Millimeter-wave (mmW)/sub-mmW/THz region of the electro-magnetic spectrum enables imaging thru clothing and other obscurants such as fog, clouds, smoke, sand, and dust. Therefore considerable interest exists in developing low cost millimeter-wave imaging (MMWI) systems. Previous MMWI systems have evolved from crude mechanically scanned, single element receiver systems into very complex multiple receiver camera systems. Initial systems required many expensive mmW integrated-circuit low-noise amplifiers. In order to reduce the cost and complexity of the existing systems, attempts have been made to develop new mmW imaging sensors employing direct detection arrays. In this paper, we report on Raytheon’s recent development of a unique focal plane array technology, which operates broadly from the mmW through the sub-mmW/THz region. Raytheon’s innovative nano-antenna based detector enables low cost production of 2D staring mmW focal plane arrays (mmW FPA), which not only have equivalent sensitivity and performance to existing MMWI systems, but require no mechanical scanning.
Transmittance spectra of solid and vapor samples of trinitrotoluene (TNT) in the spectral range 0.6 to 10 THz at
resolutions up to 1 GHz are reported. Uniform solid samples of ~100 &mgr;m thickness gave stronger absorption and more
resolved structure than previous studies. New absorption lines for TNT solid below 100 cm-1 are reported. A heated 10
m multpass White cell was used for spectroscopy of the vapor. Strong absorption bands yield unexpectedly large
absorption cross sections for the anticipated saturated vapor pressure at the cell temperature, leaving their assignment to
TNT in doubt. These results indicate that path lengths exceeding 10 m and temperatures higher than 40 C, or
significantly higher instrumental sensitivity, are needed for sensing of TNT vapor in the spectral range 0.6 to 10 THz.
The low vapor pressure and concentration of explosive such as TNT and RDX pose significant problems for the
detection of explosive vapors in the mmW bands. For the positive identification of explosive vapors using an uncooled
passive mmW imaging spectrometer with a low false alarm rate (FAR) requires an unprecedented sensitivity of <150
fW. We report on the recent development of a novel uncooled mmW antenna-coupled direct detector, which shows
promise of meeting this requirement.
We report on progress in the development of a device fabrication process for type-II strained layer superlattice IR detectors, composed of InAs/GaSb or InAs/GaInSb. Steps of the process include etching the mesas, cleaning up the surface, and applying a surface passivation treatment. Certain etchants have been evaluated and calibrated. The surface has been studied with single wavelength ellipsometry and results have been compared with modeled ellipsometry values, revealing effects of surface residues and surface roughness. An initial investigation of ammonium sulfide treatment for surface passivation has been made. Initial measurements of the IR transmission of the GaSb substrate have also been made to determine how much thinning is needed for back side illuminated operation of the IR detectors.
Wavelength tuning is demonstrated in an antenna-coupled infrared microbolometer. With a 300-mV control voltage, we observed a tuning range of 0.5 µm near 10 µm. A metal-oxide-semiconductor capacitor underneath the antenna arms causes the shift of resonance wavelength with applied voltage. We develop a device model that agrees well with measured results.
The methods of fabricating infrared antennas using electron beam lithography will be investigated. For this purpose, a process using a bi-layer lift off process and a single layer of resist has been developed. The bi-layer lift off process used allowed for antenna arm resolution of 200nm. The single layer resist process enhanced the resolution of the antenna arms to 90nm by using a Chlorine based reactive ion etcher with Chrome as an etch mask. An alignment scheme using a set of global and local marks allowed for an overlay accuracy of 25nm. An improved process was developed to further improve device yield and uniformity of the infrared detectors by sputtering the bolometer and using an oxygen descum to remove residual resist between antenna and bolometer. Two separate methods of fabrication of air-bridge microstrip antenna-coupled microbolometers using both a critical point dryer and an isotropic reactive ion etcher will also be introduced.