Z-Axis Optomechanical Accelerometer

We demonstrate a z-axis accelerometer which uses waveguided light to sense proof mass displacement. The accelerometer consists of two stacked rings (one fixed and one suspended above it) forming an optical ring resonator. As the upper ring moves due to z-axis acceleration, the effective refractive index changes, changing the optical path length and therefore the resonant frequency of the optical mode. The optical transmission changes with acceleration when the laser is biased on the side of the optical resonance. This silicon nitride "Cavity-enhanced OptoMechanical Accelerometer" (COMA) has a sensitivity of 22 percent-per-g optical modulation for our highest optical quality factor (Q_o) devicesComment: Published in Proceedings of the 25th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2012), Paris, France, January 29 - Feb 2, 2012, pp. 615-61

Monolithic sensor integration in CMOS technologies

© 2023 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Besides being mainstream for mixed-signal electronics, CMOS technology can be used to integrate micro-electromechanical system (MEMS) on a single die, taking advantage of the structures and materials available in feature sizes around 180 nm. In this article, we demonstrate that the CMOS back-end-of-line (BEOL) layers can be postprocessed and be opportunistically used to create several kinds of MEMS sensors exhibiting good or even excellent performance, such as accelerometers, pressure sensors, and magnetometers. Despite the limitations of the available mechanical and material properties in CMOS technology, due to monolithic integration, these are compensated by the significant reduction of parasitics and system size. Furthermore, this work opens the path to create monolithic integrated multisensor (and even actuator) chips, including data fusion and intelligent processing.This work was supported in part by Baolab Microsystems; in part by the Spanish Ministry of Science, Innovation and Universities (MCIN); in part by the State Research Agency (AEI); in part by the European Social Fund (ESF) under Project RTI2018-099766-B-I00; in part by MCIN/AEI/10.13039/501100011033 under Grant PID2021-123535OB-I00; and in part by ERDF, “A way of making Europe.” The associate editor coordinating the review of this article and approving it for publication was Prof. Jean-Michel Redoute.Peer ReviewedPostprint (author's final draft

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

A magneto-mechanical accelerometer based on magnetic tunnel junctions

Accelerometers have widespread applications and are an essential component in many areas such as automotive, consumer electronics and industrial applications. Most commercial accelerometers are based on micro-electromechanical system (MEMS) that are limited in downscaling and power consumption. Spintronics-based accelerometers have been proposed as alternatives, however, current proposals suffer from design limitations that result in reliability issues and high cost. Here we propose spintronic accelerometers with magnetic tunnel junctions (MTJs) as building block, which map accelerations into a measurable voltage across the MTJ terminals. The device exploits elastic and dipolar coupling as a sensing mechanism and the spintronic diode effect for the direct read out of the acceleration. The proposed technology represents a potentially competitive and scalable solution to current capacitive MEMS-based approaches that could lead to a step forward in many of the commercial applications.Comment: main document with 4 figures + supplemental informatio

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

UV-LIGA micro-fabrication of inertia type electrostatic transducers and their application

This dissertation discusses the design, working principles, static & dynamic analysis and simulation, mechanics of material, applied MEMS technology, micro-fabrication, and experimental testing of two types of micro-transducers: micro-power relay and micro-accelerometer. Several possible design concepts were proposed, and the advantages and disadvantages of electrostatic working principles were also discussed. Transducers presented in this research used electrostatic force as a driving force in the micro-relay and capacitance as a sensing parameter in the micro-accelerometer. There was an analogy between the micro-relay and the micro-accelerometer in their theoretical approach and fabrication processes. The proposed micro-transducers (micro-relay and micro-accelerometer) were fabricated using UV lithograph of SU-8 & SPR and UV-LIGA process. The advantages and disadvantages of these processes were discussed. The micro-relays fabricated by UV-LIGA technology had the following advantages compared with other reported relays: fast switching speed, high power capacity, high off-resistance, lower on-resistance, low power consumption, and low heat generation. The polymer-based micro-accelerometers were designed and fabricated. Instead of applying SU-8 only as a photo resist, cured SU-8 was used as the primary structural material in fabricating the micro-accelerometers. The great flexibility in size and aspect ratio of cured SU-8 made it feasible to produce highly sensitive accelerometers. The prototype micro-relays and micro-accelerometers were tested for the dynamic characteristics and power capacity. The experimental results in micro-relays had confirmed that reasonably large current capacity and fast response speed was able to be achieved using electromagnetic actuation and the multilayer UV-LIGA fabrication process

Thermomechanical noise of arrayed capacitive accelerometers with 300-NM-gap sensing electrodes

2017 19th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), 18-22 June 2017.Thermomechanical noise of arrayed capacitive accelerometers with sub-micrometer sensing electrodes was evaluated. The unit accelerometer of the array was 80-μm square, designed as a proportional scale-down of a conventional single-axis accelerometer. Since the size effect shows the capacitance sensitivity per unit volume increases by proportional downsizing, a 10-by-10 array of the one-tenth sized unit accelerometer would have the same sensitivity of a single accelerometer of same occupied area. However, the thermomechanical noise needs to be controlled and reduced by vacuum encapsulation because size reduction causes noise increase. By measuring the electrical impedance at the resonant frequency, the damping coefficient was estimated using electrical equivalent circuit modeling. The estimated thermomechanical noise was reduced below 3 μg√VHZ by encapsulating at 100 Pa, which is low enough for instrumentation applications

Optimal and Robust Design Method for Two-Chip Out-of-Plane Microaccelerometers

In this paper, an optimal and robust design method to implement a two-chip out-of-plane microaccelerometer system is presented. The two-chip microsystem consists of a MEMS chip for sensing the external acceleration and a CMOS chip for signal processing. An optimized design method to determine the device thickness, the sacrificial gap, and the vertical gap length of the M EMS sensing element is applied to minimize the fundamental noise level and also to achieve the robustness to the fabrication variations. In order to cancel out the offset and gain variations due to parasitic capacitances and process variations, a digitally trimmable architecture consisting of an 11 bit capacitor array is adopted in the analog front-end of the CMOS capacitive readout circuit. The out-of-plane microaccelerometer has the scale factor of 372 mV/g∼389 mV/g, the output nonlinearity of 0.43% FSO∼0.60% FSO, the input range of ±2 g and a bias instability of 122 μg∼229 μg. The signal-to-noise ratio and the noise equivalent resolution are measured to be 74.00 dB∼75.23 dB and 180 μg/rtHz∼190 μg/rtHz, respectively. The in-plane cross-axis sensitivities are measured to be 1.1%∼1.9% and 0.3%∼0.7% of the out-of-plane sensitivity, respectively. The results show that the optimal and robust design method for the MEMS sensing element and the highly trimmable capacity of the CMOS capacitive readout circuit are suitable to enhance the die-to-die uniformity of the packaged microsystem, without compromising the performance characteristics

Single- and dual-axis lateral capacitive accelerometers based on CMOS-MEMS technology

In order to have a compatible device in a sensor node in wireless sensor network, the sensors have to be made in micro-size, low cost, low power consumption and high performance. By using CMOS-MEMS technology, the micro sensors can be implemented with promising results. One of the central micro inertial sensors is an accelerometer which has the capability of sensing position change, vibration and shock of a device. A single-axis lateral capacitive accelerometer and a dual-axis in-plane capacitive accelerometer are made in this thesis. An alternative design of the single-axis accelerometer is discussed. The system designs are made through mathematical analysis in MatLab, 3D FEM simulation in CoventorWare and final layout in Cadence. The main issue is making compliant springs, large proof mass, considerable number of comb fingers, for fabricating micro sensors with high sensitivity and good noise performance. The single-axis lateral capacitive accelerometer has sensor sensitivity of 9.3mV/G, mechanical noise floor of 19uG/squareroot(Hz), linear measuring range of ±26G. The dual-axis in-plane capacitive accelerometer has sensor sensitivity of 9.3mV/G in one direction and 11.1mV/G in the cross direction. The chip is fabricated in a 0.25um BiCMOS process from STMicroelectronics. The post process is done at Carnegie Mellon University (CMU), USA and SINTEF MiNaLab, Norway