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Microelectromechanical Systems, Journal of

Issue 3 • Date June 2003

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Displaying Results 1 - 17 of 17
  • Guest editorial how to avoid the reviewer's axe: one editor's view

    Page(s): 229 - 232
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    Freely Available from IEEE
  • Shaped comb fingers for tailored electromechanical restoring force

    Page(s): 373 - 383
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    Electrostatic comb drives are widely used in microelectromechanical devices. These comb drives often employ rectangular fingers which produce a stable, constant force output as they engage. This paper explores the use of shapes other than the common rectangular fingers. Such shaped comb fingers allow customized force-displacement response for a variety of applications. In order to simplify analysis and design of shaped fingers, a simple model is developed to predict the force generated by shaped comb fingers. This model is tested using numerical simulation on several different sample shaped comb designs. Finally, the model is further tested, and the use of shaped comb fingers is demonstrated, through the design, fabrication, and testing of tunable resonators which allow both up and down shifts of the resonant frequency. The simulation and testing results demonstrate the usefulness and accuracy of the simple model. Finally, other applications for shaped comb fingers are described, including tunable sensors, low-voltage actuators, multistable actuators, or actuators with linear voltage-displacement behavior. View full abstract»

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  • Low-order modeling of resonance for fixed-valve micropumps based on first principles

    Page(s): 325 - 334
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    Micropumps that utilize fixed-valves, i.e., valves having no moving parts, are relatively easy to fabricate and inherently reliable due to their simplicity. Since fixed-valves do not close, pumps based on them need to operate in a well-designed resonant mode in order to attain flow rates and pressures comparable with other designs. However, no methodology currently exists to efficiently investigate all the design parameters including valve size to achieve optimal resonant response. A methodology that addresses this problem is 1) the determination of optimal parameters including valve size with a low-order linear model capable of nonempirical prediction of resonant behavior, and 2) the independent determination of the best valve shape for maximal valve action over a target Reynolds number range. This study addresses the first of these two steps. The hypothesis of this study is that the resonant behavior of a fixed-valve micropump can be accurately predicted from first principles, i.e., with knowledge only of geometric parameters and physical constants. We utilized a new low-order model that treats the valves as straight rectangular channels, for which the unsteady solution to the Navier-Stokes equations is exact and with which the problem was linearized. Agreement with experiment using pump-like devices with valves replaced by straight channels was found to be excellent, thereby demonstrating the efficacy of the model for describing all aspects of the pump except actual valves. Agreement with experiment using pumps with Tesla-type valves was within 20 percent. With such accuracy and without the need for empirical data, the model makes possible reliable, efficient investigation and optimization of over 30 geometric and material parameters. View full abstract»

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  • Infrared thermal velocimetry in MEMS-based fluidic devices

    Page(s): 365 - 372
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    Most MEMS (microelectromechanical system) devices are made of silicon which is transparent at infrared wavelengths. Utilizing this infrared transparency of silicon, infrared thermal velocimetry was developed to measure the velocity in MEMS based fluidic devices. The method uses an infrared laser to generate a short heating pulse in a flowing liquid. An infrared camera records the radiative images from the heated flowing liquid and the steady flow velocity is obtained from consecutive radiative images. A wide range of the velocity (1 cm/s-1 m/s or higher) in silicon (or other materials that are transparent to infrared radiation) microchannels can be measured. Numerical simulations have been carried out and are in good agreement with the experiments. Parametric studies have been carried out for different channel dimensions and laser characteristics. View full abstract»

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  • Side-opened out-of-plane microneedles for microfluidic transdermal liquid transfer

    Page(s): 296 - 301
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    We present the first hollow out-of-wafer-plane silicon microneedles having openings in the shaft rather than having an orifice at the tip. These structures are well suited for transdermal microfluidic applications, e.g., drug or vaccine delivery. The developed deep-reactive ion etching (DRIE) process allows fabrication of two dimensional, mechanically highly resistant needle arrays offering low resistance to liquid flows and a large exposure area between the fluid and the tissue. The presented process does not require much wafer handling and only two photolithography steps are required. Using a 3×3 mm2 chip in a typical application, e.g., vaccine delivery, a 100 μl volume of aqueous fluid injected in 2 s would cause a pressure drop of less than 2 kPa. The presented needles are approximately 210 μm long. View full abstract»

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  • Surface topography evolution and fatigue fracture in polysilicon MEMS structures

    Page(s): 313 - 324
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    This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and fatigue fracture in polysilicon MEMS structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO2 layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of fatigue cycling before discussing the micromechanisms of fatigue fracture. View full abstract»

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  • A MEMS electromagnetic optical scanner for a commercial confocal laser scanning microscope

    Page(s): 243 - 251
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    A MEMS electromagnetic optical scanner for horizontal scanning of a commercial confocal laser scanning microscope has been developed. The purpose is to replace the currently used commercially available scanner with our new MEMS scanner in an existing microscope product, and therefore, the scanner specifications have to be compatible with those of the current one. Electromagnetic actuation is selected because of the millimeter-sized mirror, and a single crystal silicon hinge is used for realizing high-speed scanning with sufficient scan angle. In order to maintain mirror flatness for high quality optics requirement, the whole wafer thickness (300 μm) is used as the mirror, resulting in a large moment of inertia, and this has been taken into consideration in the actuator design. Although few MEMS actuators have been commercialized to date, it has successfully satisfied all the specifications including not only the fundamentals such as resonant frequency and scan angle but also those for the commercial product such as scanning stability and reliability. It has been commercialized as a part of our product, Olympus OLS1100 (remodeled as OLS1200 in August 2002). View full abstract»

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  • Characterization of surface micromachined metallic microneedles

    Page(s): 289 - 295
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    The purpose of the paper is to provide quantitative characterization of metallic microneedles. Mechanical and fluid flow experiments were performed to evaluate the buckling force, the penetration force, and the pressure versus flow rate characteristics of the microneedles. The microneedle design variations characterized included varying the shaft lengths, varying the tip taper angles/geometries, and the inclusion of micromechanical barbs. The penetration force was found to range from 7.8 gF for a microneedle of shaft length 500 μm, to 9.4 gF for a length of 1500 μm, both with a tip taper angle of 30°. Microneedles with a linear tip taper angle of 30° penetrated 95 +% of the time without failure. The microneedles with a 15° and 20° linear tip taper penetrated 10% and 25% of the time, respectively. The buckling force was found to be 98.4 gF for a 500 μm long microneedle shaft, 72.3 gF for a needle of shaft length 1000 μm, and 51.6 gF for a 1500 μm long shaft. The results demonstrate that the penetration force was 7.9% of the buckling force for 500 μm long shafts, 11.6% for a 1000 μm long shaft, and 18.2% for a 1500 μm long microneedle shafts. The microneedle fluid flow characteristics were studied. An inlet pressure of 49.0 Pa was required for a flow rate of 1000 μL/h and 243.0 Pa for a flow rate of 4000 μL/h using air as the fluid medium. For water, an average pressure of 30.0 kPa was required for a flow rate of 1000 μL/h and 106.0 kPa for a flow rate of 4000 μL/h. View full abstract»

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  • Surface micromachined metallic microneedles

    Page(s): 281 - 288
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    In this paper, a method for fabricating surface micromachined, hollow, metallic microneedles is described. Single microneedle and multiple microneedle arrays with process enabled features such as complex tip geometries, micro barbs, mechanical penetration stops and multiple fluid output ports were fabricated, packaged and characterized. The microneedles were fabricated using electroplated metals including palladium, palladium-cobalt alloys and nickel as structural materials. The microneedles were 200 mm-2.0 cm in length with a cross-section of 70-200 μm in width and 75-120 μm in height, with a wall thickness of 30-35 μm. The microneedle arrays were typically 9.0 mm in width and 3.0 mm in height with between 3 and 17 needles per array. Using water as the fluid medium, the average inlet pressure was found to be 30.0 KPa for a flow rate of 1000 μL/h and 106 KPa for a flow rate 4000 μL/h. View full abstract»

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  • The design and operation of a MEMS differential scanning nanocalorimeter for high-speed heat capacity measurements of ultrathin films

    Page(s): 355 - 364
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    A MEMS sensor has been developed for use as a calorimetric cell in an ultra-sensitive, thin-film, differential scanning calorimetric technique. The sensor contains a freestanding, thin (30 nm to 1000 nm), low-stress silicon nitride membrane with lateral dimensions of a few millimeters. This membrane, along with a thin (50 nm) metallization layer, forms a calorimetric cell with an exceptionally small addenda. This small addenda creates a very sensitive calorimetric cell, able to make heat capacity measurements of nanometer-thick metal and polymer films. The sensor fabrication and various design considerations are discussed in detail. The calorimetric technique and examples of applications are described. View full abstract»

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  • Fracture strength of polysilicon at stress concentrations

    Page(s): 302 - 312
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    Mechanical design of MEMS requires the ability to predict the strength of load-carrying components with stress concentrations. The majority of these microdevices are made of brittle materials such as polysilicon, which exhibit higher fracture strengths when smaller volumes or areas are involved. A review of the literature shows that the fracture strength of polysilicon increases as tensile specimens get smaller. Very limited results show that fracture strengths at stress concentrations are larger. This paper examines the capability of Weibull statistics to predict such localized strengths and proposes a methodology for design. Fracture loads were measured for three shapes of polysilicon tensile specimens - with uniform cross-section, with a central hole, and with symmetric double notches. All specimens were 3.5 μm thick with gross widths of either 20 or 50 μm. A total of 226 measurements were made to generate statistically significant information. Local stresses were computed at the stress concentrations, and the fracture strengths there were approximately 90% larger than would be predicted if there were no size effect (2600 MPa versus 1400 MPa). Predictions based on mean values are inadequate, but Weibull statistics are quite successful. One can predict the fracture strength of the four shapes with stress concentrations to within ±10% from the fracture strengths of the smooth uniaxial specimens. The specimens and test methods are described and the Weibull approach is reviewed and summarized. The CARES/Life probabilistic reliability program developed by NASA and a finite element analysis of the stress concentrations are required for complete analysis. Incorporating all this into a design methodology shows that one can take "baseline" material properties from uniaxial tensile tests and predict the overall strength of complicated components. This is commensurate with traditional mechanical design, but with the addition of Weibull statistics. View full abstract»

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  • Corner-cube retroreflectors based on structure-assisted assembly for free-space optical communication

    Page(s): 233 - 242
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    We have fabricated sub-millimeter-sized quad corner-cube retroreflectors (CCRs) for free-space optical communication. Each quad CCR structure comprises three mirrors micromachined from silicon-on-insulator wafers, and is designed to facilitate manual assembly with accurate angular alignment. Assembled CCRs exhibit mirror nonflatness less than 50 nm, mirror roughness less than 2 nm, and mirror misalignment less than 1 mrad, leading to near-ideal optical performance. The quad CCR incorporates a gap-closing actuator to deflect a base mirror common to the four CCRs, allowing their reflectivity to be modulated up to 7 kb/s by a drive voltage less than 5 V. We have demonstrated a 180-m free-space optical communication link using a CCR as a passive optical transmitter. Quad CCRs have been integrated into miniature, autonomous nodes that constitute a distributed wireless sensor network. We present an analysis of the signal-to-noise ratio of CCR-based links, considering the impact of CCR dimensions, ambient light noise, and other factors. View full abstract»

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  • A new edge-detected lift force flow sensor

    Page(s): 344 - 354
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    A lift force gas flow sensor which uses the force normal to the fluid flow to measure the flow velocity has recently been introduced. Two thin plates mounted at an angle are deflected when they are subjected to fluid flow. For most mechanical flow sensors the flow sensitivity is closely connected to the time response. A weaker structure gives higher flow sensitivity but a lower natural frequency, i.e., a slower response time. The lift force sensor is designed for measurements of respiratory gas flow in ventilators, where, in addition to low flow restriction, both high sensitivity and fast response are required. A new type of suspension has now been realized for the lift force flow sensor. The detection is separated from the suspension of the airfoil plate with the strain gauges placed on separate detector beams. This leads to separate parameters for optimization of the lift force concept with "independent" control of flow sensitivity and natural frequency. This paper presents an analytical model, simulations and measurements on the new structure. The new edge-detected sensor has been experimentally evaluated for different lengths (100-600 μm), widths (20-100 μm) and thicknesses (8-20 μm) of the detector beams. In accordance with the theory, the measurements show that the new structure has approximately three times the natural frequency of the old, center detected structure and similar or improved flow sensitivity. The evaluation has also resulted in a design scheme for optimal performance. A flow sensitivity of 0.65 μV/V/(l/min)2 has been obtained for the best edge-detected sensor with a natural frequency of 3.2 kHz. View full abstract»

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  • Microfabricated preconcentrator-focuser for a microscale gas chromatograph

    Page(s): 264 - 272
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    The design, fabrication, and testing of a preconcentrator-focuser (PCF), consisting of a thick micromachined Si heater packed with a small quantity of a granular adsorbent material are described. The PCF is developed to capture and concentrate vapors for subsequent focused thermal desorption and analysis in a micro gas chromatograph. The microheater contains an array of high-aspect-ratio, etched-Si heating elements, 520 μm (h)×50 μm (w)×3000 μm (l), bounded by an annulus of Si and thermally isolated from the remaining substrate by an air gap. This structure is sandwiched between Pyrex glass plates with inlet/outlet ports that accept capillary tubes for sample flow and is sealed by anodic bonding (bottom) and rapidly annealed glass/metal/Si solder bonding (top). The large microheater surface area allows for high adsorption capacity and efficient, uniform thermal desorption of vapors captured on the adsorbent within the structure. The adsorbent consists of roughly spherical granules, ∼200 μm in diameter, of a high-surface-area, graphitized carbon. Key design considerations, fabrication technologies, and results of performance tests are presented with an emphasis on the thermal desorption characteristics of several representative volatile organic compounds as a function of volumetric flow rates and heating rates. Preconcentration factors as high as 5600 and desorbed peak widths as narrow as 0.8 s are achieved from 0.25-L samples of benzene at modest heating rates. The effects of operating variables on sensitivity, chromatographic resolution, and detection limits are assessed. Testing of this PCF with a micromachined separation column and integrated sensor array is discussed briefly. View full abstract»

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  • Design of large deflection electrostatic actuators

    Page(s): 335 - 343
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    Electrostatic, comb-drive actuators have been designed for applications requiring displacements of up to 150 μm in less than 1 ms. A nonlinear model of the actuator relates the resonant frequency and the maximum stable deflection to the actuator dimensions. A suite of experiments that were carried out on deep reactive ion etched (DRIE), single-crystal silicon, comb-drive actuators confirm the validity of the model. Four actuator design improvements were implemented. First, a folded-flexure suspension consisting of two folded beams rather than four and a U-shaped shuttle allowed the actuator area to be cut in half without degrading its performance. Second, the comb teeth were designed with linearly increasing lengths to reduce side instability by a factor of two. Third, the folded-flexure suspensions were fabricated in an initially bent configuration, improving the suspension stiffness ratio and reducing side instability by an additional factor of 30. Finally, additional actuation range was achieved using a launch and capture actuation scheme in which the actuator was allowed to swing backward after full forward deflection; the shuttle was captured and held using the backs of the comb banks as high-force, parallel-plate actuators. View full abstract»

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  • A novel integrable microvalve for refreshable Braille display system

    Page(s): 252 - 263
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    We introduce a novel integrable and electrostatic microvalve for the purpose of enabling a pneumatic refreshable Braille display system (RBDS). Physical design parameters of the microvalve such as orifice size, beam length, number of beams and beam profile are experimentally explored and found promising for use with the RBDS. Particularly, one design with an orifice of 70 μm×70 μm, beam length of 665 μm, and beam count of 20 is electrostatically closed against a differential pressure of 82.7 kPa with an applied voltage of 68 V-rms. Also introduced is a steady-state mechanical model of the microvalve established on a coupled solution of fluid and solid domains. The model and experimental test results have been used to calculate the unknown discharge coefficient, elastic deflection, and entrance pressure. The model reveals that some of the designs have remarkably low discharge coefficient and entrance pressure, implying that pressure loss occurs mostly through and around the inlet port even at fairly large supply pressures. Experimental observations concerning the practical use of the microvalve are discussed. View full abstract»

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  • A self-retracting fully compliant bistable micromechanism

    Page(s): 273 - 280
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    A new class of fully compliant bistable mechanisms with the added benefit of integrated self-retraction has been developed (hereafter identified as Self-Retracting Fully compliant Bistable Mechanism or SRFBM). A technique using tensural pivots to manage compressive loading in compliant mechanisms is introduced and implemented in the SRFBM. The elimination of traditional kinematic joints and their associated clearance allows a total displacement between stable positions of 8.5 μm, and the mechanism size is less than 300 μm square when using 2.0 μm minimum line widths. Maximum actuation force is approximately 500 μN. The SRFBM's small linear displacement and reasonable actuation force facilitate integration with efficient thermal actuators. Furthermore, fully compliant mechanisms allow greater freedom in fabrication as only one mechanical layer is needed. Systems with on-chip actuation have been fabricated and tested, demonstrating bistability and on-chip actuation, which requires approximately 150 mW. A single fatigue test has been completed, during which the SRFBM endured approximately 2 million duty cycles without failure. View full abstract»

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Aims & Scope

The topics of interest include, but are not limited to: devices ranging in size from microns to millimeters, IC-compatible fabrication techniques, other fabrication techniques, measurement of micro phenomena, theoretical results, new materials and designs, micro actuators, micro robots, micro batteries, bearings, wear, reliability, electrical interconnections, micro telemanipulation, and standards appropriate to MEMS. Application examples and application oriented devices in fluidics, optics, bio-medical engineering, etc., are also of central interest.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Christofer Hierold
ETH Zürich, Micro and Nanosystems