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  • Short-Wave IR Tunable Filters Fabricated on HgCdTe

Photonics Spectra
Feb 2006
Anne L. Fischer

Researchers from the University of Western Australia in Crawley, in collaboration with DRS Infrared Technologies LP in Dallas and the University of Texas at Austin, have designed and fabricated a tunable Fabry-Perot resonator multispectral imaging microsystem on HgCdTe, a high-performance semiconductor infrared detector material.

Short-Wave IR Tunable Filters Fabricated on HgCdTe
The demonstration of a low-temperature, substrate-independent Fabry-Perotresonator microfabrication process creates the possibility of bringing multispectral imaging to IR detector materials such as HgCdTe.

The processing methodologies for HgCdTe and typical microsystems are vastly different. The latter usually involve thin-film deposition at temperatures greater than 600 °C, which are detrimental to the electronic and optical properties of HgCdTe. The investigators addressed this by developing a low-temperature microfabrication process and associated materials science and technologies that were compatible with typical HgCdTe processing temperatures of 125 °C and lower. To verify the compatibility of the microfabrication process, they measured the optical responsivity of fabricated short-wave IR HgCdTe photoconductors before and after a 1-h vacuum bake at 125 °C.

Another challenge was to achieve device configurations with high packing densities and low power consumption, making optimum use of expensive semiconductor real estate and precious power supplies.

According to Martin T.K. Soh, an Australian Fulbright fellow at CSIRO Industrial Physics in Lindfield, Australia, and at the University of Western Australia visiting the University of Minnesota in Minneapolis, the development of a substrate-independent, low-temperature, microfabrication process to make the spectrometer widens the scope of applications for these devices. Although the scientists’ photoconductor prototype was a proof of concept of the design and microfabrication philosophy, they recently demonstrated filters on HgCdTe photodiodes with higher fill factors and lower power consumption. They will continue to engineer the process and to study the materials to facilitate denser device configurations.

The researchers noted that the mechanical membrane actuated up to 40 percent of the cavity width, snapped down and recovered upon removal of the drive voltage. Soh said that this is indicative of the mechanical strength of the low-temperature silicon nitride membrane material they deposited. The 40 percent displacement limits the wavelength tuning range to between 1.7 and 2.2 μm, and the scientists hope to expand that tuning range by extending the membrane displacement through charge-control actuation.

Soh envisions that this substrate-independent technology could become part of a portable wideband spectrometer system on a chip, consisting of several resonators covering various wavelengths. Such a system would enable the user to characterize the spectral reflectance signature of any object.

Although other substrates may be used to fabricate the microresonator, HgCdTe has proved ideal in terms of developing compatible low-temperature deposition and microfabrication methodologies.

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