The origins of surface states in E2V processed devices

Interface states influence many semiconductor device parameters. In particular, interface states generate dark signal in imaging devices (CCDs and CMOS) and affect charge transfer in the multiplication registers of Electron Multiplying Charge Coupled Devices (EMCCDs). CCDs from e2v are used in a wide variety of high performance applications, including those of space science and star trackers. Electron multiplying technology is now being considered for space applications and CMOS image sensors from e2v are also being proposed. Exposure to ionising radiation is known to increase the interface state density. Observations following ionising radiation of dark signal increase in non-inverted mode CCDs, and charge transfer degradation of EMCCDs, intended for space applications have raised many questions about the mechanisms involved. The observations cannot be explained readily using current theories for interface state formation. Back illumination is employed to maximise the quantum efficiency of CCDs, EMCCDs and CMOS sensors. This process involves thinning and implantation/annealing of the back surface. This process has been found to affect the interface state density on the opposite surface. The mechanisms may be similar to those from radiation damage. This project will investigate the build up of interface states in CCDs and EMCCDs, from both ionising radiation and processing for back illumination. This will involve the detailed characterisation of devices and test structures to establish important parameters influencing the build up, considering both material influences (investigating the influence of Hydrogen and Oxygen for example) and the effect of device architecture and design. The generation of a theoretical framework for the build up of interface states will enable the optimisation of devices for many demanding applications.