Capacitive Microelectromechanical System Accelerometer Research Articles

Utilizing porous dielectric material to enhance the performance of capacitive MEMS accelerometers for shaft monitoring

This research investigates the optimization of the performance of a capacitive-based microelectromechanical systems (MEMS) shaft monitoring accelerometer using a porous soft dielectric material with high polarization capability and low Young’s modulus. The nonlinear governing equations of a microbeam with a proof mass coupled with the squeeze motion equation of the polymeric dielectric material excited by shaft vibrations are extracted and solved in both the spatial and temporal domains. The studied model considers the stiffness, dissipative, and inertial effects of the polymeric material and incorporates porosity as a displacement-dependent variable, which introduces an additional nonlinearity. After discretizing the coupled nonlinear differential equations using the Galerkin method, the response in the frequency domain is obtained by applying an energy balance method, where the coefficients of the Fourier expansion of the response are found by arc-length continuation through a physical gradient descent learning-based approach method. The occurrence of secondary and primary resonances in different harmonic responses is studied for different harmonic acceleration inputs. The equivalent rigidity of the design is investigated for different shaft frequency magnitudes and different proof mass geometries. The effect of the initial porosity volume fraction of the elastomeric dielectric material on the dynamic response of the accelerometer is also examined. The sensor’s sensitivity is dependent on its geometry and properties and can vary based on the structure’s material parameters. Consequently, the selection of different geometries and materials may result in either an improvement or a deterioration of the sensor’s performance.

Multimode TED-tuned beam mass damper for low-g capacitive MEMS accelerometers VRE reduction

Capacitive Micro-Electro-Mechanical Systems (MEMS) accelerometers for quasi-static applications are susceptible to vibration rectification error (VRE) when the ambient AC vibration signals get rectified to DC. Studies to mitigate the VRE have relied on the sigma-delta filtering algorithm with closed-loop feedback as well as the adoption of vibration-insensitive frequency modulation (FM) instead of conventional amplitude modulation. In this study, we propose a novel solution to VRE caused by asymmetric railing introducing multimode thermoelastic damping (TED) tuned beam mass damper (TBMD) integrated into the MEMS accelerometer effectively functioning as a compensator. Our contribution breaks through the microfabrication limitation of damping elements by utilizing intrinsic TED in MEMS material. The damper elements were alternately tuned up to the third flexural mode due to MEMS manufacturing constraints. This was done through parametric sweeps in numerical simulations on single-axis capacitive MEMS accelerometer integrated with multimode TED-damped TBMD. According to simulation results, the higher the mode shapes of the TED-damped tuned beam mass damper, the higher the damping effect and the better the vibration rectification factor.

Python-Based Open-Source Electro-Mechanical Co-Optimization System for MEMS Inertial Sensors.

The surge in fabrication techniques for micro- and nanodevices gave room to rapid growth in these technologies and a never-ending range of possible applications emerged. These new products significantly improve human life, however, the evolution in the design, simulation and optimization process of said products did not observe a similarly rapid growth. It became thus clear that the performance of micro- and nanodevices would benefit from significant improvements in this area. This work presents a novel methodology for electro-mechanical co-optimization of micro-electromechanical systems (MEMS) inertial sensors. The developed software tool comprises geometry design, finite element method (FEM) analysis, damping calculation, electronic domain simulation, and a genetic algorithm (GA) optimization process. It allows for a facilitated system-level MEMS design flow, in which electrical and mechanical domains communicate with each other to achieve an optimized system performance. To demonstrate the efficacy of the methodology, an open-loop capacitive MEMS accelerometer and an open-loop Coriolis vibratory MEMS gyroscope were simulated and optimized—these devices saw a sensitivity improvement of 193.77% and 420.9%, respectively, in comparison to their original state.

An efficient design of dual-axis MEMS accelerometer considering microfabrication process limitations and operating environment variations

PurposeThis paper aims to present an efficient design approach for the micro electromechanical systems (MEMS) accelerometers considering design parameters affecting the long-term reliability of these inertial sensors in comparison to traditional iterative microfabrication and experimental characterization approach. Design/methodology/approachA dual-axis capacitive MEMS accelerometer design is presented considering the microfabrication process constraints of the foundry process. The performance of the MEMS accelerometer is analyzed through finite element method– based simulations considering main design parameters affecting the long-term reliability. The effect of microfabrication process induced residual stress, operating pressure variations in the range of 10 mTorr to atmospheric pressure, thermal variations in the operating temperature range of −40°C to 100°C and impulsive input acceleration at different input frequency values is presented in detail. FindingsThe effect of residual stress is negligible on performance of the MEMS accelerometer due to efficient design of mechanical suspension beams. The effect of operating temperature and pressure variations is negligible on energy loss factor. The thermal strain at high temperature causes the sensing plates to deform out of plane. The input dynamic acceleration range is 34 g at room temperature, which decreases with operating temperature variations. At low frequency input acceleration, the input acts as a quasi-static load, whereas at high frequency, it acts as a dynamic load for the MEMS accelerometer. Originality/valueIn comparison with the traditional MEMS accelerometer design approaches, the proposed design approach focuses on the analysis of critical design parameters that affect the long-term reliability of MEMS accelerometer.

A Novel Fabrication Method for a Capacitive MEMS Accelerometer Based on Glass-Silicon Composite Wafers.

In this paper, we report a novel teeter-totter type accelerometer based on glass-silicon composite wafers. Unlike the ordinary micro-electro-mechanical systems (MEMS) accelerometers, the entire structure of the accelerometer, includes the mass, the springs, and the composite wafer. The composite wafer is expected to serve as the electrical feedthrough and the fixed capacitance plate at the same time, to simplify the fabrication process, and to save on chip area. It is manufactured by filling melted borosilicate glass into an etched silicon wafer and polishing the wafer flat. A sensitivity of 51.622 mV/g in the range of ±5 g (g = 9.8 ), a zero-bias stability under 0.2 mg, and the noise floor with 11.28 µg/√Hz were obtained, which meet the needs of most acceleration detecting applications. The micromachining solution is beneficial for vertical interconnection and miniaturization of MEMS devices.

A high-resolution area-change-based capacitive MEMS tilt sensor

Tilt sensors have been widely used in many applications, such as instrumentation leveling, oil drilling and structural health monitoring. With the rapid development of the micro-electromechanical system (MEMS) technology, MEMS accelerometer-based high-performance tilt sensors are capable of replacing conventional tilt sensors in most of application fields. High-sensitivity MEMS resonant accelerometers (MRAs) are fully qualified for tilt sensing. However, MRA chips have to be vacuum packaged and require complex circuits; therefore, they are difficult to be miniaturized and commercialized. In this case, this paper introduces a commercial-oriented capacitive MEMS accelerometer for tilt sensing with both high-resolution and compact size. The proposed MEMS sensor utilizes simplified fabrication processes, the area-change-based capacitive displacement transducer and the capacitance-to-voltage conversion application specific integrated circuit (ASIC) for low-cost, high-sensitivity, low power consumption and miniaturization. The developed MEMS sensor has a package dimension of 14 mm × 14 mm × 3.6 mm with a single +5 V power supply and analog voltage output. Several evaluation experiments have been conducted with the results showing that the developed MEMS tilt sensor has a full measurement range of ±180°, a linear measurement range of ±30° with a nonlinearity of ±2.8%, a sensitivity of 33.6 mV/°, a noise floor of 2 × 10−4°/√Hz, a bandwidth of 100 Hz and an angle resolution of 7 × 10−4° with an integral time of 0.5 s. Compared with other commercial MEMS tilt sensors, the proposed sensor has a better resolution, a lower noise floor, a larger bandwidth, a lower power consumption and a smaller package. The simplified fabrication process and high performance suggest the developed MEMS tilt sensor is promising to be competitive in the market.

Measuring and calibrating of the parasitic mismatch in MEMS accelerometer based on harmonic distortion self-test

Capacitive MEMS (micro-electromechanical systems) accelerometers have inherent parasitic capacitors, the existence of which will increase the system bias-instability and non-linearity. The parasitic mismatch induced signal is indistinguishable to the sensed signal and cannot be canceled by traditional offset cancellation technique. In this paper, a method to realize the on-line measuring and calibrating of this parasitic effect is proposed, which is based on the digital harmonic distortion self-test. The effect of different parasitic mismatch on the system output response is analyzed in detail. As will show, in self-test mode, the parasitic mismatch will induce distinctive even-order harmonics. An on-chip digital self-test circuit is built to excite and measure this characteristic. According to the measured results, the parasitic mismatch of the front-end interface can be effectively tuned out whereby preserved calibrating unit. The test result shows, using proposed technique, the parasitic mismatch could be reduced in two orders of magnitude.

Design of Perforated Membrane for Low-Noise Capacitive MEMS Accelerometers

We propose a design method for a thick membrane with several thousand perforations suitable for low-noise capacitive micro-electro-mechanical systems (MEMS) accelerometers. Although we have already reported that a membrane with two-step perforations shows good noise performance, it is difficult to simulate mechanical noise using a conventional method because the large number of perforations complicates the membrane structure. In this work, we assumed that the number of perforations is infinite and that there are symmetric planes between them. This assumption greatly reduces computational complexity because the simulation model is simplified in the region around a perforation. We fabricated MEMS resonators with a two-step perforated membrane and evaluated the accuracy of the simulation results. The measured Q values of the fabricated resonators were in good agreement with the calculated results. Using this design method, we optimized the structure of a membrane with two-step perforations and fabricated capacitive MEMS accelerometers that showed excellent low-noise performance (less than 15 ng/Hz1/2) and low-power consumption (20 mW).

(Invited) MEMS Accelerometer Fabricated by Gold Multi-Layer Metal Technology

MEMS(microelectromechanical systems) accelerometers are key devices for IoT (Internet of Things) technology. Throughout the technical progress, the resolution of detecting acceleration has been required to be the level of micro-G (1 G = 9.8 m/s2) in spite of small sensor size. In order to solve the problem, we proposed capacitive MEMS accelerometers with gold proof mass and have been developing MEMS accelerometers [1-4] using the multi-layer metal technology [5]. Figure 1 shows the relationship between the Brownian noise (BN ) and the proof mass size of MEMS accelerometers. BN is a crucial factor to determine the resolution of MEMS accelerometer [6]. In this Fig. 1, we compare our developed MEMS accelerometers with the other reported ones. These results suggest that we can obtain the low value of 22 nG/(Hz)1/2 by utilizing gold as a high density material of the proof mass, despite the same size of gold proof mass as that of silicon proof mass [7-11]. We have proved the feasibility of the MEMS accelerometer with the gold proof mass. This paper reviews the feature of our proposed MEMS accelerometer and the issue of its reliability. Figure 2 shows a schematic illustration of the proposed MEMS accelerometer. The MEMS accelerometer consists of suspensions, stoppers, a proof mass as upper electrode, and a fixed electrode. When the upper electrode is moving to the vertical direction by the input of acceleration, the capacitance is detected by the fixed electrode. The stoppers are built to prevent mechanical destruction caused by over-swinging of the upper electrode. Figure 3 shows an SEM photograph of the MEMS accelerometer. The result shows that MEMS accelerometer fabrication process is established by the multi-layer metal technology, which consists of the gold electroplating and the photo-sensitive polyimide film [5]. On the basis of the design concept and the fabrication process, we have developed the MEMS accelerometers described above. From the viewpoint of the usage of gold material for MEMS structures, we have investigated the reliability by conducting the heat cycle and the vibration cycle tests [12, 13]. These tests were carried out on MEMS accelerometers and cantilevers for evaluating the material characteristics. Figure 4 shows the results of applying the continuous vibration to the MEMS accelerometer. It is revealed that the MEMS accelerometer is immune to the vibration. In conclusion, we confirm that our proposed MEMS accelerometer can make a contribution to a new technical development. Reference [1] T. Konishi et al., Jpn. J. Appl. Phys., 52, p. 06GL04, (2013). [2] D. Yamane et al., Appl. Phys. Lett., 104, p. 074102, (2014). [3] D. Yamane et al., APCOT 2016, pp299, (2016). [4] D. Yamane et al., Sensors and Materials, (2019). [5] K. Machida et al., IEEE Trans. On ED, 48, 2273, (2001). [6] B. E. Boser and R. T. Howe, IEEE JSC, 31, p. 366, (1996). [7] J. Chae et al., JMEMS, vol.13, pp.628-635, (2004). [8] H. Qu et al., IEEE Sensors Journal, vol.8, pp.1511(2008). [9] R. G. Walmsley et al., in Proc. IEEE SENSORS2009, (2009). [10] R. A. Dias et al., Sensors and Actuators A: pp. 47 (2011). [11] X. Zou et al., in Proc. Transducers 2015, (2015). [12] T. Konishi et al., MNC 2017, (2017). [13] D. Yamane et al., Transducers 2017,2187, (2017). Figure 1

A Review of the Capacitive MEMS for Seismology.

MEMS (Micro Electro-Mechanical Systems) sensors enable a vast range of applications: among others, the use of MEMS accelerometers for seismology related applications has been emerging considerably in the last decade. In this paper, we provide a comprehensive review of the capacitive MEMS accelerometers: from the physical functioning principles, to the details of the technical precautions, and to the manufacturing procedures. We introduce the applications within seismology and earth sciences related disciplines, namely: earthquake observation and seismological studies, seismic surveying and imaging, structural health monitoring of buildings. Moreover, we describe how the use of the miniaturized technologies is revolutionizing these fields and we present some cutting edge applications that, in the very last years, are taking advantage from the use of MEMS sensors, such as rotational seismology and gravity measurements. In a ten-year outlook, the capability of MEMS sensors will certainly improve through the optimization of existing technologies, the development of new materials, and the implementation of innovative production processes. In particular, the next generation of MEMS seismometers could be capable of reaching a noise floor under the lower seismic noise (few tenths of ng/) and expanding the bandwidth towards lower frequencies (∼0.01 Hz).

(Invited) MEMS Accelerometer Fabricated by Gold Multi-Layer Metal Technology

This paper reviews the feature of our proposed MEMS (microelectromechanical systems) accelerometer and the issue of its reliability. In order to realize micro-G (1 G = 9.8 m/s2) level sensing, we have proposed capacitive MEMS accelerometers with gold proof mass using the multi-layer metal technology. Utilizing gold as a high density material of the proof mass reduces the Brownian noise (BN ), which is a crucial factor to determine the resolution of MEMS accelerometer. As a result, we can obtain the low value of 22 nG/√Hz despite the same size of gold proof mass as silicon proof mass. Regarding the reliability, the heat cycle and the vibration cycle tests are conducted. The experimental results suggest that the MEMS accelerometer has potential for a practical use. In conclusion, we confirm that our proposed MEMS accelerometer can make a contribution to a new technical development.

Monolithic Multi Degree of Freedom (MDoF) Capacitive MEMS Accelerometers.

With the continuous advancements in microelectromechanical systems (MEMS) fabrication technology, inertial sensors like accelerometers and gyroscopes can be designed and manufactured with smaller footprint and lower power consumption. In the literature, there are several reported accelerometer designs based on MEMS technology and utilizing various transductions like capacitive, piezoelectric, optical, thermal, among several others. In particular, capacitive accelerometers are the most popular and highly researched due to several advantages like high sensitivity, low noise, low temperature sensitivity, linearity, and small footprint. Accelerometers can be designed to sense acceleration in all the three directions (X, Y, and Z-axis). Single-axis accelerometers are the most common and are often integrated orthogonally and combined as multiple-degree-of-freedom (MDoF) packages for sensing acceleration in the three directions. This type of MDoF increases the overall device footprint and cost. It also causes calibration errors and may require expensive compensations. Another type of MDoF accelerometers is based on monolithic integration and is proving to be effective in solving the footprint and calibration problems. There are mainly two classes of such monolithic MDoF accelerometers, depending on the number of proof masses used. The first class uses multiple proof masses with the main advantage being zero calibration issues. The second class uses a single proof mass, which results in compact device with a reduced noise floor. The latter class, however, suffers from high cross-axis sensitivity. It also requires very innovative layout designs, owing to the complicated mechanical structures and electrical contact placement. The performance complications due to nonlinearity, post fabrication process, and readout electronics affects both classes of accelerometers. In order to effectively compare them, we have used metrics such as sensitivity per unit area and noise-area product. This paper is devoted to an in-depth review of monolithic multi-axis capacitive MEMS accelerometers, including a detailed analysis of recent advancements aimed at solving their problems such as size, noise floor, cross-axis sensitivity, and process aware modeling.

An Electrical-Stimulus-Only BIST IC for Capacitive MEMS Accelerometer Sensitivity Characterization

Testing and calibration constitute a major part of the overall manufacturing cost of microelectromechanical system (MEMS) devices. A physical-stimulus-free built-in-self-test (BIST) integrated circuit (IC) design characterizing the sensitivity of capacitive MEMS accelerometers is presented. The BIST circuitry can extract the amplitude and phase response of the acceleration sensor’s mechanics under electrical excitation within 0.55% of error with respect to its mechanical sensitivity under physical stimulus. Sensitivity characterization is performed using a low computational complexity multi-variate linear regression model. The BIST circuitry maximizes the use of existing analog and mixed signal readout signal chain and the host processor core, without the need for computationally expensive fast Fourier transform (FFT)-based approaches. The BIST IC is designed and fabricated using the 0.18- $\mu \text{m}$ CMOS technology. The sensor analog front-end and BIST circuitry is integrated with a three axis, low- $g$ capacitive MEMS accelerometer in a single hermetically sealed package. The BIST circuitry occupies 0.3 mm $^{ { {2}}}$ with a total readout IC area of 1 mm $^{ { {2}}}$ , and consumes 8.9 mW during self-test operation.

Damping Ratio Analysis of a Silicon Capacitive Micromechanical Accelerometer

In most MEMS devices, the moving micro-structures are surrounded by air which significantly affects their dynamics behaviors. The correct prediction of the squeeze-film air damping ratio is essential in MEMS (Micro-Electro-Mechanical System) devices design. In the paper, a static test is proposed to measure the squeeze-film air damping ratios of capacitive MEMS accelerometer under different pressures. The unsealed chip of capacitive accelerometer is placed in vacuum extraction equipment and an open loop circuit is developed to apply step signal in the test. By charging the pressure and measuring the overshoot Mp and the settling time ts from the time response of the system, the damping ratio ξ under different pressures can be calculated. Finite element method (FEM) based on the modified Reynolds equation is utilized to simulate the transient response of the micro-structure. Good correlation between experiment and FEM analysis is obtained. The proposed static test in this paper provides a new method to more easily measure the dynamic performances of micro-structures under various pressures.

Design and Implementation of a MASH2-0 Electromechanical Sigma–Delta Modulator for Capacitive MEMS Sensors Using Dual Quantization Method

In this paper, a new control structure based on the dual quantization technique is presented for an electromechanical sigma–delta modulator (EM- $\Sigma \Delta \text{M}$ ) applied to a microelectromechanical system (MEMS) accelerometer. The modulator adopts a 2-0 multistage noise shaping structure (MASH2-0), taking the advantage of the intrinsic linearity of single-bit quantization and the reduced quantization error of multibit quantization in a single modulator. The sensor system is studied by the system-level modeling and the hardware implementation based on the field-programmable gate array technology. The study shows that, MASH2-0 shares the benefits of a MASH2-2 architecture of having an inherent stability, a high overload input level, and a high dynamic range compared with single-loop EM- $\Sigma \Delta \text{M}$ . However, the MASH2-0 architecture benefits from a considerably simpler implementation, while achieving a higher dynamic range and a higher signal-to-noise ratio compared with a MASH2-2 and a fourth-order single-loop $\Sigma \Delta \text{M}$ architecture. A capacitive MEMS accelerometer was designed and employed with this control system. Within a bandwidth of 1 kHz, the sensor achieved a noise floor level of −130 dB and a bias instability as low as $20~\mu \text{g}$ at an integration time of 40 s. Simulation estimated a full scale of ±20 g acceleration. The investigation confirms the concept of the MASH2-0 structure and shows its potential as a closed-loop interface for high-performance capacitive MEMS accelerometers. [2015-0071]

Real time assessment of hand–arm vibration system based on capacitive MEMS accelerometers

Vibrations are a well known potential cause of health diseases and therefore constitute a main concern for the safety of workers in a large number of activities. In order to prevent health hazards, national and international institutions have issued laws and directives which establish recommended limits to the workers’ exposure to vibrations during operations. Consequently, if the amount of adsorbed vibration exceeds the allowable daily limits the worker has to stop his job. Recent wireless sensing and communicating technologies can effectively be employed for such purpose, allowing to develop monitoring customized devices at affordable cost which could be easily employed during the workers’ activity. In such context, the present research proposes an innovative system aimed at estimating the hand–arm exposure to vibration according to the Standard EN ISO 5349-1:2004. In particular the proposed system is based on Micro Electro-Mechanical Systems (MEMS) technology and involves the design of a compact wearable unit to be attached to the waist of the operator and a fixed station for data storage and analysis. The paper reports the design of such a system and an experimental test performed on hand held shakers commonly employed for olive harvesting. The results highlight the opportunities offered by these emerging technologies in developing intelligent devices to be included in the worker’s equipment which might significantly modify the current approach to safety management in the next future.

Missile captive carry monitoring and helicopter identification using a capacitive microelectromechanical systems accelerometer

Military missiles are exposed to many sources of mechanical vibration that can affect system reliability, safety, and mission effectiveness. The US Army Aviation and Missile Research Development and Engineering Center has been developing missile health monitoring systems to assess and improve reliability, reduce life cycle costs, and increase system readiness. One of the most significant exposures to vibration occurs when the missile is being carried by a helicopter or other aviation platform, which is a condition known as captive carry. Recording the duration of captive carry exposure during the missile’s service life can enable the implementation of predictive maintenance and resource management programs. Since the vibration imparted by each class of helicopter varies in frequency and amplitude, tracking the vibration exposure from each helicopter separately can help quantify the severity and harmonic content of the exposure. To help address these needs, the authors have developed a captive carry health monitor for the Hellfire II missile. The captive carry health monitor is an embedded usage monitoring device installed on the outer skin of the Hellfire II missile to record the cumulative hours the host missile has been in captive carry mode. To classify the vibration by class of helicopter, the captive carry health monitor analyzes the amplitude and frequency content of the vibration with the Goertzel algorithm to detect the presence of distinctive rotor harmonics. This article provides an overview of the captive carry health monitor, presents vibration data collected on missiles during captive carry, describes data analysis techniques used to monitor captive carry and identify the class of helicopter, and discusses the potential application of missile health and usage data for real-time reliability analysis. More broadly, this article illuminates the challenges of developing a structural health monitor to classify transportation modes in an unstructured environment.