Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 2000"

Passive mixing in a three-dimensional serpentine microchannel

Abstract: A three-dimensional serpentine microchannel design with a "C shaped" repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a "square-wave" channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring.

Three-dimensional micro-channel fabrication in polydimethylsiloxane (PDMS) elastomer

Abstract: This paper describes a fabrication technique for building three-dimensional (3-D) micro-channels in polydimethylsiloxane (PDMS) elastomer. The process allows for the stacking of many thin (less than 100-/spl mu/m thick) patterned PDMS layers to realize complex 3-D channel paths. The master for each layer is formed on a silicon wafer using an epoxy-based photoresist (SU 8). PDMS is cast against the master producing molded layers containing channels and openings. To realize thin layers with openings, a sandwich molding configuration was developed that allows precise control of the PDMS thickness. The master wafer is clamped within a sandwich that includes flat aluminum plates, a flexible polyester film layer, a rigid Pyrex wafer, and a rubber sheet. A parametric study is performed on PDMS surface activation in a reactive-ion-etching system and the subsequent methanol treatment for bonding and aligning very thin individual components to a substrate. Low RF power and short treatment times are better than high RF power and long treatment times, respectively, for instant bonding. Layer-to-layer alignment of less then 15 /spl mu/m is achieved with manual alignment techniques that utilize surface tension driven self-alignment methods. A coring procedure is used to realize off-chip fluidic connections via the bottom PDMS layer, allowing the top layer to remain smooth and flat for complete optical access.

Quality factors in micron- and submicron-thick cantilevers

Abstract: Micromechanical cantilevers are commonly used for detection of small forces in microelectromechanical sensors (e.g., accelerometers) and in scientific instruments (e.g., atomic force microscopes). A fundamental limit to the detection of small forces is imposed by thermomechanical noise, the mechanical analog of Johnson noise, which is governed by dissipation of mechanical energy. This paper reports on measurements of the mechanical quality factor Q for arrays of silicon-nitride, polysilicon, and single-crystal silicon cantilevers. By studying the dependence of Q on cantilever material, geometry, and surface treatments, significant insight into dissipation mechanisms has been obtained. For submicron-thick cantilevers, Q is found to decrease with decreasing cantilever thickness, indicating surface loss mechanisms. For single-crystal silicon cantilevers, significant increase in room temperature Q is obtained after 700/spl deg/C heat treatment in either N/sub 2/ Or forming gas. At low temperatures, silicon cantilevers exhibit a minimum in Q at approximately 135 K, possibly due to a surface-related relaxation process. Thermoelastic dissipation is not a factor for submicron-thick cantilevers, but is shown to be significant for silicon-nitride cantilevers as thin as 2.3 /spl mu/m.

Evaluation of size effect on mechanical properties of single crystal silicon by nanoscale bending test using AFM

Abstract: This paper describes a nanometer-scale bending test for a single crystal silicon (Si) fixed beam using an atomic force microscope (AFM). This research focuses on revealing the size effect on the mechanical property of Si beams ranging from a nano- to millimeter scale. Nanometer-scale Si beams, with widths from 200 to 800 nm and a thickness of 255 nm, were fabricated on an Si diaphragm by means of field-enhanced anodization using AFM and anisotropic wet etching. The efficient condition of the field-enhanced anodization could be obtained by changing the bias voltage and the scanning speed of the cantilever. Bending tests for micro- and millimeter-scale Si beams fabricated by a photolithography technique were also carried out using an ultraprecision hardness tester and scratch tester, respectively. Comparisons of Young's modulus and bending strength, of Si among the nano-, micro-, and millimeter scales showed that the specimen size did not have an influence on the Young's modulus in the [110] direction, whereas it produced a large effect on the bending strength. Observations of the fractured surface and calculations of the clack length from Griffith's theory made it clear that the maximum peak-to-valley distance of specimen surface caused the size effect on the bending strength.

Surface-tension-driven microactuation based on continuous electrowetting

Abstract: This paper describes the first microelectromechanical systems (MEMS) demonstration device that adopts surface tension as the driving force. A liquid-metal droplet can be driven in an electrolyte-filled capillary by locally modifying the surface tension with electric potential. We explore this so-called continuous electrowetting phenomenon for MEMS and present crucial design and fabrication technology that reduce the surface-tension-driving principle, inherently powerful in microscale, into practice. The key issues that are identified and investigated include the problem of material compatibility, electrode polarization, and electrolysis, as well as the micromachining process. Based on the results from the initial test devices and the design concept for a long-range movement of the liquid-metal droplet, we demonstrate a liquid micromotor, an electrolyte and liquid-metal droplets rotating along a microchannel loop. Smooth and wear-free rotation of the liquid system is shown at a speed of /spl sim/40 mm/s (or 420 r/min along a 2-mm loop) with a driving voltage of only 2.8 V and little power consumption (10-100 /spl mu/W).

VHF free-free beam high-Q micromechanical resonators

Abstract: Free-free-beam flexural-mode micromechanical resonators utilizing nonintrusive supports to achieve measured Qs as high as 8400 at VHF frequencies from 30 to 90 MHz are demonstrated in a polysilicon surface micromachining technology. The microresonators feature torsional-mode support springs that effectively isolate the resonator beam from its anchors via quarter-wavelength impedance transformations, minimizing anchor dissipation and allowing these resonators to achieve high-Q with high stiffness in the VHF frequency range. The free-free-beam micromechanical resonators of this paper are shown to have an order of magnitude higher Q than clamped-clamped-beam versions with comparable stiffnesses.

Localized silicon fusion and eutectic bonding for MEMS fabrication and packaging

Abstract: Silicon fusion and eutectic bonding processes based on the technique of localized heating have been successfully demonstrated. Phosphorus-doped polysilicon and gold films are applied separately in the silicon-to-glass fusion bonding and silicon-to-gold eutectic bonding experiments. These films are patterned as line-shape resistive heaters with widths of 5 or 7 /spl mu/m for the purpose of heating and bonding. In the experiments, silicon-to-glass fusion bonding and silicon to gold eutectic bonding are successfully achieved at temperatures above 1000/spl deg/C and 800/spl deg/C, respectively, by applying 1-MPa contact pressure. Both bonding processes can achieve bonding strength comparable to the fracture toughness of bulk silicon in less than 5 min. Without using global heating furnaces, localized bonding process is conducted in the common environment of room temperature and atmospheric pressure. Although these processes are accomplished within a confined bonding region and under high temperature, the substrate temperature remains low. This new class of bonding scheme has potential applications for microelectromechanical systems fabrication and packaging that require low-temperature processing at the wafer level, excellent bonding strength, and hermetic sealing characteristics.

A traction stress sensor array for use in high-resolution robotic tactile imaging

Abstract: A high resolution large-area array capable of resolving the three independent components of a 2D triaxial contact stress profile has been developed. The array, composed of 4096 (64/spl times/64) individual stress sensing elements, was constructed with a fully CMOS-compatible fabrication process, allowing integration of the sensing structures with digital control circuitry. The individual array elements have been shown to demonstrate linear responses to both applied normal stress (1.59 mV/kPa, 0-35 kPa) and applied shear stress (0.32 mV/kPa, 0-60 kPa). A spatial resolution comparable to the spacing of the papillary ridges of the human dermis (/spl ap/300 /spl mu/m) has been achieved within the 1.92/spl times/1.92 cm active sensing area of the array. Descriptions of the sensor structure, the required signal conditioning, and the array architecture are presented in this paper. The results of electrical and mechanical characterization studies are also outlined.

A six-wafer combustion system for a silicon micro gas turbine engine

Abstract: As part of a program to develop a micro gas turbine engine capable of producing 10-50 W of electrical power in a package less than one cubic centimeter in volume, we present the design, fabrication, packaging, and experimental test results for the 6-wafer combustion system for a silicon microengine. Comprising the main nonrotating functional components of the engine, the device described measures 2.1 cm/spl times/2.1 cm/spl times/0.38 cm and is largely fabricated by deep reactive ion etching through a total thickness of 3800 /spl mu/m. Complete with a set of fuel plenums, pressure ports, fuel injectors, igniters, fluidic interconnects, and compressor and turbine static airfoils, this structure is the first demonstration of the complete hot flow path of a multilevel micro gas turbine engine. The 0.195 cm/sup 3/ combustion chamber is shown to sustain a stable hydrogen flame over a range of operating mass flows and fuel-air mixture ratios and to produce exit gas temperatures in excess of 1600 K. It also serves as the first experimental demonstration of stable hydrocarbon microcombustion within the structural constraints of silicon. Combined with longevity tests at elevated temperatures for tens of hours, these results demonstrate the viability of a silicon-based combustion system for micro heat engine applications.

1/f noise considerations for the design and process optimization of piezoresistive cantilevers

Abstract: Piezoresistive cantilevers are limited by two major noise sources: Johnson noise, which is independent of frequency, and conductance fluctuation noise, which has a 1/f spectrum. The 1/f fluctuations of piezoresistive cantilevers are shown to vary inversely with the total number of carriers in the piezoresistor, as formulated by Hooge in 1969. Therefore, while 1/f noise is reduced for large heavily doped cantilevers, sensitivity considerations favor thin lightly doped cantilevers. Balancing these conflicting constraints produces optima for many design and processing parameters. For a cantilever with specified spring constant and bandwidth requirements, optima are identified for the beam thickness and length, and it is shown that the legs should be between 1/3 and 2/3 of the total length with a doping depth that is 1/3 of the beam thickness. Additionally, an optimal doping concentration is identified as a function of the cantilever volume and the measurement bandwidth. Annealing reduces 1/f noise, but causes a loss in sensitivity due to dopant diffusion, and an optimal anneal is computed with a typical diffusion length 10/sup -8/ cm. The analysis, methods, and some of the conclusions of this paper are also applicable to other types of piezoresistive sensors.

Micromachining of buried micro channels in silicon

Abstract: A new method for the fabrication of micro structures for fluidic applications, such as channels, cavities, and connector holes in the bulk of silicon wafers, called buried channel technology (BCT), is presented in this paper. The micro structures are constructed by trench etching, coating of the sidewalls of the trench, removal of the coating at the bottom of the trench, and etching into the bulk of the silicon substrate. The structures can be sealed by deposition of a suitable layer that closes the trench. BCT is a process that can be used to fabricate complete micro channels in a single wafer with only one lithographic mask and processing on one side of the wafer, without the need for assembly and bonding. The process leaves a substrate surface with little topography, which easily allows further processing, such as the integration of electronic circuits or solid-state sensors. The essential features of the technology, as well as design rules and feasible process schemes, will be demonstrated on examples from the field of /spl mu/-fluidics.

Electrostatic micromechanical actuator with extended range of travel

Abstract: The practical design issues of an electrostatic micromechanical actuator that can travel beyond the trademark limit of conventional actuators are presented in this paper. The actuator employs a series capacitor to extend the effective electrical gap of the device and to provide stabilizing negative feedback. Sources of parasitics-from layout and two-dimensional nonuniform deformation-that limit the actuation range are identified and their effects quantified. Two "folded capacitor" designs that minimize the parasitics and are straightforward to implement in multiuser microelectromechanical processes are introduced. The effects of residual charge are analyzed, and a linear electrostatic actuator exploiting those effects is proposed. Extended travel is achieved in fabricated devices, but is ultimately limited by tilting instabilities. Nevertheless, the resultant designs are smaller than devices based on other extended-travel technologies, making them attractive for applications that require high fill factors.

A micromachined piezoelectric tactile sensor for an endoscopic grasper-theory, fabrication and experiments

Abstract: Present-day commercial endoscopic graspers do not have any built-in sensors, thus, the surgeon does not have the necessary tactile feedback to manipulate the tissue safely. This paper presents the design, fabrication, testing, and experimental results of a micromachined tactile sensor, which can be integrated with the tips of commercial endoscopic graspers. The prototype sensor consists of three layers. The top layer is made of micromachined silicon with a rigid tooth-like structure similar to the present-day endoscopic grasper. The bottom layer is made of flat Plexiglass serving as a substrate. Packaged between the Plexiglass and the silicon is a patterned Polyvinglidene Fluoride (PVDF) film. The proposed sensor exhibits high sensitivity, a large dynamic range, and a high signal-to-noise ratio. Through experimental results, it is shown that the magnitude and position of an applied force can be determined from the magnitude and slope of the output signals from the PVDF sensing elements. Structural analysis is also performed using the finite-element method, and the results are compared with the experimental analysis. The advantages and limitations of this sensor are also reported. A discussion of how the design of the sensor can be integrated with the design of an endoscopic grasper is also presented.

On a tunable bistable MEMS-theory and experiment

Abstract: A tunable micromechanical bistable system is presented in this paper. It consists of a long slender micromechanical beam attached to an actuator. The beam is subjected to a transverse force at the middle and a residual stress developed during fabrication. The actuator generates a force along the axial direction of the beam, and is shared by the beam and springs of the actuator. If the total axial load on the beam is compressive and exceeds a critical value, then the beam buckles along the transverse direction and it has two possible equilibrium states. Thus, the actuator and beam together become a bistable system. An analytical model is presented to characterize the system. The model is based on the first mode of buckling of the beam. The model accounts for the elastic axial shortening of the beam and the nonlinear coupling between the beam and actuator. The main result of this paper is a closed-form relation between the transverse force and corresponding equilibrium transverse displacement of the beam for a given actuator force. An experimental micromechanical device is designed and fabricated to verify the theoretical model. Excellent correspondence is found between theory and experiment, as if the experimental device emulates the mathematical model, which is an important result of this paper, since now experimental studies, both quasi-static and dynamic, of bistable systems are possible, which are otherwise difficult to conduct with macrosystems. Several examples of possible applications of the bistable system are provided, including digital sensing of physical parameters.

High aspect-ratio combined poly and single-crystal silicon (HARPSS) MEMS technology

Abstract: This paper presents a single-wafer high aspect-ratio micromachining technology capable of simultaneously producing tens to hundreds of micrometers thick electrically isolated poly and single-crystal silicon microstructures. High aspect-ratio polysilicon structures are created by refilling hundreds of micrometers deep trenches with polysilicon deposited over a sacrificial oxide layer. Thick single-crystal silicon structures are released from the substrate through the front side of the wafer by means of a combined directional and isotropic silicon dry etch and are protected on the sides by refilled trenches. This process is capable of producing electrically isolated polysilicon and silicon electrodes as tall as the main body structure with various size capacitive air gaps ranging from submicrometer to tens of micrometers. Using bent-beam strain sensors, residual stress in 80-/spl mu/m-thick 4-/spl mu/m-wide trench-refilled vertical polysilicon beams fabricated in this technology has been measured to be virtually zero. 300-/spl mu/m-long 80-/spl mu/m-thick polysilicon clamped-clamped beam micromechanical resonators have shown quality factors as high as 85 000 in vacuum. The all-silicon feature of this technology improves long-term stability and temperature sensitivity, while fabrication of large-area vertical pickoff electrodes with submicrometer gap spacing will increase the sensitivity of micro-electromechanical devices by orders of magnitude.

A high-performance planar piezoresistive accelerometer

Abstract: The micromachined piezoresistive accelerometer is now 20 years old. Design variations have been investigated, but commercial devices have generally maintained a consistent topology with incremental improvements. In this paper, a new approach is introduced to the design and construction of this device that offers functional and manufacturing advantages. Piezoresistive accelerometers are described that combine deep reactive ion etching and oblique ion implantation to form self-caging proof masses and flexures with vertical sidewalls and sidewall piezoresistive strain sensors. These devices deflect in-plane rather than out-of-plane, which allows one to form multiaxis accelerometers on one substrate. Performance is comparable to inexpensive commercial capacitive accelerometers and is limited by 1/f noise. The design, fabrication, and experimental characterization is presented. This new topology provides the foundation for a new category of piezoresistive accelerometers.

Computer-aided generation of nonlinear reduced-order dynamic macromodels. I. Non-stress-stiffened case

Abstract: Reduced-order dynamic macromodels are an effective way to capture device behavior for rapid circuit and system simulation. In this paper, we report the successful implementation of a methodology for automatically generating reduced-order nonlinear dynamic macromodels from three-dimensional (3-D) physical simulations for the conservative-energy-domain behavior of electrostatically actuated microelectromechanical systems (MEMS) devices. These models are created with a syntax that is directly usable in circuit- and system-level simulators for complete MEMS system design. This method has been applied to several examples of electrostatically actuated microstructures: a suspended clamped beam, with and without residual stress, using both symmetric and asymmetric positions of the actuation electrode, and an elastically supported plate with an eccentric electrode and unequal springs, producing tilting when actuated. When compared to 3-D simulations, this method proves to be accurate for non-stress-stiffened motions, displacements for which the gradient of the strain energy due to bending is much larger than the corresponding gradient of the strain energy due to stretching of the neutral surface. In typical MEMS structures, this corresponds to displacements less than the element thickness, At larger displacements, the method must be modified to account for stress stiffening, which is the subject of part two of this paper.

Process-dependent thin-film thermal conductivities for thermal CMOS MEMS

Abstract: The thermal conductivities /spl kappa/ of the dielectric and conducting thin films of three commercial CMOS processes were determined in the temperature range from 120 to 400 K. The measurements were performed using micromachined heatable test structures containing the layers to be characterized. The /spl kappa/ values of thermally grown silicon oxides are reduced from bulk fused silica by roughly 20%. The /spl kappa/ of phosphosilicate and borophosphosilicate glasses are 0.94/spl plusmn/0.08 W m/sup -1/ K/sup -1/ and 1.18/spl plusmn/0.06 W m/sup -1/ K/sup -1/, respectively, at 300 K. A plasma-enhanced chemical-vapor-deposition silicon-nitride layer has a thermal conductivity of 2.23/spl plusmn/0.12 W m/sup -1/ K/sup -1/ at 300 K. This value is between published data for atmospheric-pressure CVD and low-pressure CVD nitrides. For the metal layers, we found thermal conductivities between 167 W m/sup -1/ K/sup -1/ and 206 W m/sup -1/ K/sup -1/, respectively, at 300 K, to be compared with 238 W m/sup -1/ K/sup -1/ of bulk aluminum. The temperature-dependent product /spl kappa//spl rho/ of /spl kappa/ with the electrical resistivity /spl rho/ agrees better than 8.2% between 180-400 K with that of pure bulk aluminum. The /spl kappa/ values of the polysilicon layers are between 22.4 W m/sup -1/ K/sup -1/ and 37.3 W m/sup -1/ K/sup -1/ at 300 K. They are reduced from similarly doped bulk silicon by factors of between 2.0-1.3. The observed discrepancies between thin film and bulk data demonstrate the importance of determining the process-dependent thermal conductivities of CMOS thin films.

The flow structure inside a microfabricated inkjet printhead

Abstract: A micrometer resolution particle image velocimetry system has been adapted to measure instantaneous velocity fields in an inkjet printhead. The technique uses 700-nm-diameter fluorescent flow tracing particles, a pulsed Nd:YAG laser, an epi-fluorescent microscope, and a cooled interline transfer charge-coupled device camera to record images of flow tracing particles at two known instances in time. Instantaneous velocity vector fields are obtained with spatial resolutions of 5-10 /spl mu/m and temporal resolutions of 2-5 /spl mu/s. The relationship between instantaneous velocity fields is compared to instantaneous shapes of the meniscus. The flow in the nozzle is highly unsteady and characterized by a maximum velocity of 8 ms/sup -1/, Reynolds numbers of Re=500, and accelerations of up to 70 000 times gravity (i.e., 70 000 g). Since the flow field is periodic for each ejection cycle, the instantaneous measurements can be phased averaged to determine the evolution of the average flow field. The ejection cycle period is 500 /spl mu/s, and consists of four primary phases: infusion, inversion, ejection, and relaxation. During infusion, the actuator plate is deflected downward creating a low pressure that draws fluid into the inkjet cavity through the orifice and pulls the meniscus into the cavity through the nozzle. The meniscus grows, begins to decrease in size, and then deforms in shape, becoming inverted for approximately 6 /spl mu/s. The meniscus exits the cavity through the nozzle during droplet ejection. During relaxation, the flow undergoes viscously-damped oscillations, and reaches equilibrium before the next ejection cycle begins.

Stroboscopic interferometer system for dynamic MEMS characterization

Abstract: We describe a computer-controlled stroboscopic phase-shifting interferometer system for measuring out-of-plane motions and deformations of MEMS structures with nanometer accuracy. To aid rapid device characterization, our system incorporates (1) an imaging interferometer that records motion at many points simultaneously without point-by-point scanning, (2) an integrated computer-control and data-acquisition unit to automate measurement, and (3) an analysis package that generates sequences of time-resolved surface-height maps from the captured data. The system can generate a detailed picture of microstructure dynamics in minutes. A pulsed laser diode serves as the stroboscopic light source permitting measurement of large-amplitude motion (tens of micrometers out-of-plane) at kilohertz frequencies. The high out-of-plane sensitivity of the method makes it particularly suitable for characterizing actuated micro-optical elements for which even nanometer-scale deformations can produce substantial performance degradation. We illustrate the capabilities of the system with a study of the dynamic behavior of a polysilicon surface-micromachined scanning mirror that was fabricated in the MCNC MUMPS foundry process.

Surface/bulk micromachined single-crystalline-silicon micro-gyroscope

Abstract: A single-crystalline-silicon micro-gyroscope is fabricated in a single wafer using the recently developed surface/bulk micromachining (SBM) process. The SBM technology combined with deep silicon reactive ion etching allows fabricating accurately defined single-crystalline-silicon high-aspect-ratio structures with large sacrificial gaps, in a single wafer. The structural thickness of the fabricated micro-gyroscope is 40 /spl mu/m, and the sacrificial gap is 50 /spl mu/m. For electrostatic actuation and capacitive sensing of the developed gyroscope, a new isolation method which uses sandwiched oxide, polysilicon, and metal films, is developed in this paper. This triple-layer isolation method utilizes the excellent step coverage of low-pressure chemical vapor deposition polysilicon films, and thus, this new isolation method is well suited for high-aspect-ratio structures. The thickness of the additional films allows controlling and fine tuning the stiffness properties of underetched beams, as well as the capacitance between electrodes. The noise-equivalent angular-rate resolution of the SBM-fabricated gyroscope is 0.01/spl deg//s, and the bandwidth is 16.2 Hz. The output is linear to within 8% for a /spl plusmn/20/spl deg//s range. Work is currently underway to improve these performance specifications.

Design and fabrication of a cross flow micro heat exchanger

Abstract: A cross flow micro heat exchanger was designed to maximize heat transfer from a liquid (water-glycol) to a gas (air) for a given frontal area while holding pressure drop across the heat exchanger of each fluid to values characteristic of conventional scale heat exchangers. The predicted performance for these plastic, ceramic, and aluminum micro heat exchangers are compared with each other and to current innovative car radiators. The cross flow micro heat exchanger can transfer more heat/volume or mass than existing heat exchangers within the context of the design constraints specified. This can be important in a wide range of applications (automotive, home heating, and aerospace). The heat exchanger was fabricated by aligning and then bonding together two identical plastic parts that had been molded using the LIGA process. After the heat exchanger was assembled, liquid was pumped through the heat exchanger, and minimal leakage was observed.

An all-silicon single-wafer micro-g accelerometer with a combined surface and bulk micromachining process

Abstract: This paper reports an all-silicon fully symmetrical z-axis micro-g accelerometer that is fabricated on a single-silicon wafer using a combined surface and bulk fabrication process. The microaccelerometer has high device sensitivity, low noise, and low/controllable damping that are the key factors for attaining /spl mu/g and sub-/spl mu/g resolution in capacitive accelerometers. The microfabrication process produces a large proof mass by using the whole wafer thickness and a large sense capacitance by utilizing a thin sacrificial layer. The sense/feedback electrodes are formed by a deposited 2-3 /spl mu/m polysilicon film with embedded 25-35 /spl mu/m-thick vertical stiffeners. These electrodes, while thin, are made very stiff by the thick embedded stiffeners so that force rebalancing of the proof mass becomes possible. The polysilicon electrodes are patterned to create damping holes. The microaccelerometers are batch-fabricated, packaged, and tested successfully. A device with a 2-mm/spl times/1-mm proof mass and a full bridge support has a measured sensitivity of 2 pF/g. The measured sensitivity of a 1-mm/spl times/1-mm accelerometer with a cantilever support is 19.4 pF/g. The calculated noise floor of these devices at atmosphere are 0.23 /spl mu/g//spl radic/Hz and 0.16 /spl mu/g//spl radic/Hz, respectively.

An improved anodic bonding process using pulsed voltage technique

Abstract: In this study, we report a pulsed-voltage technique that is commonly employed in the electroplating industry to achieve a more efficient Si-glass anodic bonding process than the conventional constant electric field process. This technique features a less stringent voltage requirement and a shortened bonding time without compromising the tensile strength of the bonded structure. A square waveform voltage profile is used to investigate the effects of pulsed-voltage profile on the bonding time. In particular, the effects of magnitude of the base voltage and duration of the peak and base voltages are investigated. With peak and base voltages set to 400 and 300 V, respectively, and the duration of each voltage pulse fixed at 10-30 s, the bonding time is reduced to 30% of that required by a constant field process (400 V). Tensile strength of all completely bonded Si-glass pairs prepared by this technique is greater than 15 MPa. A postulated bonding mechanism based on the experimental results is presented.

Design, fabrication, and testing of a bistable electromagnetically actuated microvalve

Abstract: A bistable electromagnetically actuated microvalve was designed, processed, and tested. The valve was designed to control a water flow of 0.05-0.5 /spl mu/s from a reservoir at a pressure of 1-2000 Pa. The two valve components were fabricated in silicon, the upper piece comprises an electroplated gold coil, and the lower piece is an Ni/Fe alloy beam. The bistable capability was achieved by balancing the elastic forces on the beam with the magnetic forces due to a 46-/spl mu/m-thick rolled magnetic foil. The design includes the flow through the orifice, squeeze film damping due to beam motion, beam elasticity, and electromagnetics. The microvalve was tested for power consumption, flow rate, time response, Ni/Fe alloy composition, and magnetic foil properties. The valve operates at 1-2 V in both air and water.

The indent reflow sealing (IRS) technique-a method for the fabrication of sealed cavities for MEMS devices

Abstract: A variety of microelectromechanical system devices requires encapsulation of their crucial fragile parts in a hermetically sealed cavity for reasons of protection Hermeticity of the cavity and controllability of the ambient (gas pressure and gas composition) can be critical to the device performance In order to minimize damage during handling, the cavity is preferably realized at the same time the device is fabricated, ie, at wafer level This paper reports the development of a hermetic packaging technique satisfying all the above The method is referred to as the indent-reflow-sealing (IRS) technique, which relies on a multiple-chip fluxless solder-based joining technique and seal Key process steps are the creation of an indent in the solder, the plasma pretreatment of the bonding surfaces, the pre-bonding (or sticking) of the chips and, the closing of the indent during a low-temperature (220/spl deg/C-350/spl deg/C) solder reflow in a clean controlled ambient using a designated oven As opposed to other methods, the IRS method allows a greater flexibility with respect to the choice of the sealing gas and pressure, thereby offering a very hermetic seal and compatibility with low-cost high-throughput batch fabrication techniques Flip-chip assemblies based on SnPb (67/37) solder and Au as the top surface metallization, have been reflowed in a forming gas ambient and have next been characterized on shear strength, hermeticity, and susceptibility to thermal stresses The method has been successfully implemented in the process flow of an electromagnetic microrelay for the realization of the cavity housing the electrical contacts

Self-buckling of micromachined beams under resistive heating

Abstract: Self-buckling behavior of micromachined beams under resistive heating is described by an electromechanical model with experimental verifications. This model consists of both electrothermal and thermoelastic analyses for beam-shape polysilicon microstructures that are fabricated by a standard surface micromachining process. When an input electrical current is applied, Joule-heating effects trigger the thermal expansion of beam structures and cause mechanical buckling. The standard testing devices are clamped-clamped bridges, 2-/spl mu/m wide, 2-/spl mu/m thick, and 100-/spl mu/m long. It is found that a minimum current of 3.5 mA is required to cause beam buckling. Under an input current of 4.8 mA, a lateral deflection of 2.9/spl plusmn/0.2 /spl mu/m at the center of the bridge is measured with a computer image processing scheme. The experimental measurements are found to be consistent with analytical predictions. A discussion of modeling considerations and process variations is presented.

A new technique for producing large-area as-deposited zero-stress LPCVD polysilicon films: the MultiPoly process

Abstract: Polysilicon films deposited by low-pressure chemical vapor deposition (LPCVD) exhibit tensile or compressive residual stresses, depending on the deposition temperature. Polysilicon films composed of alternating tensile and compressive layers can display any overall stress value between those of the individual layers, including a state of zero overall residual stress, depending on the relative thickness of each layer. The residual stress gradient can be similarly controlled by the layer thicknesses and distribution. This has been demonstrated with a ten-layer near-zero stress (<10 MPa), near-zero stress gradient (/spl les/0.2 MPa//spl mu/m) polysilicon film, containing flat cantilever beams whose length-thickness ratios exceed 150. Using multilayer deposition to control the stresses and stress gradients of polysilicon films is termed the MultiPoly process.

A miniature pressure system with a capacitive sensor and a passive telemetry link for use in implantable applications

Abstract: A miniature telemetric pressure-measuring system is presented in this paper. The system uses passive telemetry to transfer power to the transponder and pressure data to the remote base unit. Such telemetric systems are becoming ever more important in the biomedical field as the interest for in-vivo measurements of different biological parameters both of humans and animals is increasing. A novel capacitive-type pressure sensor based on an SiGeB diaphragm is used as a sensing element. The merits of combining a capacitive pressure sensor and passive telemetry lies in the inherent low-power consumption of the sensor and the continuous availability of power through induction. The pressure sensor is connected to an integrated interface circuit, which includes a capacitance to frequency converter and an internal voltage regulator to suppress supply voltage fluctuations on the transponder side. In addition, the sensor and accompanying interface circuit take up very little space so as to be suitable for implantation.

An integrated liquid mixer/valve

Abstract: We present an integrated liquid mixer/valve to be used for sample preparation for bioscience analysis systems. The mixer/valve is a glass-silicon bonded structure with a wafer-bonded cantilever-plate flapper valve and deep reactive-ion etched ports. It is passively pressure actuated and is distinguished by the fact that it can perform both a mixing and valving function simultaneously to mix two liquids noncontinuously. We present the design and fabrication of the mixer/valve and show that it successfully performs both its valving and mixing functions, including the discontinuous mixing of two liquids. We propose a method for characterizing mixing in this device using fluorescence microscopy and the pH dependence of fluorescein fluorescence. This method aims to allow one to extract the mixing length from a quantifiable observable. We present modeling and results of mixing length measurements using this method.