A high-sensitivity polyimide capacitive relative humidity sensor for monitoring anodically bonded hermetic micropackages

This paper presents the design, fabrication and complete characterization of a high-sensitivity polyimide-based humidity sensor for monitoring internal humidity level in anodically bonded hermetic micropackages. This capacitive sensor is 1 mm on a side and utilizes CU1512 polyimide film with a thickness in the range from 300 Å to 1200 Å sandwiched between two metal electrodes to sense moisture. The measured sensitivity for a sensor with a 1200-Å-thick film is 0.86 pF/%RH, and for a 300-Å-thick sensor is 3.4 pF/%RH. The sensor has been exposed to and survived a one-hour test at 400°C, which is the temperature typically used to perform anodic bonding. Measurements show a drift of less than 1% RH at 50% RH and 37°C for 48 h, and a hysteresis of <2% RH over a range from 30 to 70% RH for a 1200-Å-thick polyimide film sensor. The measured breakdown voltage of the sensor (1200 Å thick) exceeds 20 V and agrees well with other results