Vacuum-encapsulated thermistor bolometer type miniature infrared sensor

Clinical tympanic thermometers detecting infrared radiation from the tympanic membrane of the ear have been used in the field of medical care. Because of the weakness of the infrared radiation, infrared sensors for clinical thermometers need high sensitivity, good resolution and small size. Therefore, we have developed a vacuum-encapsulated thermistor bolometer type infrared sensor for this purpose using semiconductor micromachining techniques. The sensor has a heat-sensitive portion on the micro-air-bridge designed to minimize heat capacity. To decrease the heat loss to the ambient air, the micro-air-bridge is encapsulated in a vacuum micro-chamber. Two thermistors on the individual micro-air-bridge are contained in the same micro-chamber. In order to cancel the thermally induced background noise, the differential output signal from the two thermistors is monitored. In this work, two types of thermistor were investigated. One was a germanium thin film thermistor (germanium thermistor) and the other a polycrystalline silicon Schottky barrier thermistor. The former has a thermistor constant B of 3500 K to 4300 K while the latter shows a larger equivalent thermistor constant of 5300 K to 6400 K. A hermetically sealed small packaging (2.0 mm×2.5 mm×0.8 mm) could be achieved. These sensors were fabricated through a successive batch process. Furthermore, the whole fabrication process, including the hermetic sealings is essentially compatible with the integrated circuit process. The sensitivity of the germanium thermistor is better than 560 V/W at 5.0 V dc, in spite of absorption loss through the silicon window