Simulation and Measurement of Advanced Semiconductor Imaging Sensors

The e2v Centre for Electronic Imaging as a collaboration between e2v and the Open University to provide training for young scientists and engineers in the field of imaging detectors for science experiments. e2v have agreed to fund up to a rolling 12 PhD CASE students as part of this collaboration, which represents one of the largest concentrations of industrially-sponsored students under the CASE scheme. At a meeting with the PPARC executive in 2004 (involving Ian Halliday and Richard Wade), the research council agreed to support this ambitious plan for PhD student training. This single application is one of 3 CASE applications proposed into the 2009 round, exploring different aspects of imaging semiconductor device R&D of relevance to STFC science. Under our STFC Gaia grant, we have recently been using the Silvaco 3D device modelling software package to help in the European effort aimed at understanding the effect of space radiation daage in the CCDs. Using this software we are producing an accurate description of the pixels in the Gaia CCDs to inform how the charge is stored in the pixel as a function of signal size. This information, in particular the signal density distribution, is then used to calculate charge trapping times in the presence of prton-induced traps, and through Monte Carlo models the impact of the spce radiation on the CTE. This work is demonstrating that such modelling (in this istance using Silvaco) can provide a very poweful tool in the understanding of pixel structures, particularly when feature sizes become comparable to the fringing fields in the device (<5 microns). Our work on Gaia indicated a protential problem with one of the implants in the CCDs, which has subsequently been backed-up by measurements. Had the modelling technique been available at the time of design, then the customised Gaia CCDs could have been improved in performance yielding better sensitivity in the Gaia data products. Here we propose a PhD CASE studentship which will perform advanced modelling in 3D of CCD and CMOS pixel designs to aid understanding of the detailed movement of signal charge within the detectors leading to improvements in future imager designs. To help ensure that the modelling work and its results are accurate, we will specify test structures based on the early modelling which will then be designed and fabricated by e2v. These test structures will then be evaluated, and the results used to benchmark and validate the output from the model. The student will work on both traditional CCD device structures, plus the newer CONS Active Pixel Structure (APS) imagers currently being developed by e2v. One of the trends in scientific imagers is toward large imaging arrays having smaller pixels. As mentioned above, when the feature sizes become less than around 5 microns, the devices are subject to non-linear effects arising from the 'narrow channel effect'. For such large area/small pixel devices in the future, a thorough understanding of their detailed operation will become increasingly important. By the end of the studentship we would anticipate that a much greater understanding is obtained into the details of charge storage and movement within CCD and CMOS structures with the creation of new 'design rules' to help the designers create new cutting-edge sensors with improved performance.