8 March 1996 FPAs and thin film binary optic microlens integration
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Abstract
The relatively large detector size of conventional focal plane arrays often acts as a limiting source of noise currents and requires these devices to run at undesirably low temperatures. To reduce the detector size without reducing the detector's quantum efficiency, we have developed efficient on-focal plane collection optics consisting of arrays of thin film binary optic microlenses on the back surface of hybrid detector array structures. P/n polarity photodiodes of an unusual `planar-mesa' geometry were fabricated in epitaxial HgCdTe deposited by molecular beam epitaxy on the `front' side of a CdZnTe substrate. Diffractive (8 - 16 phase level) Ge microlenses were deposited on 48 micrometers centers in a registered fashion (using an IR mask aligner and appropriate marks on the front surface of the CdZnTe) on the back side of the substrate using a lifting process. The lifting process circumvents some of the process limitations of the more conventional chemical etching method to diffractive microlens processing, allowing them to approach more closely their theoretical efficiency limit of > 95%. Prior to microlens deposition, but after diode fabrication, the test structures were flip- chip bonded or `hybridized' using indium interconnections to metallic strip lines which had been photolithographically deposited on sapphire dice (a process equally compatible with a silicon integrated circuit readout). After hybridization, the CdZnTe was thinned to equal the focal length of the lenses in the CdZnTe material. Optical characterization has demonstrated that the microlenses combined with the detector mesas concentrate light sufficiently to increase the effective collection area. The optical size of the mesa detectors being larger than the theoretical diffraction limit of the microlenses precludes determining whether the lenses themselves produce the theoretical diffraction-limited gain, but they clearly decrease required detector area by at least 3 - 6X. To our knowledge, this is the first successful demonstration of IR detectors and binary optics microlens integration.
© (1996) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
M. Edward Motamedi, William E. Tennant, Robert Melendes, Natalie S. Gluck, Sangtae Park, Jose M. Arias, Jagmohan Bajaj, John G. Pasko, William V. McLevige, Majid Zandian, Randolph L. Hall, Karla G. Steckbauer, Patti D. Richardson, Donald E. Cooper, "FPAs and thin film binary optic microlens integration", Proc. SPIE 2687, Miniaturized Systems with Micro-Optics and Micromechanics, (8 March 1996); doi: 10.1117/12.234638; https://doi.org/10.1117/12.234638
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