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Micro/Nano Lithography

Superconducting microbolometer captures radiation

Eye on Technology - detectors

From oemagazine February 2001
31 February 2001, SPIE Newsroom. DOI: 10.1117/2.5200102.0001

Using lithographic techniques, researchers at the University of California, Berkeley (UCB; Berkeley, CA) have developed a monolithic, large-array superconducting bolometric detector for millimeter-wave astronomy applications. Bolometers convert absorbed incident radiation to current to provide high-sensitivity detection. According to Berkeley professor Paul Richards, the key issue in astronomy applications is speed, which goes as the square of the sensitivity times the number of pixels in the array. Until recently, fabrication constraints limited array sizes to between 10 and 100 microbolometer structures, constraining the speed of the detectors. Richards and his group plan to change that.

An SEM image of a corner of an absorber grid taken at a 45° angle with respect to the bolometer plane shows the wires suspended over the underlying silicon material. (UCB)

The Berkeley research group has fabricated a monolithic filled voltage-biased superconducting bolometer (VSB) array that consists of 1024 bolometers. VSBs offer a good alternative for large bolometric arrays because the sensors can be made by standard optical lithography, and the large noise margin of the VSB superconducting quantum interference device (SQUID) combination enables readout multiplexing, rendering large arrays practical.

Each 1.5 mm X 1.5 mm bolometer (pixel) in the array consists of a rigid square 11 X 11 micromesh of 7-µm-wide silicon nitride wires covered with a sandwiched titanium/aluminum/ titanium (Ti/Al/Ti) trilayer absorber with metal resistance of 1.3 Ω. After lithographically patterning and metallizing the micromesh, the researchers used a combination of wet etch and plasma etch techniques to remove the silicon material from below the wire grid (see figure on page 9). In tests of the prototype pixel at the resistive transition temperature of 304 mK, the group measured a noise equivalent power (NEP) of 2.3 X 10-17 and a 24 ms time constant.

"We've made the physical mechanical structure of the 1024 array," says Richards. "We've wired up an individual pixel and tested it. The next stage is to wire up the whole 1024 array." The final stage will be to connect the array to the amplifiers. The group is developing a frequency-domain multiplexer that will be able to read out an entire row of 32 microbolometers through the same amplifier, analogous to a charge-coupled-device (CCD) detector.

Some device optimization would involve changing some of the materials in the device for optimal performance. Use of silicon on insulator (SOI) is under consideration to allow for thinning the silicon lying directly underneath the bolometer in a controllable fashion.