Design and simulation of micro resonator oscillator for communication circuits

In this theses design and simulation of a Micro Electro Mechanical System (MEMS) based oscillator is presented. Electrostatic comb drive is chosen as the core structure in oscillator. MicroElectroMechanical (MEM) vibrating structures such as linear drive resonators can be used as driving components in signal processing applications. The choice of these components is assisted by the fact that these MEM devices display high quality factor values when operated under vacuum. The design of a highly stable oscillator is an example utilizing the linear drive resonators and working samples are demonstrated at 16.5 kHz. For this oscillator to be used in portable communication devices, the operating frequency will have to be increased to at least IF band (> 450kHz). MEMS based microstructures are simulated and prepared for implementation by properly adjusting the physical dimensions of the micromechanical resonator. The Dimensions of the resonator is tuned to achieve higher resonance frequencies. Electrical model and governing equations of interdigitated finger structure are studied. Based on results of these studies a micromechanical oscillator is designed to attain above-mentioned frequency. The study is carried out both analytically and on the equivalent circuit. Integration of MEMS structure with Complementary Metal Oxide Semiconductor (CMOS) electronics is another motivation and driving force of this study. Therefore completely monolithic high-Q micromechanical oscillator integrated with CMOS circuits is aimed and described. As it has high Q (over 80.000) and very stable, laterally driven microresonators can be a good miniaturized replacement of a crystal and surface acoustic wave (SAW) resonator based oscillators used in telecommunication applications. The electrical model of the microresonator is given and used as a frequency selective network in the oscillator design. Different oscillator circuits are designed and simulated to estimate and compare their performance to other mechanical based oscillators (SAW, FBAR, Crystal etc.). Analog CMOS integraated circuits are designed and optimized to achieve highly stable oscillations

Readout Method And Electronic Bandwidth Control For A Silicon In-plane Tuning Fork Gyroscope

Disclosed are methods and a sensor architecture that utilizes the residual quadrature error in a gyroscope to achieve and maintain perfect mode-matching, i.e., ~0 Hz split between the drive and sense mode frequencies, and to electronically control sensor bandwidth. In a reduced-to-practice embodiment, a 6 mW, 3V CMOS ASIC and control algorithm are interfaced to a mode-matched MEMS tuning fork gyroscope to implement an angular rate sensor with bias drift as low as 0.15°/hr and angle random walk of 0.003°/√hr, which is the lowest recorded to date for a silicon MEMS gyroscope. The system bandwidth can be configured between 0.1 Hz and 1 kHz.Georgia Tech Research Coporatio

On the feasibility of integrated optical waveguide-based in situ monitoring of microelectromechanical systems (MEMS)

This dissertation explores the feasibility of using integrated optical waveguides to measure the motion of microelectromechanical structures (MEMS). MEMS are a class of silicon devices which are being developed as sensors and actuators. Because these free moving structures are fabricated using processes similar to microfabrication, MEMS devices and traditional electronics can be integrated on the same substrate. This merging of the technologies will allow the miniaturization of large scale mechanical systems. A difficulty with MEMS devices is determining the submicron motion. One method of noninvasive measurement is optical measurement. Research focused on the characterization of one particular MEMS device, a linear comb resonator. Linear comb resonators displace linearly along a single axis when drive with a sinusoidal voltage signal. This research presents how single mode and multimode guided waves have potential to yield significant positional information. Using optical fibers to create a bulk optical metrology probe, the displacement and operating frequency of this device was characterized. Integration of this an optical probe structure with the MEMS devices can create integrated optical metrology (IOM), which is an in-situ method of device characterization and can represent an enabling technology for MEMS. Co-integration of the two technologies can be achieved through either processing or post processing of integrated waveguides with the MEMS devices. The fabrication process for co-integration of polymer optical waveguides has been experimentally defined in this dissertation, however final results indicate guides wave IOM would best be explored through process interruption or hybrid techniques given existing polymer materials. Analysis yields that the co-integration of inorganic waveguide structures first requires optimization of the design of the microprobe layout

Readout Method And Electronic Bandwidth Control For A Silicon In-plane Tuning Fork Gyroscope

Disclosed are methods and a sensor architecture that utilizes the residual quadrature error in a gyroscope to achieve and maintain perfect mode-matching, i.e., ~0 Hz split between the drive and sense mode frequencies, and to electronically control sensor bandwidth. In a reduced-to-practice embodiment, a 6 mW, 3V CMOS ASIC and control algorithm are interfaced to a mode-matched MEMS tuning fork gyroscope to implement an angular rate sensor with bias drift as low as 0.15°/hr and angle random walk of 0.003°/√hr, which is the lowest recorded to date for a silicon MEMS gyroscope. The system bandwidth can be configured between 0.1 Hz and 1 kHz.Georgia Tech Research Corporatio

Nonlinear vibration of micromechanical resonators

Ph.DDOCTOR OF PHILOSOPH

The Resonant Body Transistor

With quality factors (Q) often exceeding 10,000, vibrating micromechanical resonators have emerged as leading candidates for on-chip versions of high-Q resonators used in wireless communications systems, sensor networks, and clocking sources in microprocessors. However, extending the frequency of MEMS resonators generally entails scaling of resonator dimensions leading to increased motional impedance. In this dissertation, I introduce a new transduction mechanism using dielectric materials to improve performance and increase frequency of silicon-based RF acoustic resonators. Traditionally, electrostatically transduced mechanical resonators have used air-gap capacitors for driving and sensing vibrations in the structure. To increase transduction efficiency, facilitate fabrication, and enable GHz frequencies of operation, it is desirable to replace air-gap transducers with dielectric films. In my doctoral work, I designed, fabricated, and demonstrated dielectrically transduced silicon bulk-mode resonators up to 6.2 GHz, marking the highest acoustic frequency measured in silicon to date. The concept of internal dielectric transduction is introduced, in which dielectric transducers are incorporated directly into the resonator body. With dielectric films positioned at points of maximum strain in the resonator, this transduction improves in efficiency with increasing frequency, enabling resonator scaling to previously unattainable frequencies. Using internal dielectric transduction, longitudinal-mode resonators exhibited the highest frequency-quality factor (f.Q) product in silicon to date at 5.1 x 10 exp(13) s exp(-1) . These resonators were measured by capacitively driving and sensing acoustic vibrations in the device. However, capacitive detection often requires 3-port scalar mixer measurement, complicating monolithic integration of the resonators with CMOS circuits. The internal dielectric bulk-mode resonators can be utilized in a 2-port configuration with capacitive drive and piezoresistive detection, in which carrier mobility is dynamically modulated by elastic waves in the resonator. Piezoresistive sensing of silicon-based dielectrically transduced resonators was demonstrated with 1.6% frequency tuning and control of piezoresistive transconductance gm by varying the current flowing through the device. Resonant frequency, determined by lithographically defined dimensions, was demonstrated over a wide frequency range. These degrees of freedom enable acoustic resonators spanning a large range of frequencies on a single chip, despite design restrictions of the CMOS process. As we scale to deep sub-micron (DSM) technology, transistor cut-off frequencies increase, enabling the design of CMOS circuits for RF and mm-wave applications greater than 60 GHz. However, DSM transistors have limited gain and integrated passives demonstrate low Q, resulting in poor efficiency. A successful transition into DSM CMOS requires enhanced-gain and high-Q components operating at microwave frequencies. In this work, a merged NEMS-CMOS device is introduced that can function as a building block to enhance the performance of RF circuits. The device, termed the Resonant Body Transistor (RBT), consists of a field effect transistor embedded in the body of a high-frequency NEMS resonator implementing internal dielectric transduction. The results of this work indicate improved resonator performance with increased frequency, providing a means of scaling MEMS resonators to previously unattainable frequencies in silicon. With the transduction methods developed in this work, a hybrid NEMSCMOS RBT enables low-power, narrow-bandwidth low noise amplifier design for transceivers and low phase-noise oscillator arrays for clock generation and temperature sensing in microprocessors

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