Humidity Studies on ATLAS ITk Strip Sensors

Silicon strip sensors developed for the ATLAS Inner Tracker Upgrade have shown a strong dependence of the breakdown voltage on varying levels of relative humidity (RH). The solution to this humidity sensitivity has so far been to store and test the sensors in a dry environment (RH < 10 %). This mitigates the effects of humidity but does not attempt to understand the underlying issue. To understand the silicon strip sensor humidity sensitivity, silicon diodes from the same wafer as the strip sensor were investigated using Technology Computer-Aided Design (TCAD) from Synopsys and the Top-Transient Current Technique (Top-TCT). The silicon diodes were characterized using a probe station. The full depletion voltage, active thickness, and effective silicon doping concentration were derived from Capacitance-Voltage (CV) measurements. TCAD simulations for different relative humidity values (0 %, 30 % and 40 %) were performed at a high bias voltage (900 V) to study the electric field, charge transport, and onset of early breakdown in humid conditions. TCAD proved to be a useful tool in assessing the effect of passivation openings on the leakage current and surface charge transport as a function of the relative humidity. For the first time, the Top-Transient Current Technique was used to study the charge transport in the guard ring region of silicon diodes from the same wafer as the ATLAS strip sensor by generating localized free charge carriers near the surface with picosecond pulses of red laser light. The induced transient currents were measured as a function of the relative humidity and bias voltage to determine how these factors influence the prompt current and charge profile.