Corrosion on silicon sensors

The CMS tracking detector uses about 26 000 silicon sensors to equip 206 m 2 of silicon [F. Hartmann, et al., Nucl. Instr. and Meth. A 478 (2002); J.L. Agram, et al., Nucl. Instr. and Meth. A 517 (2004) 77]. During our quality and process control, including long term testing of the silicon sensors, strange stains on the guard and bias rings have been observed. The result of a systematic investigation proved that our detectors are affected by Al 2 O 3 corrosion after time under voltage in a humid environment. Depth profiling showed that the metal structures are compromised down to the level of SiO 2 . Finally, significant concentrations of potassium (K) were found precisely at the location of the corrosion, serving as a catalyst for the electrochemical reaction. A systematic study and origin of these stains will be presented.