n+ μC–Si ohmic contact layer demonstrated that the soft contact TFT (SCTFT) design was a requirement for high voltage operation. In this research, conventional and high voltage TFT designs including the SCTFT are presented using an n+ a-Si contact layer. TFT ON-current measurements and series resistance extractions show that the conventional TFT performs equally well, if not better, at low and high voltages in comparison to all of the HVTFTs fabricated. The results indicate that the conventional space-efficient TFT design with an n+ a-Si contact layer is realizable for high fill-factor, high voltage applications such as direct detection, large area X-ray imaging. Also, the process reliability and performance for both conventional and HVTFT arrays can be improved for large area applications by the inclusion of an additional a-SiN layer.

Micro-electro-mechanical system fabrication technology applied to large area x-ray image sensor arrays

Micromachining has potential applications for large area image sensors and displays, but conventional micro-electro-mechanical system (MEMS) technology based on crystalline silicon wafers cannot be used. Instead, large area devices use deposited films on glass substrates. This presents many challenges for MEMS, both with regard to materials for micromachined structures and to integration with large area electronic devices. We are exploring the novel thick photoresist SU-8 as well as plating techniques for the fabrication of large area MEMS. As an example of its application, we have employed this MEMS technology to improve the performance of an amorphous silicon based x-ray image sensor array. SU-8 is explored as the structural material for the x-ray conversion screen and as a thick interlayer dielectric for the thin film readout electronics of the imager. Plating techniques are employed to metallize the deep contact vias in SU-8. Processing challenges that are particularly important for large area fabrication will be addressed.

Thin-film CdTe for imaging detector applications

Using the method of electrodeposition from aqueous electrolyte, we are able to grow thin films of CdTe as the basis of a direct semiconductor converter for large area X-ray imaging applications. This growth technique has the advantage of operating at low temperature with relatively low current densities and is therefore compatible, in principle, with a large-area hydrogenated amorphous silicon active matrix readout. Such a hybrid detector could prove to be an excellent optical sensor in the near term, and in the long-term advances in growth techniques may allow large area X-ray sensors to be fabricated. Currently, we are able to grow films to a thickness of 1 μm with reasonable mechanical quality. Problems that we have been identified in growing CdTe films include achieving good substrate adhesion and maintaining good stoichiometry over the full substrate area. Results will be presented to describe our approach to CdTe film growth and to show how film quality depends on both deposition time and electrolyte stirring rate.

An amorphous silicon thin-film transistor with fully self-aligned top gate structure

We have developed a novel fully self-aligned top gate amorphous silicon thin-film transistor, which shows excellent transistor characteristics. Self-alignment is achieved by patterning the gate electrode and then etching the silicon nitride gate insulator, followed by silicidation and ion implantation of the exposed a-Si in the contact regions. We obtain a long channel saturated mobility of 0.9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V/sup -1/ s/sup -1/, while for channel lengths of 6 μm, we obtain an effective mobility of 0.6 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V/sup -1/ s/sup -1/, in the saturated region and 0.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V/sup -1/ s/sup -1/, in the linear region. This high level of performance, together with the negligible parasitic capacitance of the self-aligned structure, makes this transistor suitable for new demanding applications in active matrix liquid crystal displays and large area X-ray image sensors.

Intrinsic thin film stresses in multilayered imaging pixels

When thin film devices are integrated in display or imaging arrays, the intrinsic mechanical stresses in the films can be severe enough to cause the films to either crack or even peel off the underlying substrate layer. This is a particular concern in large area x-ray imaging applications based on molybdenum/amorphous silicon (Mo/a-Si:H) Schottky diodes. This family of image sensors can be used for direct detection of x rays without the need for a phosphor layer [W. Zhao and J. A. Rowlands, Med. Phys. 22, 1595 (1995)]. In these sensors, the interaction of x-ray photons with Mo leads to injections of high energy electrons into a reverse-biased a-Si:H depletion layer, thereby producing an amplified output signal due to charge multiplication. The generated charge is read out by a thin film transistor (TFT) which is composed of intrinsic amorphous silicon (i-a-Si:H) as the active channel layer, silicon nitride (a-SiNx) as the insulating layer, and highly doped microcrystalline silicon (n+μc-Si:H) as the contact layer. For example, the i-a-Si:H layer is in compressive stress with a magnitude of 0.116 GPa, whereas low temperature, low resistivity n+μc-Si:H layer is in compressive stress of 1.47 GPa. The a-SiNx layer is in tensile stress of 0.222 GPa. The heavy metals used in the x-ray detector, such as Mo and chromium (Cr) (deposited at 5 mTorr), are found to be in tensile stress of 0.807 and 0.575 GPa, respectively. This article presents the internal mechanical stresses in thin film devices used in x-ray imaging, and more importantly, how the total film stress can be reduced without undermining device performance.

ITO/a-SiNx:H/a-Si:H Photodiode with Enhanced Photosensitivity and Reduced Leakage Current Using Polycrystalline ITO Deposited at Room Temperature

ABSTRACTWe report an ITO/a-SiNx:H/a-Si:H MIS photodiode structure based on room temperature deposition of optically transparent polycrystalline ITO for applications in large area optical and x-ray imaging. The photodiode structure exhibits device characteristics with reduced leakage current and enhanced photosensitivity giving rise to a hundred-fold improvement in dynamic range. This notable improvement in performance is believed to be due to the reduced diffusion of oxygen from the ITO to the a-Si:H layer, and thus reducing the density of defect states inside the a-Si:H layer. The behavior of photo and dark current is consistent with an elaborate transport model for the Schottky barrier. The model agrees reasonably well with measurement data for the dark current and provides a consistent picture in terms of the photo current behavior in the MIS structure, where the insulating layer serves to reduce the oxygen diffusion.

Operation of a separated-type x-ray interferometer for phase-contrast x-ray imaging

Aiming at large-area phase-contrast x-ray imaging, a separated-type x-ray interferometer system was designed and developed to produce 25×20 mm interference patterns. The skew-symmetric optical system was adopted because of the feasibility of alignment. The rotation between the separated crystal blocks was controlled within a drift of 0.06 nrad using a feedback positioning system. This interferometer generated a 25×15 mm interference pattern with 0.07 nm synchrotron x-rays. A slice of a rabbit’s kidney was observed, and its tubular structure could be revealed in a measured phase map.

Electronic transport in polycrystalline Pbl2 films

Electronic transport and optical measurements in polycrystalline Pbl2 are reported as part of a study to evaluate the material for large area x-ray imaging applications. The films are deposited by vacuum evaporation with thickness 20–100 μm and have grain sizes of up to 10 μm. The room temperature hole drift mobility measured by time-of-flight is 2×10−2–1.5×10−1 cm2/V s, depending on the specific sample, with an activation energy of 0.25 eV. Hole charge collection measurement gives about 10−6 cm2/V for the mobility–lifetime product. Details of the electron transport were not determined in this study because the mobility is too small. The hole transport is discussed in terms of a trapping model with either a discrete level above the valence band or a disorder-induced band tail. Optical absorption, photoconductivity, and Hall effect measurements are also reported.

Reverse current instabilities in amorphous silicon Schottky diodes: modeling and experiments

A new model for high bias transport is reported which describes the time-dependent reverse current variations in amorphous silicon Schottky diodes. This phenomenon is of practical importance in the design and optimization of pixels for large-area optical and X-ray imaging. In the model, the main components of the reverse current, namely thermionic emission and tunneling, are both affected by the electric field at the metal/amorphous silicon interface. Time-dependent variations in this electric field arise due to the release of charges trapped in defect states in the depletion region and to charge trapping at the interface. This effect is analyzed using the approximation that the tunneling component of the current is equivalent to a lowering of the potential barrier at the interface. The calculated time-dependent reverse current is compared with the measured data.

Recent developments and applications of fast position-sensitive gas detectors

The introduction, 30 years ago, of the multiwire proportional chamber initiated a very active and fruitful period of development of fast gas detectors. Performing position-sensitive devices have been perfected, for the needs of elementary particle physics and for applications in medical diagnostics, biology, material analysis. The high rate performance of wire counters, limited by positive ions accumulation, was largely improved with the introduction of the micro-strip gas chamber, capable of achieving position accuracies of few tens of microns at radiation fluxes exceeding 1 MHz/mm 2. The micro-strip chamber properties have been extensively studied in view of large scale use in high luminosity experiments; some interesting applications in other fields will be described here. Originally conceived as a gain booster to solve reliability problems met with micro-strips, the gas electron multiplier was invented about a year and a half ago. Progress made with high gain models is leading to a new concept in gas detectors, powerful yet cheap and reliable. Possible developments and applications will be discussed: large area position-sensitive photo detectors and X-ray imagers, including devices with non-planar geometry suited to spectrometers and crystal diffraction studies.

Stability of Amorphous Silicon Thin Film Transistors for Analog Circuit Applications

ABSTRACTAnalog circuits using amorphous silicon thin film transistors offer significant advantages for in situ signal processing in large-area optical and x-ray imagers. However such circuits are susceptible to gate-bias-induced shifts in the threshold voltages of the constituent transistors. In this work, the change of threshold voltage for devices undergoing cycles of stress, relaxation and reverse bias is measured in order to determine the feasibility of resetting the threshold voltage electrically. It is concluded that, although the reverse bias does assist the recovery of the threshold voltage, the process is still not sufficiently rapid. An analog amplifier circuit is then described which uses negative feedback to achieve a gain that is stable to within 6% over a period of 8 hours.