INFRARED SENSORS USING SILICON MICROSTRUCTURES

Interest in being able to see in the dark has spurred the exploration of infrared imaging systems. There are basically two groups of detectors: photon detectors and thermal detectors. The high costs of cooling apparatus and infrared ( lR ) sensitive materials restrict the wide application of the photon detector as an IR imager. Recent development of microbolometer detectors enables the fabrication of uncooled imagers at a relatively low cost. In this work, infrared detectors using microbolometers are fabricated by silicon micromachining. The microbolometer detects IR radiation by sensing the temperature change that the IR radiation produces in temperature dependent resistors. The resistors are sandwiched between silicon nitride and silicon oxide films, which are good absorbers of IR. The membranes are suspended over the cavities in bulk silicon to get low thermal mass and good thermal isolation. To achieve such a microstructure, the stress of the membrane is carefully compensated to get flat membranes with relatively high mechanical strength. The microstructure is obtained by bulk silicon micromachining using a polysilicon sacrificial layer. A titanium resistor is used as the bolometer sensitive element due to its low noise, mature fabrication technique and compatibility with CMOS process. The microbolometer fabrication is carried out using a standard CMOS foundry, followed by several CMOS compatible postprocessing steps. The microbolometer achieved using the techniques mentioned above has a pixel size of 50 µm x 50 µm and a membrane thickness of 2 µm. The responsivity is 10 V/W with a cut-off frequency of 400 Hz.

Author dc:creator

• ZENG WENJIANG