Medipix2 Readout Chip Research Articles

A Piggyback resistive Micromegas

A novel read-out architecture has been developed for the Micromegas detector. The anode element is made of a resistive layer on a ceramic substrate. The detector part is entirely separated from the read-out element. Without significant loss, signals are transmitted by capacitive coupling to the read-out pads. The detector provides high gas gain, good energy resolution and the resistive layer assures spark protection to the electronics. This assembly could be combined with modern pixel array electronic ASICs. This readout organization is free on how the pixels are designed, arranged and connected. We present first results taken with a Medipix2 read-out chip.

Characterization of edgeless pixel detectors coupled to Medipix2 readout chip

VTT has developed a straightforward and fast process to fabricate four-side buttable (edgeless) microstrip and pixel detectors on 6 in. (150 mm) wafers. The process relies on advanced ion implantation to activate the edges of the detector instead of using polysilicon. The article characterizes 150 μm thick n-on-n edgeless pixel detector prototypes with a dead layer at the edge below 1 μm. Electrical and radiation response characterization of 1.4×1.4 cm 2 n-on-n edgeless detectors has been done by coupling them to the Medipix2 readout chips. The distance of the detector's physical edge from the pixels was either 20 or 50 μm. The leakage current of flip-chip bonded edgeless Medipix2 detector assembles were measured to be ∼90 nA/cm 2 and no breakdown was observed below 110 V. Radiation response characterization includes X-ray tube and radiation source responses. The characterization results show that the detector's response at the pixels close to the physical edge of the detector depend dramatically on the pixel-to-edge distance.

MAXIPIX, a fast readout photon-counting X-ray area detector for synchrotron applications

A 2D photon-counting X-ray detector system with 1.4 kHz frame rate and 55 μm spatial resolution has been developed and commissionned on ESRF beamlines. The system called MAXIPIX (Multichip Area X-ray detector based on a photon-counting PIXel array) consists of a detector module implementing up to five MEDIPIX-2 or TIMEPIX photon-counting readout chips, a custom readout interface board and a Linux acquisition workstation. The detector module readout time is 290 microseconds, allowing the system to achieve sustained frame rates of 280 Hz to 1400 Hz depending on the number of connected chips. An effective time resolution of 60 ns was measured using the ESRF pulsed modes and a TIMEPIX module. The system architecture and characteristics are presented, as well as a summary of its applications on ESRF beamlines.

Pixel sensitivity variations in a CdTe-Medipix2 detector using poly-energetic x-rays

We have a 1-mm-thick cadmium telluride (CdTe) sensor bump-bonded to a Medipix2 readout chip. This detector has been characterized using a poly-energetic x-ray beam. Open beam images (i.e. without an attenuating specimen between the x-ray source and the detector) have been acquired at room temperature using the MARS-CT system. Profiles of various rows and columns were analyzed for one hundred, 35-ms exposures taken with a bias voltage of -300 V (operating in electron collection mode). A region of increased sensitivity is observed around the edges of the detector. A reasonably periodic, repeatable variation in pixel sensitivity is observed. Some small regions with very low sensitivity and others with zero signals are also observed. Surrounding these regions are circular rings of pixels with higher counts. At higher flux (higher tube current in the x-ray source) there is evidence of saturation of the detector assembly. In this paper we present our understanding of the origin of these features and demonstrate the improved image quality obtained after correcting for these variations.

Pixelized M-π-n CdTe detector coupled to Medipix2 readout chip

We have realized a simple method for patterning an M-π-n CdTe diode with a deeply diffused pn-junction, such as indium anode on CdTe. The method relies on removing the semiconductor material on the anode-side of the diode until the physical junction has been reached. The pixelization of the p-type CdTe diode with an indium anode has been demonstrated by patterning perpendicular trenches with a high precision diamond blade and pulsed laser. Pixelization or microstrip pattering can be done on both sides of the diode, also on the cathode-side to realize double sided detector configuration.The article compares the patterning quality of the diamond blade process, pulsed pico-second and femto-second lasers processes. Leakage currents and inter-strip resistance have been measured and are used as the basis of the comparison. Secondary ion mass spectrometry (SIMS) characterization has been done for a diode to define the pn-junction depth and to see the effect of the thermal loads of the flip-chip bonding process. The anode and cathode-sides of a 6 × 6 mm2 diodes were patterned with a diamond blade and flip-chip bonded to the Medipix2 readout chips. First imaging results with an X-ray source show reduced polarization effect and edgeless detector behavior for the anode-side patterned detector.

Direct growth of CdTe thick films on the Medipix2 pixel detector chip

Polycrystalline CdTe layers were directly deposited on the Medipix2 readout chip by physical vapor deposition using a Riber molecular beam epitaxy 2300. Sb2Te3 was evaporated as backside contact material, resulting in an M-π-p diode structure comprising the pixel contact material, the intentionally undoped CdTe, and Sb2Te3. It was possible to achieve x-ray images at 60 kVp. In addition, thermally oxidized Si wafers were coated with a comparable M-π-p structure grown under identical conditions, to investigate electrical properties of the films. The specific resistance of the layers was found to be 3.85×108 Ω cm.

Processing and characterization of a MEDIPIX2-compatible silicon sensor with 220 μm pixel size

Pixellated silicon detectors with a pixel size of 220 μm have been fabricated at Mid-Sweden University. The purpose was to make a detector compatible with the MEDIPIX2 readout chip with pixels large enough to avoid charge sharing to do spectral imaging. Two different guard ring structures have been tested to investigate leakage current and interpixel isolation in a sensor made on high resistivity silicon.

Spectral response of a silicon detector with 220 μm pixel size bonded to MEDIPIX2

Pixellated radiation detectors with single photon processing can be used for spectral X-ray imaging. A problem using such detectors with small pixels is that the spectral information is distorted by charge sharing. In order to get images with good spectral resolution a number of silicon sensors with a pixel size of 220 μm were fabricated and bonded to a MEDIPIX2 readout chip using only a limited number of pixels on the readout chip. The device was then used in an X-ray microscopy setup to obtain good spatial resolution as well. It is shown that spectral imaging can provide good contrast images of embedded structures by selecting an appropriate energy window.

3D Medipix2 detector characterization with a micro-focused X-ray beam

Three-dimensional (3D) photodiode detectors offer advantages over standard planar photodiodes in a wide range of applications. The main advantage of these sensors for X-ray imaging is their reduced charge sharing between adjacent pixels, which could improve spatial and spectral resolution. However, a drawback of 3D sensors structures is the loss of detection efficiency due to the presence in the pixel structure of heavily doped electrode columns which are insensitive to X-ray. In this work two types of 3D silicon detectors: n-type wafer with hole collecting readout-columns ( n-type) and p-type wafer with electron collecting readout-columns ( p-type), bump-bounded to a Medipix2 read-out chip were characterized with a 14.5 keV micro-focused X-ray beam from a synchrotron. Measurements of the detection efficiency and the charge sharing were performed at different bias voltages and Medipix2 energy thresholds and compared with those of a standard planar silicon sensor.

Study of the charge sharing in silicon pixel detector by means of heavy ionizing particles interacting with a Medipix2 device

The charge sharing effect has been investigated in a 300 μm thick silicon pixel detector bump-bonded to a Medipix2 readout chip and exposed to heavy ionizing particles of various energies, namely protons and alpha-particles. The lateral spread of the charge carriers from the interaction of a heavy ionizing particle in the silicon detector causes a sharing of the charge among adjacent pixels, giving a signal in clusters of multiple pixels of the Medipix2 device. A model of charge sharing, including the plasma effect on top of diffusion and drift processes has been compared to the measured lateral spread of the charge for bias voltage values beyond the full depletion voltage. Results of the measurements of the cluster size as a function of the energy and incidence angle of incoming heavy ionizing particles are also reported and compared with the model.

Characterisation of a GaAs(Cr) Medipix2 hybrid pixel detector

A semi-insulating GaAs(Cr) sensor of 300 μm thickness with a bulk resistivity of 10 9 Ω cm and no guard ring structures has been bump bonded to a Medipix2 readout chip. The room temperature spectroscopic response of this device to fluorescence photons in the energy range from 8 to 28 keV at different sensor biases is measured and compared to the response of a reference detector with a 300 μm thick Si sensor. The measured responses show that the full volume of the GaAs(Cr) is active and indicate a charge collection efficiency of 90%. The spatial resolution if this device is 8 lp/mm.

10 years of the Medipix2 Collaboration

The Medipix2 Collaboration was started officially in September 1999 with the aim of disseminating hybrid pixel detector technology from High Energy Physics to other fields. The Collaboration was initially composed of 13 European research institutes. Over the ensuing 10 years the Collaboration expanded to reach a peak of 17 member institutes. Although our main scientific focus has been the development of the Medipix2 and Timepix single photon counting pixel detector readout chips the Collaboration members have expanded the range of applications for the technology to many more scientific fields than initially foreseen. We have signed a number of Technology Transfer Agreements during that time, most notably with PANalytical, whose commercially available PIXcel detector is based on the second version of the Medipix2 chip. This paper will review the history of the Collaboration covering as much as possible the main technical highlights. The success of the Collaboration is testimony to the willingness of a large number of groups and individuals to pool efforts for a common purpose. The paper will also cover some of those aspects and summarize the lessons learnt.

X-ray detection with 3D Medipix2 devices

A set of double-sided 3D silicon detectors have been fabricated, bump bonded to single-photon-counting Medipix2 readout chips, and tested using X-ray sources. When tested with monochromatic X-rays, the 3D detectors show reduced charge sharing compared with standard planar pixel detectors

Patterning thick diffused junctions on CdTe

Dividing the detector crystal into discrete pixels enables making an imaging detector, in which the charge collected by each pixel can be read separately. Even if the detector is not meant for imaging, patterns on the crystal surface may be used as guard structures that control and limit the flow of charges in the crystal. This has been exceedingly hard for the detector crystals having thick diffused layers. The paper reports a patterning method of the thick diffused junctions on CdTe. The patterning method of In-diffused pn-junction on CdTe chip is demonstrated by using a diamond blade. The patterning is done by removing material from the pn-junction side of the chip, so that the trenches penetrate the diffused layer. As the trenches extend deeper into the bulk than the junction, the regions separated by the trench are electrically isolated. Electrical characterization results are reported for the strips separated by trenches with various depths. The strip isolation is clearly seen in both measured leakage currents and inter-strip resistances. The paper also represents the first images of the pixelated CdTe chips bump bonded to the Medipix2 readout chip. The images were taken with the Sr90 source. The CdTe chips had 110 μm×110 μm and 55 μm×55 μm pixel grid spacing. The images demonstrate the imaging capability of the pixelated chips and give an idea on the uniformity and quality of the pixelization method.

The RELAXd project: Development of four-side tilable photon-counting imagers

The feasibility of using photon-counting imaging techniques with X-rays, neutrons or other types of radiation for many applications in materials analysis and bio-medical sciences has already been well demonstrated. A hybrid imager consisting of an appropriate sensor chip flip-chipped on top of a Medipix2 readout ASIC is an example of such a device. It can count single X-ray photons, without any noise or dark current, at high fluxes (several Gigaphotons per cm 2 per second). The limiting factors for more widespread usage of these devices in bio-medical applications (e.g. mammography or small animal imaging) are the small size of the active area (about 2 cm 2 per chip) and the low frame rate. The aim of the RELAXd project (high REsolution Large Area X-ray Detector) is to develop a high frame-rate, fully 3D-integrated microsystem, consisting of four Medipix2 readout chips bump-bonded to one silicon sensor chip forming the basic building module. This paper presents the first results of this task focusing on the used wafer-level postprocessing technologies which are needed to achieve the 3D architecture, required for four-side tiling.

Characterisation of an epitaxial GaAs/Medipix2 detector using fluorescence photons

A high-purity GaAs sensor of 110 μm thickness has been bump bonded to a Medipix2 readout chip. The room temperature spectroscopic response of this device to fluorescence photons in the energy range from 8 to 28 keV is presented and compared to the response of a 300 μm thick Si sensor, also bonded to a Medipix2 chip. The measured photopeak responses are used to calibrate both detectors. The depth of depletion of the GaAs sensor is estimated to be ∼50 μm at 140 V sensor bias voltage from measurements made using the 8 keV Kα line of a Cu target X-ray tube.

A pixel detector-based single photon-counting system as fast spectrometer for diagnostic x-ray beams

Recent advances in semiconductor pixel detectors and read-out electronics allowed to build the first prototypes of single photon-counting imaging systems that represent the last frontier of digital radiography. Among the advantages with respect to commercially available digital imaging systems, there are direct conversion of photon energy into electrical charge and the effective rejection of electronic noise by means of a thresholding process. These features allow the photon-counting systems to achieve high imaging performances in terms of spatial and contrast resolution. Moreover, the now available deep integration techniques allow the reduction of the pixel size and the improvement of the functionality of the single cell and the read-out speed so as to cope with the high fluxes found in diagnostic radiology. In particular, the single photon-counting system presented in this paper is based on a 300-microm thick silicon pixel detector bump-bonded to the Medipix2 read-out chip to form an assembly of 256 x 256 square pixels at a pitch of 55 microm. Each cell comprises a low-noise preamplifier, two pulse height discriminators and a 14-bit counter. The maximum counting rate per pixel is 1 MHz. The chip can operate in two modalities: it records the events with energy above a threshold (single mode) or between two energy thresholds (window mode). Exploiting this latter feature, a possible application of such a system as a fast spectrometer is presented to study the energy spectrum of diagnostic beams produced by X-ray tubes.

Characterisation of a semi 3-D sensor coupled to Medipix2

A 300 μ m thick semi-3D Si sensor has been characterised after bump-bonding to a Medipix2 read-out chip. The bonding quality measured using a 90Sr-source was found to be excellent ( > 99.9 % ) . Comparative measurements with respect to a standard planar 300 μ m Si sensor comprising IV-curves, depletion voltage and energy resolution have been performed. IV-curves and depletion voltages correspond to the values expected from the sensor geometry. The depletion voltage of the semi-3D sensor is significantly lower than the one of the planar sensor. The energy resolution of the semi-3D device has been found to be lower. The uniformity of the pixel response has been measured using a pulsed 1060 nm laser.

Characterization of a high-resolution CT scanner prototype for small animals

— At the Department of Physics of the University of Pisa and INFN of Pisa, we have assembled a high-resolution CT scanner prototype for small animals. The prototype will be integrated with the YAP-(S)PET scanner, in order to realize a combined PET\SPECT\CT scanner for small animals. We have characterized the microCT scanner prototype in terms of spatial resolution and noise using three different X-ray detectors. The first detector (Hamamatsu S7199) is composed by a CsI:Tl layer coupled to a CCD via a 1:1 fiber optic plate; the second detector (Radicon Rad-Eye 4) consists in a flat-panel CMOS sensor, directly coupled to a Ga2O2S:Tb scintillator; the third detector is a 1mm thick Si detector, bump-bonded to the Medipix2 readout chip. The first two detectors are based on indirect conversion of X-rays and operate in charge integration mode, while the third is a direct conversion detector and operates in photon counting mode. The characterization was carried out using dedicated phantoms for the measurement of spatial resolution and image noise. The results obtained with the three detectors are compared, in order to evaluate the differences in tomographic performances due to the different characteristics of each detector.

Microscopic imaging of muons and [formula omitted] pion interactions in a single, low-noise [formula omitted] Si pixel detector

With a 256 × 256 pixel matrix on a 300 μ m thick silicon sensor, placed parallel to the particles in the CERN H6 beam, it proves possible to record electronically in real time the microscopic details of particle trajectories and interactions. The Medipix2 readout chip, matched to this matrix of 55 μ m square pixels, contains a tuneable discriminator and a pulse counter in each pixel. The noise of the signal processing chain in each pixel is on average 135 e - (equivalent electrons) r.m.s. A threshold not much higher than 800 e - allows discrimination of full signals ∼ 3800 e - as well as partial signals from a minimum ionizing particle in adjacent 55 μ m thick pixels. With binary pixel information, exploiting charge diffusion and redundancy in the large matrix, the vectors of trajectories can be reconstructed with angular accuracy < 1 mrad and positions with respect to the detector coordinates often with sub- μ m precision. Close tracks can be resolved down to 100 μ m distance. The width of the trail in the matrix sometimes can provide information on the energy deposition as well. A variety of applications can be imagined, the more so if several such detectors could be stacked to create a true three-dimensional position-sensitive volume.