A high-sensitivity polyimide humidity sensor for monitoring hermetic micropackages

This paper presents the fabrication and 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 of 300-1200 /spl Aring/ sandwiched between 2 metal electrodes to sense moisture. The measured sensitivity for a sensor with a 1200 /spl Aring/-thick film is 0.86 pF/%RH, and for a 300 /spl Aring/-thick sensor is 3.4 pF/%RH. The sensor has been exposed to and survived a one-hour test at 400/spl deg/C, which is the temperature typically used to perform anodic bonding. Measurements show an offset drift of less than 1% RH at 50% RH and 37/spl deg/C, and a hysteresis of <2% RH over a range of 30-70% RH for a 1200 /spl Aring/-thick polyimide film sensor.