The Soft X-Ray Performance of CCD Detectors

The Charge Coupled Device (CCD) is useful as an imaging detector in the x-ray waveband, and, when used to detect single photon events can simultaneously provide spatial imaging and energy resolution. This capability is important in the field of x-ray astronomy and the CCD has become the detector of choice for a number of future space x-ray imaging telescopes. With radiation focussed onto the front electrodes, the CCD suffers from reduced response below 1 keV due to absorption in the electrode structure; the same process that limits the blue response in the optical band. This thesis covers work performed to develop CCDs with an enhanced soft x-ray detection efficiency. Experimental results from two alternative techniques to improve the CCDs low energy response are reported. The first technique enables the CCD to be illuminated on its rear face and good detection efficiency at low energy can be achieved by removing the inert substrate of the device. The second technique studied involves fabricating an optimised front electrode structure to minimise absorption loss in the electrodes. A Monte-Carlo simulation is used to predict the response of the back illuminated CCD and compared with experimental data. The model is then used to suggest improvements to increase the charge collection efficiency of the CCD and therefore the energy resolution and detection efficiency. Results from both types of experimental devices are reported and the factors that effect the energy resolution and detection efficiency are fully investigated. In the case of back-illumination, the effects of charge loss in the rear passivating implant of the CCD and thinning position within the epitaxial layer on the detector's response are investigated. The back illuminated CCD is currently being developed for the Reflection grating Spectrometer on the XMM space mission and also the EPIC cameras. The front illuminated CCD with the optimised electrode structure is being developed for the imaging cameras for JET-X the instrument to be flown on the Russian SPECTRUM-XG mission. Through further developments in CCD technology embodied in the modelling work and experimental results of this thesis, the CCD response is shown to be able to extend down into the EUV region (100 - 10 eV) opening up a new area of EUV instrumentation. Possible future EUV space missions are described in which the EUV enhanced CCD may form the focal plane detector.