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Microelectromechanical Systems, Journal of

Issue 6 • Date Dec. 2004

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Displaying Results 1 - 25 of 27
  • Table of contents

    Page(s): c1 - c4
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  • Journal of Microelectromechanical Systems publication information

    Page(s): c2
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  • Microengineered electrically resettable circuit breaker

    Page(s): 887 - 894
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    Electromechanical circuit breakers based on microelectromechanical systems (MEMS) structures are proposed, and initial prototypes are demonstrated. The devices use laterally moving thermal actuators to set and trip low resistance contacts. The actuators are fabricated from electroplated Ni, on Si substrates, with Au-Co contact layers. Trip currents of 300 mA are obtained, with contact resistances below 1 Ω. Rapid tripping (below 50 ms) is achieved, even for modest over-current levels, and sensitivity to ambient temperature is much less than for positive temperature coefficient (PTC) devices typically used for over-current protection in electronics applications. These MEMS devices offer an ultra-miniature, potentially low cost solution for circuit protection applications at low currents, where a high degree of system control is desired. View full abstract»

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  • Wafer-scale microdevice transfer/interconnect: its application in an AFM-based data-storage system

    Page(s): 895 - 901
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    We have developed a robust, CMOS back end of the line (BEOL) compatible, wafer-scale device transfer, and interconnect method for batch fabricating systems on chip that are especially suitable for MEMS or VLSI-MEMS applications. We have applied this method to transfer arrays of 4096 free-standing cantilevers with good cantilever flatness control and high-density vertical electrical interconnects to the receiver wafer (typically CMOS). Such an array is used in a highly parallel, scanning-probe-based data-storage system, which we internally call "millipede." A very high-integration density has been achieved, even for wafer-scale transfer, thanks to the interlocking nature of the interconnect structure, which provides easy alignment with an accuracy of 2 μm. The typical integration density is 100 cantilevers/mm2 and 300 electrical interconnects/mm2. Note that only the cantilevers, not a chip with cantilevers, are transferred and, unlike flip-chip technology, our method preserves the device orientation, which is crucial for MEMS applications, where often the MEMS device should have access to its environment (in our case, the cantilever tips are in contact with the storage medium). After device transfer, the system is mechanically and electrically stable up to at least 500°C, allowing post-transfer wafer processing. View full abstract»

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  • A low-voltage lateral MEMS switch with high RF performance

    Page(s): 902 - 911
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    MEMS switches are one of the most promising future micromachined products that have attracted numerous research efforts in recent years. The majority of MEMS switches reported to date employ electrostatic actuation, which requires large actuation voltages. Few are lateral relays and those often require nonstandard post process, and none of them is intended for high-frequency applications. We have developed an electrothermally actuated lateral-contact microrelay for RF applications. It is designed and fabricated on both low-resistivity and high-resistivity silicon substrate using surface micromachining techniques. The microrelay utilizing the parallel six-beam actuator requires an actuation voltage of 2.5-3.5 V. Time response is measured to be 300 μs and maximum operating frequency is 2.1 kHz. The RF signal line has a current handling capability of approximately 50 mA. The microrelay's power consumption is in the range of 60-100 mW. The lateral contact mechanism of the microrelay provides a high RF performance. The microrelay has an off-state isolation of -20 dB at 40 GHz and an insertion loss of -0.1 dB up to 50 GHz. The simplicity of this 4-mask fabrication process enables the possibility of integrating the microrelay with other passive RF MEMS components. View full abstract»

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  • Development of a rapid-response flow-control system using MEMS microvalve arrays

    Page(s): 912 - 922
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    A method for providing high-resolution gas flow control using microelectromechanical systems (MEMS) has been developed and tested. The micromachined component consists of an array of 61 synchronized microvalves operating in parallel. A number of tests were conducted on microvalves of various designs to characterize their operation. The best performing of these was used with a prototype flow controller. Additionally, a mathematical model of the flow system and controller was derived to predict the response of the system to various changes in operating conditions. This work describes the design, modeling, and testing of a compact, stand-alone mass flow controller (MFC) to demonstrate high resolution, fast response flow control using MEMS microvalves. The device consists of a microvalve array packaged with a micro flow sensor and a microprocessor-based control system. The high bandwidth of microvalves allows an atypical flow control architecture. The controller regulates a pulsewidth-modulated (PWM) signal sent to the valve array and is capable of both open- and closed-loop control. A mathematical model was also developed to predict the dynamic performance of the system under various operating conditions. Additional advantages of the MEMS flow-control system include low-power consumption, low fabrication costs, and scalable precision. View full abstract»

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  • Vertical focusing device utilizing dielectrophoretic force and its application on microflow cytometer

    Page(s): 923 - 932
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    Focusing of particles/cells in the vertical direction inside a micromachined flow cytometer is a critical issue while using an embedded optical detection system aligned with microchannels. Even if the particles/cells have been focused centrally in the horizontal direction using coplanar sheath flows, appreciable errors may still arise if they are randomly distributed in the vertical direction. This work presents a vertical focusing device utilizing dielectrophoretic (DEP) forces and its application on micromachined flow cytometer. A pair of parallel microelectrodes is deposited on the upper and bottom surface of the microfluidic channel to drive particles/cells into the vertical center of the sample flow. This new microfluidic device is capable of three-dimensional (3-D) focusing of microparticles/cells and thus improves the uniformity of the optical detection signals. This 3-D focusing feature of the sample flow is realized utilizing the combination of dielectrophoretic and hydrodynamic forces. Initially, two sheath flows are used to focus the sample flow horizontally by means of hydrodynamic forces, and then two embedded planar electrodes apply negative DEP forces to focus the particles/cells vertically. A new micromachined flow cytometer integrated with an embedded optical detection mechanism is then demonstrated. Numerical simulation is used to analyze the operation conditions and the dimension of the microelectrodes for DEP manipulation. The dynamic trace of the moving particles/cells within a flow stream under the DEP manipulation is calculated numerically. Micro polystyrene beads and diluted human red blood cells (RBC) are used to test the performance of the proposed device. The experimental results confirm the suitability of the proposed device for applications requiring precise counting of particles or cells. Experimental data indicates the proposed method can provide more stable signals over the other types of micromachined flow cytometers that were previously reported. View full abstract»

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  • A planar approach for manufacturing cardiac stents: design, fabrication, and mechanical evaluation

    Page(s): 933 - 939
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    A new approach that uses planar batch manufacturing technologies is presented for the design and fabrication of coronary artery stents. Stent samples with different wall patterns have been fabricated from 50-μm-thick stainless steel foil using microelectrodischarge machining. Stents have been expanded to tubular shapes by using angioplasty balloons, both inside mock arteries and without external confinement (i.e., free-standing). Free-standing stents exhibit diameter variations of <±4%, almost zero radial recoil after deflation of the balloon, and longitudinal shrinkage of <3% upon expansion. A variation that uses breakable links to provide additional customization in electrical and mechanical properties is also presented. Loading tests reveal that the radial stiffness of some patterns is comparable to that of commercially available stents with greater wall thickness, while bending compliance, at 0.02 m/N for a 4-mm-long section of the stent, is also favorably large. View full abstract»

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  • Fabrication process of microsurgical tools for single-cell trapping and intracytoplasmic injection

    Page(s): 940 - 946
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    We introduce chemical etching-based processes for fabricating micromanipulators from fused silica microcapillary tubing. The resulting devices are capable of manipulating individual cells and can be used for intracytoplasmic injection. Two types of micromanipulating tools: single-cell trappers and micro-injectors, were fabricated with two single-step etching processes. These etching mechanisms, which are either surface tension controlled or diffusion rate limited, are discussed in detail with experimental verifications. The fabrication processes are reproducible and amenable to mass production due to their simplicity and low fabrication cost. Single cell capture and intracytoplasmic injection have been successfully demonstrated on brassica oleracea protoplasts. View full abstract»

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  • Site-specific enhancement of gene transfection utilizing an attracting electric field for DNA plasmids on the electroporation microchip

    Page(s): 947 - 955
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    Site-specific enhancement of in vitro gene delivery using electrostatic force and ElectroPoration (EP) microchips is analyzed in this study. Electroporation is a technique that introduces foreign materials into cells by applying impulses with an electric field to create multiple transient pores in the cell membrane through a dielectric breakdown of the cell membrane. The electroporation chip employed in this process consists of a defined cell culture cavity region with thin film electrodes made of titanium and gold, and was fabricated on a glass slide using microfabrication technologies. Four μg of DNA plasmids were added into the well of the microchip prior to electroporation. The electric field for attracting DNA was generated using a gold plate electrode on the top of the cell culture cavity, and one side of interdigitated electrodes. As the anode was connected to one side of the interdigitated electrodes, the negative-charged DNA plasmids were attracted and accumulated at the finger electrodes with positive polarity, thereby increasing the DNA concentration on the surface of these powered electrodes. After the DNA plasmids were accumulated, the electric power was switched to the interdigitated electrodes to perform the cell electroporation process. This paper investigates the DNA concentration during electrophoresis on the micro electroporation chip, based on a one-dimensional (1-D) steady-state approximation and a two-dimensional (2-D) transient simulation. This study demonstrates that the attracting electric field increases the concentration of negative-charged DNA plasmids near the cell surface up to several thousand-fold prior to electroporation, which enhances the gene transfection efficiency up to 6.3-fold compared to that without an attracting electric field. The pEGFP-N1 plasmids coding for green fluorescent protein (GFP) were transfected into an osteoblast-like cell line (MC3T3E-1) to exhibit the site-specific and enhanced gene delivery on the microchip. The experiments of in vitro gene transfection on a microchip successfully verified the numerical study in this paper. The most important issue of gene therapy is to develop a site-specific gene delivery platform for the controlled expression of transgenes in specific cells or tissues. This- present work successfully demonstrates the enhancement and site-specific transfection utilizing the attracting electric field on an electroporation microchip. View full abstract»

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  • Microstenciling: a generic technology for microscale patterning of vapor deposited materials

    Page(s): 956 - 962
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    The fabrication of microstencils for patterning on unconventional substrates was demonstrated. Stencil feature sizes ranging from 6 to 370 μm with aspect ratios (stencil feature height :width) in the range of 0.5 : 1 to 15 : 1 were fabricated using ICP etching of silicon. The stenciling process was demonstrated for the deposition of metals (Ti/Au) and dielectrics (silicon dioxide) onto silicon, glass, and polymer based substrates for microfluidic system development. The results demonstrated some dependency of the deposition rate on the stencil feature size and aspect ratio. Results from adhesion studies showed excellent adhesion on all substrates with the exception of PMMA. View full abstract»

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  • Characterization of wafer-level thermocompression bonds

    Page(s): 963 - 971
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    Thermocompression bonding joins substrates via a bonding layer. In this paper, silicon substrates were bonded using gold thin films. Experimental data on the effects of bonding pressure (30 to 120 MPa), temperature (260 and 300°C), and time (2 to 90 min) on the bond toughness, measured using the four-point bend technique, are presented. In general, higher temperature and pressure lead to higher toughness bonds. Considerable variation in toughness was observed across specimens. Possible causes of the nonuniform bond quality were explored using finite element analysis. Simulation results showed that the mask layout contributed to the pressure nonuniformity applied across the wafer. Finally, some process guidelines for successful wafer-level bonding using gold thin films are presented. View full abstract»

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  • Fracture of polycrystalline 3C-SiC films in microelectromechanical systems

    Page(s): 972 - 976
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    The fracture of polycrystalline SiC films is investigated using a micrometer-sized fracture tester fabricated by micromachining techniques. A series of SiC cantilever beams varying in length are carried by a moving shuttle tethered to the substrate, and are bent in plane until fracture. The fracture strain of SiC films is calculated from the deflection of bending beams using nonlinear beam theory and determined to be 3.3%±0.2%, which corresponds to a fracture stress of 23.4±1.4 GPa. These values are significantly higher than those for polycrystalline silicon. In addition, the crack propagation in the polycrystalline SiC films is observed to be transgranular. View full abstract»

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  • Electrical contact resistance as a diagnostic tool for MEMS contact interfaces

    Page(s): 977 - 987
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    The electrical contact resistance (ECR) was evaluated as an in situ diagnostic tool for the contact interface behavior of microelectromechanical systems (MEMS). Special polycrystalline silicon (polysilicon) MEMS devices fabricated by surface micromachining were used to study polysilicon/native oxide/polysilicon contact interfaces. ECR measurements obtained during monotonic contact loading and unloading and cyclic contact loading are interpreted in the context of a previous ECR theory. For monotonic contact loading and unloading, the ECR was measured as a function of apparent contact pressure and was found to be on the order of 105 Ω. The fairly moderate decrease of the ECR with the increase of the contact load is attributed to the intrinsic nonohmic behavior of the native oxide film. Experimental ECR results are correlated with analytical solutions to determine the oxide film thickness. The results indicate that the oxide film remains nearly intact under monotonic contact loading and unloading. Differences in the ECR behavior during unloading are discussed in light of the statistical distribution of asperity nanocontacts and the prevailing deformation mode. During cyclic contact loading, rupture of the oxide film leads to the formation of polysilicon/polysilicon nanocontacts, which produces ECR values in the range of 102-103 Ω. The erratic behavior of the ECR during cyclic contact loading is related to the pronounced effects of the insulating oxide film and oxide debris trapped at the contact interface. View full abstract»

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  • Tilt-angle stabilization of electrostatically actuated micromechanical mirrors beyond the pull-in point

    Page(s): 988 - 997
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    Recently proposed optical subsystems utilizing microelectromechanical system (MEMS) components are being developed for use in optical crossconnects, add-drop multiplexers, and spectral equalizers. Common elements to these subsystems are electrostatically actuated micromechanical mirrors that steer optical beams to implement the subsystem functions. In the past, feedback control methods were used to obtain precise mirror orientations to minimize loss through optical switch fabrics or to stabilize attenuation through spectral equalizers. However, the mirror tilt angle range is limited because of inherent instability beyond a critical tilt angle (pull-in angle), and the usual feedback schemes do not counteract this effect. This work presents a feedback control method to enable operation of electrostatic micromirrors beyond the pull-in angle, yielding advantages including greater scalability of switch arrays and increased dynamic range of optical attenuators. Both static and dynamic tilting behaviors of electrostatic micromirrors under the feedback control are studied. In addition, a practical implementation of the feedback control system by using linear voltage control law is developed. A voltage slightly larger than the pull-in voltage is first applied when the mirror is at small angle positions, and the voltage is then linearly reduced as the mirror approaches the desired position. Experimental measurements, showing that tilt angles beyond the pull-in point can be achieved, are in good agreement with theoretical analysis. View full abstract»

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  • Analog piezoelectric-driven tunable gratings with nanometer resolution

    Page(s): 998 - 1005
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    This work presents the design, fabrication, and characterization of a piezoelectrically actuated MEMS diffractive optical grating, whose spatial periodicity can be tuned in analog fashion to within a fraction of a nanometer. The fine control of the diffracted beams permits applications in dense wavelength-division multiplexing (DWDM) optical telecommunications and high-resolution miniaturized spectrometers. The design concept consists of a diffractive grating defined on a deformable membrane, strained in the direction perpendicular to the gratings grooves via thin-film piezoelectric actuators. The tunable angular range for the first diffracted order is up to 400 μrad with 0.2% strain (∼8 nm change in grating periodicity) at 10 V actuation, as predicted by device modeling. The actuators demonstrate a piezoelectric d31 coefficient of -100 pC/N and dielectric constant εr of 1200. Uniformity across the tunable grating and the out-of-plane deflections are also characterized and discussed. View full abstract»

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  • Two-axis single-crystal silicon micromirror arrays

    Page(s): 1006 - 1017
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    This work presents the design, fabrication, and testing of a two-axis 320 pixel micromirror array. The mirror platform is constructed entirely of single-crystal silicon (SCS) minimizing residual and thermal stresses. The 14-μm-thick rectangular (750×800 μm2) silicon platform is coated with a 0.1-μm-thick metallic (Au) reflector. The mirrors are actuated electrostatically with shaped parallel plate electrodes with 86 μm gaps. Large area 320-mirror arrays with fabrication yields of 90% per array have been fabricated using a combination of bulk micromachining of SOI wafers, anodic bonding, deep reactive ion etching, and surface micromachining. Several type of micromirror devices have been fabricated with rectangular and triangular electrodes. Triangular electrode devices displayed stable operation within a (±5°, ±5°) (mechanical) angular range with voltage drives as low as 60 V. View full abstract»

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  • Limit cycle oscillations in CW laser-driven NEMS

    Page(s): 1018 - 1026
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    Limit cycle, or self-oscillations, can occur in a variety of NEMS devices illuminated within an interference field. As the device moves within the field, the quantity of light absorbed and hence the resulting thermal stresses changes, resulting in a feedback loop that can lead to limit cycle oscillations. Examples of devices that exhibit such behavior are discussed as are experimental results demonstrating the onset of limit cycle oscillations as continuous wave (CW) laser power is increased. A model describing the motion and heating of the devices is derived and analyzed. Conditions for the onset of limit cycle oscillations are computed as are conditions for these oscillations to be either hysteretic or nonhysteretic. An example simulation of a particular device is discussed and compared with experimental results. View full abstract»

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  • Influence of van der Waals and Casimir forces on electrostatic torsional actuators

    Page(s): 1027 - 1035
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    The influence of van der Waals (vdW) and Casimir forces on the stability of the electrostatic torsional nanoelectromechanical systems (NEMS) actuators is analyzed in the paper. With the consideration of vdW and Casimir effects, the dependence of the critical tilting angle and pull-in voltage on the sizes of structure is investigated. The influence of vdW torque is compared with that of Casimir torque. The modified coefficients of vdW and Casimir torques on the pull-in voltage are, respectively, calculated. When the gap is sufficiently small, pull-in can still take place with arbitrary small angle perturbation because of the action of vdW and Casimir torques even if there is not electrostatic torque. And the critical pull-in gaps for two cases are, respectively, derived. View full abstract»

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  • The SiOG-based single-crystalline silicon (SCS) RF MEMS switch with uniform characteristics

    Page(s): 1036 - 1042
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    This paper details single-crystalline silicon (SCS) direct contact radio frequency microelectromechanical systems (RF MEMS) switch designed and fabricated using an SiOG (silicon-on-glass) substrate, so as to obtain higher fabrication and performance uniformity compared with a conventional metal switch. The mechanical and electrical performances of the fabricated silicon switch have been tested. In comparison with a conventional metallic MEMS switch, we can obtain higher productivity and uniformity by using SCS, because it has very low stresses and superior thermal characteristics as a structural material of the switch. Also, by using the SiOG substrate instead of an SOI substrate, fabrication cost can be significantly reduced. The proposed switch is fabricated on a coplanar waveguide (CPW) and actuated by electrostatic force. The designed chip size is 1.05 mm×0.72 mm. Measured pull-in voltage and actuation voltage were 19 V and 26 V, respectively. Eighteen identical switches taken randomly throughout the wafer showed average and standard deviation of the measured pull-in voltage of 19.1 and 1.5 V, respectively. The RF characteristics of the fabricated switch from dc to 30 GHz have been measured. The isolation and insertion loss measured on the four identical samples were -38 to -39 dB and -0.18 to -0.2 dB at 2 GHz, respectively. Forming damping holes on the upper electrode leads to a relatively fast switching speed. Measured ON and OFF time were 25 and 13 μs, respectively. In the switch OFF state, self-actuation does not happen up to the input power of 34 dBm. The measured holding power of the fabricated switch was 31 dBm. Stiction problem was not observed after 108 cycles of repeated actuation, but the contact resistance varied about 0.5-1 Ω from the initial value. View full abstract»

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  • VHF single-crystal silicon elliptic bulk-mode capacitive disk resonators-part I: design and modeling

    Page(s): 1043 - 1053
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    This work, the first of two parts, presents the design and modeling of VHF single-crystal silicon (SCS) capacitive disk resonators operating in their elliptical bulk resonant mode. The disk resonators are modeled as circular thin-plates with free edge. A comprehensive derivation of the mode shapes and resonant frequencies of the in-plane vibrations of the disk structures is described using the two-dimensional (2-D) elastic theory. An equivalent mechanical model is extracted from the elliptic bulk-mode shape to predict the dynamic behavior of the disk resonators. Based on the mechanical model, the electromechanical coupling and equivalent electrical circuit parameters of the disk resonators are derived. Several considerations regarding the operation, performance, and temperature coefficient of frequency of these devices are further discussed. This model is verified in part II of this paper, which describes the implementation and characterization of the SCS capacitive disk resonators. View full abstract»

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  • VHF single crystal silicon capacitive elliptic bulk-mode disk resonators-part II: implementation and characterization

    Page(s): 1054 - 1062
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    This work, the second of two parts, reports on the implementation and characterization of high-quality factor (Q) side-supported single crystal silicon (SCS) disk resonators. The resonators are fabricated on SOI substrates using a HARPSS-based fabrication process and are 3 to 18 μm thick. They consist of a single crystal silicon resonant disk structure and trench-refilled polysilicon drive and sense electrodes. The fabricated resonators have self-aligned, ultra-narrow capacitive gaps in the order of 100 nm. Quality factors of up to 46 000 in 100 mTorr vacuum and 26000 at atmospheric pressure are exhibited by 18 μm thick SCS disk resonators of 30 μm in diameter, operating in their elliptical bulk-mode at ∼150 MHz. Motional resistance as low as 43.3 kΩ was measured for an 18-μm-thick resonator with 160 nm capacitive gaps at 149.3 MHz. The measured electrostatic frequency tuning of a 3-μm-thick device with 120 nm capacitive gaps shows a tuning slope of -2.6 ppm/V. The temperature coefficient of frequency for this resonator is also measured to be -26 ppm/°C in the temperature range from 20 to 150°C. The measurement results coincide with the electromechanical modeling presented in Part I. View full abstract»

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  • Microplastic lens array fabricated by a hot intrusion process

    Page(s): 1063 - 1071
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    A microplastic lens array has been successfully constructed on top of a 500-μm-thick PC (Polycarbonate film) by using a micro hot intrusion process. A single-layer LIGA process is used to fabricate the high-aspect-ratio nickel mold insert that has circular hole patterns of 80 μm in diameter and 200 μm in depth. Under the hot intrusion process, plastic material can be intruded into these circular-shape holes and stopped at desired depth under elevated temperature and pressure to fabricate microlenses. By adjusting the embossing load, temperature and time, the curvature and height of the lens are controllable when the same mold insert is used. The optical properties of these microlenses have been characterized and the average radius of curvature is found as 41.4 μm with a standard deviation of 1.05 μm. Experimental characterization and theoretical model are conducted and developed for the micro-intrusion process in terms of the radius of curvature and height of the lenses and they correspond well with experimental data within 5% of variations. The focusing capability of the lenses is demonstrated by comparing the images of laser light with and without using the lenses. When the projection screen is placed 200 μm away from the lens, the full-width at half-maximum (FWHM) for the lens is 110 μm while the original FWHM of the optical fiber is 300 μm. 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.

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