Ultra thin fully depleted active pixel sensors processed on SOI wafers

The production of the latest generation of fully depleted DEPFET active pixel sensors designed for the Belle-II experiment at KEK, Japan, is currently being finalized. For the first time a thinning technology based on SOI wafers finds now its application in a high energy physics experiment. The DEPFET (DEpleted P-channel FET) is a field effect transistor with an additional implant underneath the channel and integrated on a fully depleted substrate. It combines the functions of a detector and the first amplification stage in one single device. The in-sensor amplification makes it possible to create very thin sensors with an excellent signal/noise ratio for minimum ionizing particles. The fabrication of thin wafer-scale active pixel sensors requires the combination of a highly specialized MOS technology, including two poly-silicon and three metal layers, on fully depleted bulk with MEMS technologies. The devices are realized on custom-made SOI wafers with structured back side implant supplied by Soitec, France. Initially developed for thin DEPFETs, the technology is now being used for the production of other high performance sensors in High Energy Physics (strip and passive pixel detectors) and photon counting devices based on Geiger-mode avalanche photo diodes. Other fields of application are all kind of thin imaging devices for low energy particles (electron microscopy or in medical hadron therapy).