Tuned Infrared Emission From Lithographically-Defined Silicon Surface Structures
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Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
wavelengths shorter than the cut-off wavelength. The resulting emission band is fairly broad, and in this work, we attempt to narrow the emission width by enforcing a short wavelength cut-off. 2*
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Figure 1 Inductive (b, c) and capacitive (eJ) metal mesh filters (micro-antennaarrays) and their respective spectraltransmission[8]. Our goal then was to develop an infrared thermal emitter with high emissivity over a narrow band of wavelengths (AMA - 0.1) and low emissivity everywhere else. Such sources, emitting hundreds of milliwatts of power in-band, are attractive alternatives to infrared light emitting diodes (IRLEDs) which have low quantum efficiencies (-2-4%) and only tens of microwatts output power. We chose to make the tuned emitters by making non-random patterns, periodic arrays similar to metal mesh and photonic bandgap filters, on the emitter's surface. Since we needed wavelengths in the range 2-14 gim for spectroscopy, this implies sizes as small as 0.5 gm which are readily fabricated by lithography on silicon. To our knowledge, this is the first report of photonic bandgap structures as tuned thermal emitters. In particular, we find the peak emission wavelength is proportional to the spacing of the lithographic pattern. 0.6 0.5 &.4
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Figure 2 Ion Optics current TO-8 sized pulsIR product, SEM micrograph of surface texture, and grating monochromator scan of thermal emission (compared to Planck radiation) shows the long wavelength emissivity cut-off achieved by random texture.
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EXPERIMENTAL We selected patterns based on the previous work of Byrne, et al. [8,9] because of the relatively simple relation given and demonstrated between feature size and transmission wavelength. Specifically, we used the crossed dipole pattern depicted in figure 1 and fabricated a variety of feature sizes and spacings. A summary of the patterns used is given in table 1 and Figure 3. Table I Summary of crosseddipole patternsetched into silicon wafers. cross length, L (pm) 4.0 5.0 6.0 3.8 4.8 5.7 2.5 1.6
pattern designation 8B 10B 12B 8N ION 12N 5.25N 3.4N
line width, 2b (jrm) 1.7 1.9 2.2 1.0 1.2 1.4 0.6 0.4
Center-to-center spacing, g (jrm) 5.0 6.0 7.1 6.4 8.0 9.5 4.2 2.7
Photomasks, made by direct write e-beam lithography, were purchased from Dupont. Lithography and etching was performed at the Microdevices Laboratory at JPL. The basic fabrication sequence was: deposit photoresist; expose pattern on resist; reactive ion etch (RIE) for 5, 10 or 15 minutes to form the cross-shaped cavities of different depths with straight sidewalls; and remove photoresist. Five n" and two n' wafers were processed. One n- and one n' wafer were coated with 500A chromium (for adhesion) followed by IOOOA gold before liftingoff with removal of the photoresist, leaving gold coating at the bottom of the etched cavities. In another case, an n" wafer was coated with alumi
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