Optical MEMS Group in print

A list of recent publications and literature contributions by Optical MEMS group members. Click the PDF icon to download a document, or use the CrossRef reference number (where available) to view the document from the publisher.

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Surface heterostructure nanomechanical actuators with atomic resolution

Jan D. Makowski, Joseph J. Talghader; Applied Physics Letters, 2007

A continuously tunable vertical actuator with subnanometer resolution is presented. It consists of a heterostructure cantilever which has collapsed over a 125 nm thick nanogap. Its operating principle relies on the temperature dependence of the adhesion energy between two InGaAs surface quantum wells. Deflections from -17 to 5 nm with a precision better than three atomic layers have been measured. 2007 American Institute of Physics.

CrossRef DOI: 10.1063/1.2735675

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Adhesion energy in nanogap InP/InGaAs microcantilevers

Jan D. Makowski, Anand Gawarikar, and Joseph J. Talghader; Applied Physics Letters, 2006

The adhesion energy is measured between InGaAs quantum wells that have collapsed across a 125 nm air gap in an InP/InGaAs heterostructure. The method relies on measuring the unadhered length and shape of collapsed microcantilevers with optical interferometry. The adhesion energy is found to be 7216 mJ m-2. Since the air gap is much smaller than has been measured previously, the influence of van der Waals forces across the gap was included in theoretical modeling. It was found that the forces should not cause significant deviation from the standard adhesion models unless the adhesion energy drops below 25 mJ m-2. 2006 American Institute of Physics.

CrossRef DOI: 10.1063/1.2405854

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Thermally induced stress hysteresis and co-efficient of thermal expansion changes in nanoporous SiO2

Martin T K Soh, Jeremy Thurn, J H Thomas III and Joseph J. Talghader; Journal of Applied Physics, 2007

The thermomechanical response of electron beam deposited nanoporous silicon dioxide is examined using substrate curvature measurements and nanoindentation. Analysis of the thin film bond angle strain distributions versus temperature indicates that low temperature (T < 100 °C) stress hysteresis and tensioning are primarily attributed to hydrogen bonded water desorption. However, at higher temperatures, the absence of water desorption suggests that the thermomechanical behaviour is related to thermally induced bond angle strain redistributions towards the local bonding environment of quartz and thermally grown silicon dioxide. This is supported by the co-efficient of thermal expansion data that trend lower with higher annealing temperatures. The re-absorption of water into the thin film accounts for the reproducibility of the open-loop stress hysteresis and tensioning observations.

CrossRef DOI: 10.1088/0022-3727/40/7/048

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Micromachined Particles for Detecting Metal-Ion Concentration in Fluids

Ryan Supino and Joseph J. Talghader; Journal of Microelectromechanical Systems, 2006

A remote microfluidic metal-ion sensor is developed using an electrochemical system integrated with a compact photovoltaic cell power supply. The sensor is designed to detect the sum of metal ions in a remote environment. The sensor uses electrodeposition to remove ions from the fluid around the sensor and deposit them on an electrode at the tip of a cantilever. The electrodeposited mass changes the resonant frequency of the cantilever, which can be determined upon read-out. The sensor is designed to be dropped in liquids or flow through microfluidic systems and can be used in parallel with many other similar sensors. The photovoltaic cells are directly integrated on the device and are capable of producing tens of microwatts of power at about 15% efficiency with laser excitation. However, the sensor operates at power levels of 50 nW with small voltages and currents using only scavenged daylight or room light. The complete device is integrated into a total volume below 0.046 mm3, which is more than two orders of magnitude smaller than other remote electrically-powered sensors reported to date. Although it is expected that multiple devices will be used in parallel to gain statistical data, individual particles detect metal-ion concentration within 24% of the actual concentration, making them suitable for safety testing and endpoint monitoring among other applications.

CrossRef DOI: 10.1109/JMEMS.2006.880246

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Spatial-Mode Analysis of Micromachined Optical Cavities Using Electrothermal Mirror Actuation

Wei Liu and Joseph J. Talghader; Journal of Microelectromechanical Systems, 2006

The performance of optical microcavities is limited by spectral degradation resulting from thermal deformation and fabrication imperfections. In this paper, we study the spatial-mode properties of micromirror optical cavities with respect to commonly seen aberrations. Electrothermal actuation is used to slightly adjust the shape and position of micromirrors and study the effects this has on the spatial-mode structure of the cavity spectrum. The shapes of the micromirrors are changed using Joule heating with thermal expansion deformation. Significant differences in mirror tilt, curvature, and astigmatism are measured, but the tilt has by far the biggest impact on cavity finesse and resolution. We demonstrate that unwanted higher order spatial modes can be suppressed electrically and an amplitude reduction for the higher order modes of over 60% has been obtained with a tuning current of 5.5 mA. A fundamental mode finesse of approximately 60 is maintained throughout tuning. These tunable cavities have great potential in applications using cavity arrays or requiring dynamic mode control.

CrossRef DOI: 10.1109/JMEMS.2006.878881

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Coupled absorption filters for thermal detectors

Yuyan Wang, B.J. Potter, and Joseph J. Talghader; Optics Letters, 2006

A resonant absorption cavity that couples long-wavelength infrared (LWIR) light into a movable plate has been demonstrated for thermal detectors, especially microbolometers. Each device is continuously tunable over 8.711.1m by using electrostatic actuation with voltages from 0 to 42 V. The width of the resonance is relatively broad, approximately 1.5m, to match the large widths of many spectral features in the LWIR. At an actuation voltage of 45 V, the device switches into a broadband mode with an absorption width of 2.83m. This latter mode is used to enhance sensitivity in low-light situations in which little spectral information is present. 2006 Optical Society of America

CrossRef DOI: not available

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Design and characterization of adaptive microbolometers

Woo-Bin Song and Joseph J. Talghader; Journal of Micromechanics and Microengineering, 2006

We report the first practical results on design and characterization of adaptive microbolometers with a thermally tuned responsivity. Such devices are needed to simultaneously image scenes that contain objects at ambient and extremely hot temperatures. In the high detectivity state, the microbolometers are operated similar to standard commercial devices. In the low detectivity state, portions of the support beams are brought in contact with the substrate, which thermally shorts the devices on a pixel-by-pixel basis. This is the first time this concept has been practically implemented in a microbolometer device architecture as opposed to simple cantilever beams and plates. The maximum actuation voltage is set to 17 V, and the thermal conductances, responsivities and detectivities of a typical device can be switched more than an order of magnitude between 1.65 10-5WK-1 and 2.99 10-4WK-1, between 1.5 V W-1 and 0.2 V W-1 and between 1.8 106 cm Hz1/2 W-1 and 1.5 105 cm Hz1/2 W-1, respectively. This extends the dynamic range of the device more than an order of magnitude. The device pixel size is 100 m 100 m.

CrossRef DOI: 10.1088/0960-1317/16/5/028

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Stress-mapping sensors for high-power adaptive micro-optics

Mohd Suffian B. Zamali and Joseph J. Talghader; Journal of Micromechanics and Microengineering, 2006

Continuous deformable membrane mirrors are becoming more attractive for use in adaptive optics because they cause no diffraction in the reflected beam and ensure smooth and continuous phase variations across the mirrors. However, when such mirrors are used to correct a high-power incident wave front, the absorption in the coatings causes the temperature of the membrane to increase, thereby creating in-plane thermal stress due to the rigidly clamped boundaries. We present a technique to measure thermal stress in such nondeforming membrane structures. The directional stress and temperature effects are simultaneously measured and decoupled in micromachined membrane mirrors by using a group of three ion-implanted silicon resistors with different orientations. In stress measurements made with incident power, the sensors measure changes in compressive thermal stress to within 8090 kPa. 2006 Optical Society of America

CrossRef DOI: not available

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Thermally induced structural changes in nanoporous silicon dioxide from x-ray photoelectron spectroscopy

Martin T.K. Soh, J.H. Thomas III, and Joseph J. Talghader; Journal of Vacuum Science and Technology A, 2006

Morphological changes due to adsorbed gases in nanoporous silicon dioxide thin films are demonstrated using in situ x-ray photoelectron spectroscopy at temperatures in the range 20≤T≤ 300 °C. Adsorbed hydrogen bonded water vapor is observed to relax the surface bond strain of low-temperature electron-beam deposited silicon dioxide up to 100 °C. This was determined by measuring the width of the Si 2p and O 1s photoemission peak full widths at half maximum, which are distinctly smaller for films with adsorbed water vapor than for the same films after vapor has been outgassed by heating above 100 °C. In situ heating in the range 100≤T≤200 °C decreases the peak width as the atoms gain sufficient energy to establish a more homogeneous local bonding environment. This process is overshadowed above 200 °C as thermally induced localized bond strains and charge inhomogeneities at the surface begin to introduce disorder, as demonstrated in the repeatable increase in peak spread with temperature for thermally grown silicon dioxide and quartz. The in vacuo peak width behavior in subsequent thermal cycles is repeatable for the nanoporous thin films. However, if the films are reexposed to atmosphere, the initial increase in peak width is seen again. 2006 American Vacuum Society.

CrossRef DOI: 10.1116/1.2359734

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Zirconium Tungstate (ZrW2O8)-Based Micromachined Negative Thermal-Expansion Thin Films

Michael S. Sutton and Joseph J. Talghader; Journal of Microelectromechanical Systems, 2004

Negative expansion materials are relatively rare but promise to be particularly useful in designing thermally sensitive mechanical devices. Although negative thermal expansion (NTE) in bulk materials such as ZrW2O8 has been extensively studied, this paper reports the first deposition of a NTE material thin film. ZrWxOy films were deposited by electron beam evaporation and reactive cosputtering. The films were processed and patterned for various microstructures. The coefficients of thermal expansion of the deposited thin films were determined by measuring the change in curvature with temperature. It was found that evaporated films but not sputtered films, which were denser than the evaporated films, exhibited NTE. It was also found that NTE behavior occurred across a variety of stoichiometries. Since crystalline ZrW2O8 and thin film ZrWxOy both have low densities and show negative expansion, it is speculated that similar physical mechanisms, as discussed in the text, are at work. Further, since the deposition conditions of a thin film can often be changed to control density, it is speculated that a wider variety of thin films than bulk crystals might be made to have NTE.

CrossRef DOI: 10.1109/JMEMS.2004.832191

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Current-controlled curvature of coated micromirrors

Wei Liu and Joseph J. Talghader; Optics Letters, 2003

Precise control of micromirror curvature is critical in many optical microsystems. Micromirrors with current controlled curvature are demonstrated. The working principle is that resistive heating changes the temperature of the micromirrors and thermal expansion induces a controlled curvature whose magnitude is determined by coating design. For example, for wide focal-length tuning, the radius of curvature of a gold-coated mirror was tuned from 2.5 to 8.2 mm over a current-induced temperature range from 22 to 72 C. For fine focal-length tuning, the radius of curvature of a dielectric-coated (SiO2Y2O3 λ/4 pairs) mirror was tuned from 20.68 to 20.64 mm over a current-induced temperature range from 22 to 84 C. These results should be readily extendable to mirror flattening or real-time adaptive shape control. 2003 Optical Society of America

CrossRef DOI: not available

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