Moisture sensitivity of AC-coupled silicon strip sensors

A study is presented on the I–V behaviour of a part of the AC-coupled silicon strip sensors used in the ATLAS semiconductor tracker (SCT) at the LHC. For these sensors the metal strips are narrower than the implants while for the majority of the SCT sensors the strips are wider than the implants (fieldplate layout). The I–V curves of these sensors show an early I–V breakdown in a dry environment that did not occur at the I–V test by the manufacturer done under normal laboratory conditions. The breakdown behaviour was not observed at the sensors with the fieldplate layout. In this paper the dependence of the breakdown on the environmental moisture level will be described and a possible explanation will be given that is based on the detector layout. It will also be shown that the breakdown behaviour does not cause operational problems for the SCT.