Abstract: A backside cannelure of an electrode to provide for detecting semiconductor wafer shift after the wafer has been positioned over the cannelure of the electrode is disclosed. The wafer has a backside and a proper position over the cannelure. The cannelure exposes the backside of the wafer to a gas piped in through one or more holes of the electrode. The cannelure has a size such that deviation of the wafer from its proper position by more than a threshold partially exposes the cannelure, such that the gas leaks from the cannelure as now partially exposed. A gas flow detector may detect the gas leaking from the cannelure, and provide corresponding detection of the wafer deviating from its proper position.