Microelectromechanical Isolation of Acoustic Wave Resonators

Microelectromechainical systems (MEMS) is a rapidly expanding field of research into the design and fabrication of actuated mechanical systems on the order of a few micrometers to a few millimeters. MEMS potentially offers new methods to solve a variety of engineering problems. A large variety of MEMS systems including flip-up platforms, scanning micromirrors, and rotating micromirrors are developed to demonstrate the types of MEMS that can be fabricated. The potential of MEMS for reducing the vibration sensitivity of surface acoustic wave and surface transverse wave resonators is then evaluated. A micromachined vibration isolation system is designed and modeled. A fabrication process utilizing two sided anisotropic etching of {110} silicon wafers is developed. The process utilizes standard microelectronic fabrication equipment to batch fabricate the isolation systems. The fabricated systems are only 1 cm by 1 cm by 1 mm. Several oscillators are fabricated using commercially fabricated STW resonators mounted on the isolation systems. The resonators are driven by their standard oscillator circuit. Incorporating the isolation system into the oscillator does not result in an appreciable increase the size or the weight of the oscillator. Testing of the oscillators shows that the isolators successfully function as passive vibration isolation systems

Advanced single-chip temperature stabilization system for silicon MEMS resonators and gyroscopes

The main objective of this research is to develop temperature and frequency stabilization techniques for silicon MEMS oven-controlled crystal oscillators (MEMS OCXO) with high-frequency stability. The device was built upon an ovenized platform that used a micro-heater to adjust the temperature of the resonator. Structural resistance-based (Rstruc) temperature sensing was used to improve the self-temperature monitoring accuracy of the silicon MEMS resonator. An analog feedback micro-oven control loop and a feedforward digital calibration scheme were developed for a 77MHz MEMS oscillator, which achieved a ±0.3ppm frequency stability from -25°C to 85°C. An AC heating scheme was also developed to enable tighter integration of the resonator, temperature sensor (Rstruc) and heaters. This temperature stabilization technique was also applied to silicon MEMS mode-matched vibratory x/y-axis and z-axis gyroscopes on a single chip. The temperature-induced frequency change, scale factor and output bias variations were all reduced significantly. The complete interface circuit for the single-chip three axes gyroscopes were also developed with an innovative trans-impedance amplifier to reduce the input-referred noise. For the first time, the simultaneous operation of mode-matched vibratory 3-axis MEMS gyroscopes on a single chip was demonstrated.Ph.D

BULK-PIEZOELECTRIC TRANSDUCTION OF MICROSYSTEMS WITH APPLICATIONS TO BATCH-ASSEMBLY OF MICROMIRRORS, CAPACITIVE SENSING, AND SOLAR ENERGY CONCENTRATION

Electromechanical modeling, actuation, sensing and fabrication aspects of bulkpiezoelectric ceramic integration for microsystems are investigated in this thesis. A small-signal model that describes the energy exchange between surface micromachined beams and bulk-lead zirconium titanate (PZT) actuators attached to the silicon substrate is presented. The model includes detection of acoustic waves launched from electrostatically actuated structures on the surface of the die, as well as their actuation by bulk waves generated by piezoelectric ceramics. The interaction is modeled via an empirical equivalent circuit, which is substantiated by experiments designed to extract the model parameters. As a die level application of bulk-PZT, an Ultrasound Enhanced Electrostatic Batch Assembly (U2EBA) method for realization of 3-D microsystems is demonstrated. U2EBA involves placing the die in an external DC electric field perpendicular to the substrate and actuating the die with an off-chip, bulk-piezoelectric ceramic. Yield rates reaching up to 100% are reported from 8×8 arrays of hinged mirrors with dimensions of 180 × 100 micrometre-squared. U2EBA is later improved to provide temporary latching at intermediate angles between fully horizontal and vertical states, by using novel latching structures. It is shown that the micromirrors can be trapped and freed from different rotation angles such that zero static power is needed to maintain an angular position. The zero-idle-power positioning of large arrays of small mirrors is later investigated for energy redirection and focusing. All-angle LAtchable Reflector (ALAR) concept is introduced, and its application to Concentrated Solar Power (CSP) systems is discussed. The main premise of ALAR technology is to replace bulky and large arrays of mirrors conventionally used in CSP technologies with zeroidle- power, semi-permanently latched, low-profile, high-fill factor, micrometer to centimeter scale mirror arrays. A wirelessly controlled prototype that can move a 2-D array of mirrors, each having a side length of less than 5 cm, in two degrees of freedom to track the brightest spot in the ambient is demonstrated. Capacitive sensing using bulk-piezoelectric crystals is investigated, and a Time- Multiplexed Crystal based Capacitive Sensing (TM-XCS) method is proposed to provide nonlinearity compensation and self-temperature sensing for oscillator based capacitive sensors. The analytical derivation of the algorithm and experimental evidence regarding the validity of some of the relations used in the derivation are presented. This thesis also presents results on microfluidic particle transport as another application of bulk-PZT in microsystems. Experiments and work regarding actuation of micro-scale, fluorescent beads on silicon nitride membranes are described

Study and characetrization of plastic encapsulated packages for MEMS

Technological advancement has thrust MEMS design and fabrication into the forefront of modern technologies. It has become sufficiently self-sustained to allow mass production. The limiting factor which is stalling commercialization of MEMS is the packaging and device reliability. The challenging issues with MEMS packaging are application specific. The function of the package is to give the MEMS device mechanical support, protection from the environment, and electrical connection to other devices in the system. The current state of the art in MEMS packaging transcends the various packaging techniques available in the integrated circuit (IC) industry. At present the packaging of MEMS includes hermetic ceramic packaging and metal packaging with hermetic seals. For example the ADXL202 accelerometer from the Analog Devices. Study of the packaging methods and costs show that both of these methods of packaging are expensive and not needed for majority of MEMS applications. Due to this the cost of current MEMS packaging is relatively high, as much as 90% of the finished product. Reducing the cost is therefore of the prime concern. This Thesis explores the possibility of an inexpensive plastic package for MEMS sensors like accelerometers, optical MEMS, blood pressure sensors etc. Due to their cost effective techniques, plastic packaging already dominates the IC industry. They cost less, weigh less, and their size is small. However, porous nature of molding materials allows penetration of moisture into the package. The Thesis includes an extensive study of the plastic packaging and characterization of three different plastic package samples. Polymeric materials warp upon absorbing moisture, generating hygroscopic stresses. Hygroscopic stresses in the package add to the thermal stress due to high reflow temperature. Despite this, hygroscopic characteristics of the plastic package have been largely ignored. To facilitate understanding of the moisture absorption, an analytical model is presented in this Thesis. Also, an empirical model presents, in this Thesis, the parameters affecting moisture ingress. This information is important to determine the moisture content at a specific time, which would help in assessing reliability of the package. Moisture absorption is modeled using the single phase absorption theory, which assumes that moisture diffusion occurs freely without any bonding with the resin. This theory is based on the Fick\u27s Law of diffusion, which considers that the driving force of diffusion is the water concentration gradient. A finite difference simulation of one-dimensional moisture diffusion using the Crank-Nicolson implicit formula is presented. Moisture retention causes swelling of compounds which, in turn, leads to warpage. The warpage induces hygroscopic stresses. These stresses can further limit the performance of the MEMS sensors. This Thesis also presents a non invasive methodology to characterize a plastic package. The warpage deformations of the package are measured using Optoelectronic holography (OEH) methodology. The OEH methodology is noninvasive, remote, and provides results in full-field-of-view. Using the quantitative results of OEH measurements of deformations of a plastic package, pressure build up can be calculated and employed to assess the reliability of the package

Degree-per-hour mode-matched micromachined silicon vibratory gyroscopes

The objective of this research dissertation is to design and implement two novel micromachined silicon vibratory gyroscopes, which attempt to incorporate all the necessary attributes of sub-deg/hr noise performance requirements in a single framework: large resonant mass, high drive-mode oscillation amplitudes, large device capacitance (coupled with optimized electronics), and high-Q resonant mode-matched operation. Mode-matching leverages the high-Q (mechanical gain) of the operating modes of the gyroscope and offers significant improvements in mechanical and electronic noise floor, sensitivity, and bias stability. The first micromachined silicon vibratory gyroscope presented in this work is the resonating star gyroscope (RSG): a novel Class-II shell-type structure which utilizes degenerate flexural modes. After an iterative cycle of design optimization, an RSG prototype was implemented using a multiple-shell approach on (111) SOI substrate. Experimental data indicates sub-5 deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 30,000 at 23ºC (in vacuum). The second micromachined silicon vibratory gyroscope presented in this work is the mode-matched tuning fork gyroscope (M2-TFG): a novel Class-I tuning fork structure which utilizes in-plane non-degenerate resonant flexural modes. Operated under vacuum, the M2-TFG represents the first reported high-Q perfectly mode-matched operation in Class-I vibratory microgyroscope. Experimental results of device implemented on (100) SOI substrate demonstrates sub-deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 50,000 at 23ºC. In an effort to increase capacitive aspect ratio, a new fabrication technology was developed that involved the selective deposition of doped-polysilicon inside the capacitive sensing gaps (SPD Process). By preserving the structural composition integrity of the flexural springs, it is possible to accurately predict the operating-mode frequencies while maintaining high-Q operation. Preliminary characterization of vacuum-packaged prototypes was performed. Initial results demonstrated high-Q mode-matched operation, excellent thermal stability, and sub-deg/hr Allan variance bias instability.Ph.D.Committee Chair: Dr. Farrokh Ayazi; Committee Member: Dr. Mark G. Allen; Committee Member: Dr. Oliver Brand; Committee Member: Dr. Paul A. Kohl; Committee Member: Dr. Thomas E. Michael

Vacuum Packaged Low-Power Resonant MEMS Strain Sensor

This paper describes a technical approach toward the realization of a low-power temperature-compensated micromachined resonant strain sensor. The sensor design is based on two identical and orthogonally-oriented resonators where the differential frequency is utilized to provide an output proportional to the applied strain with temperature compensation achieved to first order. Interface circuits comprising of two front-end oscillators, a mixer, and low-pass filter are designed and fabricated in a standard 0.35-μm CMOS process. The characterized devices demonstrate a scale factor of 2.8 Hz/με over a strain range of 1000 με with excellent linearity over the measurement range. The compensated frequency drift due to temperature is reduced to 4% of the uncompensated value through this scheme. The total continuous power consumption of the strain sensor is 3 μW from a 1.2 V supply. This low power implementation is essential to enable battery-powered or energy harvesting enabled monitoring applications.This work was supported in part by the U.K. Engineering and Physical Sciences Research Council under Grant EP/K000314/1 and in part by the Cambridge Centre for Smart Infrastructure and Construction.This is the author accepted manuscript. The final version is available from IEEE via https://doi.org/10.1109/JMEMS.2016.258786

Classical and fluctuation-induced electromagnetic interactions in micronscale systems: designer bonding, antibonding, and Casimir forces

Whether intentionally introduced to exert control over particles and macroscopic objects, such as for trapping or cooling, or whether arising from the quantum and thermal fluctuations of charges in otherwise neutral bodies, leading to unwanted stiction between nearby mechanical parts, electromagnetic interactions play a fundamental role in many naturally occurring processes and technologies. In this review, we survey recent progress in the understanding and experimental observation of optomechanical and quantum-fluctuation forces. Although both of these effects arise from exchange of electromagnetic momentum, their dramatically different origins, involving either real or virtual photons, lead to different physical manifestations and design principles. Specifically, we describe recent predictions and measurements of attractive and repulsive optomechanical forces, based on the bonding and antibonding interactions of evanescent waves, as well as predictions of modified and even repulsive Casimir forces between nanostructured bodies. Finally, we discuss the potential impact and interplay of these forces in emerging experimental regimes of micromechanical devices.Comment: Review to appear on the topical issue "Quantum and Hybrid Mechanical Systems" in Annalen der Physi

Design of a MEMS-based 52 MHz oscillator

Mechanical resonators are widely applied in time-keeping and frequency reference applications. Mechanical resonators are preferred over electrical resonators because of their high Q. In the $4.1 billion (2008) timing market, quartz crystals are still ubiquitous in electronic equipment. Quartz crystals show excellent performance in terms of stability (shortterm and long-term), power handling, and temperature drift. MEMS resonators are investigated as a potential alternative to the bulky quartz crystals, which cannot be integrated with IC technology. MEMS offer advantages in terms of size, cost price, and system integration. Efforts over recent years have shown that MEMS resonators are able to meet the high performance standards set by quartz. Critical success factors are high Q-factor, low temperature drift, low phase noise, and low power. This PhD thesis addresses the feasibility of scaling MEMS resonators/oscillators to frequencies above 10 MHz. The main deliverable is a 52 MHz MEMS-based oscillator. The MEMS resonators at NXP are processed on 8-inch silicon-on-insulator (SOI) wafers, with a SOI layer thickness of 1.5 µm and a buried oxide layer thickness of 1 µm. The strategic choice for thin SOI substrates has been made for two reasons. First, MEMS processing in thin silicon layers can be done with standard CMOS processing tools. The silicon dioxide layer serves as a sacrificial layer. Second, identical substrates are used for the Advanced Bipolar CMOS DMOS (ABCD) IC-processes. This class of processes can handle high voltages (ABCD2 up to 120V). The high voltage capability is suitable for the transduction of the mechanical resonator. Both MEMS and IC are processed on a similar substrate, since the strategic aim is to integrate the MEMS structure with the IC-process in the long run. Frequency scaling is investigated for both the capacitive and the piezoresistive MEMS resonator. MEMS resonators have been successfully tested from 13 MHz to over 400 MHz. This is achieved by decreasing the size of the resonator with a factor 32. We show that the thin SOI layer and the decreasing size of the resonator increase the effective impedance of the capacitive resonator at higher frequencies. For the piezoresistive resonator, we show that this readout principle is insensitive to geometrical scaling and layer thickness. Therefore, the piezoresistive readout is preferred at high frequencies. The effective impedance can be kept low, at the expense of higher power consumption. Frequency accuracy can be improved by decreasing the initial frequency spread and the temperature drift of the MEMS resonator. The main source of initial frequency spread is geometrical offset, due to the non-perfect pattern transfer from mask layout to SOI. A FEM tool has been developed in Comsol Multiphysics to obtain compensated layouts. The resonance frequency of these designs is first-order compensated for geometric offset. The FEM tool is used to obtain compensated resonators of various designs. We show empirically that the compensation by design is effective on a 52 MHz square plate design. For the compensated design, frequency spread measurements over a complete wafer show that there are other systematic sources of frequency spread. The resonance frequency of the silicon MEMS resonator drifts about –30 ppm/K. This is due to the Young’s modulus of silicon that depends on temperature. We have investigated two compensation methods. The first is passive compensation by coating the silicon resonator with a silicon dioxide skin. The Young’s modulus of silicon dioxide has a positive temperature drift. Measurements on globally oxidized structures show that the right oxide thickness reduces the linear temperature drift of the resonator to zero. A second method uses an oven-control principle. The temperature of the resonator is fixed, independent of the ambient temperature. A demo of this principle has been designed with a piezoresistive resonator in which the dc readout current through the resonator is used to control the temperature of the resonator. With both concepts, more than a factor 10 reduction in temperature drift is achieved. To demonstrate the feasibility of high-frequency oscillators, a MEMS-based 56 MHz oscillator has been designed for which a piezoresistive dogbone resonator is used. The amplifier has been designed in the ABCD2 IC-process. The MEMS oscillator consumes 6.1 mW and exhibits a phase noise of –102 dBc/Hz at 1 kHz offset from the carrier and a floor of –113 dBc/Hz. This demonstrates feasibility of the piezoresistive MEMS oscillator for lowpower, low-noise applications. Summarizing, this PhD thesis work as part of the MEMSXO project at NXP demonstrates a MEMS oscillator concept based on the piezoresistive resonator in thin SOI. It shows that by compensated designs for geometric offset and oven-control to reduce temperature drift, a frequency accuracy can be achieved that can compete with the performance of crystal oscillators. In a benchmark with MEMS competitors the concept shows the lowest phase noise, making it the most suited concept for wireless applications