Technology development of 3D detectors for high energy physics and medical imaging

This thesis is concerned with the fabrication, characterisation and simulation of 3D semiconductor detectors. Due to their geometry, these detectors have more efficient charge collection properties than current silicon and gallium arsenide planar detectors. The unit cell of these detectors is hexagonal with a central anode surrounded by six cathode contacts. This geometry gives a uniform electric field with the maximum drift and depletion distance set by electrode spacing, 85m in this project, rather than detector thickness, as in the case of planar detectors (typically 100-300m). This results in lower applied biases (35-40 V in the work of this project) compared to >200 V in typical planar detectors. The reduction in bias offers the possibility of improved detector operation in the presence of bulk radiation damage as lower voltage reduces leakage current which limits the signal to noise ratio and hence the overall detector efficiency. In this work, 3D detectors realised in Si, GaAs and SiC have been studied. As part of fabrication of 3D detectors, dry etching, laser drilling and photoelectrochemical etching have been investigated in detail, in order to create a matrix of holes of uniform diameter (in the range of 10-30m) and depth in the range of 100-500m in Si, GaAs and SiC substrates. Holes with an aspect ratio up to 14:1 (depth to diameter ratio) have been successfully created in silicon wafers using an plasma etching technique. A femtosecond laser has been used to achieve holes with higher aspect ratio, up to 50:1, in Si, GaAs and SiC. Using this technique, the first working 3D detectors in GaAs and SiC have been demonstrated in this work. A photoelectrochemical etching system was successfully developed in the Physics Department with which matrices of uniform holes have been etched successfully in n-type silicon wafers with aspect hole ratios up to 15:1. Process steps have been developed for the creation of Schottky contacts within the holes created. Thin films of metals were evaporated inside high aspect ratio holes in Si, GaAs and SiC substrates which were subsequently filled with gold using a standard electroplating technique. Preliminary fabrication steps that make use of the spin on glass technique and polysilicon deposition were investigated in order to create p+-n junctions. In particular, the possibility of growing layers of polysilicon inside high aspect ratio holes has been demonstrated. As part of the characterisation of the 3D detectors, the formation of the Schottky junction has been verified by measuring the value of the barrier heights. These values agreed with the data obtained for planar detectors reported in the literature. Pulse height spectrum measurements were carried out for both alpha particles and X-rays on silicon and gallium arsenide detectors. An energy separation for X-ray illumination similar to standard planar detectors and an intrinsic efficiency close to 100% has been found for the GaAs detector. As part of the simulation, the MEDICI software package was used to analyse the effect of the sidewall damage introduced by the etching in the 3D detectors. The simulation showed that sidewall damage induced by dry etching reduces the leakage current in the Schottky junction by two orders of magnitude. Finally, the techniques developed for 3D detectors were successfully employed for the fabrication of Gaseous Electron Multiplier for photomultiplier devices.