Micromachines create IR video image sensor

Micromachines create IR video image sensorDevelopment programme by US firm Sarcon promises an infra-red video image sensor that needs no cooling and could cut the price of associated optics to one tenth; prototype sensor exists. Steve Bush. An infra-red video image sensor based on micromachining could see the light of day if a development programme by Sarcon Microsystems in the US is successful. The key to its operation is a micro-version of the bimetallic strip temperature sensor found in the heart of most low-tech central heating controllers. In this case the bimetallic strips are actually silicon carbide and aluminium cantilevers, 30?m wide and 50?m long. The cantilevers are fabricated in a rectangular array and the image to be viewed is focussed on to them. Response time is short because of the small amount of material present. It is tuneable between 5 and 50ms. and prototype structures have been set at 10ms to fit in with a 30Hz frame rate. Sensing is capacitive and sensitivity is claimed to be high, with a 40 per cent change in sensor capacitance for a 1?C temperature change. The assembly is fabricated on a CMOS wafer. All necessary signal conditioning circuitry is made in this wafer then the SiC cantilevers are built on top. These are around 0.15?m thick and are suspended 0.3?m above the CMOS surface. Aluminium deposited on top of the cantilevers acts as the differential expansion material and also forms one electrode of the sensing capacitor. The other electrode is another aluminium layer, immediately below the first, on the surface of the CMOS . Oak Ridge National Laboratory (ORNL) in Tennessee developed the cantilever structures and Sarnoff (formally RCA Laboratories) introduced the idea of using silicon carbide. It is claimed that SiC is the key to the devices high sensitivity. According to Sarcon, previous attempts at making similar devices have failed because heat from the cantilever was quickly conducted to the substrate. SiC is an order of magnitude less thermally conductive than SiN, the alternative material, and blocks the leakage of heat. High sensitivity pays off in cheaper lenses. Lenses for traditional infra-red cameras are made from germanium and need to be large (F1 of F0.7). These cost approximately $5,000. The Sarcon sensor is said to allow smaller (F5) lenses to be used costing a few hundred dollars. One more advantage of the cantilever-based sensor is that it does not require cooling. Noise equivalent temperature is 5-10mK, comparable with that of cryo-sensors that are liquid nitrogen-cooled . A prototype sensor with 16 x 16 cantilevers on a 50 x 50?m pitch exists. The next development is a 30 x 30?m pitch sensor element and a 320 x 240 array. VGA resolution (640 x 480) is the eventual target. Single cantilever chips for temperature sensing and linear arrays for spectrometers are also planned.


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