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

Development and implementation of a deflection amplification mechanism for capacitive accelerometers

Micro-Electro-Mechanical-Systems (MEMS) and especially physical sensors are part of a flourishing market ranging from consumer electronics to space applications. They have seen a great evolution throughout the last decades, and there is still considerable research effort for further improving their performance. This is reflected by the plethora of commercial applications using them but also by the demand from industry for better specifications. This demand together with the needs of novel applications fuels the research for better physical sensors.Applications such as inertial, seismic, and precision tilt sensing demand very high sensitivity and low noise. Bulk micromachined capacitive inertial sensors seem to be the most viable solution as they offer a large inertial mass, high sensitivity, good noise performance, they are easy to interface with, and of low cost. The aim of this thesis is to improve the performance of bulk micromachined capacitive sensors by enhancing their sensitivity and noise floor.MEMS physical sensors, most commonly, rely on force coupling and a resulting deflection of a proof mass or membrane to produce an output proportional to a stimulus of the physical quantity to be measured. Therefore, the sensitivity to a physical quantity may be improved by increasing the resulting deflection of a sensor. The work presented in this thesis introduces an approach based on a mechanical motion amplifier with the potential to improve the performance of mechanical MEMS sensors that rely on deflection to produce an output signal.The mechanical amplifier is integrated with the suspension system of a sensor. It comprises a system of micromachined levers (microlevers) to enhance the deflection of a proof mass caused by an inertial force. The mechanism can be used in capacitive accelerometers and gyroscopes to improve their performance by increasing their output signal. As the noise contribution of the electronic read-out circuit of a MEMS sensor is, to first order, independent of the amplitude of its input signal, the overall signal-to-noise ratio (SNR) of the sensor is improved.There is a rather limited number of reports in the literature for mechanical amplification in MEMS devices, especially when applied to amplify the deflection of inertial sensors. In this study, after a literature review, mathematical and computational methods to analyse the behaviour of microlevers were considered. By using these methods the mechanical and geometrical characteristics of microlevers components were evaluated. In order to prove the concept, a system of microlevers was implemented as a mechanical amplifier in capacitive accelerometers.All the mechanical structures were simulated using Finite Element Analysis (FEA) and system level simulations. This led to first order optimised devices that were used to design appropriate masks for fabrication. Two main fabrication processes were used; a Silicon on Insulator (SOI) process and a Silicon on Glass (SoG) process. The SOI process carried out at the University of Southampton evolved from a one mask to a two mask dicing free process with a yield of over 95%, in its third generation. The SoG is a well-established process at the University of Peking that uses three masks.The sensors were evaluated using both optical and electrical means. The results from the first prototype sensor design (1HAN) revealed an amplification factor of 40 and a mechanically amplified sensitivity of 2.39V/g. The measured natural frequency of the first mode of the sensor was at 734Hz and the full-scale measurement range was up to 7g with a maximum nonlinearity of 2%. The measurements for all the prototype sensor designs were very close to the predicted values with the highest discrepancy being 22%. The results of this research show that mechanical amplification is a very promising concept that can offer increased sensitivity in inertial sensors without increasing the noise. Experimental results show that there is plenty of room for improvement and that viable solutions may be produced by using the presented approach. The applications of this scheme are not restricted only to inertial sensors but as the results show it can be used in a broader range of micromachined devices

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

High Aspect-ratio Biomimetic Hair-like Microstructure Arrays for MEMS Multi-Transducer Platform

Many emerging applications of sensing microsystems in health care, environment, security and transportation systems require improved sensitivity and selectivity, redundancy, robustness, increased dynamic range, as well as small size, low power and low cost. Providing all of these features in a system consisting of one sensor is not practical or possible. Micro electro mechanical microsystems (MEMS) that combine a large sensor array with signal processing circuits could provide these features. To build such multi-transducer microsystems we get inspiration from “hair”, a structure frequently used in nature. Hair is a simple yet elegant structure that offers many attractive features such as large length to cross-sectional area ratio, large exposed surface area, ability to include different sensing materials, and ability to interact with surrounding media in sophisticated ways. In this thesis, we have developed a microfabrication technology to build 3D biomimetic hair structures for MEMS multi-transducer platform. Direct integration with CMOS will enable signal processing of dense arrays of 100s or 1000s of MEMS transducers within a small chip area. We have developed a new device structure that mimics biological hair. It includes a vertical spring, a proof-mass atop the spring, and high aspect-ratio narrow electrostatic gaps to adjacent electrodes for sensing and actuation. Based on this structure, we have developed three generations of 3D high aspect-ratio, small-footprint, low-noise accelerometers. Arrays of both high-sensitivity capacitive and threshold accelerometers are designed and tested, and they demonstrate extended full-scale detection range and frequency bandwidth. The first-generation capacitive hair accelerometer arrays are based on Silicon-on-Glass (SOG) process utilizing 500 µm thick silicon, achieving a highest sensor density of ~100 sensors/mm2 connected in parallel. Minimum capacitive gap is 5 μm with device height of 400 μm and spring length of 300 μm. A custom-designed Bosch deep-reactive-etching (DRIE) process is developed to etch ultra-deep (> 500 µm) ultra-high aspect-ratio (UHAR) features (AR > 40) with straight sidewalls and reduced undercut across a wide range of feature sizes. A two-gap dry-release process is developed for the second-generation capacitive hair accelerometers. Due to the large device height at full wafer thickness of 1 mm and UHAR capacitive transduction gaps at 2 µm that extend > 200 µm, the accelerometer achieves sub-µg resolution (< 1µg/√Hz) and high sensitivity (1pF/g/mm2), having an area smaller than any previous precision accelerometers with similar performance. Each sensor chip consists of devices with various design parameter to cover a wide range. Bonding with metal interlayers at < 400 °C allows direct integration of these devices on top of CMOS circuits. The third-generation digital threshold hair accelerometer takes advantage of large aspect-ratio of the hair structure and UHAR DRIE structures to provide low noise (< 600 ng/√Hz per mm2 footprint proof-mass due to small contact area) and low power threshold acceleration detection. 16-element (4-bit) and 32-element (5-bit) arrays of threshold devices (total chip area being < 1 cm2) with evenly-spaced threshold gap dimensions from 1 µm to 4 µm as well as with hair spring cross-sectional area from 102 µm to 302 µm are designed to suit specific g-ranges from < 100 mg to 50 g. This hair sensor and sensor array technology is suited for forming MEMS transducer arrays with circuits, including high performance IMUs as well as miniaturized detectors and actuators that require high temporal and spatial resolution, analogous to high-density CMOS imagers.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143975/1/yemin_1.pd

3D Energy Harvester Evaluation

This paper discusses the characterization and evaluation of an MEMS based electrostatic generator, a part of the power supply unit of the self-powered microsystem[1,2,3]. The designed generator is based on electrostatic converter and uses the principle of conversion of non-electric energy into electrical energy by periodical modification of gap between electrodes of a capacitor [4]. The structure is designed and modeled as three-dimensional silicon based MEMS. Innovative approach involving the achievement of very low resonant frequency of the structure (about 100Hz) by usage of modified long cantilever spring design, minimum area of the chip, 3D work mode, the ability to be tuned to reach desired parameters, proves promising directions of possible further development

Performance of compliant mechanisms applied to a modified shape accelerometer of single and double layer

Accelerometers are widely used in several mechanisms of high sensitivity. They are employed for example in tilt-control in spacecraft, inertial navigation, oil exploration, seismic monitoring, etc. In order to improve the sensitivity of the measurements, implementation of Displacement-amplifying Compliant Mechanisms (DaCMs) in a capacitive accelerometer have been reported in the literature. In this paper, a system composed of two elements; capacitive accelerometer with extended beams (CAEB) and a DaCM geometry, of single and souble layer, are analysed. Three materials were considered, in the case, for the second layer. The DaCM implementation improves the operation frequency and displacement sensitivity, under different proportions, at the same time. Furthermore, three sweeps were performed: a range of thickness from 25 µm up to 30 µm (to determine the appropriate silicon mass value, using SOI technology), a range of second layer thickness (to choose the more appropriate material and its thickness) and a range of gravity values (to determine the maximum normal stress in the beams, which defines the superior value of the g operation range). The in-plane mode (y-axis) was considered in all analysed cases. This characterization was developed using the Finite Element Method. Structural and modal analysis responses were under study

A high-resolution micro-electro-mechanical resonant tilt sensor

This paper reports on the design and experimental evaluation of a high-resolution Micro-Electro-Mechanical (MEM) tilt sensor based on resonant sensing principles. The sensors incorporate a pair of double-ended tuning fork (DETF) resonant strain gauges, the mechanical resonant frequencies of which shift in proportion to an axial force induced by variations in the component of gravitational acceleration along a specified input axis. An analysis of the structural design of such sensors (using analytical and finite element modelling) is presented, followed by experimental test results from device prototypes fabricated using a silicon-on-insulator (SOI) MEMS technology. This paper reports measurement conducted to quantify sensor scale factor, temperature sensitivity, scale factor linearity and resolution. It is demonstrated that such sensors provide a ±90 degree dynamic range for tilt measurements with a temperature sensitivity of nearly 500 ppb/K (equating to systematic sensitivity error of approximately 0.007 degree/K). When configured as a tilt sensor, it is also shown that the scale factor linearity is better than 1.4% for a ±20° tilt angle range. The bias stability of a micro-fabricated prototype is below 500 ng for an averaging time of 0.8 seconds making these devices a potentially attractive option for numerous precision tilt sensing applications.This is the author's accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0924424714004385

Silicon microaccelerometer fabrication technologies

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1997.Includes bibliographical references (leaves 275-282).by Charles Heng-Yuan Hsu.Ph.D

Development of a 3-axis MEMS magnetometer based on Lorentz force

Dissertação de mestrado em Physics Engineering, (especialização em Devices, Microsystems and Nanotechnologies)Typical magnetometers found in the magnetic fields research are highly incompatible with the massive MEMS technology industry that has been the object of study in the past years. This aspect leads to the rapid increase in production costs and reliability reduction. Furthermore, most of the magnetometers that are adapted to this technology are highly complex and with little to no adaptation to outer-space research. In this work, a novel single-axis MEMS magnetometer based on the principle of the Lorentz force capable of reading fields in the X or Y direction is designed and simulated with the description of a fabrication method to be used. This magnetometer uses an innovative design for a current-carrying-bar that’s highly adaptable to a variety of scenarios with a low 100Ω current resistance in each of its paths. An amplitude-modulated method is approached through the use of a capacitive-readout system and an off-resonance frequency of operation to achieve the detection baseline of a 1aF capacitive variation at a 20nT magnetic field. This involves the use of various mechanisms to increase the quality factor and reduce the overall stiffness of the device to increase its displacement caused by the Lorentz force. The device is also to be operated at a 500Pa atmosphere to reduce the damping and, at the same time, increase the quality factor. A thermomechanical noise below 3 /√ with a frequency of operation at around 4977 Hz was deemed necessary to adapt the design to another previously designed single-axis MEMS magnetometer capable of reading fields in the Z direction. Various simulation and design tools are used to predetermine the best properties at which the magnetometer will be operated to its highest capabilities. Through these simulations, a 50Hz bandwidth magnetometer, required for spatial research, is achieved with a capacitance variation of 1.37aF at 20nT surpassing the initial requirements. A 1.77 /√ thermomechanical noise is obtained, well below the baseline that was defined for this work. A fabrication layout was developed with all lithography masks designed, and a microfabrication process flow was devised. The microfabrication process run was partially completed and it’s still ongoing.Os magnetómetros típicos encontrados na investigação de campos magnéticos são altamente incompatíveis com a enorme indústria da tecnologia MEMS que tem sido objeto de estudo nos últimos anos. Este aspeto leva ao rápido aumento dos custos de produção e à redução da fiabilidade. Para além disso a maioria dos magnetómetros adaptados a esta tecnologia são altamente complexos e com pouca ou nenhuma adaptação à investigação espacial. Neste trabalho, um novo magnetómetro MEMS de um único eixo baseado no princípio da força de Lorentz capaz de ler campos na direção X ou Y é concebido e simulado com a descrição de um método de fabrico a ser utilizado. Este magnetómetro utiliza um desenho inovador para uma barra condutora que é altamente adaptável a uma variedade de cenários com uma baixa resistência de 100Ω em cada um dos seus caminhos. Um método de modulação em amplitude é abordado através da utilização de um sistema de leitura capacitiva e uma frequência de operação com um desvio da ressonância para alcançar a linha de base de deteção de uma variação capacitiva de 1aF para um campo magnético de 20nT. Isto envolve a utilização de vários mecanismos para aumentar o fator de qualidade e reduzir a rigidez geral do dispositivo para aumentar o deslocamento causado pela força de Lorentz. O dispositivo deve também ser operado a uma atmosfera de 500Pa para reduzir o amortecimento e, ao mesmo tempo, aumentar o factor de qualidade. Um ruído termomecânico inferior a 3 /√ com uma frequência de operação de cerca de 4977 Hz foram consideradas necessárias para adaptar o desenho a outro magnetómetro MEMS de um eixo, previamente concebido, capaz de ler campos na direção Z. Várias ferramentas de simulação e desenho são utilizadas para pré-determinar as melhores propriedades em que o magnetómetro será operado até às suas capacidades mais elevadas. Através destas simulações, um magnetómetro de 50Hz de largura de banda, necessário para a investigação espacial, é alcançado com uma variação de capacidade de 1.37aF a 20nT, ultrapassando os requisitos iniciais. É obtido um ruído termomecânico de 1.77 /√, bem abaixo da linha de base que foi definida para este trabalho. Foi desenvolvido um esquema de fabricação com todas as máscaras litográficas concebidas, e foi concebido um fluxo de processo de microfabricação. A execução do processo de microfabricação foi parcialmente concluída e ainda está em curso.This work was framed in the scope of the Project (Link4S)ustainability - A new generation connectivity system for creation and integration of networks of objects for new sustainability paradigms [POCI-01- 0247-FEDER-046122 | LISBOA-01-0247-FEDER-046122], financed by the Operational Competitiveness and Internationalization Programmes COMPETE 2020 and LISBOA 2020, under the PORTUGAL 2020 Partnership Agreement, and through the European Structural and Investment Funds in the FEDER component