Issues in imaging keV electrons using back illuminated CCDs are discussed. A proximity focused Gen III image intensifier has been built using a back illuminated 512 x 512 CCD in place of the phosphor screen. Signal to noise, spatial resolution, minimum resolvable signal and life are discussed. In addition, a model of the back thinned CCD in the electron bombarded mode is presented.

Very low light level imaging is driven by the need for high quantum efficiency detection of single photons at the highest possible speed and the highest spatial resolution. The scientific back-illuminated CCD provides excellent quantum efficiency, spatial resolution and low noise at slow-scan speeds and has found application in astronomy, spectroscopy and microscopy. In areas such as surveillance, where video rates are necessary, the Gen III image intensifier fiber-optically coupled to a front illuminated video rate CCD (ICCD) has found application. The GaAs Gen III photocathode quantum efficiencies can approach 30% at visible wavelengths. However, the image intensifier uses a micro-channel plate (MCP) as a gain stage, which increases noise above shot limits, is not linear and contributes to the poor spatial resolution of the system. The numerous interfaces and photon to electron conversions cause scattering which further degrades spatial resolution. It has long been realized that direct imaging of a GaAs photocathode by a back-illuminated CCD (EBCCD) will improve detective quantum efficiency to the level of the GaAs photocathode due to the 'built in' gain in the electron-bombarded silicon (EBS) process, thereby providing the highest possible sensitivity. Elimination of the MCP will increase noise performance to near theoretical limits throughout the dynamic range of the device and the simplicity of the design will eliminate the interfaces that cause poor spatial resolution. Perhaps most importantly, the EBCCD may be run at high speeds.

Through a joint effort between SITe and Intevac EO Sensors, two proximity focused GaAs image intensifiers have been built using the SITe SI502AB in place of the phosphor screen. The performance of these tubes, one, a bare SI502AB and the other, an SI502AF fiber-optically coupled to a high quality Intevac GaAsP image intensifier, has been measured and compared. In addition, a model of MTF and signal to noise vs spatial frequency has been developed for the three types of sensors. In earlier work a model for the EBS gain analogous to a photon quantum efficiency model for the back-illuminated CCD has been developed and fit to match both EBS gain data and QE data using the same set of physical parameters.