Corrosion on silicon sensors

The CMS tracking detector uses about 26000 silicon sensors to equip 206m2 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 Al2O3 corrosion after time under voltage in a humid environment. Depth profiling showed that the metal structures are compromised down to the level of SiO2. 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