Investigation of silicon sensors 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 a strong irradiation. Studies made by several groups 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 depth of the effective “dead” n⁺ layer. By increasing this thickness from 1 μm to 2.5 μm, the fraction of sensors with breakdown voltage higher than 500 V increased from 22% to more than 80%. On the other hand, it was noticed that the starting surface quality of the wafer (double side polished or single side polished) does not affect the detectors parameters for a given production technology. The thick n ⁺-layer protects against initial wafer surface and defects caused by the technological treatment during detector production