Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 1995"

Modeling of a three-layer piezoelectric bimorph beam with hysteresis

Abstract: Piezoelectric actuators are usually stacked or bimorph in configuration. In this paper the mechanics of a three-layer piezoelectric bimorph is discussed and its dynamic model with hysteresis is presented. The results can be used to analyze piezoelectric actuators constructed with three-layer piezoelectric bimorphs. A piezoelectric bimorph actuator has been fabricated and experiments have been carried out to verify the model. The calculated results of this model closely matched the tested results. This model can also be used with other types of piezoelectric actuators with a slight modification.

A new analytical solution for the load-deflection of square membranes

Abstract: Accurate models are essential for the determination of the elastic properties of thin films from load-deflection experiments. Although finite element method (FEM) models have the potential to be very accurate, analytical models are desirable because of their simplicity. In this paper we present a new analytical solution for the load-deflection of membranes. Our solution yields the same relationship between the load and the deflection as the known analytical solution. However, the values of two constants are up to 35% higher and correspond well with the results from FEM analysis. In addition, the new solution yields analytical forms of the bending lines, which agree well with experimental measurements carried out with silicon nitride membranes.

TiNi (shape memory) films silicon for MEMS applications

Abstract: TiNi shape memory alloy in thin film form is an excellent candidate for MEMS microactuation. Using RF sputter deposition, thin films of TiNi (51.7 at% Ti-48.3 at% Ni) have been formed on silicon substrates and produced shape memory behavior at approximately 60/spl deg/C. Films were amorphous when deposited and were subsequently annealed at 515/spl deg/C for 30 min. to crystallize the shape memory microstructure. Excellent adherence was achieved onto silicon, SiO/sub 2/ and poly-silicon surfaces. Microfabrication was used to create TiNi diaphragms, which exhibited useful shape memory microactuation and other desirable mechanical properties. The diaphragms recovered greater than 2% strain when heated through the phase transformation temperature, providing a maximum work density of at least 5/spl times/10/sup 6/ J/m/sup 3/. This work density is higher than that of any other type of microactuation.

The anharmonic Casimir oscillator (ACO)-the Casimir effect in a model microelectromechanical system

Abstract: The Casimir effect is the attractive pressure between two flat parallel plates of solids that arises from quantum fluctuations in the ground state of the electromagnetic field. The magnitude of this pressure varies as the inverse fourth power of the separation between the plates. At a 20 nm separation between two metallic plates, the attraction is approximately 0.08 atmosphere. If one or both plates are nonconducting the pressure is smaller, roughly by an order of magnitude. As an idealized MEMS component that takes account of the Casimir effect, the anharmonic Casimir oscillator (ACO) is introduced and shown to be a bi-stable system for certain values of the dimensionless parameter, C, which characterizes the system. The phenomenon of "stiction" in MEMS is then explained as analogous to an ACO energetically descending to and settling in an equilibrium state that is very stable against perturbations for all values of C. A micromechanical switch based on the bistable ACO is proposed and modeled. The dynamics of an ACO, executing undamped periodic motion, are studied using numerical and analytical solutions of the differential equation of motion. Frequencies and amplitudes vary with C. C, in turn, is inversely proportional to the fifth power of the parallel plate separation. This extreme sensitivity makes the ACO an attractive platform for designing rather sensitive sensors and detector systems, such as submicrometer proximity sensors and microlever deflection detectors for scanning probe microscopes.

Magnetic microactuation of polysilicon flexure structures

Abstract: A microactuator technology that combines magnetic thin films with polysilicon flexural structures is described. Devices are constructed in a batch-fabrication process that combines electroplating with conventional lithography, materials, and equipment. A microactuator consisting of a 400/spl times/(47-40)/spl times/7 /spl mu/m/sup 3/ rectangular plate of NiFe attached to a 400/spl times/(0.9-1.4)/spl times/2.25 /spl mu/m/sup 3/ polysilicon cantilever beam has been displaced over 1.2 mm, rotated over 180/spl deg/, and actuated with over 0.185 nNm of torque. The microactuator is capable of motion both in and out of the wafer plane and has been operated in a conductive fluid environment. Theoretical expressions for the displacement and torque are developed and compared to experimental results.

Electrophysics of micromechanical comb actuators

Abstract: A simple approximate theory is developed for the electrostatic forces operating in a micromechanical comb actuator. The comb drive is considered both without (for simplicity) and with an underlying ground plane. The forces are partitioned into local forces (electric fields confined to the cross-sections of the individual comb fingers) and global force corrections (electric fields resulting from effective equipotential sheets representing the engaged and unengaged comb finger regions). The local forces are obtained by applying the principle of virtual work (both engaged and unengaged regions are involved when a ground plane is present beneath the comb fingers). The global forces are obtained from the force between magnetic current filaments introduced to model the electric-potential discontinuities in the effective equipotential sheets of the engaged and unengaged finger regions. Conformal mapping, in addition to a static mode decay approximation, is used to obtain simple and accurate formulas for the local charge per unit length (local forces) and, when a ground plane is present, for the effective sheet potentials (magnetic currents and global forces). The forces in the separated case (which are also global in nature) are also obtained by the principle of virtual work. The results of the paper show that the attractive local forces are independent of engagement distance and the smaller repulsive global forces are inversely proportional to engagement distance. The attractive separated forces are inversely proportional to the separation distance without the ground plane and inversely proportional to the square of the separation distance with the ground plane. >

A miniaturized high-voltage solar cell array as an electrostatic MEMS power supply

Abstract: A hydrogenated amorphous silicon (a-Si:H) solar cell array that is designed as an on-board power source for electrostatic microelectromechanical systems (MEMS) is presented. A single cell consists of a triple layer of p-i-n/p-i-n/p-i-n a-Si:H and produces an open circuit voltage (V/sub OC/) of 1.8/spl sim/2.3 V, a short circuit current density (J/sub SC/) of 2.8 mA/cm/sup 2/, and fill factor (FF) of 0.495. A series interconnected array of 100 single solar cells (total array area of 1 cm/sup 2/) is fabricated in an integrated fashion and produces an array V/sub OC/ of 150 V, and array short circuit current (I/sub SC/) of 2.8 /spl mu/A under Air Mass (AM) 1.5 illumination. To demonstrate the usefulness of this solar cell array as an on-board power source for electrostatically driven micromachined devices, it has been packaged with a movable micromachined silicon (Si) mirror in a hybrid manner. The movable Si mirror is directly driven by the cell array electrical output, and the motion of the mirror plate has been observed reproducibly. Variation of light intensity and/or number of illuminated cells produces different values of array V/sub OC/, thus enabling control of the deflection of the Si mirror by variation of incident light intensity. >

Demonstration of three-dimensional microstructure self-assembly

Abstract: Self-assembly of three-dimensional microstructures using the surface tension force of molten solder to produce out-of-plane rotation is demonstrated. The generic nature of the technique is illustrated by reconfiguring structures formed in both Ni metal and single crystal Si. The structures do not have a hinge to constrain the rotation. This considerably simplifies fabrication and eliminates problems associated with the compatibility of a suitable hinge material. Details of the fabrication processes are given and results are presented for rotated structures.

Fabrication of 45/spl deg/ mirrors together with well-defined v-grooves using wet anisotropic etching of silicon

Abstract: The most commonly used microstructure for passive fiber alignment is the ordinary v-groove, defined by {111} planes on a (100) silicon wafer. The plane at the end of the groove, having a 54.7/spl deg/ angle to the surface, can be used as a reflecting mirror. For single-mode fiber applications, a 45/spl deg/ mirror is advantageous together with high accuracy in the position of the fiber, i.e. a smooth mirror and good control of the groove geometry is needed. Two techniques are presented to form 45/spl deg/ mirrors along with well-defined grooves in silicon, using the wet anisotropic etchants EDP and KOH. These techniques are used: (1) to reveal {110} planes on (100) silicon and (2) to make {111} mirrors on wafers that are cut 9.7/spl deg/ off the [100] axis. On (100) silicon, EDP without pyrazine gave the best result. The best mirror and groove reproducibility was found on off-axis cut silicon, using 36 wt.% KOH, with isopropyl alcohol added.

High-aspect-ratio photolithography for MEMS applications

Abstract: High-aspect-ratio photolithography using a commercially available positive photoresist and a conventional contact mask aligner with standard UV light source is described. A multiple coating process is developed to obtain a photoresist thickness up to 23 /spl mu/m while maintaining a smooth photoresist surface. Intimate contact between the mask and wafer is found to be most critical for high-resolution photolithography. Vacuum contact is found to be well-suited for this purpose. Additionally, edge bead removal is found to be of significant importance for intimate contact between the mask and the substrate. Prebake, exposure, and development conditions are optimized for resolution and aspect ratio. Maximum prebake temperature still allowing the photoresist to be developed is found to be the optimal temperature for obtaining high resolution. Prebake time distribution is optimized for avoiding residual stress in the photoresist, as well as maintaining high resolution, when multiple coating is applied. Minimum linewidth and spacing of 3.5 /spl mu/m and 2.5 /spl mu/m, respectively, and a maximum aspect ratio of 7.7 have been achieved in a photoresist thickness of 23 /spl mu/m. Postbake improves the chemical resistance to subsequent processes, for example, electroless nickel plating using the photoresist as a mold. However, postbake also causes pattern distortion, which can be severe at times. Therefore, optimal process and design conditions for minimizing the pattern distortion have been studied.

Object imaging with a piezoelectric robotic tactile sensor

Abstract: A two-dimensional, electrically multiplexed robotic tactile sensor was realized by coupling a piezoelectric polyvinylidene fluoride (PVDF) polymer film to a monolithic silicon integrated circuit (IC). The IC incorporates 64 sensor electrodes arranged in a symmetrical 8/spl times/8 matrix. Each electrode occupies a 400/spl times/400 /spl mu/m square area, and they are separated from each other by 300 /spl mu/m. A 40-/spl mu/m-thick piezoelectric PVDF polymer film was attached to the electrode array with an electrically nonconductive methane adhesive. The response of the tactile sensor is linear for loads spanning 0.8-135 grams-of-force (gmf) (0.00-1.35 Newtons (N)). The response bandwidth is 25 Hz, the hysteresis level is tolerable, and, for operation in the sensor's linear range, taxel crosstalk is negligible. The historically persistent stability and response reproducibility limitation associated with piezoelectric-based tactile sensors has been solved by implementing a novel pre-charge voltage bias technique to initialize the pre- and post-load sensor responses. A rudimentary tactile object image measurement procedure for applied loads has been devised to recognize the silhouette of a sharp edge, square, trapezoid, isosceles triangle, circle, toroid, slotted screw, and cross-slotted screw. >

Effects of elevated temperature treatments in microstructure release procedures

Abstract: Elevated temperature solvent rinses are shown to reduce stiction of micromechanical structures after release etch processing. Rinsing the structures in boiling methanol after rinsing in water significantly improves yields. Photographs taken during the drying process indicate two distinct modes of solvent vaporization, one of which leads to stiction. This process can be modeled and used to predict the critical length of cantilevered beams above which stiction occurs. A key element of this model is that the dynamics of the drying process are strongly influenced by the shape of the microstructure perimeter. The decrease in stiction cannot be explained solely by liquid bridging modeling in case of elevated rinse temperature. Stiction can be further decreased by drying at high temperature in a rapid thermal annealer; this suggests that instability of the trapped liquid under tension at elevated temperatures may be a dominant factor in reducing the stiction. >

Design, fabrication, and testing of silicon microgimbals for super-compact rigid disk drives

Abstract: This paper documents results related to design optimization, fabrication process refinement, and micron-level static/dynamic testing of silicon micromachined microgimbals that have applications in super-compact computer disk drives as well as many other engineering applications of microstructures and microactuators requiring significant out-of-plane motions. The objective of the optimization effort is to increase the in-plane to out-of-plane stiffness ratio in order to maximize compliance and servo bandwidth and to increase the displacement to strain ratio to maximize the shock resistance of the microgimbals, while that of the process modification effort is to simplify in order to reduce manufacturing cost. The testing effort is to characterize both the static and dynamic performance using precision instrumentation in order to compare various prototype designs. >

Thermally driven phase-change microactuation

Abstract: This paper describes a microactuation scheme based on thermally driven liquid-vapor phase-change in a partially filled sealed cavity. A test structure for studying this system has been designed and fabricated. The cavity is 900 /spl mu/m by 900 /spl mu/m by 300 /spl mu/m in size with a thin, 600 /spl mu/m by 800 /spl mu/m grid-shaped heater located on the floor of the cavity and elevated approximately 8 /spl mu/m above it. The heater is composed of open diamond-shaped unit cells defined by 12-/spl mu/m-wide, 3-/spl mu/m-thick bulk-silicon beams, giving an overall electrical heater resistance of 3-10 /spl Omega/. Using methanol as the cavity fluid with partial filling, drive levels of 10 mW sustain a 1.2-Atm pressure rise within the cavity. Real-time measurements demonstrate a pressure response time on the order of 100 ms for an input power of 100 mW. Simulated pressure response calculations indicate the potential for an optimized response time on the order of 40 ms at this power level. >

Equilibrium of hinged and hingeless structures rotated using surface tension forces

Abstract: Two different geometries for the rotation of microstructures by the surface tension force of molten solder are investigated theoretically and experimentally. The geometries are based on structures with and without a hinge acting to constrain the motion of a flap to pure rotation. The equilibrium geometry of a hinged structure is first found analytically by considering the surface energy of the solder. An analysis of hingeless structures is then performed that shows a hinge to be unnecessary under certain conditions. Macroscopic experiments performed using printed circuit board parts are then described; the results show that the behaviors of hinged and hingeless structures are similar.

Fabrication of vapor-deposited micro heat pipe arrays as an integral part of semiconductor devices

Abstract: Vapor-deposited micro heat pipe arrays (VDMHP) were fabricated as an integral part of semiconductor devices to act as efficient heat spreaders by reducing the thermal path between the heat sources and heat sink. Fabrication of the VDMHP was accomplished by first establishing a series of grooves in a silicon wafer. Orientation dependent etching (ODE) using a KOH-1-propanol-H/sub 2/O solution on a (100) wafer with a (111) flat covered with an oxide mask, resulted in grooves 25 /spl mu/m wide and 25 /spl mu/m deep with sharp, perpendicular edges. The wafers were predeposited with a layer of chromium followed by a layer of gold to improve the adhesion characteristics. Dual electron beam vapor deposition, followed by planetary process using molybdenum crucibles, were used to deposit copper 31.5-33.0 /spl mu/m thick, and provide complete closure of the grooves. A glass cover slip was bonded on the top of the deposited layer. The grooves were finally charged and sealed. A computer model Simulation and Modeling of Evaporated Deposition Profiles (SAMPLE) was used to optimize the metal step coverage and successfully predict the cross-sectional profile of the VDMHP. >

Polycrystalline diamond pressure sensor

Abstract: The piezoresistance and other characteristics of boron doped polycrystalline diamond films (PDF's) were determined by analyzing free-standing films that had been formed on silicon. These structures were adhered to a dielectric substrate, and from bending stresses a gauge factor was estimated. Subsequently, a monolithic all-diamond pressure sensor was designed and fabricated, whereby doped diamond resistors reside on a dielectric diamond substrate diaphragm. The process and piezoresistance behavior of their structure is described. >

Adhesion release and yield enhancement of microstructures using pulsed Lorentz forces

Abstract: Adhesion of microstructures is an important failure mechanism in surface-micromachined devices. In this paper, a simple and effective method for releasing pinned microstructures is presented. The method uses the Lorentz force due to the interaction of a current with an external magnetic field to generate an upward force that frees the microstructures. The static and transient behavior of beams under the Lorentz force is examined. Critical values of current and pulse durations needed to release the microstructures are determined and verified with experimental data. Using this technique, previously pinned beams and rectangular plates have been released. The release technique is suitable for mass production environments since it is easily applied during the electrical testing of the device, thereby increasing the manufacturing yield.

A piezoelectrically operated optical chopper by quartz micromachining

Abstract: The fabrication and operation of a piezoelectrically operated monolithic optical chopper (6 mm/spl times/7 mm) are reported. The chopper is made by anisotropic etching of a Z-cut quartz wafer (100 /spl mu/m thick) in an aqueous solution of NH/sub 4/HF/sub 2/. Newly developed piezoelectric operating scheme using meandering electrodes on a flexible quartz suspension (10/spl sim/50 /spl mu/m wide, 2 mm long) was found to produce large displacement (deforming ratio 3% by size) at a resonant frequency. Typical operational parameters are the amplitude of oscillation of 55 /spl mu/m (peak to peak) at a driving voltage of 100 V and the chopping frequency in the 2-5 kHz range depending on the suspension width and length. The resonant frequency is accurately calculated when the cross-sectional profile of the suspension is taken into account. We also successfully demonstrated a chopping operation on a collimated laser beam with the chopper. >

Performance of a 7-mm microfabricated car

Abstract: A 7-mm-scale miniature car was manufactured as a conceptional model of a micromachine. The car consists of a chassis, a shell body, and an electromagnetic motor whose diameter is 1.0 mm. The motor consists of a core shaft, a coil, and a cylindrical permanent magnet magnetized by a special tool. The car runs at the maximum speed of 100 mm/sec by electric power. This is the smallest wheel driven mechanism in the world. The moving characteristics are investigated in detail by a high-speed video camera. Through the experiments, problems of the moving mechanism for micromachines are studied. >

Fabrication of metallic microstructures by electroplating using deep-etched silicon molds

Abstract: A new technology is presented here to fabricate three-dimensional micromachined metal structures. The microstructures are manufactured by electroplating in deep-etched silicon structures followed by a separation from their mold. Up to 140-/spl mu/m-deep silicon structures with vertical sidewalls are realized by an anisotropic plasma etching process producing the mold for electroplating. An etching gas mixture of SF/sub 6s/ and CBrF/sub 3/ is used to achieve both an anisotropic etching behavior by protective film formation of CF/sub 2/-radicals and high etching rates. The anisotropy is due to photoresist masking, which enhances the polymer formation. The vertical trenches are electroplated from the trench base filling the structures uniformly to the substrate surface. By avoiding overplating across the whole substrate the resulting structures are suitable for micromechanical devices. If needed, released microstructures from the silicon mold can be obtained by direct lift-off. >

Integrated force arrays: theory and modeling of static operation

Abstract: Integrated force arrays (IFAs) are a novel means of microelectromechanical actuation. They are membranes that consist of thousands of micron-scale deformable capacitors and are capable of contraction and force exertion in one dimension by application of an applied voltage. The theory and modeling of the static operation of a representative unit device and an array is presented. The electrostatic and elastic forces present in the system are considered and classical beam theory and superposition are applied to calculate the voltage-induced closure and force generation of the networks without the aid of finite element analysis. It is shown that contraction and force generation are expected at reasonable voltages for these modular structures: Ideal system contraction on the order of 40% is possible and minimum external forces of 4500 /spl mu/N/mm/sup 2/ are expected with the application of 50 V. >

Sacrificial wafer bonding for planarization after very deep etching

Abstract: A new technique is presented that provides planarization after a very deep etching step in silicon. This offers the possibility for resist spinning and layer patterning as well as realization of bridges or cantilevers across deep holes or grooves. The sacrificial wafer bonding technique contains a wafer bond step followed by an etch back. Results of polymer bonding followed by dry etching and anodic bonding combined with KOH etching are discussed. The polymer bonding has been applied in a strain based membrane pressure sensor to pattern the strain gauges and to provide electrical connections across a deep corrugation in a thin silicon nitride membrane by metal bridges. >

Plasma planarization for sensor applications

Abstract: Filling trenches in silicon using phosphosilicate glass (PSG) provides many possibilities for novel device structures for sensors and actuators. This paper describes a plasma planarization technique that provides fully planarized PSG filled silicon trenches for sensor applications. The technique consists of planarizing the substrate using two photoresist layers and plasma etching-back. The lower resist layer is the AZ5214 image reversal resist, which is patterned and then thermally cured. The upper resist layer is a global HPR204 coating. The plasma etching-back is carried out using CHF/sub 3//C/sub 2/F/sub 6/ gas mixture with an O/sub 2/ addition. It is shown that by using the image reversal photoresist approach, fully planarized surface coating can be obtained without resorting to an additional mask. By adding 25 sccm (14%) O/sub 2/ into the 137 sccm CHF/sub 3/+18 sccm C/sub 2/F/sub 6/ gas mixture, the etch rates for the photoresist and PSG can be matched. Process optimization for the two layer resist coating and plasma etching is discussed. >

Microphysical investigations on mechanical structures realized in p/sup +/ silicon

Abstract: Secondary Ion Mass Spectroscopy (SIMS) and Microprobe Raman Spectroscopy (MRS) together with optical evaluation of cantilever bending were used for the characterization of micromechanical structures realized in p/sup +/ silicon. Boron profiles in different steps of the fabrication process were accurately traced by SIMS. SIMS is, however, a destructive method. MRS is a noncontact, nondestructive method with high spatial resolution. It was used to evaluate boron concentration in the near surface region, together with lattice disorder and stresses. Mechanical behavior of p/sup +/ microstructures was found to be strongly related to the history of boron concentration profile. Parameters of Raman spectra are in direct relation with the level of boron in the near surface region. Such measurements can be used for p/sup +/ layer monitoring during the fabrication process and for the prediction of mechanical behavior in the final structure. A new explanation is suggested for the observed compressive behavior of oxidized/annealed p/sup +/ microstructures. It is based on the hysteresis introduced by the plastic deformations in the strain-concentration characteristic of the diffused layer. Detailed investigations in better technological environment could bring an accurate quantitative description of the phenomena with useful connections between Raman frequencies and elastic constants of the p/sup +/ layer. >

Strain analysis of silicon-on-insulator films produced by zone melting recrystallization

Abstract: Silicon-on-Insulator (SOI) wafers produced by the Zone-Melting-Recrystallization (ZMR) method were evaluated to determine the level of built-in strain. Micromechanical strain measurement structures were produced by surface micromachining the thin film silicon epitaxial layer. A variety of test structures and a new tensile strain measurement device were used to determine the level of strain in the material. Results indicated that the maximum strain in the ZMR material is less than 2/spl times/10/sup -4/ and that there is a significant orientation dependence. >

Millimeter-scale actuator with fiber-optic roller bearings

Abstract: A technique combining semiconductor processing and fiber-optic technology has been developed to micromachine a 1-cm/sup 2/ silicon die that rolls on two wheels above a flat substrate. Each wheel consists of a glass capillary surrounding a fixed solid glass fiber axle. A 100-silicon die is anisotropically etched to create two variable width v-grooves. Each v-groove has a wide center section and two narrow ends, which is schematically illustrated as -==-. The capillary is free to rotate about the axle in the wide v-groove section while the axle is anodically bonded into the narrow v-groove ends. The gap between the die and the substrate is determined by the narrow v-groove width, fiber diameter, and capillary wall thickness. Several rolling die have been fabricated with 210-120 /spl mu/m gaps. The coefficient of static friction (/spl mu//sub S/) has been investigated on several substrates as a function of the load on the die. Values for /spl mu//sub S/ are compared to an unetched die with a silicon nitride coating. With loads ranging from 0-10 grams, the wheels reduce /spl mu//sub S/ by more than 50% on borosilicate glass. >