Detectors fabricated with SI-GaAs and Si bulk material were bonded to Photon Counting Chips (PCC), developed in the framework of the MEDIPIX Collaboration. The PCC consists of a matrix of 64×64 identical square pixels (170 μm×170 μm) with a 15-bit counter in each cell. We investigated the imaging properties of these detector systems under exposure of a dental X-ray tube at room temperature. The image homogeneity and the mean count rate were determined via flood exposure images and compared. Exposures for GaAs detectors exhibit a 3 times larger spread in count rate per image in comparison to Si detectors. This also results in a 3 times worse signal to noise ratio. IV -characteristics and X-ray images at different values of the detectors bias voltage were also taken and show a 30 times higher leakage current for GaAs. The Si detector is fully active beginning from 70 V, whereas the GaAs detector does not reach full charge collection. The presampling modulation transfer function of both assembly types was measured via slit images and gives a spatial resolution of 4.3 lp/mm for both detector systems.

Measurements with Si and GaAs pixel detectors bonded to photon counting readout chips

A hybrid active pixel sensor (HAPS) consists of an array of sensing elements which is connected to an electronic read-out unit. The most used way to connect these two different devices is bump bonding. This interconnection technique is very suitable for these systems because it allows a very fine pitch and a high number of I/Os. However, there are other interconnection techniques available such as direct bonding. This paper, as a continuation of a review [M. Lozano, E. Cabruja, A. Collado, J. Santander, M. Ullan, Nucl. Instr. and Meth. A 473 (1–2) (2001) 95–101] published in 2001, presents an update of the different advanced bonding techniques available for manufacturing a hybrid active pixel detector.

Technical reviews Bonding techniques for hybrid active pixel sensors (HAPS)

This article describes the theoretical basis, design and implementation of X-ray microdetectors based on scintillating materials and CMOS technology. The working principle of such microdetectors consists in the absorption of X-rays by scintillators, which produce visible light. The visible light is then detected and converted into electric signals by means of photodetectors. In order to understand such detectors, several issues related to its implementation are presented in this article, namely: Production of X-rays and interaction between them and matter - the first step necessary to the detection of X-rays is that they must be absorbed by some material, in this case by a scintillator; Radiation detectors - there are several types of detectors, namely: pn junctions, photoconductors, based on thermal effects and scintillators; Fabrication of scintillator arrays - after the X-ray radiation is absorbed by a scintillator, this material emits visible light whose intensity is proportional to the total energy of the absorbed X-rays; Optical interfaces between scintillators and photodetectors - the visible light generated by scintillators must arrive to the photodetectors, so, it is necessary to have an interface between the scintillators and the photodetectors that ideally does not introduce losses; Photodetectors and interface electronics - the visible light is absorbed by the photodetectors and converted into electrical signals, which are finally converted into digital images by means of interface electronics. The article presents some promising patents on X-ray detectors based on scintillators and CMOS technology.

/pdf/review-on-x-ray-detectors-based-on-scintillators-and-cmos-4fi1ig5wsx.pdf

Review on X-ray Detectors Based on Scintillators and CMOS Technology

A digital mammography system based on a GaAs pixel detector has been developed by the INFN (Istituto Nazionale di Fisica Nucleare) collaboration MED46. The high atomic number makes the GaAs a very efficient material for low energy X-ray detection (10-30 keV is the typical energy range used in mammography). Low contrast details can be detected with a significant dose reduction to the patient. The system presented in this paper consists of a 4096 pixel matrix built on a 200 /spl mu/m thick semi-insulating GaAs substrate. The pixel size is 170/spl times/170 /spl mu/m/sup 2/ for a total active area of 1.18 cm/sup 2/. The detector is bump-bonded to a VLSI front-end chip which implements a single-photon counting architecture. This feature allows to enhance the radiographic contrast detection with respect to charge integrating devices. The system has been tested by using a standard mammographic tube. Images of mammographic phantoms will be presented and compared with radiographs obtained with traditional film/screen systems. Monte Carlo simulations have been also performed to evaluate the imaging capability of the system. Comparison with simulations and experimental results will be shown.

Low contrast imaging with a GaAs pixel digital detector

The performance of hybrid GaAs pixel detectors as X-ray imaging sensors were investigated at room temperature. These hybrids consist of 300 μm thick GaAs pixel detectors, flip-chip bonded to a CMOS Single Photon Counting Chip (PCC). This chip consists of a matrix of 64 × 64 identical square pixels (170 μm × 170 μm) and covers a total area of 1.2 cm 2 . The electronics in each cell comprises a preamplifier, a discriminator with a 3-bit threshold adjust and a 15-bit counter. The detector is realized by an array of Schottky diodes processed on semi-insulating LEC-GaAs bulk material. An IV-characteristic and a detector bias voltage scan showed that the detector can be operated with voltages around 200 V. Images of various objects were taken by using a standard X-ray tube for dental diagnostics. The signal to noise ratio (SNR) was also determined. The applications of these imaging systems range from medical applications like digital mammography or dental X-ray diagnostics to non destructive material testing (NDT). Because of the separation of detector and readout chip, different materials can be investigated and compared.

X-ray imaging using a hybrid photon counting GaAs pixel detector

We present room temperature measurements on 200 /spl mu/m thick GaAs pixel detectors, which were hybridized to silicon readout circuits. The whole detector array contains 320/spl times/240 square shaped pixel with a pitch of 38 /spl mu/m and is based on semi-insulating liquid-encapsulated Czochralski (LEC) GaAs material. After fabricating and dicing, the detector chips were indium bump flip chip bonded to CMOS readout circuits based on charge integration and finally evaluated. This readout chip was originally designed for the readout of flip chip bonded infrared detectors, but appears to be suitable for X-ray applications as well. A bias voltage between 50 V and 100 V was sufficient to operate the detector at room temperature. The detector array did respond to X-ray radiation by an increase in current due to production of electron hole pairs by the ionization processes. Images of various objects and slit patterns were acquired by using a standard X-ray source for dental imaging. The new X-ray hybrid detector was analyzed with respect to its imaging properties. Due to the high absorption coefficient for X-rays in GaAs and the small pixel size, the sensor shows a high modulation transfer function up to the Nyquist frequency.

X-ray imaging using a 320/spl times/240 hybrid GaAs pixel detector

X-ray imaging using a 320×240 hybrid GaAs pixel detector

We report on gain and offset corrections for GaAs X-ray pixel detectors, which were hybridised to silicon CMOS readout integrated circuits (ROICs). The whole detector array contains 320×240 square-shaped pixels with a pitch of 38 μm. The GaAs pixel detectors are based on semi-insulating and VPE grown substrates. The ROIC operates in the charge integration mode and provides snapshot as well as real time video images. Previously we have reported that the image quality of semi-insulating GaAs pixel detectors suffer from local variations in X-ray sensitivity. We have now developed a method to compensate for the sensitivity variations by applying suitable offset and gain corrections. The improvement in image quality is demonstrated in the measured signal-to-noise ratio of flood exposure images.

320×240 GaAs pixel detectors with improved X-ray imaging quality

Infrared pixel detectors sensitive to radiation in the wavelength range 8 to 9 µm have been fabricated and evaluated. The detectors have 320 × 240 pixels and consist of two parts. One is the infrared sensitive part with a quantum well structure based on n-doped AlGaAs/GaAs. The other part is a readout circuit in standard CMOS technology. The two parts are hybridized by a flip-chip technique using indium bumps. Optical coupling into the detectors is performed by using optimized, etched, two-dimensional gratings combined with GaAs substrate thinning down. The thinning removes the problem with thermal mismatch, enhances the absorption and eliminates optical crosstalk between pixels. The intended operating temperature range is 70–73 K, achievable by a miniature Stirling cooler. Excellent wafer uniformities resulting in responsivity uniformities of 3.3% across an array are found, and a temperature resolution NETD (noise equivalent temperature difference) = 20 mK is achieved.

https://iopscience.iop.org/article/10.1238/Physica.Topical.079a00138/pdf

A 320 × 240 pixels quantum well infrared photodetector array for thermal imaging

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X-ray imaging using a 320/spl times/240 hybrid GaAs pixel detector

X-ray imaging using a 320×240 hybrid GaAs pixel detector

320×240 GaAs pixel detectors with improved X-ray imaging quality

A 320 × 240 pixels quantum well infrared photodetector array for thermal imaging