An overview on the vise of microelectromechanical systems (MEMS) technologies for timing and frequency control is presented. In particular, micromechanical RF filters and reference oscillators based on recently demonstrated vibrating on-chip micromechanical resonators with Q's > 10,000 at 1.5 GHz are described as an attractive solution to the increasing count of RF components (e.g., filters) expected to be needed by future multiband, multimode wireless devices. With Q's this high in on-chip abundance, such devices might also enable a paradigm shift in the design of timing and frequency control functions, where the advantages of high-Q are emphasized, rather than suppressed (e.g., due to size and cost reasons), resulting in enhanced robustness and power savings. Indeed, as vibrating RF MEMS devices are perceived more as circuit building blocks than as stand-alone devices, and as the frequency processing circuits they enable become larger and more complex, the makings of an integrated micromechanical circuit technology begin to take shape, perhaps with a functional breadth not unlike that of integrated transistor circuits. With even more aggressive three-dimensional MEMS technologies, even higher on-chip Q's are possible, such as already achieved via chip-scale atomic physics packages, which so far have achieved Q's > 107 using atomic cells measuring only 10 mm3 in volume and consuming just 5 mW of power, all while still allowing atomic clock Allan deviations down to 10-11 at one hour

/pdf/mems-technology-for-timing-and-frequency-control-4x49gry32c.pdf

MEMS technology for timing and frequency control

This paper reports theoretical analysis and experimental results on a new class of rectangular plate and ring-shaped contour-mode piezoelectric aluminum nitride radio-frequency microelectromechanical systems resonators that span a frequency range from 19 to 656 MHz showing high-quality factors in air (Qmax=4300 at 229.9 MHz), low motional resistance (ranging from 50 to 700 Omega), and center frequencies that are lithographically defined. These resonators achieve the lowest value of motional resistance ever reported for contour-mode resonators and combine it with high Q factors, therefore enabling the fabrication of arrays of high-performance microresonators with different frequencies on a single chip. Uncompensated temperature coefficients of frequency of approximately -25 ppm/degC were also recorded for these resonators. Initial discussions on mass loading mechanisms induced by metal electrodes and energy loss phenomenon are provided

/pdf/piezoelectric-aluminum-nitride-vibrating-contour-mode-mems-o0680bpeg2.pdf

Piezoelectric Aluminum Nitride Vibrating Contour-Mode MEMS Resonators

The importance of thermoelastic damping as a fundamental dissipation mechanism for small-scale mechanical resonators is evaluated in light of recent efforts to design high-Q micrometer- and nanometer-scale electromechanical systems. The equations of linear thermoelasticity are used to give a simple derivation for thermoelastic damping of small flexural vibrations in thin beams. It is shown that Zener’s well-known approximation by a Lorentzian with a single thermal relaxation time slightly deviates from the exact expression.

Thermoelastic Damping in Micro- and Nano-Mechanical Systems.

Pull-in instability as an inherently nonlinear and crucial effect continues to become increasingly important for the design of electrostatic MEMS and NEMS devices and ever more interesting scientifically. This review reports not only the overview of the pull-in phenomenon in electrostatically actuated MEMS and NEMS devices, but also the physical principles that have enabled fundamental insights into the pull-in instability as well as pull-in induced failures. Pull-in governing equations and conditions to characterize and predict the static, dynamic and resonant pull-in behaviors are summarized. Specifically, we have described and discussed on various state-of-the-art approaches for extending the travel range, controlling the pull-in instability and further enhancing the performance of MEMS and NEMS devices with electrostatic actuation and sensing. A number of recent activities and achievements methods for control of torsional electrostatic micromirrors are introduced. The on-going development in pull-in applications that are being used to develop a fundamental understanding of pull-in instability from negative to positive influences is included and highlighted. Future research trends and challenges are further outlined.

Electrostatic pull-in instability in MEMS/NEMS: A review

MEMS-based oscillators are an emerging class of highly miniaturized, batch manufacturable timing devices that can rival the electrical performance of well-established quartz-based oscillators. In this review, a description is given of the key properties of a MEMS resonator that determine the overall performance of a MEMS oscillator. Piezoelectric, capacitive and active resonator transduction methods are compared and their impact on oscillator noise and power dissipation is explained. An overview is given of the performance of MEMS resonators and MEMS-based oscillators that have been demonstrated to date. Mechanisms that affect the frequency stability of the resonator, such as temperature-induced frequency drift, are explained and an overview is given of methods that have been demonstrated to improve the frequency stability. The aforementioned performance indicators of MEMS-based oscillators are benchmarked against established quartz and CMOS technologies.

https://iopscience.iop.org/article/10.1088/0960-1317/22/1/013001/pdf

A review of MEMS oscillators for frequency reference and timing applications


 With enormous amounts of data being generated every day from countless sensors and sensor networks, the need for intelligent devices to process and make use of this data continues to grow and is only projected to increase. The advent of wearable technologies has exacerbated this problem, and with researchers struggling to process data locally with small power budgets, it is clear a solution is needed. Micro-electromechanical (MEMS)-based innovation will have high impact on these issues. MEMS devices can process computing taskes in the hardware level which consumes almost no power (nW). They are very small in size and do the classification without the need of storing the data which boosts up the power saving. Toward this goal, simulation results for a MEMS network to perform basic neural computing is shown in this paper. The network is made up of a mechanically connected network of three electrostatically controlled microstructures, two of which serve as input layers and the third as output (computing) layers. The mechanical coupling was achieved through stiffnesses connecting the masses of the MEMS. It has been demonstrated that such a device may be programmed to distinguish between a ramp (gradually growing) input signal and a step (abruptly rising) by applying suitable bias voltages to the electrostatic control electrodes. The findings serve as a proof of concept and founding to completing more sophisticated computational tasks using MEMS and opening a new direction for alternative efficient computing technologies compared to current digital computing.

A Three Degree of Freedom Model Approach to Enable a MEMS-Based Neural Computing Unit


 The next frontier of MEMS applications is their use as computing units to harness data at the sensor level. In particular, MEMS intelligent computing unit has the unique promise of significantly increasing energy efficiency while simultaneously increasing data processing speeds and eliminating data latency in many applications such as wearable devices. Along this line, this paper presents a demonstration of the first simulated microelectromechanical (MEMS) network capable of classifying real-life experimental data based on acceleration measurement to distinguish between sitting and standing behaviors without the need for any computing or processing unit. The MEMS network is made of four MEMS; two MEMS in the input layer and two in the output layer. The first MEMS in the input layer is responsible for detecting a rising edge acceleration and allows the first MEMS in the output layer to be triggered at a following falling edge of the acceleration signal. This signature corresponds to the sitting activity. On the other hand, the second MEMS in the input layer is responsible for detecting the falling edge of the acceleration signal and allows the second output MEMS to declare a standing activity if it detects a following falling edge signal. This work demonstrates the potential of distinguishing between the sit and stand activities without any computing unit.

A Small MEMS Neural Network to Classify Human Sitting and Standing Activities

https://link-springer-com-443.webvpn.jmu.edu.cn/content/pdf/10.1007%2F978-94-007-6178-0_308-2.pdf

Thermally Actuated Silicon Resonators

http://ieeexplore.ieee.org/iel5/9868/31227/01453881.pdf

Low-motional-impedance higheftunable i2 resonators for temperature-

The paper describes a 4-MHz temperature compensated reference oscillator based on a capacitive silicon micro-mechanical resonator. The design of the resonator has been optimized to offer large quality factors (22000), while maintaining tunability in excess of 3000 ppm for fine tuning and temperature compensation. Oscillations are sustained with a CMOS amplifier and temperature compensation is performed with a novel resonator bias generator. When interfaced with the bias circuit, the oscillator exhibits a temperature drift of 380 ppm over a 90/spl deg/C range, a 6 times improvement in stability over an uncompensated oscillator. The sustaining amplifier and compensation circuitry were fabricated in a 2P3M 0.5 /spl mu/m CMOS process. The oscillator is designed to prototype highly stable, low phase-noise reference oscillators integrated at the chip or package level.

A two-chip, 4-MHz, microelectromechanical reference oscillator

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