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 irradiation to 2×10¹⁴ n/cm² expected during the ten years of Large Hadron Collider operation (CMS ECAL Group, Bloch et al.). Studies made by several groups (Bischoff et al., Evensen et al., Da Rold et al., Passeri et al., Li et al., Militaru et al.) have underlined the importance of the p⁺ side geometrical parameters, such as the metal width and the number and spacing of guard rings. We have in addition investigated the effects related to the ohmic side processing and found that the breakdown voltage depends strongly on the thickness of the effective "dead" n⁺ layer. By increasing this thickness from 1 to 2.5 μm, the fraction of sensors with breakdown voltage higher than 500 V increased from 22% to more than 80%. In addition, a thick n⁺ layer protects against defects caused by the technological treatment during detector production and assembly