34 research outputs found

    Design and Analysis of Extremely Low-Noise MEMS Gyroscopes for Navigation

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    Inertial measurement sensors that include three gyroscopes and three accelerometers are key elements of inertial navigation systems. Miniaturization of these sensors is desirable to achieve low manufacturing cost, high durability, low weight, small size, and low energy consumption. However, there is a tradeoff between miniaturization of inertial sensors and their performance. Developing all the necessary components for navigation using inertial sensors in a small volume requires major redesign and innovation in these sensors. The main goal of this research is to identify, analyze and optimize parameters that limit the performance of miniaturized inertial gyroscopes and provide comprehensive design guidelines for achieving multi-axis navigation-grade MEMS gyroscopes. It is shown that the fundamental performance limit of inertial gyroscopes is angle random walk (ARW) due to thermo-mechanical and electronic noises. Theoretical models show that resonant frequency, frequency mismatch between sensing and driving modes, effective mass, quality factor (Q), driving amplitude, sensing gap, sensing area and angular gain are the most important parameters that need to be optimized for best noise and most practical device design. In this research, two different structures are considered for low-noise MEMS gyroscopes: 1) shell gyroscopes in yaw direction, and 2) a novel super sensitive stacked (S3) gyroscope for pitch/roll directions. Extensive analytical and FEM numerical modeling was conducted throughout this research to investigate the mechanisms that affect Q and noise in shell resonators used in yaw-rate gyroscopes. These models provided insight into ways to significantly improve resonator design, structure, fabrication, and assembly and helped fabricate fused silica shells with Qs as high as 10 million (at least an order of magnitude larger than other similar shells). Noise performance of these fused silica shell gyroscopes with 5 mm dimeter improved by about two orders of magnitude (< 5×10-3 °/√hr), representing one of the best noise performances reported for a MEMS gyroscope. To build a high-performance MEMS-based planar vibratory pitch/roll gyroscope, it is critical to have a resonator with high Q in the out-of-plane resonant mode. Existing out-of-plane resonators suffer from low Q due to anchor loss or/and thermoelastic dissipation (TED). Increasing the thickness of the out-of-plane resonator reduces TED, but this increases the anchor loss. To reduce anchor loss significantly, a novel structure called S3 is designed. In this structure, two similar resonators are stacked on top of each other and move in opposite directions, thus providing a balanced stacked resonator with reduced anchor loss. The reduction of anchor loss allows larger thickness of silicon S3 gyroscopes, leading to a very low TED. A large-scale model of a stacked balanced resonator is fabricated and tested. The initial results show more than 50 times improvement in Q (measured in air) when resonators are stacked. It is expected that by testing this device in vacuum, Q would improve by more than three orders of magnitude. The S3 design also has an extremely large effective mass, a very large angular gain, a large driving amplitude, a very small sensing gap, and a large sensing area. It is estimated that a 500 µm thick silicon S3 gyroscope provides ARW of about 1.5×10-5 °/√hr (more than two orders of magnitude better performance than a navigation-grade gyroscope). This extraordinary small value can be improved for 1mm thick fused silica to 7.6×10-7 °/√hr if the technology for etching fused silica could be developed in the future.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147701/1/darvishi_1.pd

    Thin-Film AlN-on-Silicon Resonant Gyroscopes: Design, Fabrication, and Eigenmode Operation

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    Resonant MEMS gyroscopes have been rapidly adopted in various consumer, industrial, and automotive applications thanks to the significant improvements in their performance over the past decade. The current efforts in enhancing the performance of high-precision resonant gyroscopes are mainly focused on two seemingly contradictory metrics, larger bandwidth and lower noise level, to push the technology towards navigation applications. The key enabling factor for the realization of low-noise high-bandwidth resonant gyroscopes is the utilization of a strong electromechanical transducer at high frequencies. Thin-film piezoelectric-on-silicon technology provides a very efficient transduction mechanism suitable for implementation of bulk-mode resonant gyroscopes without the need for submicron capacitive gaps or large DC polarization voltages. More importantly, in-air operation of piezoelectric devices at moderate Q values allows for the cointegration of mode-matched gyroscopes and accelerometers on a common substrate for inertial measurement units. This work presents the design, fabrication, characterization, and method of mode matching of piezoelectric-on-silicon resonant gyroscopes. The degenerate in-plane flexural vibration mode shapes of the resonating structure are demonstrated to have a strong gyroscopic coupling as well as a large piezoelectric transduction coefficient. Eigenmode operation of resonant gyroscopes is introduced as the modal alignment technique for the piezoelectric devices independently of the transduction mechanism. Controlled displacement feedback is also employed as the frequency matching technique to accomplish complete mode matching of the piezoelectric gyroscopes.Ph.D

    Technologies for single chip integrated optical gyroscopes

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    Optical gyroscopes are being employed for navigational purposes for decades now and have achieved comparable or better reliability and performance than rotor-based gyroscopes. Mechanical gyros are however generally bulky, heavy and consume more power which make them unsuitable for miniaturized applications such as cube satellites and drones etc. Therefore, much effort is being expended worldwide to fabricate optical gyros having tactical grade robustness and reliability, small size, weight, cost and power consumption with minimal sacrifice of sensitivity. Integrated optics is an obvious approach to achieving this. This work comprises detailed comparative analysis of different types and structures of integrated optical gyroscopes to find out the suitable option for applications which require a resolution of <10 o/h. Based on the numerical analysis, Add-drop ring resonator-based gyro is found to be a suitable structure for integration which has a predicted shot noise limited resolution of 27 o/h and 2.71 o/h for propagation losses of 0.1 dB/cm and 0.01 dB/cm respectively. An integrated gyro is composed of several optical components which include a laser, 3dB couplers, phase/frequency modulators, sensing cavity and photodetectors. This requires hybrid integration of multiple materials technologies and so choices about which component should be implemented in which technology. This project also undertakes theoretical optimization of few of the above-mentioned optical components in materials systems that might offer the most convenient/tolerant option, this including 3dB coupler, thermo-optic phase modulator and sensing cavity (resonator and waveguide loop). In particular, the sensing element requires very low propagation loss waveguides which can best be realised from Si3N4 or Ta2O5. The optimised Si3N4 or Ta2O5 waveguides however are not optimal for other functions and this is shown and alternatives explored where the Si3N4 or Ta2O5 can easily be co-integrated. The fabrication process of low loss Si3N4 and Ta2O5 waveguides are also reported in this thesis. Si3N4 films were grown by using low pressure chemical vapor deposition (LPCVD) technique. Dry etching of Si3N4 films have been optimized to produce smooth and vertical sidewalls. Experimental results predicted that the propagation loss of 0.009 dB/cm is achievable by using optimum waveguide dimensions and silica cladding with the relatively standard processes available within the Laser Physics Centre at the Australian National University. A CMOS back end of line compatible method was developed to deposit good quality Ta2O5 films and silica claddings through ion beam sputtering (IBS) method. Plasma etching of Ta2O5 waveguides has been demonstrated by using a gas combination of CHF3/SF6/Ar/O2. Oxygen was introduced into the chamber to produce non-vertical sidewalls, so the waveguides could be cladded without voids with IBS silica. Average propagation losses of 0.17 dB/cm were achieved from Ta2O5 waveguides which appeared after extensive investigation to be limited by the spatial inhomogeneity of the processing. Lastly, a detailed theoretical and experimental analysis was performed to find out the possible causes of the higher average propagation loss in Ta2O5 waveguides, some sections being observed with 0.02 dB/cm or lower losses

    Micro-Resonators: The Quest for Superior Performance

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    Microelectromechanical resonators are no longer solely a subject of research in university and government labs; they have found a variety of applications at industrial scale, where their market is predicted to grow steadily. Nevertheless, many barriers to enhance their performance and further spread their application remain to be overcome. In this Special Issue, we will focus our attention to some of the persistent challenges of micro-/nano-resonators such as nonlinearity, temperature stability, acceleration sensitivity, limits of quality factor, and failure modes that require a more in-depth understanding of the physics of vibration at small scale. The goal is to seek innovative solutions that take advantage of unique material properties and original designs to push the performance of micro-resonators beyond what is conventionally achievable. Contributions from academia discussing less-known characteristics of micro-resonators and from industry depicting the challenges of large-scale implementation of resonators are encouraged with the hopes of further stimulating the growth of this field, which is rich with fascinating physics and challenging problems

    High-Q Fused Silica Micro-Shell Resonators for Navigation-Grade MEMS Gyroscopes

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    This research aims to develop the resonator for a navigation-grade microelectromechanical system (MEMS) Coriolis vibratory gyroscope (CVG) that will bring inertial navigation capabilities to a wider range of applications by reducing gyroscope size and cost. To achieve the desired gyroscope performance, the gyroscope resonator must have low energy dissipation and a highly symmetric structure. Several challenges arise at the micro-scale due to the increased sensitivity to imperfections and increased susceptibility to energy loss mechanisms. This work investigates the lower limit on energy dissipation in a micro-shell resonator known as the birdbath (BB) resonator. The BB resonator is designed to mitigate the energy loss mechanisms that commonly limit MEMS resonators, including anchor loss and thermoelastic dissipation, through a unique shape and fabrication process and through the use of fused silica as the structural material. A blowtorch molding process is used to form high aspect ratio fused silica shells with a range of wall profiles, providing a high level of control in three dimensions that is not possible with conventional micromachining techniques. Prototype BB resonators were developed prior to this dissertation work but they achieved low quality factors (Q) and low ring-down time constants (T) on the order of 100 thousand and 1 s, respectively. The goal of this work is to drastically increase performance above these initial results. Each relevant energy loss mechanism is considered in order to identify the dominant loss mechanism for a given device. Process improvements are implemented to mitigate each loss mechanism, including improved thermal management during blowtorch molding, cleaner lapping and polishing, reduced upfront surface contamination, and methods to remove contaminants after fabrication. Following optimization, Qs up to 10 million and Ts up to 500 s are measured, representing a marked improvement over the prototype resonators. It is found that BB resonators are now limited by surface loss, as indicated by the observed inverse relationship between Q and surface-to-volume ratio. The surface-loss-limited regime results in a high sensitivity to added surface layers. The addition of a conductive layer to enable electrostatic transduction is found to have a large impact, decreasing Q by 50% with the addition of only 30 angstroms of metal. It is suggested that the origin of this loss may be interfacial slippage due to a large increase in stress that occurs at the interface during oscillation. Experimental investigation into the dependence of Q on conductive layer composition, thickness, deposition conditions, and post-deposition treatments is carried out. Following treatments to removed adsorbed contaminants from the surface, resonators with a 15/50 angstrom Ti/Pt layer are found to maintain 60% of their initial Qs. Indium tin oxide (ITO) is identified as a promising conductive layer candidate, with initial experiments producing shells that maintain 70% of their initial Q. The values of Q and T produced in this work are unprecedented for MEMS resonators. Even accounting for the losses that accompany conductive layer deposition, birdbath resonator gyroscopes are expected to achieve navigation-grade performance.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/146096/1/taln_1.pd

    Light-assisted domain engineering, waveguide fabrication and microstructuring of lithium niobate

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    The thesis is focussing on the interaction of lithium niobate with UV and ultrafast laser radiation to achieve 1) ferroelectric domain inversion, 2) waveguide fabrication, and 3) surface microstructuring. Preferential ferroelectric domain inversion has been demonstrated by 'latent light-assisted poling' and 'inhibition of poling' using ultrafast laser irradiation at 400 nm and CW highly absorbed UV radiation (305..244 nm) respectively. The characteristics of the resultant domains have been experimentally investigated as a function of the fabrication conditions and a theoretical model have been proposed to explain the experimental observations. UV radiation in the 305 nm to 244 nm range have been used for the fabrication of optical waveguides in lithium niobate. The waveguiding characteristics and electro-optic response of the UV written optical channel waveguides have been investigated experimentally. Inhibition of poling and post processing has been used for the fabrication of ridge waveguide structures with enhanced refractive index change. Finally, a method for the fabrication of ultra-smooth lithium niobate single crystal photonic microstructures has been proposed. The method is based on surface tension reshaping of surface microstructures which are produced by preferential poling and subsequent etching. Whispering gallery mode resonators have been fabricated and characterised here

    Studies in fiber-optic couplers and resonators

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    Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1993.Title as it appears in the Feb. 1993 MIT Graduate List: Studies in polished couplers and resonators. Vita.Includes bibliographical references.Robert Paul Dahlgren.M.S

    Novel Photostructurable Polymer for On-Board Optical Interconnects Enabled by Femtosecond Direct Laser Writing

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    Die integrierte Optik hat sich als vielversprechende Lösung für elektronische Verbindungen erwiesen, die eine hohe Bandbreitendichte und einen geringen Stromverbrauch ermöglicht. Seit kurzem ist es möglich photochemische und physikalische Reaktionen auf ein Mikrovolumen zu begrenzen. Dies hat der optischen Verbindungstechnik unter Verwendung von Glas oder Polymer eine zusätzliche Dimension verliehen. Dreidimensionale Wellenleiter können das optische Signal zwischen Blöcken aller Dimensionen verbinden, kombinieren oder aufteilen. Die Erhöhung des Brechungsindex ist jedoch immer noch eine Herausforderung für die Herstellung stabiler Freiform- und monomodaler Wellenleiter mit dreidimensionaler Ausdehnung, welche sich innerhalb der Platine befinden. Diese Dissertation stellt ein neues Konzept vor, um dieser Herausforderung zu begegnen, indem direktes Femtosekunden-Laserschreiben in Polymer und externe Diffusion eines gasförmigen Monomers verwendet wird. Direktes Laserschreiben mit Zwei-Photonen-Absorption wurde verwendet, um die Vernetzung entlang eines vorher definierten Pfades zur Bildung des Wellenleiterkerns zu initiieren. Es wurde ein ausreichender Brechungsindexkontrast erzeugt, um gaußförmige Strahlen mit einem Modus zu führen. Feature-Größen konnten durch Variieren der Scangeschwindigkeit und der Laserintensität linear angepasst werden. Dieses Herstellungsverfahren erfordert nur eine Schicht eines einzelnen Materials ohne Masken-, Kontakt- oder Nassbearbeitung. Durch Verwendung dieser neuartigen Methode wurden dreidimensionale optische Wellenleiter-Arrays, Fan-in/Fan-out- und Splitter-Strukturen hergestellt. Dreidimensionale freiforme Wellenleiter haben ein hohes Potential zur Verbesserung der Packungsdichte und Flexibilität optischer Verbindungen auf Platinenebene
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