Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 2010"

What is the Young's Modulus of Silicon?

Abstract: The Young's modulus (E) of a material is a key parameter for mechanical engineering design. Silicon, the most common single material used in microelectromechanical systems (MEMS), is an anisotropic crystalline material whose material properties depend on orientation relative to the crystal lattice. This fact means that the correct value of E for analyzing two different designs in silicon may differ by up to 45%. However, perhaps, because of the perceived complexity of the subject, many researchers oversimplify silicon elastic behavior and use inaccurate values for design and analysis. This paper presents the best known elasticity data for silicon, both in depth and in a summary form, so that it may be readily accessible to MEMS designers.

Wireless Intraocular Pressure Sensing Using Microfabricated Minimally Invasive Flexible-Coiled LC Sensor Implant

Abstract: This paper presents an implant-based wireless pressure sensing paradigm for long-range continuous intraocular pressure (IOP) monitoring of glaucoma patients. An implantable parylene-based pressure sensor has been developed, featuring an electrical LC-tank resonant circuit for passive wireless sensing without power consumption on the implanted site. The sensor is microfabricated with the use of parylene C (poly-chloro-p-xylylene) to create a flexible coil substrate that can be folded for smaller physical form factor so as to achieve minimally invasive implantation, while stretched back without damage for enhanced inductive sensor-reader coil coupling so as to achieve strong sensing signal. A data-processed external readout method has also been developed to support pressure measurements. By incorporating the LC sensor and the readout method, wireless pressure sensing with 1-mmHg resolution in longer than 2-cm distance is successfully demonstrated. Other than extensive on-bench characterization, device testing through six-month chronic in vivo and acute ex vivo animal studies has verified the feasibility and efficacy of the sensor implant in the surgical aspect, including robust fixation and long-term biocompatibility in the intraocular environment. With meeting specifications of practical wireless pressure sensing and further reader development, this sensing methodology is promising for continuous, convenient, direct, and faithful IOP monitoring.

Design, Fabrication, and Characterization of CMOS MEMS-Based Thermoelectric Power Generators

Abstract: This paper presents the design, modeling, fabrication, and characterization of CMOS microelectromechanical-systems-based thermoelectric power generators (TPGs) to convert waste heat into a few microwatts of electrical power. Phosphorus and boron heavily doped polysilicon thin films are patterned and electrically connected to consist thermopiles in the TPGs. To optimize heat flux, the thermal legs are embedded between the top and bottom vacuum cavities, which are sealed on the wafer level at low temperature. A heat-sink layer is coated on the cold side of the device to effectively disperse heat from the cold side of the device to ambient air. The peripheral cavity is designed to isolate heat from the surrounding silicon substrate. Both simulation and experiments are implemented to validate that the energy conversion efficiency is highly improved due to the aforementioned three unique designs. The device has been fabricated by a CMOS-compatible process. Properties of thermoelectric material, such as the Seebeck coefficient, electrical resistivity, and specific contact resistance are measured through test structures. For a device in the size of 1 cm2 and with a 5-K temperature difference across the two sides, the open-circuit voltage is 16.7 V and the output power is 1.3 ?W under matched load resistance. Such energy can be efficiently accumulated as useful electricity over time and can prolong the battery life.

High- $Q$ Tunable Microwave Cavity Resonators and Filters Using SOI-Based RF MEMS Tuners

Abstract: This paper presents the modeling, design, fabrication, and measurement of microelectromechanical systems-enabled continuously tunable evanescent-mode electromagnetic cavity resonators and filters with very high unloaded quality factors (Qu). Integrated electrostatically actuated thin diaphragms are used, for the first time, for tuning the frequency of the resonators/filters. An example tunable resonator with 2.6:1 (5.0-1.9 GHz) tuning ratio and Qu of 300-650 is presented. A continuously tunable two-pole filter from 3.04 to 4.71 GHz with 0.7% bandwidth and insertion loss of 3.55-2.38 dB is also shown as a technology demonstrator. Mechanical stability measurements show that the tunable resonators/filters exhibit very low frequency drift (less than 0.5% for 3 h) under constant bias voltage. This paper significantly expands upon previously reported tunable resonators.

Real-Time Temperature Compensation of MEMS Oscillators Using an Integrated Micro-Oven and a Phase-Locked Loop

Abstract: We present a new temperature compensation system for microresonator-based frequency references. It consists of a phase-locked loop (PLL) whose inputs are derived from two microresonators with different temperature coefficients of frequency. The resonators are suspended within an encapsulated cavity and are heated to a constant temperature by the PLL controller, thereby achieving active temperature compensation. We show repeated real-time measurements of three 1.2-MHz prototypes that achieve a frequency stability of ± 1 ppm from -20°C to +80°C, as well as a technique to reduce steady-state frequency errors to ±0.05 ppm using multipoint calibration.

Nonlinear Behavior of an Electrostatic Energy Harvester Under Wide- and Narrowband Excitation

Abstract: This paper investigates an electrostatic vibration energy harvester that displays rich nonlinear behavior including jumps during frequency sweeps and broadening of the spectrum with increasing levels of broadband vibration. We demonstrate that the measured nonlinear phenomena can be adequately described by a lumped model with a nonlinear beam displaying both spring softening and hardening. Our results show that considerable bandwidth enhancements can be achieved by use of nonlinear springs without relying on mechanical stopper impacts, resonance tuning, or large electromechanical coupling.

Fabrication and Characterization of the Capillary Performance of Superhydrophilic Cu Micropost Arrays

Abstract: We report the fabrication of dense arrays of super-hydrophilic Cu microposts at solid fractions as high as 58% and aspect ratios as high as four using electrochemical deposition and chemical oxidation techniques. Oxygen surface plasma treatments of photoresist molds and a precise control of the initial electrodeposition current are found to be critical in creating arrays of nearly defect-free Cu posts. The capillary performance of the micropost arrays is characterized using capillary rate of rise experiments and numerical simulations that account for the finite curvatures of liquid menisci. For the given wick morphology, the capillary performance generally decreases with increasing solid fraction and is enhanced by almost an order of magnitude when thin nanostructured copper oxide layers are formed on the post surface. The present work provides a useful starting point to achieve optimal balance between the capillary performance and the effective thermal conductivity of advanced wicks for micro heat pipes.

Modeling and Characterization of Piezoelectric $d_{33}$ -Mode MEMS Energy Harvester

Abstract: This paper presents the modeling, fabrication, and characterization of a piezoelectric microelectromechanical systems (MEMS) energy harvester using a d33 piezoelectric mode. A theoretical analysis and an analytical modeling for the d33-mode device were first performed to estimate the output power as a function of the material parameters and device geometry. A PbTiO3 seed layer was newly applied as an interlayer between the ZrO2 and Pb(Zr0.52Ti0.48)O3 (PZT) thin films to improve the piezoelectric property of the sol-gel spin-coated PZT thin film. The fabricated cantilever PZT film with an interdigital shaped electrode exhibited a remnant polarization of 18.5 C/cm2, a coercive field of less than 60 kV/cm, a relative dielectric constant of 1125.1, and a d33 piezoelectric constant of 50 pC/N. The fabricated energy-harvesting device generated an electrical power of 1.1 W for a load of 2.2 M with 4.4 Vpeak-to-peak from a vibration with an acceleration of 0.39 g at its resonant frequency of 528 Hz. The corresponding power density was 7.3 mW cm-3 · g-2. The experimental results were compared with those numerically calculated using the equations derived from the dynamic and analytical modeling. The fabricated device was also compared with other piezoelectric MEMS energy-harvesting devices.

A Resonant Microaccelerometer With High Sensitivity Operating in an Oscillating Circuit

Abstract: A new micromachined uniaxial silicon resonant accelerometer characterized by a high sensitivity and very small dimensions is presented. The device's working principle is based on the frequency variations of two resonating beams coupled to a proof mass. Under an external acceleration, the movement of the proof mass causes an axial load on the beams, generating opposite stiffness variations, which, in turn, result in a differential separation of their resonance frequencies. A high level of sensitivity is obtained, owing to an innovative and optimized geometrical design of the device that guarantees a great amplification of the axial loads. The acceleration measure is obtained, owing to a properly designed oscillating circuit. In agreement with the theoretical prediction, the experimental results show a sensitivity of 455 Hz/ ( g being the gravity acceleration) with a resonant frequency of about 58 kHz and a good linearity in the range of interest.

The Nanoaquarium: A Platform for In Situ Transmission Electron Microscopy in Liquid Media

Abstract: Transmission electron microscopes (TEMs) and scanning transmission electron microscopes (STEMs) are powerful tools for imaging on the nanoscale. These microscopes cannot be typically used to image processes taking place in liquid media because liquid simply evaporates in the high-vacuum environment of the microscope. In order to view a liquid sample, it is thus necessary to confine the liquid in a sealed vessel to prevent evaporation. Additionally, the liquid layer must be very thin to minimize electron scattering by the suspending medium. To address these issues, we have developed a flow cell with a height of tens of nanometers, sandwiched between two thin silicon nitride membranes. The cell is equipped with electrodes for actuation and sensing. The cell is thin enough to allow the transmission of electrons and the real-time imaging of nanoparticles suspended in liquid. This paper details the fabrication process, which relies on plasma-activated wafer bonding. Some of the advantages of our nanoaquarium include the thinnest observation chamber of any reported in situ TEM/STEM device, integrated electrodes for sensing and actuation, and wafer-scale processing that allows bulk device production. Device performance was demonstrated by STEM imaging of gold and polystyrene nanoparticles suspended in water with excellent resolution. Potential applications of the device include imaging of colloidal crystal formation, aggregation, nanowire growth, electrochemical deposition, and biological interactions.

Nonlinear Dynamics of MEMS Arches Under Harmonic Electrostatic Actuation

Abstract: We present an investigation of the nonlinear dynamics of clamped-clamped micromachined arches when actuated by a dc electrostatic load superimposed on an ac harmonic load. The Galerkin method is used to discretize the distributed-parameter model of a shallow arch to obtain a reduced-order model. The static response of the arch due to a dc load actuation is simulated, and the results are validated by comparing them to experimental data. The dynamic response of the arch to a combined dc load and ac harmonic load is studied when excited near its fundamental natural frequency, twice its fundamental natural frequency, and near other higher harmonic modes. The results show a variety of interesting nonlinear phenomena, such as hysteresis, softening behavior, dynamic snap-through, and dynamic pull-in. The results are also shown demonstrating the potential to use microelectromechanical systems (MEMS) arches as bandpass filters and low-powered switches. An experimental work is conducted to test arches realized of curved polysilicon microbeams when excited by dc and ac loads. Experimental data are shown for the softening behavior and the dynamic pull-in of the curved microbeams.

An Experimental and Theoretical Investigation of Dynamic Pull-In in MEMS Resonators Actuated Electrostatically

Abstract: We present experimental and theoretical investigations of dynamic pull-in of electrostatically actuated resonators. Several experimental data are presented, showing regimes of ac forcing amplitude versus ac frequency, where a resonator is forced to pull in if operated within these regimes. Results are shown for primary and secondary resonance excitations. The influences of the initial conditions of the system, the ac excitation amplitude, the ac frequency, the excitation type, and the sweeping type are investigated. A shooting technique to find periodic motions and a basin-of-attraction analysis are used to predict the limits of the pull-in bands. When compared with the experimental data, the results have shown that the pull-in limits coincide with 30%-40% erosion lines of the safe basin in the case of primary resonance and 5%-15% erosion lines of the safe basin in the case of subharmonic resonance. Bifurcation diagrams have been constructed, which designers can use to establish factors of safety to reliably operate microelectromechanical-systems resonators away from pull-in bands and the danger of pull-in, depending on the expected disturbances and noise in the systems.

Carbon Nanotubes as a Framework for High-Aspect-Ratio MEMS Fabrication

Abstract: A class of carbon-nanotube (CNT) composite materials was developed to take advantage of the precise high-aspect-ratio shape of patterned vertically grown nanotube forests. These patterned forests were rendered mechanically robust by chemical vapor infiltration and released by etching an underlying sacrificial layer. We fabricated a diverse variety of functional MEMS devices, including cantilevers, bistable mechanisms, and thermomechanical actuators, using this technique. A wide range of chemical-vapor-depositable materials could be used as fillers; here, we specifically explored infiltration by silicon and silicon nitride. The CNT framework technique may enable high-aspect-ratio MEMS fabrication from a variety of materials with desired properties such as high-temperature stability or robustness. The elastic modulus of the silicon-nanotube and silicon nitride-nanotube composites is dominated by the filler material, but they remain electrically conductive, even when the filler (over 99% of the composite's mass) is insulating.

A MEMS Reactor for Atomic-Scale Microscopy of Nanomaterials Under Industrially Relevant Conditions

Abstract: We present a microelectromechanical systems (MEMS) nanoreactor that enables high-resolution transmission electron microscopy (TEM) (HRTEM) of nanostructured materials with atomic-scale resolution during exposure to reactive gases at 1 atm of pressure. This pressure exceeds that of existing HRTEM systems by a factor of 100, thereby entering a pressure range that is relevant to industrial purposes. The nanoreactor integrates a shallow flow channel (35 ?m high) with a microheater and with an array of electron transparent windows of silicon nitride. The windows are only 10 nm thick but are mechanically robust. The heater has the geometry of a microhotplate and is made of Pt embedded in a silicon nitride membrane. To interface the nanoreactor, a dedicated TEM specimen holder has been developed. The performance is demonstrated by the live formation of Cu nanoparticles in a catalyst for the production of methanol. At 120 kPa and for temperatures of up to 500°C , the formation of these nanoparticles can be observed clearly and with an exceptionally low thermal drift. HRTEM images of the nanoparticles show atomic lattice fringes with spacings down to 0.18 nm.

Thermal Excitation and Piezoresistive Detection of Cantilever In-Plane Resonance Modes for Sensing Applications

Abstract: Thermally excited and piezoresistively detected bulk-micromachined cantilevers vibrating in their in-plane flexural resonance mode are presented. By shearing the surrounding fluid rather than exerting normal stress on it, the in-plane mode cantilevers exhibit reduced added fluid mass effects and improved quality factors in a fluid environment. In this letter, different cantilever geometries with in-plane resonance frequencies from 50 kHz to 2.2 MHz have been tested, with quality factors as high as 4200 in air and 67 in water.

A Parylene Bellows Electrochemical Actuator

Abstract: We present the first electrochemical actuator with Parylene bellows for large-deflection operation. The bellows diaphragm was fabricated using a polyethylene-glycol-based sacrificial molding technique followed by coating in Parylene C. Bellows were mechanically characterized and integrated with a pair of interdigitated electrodes to form an electrochemical actuator that is suitable for low-power pumping of fluids. Pump performance (gas generation rate and pump efficiency) was optimized through a careful examination of geometrical factors. Overall, a maximum pump efficiency of 90% was achieved in the case of electroplated electrodes, and a deflection of over 1.5 mm was demonstrated. Real-time wireless operation was achieved. The complete fabrication process and the materials used in this actuator are biocompatible, which makes it suitable for biological and medical applications.

A Flat Heat Pipe Architecture Based on Nanostructured Titania

Abstract: A novel 3 cm × 3 cm × 600 μm-thick Ti-based flat heat pipe is developed for Thermal Ground Plane (TGP) applications. The Ti-based heat pipe architecture is constructed by laser welding two microfabricated titanium substrates to form a hermetically sealed vapor chamber. The scalable heat pipes' flat geometry facilitates contact with planar heat sources, such as microprocessor chip surfaces, thereby reducing thermal contact resistance and improving system packaging. Fluid transport is driven by the wicking structure in the TGP, which consists of an array of Ti pillars that are microfabricated from a titanium substrate using recently developed high-aspect-ratio Ti processing techniques. The hydrophilic nature of the Ti pillars is increased further by growing ~200-nm hairlike nanostructured titania of the pillar surfaces. The resulting super hydrophilic wick offers the potential to generate high wicking velocities of ~27.5 mm/s over distances of 2 mm. The experimental wetting results show a diffusive spreading behavior that is predicted by Washburn dynamics. The maximum effective thermal conductivity of a heat pipe is directly related to the speed of capillary flow of the working fluid through the wick and is measured experimentally in the first-generation device to be k = 350 W/m · K. A dummy TGP with a cavity volume of ~170 μL was used to test the hermiticity level of the laser packaging technique. The device gave a 0.067% of water loss based on ~60 μL of charged water at 100°C in air for over a year.

Piezoresistive Cantilever Performance—Part I: Analytical Model for Sensitivity

Abstract: An accurate analytical model for the change in resistance of a piezoresistor is necessary for the design of silicon piezoresistive transducers. Ion implantation requires a high-temperature oxidation or annealing process to activate the dopant atoms, and this treatment results in a distorted dopant profile due to diffusion. Existing analytical models do not account for the concentration dependence of piezoresistance and are not accurate for nonuniform dopant profiles. We extend previous analytical work by introducing two nondimensional factors, namely, the efficiency and geometry factors. A practical benefit of this efficiency factor is that it separates the process parameters from the design parameters; thus, designers may address requirements for cantilever geometry and fabrication process independently. To facilitate the design process, we provide a lookup table for the efficiency factor over an extensive range of process conditions. The model was validated by comparing simulation results with the experimentally determined sensitivities of piezoresistive cantilevers. We performed 9200 TSUPREM4 simulations and fabricated 50 devices from six unique process flows; we systematically explored the design space relating process parameters and cantilever sensitivity. Our treatment focuses on piezoresistive cantilevers, but the analytical sensitivity model is extensible to other piezoresistive transducers such as membrane pressure sensors.

Multifrequency Pierce Oscillators Based on Piezoelectric AlN Contour-Mode MEMS Technology

Abstract: This paper reports on the first demonstration of multifrequency (176-, 222-, 307-, and 482-MHz) oscillators based on the piezoelectric AlN contour-mode microelectromechanical systems technology. All the oscillators show phase noise values between -88 and -68 dBc/Hz at 1-kHz offset frequency from the carriers and phase noise floor values as low as -160 dBc/Hz at 1 MHz offset. The same Pierce circuit design is employed to sustain oscillations at the four different frequencies; on the other hand, the oscillator core consumes 10 mW. The AlN resonators are currently wire bonded to the integrated circuit realized in the AMIS 0.5-?m 5-V complimentary metal-oxide-semiconductor process. Limits on phase noise and power consumption are discussed and compared with other competing technologies. This paper constitutes a substantial step forward toward the demonstration of a single-chip multifrequency reconfigurable timing solution that can be used in wireless communications and sensing applications.

Wafer-Level Parylene Packaging With Integrated RF Electronics for Wireless Retinal Prostheses

Abstract: This paper presents an embedded chip integration technology that incorporates silicon housings and flexible Parylene-based microelectromechanical systems (MEMS) devices. Accelerated-lifetime soak testing is performed in saline at elevated temperatures to study the packaging performance of Parylene C thin films. Experimental results show that the silicon chip under test is well protected by Parylene, and the lifetime of Parylene-coated metal at body temperature (37°C) is more than 60 years, indicating that Parylene C is an excellent structural and packaging material for biomedical applications. To demonstrate the proposed packaging technology, a flexible MEMS radio-frequency (RF) coil has been integrated with an RF identification (RFID) circuit die. The coil has an inductance of 16 μH with two layers of metal completely encapsulated in Parylene C, which is microfabricated using a Parylene-metal-Parylene thin-film technology. The chip is a commercially available read-only RFID chip with a typical operating frequency of 125 kHz. The functionality of the embedded chip has been tested using an RFID reader module in both air and saline, demonstrating successful power and data transmission through the MEMS coil.

In Situ Electron Microscopy Mechanical Testing of Silicon Nanowires Using Electrostatically Actuated Tensile Stages

Abstract: Two types of electrostatically actuated tensile stages for in situ electron microscopy mechanical testing of 1-D nanostructures were designed, microfabricated, and tested. Testing was carried out for mechanical characterization of silicon nanowires (SiNWs). The bulk micromachined stages consist of a comb-drive actuator and either a differential capacitive sensor or a clamped-clamped beam force sensor. High-aspect-ratio structures (height/gap = 20) were designed to increase the driving force of the geometrically optimized actuator and the sensitivity of the capacitive sensor. The actuator stiffness is kept low to enable high tensile force to be exerted in the specimen rather than in the suspensions of the comb drive. Individual SiNWs were mounted on the devices by in situ scanning electron microscopy nanomanipulation, and their tensile properties were determined to demonstrate the device capability. The phosphorus-doped SiNWs, which were grown in a bottom-up manner by the vapor-liquid-solid process, show an average Young's modulus of (170.0 ± 2.4) GPa and a tensile strength of at least 4.2 GPa. Top-down electroless chemically etched SiNWs, with their long axis along the [100] direction, show a fracture strength of 5.4 GPa.

Comb-Actuated Resonant Torsional Microscanner With Mechanical Amplification

Abstract: A comb-actuated torsional microscanner is developed for high-resolution laser-scanning display systems. Typical torsional comb-drive scanners have fingers placed around the perimeter of the scanning mirror. In contrast, the structure in this paper uses cascaded frames, where the comb fingers are placed on an outer drive frame, and the motion is transferred to the inner mirror frame with a mechanical gain. The structure works only in resonant mode without requiring any offset in the comb fingers, keeping the silicon-on-insulator-based process quite simple. The design intent is to improve actuator efficiency by removing the high-drag fingers from the high-velocity scanning mirror. Placing them on the lower velocity drive frame reduces their contribution to the damping torque. Furthermore, placement on the drive frame allows an increase of the number of fingers and their capacity to impart torque. The microscanner exhibits a parametric response, and as such, the maximum deflection is found when actuated at twice its natural frequency. Analytical formulas are given for the coupled-mode equations and frame deflections. A simple formula is derived for the mechanical-gain factor. For a 1-mm × 1.5-mm oblong scanning mirror, a 76° total optical scan angle is achieved at 21.8 kHz with 196-V peak-to-peak excitation voltages.

Thermally Actuated Omnidirectional Walking Microrobot

Abstract: We describe a walking microrobot that is propelled by cilialike thermal bimorph actuator arrays. The robot consists of two actuator array chips, each having an 8 × 8 array of “motion pixels,” which are composed of four orthogonally oriented cilia. Each group of unidirectional cilia is controlled independently for each chip, which provides planar motion with three degrees of freedom (x, y, θ). The robot is approximately 3 cm in length, 1 cm in width, and 0.9 mm in height and has a mass of 0.5 g. By varying the actuation frequency and motion gait strategy, the direction and velocity of the motion can be controlled. In this paper, we present the system architecture, control mechanism, and modeling of the robot, as well as experimental results, during linear and rotary motion. The robot can carry loads up to seven times its own mass, and it can operate at speeds up to 250 μm/s with step sizes from 1 to 4 μm.

Development and Application of a Novel Microfabricated Device for the In Situ Tensile Testing of 1-D Nanomaterials

Abstract: We report on the development and application of a silicon microdevice for the in situ quantitative mechanical characterization of single 1-D nanomaterials within a scanning electron microscope equipped with a quantitative nanoindenter. The design makes it possible to convert a compressive nanoindentation force applied to a shuttle to uniaxial tension on a specimen attached to a sample stage. Finite-element analysis and experimental calibration have been employed to extract the specimen stress versus strain curve from the indentation load versus displacement curve. The stress versus strain curves for three 200-300-nm-diameter Ni nanowire specimens are presented and analyzed.

CMOS-MEMS Capacitive Humidity Sensor

Abstract: A high-sensitivity capacitive humidity sensor intended for use as part of a respirator end-of-service-life indicator system is presented. This paper reports a method for improving the sensitivity of integrated capacitive chemical sensors by removing the underlying substrate. The sensor is integrated with CMOS testing electronics using maskless postprocessing followed by ink-jet deposition of a sensitive polymer. Two different methods of depositing polymer, namely, capillary wicking and coating the top surface directly, were investigated. The sensors had measured sensitivities of 0.16% to 0.18% change in capacitance per percent relative humidity, which is the highest demonstrated for an integrated capacitive humidity sensor. Temperature sensitivity of the sensor, which is an important criterion for a sensor intended for a variety of different ambient conditions, was measured to be 0.07%/°C. The cross sensitivities to toluene and acetone, which are two common industrial solvents that are filtered by respirator cartridges, were measured to be 2.4 × 10-4 and 9.0 × 10-5%/ppm, respectively.

Fabrication and Characterization of a Parylene-Based Three-Dimensional Microelectrode Array for Use in Retinal Prosthesis

Abstract: A novel Parylene-based shape-transferring technique was developed to fabricate 3-D microelectrodes, which fulfilled the requirements of the high-density microelectrode array (MEA) used in retinal prosthesis. A process combining anisotropic and isotropic etching was utilized to form the 3-D silicon-tip array whose shape was transferred to the Parylene C film as a flexible and biocompatible substrate. Platinum was chosen as the electrode material owing to its high charge delivery capacity and chemical inertness in the physiological environment. An aluminum-photoresist dual-layer lift-off process was employed for platinum patterning on the substrate with high tips. To avoid the cracking problem existing in the platinum-Parylene C structure, a gold intermediate layer was introduced to retard the tendency. To test the superiority of the present 3-D flexible MEA compared with the planar one, electrical properties of MEAs were characterized both in vitro and in vivo. The experimental results showed that the impedance of the 3-D electrode was about 72.7% of the planar one at 1 kHz with the same footprint in vitro, and 71.5% in vivo. The preliminary surgical experiment validated the feasibility of the 3-D MEA in retinal prosthesis.

Micromechanical IBARs: Tunable High- $Q$ Resonators for Temperature-Compensated Reference Oscillators

Abstract: This paper presents a unique capacitive micromechanical resonator and oscillator architecture for temperature-compensated frequency references. The I-shaped bulk acoustic resonator (IBAR) is designed to have excellent electrical tunability for temperature compensation (TC) and dynamic frequency control. High quality factor and low motional resistance are also achieved. The applicable range of frequencies is 1-30 MHz, in which quality factors exceeding 100 000 have been measured. Resonator metrics, including the electrostatic tuning coefficient, normalized dynamic stiffness, and relative dynamic compliance, are introduced. A small-signal resistance in the resonator is reported and explained. This unexpected resistance is beneficial for oscillator functionality over a large temperature range. The interface IC, inclusive of all blocks for sustaining oscillations and TC, is also presented. A two-chip 6-MHz oscillator with a temperature stability of 39 ppm over 100°C is demonstrated. The interface IC consumes 1.9 mW.

Subnanometer Translation of Microelectromechanical Systems Measured by Discrete Fourier Analysis of CCD Images

Abstract: In-plane linear displacements of microelectromechanical systems are measured with subnanometer accuracy by observing the periodic micropatterns with a charge-coupled device camera attached to an optical microscope. The translation of the microstructure is retrieved from the video by phase-shift computation using discrete Fourier transform analysis. This approach is validated through measurements on silicon devices featuring steep-sided periodic microstructures. The results are consistent with the electrical readout of a bulk micromachined capacitive sensor, demonstrating the suitability of this technique for both calibration and sensing. Using a vibration isolation table, a standard deviation of σ = 0.13 nm could be achieved, enabling a measurement resolution of 0.5 nm (4σ) and a subpixel resolution better than 1/100 pixel.

A Novel Method to Guarantee the Specified Thickness and Surface Roughness of the Roll-to-Roll Printed Patterns Using the Tension of a Moving Substrate

Abstract: Roll-to-roll (R2R) continuous printing is an attractive technology for mass-producing flexible printed electronics. Many studies have been conducted in this field. The application of the R2R printing process, however, requires information pertaining to system parameters such as substrate flexibility, ink formulation, and printing method as well as the curing method for conductive ink. We show that the quality of a printed pattern (thickness and surface roughness) could be affected by tension variation of the flexible bare substrate in spite of the optimal settings of the ink, substrate, and printing method. In addition, an ink-transfer mechanism for the R2R printed electronics is analyzed to reveal the relationships between the dynamic surface roughness and tension of a moving web. Since the dynamics of the physical problem are complex, simple meta models using a design of experiment are developed. The experimental results are found to be in agreement with the meta models. It is found that the two important factors for achieving the desired thickness and surface roughness of the R2R printed patterns are optimal tension and control accuracy of the operating tension.

Bioinspired 3-D Tactile Sensor for Minimally Invasive Surgery

Abstract: This paper reports the development of a bioinspired 3-D tactile sensor for minimally invasive surgery. Inspired by the principle of hair cells, the sensor is composed of a central silicon post and a piezoresistor-embedded polyimide diaphragm to which the silicon post is attached. The high-aspect-ratio silicon post helps to increase the sensitivity to shear force significantly. Another unique advantage is that the silicon post is surrounded by a cylindrical cavity that provides a safety stop for the excessively large force/displacement. A fabrication process using deep reactive-ion etching, as well as a simple but effective packaging scheme, was demonstrated. The packaged tactile sensor was characterized by monitoring the resistance change of the piezoresistors when the central post was pushed in different directions. The sensor exhibited a shear force sensitivity of 10.8 N-1 and a normal force sensitivity of 3.5 N-1. The measured displacement sensitivities in shear and normal directions are 1.2 × 10-3 and 6.0 × 10-3, respectively. The responses of single and multiple sensors to the rubber scratching test were studied to demonstrate the capability of the sensor to detect the direction and other information of the scratching.