Back-illuminated CCDs made by gas immersion laser doping

Abstract

Back-illuminated CCDs with high quantum efficiency in the soft X-ray range have been developed by EEV in collaboration with the Space Research Organisation Netherlands. By using Gas Immersion Laser Doping (GILD) for producing the backside accumulation layer very shallow doping profiles can easily be achieved. Additionally the GILD process does not affect the silicon behind the p + layer in contrast to the commonly used ion implantation process. This implies that only the electrons generated in, or reaching the very small accumulation layer will have a probability to recombine at the surface or in the accumulation layer itself. Therefore only a small fraction of the electron clouds produced by the absorbed soft X-rays will suffer charge loss, resulting in a high quantum efficiency. X-ray measurements of back-illuminated CCDs with doping profiles of 50 and 100 nm depth are presented and shown to be consistent with calculations based on minority carrier transport.