Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 1996"

Etch rates for micromachining processing

Abstract: The etch rates for 317 combinations of 16 materials (single-crystal silicon, doped, and undoped polysilicon, several types of silicon dioxide, stoichiometric and silicon-rich silicon nitride, aluminum, tungsten, titanium, Ti/W alloy, and two brands of positive photoresist) used in the fabrication of microelectromechanical systems and integrated circuits in 28 wet, plasma, and plasmaless-gas-phase etches (several HF solutions, H/sub 3/PO/sub 4/, HNO/sub 3/+H/sub 2/O+NH/sub 4/F, KOH, Type A aluminum etchant, H/sub 2/O+H/sub 2/O/sub 2/+HF, H/sub 2/O/sub 2/, piranha, acetone, HF vapor, XeF/sub 2/, and various combinations of SF/sub 6/, CF/sub 4/, CHF/sub 3/, Cl/sub 2/, O/sub 2/, N/sub 2/, and He in plasmas) were measured and are tabulated. Etch preparation, use, and chemical reactions (from the technical literature) are given. Sample preparation and MEMS applications are described for the materials.

Thin film shape memory alloy microactuators

Abstract: Thin film shape memory alloys (SMAs) have the potential to become a primary actuating mechanism for mechanical devices with dimensions in the micron-to-millimeter range requiring large forces over long displacements. The work output per volume of thin film SMA microactuators exceeds that of other microactuation mechanisms such as electrostatic, magnetic, thermal bimorph, piezoelectric, and thermopneumatic, and it is possible to achieve cycling frequencies on the order of 100 Hz due to the rapid heat transfer rates associated with thin film devices. In this paper, a quantitative comparison of several microactuation schemes is made, techniques for depositing and characterizing Ni-Ti-based shape memory films are evaluated, and micromachining and design issues for SMA microactuators are discussed. The substrate curvature method is used to investigate the thermo-mechanical properties of Ni-Ti-Cu SMA films, revealing recoverable stresses up to 510 MPa, transformation temperatures above 32/spl deg/C, and hysteresis widths between 5 and 13/spl deg/C. Fatigue data shows that for small strains, applied loads up to 350 MPa can be sustained for thousands of cycles. Two micromachined shape memory-actuated devices-a microgripper and microvalve-also are presented.

Electrostatic micro torsion mirrors for an optical switch matrix

Abstract: We have developed a new type of compact optical switch using silicon micromachining technique. Torsion mirrors (300 /spl mu/m/spl times/600 /spl mu/m) supported by thin polysilicon beams (16 /spl mu/m wide, 320 /spl mu/m long, and 0.4 /spl mu/m thick) are arranged in a 2/spl times/2 matrix (total size 3 mm/spl times/5 mm, t 0.3 mm). The mirrors are independently attracted by electrostatic force of applied bias voltage to redirect the incident light in a free space. Using collimated beam fibers for optical coupling, we obtained small insertion loss (/spl les/-7.66 dB), considering the length of a light path (/spl ges/10 mm), a large switching contrast (/spl ges/60 dB), and small crosstalk (/spl les/-60 dB). The fabrication yield was higher than 80% thanks to the newly developed releasing technique that used a silicon oxide diaphragm as an etch-stop layer and as a mechanical support in the process. Holding voltage (/spl les/50 V) was lower than the voltage to attract the mirror (100/spl sim/150 V) because of the hysteresis of angle-voltage characteristic of electrostatic operation.

Three-dimensional microfabrication by localized electrochemical deposition

Abstract: A microfabrication technology capable of electrodepositing truly three-dimensional metal structures is introduced. Micrometer-scale nickel structures including a multicoiled helical spring have been fabricated. Electrodeposition is localized by placing a sharp-tipped electrode in a plating solution, near a substrate, and applying a voltage. Structures are built by moving the electrode appropriately with respect to the substrate. Vertical deposition rates of 6 /spl mu/m/s are observed, two orders of magnitude greater than those of conventional electrodeposition. The theory of mass transport to a region of localized field is discussed, and a model of deposition profile is presented. The process can potentially produce submicrometer feature sizes using a range of materials including pure metals, alloys, and polymers.

Bent-beam strain sensors

Abstract: We examine a new class of sensitive and compact passive strain sensors that utilize a pair of narrow bent beams with an apex at their mid-points. The narrow beams amplify and transform deformations caused by residual stress into opposing displacements of the apices, where vernier scales are positioned to quantify the deformation. An analytical method to correlate vernier readings to residual stress is outlined, and its results are corroborated by finite-element modeling. It is shown that tensile and compressive residual stress levels below 10 MPa, corresponding to strains below 6/spl times/10/sup -5/ can be measured in a 1.5-/spl mu/m-thick layer of polysilicon using a pair of beams that are 2 /spl mu/m wide, 200 /spl mu/m long, and bent 0.05 radians (2.86/spl deg/) to the long axis of the device. Experimental data is presented from bent-beam strain sensors that were fabricated from boron-doped single crystal silicon using the dissolved wafer process and from polycrystalline silicon using surface micromachining. Measurements from these devices agree well with those obtained by other methods.

A fully integrated micromachined magnetic particle separator

Abstract: A prototype micromachined magnetic particle separator that can separate magnetic particles from suspended liquid solutions has been realized on a silicon wafer. The requisite magnetic field gradients are generated by integrated inductive components in place of permanent magnets, which yields several advantages in design flexibility, compactness, electrical and optical monitoring, and integration feasibility (thus enabling mass production). Preliminary experiments have been performed on aqueous suspensions of magnetic beads. At 500 mA of dc current, approximately 0.03 Tesla of magnetic flux density is achieved at the gap between the quadrupoles, and the magnetic particles rapidly move toward the quadrupoles, separate from the buffer solution, and clump on the poles. The magnetic particles clumped on the poles are also easily released when the dc current is removed, achieving the primary purpose of a separator. The device shows that micromachined magnetic components have a high potential in biological or biomedical applications, especially in separating small amounts of cells or DNA that are marked with magnetic beads, especially when close monitoring and control of the process is important.

Fracture testing of bulk silicon microcantilever beams subjected to a side load

Abstract: A custom experimental system was developed to fracture silicon microcantilever beams in side loading (i.e., the load was applied in the noncompliant direction), and the resulting force/deflection (stiffness) characteristics were obtained. A finite element model of these structures was analyzed using ABAQUS, and the resulting model stiffness correlated well with the experimental data. Fracture types were divided into two categories, {111} and {110}, according to the type of silicon crystalline plane along which fracture occurred. The initiation location of each fracture type was identified. The fracture stress (strength) in the beam was obtained from the stress produced in the model at the fracture initiation site for a load equivalent to the experimental fracture force. Numerous beams were tested, and the statistical results were compiled. The distributions and statistical data from each of the fracture types were compared to each other and to previously acquired results from front/back loading (i.e., loading in the compliant direction) of these same structures. Side-loading results indicated that the {110} fracture type had a greater fracture strength than the {111} type. Based on a comparison of the side loading data with the front/back loading data, it was concluded that side wall roughness and especially the edge roughness greatly affected the fracture strength of the silicon micromechanical structures.

Development of the micromilling process for high-aspect-ratio microstructures

Abstract: At the macroscale, the milling process is very versatile and capable of creating three-dimensional features and structures. Adaptation of this process at the microscale could lead to the rapid and direct fabrication of micromolds and masks to aid in the development of microcomponents. This task has been undertaken, and results of the process indicate it can become an increasingly useful method. The micromilling process is characterized by milling tools that are currently in the range from 22-100 /spl mu/m in diameter and made by the focused-ion beam machining process. The tools are used in a specially designed, high-precision milling machine. Results are comparable to other processes currently used to fabricate mold and mask features. The micromilling process can create trench-like features with nearly vertical sidewalls and good smoothness. External corners are sharp and stepped features can be machined simply by programming those shapes. The process is direct, and therefore dimensional errors do not accumulate as can occur with serial fabrication processes.

Piezoelectric cantilever microphone and microspeaker

Abstract: A micromachined piezoelectric cantilever transducer, which works both as a microphone and as a microspeaker, has been fabricated and tested. The 2000/spl times/2000/spl times/4.5 /spl mu/m/sup 3/ cantilever has a zinc oxide (ZnO) piezoelectric thin film on a supporting layer of low-pressure chemical-vapor-deposited (LPCVD) low-stress silicon nitride. A highlight of the fabrication process, which may also be relevant for other micromachined structures, is the technique for producing a flat, multilayer cantilever. The measured microphone sensitivity is fairly constant at 3 mV//spl mu/bar in the low frequency range and rises to 20 mV//spl mu/bar at the lowest resonant frequency of 890 Hz. The 3 mV//spl mu/bar sensitivity is the highest reported to date for a microphone with a micromachined diaphragm. When measured into a 2 cm/sup 3/ coupler with 4 V(zero-peak) drive, the microspeaker output sound pressure level (SPL) is 75 dB at 890 Hz. It increases to approximately 100 dB SPL at 4.8 kHz with 6 V(zero-peak) drive. The measured microphone frequency response agrees well with the results of an ABAQUS simulation.

A hermetic glass-silicon micropackage with high-density on-chip feedthroughs for sensors and actuators

Abstract: This paper describes the development of a hermetic micropackage with high-density on-chip feedthroughs for sensor and actuator applications. The packaging technique uses low-temperature (320/spl deg/C) electrostatic bonding of a custom-made glass capsule (Corning 7740, 2/spl times/2/spl times/8 mm/sup 3/) to fine grain polysilicon in order to form a hermetically sealed cavity. High-density on-chip multiple polysilicon feedthroughs (200 per millimeter) are used for connecting external sensors and actuators to the electronic circuitry inside the package. A high degree of planarity over feedthrough areas is obtained by using grid-shaped polysilicon feedthrough lines that are covered with phosphosilicate glass (PSG), which is subsequently reflown at 1100/spl deg/C in steam for 2 h. Saline and DI water soak tests at elevated temperatures (85 and 95/spl deg/C) were performed to determine the reliability of the package. Preliminary results have shown a mean time to failure (MTTF) of 284 days and 118 days at 85 and 95/spl deg/C, respectively, in DI water. An Arrhenius diffusion model for moisture penetration yields an expected lifetime of 116 years at body temperature (37/spl deg/C) for these packages. In vivo tests in guinea pigs and rats for periods ranging from one to two months have shown no sign of infection, inflammation, or tissue abnormality around the implanted package.

An electrochemical microactuator: principle and first results

Abstract: A novel electrochemical microactuator made with the use of silicon micromachining techniques, and its feasibility, are presented. Gas pressure is generated by electrolysis of an aqueous electrolyte solution. The pressure built up is used to change the deflection of a membrane. The actuator has three states: the electrolysis state, in which the pressure is built up; the passive state, in which the circuit is open and the pressure is maintained; and the pressure reduction state, in which the electrodes are short-circuited in order to reverse the electrolysis reaction. The advantage of this type of actuation is a relatively large pressure generation with low energy consumption. Power is required only for pressure build-up and for changing the states. Therefore, this type of actuation has promising applications in pumps or active valves.

Surface-micromachined components for articulated microrobots

Abstract: A class of articulated micromanipulator robots with multiple degrees of freedom, workspaces on the order of a cubic millimeter, and payloads on the order of a milligram are proposed. Rigid links, mechanical couplings, and large-force, large-displacement micromotors have been created. Hollow triangular beams made from rotated microhinged polysilicon plates with polysilicon locks can withstand axial loads of up to 2.6 gm. Mechanical couplings with sliding mechanisms are used to rotate hinged structures off the substrate. The typical frictional force observed is approximately 2 /spl mu/N. Linear electrostatic stepper motors with an estimated force of 6.5 /spl mu/N at 35 V and a travel of 40 /spl mu/m have also been demonstrated.

On hillocks generated during anisotropic etching of Si in TMAH

Abstract: Hillocks on etched Si{100} surfaces produced by anisotropic etching are a common irritant in the creation of micromachined devices. Close inspection of typical pyramidal hillock shapes reveals that they are usually bounded by convex -directed edges and {111} or near-{111} planes. Underetch experiments at varying TMAH etchant composition confirm that the etch rates of {101} planes and {100} planes vary with etchant conditions. Hillocks are suppressed when {101} etches faster than {100}, which occurs when the TMAH concentration is low. A simple model involving kinks and ledges is proposed and allows direct relation of hillock features to etch anisotropy. Hillocks are hypothesized to be stable due to a lower etch rate for ledges adjacent to the etched surface. The apex of the pyramids may be protected by impurities or defects. Re-etch experiments indicate that hillock-producing conditions are quite sensitive to etchant conditions.

Design and fabrication of silicon condenser microphone using corrugated diaphragm technique

Abstract: A novel silicon condenser microphone with a corrugated diaphragm has been proposed, designed, fabricated and tested. The microphone is fabricated on a single wafer by use of silicon anisotropic etching and sacrificial layer etching techniques, so that no bonding techniques are required. The introduction of corrugations has greatly increased the mechanical sensitivities of the microphone diaphragms due to the reduction of the initial stress in the thin films, For the purpose of further decreasing the thin film stress, composite diaphragms consisting of multilayer (polySi/Si/sub x/N/sub y//polySi) materials have been fabricated, reducing the initial stress to a much lower level of about 70 MPa in tension. Three types of corrugation placements and several corrugation depths in a diaphragm area of 1 mm/sup 2/ have been designed and fabricated. Microphones with flat frequency response between 100 Hz and 8/spl sim/16 kHz and open-circuit sensitivities as high as 8.1/spl sim/14.2 mV/Pa under the bias voltages of 10/spl sim/25 V have been fabricated in a reproducible way. The experimental results proved that the corrugation technique is promising for silicon condenser microphone.

Multimode digital control of a suspended polysilicon microstructure

Abstract: Digital control of a suspended 350 /spl mu/m/spl times/380 /spl mu/m/spl times/1.6 /spl mu/m-thick surface-micromachined polysilicon plate is demonstrated in three degrees of freedom, with application to multimode accelerometers, vibratory rate gyroscopes, and actively positioned micromirrors. Plate displacement about the 2.2 /spl mu/m nominal position above the substrate is measured with shielded capacitive sensors connected to CMOS buffer circuits fabricated adjacent to the microstructure. Four micromechanical sigma-delta loops are used to control eight electrostatic actuators that drive the plate vertically (z) and in out-of-plane rotation (/spl theta/ and /spl phi/). Resonant frequencies are 2.7 kHz for the /spl theta/ rotational mode and 3.7 kHz for both z and /spl phi/ modes. The system is evaluated using a mixed mechanical/electromechanical/circuit simulation in SPICE. Closed-loop transient simulation of a 150-Hz square-wave position input signal is in good agreement with experimental results. Squeeze-film damping limits the plate slew rate to 0.83 mm/s in air. Position is controlled to within /spl plusmn/25 mm, being limited by quantization noise at the 50 kHz sampling rate.

Surface-micromachined capacitive differential pressure sensor with lithographically defined silicon diaphragm

Abstract: A capacitive surface-micromachined sensor suitable for the measurement of liquid and gas pressures was fabricated. The structure consists of a polysilicon stationary electrode suspended 0.7 /spl mu/m above a 20-/spl mu/m-thick lightly doped silicon diaphragm formed by a patterned etch stop. The a priori patterning of the buried etch stop yields diaphragm widths independent of wafer thickness variations with excellent alignment. The design described here has a pressure range of 100 PSI, a nominal capacitance of 3.5 pF with a full scale span of 0.8 pF, and a temperature coefficient of 100 ppm/spl deg/C/sup -1/. Each device, including a matched reference capacitor, occupies 2.9 mm/sup 2/, yielding approximately 2000 devices per 100-mm wafer.

A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes

Abstract: This paper discusses a noninvasive sensing technique for the direct measurement of low-magnitude shear stresses in laminar and turbulent air flows. The sensing scheme detects the flow-induced in-plane displacement of a microfabricated floating-element structure (500 /spl mu/m/spl times/500 /spl mu/m/spl times/7 /spl mu/m), using integrated photodiodes. The wall-mounted floating-element sensors were fabricated using a wafer-bonding technology. The sensors were calibrated in a custom-designed laminar flow cell and subsequently shown to be able to transduce shear stresses of 0.01 Pa during tests in a low-speed wind tunnel.

Characterization and modeling of metal-film microbolometer

Abstract: The characteristic thermal parameters of a platinum-film microbolometer are extracted from the data of two measuring methods. A simple and accurate equivalent circuit model, along with its thermal behavior, is proposed for the device. Applying the model to simulate some device circuits results in good agreement with the experimental data. Furthermore, an effective method of ambient temperature compensation, proposed previously by our laboratory, is demonstrated both experimentally and by simulation using the same model. The established electro-thermal model therefore serves as an useful tool for SPICE simulations in the design of microbolometers.

The development of a low-stress polysilicon process compatible with standard device processing

Abstract: When surface micromachined devices are combined with on-chip circuitry, any high-temperature processing must be avoided to minimize the effect on active device characteristics. High-temperature stress annealing cannot be applied to these structures. This work studies the effects of deposition parameters and subsequent processing on the mechanical properties of the polysilicon film in the development of a low-strain polysilicon process, without resorting to high-temperature annealing. The films are deposited as a semi-amorphous film and then annealed, in situ at 600/spl deg/C for 1 h, to ensure the desired mechanical characteristics for both doped and undoped samples. This low temperature anneal changes the strain levels in undoped films from -250 to +1100 /spl mu//spl epsi/. The best results have been obtained for an 850/spl deg/C anneal for 30 min which is used to activate the dopant (both phosphorus and boron). No further stress annealing was used, and 850/spl deg/C does not present problems in terms of thermal budget for the electrical devices. It is shown that these mechanical characteristics are achieved by forming the grain boundaries during subsequent low temperature annealing, and not during deposition. TEM (transmission electron microscopy) studies have been used to investigate the link between the structure and mechanical strain. This has shown that it is the formation of the grain boundary rather than the grain size which has a significant effect on strain levels, contrary to reports in the literature. Using the above-mentioned deposition process, a series of experiments have been performed to establish the flexibility in subsequent processing available to the designer. Therefore, by careful consideration of the processing, a low-temperature polysilicon process, which can be used to fabricate thin micromachined structures, has been developed.

Uncooled IR imaging array based on quartz microresonators

Abstract: A quartz crystal resonator's resonance frequency is sensitive to temperature. This sensitivity has been exploited in the past in thermometers made of single, macroscopic quartz resonators that can accurately detect temperature changes of /spl mu/K. Using semiconductor microfabrication techniques, it is now possible to fabricate a large number of microresonators from a single quartz wafer. It is shown that combining the small thermal mass and high thermal isolation capability of such microresonators, the steep frequency versus temperature characteristics of resonators made of certain cuts of quartz and the low-noise characteristics of quartz crystal oscillators can result in high-performance infrared (IR) sensors and sensor arrays. In a microresonator sensor, the temperature change produced by the absorption of IR energy results in a frequency change that can be measured with a resolution that corresponds to a change in the resonator's temperature of less than a /spl mu/K. Calculation shows that an array of microresonators in the 200 MHz-1 GHz range can be the basis of an uncooled IR imaging system with a noise equivalent temperature difference, NETD, of <0.01 K. The design and fabrication problems to be overcome before such microresonator arrays can be realized are discussed.

Resonant silicon accelerometers in bulk micromachining technology-an approach

Abstract: The fabrication and characterization of resonant silicon accelerometers, made in bulk micromachining technology, is presented. The devices consist of a silicon mass, coupled axially to a strain-sensitive vibrating silicon beam. The beam is driven electrothermally and sensed piezoresistively by means of implanted piezoresistors. Two different accelerometer types are shown, differing in the complexity of the respective fabrication processes and in performances. Closed-loop operation of the devices is demonstrated. Also in the closed loop, static and dynamic measurements of prototypes have been performed. The sensor types presented are compared, and the resonant acceleration sensor concept is discussed.

Silicon-based microelectrodes for neurophysiology fabricated using a gold metallization/nitride passivation system

Abstract: The multimicroelectrode probe (microprobe) is a device used in neurophysiology to record signals from nerve cells. Microprobes typically have a number of gold recording sites supported on a narrow cantilever beam which is inserted into the tissue. Conducting tracks connect the recording sites to bonding pads on the body of the device. The metallization is insulated, except at the recording sites and bonding pads, by a passivation layer. Boron etch stop techniques can be used to produce narrow cantilever beams upon which recording sites are situated. Previously, polysilicon interconnects were used on microprobes fabricated using boron etch stop techniques, with gold inlaid onto the recording sites using a lift-off technique. This meant that mechanical jigging was required before the final shaping of the probes in potassium hydroxide (or other etch) to prevent the etch from attacking the polysilicon conductors beneath the inlaid gold. The process reported here incorporates a gold metallization layer, in conjunction with a plasma-enhanced chemical vapor deposition (PECVD) nitride passivation layer. Since both these materials etch very slowly in potassium hydroxide, no mechanical jigging, or other steps, need to be taken to protect the front of the wafer during the shaping stage. This simplifies the fabrication of these devices.

Mechanisms of etch hillock formation

Abstract: We have studied the formation of etch hillock defects during anisotropic etching of (100) silicon in KOH. Defect density is correlated with low etchant concentration and high etch temperature. Cathodic etch experiments indicate that hillocks form under conditions of decreased OH/sup -/ ion concentration. The activation energy for defect formation is 1.2 eV, considerably higher than the energy associated with silicon removal. We propose a mechanism to explain hillock formation that involves nucleation by silicon redeposited from the etch solution. The incidence of hillocks in this model is the result of a competition between the forward and reverse etch reactions. Examination of defects by electron microscopy suggests that growth occurs preferentially on slow-etching planes, in agreement with the model predictions.

Fabrication of a mechanical antireflection switch for fiber-to-the-home systems

Abstract: We present the methods used to fabricate a micromechanical silicon optical modulator for use in a fiber-to-the-home applications. We emphasize the efforts made to realize a practical, robust, manufacturable, and easily packaged device. In addition, recent speed, temperature stability, and reliability results are presented. Rise and fall times of 132 and 125 ns, respectively, have been observed in response to a square wave drive signal. The device has been temperature cycled from -50/spl deg/C to 90/spl deg/C and shown greater than 10-dB optical contrast ratio over this temperature range. Finally, the device has been cycled at 500 kHz for a period of nearly two months (two-trillion cycles) without a noticeable loss in performance.

Computation of static shapes and voltages for micromachined deformable mirrors with nonlinear electrostatic actuators

Abstract: In modeling micromachined deformable mirrors with electrostatic actuators whose gap spacings are of the same order of magnitude as those of the surface deformations, it is necessary to use nonlinear models for the actuators. In this paper, we consider micromachined deformable mirrors modeled by a membrane or plate equation with nonlinear electrostatic actuator characteristics. Numerical methods for computing the mirror deformation due to given actuator voltages and the actuator voltages required for producing the desired deformations at the actuator locations are presented. The application of the proposed methods to circular deformable mirrors whose surfaces are modeled by elastic membranes is discussed in detail. Numerical results are obtained for a typical circular micromachined mirror with electrostatic actuators.

X-ray lathe: An X-ray lithographic exposure tool for nonplanar objects

Abstract: An X-ray lithography lathe has been developed that can pattern cylindrical, ellipsoidal, and other nonplanar objects. This lathe is capable of patterning on a micron scale a wide variety of shapes including shapes impossible to achieve with a conventional lathe. A cylindrical core covered with a suitable resist is rotated while being exposed with a collimated X-ray source through a mask. The mask absorbs X rays up to a particular radius from the center of the core and the resist beyond that radius is removed in a developer. Several cylindrical cores were coated with poly(methylmethacrylate) (PMMA) 5 to 125 /spl mu/m thick and patterned with X-rays down to a 250-/spl mu/m horizontal scale (along the lathe axis). The exposure time for a cylindrical PMMA layer is /spl sim/three-four times longer than a planar layer with the same thickness. The capabilities of this technology, lathe apparatus, dose calculations, and initial exposure results are described.

Planarity of large MEMS

Abstract: A systematic approach is developed to study the planarity of large (few mm long) micromechanical cantilever beams made of /spl mu/m-size features. The beams are made by the SCREAM (single crystal reactive etching and metallization) process. SCREAM beams consist of a single crystal silicon (SCS) core coated on top and sides by oxide or nitride and a metal. The sidewalls overhang the SCS core. The beams deform out of plane due to thermal and intrinsic strains of the coating films. These strains are defined and measured for plasma deposited SiO/sub 2/ and sputtered aluminum films. A linear elastic model of SCREAM cantilever beams is then developed to evaluate the deformation of the beams caused by film strains. The model predicts that the beams may bend up or down or remain planar depending on their cross-sectional design. Also, the greater the depth of the beams, the more planar they are, and a change in temperature (room temp-100/spl deg/C) has little influence on planarity for beams with thin (/spl sim/.2 /spl mu/m) metallization. The model is validated by fabricating large (up to 2 mm long) cantilever beams, 1 and 2 /spl mu/m wide, with PECVD SiO/sub 2/ and sputtered Al coatings. The deformations of the beams prior to metallization as well as before and after annealing of the metallized beams are measured. Good agreement is obtained between the experimental deformations and those predicted by the model. The paper is concluded with an example of a working, large (4/spl times/5 mm/sup 2/), planar MEM device fabricated by the SCREAM process.

Released Si microstructures fabricated by deep etching and shallow diffusion

Abstract: A novel etch-diffusion process is developed for fabricating high-aspect-ratio Si structures for microsensors. This is accomplished by first dry etching narrow gap Si microstructures using an electron cyclotron resonance (ECR) source, followed by a shallow B diffusion to fully convert the etched microstructures to p/sup ++/ layer. Microstructures up to 40 /spl mu/m deep with 2-/spl mu/m-wide gaps were etched with a Cl/sub 2/ plasma generated using the ECR source. Vertical profile and smooth morphology were obtained at low pressure. A shallow B diffusion at 1175/spl deg/C for 5.5 h. was then carried out to convert the 40-/spl mu/m-thick resonant elements to p/sup ++/ layer. A second dry etching step was used to remove the thin p/sup ++/ layer around the bottom of the resonant elements, followed by bonding to glass and selective wet etch. Released high-aspect-ratio Si microsensors with thicknesses of 35 /spl mu/m have been demonstrated. At atmospheric pressure, only 5 V/sub dc/ driving voltage is needed for 2.5 /spl mu/m vibration amplitude, which is less than the 10 V/sub dc/ required to drive 12-/spl mu/m-thick resonators fabricated by conventional dissolved wafer process.

Vibration modes of a resonant silicon tube density sensor

Abstract: We present an investigation of the resonance parameters for a new sensor for on-line measurements of fluid density. The sensor consists of a tube system made of single crystalline silicon. The tube system is excited electrostatically into mechanical resonance and the vibration is detected optically. Using a simplified theoretical analysis, the resonance frequency can be shown to be proportional to 1/spl radic//spl rho/, where /spl rho/ is the density of the silicon and the fluid weighted according to their areas in a cross section of the tube. Thus, a change in fluid density results in a change in the resonance frequency. This dependence is demonstrated by measurements for four different vibrations modes. The quality of the vibration is also investigated through measurements of the Q-values of the vibration modes. The tubes are made using anisotropic silicon KOH etching and silicon-to-silicon fusion bonding micromachining techniques. The dimensions of the tube system are 8.6/spl times/17.7 mm with an outer tube thickness of 1 mm and a wall thickness of 100 /spl mu/m. Total tube length is 61 mm, and the sample volume is 0.035 ml. The sensor has a very good density sensitivity of the order of -200 ppm/(kgm/sup -3/) and a high Q of the order of 3000 for air in the tube.

Measurement of residual stresses in a plate using a vibrational technique-application to electrolytic nickel coatings

Abstract: A vibration method for the evaluation of the stress in a membrane is presented. The principle is based on the shift of the eigenfrequency of the modes due to the stress. The identification of the modes is obtained by an optical technique. The proposed method is applied to electrolytic nickel coatings. The results are compared with those obtained by X-ray analysis. The application of this type of measurement to other structures is discussed.