Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 2004"

Architectures for vibration-driven micropower generators

Abstract: Several forms of vibration-driven MEMS microgenerator are possible and are reported in the literature, with potential application areas including distributed sensing and ubiquitous computing. This paper sets out an analytical basis for their design and comparison, verified against full time-domain simulations. Most reported microgenerators are classified as either velocity-damped resonant generators (VDRGs) or Coulomb-damped resonant generators (CDRGs) and a unified analytical structure is provided for these generator types. Reported generators are shown to have operated at well below achievable power densities and design guides are given for optimising future devices. The paper also describes a new class-the Coulomb-force parametric generator (CFPG)-which does not operate in a resonant manner. For all three generators, expressions and graphs are provided showing the dependence of output power on key operating parameters. The optimization also considers physical generator constraints such as voltage limitation or maximum or minimum damping ratios. The sensitivity of each generator architecture to the source vibration frequency is analyzed and this shows that the CFPG can be better suited than the resonant generators to applications where the source frequency is likely to vary. It is demonstrated that mechanical resonance is particularly useful when the vibration source amplitude is small compared to the allowable mass-to-frame displacement. The CDRG and the VDRG generate the same power at resonance but give better performance below and above resonance respectively. Both resonant generator types are unable to operate when the allowable mass frame displacement is small compared to the vibration source amplitude, as is likely to be the case in some MEMS applications. The CFPG is, therefore, required for such applications.

A curved-beam bistable mechanism

Abstract: This paper presents a monolithic mechanically-bistable mechanism that does not rely on residual stress for its bistability. The typical implementation of this mechanism is two curved centrally-clamped parallel beams, hereafter referred to as "double curved beams". Modal analysis and finite element analysis (FEA) simulation of the curved beam are used to predict, explain, and design its bistable behavior. Microscale double curved beams are fabricated by deep-reactive ion etching (DRIE) and their test results agree well with the analytic predictions. Approaches to tailor the bistable behavior of the curved beams are also presented.

Nonlinear limits for single-crystal silicon microresonators

Abstract: Nonlinear effects in single-crystal silicon microresonators are analyzed with the focus on mechanical nonlinearities. The bulk acoustic wave (BAW) resonators are shown to have orders-of-magnitude higher energy storage capability than flexural beam resonators. The bifurcation point for the silicon BAW resonators is measured and the maximum vibration amplitude is shown to approach the intrinsic material limit. The importance of nonlinearities in setting the limit for vibration energy storage is demonstrated in oscillator applications. The phase noise calculated for silicon microresonator-based oscillators is compared to the conventional macroscopic quartz-based oscillators, and it is shown that the higher energy density attainable with the silicon resonators can partially compensate for the small microresonator size. Scaling law for microresonator phase noise is developed.

New thermoelectric components using microsystem technologies

Abstract: This work describes the first thermoelectric devices based on the V-VI-compounds Bi/sub 2/Te/sub 3/ and (Bi,Sb)/sub 2/Te/sub 3/ which can be manufactured by means of regular thin film technology in combination with microsystem technology. Fabrication concept, material deposition for some 10-μm-thick layers and the properties of the deposited thermoelectric materials will be reported. First device properties for Peltier-coolers and thermogenerators will be shown as well as investigations on long term and cycling stability. Data on metal/semiconductor contact resistance were extracted form device data. Device characteristics like response time for a Peltier-cooler and power output for a thermogenerator will be compared to commercial devices.

Microassembly of 3-D microstructures using a compliant, passive microgripper

Abstract: This paper describes a novel microassembly system that can be used to construct out-of-plane three-dimensional (3-D) microstructures. The system makes use of a surface-micromachined microgripper that is solder bonded to a robotic manipulator. The microgripper is able to grasp a micropart, remove it from the chip, reorient it about two independent axes, translate it along the x, y and z axes to a secondary location, and join it to another micropart. In this way, out-of-plane 3-D microstructures can be assembled from a set of initially planar and parallel surface micromachined microparts. The microgripper is 380 /spl times/ 410 /spl mu/m in size. It utilizes three geometric features for operation: 1) compliant beams to allow for deflection at the grasping tips; 2) self-tightening geometry during grasping; and 3) 3-D interlocking geometry to secure a micropart after the grasp. Each micropart has three geometric features built into its body. The first is the interlock interface feature that allows it to be grasped by the microgripper. The second is a tether feature that secures the micropart to the substrate, and breaks away after the microgripper has grasped the micropart. The third is the snap-lock feature, which is used to join the micropart to other microparts.

A microreactor for hydrogen production in micro fuel cell applications

Abstract: A silicon-chip based microreactor has been successfully fabricated and tested for carrying out the reaction of methanol reforming for microscale hydrogen production. The developed microreactor in combination with a micro fuel cell is proposed as an alternative to conventional portable sources of electricity such as batteries due to its ability to provide an uninterrupted supply of electricity as long as a supply of methanol and water can be provided. The microreformer-fuel cell combination has the advantage of not requiring the tedious recharging cycles needed by conventional rechargeable lithium-ion batteries. It also offers significantly higher energy storage densities, which translates into less frequent "recharging" through the refilling of methanol fuel. The microreactor consists of a network of catalyst-packed parallel microchannels of depths ranging from 200 to 400 /spl mu/m with a catalyst particle filter near the outlet fabricated using photolithography and deep-reactive ion etching (DRIE) on a silicon substrate. Issues related to microchannel and filter capping, on-chip heating and temperature sensing, introduction and trapping of catalyst particles in the microchannels, flow distribution, microfluidic interfacing, and thermal insulation have been addressed. Experimental runs have demonstrated a methanol to hydrogen molar conversion of at least 85% to 90% at flow rates enough to supply hydrogen to an 8- to 10-W fuel cell.

Micromechanical mixer-filters ("mixlers")

Abstract: A device comprised of interlinked micromechanical resonators with capacitive mixer transducers has been demonstrated to perform both frequency translation (i.e., mixing) and highly selective low-loss filtering of applied electrical input signals. In particular, successful downconversion of a 200-MHz radio frequency (RF) signal down to a 37-MHz intermediate frequency (IF) and subsequent high-Q bandpass filtering at the IF are demonstrated using this single, passive, micromechanical device, all with less than 13 dB of combined mixing conversion and filter insertion loss. The mixer-filter (or "mixler") RF-to-IF voltage transfer function is shown to depend upon a ratio of local oscillator amplitude and applied bias voltages.

Three-dimensional hydrodynamic focusing in polydimethylsiloxane (PDMS) microchannels

Abstract: This paper presents a generalization of the hydrodynamic focusing technique to three-dimensions. Three-dimensional (3-D) hydrodynamic focusing offers the advantages of precision positioning of molecules in both vertical and lateral dimensions and minimizing the interaction of the sample fluid with the surfaces of the channel walls. In an ideal approach, 3-D hydrodynamic focusing could be achieved by completely surrounding the sample flow by a cylindrical sheath flow that constrains the sample flow to the center of the channel in both the lateral and the vertical dimensions. We present here design and simulation, 3-D fabrication, and experimental results from a piecewise approximation to such a cylindrical flow. Two-dimensional (2-D) and 3-D hydrodynamic focusing chips were fabricated using micromolding methods with polydimethylsiloxane (PDMS). Three-dimensional hydrodynamic focusing chips were fabricated using the "membrane sandwich" method. Laser scanning confocal microscopy was used to study the hydrodynamic focusing experiments performed in the 2-D and 3-D chips with Rhodamine 6G solution as the sample fluid and water as the sheath fluid.

Shape-memory polymers for microelectromechanical systems

Abstract: This paper investigates the use of shape-memory polymer thin films in microelectromechanical systems (MEMS). shape-memory polymers possess the capacity to recover large-strain deformations by the application of heat and are candidates for small-scale transduction. The key advantages of shape-memory polymers are their low material/fabrication cost coupled with their simplicity of integration/operation. In the present study, shape-memory polymers are spin coated onto a standard Si wafer and polymerized by thermal annealing. The thermomechanics of strain storage and recovery in the polymer films are studied using instrumented microindentation. The sharp microindents demonstrate full recovery at all load levels, establishing the feasibility of microscale actuation. The microindentation response of the polymer film is shown to depend on temperature and the cooling cycle during indentation. In turn, the subsequent recovery behavior of an indent depends on the thermal history during indentation. Indents performed at higher temperatures are larger in size, but have smaller stored strain energy compared to indents performed at low temperature. The larger stored strain energy in low temperature indents results in lower shape recovery temperatures. The effects of indentation temperature and load are systematically investigated to provide a framework for the use of shape-memory polymers in microsystems. Application of shape-memory polymers is demonstrated through the development of an active microfluidic reservoir. The reservoir was created by indentation at the end of a microfluidic channel and was activated by local heating. The collapse of the filled reservoir caused the motion of fluid down the microfluidic channel.

Fabrication and characterization of a nanowire/polymer-based nanocomposite for a prototype thermoelectric device

Abstract: This paper discusses the design, fabrication and testing of a novel thermoelectric device comprised of arrays of silicon nanowires embedded in a polymer matrix. By exploiting the low-thermal conductivity of the composite and presumably high-power factor of the nanowires, a thermoelectric figure of merit, higher than the corresponding bulk value, should result. Arrays were first synthesized using a vapor-liquid-solid (VLS) process leading to one-dimensional (1-D) growth of single-crystalline nanowires. To provide structural support while maintaining thermal isolation between nanowires, parylene, a low thermal conductivity and extremely conformal polymer, was embedded within the arrays. Mechanical polishing and oxygen plasma etching techniques were used to expose the nanowire tips and a metal contact was deposited on the top surface. Scanning electron micrographs (SEMs) illustrate the results of the fabrication processes. Using a modification of the 3/spl omega/ technique, the effective thermal conductivity of the nanowire matrix was measured and 1 V characteristics were also demonstrated. An assessment of the suitability of this nanocomposite for high thermoelectric performance devices is given.

Micromachined jets for liquid impingement cooling of VLSI chips

Abstract: Two-phase microjet impingement cooling is a potential solution for removing heat from high-power VLSI chips. Arrays of microjets promise to achieve more uniform chip temperatures and very high heat transfer coefficients. This paper presents the design and fabrication of single-jets and multijet arrays with circular orifice diameters ranging from 40 to 76 /spl mu/m, as well as integrated heater and temperature sensor test devices. The performance of the microjet heat sinks is studied using the integrated heater device as well as an industry standard 1 cm/sup 2/ thermal test chip. For single-phase, the silicon temperature distribution data are consistent with a model accounting for silicon conduction and fluid advection using convection coefficients in the range from 0.072 to 4.4 W/cm/sup 2/K. For two-phase, the experimental results show a heat removal of up to 90 W on a 1 cm/sup 2/ heated area using a four-jet array with 76 /spl mu/m diameter orifices at a flowrate of 8 ml/min with a temperature rise of 100/spl deg/C. The data indicate convection coefficients are not significantly different from coefficients for pool boiling, which motivates future work on optimizing flowrates and flow regimes. These microjet heat sinks are intended for eventual integration into a closed-loop electroosmotically pumped cooling system.

A chaotic mixer for magnetic bead-based micro cell sorter

Abstract: An efficient magnetic force driven mixer with simple configuration is designed, fabricated, and tested. It is designed to facilitate the mixing of magnetic beads and biomolecules in a microchannel, where mixing is unavoidably inefficient due to its low Reynolds number. With appropriate temporal variations of the force field, chaotic mixing is achieved, hence the mixing becomes effective. The mixing device consists of embedded microconductors as a magnetic field source and a microchannel that guides the streams of working fluid. It is demonstrated that a pair of integrated micro conductors provides a local magnetic field strong enough to attract nearby magnetic beads. Mixing of magnetic beads is accomplished by applying a time-dependent control signal to a row of conductors, at the Reynolds number of as low as 10/sup -2/. Two-dimensional numerical simulation has been performed to design the configuration of the channel and electrodes, which creates chaotic motion of beads. It is found that a simple two-dimensional serpentine channel geometry with the transverse electrodes is able to create the stretching and folding of material lines, which is a manifestation of chaos. The mixing pattern predicted by the simulation has been confirmed by both flow visualization and PTV (particle tracking velocimetry) in the chaotic mixer fabricated, which should greatly increase the attachment of beads onto the target biomolecules. The optimum frequency of applied control signal is searched by evaluating the Lyapunov exponent in both numerical and experimental particle tracking. It is found that the range of optimum Strouhal number is 5

A 2-D microcantilever array for multiplexed biomolecular analysis

Abstract: An accurate, rapid, and quantitative method for analyzing variety of biomolecules, such as DNA and proteins, is necessary in many biomedical applications and could help address several scientific issues in molecular biology Recent experiments have shown that when specific biological reactions occur on one surface of a microcantilever beam, the resulting changes in surface stress deflect the cantilever beam To exploit this phenomenon for high-throughput label-free biomolecular analysis, we have developed a chip containing a two-dimensional (2-D) array of silicon nitride cantilevers with a thin gold coating on one surface Integration of microfluid cells on the chip allows for individual functionalization of each cantilever of the array, which is designed to respond specifically to a target analyte An optical system to readout deflections of multiple cantilevers was also developed The cantilevers exhibited thermomechanical sensitivity with a standard deviation of seven percent, and were found to fall into two categories-those whose deflections tracked each other in response to external stimuli, and those whose did not due to drift The best performance of two "tracking" cantilevers showed a maximum difference of 4 nm in their deflections Although "nontracking" cantilevers exhibited large differences in their drift behavior, an upper bound of their time-dependent drift was determined, which could allow for rapid bioassays Using the differential deflection signal between tracking cantilevers, immobilization of 25mer thiolated single-stranded DNA (ssDNA) on gold surfaces produced repeatable deflections of 80 nm or so on 05-/spl mu/m-thick and 200-/spl mu/m-long cantilevers

High-performance surface-micromachined inchworm actuator

Abstract: This work demonstrates a polycrystalline silicon surface-micromachined inchworm actuator that exhibits high-performance characteristics such as large force (/spl plusmn/0.5 millinewtons), large velocity range (0 to /spl plusmn/4.4 mm/sec), large displacement range (/spl plusmn/100 microns), small step size (/spl plusmn/10, /spl plusmn/40 or /spl plusmn/100 nanometers), low power consumption (nanojoules per cycle), continuous bidirectional operation and relatively small area (600 /spl times/ 200/spl mu/m/sup 2/). An in situ load spring calibrated on a logarithmic scale from micronewtons to millinewtons, optical microscopy and Michelson interferometry are used to characterize its performance. The actuator consists of a force-amplifying plate that spans two voltage-controlled clamps, and walking is achieved by appropriately sequencing signals to these three components. In the clamps, normal force is borne by equipotential rubbing counterfaces, enabling friction to be measured against load. Using different monolayer coatings, we show that the static coefficient of friction can be changed from 0.14 to 1.04, and that it is load-independent over a broad range. We further find that the static coefficient of friction does not accurately predict the force generated by the actuator and attribute this to nanometer-scale presliding tangential deflections.

Development of a deep silicon phase Fresnel lens using Gray-scale lithography and deep reactive ion etching

Abstract: We report the first fabrication and development of a deep phase Fresnel lens (PFL) in silicon through the use of gray-scale lithography and deep-reactive ion etching (DRIE). A Gaussian tail approximation is introduced as a method of predicting the height of photoresist gray levels given the relative amount of transmitted light through a gray-scale optical mask. Device mask design is accomplished through command-line scripting in a CAD tool to precisely define the millions of pixels required to generate the appropriate profile in photoresist. Etch selectivity during DRIE pattern transfer is accurately controlled to produce the desired scaling factor between the photoresist and silicon profiles. As a demonstration of this technology, a 1.6-mm diameter PFL is etched 43 /spl mu/m into silicon with each grating profile designed to focus 8.4 keV photons a distance of 118 m.

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

Abstract: 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.

Full-Lagrangian schemes for dynamic analysis of electrostatic MEMS

Abstract: Dynamic analysis of microelectromechanical systems (MEMS) is characterized by the nonlinear coupling of electrical and mechanical domains. The nonlinear coupling between the two domains gives rise to several interesting dynamic phenomena besides the well established pull-in phenomenon in electrostatic MEMS. For proper understanding and detailed exploration of MEMS dynamics, it is important to have a reliable and efficient physical level simulation method. In this paper, we develop relaxation and Newton schemes based on a Lagrangian description of both the mechanical and the electrical domains for the analysis of MEMS dynamics. The application of a Lagrangian description for both mechanical and electrostatic analysis makes this method far more efficient than standard semi-Lagrangian scheme-based analysis of MEMS dynamics. A major advantage of the full-Lagrangian scheme is in the accurate computation of the interdomain coupling term (mechanical to electrical) in the Jacobian matrix of the Newton scheme which is not possible with a semi-Lagrangian scheme. The full-Lagrangian based relaxation and Newton schemes have been validated by comparing simulation results with published data for cantilever and fixed-fixed MEM beams. The Newton scheme has been used for the dynamic analysis of two classes of comb-drives widely used in MEMS, namely, transverse and lateral comb-drives. Several interesting MEM dynamic phenomena and their possible applications have been presented. Spring-hardening and softening of MEM devices has been shown. The existence of multiple resonant peaks in MEM devices has been analyzed under different electrical signals and their possible applications in multiband/passband MEM filters/oscillators is discussed. Switching speed is a serious constraint for capacitive based RF-MEM switches. We have shown that a DC bias along with an ac bias at the resonant frequency can give very fast switching at a considerably less peak power requirement.

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

Abstract: 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 /spl mu/m 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 /spl mu/m thick SCS disk resonators of 30 /spl mu/m in diameter, operating in their elliptical bulk-mode at /spl sim/150 MHz. Motional resistance as low as 43.3 k/spl Omega/ was measured for an 18-/spl mu/m-thick resonator with 160 nm capacitive gaps at 149.3 MHz. The measured electrostatic frequency tuning of a 3-/spl mu/m-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//spl deg/C in the temperature range from 20 to 150/spl deg/C. The measurement results coincide with the electromechanical modeling presented in Part I.

Low-voltage, large-scan angle MEMS analog micromirror arrays with hidden vertical comb-drive actuators

Abstract: This paper reports on novel polysilicon surface-micromachined one-dimensional (1-D) analog micromirror arrays fabricated using Sandia's ultraplanar multilevel MEMS technology-V (SUMMiT-V) process. Large continuous DC scan angle (23.6/spl deg/ optical) and low-operating voltage (6 V) have been achieved using vertical comb-drive actuators. The actuators and torsion springs are placed underneath the mirror (137/spl times/120 /spl mu/m/sup 2/) to achieve high fill-factor (91%). The measured resonant frequency of the mirror ranges from 3.4 to 8.1 kHz. The measured DC scanning characteristics and resonant frequencies agree well with theoretical values. The rise time is 120 /spl mu/s and the fall time is 380 /spl mu/s. The static scanning characteristics show good uniformity (

Mechanical characterization of polysilicon through on-chip tensile tests

Abstract: Two new types of on-chip tests have been designed in order to evaluate the elastic Young modulus and the fracture strength of polysilicon used in microelectromechanical systems (MEMS). The former is a pure tension test, while the latter is a single-edge-notched tension test. The actuation in both tests is obtained by means of an ad hoc designed layout of parallel plates capacitors applying sufficiently high forces to reach significant strains in the tensile specimens and complete failure of the notched specimens. The pure tension tests on 20 specimens showed a low dispersion and gave a Young modulus for the polysilicon of 143 GPa. A total of 92 notched specimens were tested up to failure. The experimental results, supported by finite-element simulations, gave a value of the maximum stress for the notched specimens in the range 4144-4568 MPa.

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

Abstract: 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.

Position control of parallel-plate microactuators for probe-based data storage

Abstract: In this paper, we present the use of closed-loop voltage control to extend the travel range of a parallel-plate electrostatic microactuator beyond the pull-in limit. Controller design considers nonlinearities from both the parallel-plate actuator and the capacitive position sensor to ensure robust stability within the feedback loop. Desired transient response is achieved by a pre-filter added in front of the feedback loop to shape the input command. The microactuator is characterized by static and dynamic measurements, with a spring constant of 0.17 N/m, mechanical resonant frequency of 12.4 kHz, and effective damping ratio from 0.55 to 0.35 for gaps between 2.3 to 2.65 /spl mu/m. The minimum input-referred noise capacitance change is 0.5 aF//spl radic/Hz measured at a gap of 5.7 /spl mu/m, corresponding to a minimum input-referred noise displacement of 0.33 nm//spl radic/Hz. Measured closed-loop step response illustrates a maximum travel distance up to 60% of the initial gap, surpassing the static pull-in limit of one-third of the gap.

Limit cycle oscillations in CW laser-driven NEMS

Abstract: 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.

A water-powered micro drug delivery system

Abstract: A plastic micro drug delivery system has been successfully demonstrated by utilizing the principle of osmosis without any electrical power consumption. The system has an osmotic microactuator (see Su, Lin, and Pisano, J. Microelectromech., vol. 11, pp. 736-7462, Dec. 2002) and a polydimethylsiloxane (PDMS) microfluidic cover compartment consisting of a reservoir, a microfluidic channel and a delivery port. The typical dimension of the microfluidic channel is 1 cm in length with a cross-sectional area of 30/spl times/100 /spl mu/m/sup 2/ to minimize the diffusive drug flow while pressure drop remains moderate. Using oxygen plasma to activate the surfaces of polymers for bonding, the osmotic actuator is bonded with the PDMS cover while liquid drug can be encapsulated during the bonding process. Employing the net water flow induced by osmosis, the prototype drug delivery system has a measured constant delivery rate at 0.2 /spl mu/L/h for 10 h with an accumulated delivery volume of 2 /spl mu/L. Both the delivery rate and volume could be altered by changing the design and process parameters for specific drug delivery applications up to a few years. Moreover, the induced osmotic pressure can be as high as 25 MPa to overcome possible blockages caused by cells or tissues during drug delivery operations.

A low-temperature thin-film electroplated metal vacuum package

Abstract: This paper presents a packaging technology that employs an electroplated nickel film to vacuum seal a MEMS structure at the wafer level. The package is fabricated in a low-temperature (<250/spl deg/C) 3-mask process by electroplating a 40-/spl mu/m-thick nickel film over an 8-/spl mu/m sacrificial photoresist that is removed prior to package sealing. A large fluidic access port enables an 800/spl times/800 /spl mu/m package to be released in less than three hours. MEMS device release is performed after the formation of the first level package. The maximum fabrication temperature of 250/spl deg/C represents the lowest temperature ever reported for thin film packages (previous low /spl sim/400/spl deg/C). Implementation of electrical feedthroughs in this process requires no planarization. Several mechanisms, based upon localized melting and Pb/Sn solder bumping, for sealing low fluidic resistance feedthroughs have been investigated. This package has been fabricated with an integrated Pirani gauge to further characterize the different sealing technologies. These gauges have been used to establish the hermeticity of the different sealing technologies and have measured a sealing pressure of /spl sim/1.5 torr. Short-term (/spl sim/several weeks) reliability data is also presented.

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

Abstract: 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 /spl mu/m. The typical integration density is 100 cantilevers/mm/sup 2/ and 300 electrical interconnects/mm/sup 2/. 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/spl deg/C, allowing post-transfer wafer processing.

Mechanical property characterization of LPCVD silicon nitride thin films at cryogenic temperatures

Abstract: T-shape, LPCVD silicon nitride cantilevers are fabricated to determine Young's modulus and fracture strength of silicon nitride thin films at room and cryogenic temperatures. A helium-cooled measurement setup is developed and installed inside a focused-ion-beam (FIB) system. A lead-zirconate-titanate (PZT) translator powered by a function generator and a dc voltage is utilized as an actuator, and a silicon diode is used as a temperature sensor in this setup. Resonant frequencies of identical cantilevers with different "milling masses" are measured to obtain thickness and Young's modulus of the silicon nitride thin films, while a bending test is performed to obtain fracture strength. From the experiment, the average Young's modulus of low-pressure chemical-vapor deposition (LPCVD) silicon nitride thin films varies from 260.5 GPa at room temperature (298 K) to 266.6 GPa at 30 K, and the average fracture strength ranges from 6.9 GPa at room temperature to 7.9 GPa at 30 K. The measurement setup and technique presented here can be used to characterize the mechanical properties of different MEMS materials at cryogenic temperatures.

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

Abstract: 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.

Characterization of wafer-level thermocompression bonds

Abstract: 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/spl deg/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.

Leak-tight piezoelectric microvalve for high-pressure gas micropropulsion

Abstract: This paper describes the results of our development of a leak-tight piezoelectric microvalve, operating at extremely high upstream pressures for microspacecraft applications. The device is a normally closed microvalve assembled and fabricated primarily from micromachined silicon wafers. The microvalve consists of a custom-designed piezoelectric stack actuator bonded onto silicon valve components (such as the seat, boss, and tether) with the entire assembly contained within a stainless steel housing. The valve seat configurations include narrow-edge seating rings and tensile-stressed silicon tethers that enable the desired, normally closed, leak-tight operation. Leak testing of the microvalve was conducted using a helium leak detector and showed leak rates of 5/spl times/10/sup -3/ sccm at 800 psi (5.516 MPa). Dynamic microvalve operation (switching rates of up to 1 kHz) was successfully demonstrated for inlet pressures in the range of 0/spl sim/1000 psi. The measured static flow rate for the microvalve under an applied potential of 10 V was 52 sccm at an inlet pressure of 300 psi. The measured power consumption, in the fully open state, was 3 mW at an applied potential of 30 V. The measured dynamic power consumption was 180 mW for 100 Hz continuous operation at 100psi.