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We describe an aluminum nitride (AlN)-based resonant switch for use in a near zero power radio frequency (RF) wake-up receiver. A folded beam structure with a slot compensates for the curvature caused by the stress gradient in the sputtered AlN film and ensures that the released contact gap is approximately equal to the designed contact gap. A 80 kHz resoswitch with a Q of 8600 and an actuation gap of approximately 600 nm turns on when a −4 dBm, 800 MHz signal, square wave modulated at 80 kHz, is applied to the actuator. This AlN electrostatic resonant switch is designed to enable integration with a high gain AlN RF piezoelectric transformer to form a complete ultra-low power RF receiver.

This paper demonstrates, for the first time, a zero-power acoustic wake-up receiver for intrabody links, allowing Implanted Medical Devices (IMDs) to minimize the IDLE power consumption. This architecture is based on Directly Modulated Ultrasonic Transducers (dMUT) made of Lead Zirconate Titanate (PZT) as transmitter (TX), a Piezoelectric Micro Machined Ultrasonic Transducer (pMUT) array as receiver (RX), and a Micro Electro-Mechanical System (MEMS) switch. The role of the dMUTs and the pMUT array is to establish an acoustic link sensitive to signals as low as 50mV pp peak-peak at a distance of 10cm. This low signal triggers the switch which will turn on the IMDs from an IDLE-state to a full power mode.

Residual stress gradients often negatively affect the performance of MEMS devices, causing film curvature and changing the designed gaps of released structures. In this work, we built folded beams designed to compensate for the film curvature and keep the actuator gaps of sensitive resonant switches constant. While the average stress gradient is cancelled by our designs, we find that random variations in the stress gradient (rather than random variations in device dimensions) cause the majority of the observed variation in actuator gap. To our knowledge, this has not previously been reported, and represents an important limitation on MEMS designs using sputtered films. The standard deviation of the 400 nm contact gap for a folded beam of total length 152 µm and width 108 µm was measured to be about 134 nm. Using parameters measured from test cantilevers, our simulations predict that about 98% of the variation in contact gap is due to stress gradient variation, rather than variations in device geometry.

This paper reports on the first demonstration of an infrared (IR) wireless sensor node (WSN) with near-zero standby power consumption. The prototype presented here employs a plasmonically-enhanced micromechanical photoswitch (PMP) that exploits the energy contained in the impinging IR spectral band of interest itself, to perform passive sensing and digitizing functions. The palm-sized IR WSN demonstrated in this work comprises a vacuum-packaged PMP (IR power threshold ∼284 nW, lithographically-defined IR absorptance: ∼89% at 4.6 μm, 0.75 μm bandwidth) connected to a CMOS load-switch which wakes up a coin battery-powered sub 1-GHz wireless microcontroller to transmit data when the IR signal of interest is detected. The standby power consumption for the IR WSN was measured to be just ∼2.6 nW: a >1900X improvement over state-of-the-art pyroelectric IR WSNs, while offering integrated spectral-selectivity. This work represents the first demonstration of an OFF-but-alert WSN that awakens only in the presence of a signal of interest, resulting in near-unlimited battery-life when deployed to detect infrequent but time-critical events.

Due to fundamental scaling laws, the effect of surface forces becomes relatively large for microscale structures, such as MEMS. The operation of an ohmic MEMS microswitch relies on repeated make-and-break contacts, typically between each of two metal tips and a metal drain. Large contact forces are desirable for low contact resistance, but the resulting plastic deformation at this small scale, as well as the breaking through of contaminant films, further increases the adhesion force which can then exceed the restoring force of the actuator. This stuck-closed failure, also known as stiction, is a major hindrance to switch reliability and to the ultimate commercialization of these devices. In this article we review the experimental and modeling efforts which have been directed to achieving an improved understanding of adhesion in an ohmic MEMS microswitch.

We have designed and characterized a MEMS-based Fabry–Perot device (MFPD) to measure vibration at high temperatures. The MFPD consists of a micromachined cavity formed between a substrate and a top thin film structure in the form of a cantilever beam. When affixed to a vibrating surface, the amplitude and frequency of vibration are determined by illuminating the MFPD top mirror with a monochromatic light source and analyzing the back-reflected light to determine the deflection of the beam with respect to the substrate. Given the device geometry, a mechanical transfer function is calculated to permit the substrate motion to be determined from the relative motion of the beam with respect to the substrate. Because the thin film cantilever beam and the substrate are approximately parallel, this two-mirror cavity arrangement does not require alignment or sophisticated stabilization techniques. The uncooled high-temperature operational capability of the MFPD provides a viable low-cost alternative to sensors that require environmentally controlled packages to operate at high temperature. The small size of the MFPD (85–175 µm) and the choice of materials in which it can be manufactured (silicon nitride and silicon carbide) make it ideal for high-temperature applications. Relative displacements in the sub-nanometer range have been measured and close agreement was found between the measured sensor frequency response and the theoretical predictions based on analytical models.

As an essential part of an optical imager project, there was the need for a very high aspect ratio MEMS optical scanning mirror (5 mm ? 150 ?m clear aperture), capable of a ?2? sweep at 1 kHz with an applied voltage less than 200 V. This paper reports on the design, fabrication, modeling and testing of such an electrostatically actuated MEMS mirror. Fabrication involves using a 1?0?0 n-type double side polished silicon wafer, along with surface and bulk micromachining techniques, to produce a mirror with nickel torsion springs and nickel electrostatic actuators. There were 8 masks and 20 processing steps required. The performance of these devices was measured and found to be within the required specifications. Analysis involved developing models to predict the dynamic behavior of these MEMS micromirrors. A basic parallel plate capacitor model was adjusted with finite element analysis to account for fringing fields. Young's modulus of the electroplated nickel was determined to be 110 GPa from a comparison of the model with the results of dynamic testing, and the residual strain test structures led to a value of 0.0029 for the residual strain. Each of these values is within the rather wide range of published values. Once these values were determined, the model agreed very well with the measurements of the dynamic angular response of the mirror.

In this paper, we discuss our work on surface micromachined microswitches and microrelays that are based on electroplated materials. We have successfully fabricated devices with lifetimes exceeding 107 cycles cold switched at current levels exceeding 10 ma. Contact resistance is consistently below one ohm for a single contact. Multiple contact devices have much lower effective contact resistance. ©1999 John Wiley & Sons, Inc. Int J RF and Microwave CAE 9: 338–347, 1999.

Although metal-to-metal direct contact MEMS switches are a promising alternative to solid state switches in RF communication systems, the reliability of their electrical contacts has proven to be a barrier to entry to the market. Hot switching is known to have the most damaging effect on the contacts although an understanding of all the mechanisms leading to this damage is a work-in-progress. In this investigation, a customized Atomic Force Microscope was used to study the electrical contacts. The contacts were cycled at approach rates of 115 μm/s, 440 μm/s and 4400 μm/s and at different polarities. While material transfer is confirmed to be the primary source of contact damage, the results suggest that two different mechanisms could be at play at the leading and trailing edge of a hot switched cycle. A polarity driven mechanism and a contact separation rate dependent mechanism were observed. Low-level transient currents up to a few μA that may or may not be related to the material transfer, were observed at the leading edge of the switching cycle.

This paper describes the hot switching damage in a MEMS switch contact. Under hot switched conditions, material transfer leading to contact erosion causes the performance of RF MEMS switches to degrade over their lifetime. Hot switching performance of RF MEMS switches is one of the major reliability issues and is presently affecting the marketability of these devices despite their proven advantages over solid state switches which are currently used in RF systems. The phenomena leading to such damage both at the leading and trailing edge of a switching cycle are investigated in this paper.

A multifunctional optical MEMS sensor system that can be used for the simultaneous measurement of displacement, temperature, and pressure has been fabricated. The system is based on an array of micromachined Fabry-Perot devices suitable for high temperature and pressure operations. The central concept of the sensor system is that airflow force around the moving structure (air viscous damping) strongly affects the oscillating height of the microcavity and hence, the mechanical performance of the device. The major effect that the viscosity and the density of the air have on the damping provides the mechanism for the simultaneous detection of pressure and temperature in addition to displacement. Since the air is trapped by the viscous damping effects there is no need for a sealed cavity. In addition, the very simple configuration in the optical interferometric system can be considered for the integration in the sensor system package. A model to accurately predict the frequency response of the Fabry-Perot structure subjected to a mechanical excitation on the substrate at different pressures and temperatures is presented. The model shows that shifts of the order of 10.5°C and 7.46psi can be detected for temperatures and pressures around 536°C and 446psi respectively using 90μm long microcantilever beams vibrating at a frequency of 97.4 kHz. To the best of our knowledge, this sensor array is one of the few, if not the only MOEMS device in the literature capable of simultaneous measurement of displacement, temperature, and pressure in high temperature environments.

In this paper, we present some current research issues in contact failures in ohmic RF MEMS microswitches. Such experimental and modeling research encompasses contact mechanics (including elastic, plastic, and adhesive contacts) and the related phenomena of brittle and ductile separation. Furthermore the contact interface is not static, but rather is known to evolve over the lifetime of the switch. Also discussed are contamination failures.Copyright © 2007 by ASME

A dynamic model has been developed to investigate the transient mechanical response of an ohmic contact-type RF MEMS switch. The model combines the built-in characteristics of a commercial finite element package and a finite difference method. The model includes a realistic geometry, the electrostatic actuation, two-dimensional squeeze-film damping, slip-flow terms due to molecular effects, the nonlinear elastic contact, and the JKR adhesive force. The time-dependent characteristics of the microswitch, which include switching speed, electrostatic force, squeeze-film damping force, slip-flow, impact force, and contact tip bounce have been simulated using this model. Contact tip bounce is observed for a single-step actuation voltage whereas a properly tailored dual-actuation pulse can be used to eliminate bounce while maintaining a fast closing time. The slip-flow of the gas film between the two sides of the electrostatic actuator results in a larger correction at low actuation voltages than at high actuation voltages. The simulated results are in good agreement with experimental measurements of the microswitch.

Microswitch performance and reliability are affected by the coupled influences of actuator properties, material and process properties, and device thermal properties. Different contact materials show large differences in immunity to contamination. Contact shape and the adherence force between contact surfaces both evolve as the contact pair is cycled, with the adherence force often reaching a maximum between 105and 107cycles for gold. A typical switch actuation with a step function voltage results in an contact impact force 5 times greater than the static force, increasing the adherence force and the rate of change of the shape of the contact. Heating of the conductors in the switch results in intermodulation products, which are small at a transmitted power of 1 W, but increase with increasing power levels.

This paper presents the design, fabrication and testing results of a Fabry-Perot Micro-Opto-Mechanical Device (FPMOD) used as a vibration sensor. The un-cooled high-temperature operational capability of the FPMOD provides a viable low-cost alternative to sensors that require environmentally controlled packages to operate at high temperature. The FPMOD is a passive MEMS device that consists of a micromachined cavity formed between a substrate and a top thin film structure in the form of a cantilever beam. When affixed to a vibrating surface, the amplitude and frequency of vibration are determined by illuminating the FPMOD with a monochromatic light source and analyzing the back reflected light to determine the deflection of the beam with respect to the substrate. Given the Fabry-Perot geometry, a mechanical transfer function is calculated to permit the substrate motion to be determined from the relative motion of the beam with respect to the substrate. Because the thin film cantilever beam and the substrate are approximately parallel, this convenient two-mirror cavity arrangement needs no alignment, no reference arm, and no sophisticated stabilization techniques. The small size of the FPMOD (85-175μm), the choice of materials in which it can be manufactured (Silicon Nitride and Silicon Carbide), and its simple construction make it ideal for harsh high-temperature applications. Relative displacements in the sub-nanometer range have been measured and close agreement was found between the measured sensor frequency response and the theoretical predictions based on analytical models.

A multi-asperity model of the contact resistance in a MEMS microswitch has been developed which includes the effects of elastic and plastic deformation, adhesion, and constrictive resistance. The number of asperities in contact is small enough that a discrete distribution of asperity heights is used. Adhesion produces significant hysteresis in the contact resistance vs. contact force characteristics. Due to the combined effects of plasticity and adhesion, the surface profile changes during the first load/unload cycle. Measurements of the contact resistance as a function of actuation voltage show good qualitative agreement with the model.

This paper presents a method for predicting the size of intermodulation products due to contact heating in a contact type MEMS microswitch. The primary origin of intermodulation distortion in the microswitch is the variation of the switch resistance at the difference frequency due to the temperature dependence of the resistivity and thermal conductivity of materials in the switch. This varying resistance modulates the high frequency signals, resulting in intermodulation products. The resistance of the switch was modeled using a finite element simulation that predicted resistance variations at a variety of frequencies and power levels. This procedure leads to IIP3 predictions on the order of +64.8 dBm for this switch. This result corresponds to a sideband power of -73.3 dBc for a 1 W input.

Electrostatically actuated microswitches and relays have been developed at Northeastern University and Analog Devices, Inc. Here, we report a steady-state thermal-electrical finite element model of microswitches with gold-gold contacts. The modeling results show that in a microswitch with a typical geometry, the thermal constriction occurs in the thin film trace leading up to the contact, and not at the contact interface. The model correctly predicts the switch voltage at which the drain trace melts, but underestimates the switch resistance, and therefore overestimates the failure current. SEM images indicate that the contact area increases significantly with current.

Using an AFM-based test setup, experiments were performed on Ru microcontacts under a variety of leading and trailing edge hot switching conditions, including different voltages, different currents, different polarities (including bipolar and ac up to 20?MHz), and different approach and separation rates. It was found that hot switching damage is a complex phenomenon for microcontacts. It consists of a number of different mechanisms occurring simultaneously to different degrees depending on the hot switching conditions. It was determined through a combination of experiments and models that the mechanisms leading to contact erosion operate when the electrodes are separated by less than a few ? or are barely touching. For leading edge hot switching, i.e. hot switching when the contacts are closing, the main damage mechanism was found to be associated with currents less than 0.15?mA. Pre-contact currents were observed on uncleaned contacts and were not found to contribute to contact damage. Despite the damage caused by hot switching, it was found that unless the contact material is almost or completely eroded, hot switching does not lead to high contact resistance or high adhesion on Ru contacts. Under bipolar hot switching conditions, microcontacts with a 400??N contact force maintained a contact resistance of less than 1?? and a pull-off force less than 60??N for more than 100 million cycles.

An improved scanning probe microscope based contact tester has been designed, constructed and used for cyclic testing of various metal contacts. The tester is designed for contact material evaluation, especially for metal-contact MEMS switch applications, and is capable of simultaneous measurements of contact force and resistance. The tester uses a specially designed silicon force sensor with an integrated contact bump and a mating silicon pillar in order to simulate switch operation. The sensor and the pillar are coated with contact materials, allowing a wide range of contact metals and metal pairs to be evaluated. The testing takes place within a custom-built test chamber in which both plasma and UV-ozone treatments are available for contact cleaning and surface modification. It was found that a dissimilar contact pair of Au?Ru with O2?plasma cleaning can provide low contact adhesion and low resistance as compared with Au, Ru and Ir contact pairs. Layered structures with a thin layer of Ru on top of Au were also modeled and tested. Ru layers between 50 and 100 nm were effective in reducing adhesion and contact resistance provided the contact force is greater than 300 ?N.

The authors demonstrate precise alignment and controlled assembly of single wall nanotube (SWNT) bundles at a fast rate over large areas by combining electrophoresis and dip coating processes. SWNTs in solution are assembled on prepatterned features that are 80nm wide and separated by 200nm. The results show that the direction of substrate withdrawal significantly affects the orientation and alignment of the assembled SWNT bundles. I-V characterization is carried out to demonstrate electrical continuity of these assembled SWNT bundles.