Subwavelength resonant structures designed for long-wave infrared (LWIR) absorption have been integrated with a
standard vanadium-oxide microbolometer. Dispersion of the dielectric refractive index provides for multiple
overlapping resonances that span the 8-12 μm LWIR wavelength band, a broader range than can be achieved using the
usual quarter-wave resonant cavity engineered into the air-bridge structures. Experimental measurements show a 49%
increase in responsivity for LWIR and a 71% increase across a full waveband as compared to a similar device designed
for only LWIR absorption, using a 300°C blackbody at 35 Hz chopping rate. Increased thermal time constant due to
additional mass is shown to lessen this enhancement at higher chopping rates.
Resonantly absorbing thin films comprising periodically sub-wavelength structured metal surface, dielectric spacer, and metal ground plane are a topic of current interest with important applications. These structures are frequently described as “metamaterials”, where effective permittivity and permeability with dispersion near electric and magnetic resonances allow impedance matching to free space for maximum absorption. In this paper, we compare synchrotron-based infrared spectral microscopy of a single isolated unit cell and a periodic array, and we show that the resonances have little to do with periodicity. Instead, the observed absorption spectra of usual periodically structured thin films are best described as due to standing-wave resonances within each independent unit cell, rather than as due to effective optical constants of a metamaterial. The effect of having arrays of unit cells is mainly to strengthen the absorption by increasing the fill factor, and such arrays need not be periodic. Initial work toward applying the subject absorbers to room-temperature bolometer arrays is presented.
Patterned highly absorbing gold black film has been selectively deposited on the active surfaces of a vanadium-oxide-based infrared bolometer array. Patterning by metal lift-off relies on protection of the fragile gold black with an evaporated oxide, which preserves gold black’s near unity absorption. This patterned gold black also survives the dry-etch removal of the sacrificial polyimide used to fabricate the air-bridge bolometers. Infrared responsivity is substantially improved by the gold black coating without significantly increasing noise. The increase in the time constant caused by the additional mass of gold black is a modest 14%.
We present a design for a low-noise bolometer linear array based on the temperature-dependent conductivity of a VOx- Au film. Typical thin film bolometers must compromise between low resistivity to limit Johnson noise and high temperature coefficient of resistivity (TCR) to maximize responsivity. Our vanadium oxide is alloyed with a small concentration of gold by co-sputtering, which gives very low resistivity and very high TCR simultaneously. The film is fabricated on an air bridge device having high thermal conductivity and small thermal time constant optimized for 30 to 60 Hz frame rates. The linear array functions as a low-power profile sensor with a modulated bias. For 1 V bias, we predict responsivity exceeding 1200 V/W. Johnson noise dominates with predicted NEP values as low as 1.0 × 10-11 W/Hz1/2. Preliminary device testing shows film resistivity below 2.5 Ω-cm with TCR exceeding -2.0%. Preliminary measurements of NEP and D* are reported.
A MEMS cantilever IR detector that repetitively lifts from the surface under the influence of a saw-tooth electrostatic force, where the contact duty cycle is a measure of the absorbed IR radiation, is analyzed. The design is comprised of three parallel conducting plates. Fixed buried and surface plates are held at opposite potential. A moveable cantilever is biased the same as the surface plate. Calculations based on energy methods with position-dependent capacity and electrostatic induction coefficients demonstrate the upward sign of the force on the cantilever and determine the force magnitude. 2D finite element method calculations of the local fields confirm the sign of the force and determine its distribution across the cantilever. The upward force is maximized when the surface plate is slightly larger than the other two. The electrostatic repulsion is compared with Casimir sticking force to determine the maximum useful contact area. MEMS devices were fabricated and the vertical displacement of the cantilever was observed in a number of experiments. The approach may be applied also to MEMS actuators and micromirrors.
Thermomechanical noise for a MEMs-based infrared detector using null switching (US patent 7977635) depends on vibrational amplitude, since IR radiation is transduced to a change in the duty cycle of a repetitively closing switch. Equipartition theorem gives a maximum rms vibrational amplitude of 45 pm for the fabricated cantilever switch at its natural frequency. This gives a worst case timing uncertainty of 700 ns and an NEP of 2 pW/Sqrt[Hz].
We present performance calculations for a MEMS cantilever device for sensing heat input from convection or radiation. The cantilever deflects upwards under an electrostatic repulsive force from an applied periodic saw-tooth bias voltage, and returns to a null position as the bias decreases. Heat absorbed during the cycle causes the cantilever to deflect downwards, thus decreasing the time to return to the null position. In these calculations, the total deflection with respect to absorbed heat is determined and is described as a function of time. We present estimates of responsivity and noise.
We experimentally demonstrate a structured thin film that selectively absorbs incident electromagnetic waves in discrete bands, which by design occur in any chosen range from near UV to far infrared. The structure consists of conducting islands separated from a conducting plane by a dielectric layer. By changing dimensions and materials, we have achieved broad absorption resonances centered at 0.36, 1.1, 14, and 53 microns wavelength. Angle-dependent specular reflectivity spectra are measured using UV-visible or Fourier spectrometers. The peak absorption ranges from 85 to 98%. The absorption resonances are explained using the model of an LCR resonant circuit created by coupling between dipolar plasma resonance in the surface structures and their image dipoles in the ground plane. The resonance wavelength is proportional to the dielectric permittivity and to the linear dimension of the surface structures. These absorbers have application to thermal detectors of electromagnetic radiation.
Patterning of gold-black infrared absorbing films by stencil lithography and hardening by polymer infusion is reported. Gold black nano-structured films are deposited through a thin metal shadow mask in a thermal evaporator in ~400 mTorr pressure of inert gas, followed by ethyl cyanoacrylate fuming through the same mask to produce rugged IR absorptive patterns of ~100 micron scale dimensions. Infrared absorptivity is determined by transmission and reflectivity measurements using a Fourier spectrometer and infrared microscope. Results indicate that the optimized hardening process reduces the usual degradation of the absorptivity with age. This work has potential application to infrared array bolometers.
Removal of polyimides used as sacrificial layer in fabricating MEMS devices can be challenging after hardbaking, which may easily result by the end of multiple-step processing. We consider the specific commercial co-developable polyimide ProLift 100 (Brewer Science). Excessive heat hardens this material, so that during wet release in TMAH based solvents, intact sheets break free from the substrate, move around in the solution, and break delicate structures. On the other hand, dry reactive-ion etching of hard-baked ProLift is so slow, that MEMS structures are damaged from undesirably-prolonged physical bombardment by plasma ions. We found that blanket exposure to ultraviolet light allows rapid dry etch of the ProLift surrounding the desired structures without damaging them. Subsequent removal of ProLift from under the devices can then be safely performed using wet or dry etch. We demonstrate the approach on PECVD-grown silicon-oxide cantilevers of 100 micron × 100 micron area supported 2 microns above the substrate by ~100-micron-long 8-micron-wide oxide arms.
The convergence of silicon photonics and infrared plasmonics allows compact, chip-scale spectral sensors. We report on
the development of a compact mid-IR spectrometer based on a broad-band IR source, dielectric waveguides, a
transformer to convert between waveguide modes and surface plasmon polaritons (SPP), an interaction region where
analyte molecules are interrogated by SPPs, an array of ring resonators to disperse the light into spectral components,
and photodetectors. The mid-IR light source emits into a dielectric waveguide, leading to a region that allows coupling
of the incident photons into SPPs. The SPPs propagate along a functionalized metal surface within an interaction region.
Interactions between the propagating SPP and any analytes bound to the surface increase loss at those wavelengths that
correspond to the analyte vibrational modes. After a suitable propagation length the SPP will be coupled back into a
dielectric waveguide, where specific wavelength components will be out-coupled to detectors by an array of ring
resonators. We have selected a 3.4 micron LED as the IR source, based on both cost and performance. Initial
experiments with circular waveguides formed from GLSO glass include measurement of the loss per mm.
Electrodynamic simulations have been performed to inform the eventual Si taper design of the proposed
photonic/plasmonic transformer. The SPP propagation length necessary for a discernible change in the signal due to
absorption in the interaction region has been estimated to be on the order of 1 mm, well within the bounds of calculated
propagation lengths for SPPs on Au.
Cathodoluminescence (CL) spectroscopy is performed on conducting 1- and 2-dimensional gratings of metals, semimetals
and semi-conductors of varying periods from 0.5 to 20 microns for a range of grating amplitudes from 0.1 to 4.6
microns. The overall emission spectrum consists of a 400 nm wide band centered at ~600 nm which depends little on the
grating period, grating amplitude, material, e-beam energy, or temperature. CL intensity increases and the center
wavelength blue shifts with increasing excitation beam current. For the larger amplitude 1-dimensional gratings fringes
appear in the emission spectrum, which is due to interference between emission from grating bars and grooves. Surface
corrugation is necessary to the emission as none is observed from smooth surfaces. The same band appears weakly in CL
of a Cu sample with random ~1 micron surface roughness, but this emission is strongly reduced when the same sample is
highly polished. The CL signal appears even when the ~10 nm electron-beam is at least 2 mm away from the grating
edge, suggesting electron-beam induced currents are important to the emission, whose precise mechanism remains
unclear. Previously suggested mechanisms--electron collision with image charge, transition radiation, surface
contamination, and inverse photoemission effect--all fail to explain the observed spectrum and its lack of beam-energy
dependence.
Cathodo-luminescence spectroscopy is performed on silver and gold lamellar gratings of period 7.5 or 20
microns for a range of grating amplitudes from 0.1 to 4.6 microns. The overall emission spectrum consists of a 400 nm
wide band centered at ~600 nm which depends little on the grating amplitude, metal, or e-beam energy. For the larger
grating amplitudes the emission spectrum is periodically modulated as a function of wavelength. Both the strength of the
emission envelop and the depth and phase of the modulation depend on grating orientation with respect to the light
collection axis, the distance of the excitation spot from the grating, and the distance between the grating and the
collection optics. The modulation can be explained as interference of surface emission from grating bars and grooves.
The origin of the emission remains unclear, as mechanisms of electron collision with image charge, transition radiation,
surface contamination, and inverse photo-electron effect all fail to explain the observed spectrum. This work is relevant
to the interpretation of cathodoluminescence studies of surface plasmons on structured metals for nano-photonic
applications.
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