Investigation of an abnormal pattern of leakage currents in silicon microstrip detectors

Some batches of microstrip detectors fabricated by FBK-irst showed an odd and peculiar pattern of the strip leakage currents: the first and last few strips of the detector had low leakage, whereas all the strips in between showed very high current (3-5 orders of magnitude higher). Due to the peculiar shape of the strip current plot, the phenomenon has been called 'panettone effect'. This characteristic was common to all detectors within the affected batches. A detailed study of detectors and test structures made with a few variations of the fabrication process led us to conclude that the high strip currents were due to stress induced in the silicon by the combination of LPCVD-deposited layers of TEOS oxide and nitride. The strips close to detector ends had low current because there the stress was locally relaxed by the presence along the bias ring of a continuous contact opening through the dielectric layers. In accordance with this interpretation, some modifications have been introduced in the fabrication process, resulting in detectors with low leakage current for all strips, showing no particular distribution pattern.