Enhancements of MEMS design flow for Automotive and Optoelectronic applications

In the latest years we have been witnesses of a very rapidly and amazing grown of MicroElectroMechanical systems (MEMS) which nowadays represent the outstanding state-of-the art in a wide variety of applications from automotive to commercial, biomedical and optical (MicroOptoElectroMechanicalSystems). The increasing success of MEMS is found in their high miniaturization capability, thus allowing an easy integration with electronic circuits, their low manufacturing costs (that comes directly from low unit pricing and indirectly from cutting service and maintaining costs) and low power consumption. With the always growing interest around MEMS devices the necessity arises for MEMS designers to define a MEMS design flow. Indeed it is widely accepted that in any complex engineering design process, a well defined and documented design flow or procedure is vital. The top-level goal of a MEMS/MOEMS design flow is to enable complex engineering design in the shortest time and with the lowest number of fabrication iterations, preferably only one. These two characteristics are the measures of a good flow, because they translate directly to the industry-desirable reductions of the metrics “time to market” and “costs”. Like most engineering flows, the MEMS design flow begins with the product definition that generally involves a feasibility study and the elaboration of the device specifications. Once the MEMS specifications are set, a Finite Element Method (FEM) model is developed in order to study its physical behaviour and to extract the characteristic device parameters. These latter are used to develop a high level MEMS model which is necessary to the design of the sensor read out electronics. Once the MEMS geometry is completely defined and matches the device specifications, the device layout must be generated, and finally the MEMS sensor is fabricated. In order to have a MEMS sensor working according to specifications at first production run is essential that the MEMS design flow is as close as possible to the optimum design flow. The key factors in the MEMS design flow are the development of a sensor model as close as possible to the real device and the layout realization. This research work addresses these two aspects by developing optimized custom tools (a tool for layout check (LVS) and a tool for parasitic capacitances extraction) and new methodologies (a methodology for post layout simulations) which support the designer during the crucial steps of the design process as well as by presenting the models of two cases studies belonging to leading MEMS applications (a micromirror for laser projection system and a control loop for the shock immunity enhancement in gyroscopes for automotive applications)

Design, Fabrication and Levitation Experiments of a Micromachined Electrostatically Suspended Six-Axis Accelerometer

A micromachined electrostatically suspended six-axis accelerometer, with a square plate as proof mass housed by a top stator and bottom stator, is presented. The device structure and related techniques concerning its operating principles, such as calculation of capacitances and electrostatic forces/moments, detection and levitation control of the proof mass, acceleration measurement, and structural parameters design, are described. Hybrid MEMS manufacturing techniques, including surface micromachining fabrication of thin film electrodes and interconnections, integration fabrication of thick nickel structures about 500 μm using UV-LIGA by successful removal of SU-8 photoresist mold, DRIE of silicon proof mass in thickness of 450 μm, microassembly and solder bonding, were employed to fabricate this prototype microdevice. A levitation experiment system for the fabricated microaccelerometer chip is introduced, and levitation results show that fast initial levitation within 10 ms and stable full suspension of the proof mass have been successfully demonstrated

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

Development of a low damping MEMS resonator

MEMS based low damping inertial resonators are the key element in the development of precision vibratory gyroscopes. High quality factor (Q factor) is a crucial parameter for the development of high precision inertial resonators. Q factor indicates how efficient a resonator is at retaining its energy during oscillations. Q factor can be limited by different types of energy losses, such as anchor damping, squeeze-film damping, and thermoelastic damping (TED). Understanding the energy loss-mechanism can show a path for designing high Q resonator. This thesis explores the effects of different design parameters on Q factor of 3D inertial resonators. TED loss mechanisms in a 3D non-inverted wineglass (hemispherical) shell resonator and a disk resonator were investigated. Both the disk and shell share the same vibration modes, and they are widely used as a vibratory resonator shape. Investigation with loss-mechanism shows that robust mechanical materials such as fused silica can offer ultra-low damping during oscillation. TED loss resulting from the effects of geometric parameters (such as thickness, height, and radius), mass imbalance, thickness non-uniformity, and edge defects were investigated. Glassblowing was used to fabricate hemispherical 3D shell resonators and conventional silicon based dry etching was used to fabricate micro disk resonators. The results presented in this thesis can facilitate selecting efficient geometric and material properties for achieving a higher Q-factor in 3D inertial resonators. Enhancing the Q-factor in MEMS based 3D resonators can further enable the development of high precision resonators and gyroscopes

MEMS suljenta kuparin lämpöpuristusliitännällä

Copper thermocompression is a promising wafer-level packaging technique, as it allows the bonding of electric contacts simultaneously to hermetic encapsulation. In thermocompression bonding the bond is formed by diffusion of atoms from one bond interface to another. The diffusion is inhibited by barrier forming surface oxide, high surface roughness and low temperature. Aim of this study was to establish a wafer-level packaging process for MEMS (Mi-croElectroMechanical System) mirror and MEMS gyroscope. The cap wafer of the MEMS mirror has an antireflective coating that limits the thermal budget of the bonding process to 250°C. This temperature is below the eutectic temperature of most common eutectic bonding materials, such as Au-Sn (278°C), Au-Ge (361°C) and Au-Si (370°C). Thus a thermocompression bonding method needed to be developed. Copper was used as a bonding material due to its low cost, high self-diffusivity and resistance to oxidation in ambient air. The bond structures were fabricated using three different methods and the bonding was further enhanced by annealing. The bonded structures were characterized with scanning acoustic microscopy, scanning electron microscope and the bond strength was determined by shear testing. Exposing the bond structures to etchant during Cu seed layer removal was found to drastically increase the surface roughness of bond structures. This increase proved detrimental to bond strength and dicing yield and thus covering the bond surface during wet etching is recommended. The native oxidation on copper surfaces was completely removed with combination of ex situ acetic acid wet etch and in situ forming gas anneal. Successful thermocompression bonding process using sputtered copper films was established at a low temperature of 200°C, well below the thermal limitation set by the antireflective coating. The established wafer bonding process had high yield of 97% after dicing. The bond strength was evaluated by maximum shear strength and recorded at 75 MPa, which is well above the MIL-STD-883E standard (METHOD 2019.5) rejection limit of 6.08 MPa.Kuparin lämpöpuristusliitäntä on lupaava kiekkotason pakkausmenetelmä, sillä se mahdollistaa sekä sähköisten liitäntöjen, että hermeettisen suljennan toteuttamisen samanaikaisesti. Lämpöpuristusliitännässä sidos muodostuu atomien diffuusiosta liitospinnalta toiselle. Diffuusiota rajoittavat estokerroksen muodostava pinta oksidi, korkea pinnan karheus ja matala lämpötila. Diplomityön tavoitteena oli luoda kiekkotason pakkausmenetelmä mikroelektromekaaniselle (MEMS, MicroElectroMechanical System) peilille ja MEMS gyroskoopille. Peilin lasisen kansikiekon pinnalla oleva antiheijastava kalvo rajoitti liitännässä käytettävän lämpötilan korkeintaan 250°C:een, mikä on alempi lämpötila kuin useimpien kiekkoliitännässä käytettyjen materiaaliparien eutektinen piste. Esimerkkinä mainittakoon mm. Au-Sn (278°C), Au-Ge (361°C) ja Au-Si (370°C). Kuparin alhainen hinta, korkea ominaisdiffuusio ja hidas hapettuminen ilmakehässä puoltavat sen valintaa liitäntämateriaaliksi. Liitäntärakenteet valmistettiin kolmella menetelmällä ja liitännän vahvuutta parannettiin lämpökäsittelyllä. Liitetyt rakenteet karakterisoitiin pyyhkäisy elektronimikroskoopin, akustisen mikroskoopin ja liitoslujuus-mittauksen avulla. Liitospintojen altistamisen hapolle havaittiin lisäävän pinnankarkeutta ja olevan siten haitallista liitokselle ja laskevan saantoa. Liitospintojen suojaaminen siemenkerroksen syövytyksen aikana on suotavaa. Pintaoksidi pystytään poistamaan täysin suorittamalla oksidin märkäetsaus jääetikalla sekä lämpökäsittely N2/H2 atmosfäärissä. Sputteroidut kuparikalvot pystyttiin liittämään onnistuneesti yhteen 200°C lämpötilassa, mikä on alle anti-heijastavan pinnan asettaman lämpötilarajan. Tällä liitäntä menetelmällä saavutettiin kiekkoliitoksella yhteen liitettyjen sirujen sahauksessa korkea 97% saanto. Liitoslujuus määritettiin maksimi-leikkausvoiman avulla ja sen suuruudeksi mitattiin 75 MPa. Lujuus oli yli kymmenkertainen MIL-STD-883E standardin (METHOD 2019.5) asettamaan hylkäysrajaan 6.08 MPa nähden

Overview of the technologies used in the fabrication of MEMS/NEMS actuators for space applications

Given the advantages in terms of weight, size and cost and because it can withstand severe shocks and temperature changes, the MEMS/NEMS sensors are widely used in the aerospace domain. This paper presents a brief history of the scientists who made reference to micro and nano technologies for the first time, followed by a synthesis of the leading technologies used in the manufacture of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) intensively used in aerospace industry. After reviewing the latest technologies used in the manufacture of MEMS/NEMS sensors, the paper continues with predicting the current state regarding the development of NEMS and MEMS, respectively

Micro-Resonators: The Quest for Superior Performance

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

MEMS Accelerometers

Micro-electro-mechanical system (MEMS) devices are widely used for inertia, pressure, and ultrasound sensing applications. Research on integrated MEMS technology has undergone extensive development driven by the requirements of a compact footprint, low cost, and increased functionality. Accelerometers are among the most widely used sensors implemented in MEMS technology. MEMS accelerometers are showing a growing presence in almost all industries ranging from automotive to medical. A traditional MEMS accelerometer employs a proof mass suspended to springs, which displaces in response to an external acceleration. A single proof mass can be used for one- or multi-axis sensing. A variety of transduction mechanisms have been used to detect the displacement. They include capacitive, piezoelectric, thermal, tunneling, and optical mechanisms. Capacitive accelerometers are widely used due to their DC measurement interface, thermal stability, reliability, and low cost. However, they are sensitive to electromagnetic field interferences and have poor performance for high-end applications (e.g., precise attitude control for the satellite). Over the past three decades, steady progress has been made in the area of optical accelerometers for high-performance and high-sensitivity applications but several challenges are still to be tackled by researchers and engineers to fully realize opto-mechanical accelerometers, such as chip-scale integration, scaling, low bandwidth, etc

Fabrication of high aspect ratio vibrating cylinder microgyroscope structures by use of the LIGA process

Inertial grade microgyroscopes are of great importance to improve and augment inertial navigation systems based on GPS for industrial, automotive, and military applications. The efforts by various research groups worldwide to develop inertial grade microgyroscopes have not been successful to date. In 1994, the Department of Mechanical Engineering at Louisiana State University and SatCon Technology Corporation (Boston, Massachusetts) proposed a series of shock tolerant micromachined vibrating cylinder rate gyroscopes with aspect ratios of up to 250:1 to meet the needs of inertial navigation systems based on existing conventional vibrating cylinder gyroscopes. Each microgyroscope consisted of a tall thin shell metallic cylinder attached to a substrate at one end and surrounded by four drive- and four sense-electrodes. The proposed drive- and sense-mechanisms were capacitive-force and capacitance-change, respectively. Since the high aspect ratio metallic microgyroscope structures could not be fabricated by using traditional silicon-based MEMS processes, a LIGA-based two layer fabrication process was developed. A wiring layer was constructed by using a combination of thick film photolithography and electroplating (nickel and gold) on a silicon substrate covered with silicon nitride and a tri-layer plating base; aligned X-ray lithography and nickel electroplating were used to build the high aspect ratio cylinders and electrodes. Deficiencies in the LIGA process were also addressed in this research. Three types of X-ray mask fabrication processes for multi-level LIGA were developed on graphite, borosilicate glass and silicon nitride substrates. Stable and reliable gold electroplating methods for X-ray masks were also established. The plating rate and internal stress of deposits were thoroughly characterized for two brands of commercially available sulfite-based gold electroplating solutions, Techni Gold 25E and NEUTRONEX 309. The gaps between the cylinders and electrodes, which are defined by thin PMMA walls during electroplating, were found to be smaller than designed and deformed in many of the microgyroscope structures. The lateral dimensional loss (LDL) and deformation were identified to be related to the overall thickness and lateral aspect ratio (LAR) of the thin PMMA walls