V. O'Shea

A preliminary study of GaAs solid-state detectors for high-energy physics

Abstract The first phase of a study of GaAs as a base material for solid-state detectors has been completed. The main motivation behind this study is the greater radiation resistance of integrated circuits made of GaAs (compared with Si). Many diodes, of different sizes and shapes but built with the same technique, have been tested electrically and as detectors, using α sources and minimum-ionizing particles. The tests show that these devices work with a full detection effeciency, although there is evidence for trapping of a fraction of the charge produced by the particle inside the semiconductor.

GaAs solid state detectors for particle physics

Abstract We report on progress with Schottky diode and p-i-n diode GaAs detectors for minimum ionising particles. The radiation hardness and potential speed of simple diodes is shown to be more than competitive with silicon detector. A discussion is given of the present understanding of the charge transport mechanism in the detectors since it influences their charge collection efficiency. Early results from microstrip detectors are also described.

Pixel readout electronics development for the ALICE pixel vertex and LHCb RICH detector

The ALICE1LHCB pixel readout chip emerged from previous experience at CERN. The RD-19 collaboration provided the basis for the installation of a pixel system in the WA97 and NA57 experiments. Operation in these experiments was key in the understanding of the system issues. In parallel the RD-49 collaboration provided the basis to obtain radiation tolerance in commercial submicron CMOS through special circuit layout. The new ALICE1LHB chip was developed to serve two different applications: particle tracking in the ALICE Silicon Pixel Detector and particle identification in the LHCb Ring Imaging Cherenkov detector. To satisfy the different needs for these two experiments, the chip can be operated in two different modes. In tracking mode all the 50 μm×425 μm pixel cells in the 256×32 array are read out individually, whilst in particle identification mode they are combined in groups of 8 to form a 32×32 array of 400 μm×425 μm cells. The circuit is currently being manufactured in a commercial 0.25 μm CMOS technology.

Simulation Results From Double-Sided 3-D Detectors

A new ldquodouble sidedrdquo 3-D solid-state detector structure, intended to simplify the 3-D fabrication process, is proposed. In this structure, electrode columns of different doping types are etched from opposite sides of the substrate, with neither set of columns passing through the full substrate thickness. The finite-element simulation package ISE-TCAD is used to determine the performance of this structure. The double-sided detector shows similar electrostatic behavior to a standard 3-D detector, giving a low depletion voltage and fast charge collection. However, unless the electrode column length is very close to the substrate thickness, charge deposited around the front and back surfaces of the device is collected less quickly (though still rapidly compared with a planar geometry device). The breakdown voltage is dominated by high-field regions around the tips of the electrode columns and shows little change when the oxide charge is increased.

Large-area microelectrode arrays for recording of neural signals

To understand the neural code, that the retina uses to communicate the visual scene to the brain, large-area microelectrode arrays are needed to record retinal signals simultaneously from many recording sites. This will give a valuable insight into how large biological neural networks (such as the brain) process information, and may also be important in the development of a retinal prosthesis as a potential cure for some forms of blindness. We have used the transparent conductor indium tin oxide to fabricated electrode arrays with approximately 500 electrodes spaced at 60 /spl mu/m. The fabrication procedures include photolithography, electron-beam lithography, chemical etching and reactive-ion etching. These arrays have been tested electrically using impedance measurements over the range of frequencies important when recording extracellular action potentials (0.1-100kHz). The data has been compared to a circuit model of the electrode/electrolyte interface. One type of array (512 electrodes) behaves as theory would dictate and exhibits an impedance of 200 k/spl Omega/ at 1kHz. The other array (519 electrodes) has an impedance of 350 k/spl Omega/ at this frequency, which is higher than predicted by the models. This can perhaps be attributed to the difference in fabrication techniques. The 512-electrode array has been coupled to low-noise amplification circuitry and has recorded signals from a variety of retinal tissues. Example in vitro recordings are shown here.

The ZEUS vertex detector: Design and prototype test

Abstract A gas vertex detector, operated with dimethylether (DME) at atmospheric pressure, is presently being built for the ZEUS experiment at HERA. Its main design features, together with the performances of a prototype measured at various operating voltages, particle rates and geometrical conditions on a CERN Proton Synchrotron test beam, are presented. A spatial resolution down to 35 μm and an average wire efficiency of 96% have been achieved, for a 3 mm gas gap relative to each sense wire.

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

The authors present room temperature measurements on 200 {micro}m thick GaAs pixel detectors, which were hybridized to silicon readout circuits. The whole detector array contains 320 x 240 square shaped pixel with a pitch of 38 {micro}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.

Development of the LPD, a high dynamic range pixel detector for the European XFEL

We present the development and prototype test of the LPD instrument, a novel pixel detector for the European XFEL. At XFEL the LPD detector must be capable of operating with a frame rate of 4.5MHz and record images with a dynamic range of 1:100,000 photons (12keV) whilst maintaining low noise. The prototype LPD system has a large in pixel memory depth of 512 images that can be selected with a flexible veto system. Data is then transferred off the detector head in between XFEL pulses with an accompanying high rate data acquisition system. The system has been prototyped and assembled into an LPD detector head that contains custom silicon sensors and ASICs as well as a programmable data acquisition cards and supporting electronics and mechanics. A second version of the ASIC has also been submitted for manufacture. The experiences with our first prototype are presented.

Pixel readout chips in deep submicron CMOS for ALICE and LHCb tolerant to 10 Mrad and beyond

The ALICE1LHCB chip is a mixed-mode integrated circuit designed to read out silicon pixel detectors for two different applications: particle tracking in the ALICE Silicon Pixel Detector and particle identification in the LHCb Ring Imaging Cherenkov detector. To satisfy the different needs for these two experiments, the chip can be operated in two different modes. In tracking mode all the 50 μm×425 μm pixel cells in the 256×32 array are read out individually, whilst in particle identification mode they are combined in groups of 8 to form a 32×32 array of 400 μm×425 μm cells. Radiation tolerance was enhanced through special circuit layout. Sensitivity to coupling of digital signals into the analog front end was minimized. System issues such as testability and uniformity further constrained the design. The circuit is currently being manufactured in a commercial 0.25 μm CMOS technology.

Test beam results of 3D silicon pixel sensors for the ATLAS upgrade

Results on beam tests of 3D silicon pixel sensors aimed at the ATLAS Insertable B-Layer and High Luminosity LHC (HL-LHC) upgrades are presented. Measurements include charge collection, tracking efficiency and charge sharing between pixel cells, as a function of track incident angle, and were performed with and without a 1.6 T magnetic field oriented as the ATLAS inner detector solenoid field. Sensors were bump-bonded to the front-end chip currently used in the ATLAS pixel detector. Full 3D sensors, with electrodes penetrating through the entire wafer thickness and active edge, and double-sided 3D sensors with partially overlapping bias and read-out electrodes were tested and showed comparable performance.