This paper studies the use of Microsphere Photolithography (MPL) as an alternative to Focused Ion Beam milling or e-beam lithography to pattern plasmonic fiber-optic based sensors. In the MPL approach, silica microspheres are self-assembled to form a Hexagonal Close-Packed (HCP) array on top of a layer of photoresist. The microspheres serve as an optical element and focus collimated UV radiation to an array of photonic jets inside the photoresist layer. The exposed region is dependent on the angle of incidence of the UV radiation which facilitates hierarchical patterning. Pattern transfer can be accomplished using either etching or lift-off with the size of the features dependent on the exposure dose. While low-cost and very scalable, the use of MPL influences the design and performance of the fiber probe in several ways. Specifically, the entire cleaved face is patterned without alignment to the fiber core and any defects in the self-assembled microsphere lattice are transferred to the surface. This paper presents the low-cost fabrication of Extraordinary Optical Transmission (EOT) type plasmonic fiber-optic based sensors. The sensors consist of a thin aluminum film on the cleaved face of single mode optical fiber, perforated with a HCP hole-array. At resonance, Extraordinary Optical Transmission (EOT), decreases the reflection from the fiber tip. The conditions for resonance are dependent on the local environment surrounding the fiber tip and the resonant wavelength can be used to measure the index of refraction of a liquid. Experiments show that viable sensors can be created with MPL. The reflection spectra of the sensors was measured in various concentrations of sugar water with a measured sensitivity of 8.33 mg/mL/nm. These results are compared to simulation results which provides a sensitivity analysis of the sensors.
This paper reports the design, fabrication, characterization, and noise reduction of metasurface based uncooled infrared microbolometers with focus on device architecture. Two designs are investigated. In the first design, the devices are fabricated with the legs positioned underneath the microbolometer pixel. This is facilitated by the use of the metasurface which removes the need for a Fabry-Perot 1/4 cavity. Placing the legs underneath the pixel permits longer legs without sacrificing fill factor and raises the thermal resistance between the microbolometer and the substrate. The metasurface potentially allows spectrally dependent IR absorption. The second design extends this architecture to include a second microbolometer suspended above the first microbolometer to form a single pixel. Metasurfaces on each microbolometer can be designed to capture a portion of the spectrum with the combined structure maximizing the total absorptance across the Long Wave Infrared (LWIR) band. The TCR and resistivity are measured on the fabricated devices with and without the addition of the metasurface for both designs. The metasurface produces a slight increase in the TCR 5% to 12% and a dramatic reduction in the resistivity (>5×) which leads to a two order of magnitude reduction in the microbolometer noise voltage Power Spectral Density (PSD) after annealing in vacuum. The measured single cavity microbolometer has a voltage responsivity of 4.1×104 V/W and detectivity of 3.57×108 cm·Hz1/2/W.
This paper presents a study of metasurface integrated microbolometers. The semiconductor absorber is sandwiched between a metal Frequency-Selective Surface (FSS) and ground plane. When the semiconductor absorber is electrically isolated from the ground plane by a thin dielectric it can be used to measure the temperature of the pixel. The integration with the FSS removes the need for a Fabry-Perot cavity. The FSS allows control the attributes of radiation absorbed by the microbolometer on a pixel-by-pixel basis which provides the potential for spectral or polarimetric imaging. The FSS also affects he electrical performance of the semiconductor absorber and the thermal performance of the microbolometer. In addition, the complex permittivity of the semiconductor affects the optimal design of the FSS. The Si/Ge/O system is selected because it allows the properties of the absorber to be engineered (e.g., less oxygen gives lower absorptance and higher resistivity). This paper explores the absorber/FSS parameter space with an emphasis on the electrical and noise properties of the integrated system. Models are developed to explain results. Preliminary results show that the addition of the FSS improves TCR of the microbolometer by 10% while dramatically lowering its resistivity (factor of 5×). The resistivity reduction leads to a dramatic reduction of the noise power spectral density with the addition of FSS improving the measured 1/f noise by two orders of magnitude over an identical sample without the FSS. In addition, this paper will present the microbolometer figures of merits including voltage responsivity, detectivity, and thermal response time.
Frequency Selective Surfaces (FSS) are periodic array of sub-wavelength antenna elements. They allow the absorptance
and reflectance of a surface to be engineered with respect to wavelength, polarization and angle-of-incidence. This paper
applies this technique to microbolometers for uncooled infrared sensing applications. Both narrowband and broadband
near perfect absorbing surfaces are synthesized and applied engineer the response of microbolometers. The paper
focuses on simple FSS geometries (hexagonal close packed disk arrays) that can be fabricated using conventional
lithographic tools for use at thermal infrared wavelengths (feature sizes > 1 μm). The affects of geometry and material
selection for this geometry is described in detail. In the microbolometer application, the FSS controls the absorption
rather than a conventional Fabry-Perot cavity and this permits an improved thermal design. A coupled full wave
electromagnetic/transient thermal model of the entire microbolometer is presented and analyzed using the finite element
method. The absence of the cavity also permits more flexibility in the design of the support arms/contacts. This
combined modeling permits prediction of the overall device sensitivity, time-constant and the specific detectivity.
This paper presents a detailed characterization of silicon germanium oxide (SixGeyO1-x-y) thin films with an Oxygen
concentration below 10%. The results demonstrated that a high TCR and a low corresponding resistivity can be achieved
using various compositions, for example, Si0.054Ge0.877O0.069 film has achieved a TCR and a resistivity of -3.516/K, and
629 Ω-cm, respectively. The lowest measured resistivity and the corresponding TCR were 119.6 Ω-cm and -2.202 %/K
respectively, using Si0.136Ge0.838O0.026 for film deposited at room temperature, whereas the highest achieved TCR and the
corresponding resistivity at room temperature were -5.017 %/K, and 39.1×103 Ω-cm, respectively, using
Si0.167Ge0.762O0.071 for films deposited at room temperature. The calculated activation energy (Ea) from the slope of
Arrhenius plots were varied between 0.1232 eV to 0.3788 eV. The X-ray diffraction study demonstrated that the films
are amorphous but did not show any dependence on varying silicon at fixed oxygen concentration. The noise study
demonstrated that these films exhibit relatively high 1/f.
We have designed, modeled, fabricated and tested novel MEMS variable capacitors with two air cavities (two capacitors) for electrostatic power harvesting utilizing mechanical vibration in environment. The device is unique in the use of an innovative two-cavity design and electroplated nickel as the main structural material, which allows using both up and down directions to generate energy. The prototype of two-cavity MEMS variable capacitors have been successfully fabricated using surface micromachining. The initial testing for investigating electrical dynamic behaviors and power generation from the fabricated devices was implemented.
The modeling, fabrication and testing of a two-cavity MEMS capacitor utilizing inertial forces from unwanted
ambient vibrations has been performed. The device was designed with two air cavities and a thick movable metallic
plate in order to increase the efficiency of the energy conversion from mechanical vibration. The moving plate was
sandwiched between two fixed plates to construct a two cavity capacitor. The improved model verified the fact that
that two-cavity model enhances the average output power by 2 to 5 times of a single cavity model. The device was
designed with soft suspension beams and with a thick plate in order to achieve a natural frequency close to the ambient
vibration frequencies. The FEM analysis showed that a thick electroplated nickel plate and beams can results in a
natural frequency less than 1 kHz. The behavior of the plate under damping was also calculated using FEM analysis.
The MEMS converters were fabricated using surface micromachining technology, nickel electroplating and photoresist
sacrificial layer. The moving plate and suspension beams were grown on the photoresist sacrificial layer and nickel
anchors. The structure was released by removing the photoresist sacrificial layers using photoresist strip remover. To
form the top cavity, nickel bonding tabs with sufficient thickness were grown by electroplating on another substrate
followed by indium electroplating with a thickness of 1 μm. The two substrates were then aligned and bonded
together. A good control of the height of the two cavity MEMS capacitor is possible with the control of Ni deposition
and sacrificial layer thicknesses.
An impedance biosensor was designed, fabricated and tested for detection of viable Escherichia coli O157:H7 in food
samples. This device consists of interdigitated microelectrode array (IDEA) fabricated using thin layer of sputtered gold,
embedded under a polydimethylsiloxane (PDMS) microchannel. The array of electrodes is designed to detect viable EColi
in different food products. The active surface area of the detection array was modified using goat anti-E.coli
polyclonal IgG antibody. Contaminated food samples were tested by infusing the supernatant containing bacteria over
the IDEA's, through the microchannel. Antibody-antigen binding on the electrodes results in impedance change. Four
serial concentrations of E.coli contaminated food samples (3x102 CFUmL-1 to 3x105 CFUmL-1) were tested. The
biosensor successfully detected the E.coli samples, with the lower detection limit being 3x103 CFUmL-1 (up to 3cells/μl).
Comparing the test results with an IDEA impedance biosensor without microchannel (published elsewhere) indicates that
this biosensor have two order of magnitude times higher sensitivity. The proposed biosensor provides qualitative and
quantitative detection, and potentially could be used for detection of other type of bacteria by immobilizing the specific
type of antibody.
We demonstrate the rapid detection of explosive vapors based on a fiber-based optical Fabry-Pérot (FP) gas sensor. The
sensing probe of the FP sensor is composed of a thin metal layer and a vapor-sensitive polymer layer that are deposited
sequentially on a cleaved fiber endface to form an FP cavity. The interference spectrum generated from the reflected
light at the metal-polymer and polymer-air interfaces changes upon the absorption of gas analyte. By monitoring the
interference shift, we are able to obtain quantitative and knetic information of the interaction between the analyte and the
polymer layer. We further assemble the FP sensor with a short fused silica capillary into a sensor module, and employ it
in a gas chromotgraphy (GC) system for selevtive rapid on-column detection. In this report, we specifically target 2, 4-
dinitrotoluene (DNT) and 2, 4, 6-trinitrotoluene (TNT) for their obvious defense applications. This work could lead to a
portable sensor capable of detecting low concentrations of DNT, TNT, and other explosive chemicals.
Thin film SixGe1-xOy infrared sensitive material was grown by RF magnetron sputtering, by depositing Si and Ge thin
film simultaneously from two deposition targets in an oxygen (O) and argon environment at room temperature and at
400°C. Film composition was varied by adjusting RF power applied to the silicon target and by varying the oxygen
flow of the gas mixture in the deposition chamber. The atomic compositions of Si, Ge, and O in the deposited thin film
were determined and analyzed using energy dispersive X-ray spectroscopy (EDS). The influence of changing Ge and
Si and O compositions on temperature coefficient of resistance (TCR), and resistivity were studied. Different
fabrication scenarios have been used to vary the Ge, Si and O concentrations. The highest achieved TCRs and the
corresponding resistivities at room temperature were -4.86 %/K and -6.43 %/K, and 2.45×102 Ω cm and 3.34×102 Ω
cm using Si0.195Ge0.706O0.099and Si0.127Ge0.835O0.038 for films deposited at room temperature and at 400 oC, respectively.
Thin film SixGe1-xOy were deposited on glass, silicon and SiO2 by RF magnetron sputtering using co-sputtering of silicon and germanium targets in an environment of oxygen and argon. Silicon percentage was varied from ~7% to 22%. Exact contents of each material were determined by XRD/EDS and electrical properties of amorphous
compound were studied. High values of temperature coefficient of resistance were obtained in specific conditions.
the highest achieved TCR at room temperature was (5.8%/K) using Si0.177Ge0.726O0.097 (film deposited at 400 °C).
The measured resistivity on this sample was 14.6 Ω cm.
This paper presents the design and fabrication of MEMS based Coulter counter for monitoring cellular volumetric
changes after an exposure to various media. The design consists of a thick SU8 channel which is divided into mixing,
focusing, and measuring regions. The mixing region is a serpentine shaped channel, enabling complete mixing of a
sample and a reactant before entering the focusing region. The focusing region consists of an electrode pair used to
generate AC fields that result in negative dielectrophoretic forces directing cells from all directions to the center of the
channel to prevent clogging of the Coulter channel. Finally, the measuring region consists of a channel of width ranging
from 20-25 μm, with multiple electrode pairs fabricated using electroplated gold in order to measure the change in
impedance at different points along the channel as a cell passes through. This device improves upon existing macro-scale
Coulter counter technology by allowing extremely small sample sizes (101 compared to 105 cells per experiment), an
extremely short time frame from the exposure to reactant media to the initial measurement, serial time series
measurements of a single cell, and optical microscopic monitoring of the experiment. The design of this chamber will
allow for the manufacture of cell specific channel diameters in order to maximize measurement precision for each cell
type. This design also eliminates the sheath flow and complex fluid control systems that make conventional cytometers
bulky and complicated.
A new micromachined one dimensional (1-D) micromirror array structure is presented that utilizes primarily
electroplated nickel, a mechanically durable material with a high glass transition temperature and with controllable
residual stress as the main structural material. The goal of this research is to develop custom micromirror array for use in
epitaxial growth systems to define the device structure and hence eliminate the need for etching and lithography, the
same micromirror can be used for switches and optical cross-connects. The high glass transition temperature of nickel
allows it to be used at high temperature without causing any contamination to the epitaxial systems or to the deposited
materials. Micromirror arrays with 5×5 and 1×5 pixels were designed with square shape with an area of 500 μm2 to
provide high fill factor and uniform stress distribution. The focus of this paper is on improved design for reducing
actuation voltage and increasing the rotation angle. The micromirror was previously fabricated using surface
micromachining technologies with a thick photoresist sacrificial layer [1]. The torsion beams were designed with a
serpentine shape in order to optimize the voltage necessary to tilt the micromirror by ± 10°. The micromirrors were
simulated using Coventor finite element tool in order to determine their geometries and performance. A voltage of 20
volts was required to rotate the mirror with a pixel pitch of 500 μm by 7.68° with resonance frequency of 221.52 Hz.
This paper describes the design, fabrication and testing of tunable Fabry-Perot filters. The goal of this research is to develop novel tunable filter with an area of 5x5 mm2 that will be used in infrared gas sensors. This exploits the fact that most gases have unique infrared absorption signatures in the 2-14 µm wavelength region. The filter consists of two thin silicon wafers coated with quarter wave dielectric layers to form wavelength dependent high reflection mirrors and separated by air gaps with an average height of 8, 5.1 and 3.5 μm. The mirrors are supported by four elastic polymer posts (springs) each with an area of 100×100 μm2 made by using photo definable polydimethylsilxane (PDMS). An
electrostatic voltage is used to compress the springs, change the airgap height and hence shift the transmission peaks to a shorter wavelength. A finesse of 12 with full width at half maximum (FWHM) of 70 nm, and a peak transmission of 63% were achieved by applying 100 volts on a device with 8 µm post height and wafer thickness of 125 µm. In addition, the measured tunability before and after hard baking of the device was 210 nm and 130 nm respectively. The tunability
stayed constant after hard baking the devices and did not show any changes with time. The tunability was also measured on a thinner silicon mirror with 3.5 µm post height. In this case, the filter was tuned 180 nm by applying 10 volts. However, the filter finesse was 3, transmission peak was 40% and FWHM over 200 nm. An antireflection coating was deposited on one side of silicon wafers and a Fabry-Perot filter to study transmission enhancement and satisfactory results were achieved.
This paper describes the modeling, design, fabrication and testing of advanced uncooled thermal detectors, based on semiconducting YBaCuO. The aim is to provide NASA with advanced broad-band infrared (IR) detectors to replace the current CERES (Clouds and the Earth's Radiant Energy System) hardware that utilizes three channels, each housing a 1.5 mm X 1.5 mm thermister bolometer with 1 X 4 array of detectors in each of the three channels, thus yielding a total of 12 channels. A double mirror structure is used to obtain uniform spectral response from 0.3-100 μm wavelength. Double absorbers are utilized to further flatten the spectral response and to enhance the absorption of infrared radiation. The devices were fabricated using a polyimide sacrificial layer to achieve thermal isolation of the detector. A low thermal conductivity to the substrate enables the detector to integrate the energy from the incident radiation. An air gap was created by ashing the polyimide sacrificial layer from underneath the thermometer. A passivation layer was used to protect YBaCuO during ashing process and maintain a relatively high temperature coefficient of resistance of around 2.8%. These devices have successfully demonstrated voltage responsivities over 103 V/W, detectivities above 108 cm Hz1/2/W, NEP per root Hertz bandwidth less than 4 X 10-10 W/Hz1/2 and thermal time constant less than 15 ms. Several specific designs were fabricated and tested. Relatively uniform response in the wavelength range of 0.6 to 15 μm was measured.
Amorphous semiconducting Y-Ba-Cu-O is attractive as the temperature sensitive element for uncooled IR bolometers and pyroelectric detectors. Thin films can be easily fabricated by RF magnetron sputtering at room temperature from a composite target. It is compatible with micromachining techniques for the fabrication of thermally isolated structures. As a bolometer, Y-Ba-Cu-O possesses a relatively high temperature coefficient of resistance of 3.5% K-1 near room temperature. This paper will present the IR characteristics of 40-micrometer X 40-micrometer microbolometer arrays fabricated in thermal isolation structures. These detectors are aimed at thermal imaging at 10-micrometer wavelength. Recently, self-supporting YBaCuO pixels have been developed. In this case, the Y-Ba-Cu-O thin film pixel requires no underlying bridge material to provide structural support. The Y-Ba-Cu-O thin film is supported solely by the electrode arms. Responsivity and detectivity greater than 4 X 103 V/W and 108 cmHz1/2/W respectively have been measured in these detectors. The development of large area 1.5-mm and 0.4- mm square YBaCuO bolometers for NASA's global warming studies in low-orbiting satellites will also be presented. These large area detectors require large optical bandwidths covering the 0.3-micrometer to 100-micrometer wavelength band.
Amorphous semiconducting Y-Ba-Cu-O has shown promise as the temperature sensitive element for uncooled IR detectors as both a bolometer and pyroelectric material. Thin films can be easily fabricated by RF magnetron sputtering at room temperature from a composite target. As a bolometer, Y-Ba- Cu-O possesses a relatively high temperature coefficient of resistance of 3.5% K-1 near room temperature. As a pyroelectric detector, pyroelectric coefficients as high as 20 (mu) C/cm2-K have been measured yielding a pyroelectric figure of merit of 0.065 (cm3/J)1/2. In Y-Ba-Cu-O, the oxygen concentration has been shown to determine the hole concentration and mobility. However, the anion stoichiometry plays an equally important role in determining the electronic characteristics. In this work, we have explored the effects of substitution for Cu and the corresponding changes on the electronic properties affecting the performance as an IR detector. Further, we have fabricated micromachined 1 X 10 arrays in which utilize a self-supporting Y-Ba-Cu-O thin film geometry. In this case, the Y-Ba-Cu-O film is held above the substrate only by the electrode arms, without the need of any underlying bridge material. These detectors posses a low thermal mass and have yielded detectivities as high as 108 cm-Hz1/2/W, which extrapolates to NETDs less than 20 mK.
This paper reports the fabrication of microbolometers using semiconducting YBaCuO as the IR sensing material. The detectors are operable at room temperature and thus are suitable for lost-cost and high performance imaging applications. Semiconducting YBaCuO is promising as a bolometric material as it has a thermal coefficient of resistance near 3% and relatively low noise. Two different bolometer structures will be reported here. First generation YBaCuO microbolometers were built on micromachined SiO2 bridges using wet etching techniques to undercut the silicon. The second generation structures were processed upon micromachined Si3N4 membranes with sputtered MgO films used as sacrificial layers. The membrane structures are the first of its kind to incorporate MgO as a sacrificial layer, and they offer a fabrication technique that is fully CMOS compatible, with all processing at ambient temperatures. Detectivities in the order of 108 cm Hz1/2/W were measured at 30 Hz chopping frequency in both structures. The thermal conductance of the suspended membranes was on the order of 10-7 W/K, which is desirable as low thermal conductance yields high responsivities. There are realizable optimizations for both applications to yield detectivities over 109 cm Hz 1/2/W. All measurements reported here were performed at ambient temperature with no temperature stabilization.
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