Novel test structure to study location of breakdown for trench capacitor

A test structure for detecting the location of the breakdown of a trench capacitor electrically is introduced. It is very important to know the location of the breakdown in order to improve oxide integrity. However, the conventional failure analysis method for a plane capacitor cannot be applied to a trench capacitor. A vertical n-channel MOS transistor is formed in the trench. An n-type substrate and surface n-type region are used as source and drain, respectively. The side surface of the trench in a p-well acts as a channel region. If the gate potential is lower than the threshold voltage of the n-channel MOSFET, they are electrically separated from each other. The location of oxide breakdown in the trench can be detected by checking the leakage current between gate electrode and drain, p-well and source. All capacitors show breakdown at either the bottom or the top of trench, not the side.