3D detectors for synchrotron applications

3D detectors are a novel variety of photodiode radiation detector, invented by Parker, Kenney and Segal (1997). Instead of having n- and p-type contacts on the front and back surfaces of a silicon substrate, like a standard photodiode, they have columns of doped material passing through the thickness of the silicon. This structure means that the detector can combine a reasonable substrate thickness with a very small electrode spacing, resulting in a low depletion voltage, fast charge collection and low charge sharing. These detectors have a couple of promising applications. Their fast charge collection and low depletion voltage should make them very radiation-tolerant. So, they could be used for future particle physics experiments at the Super Large Hadron Collider (SLHC), where high levels of radiation damage are expected. Also, their low charge sharing means they could potentially improve X-ray diffraction measurements at synchrotrons such as Diamond Light Source. This would allow these experiments, for example, to determine the structures of biological molecules more accurately. However, before 3D devices can be used in practical experiments, their design and fabrication must be optimised to ensure that reliable, high-performance detectors can be produced on a reasonably large scale. The aim of this thesis is to evaluate and understand the behaviour of a variety of 3D detectors using a combination of lab tests and computer simulations. Using these results, future fabrication runs can then be re-designed to improve their performance. Firstly, the "Synopsys TCAD" simulation package was used to determine the optimum design for 3D detectors at the SLHC. It was found that the device behaviour depends strongly on the electrode spacing, and the choice of spacing requires a trade-off between different effects. Using a smaller spacing reduces the detector's operating voltage, and improves the charge collection efficiency by reducing carrier trapping. However, reducing the spacing also increases the capacitance, resulting in greater noise, and also increases the insensitive volume occupied by the columns. At SLHC radiation damage levels, the optimal electrode spacing was found to be 40-55 micrometres. CNM (Centro Nacional de Microelectronica) in Barcelona have produced a set of "double sided" 3D detectors. The n- and p-type columns in these devices are etched from opposite sides of the substrate and do not pass through the full substrate thickness. Computer simulations show that these detectors should give similar performance to full-3D detectors. The main difference is that these devices have slower charge collection around their front and back surfaces. Basic electrical characterisation of the detectors showed that they have low depletion voltages. However, the guard ring current varied a great deal between detectors, though this was fixed by using better guard structures. Charge collection tests on these detectors using beta particles gave mixed results. A heavily-irradiated detector gave a relatively high collection signal, similar to the simulated value, which demonstrated the structure's radiation hardness. However, an unirradiated detector gave an unexpectedly low collection signal. This was perhaps due to poor coupling between this detector and the readout chip. Three of these "double-sided" 3D detectors were bonded to Medipix2 pixel readout chips. These chips are specifically designed for X-ray detection, and can count individual photon hits. The detectors worked successfully, and initial lab tests demonstrated that they depleted extremely rapidly. The detectors were then tested in an X-ray beam at Diamond Light Source. These tests showed that the detectors have lower charge sharing than a standard planar photodiode. For example, 24% of the hits on a double-sided 3D detector at 22V were shared, compared to 40% on a planar detector at 100V. A set of devices with a simplified "single-type-column" structure, fabricated by FBK-IRST in Trento, were also tested. Simulations showed that although this structure will have a low depletion voltage and fast electron collection, the hole collection will be slow. This will result in poorer behaviour than full- and double-sided 3D detectors. This was confirmed by lab tests, which showed that when the detector was coupled to fast readout electronics, the charge collection efficiency was reduced due to ballistic deficit

Optimization of radiation hardness and charge collection of edgeless silicon pixel sensors for photon science

Recent progress in active-edge technology of silicon sensors enables the development of large-area tiled silicon pixel detectors with small dead space between modules by utilizing edgeless sensors. Such technology has been proven in successful productions of ATLAS and Medipix-based silicon pixel sensors by a few foundries. However, the drawbacks of edgeless sensors are poor radiation hardness for ionizing radiation and non-uniform charge collection by edge pixels. In this work, the radiation hardness of edgeless sensors with different polarities has been investigated using Synopsys TCAD with X-ray radiation-damage parameters implemented. Results show that if no conventional guard ring is present, none of the current designs are able to achieve a high breakdown voltage (typically < 30 V) after irradiation to a dose of ~10 MGy. In addition, a charge-collection model has been developed and was used to calculate the charges collected by the edge pixels of edgeless sensors when illuminated with X-rays. The model takes into account the electric field distribution inside the pixel sensor, the absorption of X-rays, drift and diffusion of electrons and holes, charge sharing effect, and threshold settings in ASICs. It is found that the non-uniform charge collection of edge pixels is caused by the strong bending of electric field and the non-uniformity depends on bias voltage, sensor thickness and distance from active edge to the last pixel ("edge space"). In particular, the last few pixels close to the active edge of the sensor are not sensitive to low-energy X-rays (< 10 keV) especially for sensors with thicker Si and smaller edge space. The results from the model calculation have been compared to measurements and good agreement was obtained. The model has been used to optimize the edge design.Comment: 12 pages, 8 figure

<i>miniPixD</i>: a compact sample analysis system which combines X-ray imaging and diffraction

This paper introduces miniPixD: a new, compact system that utilises transmission X-ray imaging and X-ray diffraction (XRD) to locate and identify materials of interest within an otherwise opaque volume. The system and the embodied techniques have utility in security screening, medical diagnostics, non-destructive testing (NDT) and quality assurance (QA). This paper outlines the design of the system including discussion on the choice of components and presents some data from relevant samples which are compared to other energy dispersive and angular dispersive XRD techniques

Precision scans of the pixel cell response of double sided 3D pixel detectors to pion and x-ray beams

hree-dimensional (3D) silicon sensors offer potential advantages over standard planar sensors for radiation hardness in future high energy physics experiments and reduced charge-sharing for X-ray applications, but may introduce inefficiencies due to the columnar electrodes. These inefficiencies are probed by studying variations in response across a unit pixel cell in a 55μm pitch double-sided 3D pixel sensor bump bonded to TimePix and Medipix2 readout ASICs. Two complementary characterisation techniques are discussed: the first uses a custom built telescope and a 120GeV pion beam from the Super Proton Synchrotron (SPS) at CERN; the second employs a novel technique to illuminate the sensor with a micro-focused synchrotron X-ray beam at the Diamond Light Source, UK. For a pion beam incident perpendicular to the sensor plane an overall pixel efficiency of 93.0±0.5% is measured. After a 10o rotation of the device the effect of the columnar region becomes negligible and the overall efficiency rises to 99.8±0.5%. The double-sided 3D sensor shows significantly reduced charge sharing to neighbouring pixels compared to the planar device. The charge sharing results obtained from the X-ray beam study of the 3D sensor are shown to agree with a simple simulation in which charge diffusion is neglected. The devices tested are found to be compatible with having a region in which no charge is collected centred on the electrode columns and of radius 7.6±0.6μm. Charge collection above and below the columnar electrodes in the double-sided 3D sensor is observed

R&D Paths of Pixel Detectors for Vertex Tracking and Radiation Imaging

This report reviews current trends in the R&D of semiconductor pixellated sensors for vertex tracking and radiation imaging. It identifies requirements of future HEP experiments at colliders, needed technological breakthroughs and highlights the relation to radiation detection and imaging applications in other fields of science.Comment: 17 pages, 2 figures, submitted to the European Strategy Preparatory Grou

Characterization of proton irradiated 3D-DDTC pixel sensor prototypes fabricated at FBK

In this paper we discuss results relevant to 3D Double-Side Double Type Column (3D-DDTC) pixel sensors fabricated at FBK (Trento, Italy) and oriented to the ATLAS upgrade. Some assemblies of these sensors featuring different columnar electrode configurations (2, 3, or 4 columns per pixel) and coupled to the ATLAS FEI3 read-out chip were irradiated up to large proton fluences and tested in laboratory with radioactive sources. In spite of the non optimized columnar electrode overlap, sensors exhibit reasonably good charge collection properties up to an irradiation fluence of 2 x 10**15 neq/cm2, while requiring bias voltages in the order of 100 V. Sensor operation is further investigated by means of TCAD simulations which can effectively explain the basic mechanisms responsible for charge loss after irradiation.Comment: Preprint submitted to Nuclear Instruments and Methods A, 11 pages, 13 fig