Investigation of Silicon Sensor Quality as a Function of the Ohmic Side Processing Technology

Silicon sensors designed for the CMS Preshower detector must have a high breakdown voltage in order to be fully efficient after strong irradiation. Studies madeby several groups [1-7] have underlined the importance of the p+ side geometrical parameters, such as the metalwidth and the number and spacing of guardrings. We have in addition investigated the effects related tothe ohmic side processing andfound that the breakdown voltage depends strongly on the thickness of the effective “dead” n+ layer. Byincreasing this thicknessfrom 1Pm to2.5 Pm, thefraction of sensors with breakdown voltagehigher than 500V increasedfrom22% to more than 80%. In addition a thick n+ layer protects against defects caused by the technological treatment during detector production and assembly.