Silicon Microelectromechanical Systems Research Articles

Active optical phased array integrated within a micro-cantilever

Three dimensional sensing is essential in order that machines may operate in and interact with complex dynamic environments. Solid-state beam scanning devices are seen as being key to achieving required system specifications in terms of sensing range, resolution, refresh rate and cost. Integrated optical phased arrays fabricated on silicon wafers are a potential solution, but demonstrated devices with system-level performance currently rely on expensive widely tunable source lasers. Here, we combine silicon nitride photonics and micro-electromechanical system technologies, demonstrating the integration of an active photonic beam-steering circuit into a piezoelectric actuated micro cantilever. An optical phased array, operating at a wavelength of 905 nm, provides output beam scanning over a range of 17° in one dimension, while the inclination of the entire circuit and consequently the angle of the output beam in a second dimension can be independently modified over a range of up to 40° using the piezoelectric actuator.

Five Low-Noise Stable Oscillators Referenced to the Same Multimode AlN/Si MEMS Resonator.

We report on the first experimental demonstration of five self-sustaining feedback oscillators referenced to a single multimode resonator, using piezoelectric aluminum nitride on silicon (AlN/Si) microelectromechanical systems (MEMS) technology. Integrated piezoelectric transduction enables efficient readout of five resonance modes of the same AlN/Si MEMS resonator, at 10MHz, 30MHz, 65MHz, 95MHz, and 233MHz with quality (Q) factors of 18600, 4350, 4230, 2630, and 2138, respectively. Five stable self-sustaining oscillators are built, each referenced to one of these high-Q modes, and their mode-dependent phase noise and frequency stability (Allan deviation) are measured and analyzed. The 10, 30, 65, 95 and 233MHz oscillators exhibit low phase noise of -116, -100, -105, -106 and -92dBc/Hz at 1kHz offset frequency, respectively. The 65MHz oscillator yields Allan deviation of 4×10-9 and 2×10-7 at 1s and 1000s averaging time, respectively. The 10MHz oscillator's low phase noise holds strong promise for clock and timing applications. The five oscillators' overall promising performance also suggests suitability for multimode resonant sensing and tracking. This work also elucidates mode dependency in oscillator noise and stability, one of the key attributes of mode-engineerable resonators.

Mechanically Coupled Single-Crystal Silicon MEMS Resonators for TCF Manipulation

This paper presents mechanically coupled single-crystal silicon (SCS) microelectromechanical system (MEMS) resonators for temperature coefficient of frequency (TCF) manipulation. The reported mechanically coupled square extensional (SE) mode resonator consists of two square plates coupled with a beam or a third square plate purposely vibrating in another bulk mode. The SE mode resonators and the coupled structure can be considered as two kinds of individual resonators with different TCFs. Within the specific dimensions of the coupled resonator, the resonant frequencies of the two components are assumed to be equal. Hence, the TCF of the mechanically coupled resonator could be manipulated by adjusting the sizes of the coupling parts. The design and fabrication of coupled resonators have been completed in a degenerate-doped (100) SCS wafer. Measurement results illustrate that the TCFs of the coupled resonators can be purposely manipulated by adjusting the volume and equivalent spring constant ratios of the individual resonators. The minimal measured frequency shift is roughly <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 10 ppm for an 18 MHz coupled SE mode resonator, over the entire industrial temperature range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 40 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C to 85 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C. This work highlights a novel and effective way to manipulate the TCFs of MEMS resonators via mechanical coupling. 2022-0197

Automatic Pico Laser Trimming System for Silicon MEMS Resonant Devices Based on Image Recognition

Laser trimming promises an increasing yield in microfabrication for the high performance Micro Electro-Mechanical Systems (MEMS) resonant devices by overcoming manufacturing tolerances and tuning sensors or actuators for certain operating conditions. How to achieve a high precision and efficiency trimming at low cost is the critical issue that the technical communities are most concerned. In this paper, we proposed an automatic laser trimming system for silicon MEMS devices based on a picosecond laser source and image recognition technique (IRT). Through the design of laser focusing optical path and coaxial imaging optical path, the observation of laser machining process is realized. A set of picosecond laser trimming parameters is determined by experiments, which enables a 4um width trench on the silicon resonator. Compared with the femtosecond laser trimming systems, the cost of the pico-laser system is reduced by more than 60%. The shape matching algorithm based on OpenCV and the chord-to-point distance accumulation (CPDA) algorithm for MEMS resonators image measurement solve the problem of inefficiency of manual trimming for complex resonators. A dual-mass tuning fork resonator is used to demonstrate the feasibility of the trimming approach. It is shown that the system realizes a fine-tuning (< 0.3Hz) of the resonant frequencies. The trimming time of a single device using the automatic trimming method is reduced from more than 1 hour manual trimming to less than 1 minute. Finally, the structural stiffness imbalance of tuning fork resonator caused by the manufacturing error is eliminated, which verifies the feasibility of the system.

Wafer level hermetic bonding and packaging using recrystallized parylene

This work reports a wafer-level vacuum packaging technique for microelectromechanical system (MEMS) using recrystallized parylene material as a bonding layer. The effect of thermal annealing on the crystallinity of the parylene surface was demonstrated. The low-temperature, stable, homogeneous, and defect-free recrystallized parylene is found as an excellent bonding material for hermetic packages, suitable for the packaging of MEMS sensors. The material’s recrystallization improves its capabilities as a moisture and air barrier. The mechanical stability of the bond interface was also investigated by measuring the package’s tensile and shear strength. In the absence of a vacuum bonding tool, a Ti getter was integrated into the cavity of the glass capping wafer to create the vacuum and test the hermeticity. The MEMS silicon Pirani gauge was used to monitor the pressure changes inside the sealed cavity. After the getter activation, it was observed that there was a decrease in the pressure from atmospheric pressure to 0.2 mbar for the first few days; after that, no noticeable change was observed. The hermeticity of the packaged device was examined, and the vacuum level inside the package remained the same for the last 70 d. The recrystallized parylene bonded micro package shows better hermeticity than the non-recrystallized parylene micro package.

Digital Control and Readout of MEMS Gyroscope Using Second-Order Sliding Mode Control

Microelectromechanical system (MEMS) is a kind of MEMS that has been utilized to monitor the angular speed of moving objects on several occasions and has attracted the interest of several organizations across the globe. This type of sensor has the benefits of excellent integration, cheap cost, and low power utilization. This study begins by describing the research and development of silicon MEMS gyroscopes since the 1980s, including the work of several universities, such as Draper Laboratory and UC Berkeley, as well as various design ideas, control systems, and architectures. Matching resonance frequencies of the driving and sense mechanical resonators may significantly improve the efficiency of MEMS gyroscopes. In a closed-loop readout circuit, however, exact control of the resonance frequency is difficult. Because of their inexpensive production costs in large numbers, MEMS gyroscopes are gaining appeal. Because of substantial nonstationary noise processes in the output of MEMS gyros, such as 1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}$ </tex-math></inline-formula> noise, this creates difficulty for navigation system designing. In the implementation, relative error in ramp and step function is determined through the proposed methodology. Furthermore, the second-order sliding mode control (SoSMC) is used in the sliding mode of the gyroscope to minimize the chattering effect.

Evaluation of Polymer Materials for MEMS Microphones

Polymers are attractive materials for the fabrication of micro-electro-mechanical system (MEMS) microphones because of their low Young’s moduli, generally lower processing temperatures than silicon, and simple fabrication; such microphones are sensitive and economical. However, an earlier polymer diaphragm revealed lower sensitivity than expected. In addition, the viscoelastic properties of polymer diaphragm have been considered to degrade the sensing performance due to a high-level mechanical noise. Here, we investigated the possibility of the polymer material as a MEMS microphone in terms of stiffness and damping. Stiffness was analyzed and experimentally verified. The obtained polymer diaphragm damping coefficient was compared with the coefficient of a silicon MEMS microphone to explore whether performance was degraded by material damping. All samples were fabricated using a thin film transfer method. SU-8 was mainly used but Polymethyl methacrylate (PMMA) was also selected for comparing the material damping effect. The measured residual stress of an SU-8 diaphragm was 18 MPa; the measured flexibility was similar to the state-of-art of a silicon-based MEMS microphone, and it was proven that there is room for further improvement through process optimization. The damping effect induced by viscoelasticity was very small, compared with the effect of air damping; indeed, the intrinsic damping effect of the polymer on device performance was negligible. [2021-0213]

Experimental Realization of 16-Pixel Terahertz Receiver Front-End Based on Bulk Silicon MEMS Power Divider and AlGaN/GaN HEMT Linear Detector Array

A 16-pixel terahertz (THz) receiver front-end working at room temperature was designed, built, and measured in this paper. The designed receiver front-end is based on the antenna-coupled AlGaN/GaN high-electron-mobility transistor (HEMT) THz linear detector array (TeraLDA) and a 16-way THz power divider. The local oscillator (LO) signal is divided by the power divider into 16 ways and transmits to the TeraLDA. Each detector contains a planar unified antenna printed on a 150 μm-thick sapphire substrate and a transistor fabricated on AlGaN/GaN heterostructure. There are 16 silicon hemispheric lenses located on the TeraLDA to increase the responsivity of the TeraLDA. The focus of each lens is aligned in the center of the TeraLDA pixels. Depending on different read out circuits, the receiver front-end could work in homodyne and heterodyne modes. The 16-way power divider is a four-stage power divider that consists of fifteen same 2-way dividers, and was fabricated by bulk silicon microelectromechanical systems (MEMS) technology to achieve low insertion loss (IL). This designed receiver front-end could be a key component of a THz coherent focal plane imaging radar system, that may play a crucial role in nondestructive 3D imaging application.

Integrated 1 × 3 MEMS silicon nitride photonics switch.

We present a 1 × 3 optical switch based on a translational microelectromechanical system (MEMS) platform with integrated silicon nitride (SiN) photonic waveguides. The fabricated devices demonstrate efficient optical signal transmission between fixed and suspended movable waveguides. We report a minimum average insertion loss of 4.64 dB and a maximum average insertion loss of 5.83 dB in different switching positions over a wavelength range of 1530 nm to 1580 nm. The unique gap closing mechanism reduces the average insertion loss across two air gaps by a maximum of 7.89 dB. The optical switch was fabricated using a custom microfabrication process developed by AEPONYX Inc. This microfabrication process integrates SiN waveguides with silicon-on-insulator based MEMS devices with minimal stress related deformation of the MEMS platform.

Thickness dependence of the response of metal-dioxide-based micromechanical sensors for sensitive gamma-ray detection

In this study, we investigated the effect of the thickness of metal-dioxide thin films on silicon (Si) micro-electromechanical systems (MEMS) for sensitive, accurate, and reproducible detection of gamma-radiation dose. Silicon wafers and microcantilevers were coated with various thicknesses of titanium dioxide (TiO2) thin films and controlled by the number of cycles in atomic layer deposition (ALD) under 17-mbar pressure and temperature of 200 °C. All samples were exposed to different doses of gamma (0, 10, and 20 kGy) using a60Co source. Before and after gamma irradiation, the optical and mechanical properties of the TiO2 thin films on Si wafers were studied by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). The resonance frequency shift (RFS) resulting from exposing different thicknesses of TiO2 thin films on MEMS-based cantilevers to gamma-radiation doses were evaluated by AFM. SE results revealed the film thicknesses of 11.91, 21.77, 62.91, and 218.23 nm at 250, 500, 1250, and 2500 coating cycles, respectively. Through SE, other optical constants, such as surface roughness (SR) and refractive index (n), were obtained. The root-mean-square (RMS) SR obtained from AFM images and SE measurements on Si wafers showed the same behavior under gamma radiation according to the TiO2 thin-films thicknesses. The RFS results show that the best film thickness for reproducible and sensitive gamma-radiation detection was 218.23 nm. The frequency shift as the gamma dose increase from 0 to 20 kGy was ∼60 Hz. It is a very palpable and linear shift; hence, it can be used as a dosimeter sensor. The results were verified by the statistical correlation coefficient method to find the correlation between RFS and the gamma dose at different film thicknesses. Correlation results are consistent with other results.

Temperature frequency stability study of extensional mode N-doped silicon MEMS resonator

Analytical and numerical modeling techniques are presented to predict n-type-doped silicon resonators’ temperature coefficient frequencies for the extensional mode of vibration by modeling of single-crystal silicon ([100] and [110]) directions. We utilized a previously reported empirical exponential expression for the uniaxial deformation potential that allows us to determine the deformation potential constant. It was observed that by using the empirical model of exponential expression for the uniaxial deformation potential, our result shows that the frequency variation with temperature is closely dependent on the direction and the variation of the different doping levels. We apply our analytical model to the [100] and [110] length extensional modes of a Microelectromechanical Systems (MEMS) resonator. We calculated the resonance frequency with temperature dependence and found that the first-order temperature coefficient frequency shows zero values at doping concentration levels of 0.5 and 1 × 1019 cm−3, which indicates the existence of frequency turnover at room temperature. We also extended our study by finite element analysis of MEMS resonators to investigate the mode shapes and resonant frequency for different aspect ratios in the extensional mode of vibration. It is observed that the mode shapes are more stressed with higher aspect ratios, and the frequency variation (ppm) of the resonance frequency between the [100] and [110] crystal directions of the MEMS resonator increases with increasing aspect ratios.

Effects of material and dimension on TCF, frequency, and Q of radial contour mode AlN-on-Si MEMS resonators

This paper investigates the effects of material and dimension parameters on the frequency splitting, frequency drift, and quality factor (Q) of aluminium nitride (AlN)-on-n-doped/pure silicon (Si) microelectromechanical systems (MEMS) disk resonators through analysis and simulation. These parameters include the crystallographic orientation, dopant, substrate thickness, and temperature. The resonators operate in the elliptical, higher order, and flexural modes. The simulation results show that i) the turnover points of the resonators exist at 55 ​°C, –50 ​°C, 40 ​°C, and –10 ​°C for n-doped silicon with the doping concentration of 2 ​× ​1019 ​cm–3 and the Si thickness of 3.5 ​μm, and these points are shifted with the substrate thickness and mode variations; ii) compared with pure Si, the modal-frequency splitting for n-doped Si is higher and increases from 5% to 10% for all studied modes; iii) Q of the resonators depends on the temperature and dopant. Therefore, the turnover, modal-frequency splitting, and Q of the resonators depend on the thickness and material of the substrate and the temperature. This work offers an analysis and design platform for high-performance MEMS gyroscopes as well as oscillators in terms of the temperature compensation by n-doped Si.

Quantification of Energy Dissipation Mechanisms in Toroidal Ring Gyroscope

We present a study on the quantification of energy dissipation of Micro-Electro-Mechanical System (MEMS) resonators. Toroidal Ring Gyroscope (TRG) was used as a platform to conduct the study. The main energy dissipation mechanisms in TRG include viscous air damping, Thermo-Elastic Damping (TED), anchor loss, and surface loss. During our experimental study, these energy dissipation mechanisms were minimized and controlled by venting the encapsulation and actively pumping down to high vacuum, cooling the temperature down to around 123 K, adjusting modal balance by electrostatic tuning, and pre-baking the device at high temperature (425 °C), respectively. At room temperature, the quality factor related to viscous air damping was measured to be 625,000, TED to be 170,000, and anchor loss to be 1,350,000. Finite Element Analysis (FEA) was conducted to support these findings. Relation between the anchor loss and electrostatic tuning was also explored. The effects of moisture-related surface loss have also been demonstrated by monitoring characteristics over a 2-year period of time. High temperature bake-out was proven to be effective in removing the moisture and reducing the surface loss. This paper combines topics that are scattered in literature on identification of energy dissipation mechanisms in kilohertz-range silicon MEMS resonators and presents the topic as a single methodology illustrating how the contribution of each energy dissipation mechanism can be quantified independently. To the best of our knowledge, this study is the first to experimentally quantify all major energy dissipation mechanisms in a kilohertz-range silicon MEMS resonator. [2020-0294]

Exploiting the properties of TiO2 thin films as a sensing layer on (MEMS)-based sensors for radiation dosimetry applications

In this work, we investigate the potential of exploiting TiO2 thin films as sensing layers on silicon micro-electromechanical systems for the detection of gamma radiations. All samples are exposed to gamma rays produced by 60Co, with different doses ranging from 0 kGy to 40 kGy. Properties of silicon coated with a 200-nm-thick layer of TiO2 grown at 200 °C by atomic layer deposition are studied before and after its gamma irradiation using x-ray diffraction (XRD), scanning electron microscopy, and spectroscopic ellipsometry. Atomic force microscopy (AFM) is carried out on functionalized microcantilevers to measure the resonance frequency shift (Δf 0) resulting from irradiation of the TiO2 thin film. XRD results show a change in the films from a mixture of rutile and anatase phases to an anatase phase upon irradiation. Spectroscopic ellipsometry results show a change with a fixed pattern in the film thickness, roughness, void, and optical constants with different irradiation doses. This pattern appears as Δf 0 in AFM, where the response of sensors to doses between 0 kGy and 20 kGy was linear. The values of Δf 0 are convenient to control parameters for the proposed dosimeter, which is characterized by the reproducibility and sensitivity of measurements. The maximum detectable linear effect of the proposed dosimeter was found at a dose of 20 kGy. This makes a 200-nm thin layer of TiO2 coated on a microcantilever surface, a possible candidate for dosimetry for the range lower than 20 kGy applications, such as in personal dosimeters.

Enhancement of the Start-Up Time for Microliter-Scale Microbial Fuel Cells (µMFCs) via the Surface Modification of Gold Electrodes.

Microbial Fuel Cells (MFCs) are biological fuel cells based on the oxidation of fuels by electrogenic bacteria to generate an electric current in electrochemical cells. There are several methods that can be employed to improve their performance. In this study, the effects of gold surface modification with different thiol molecules were investigated for their implementation as anode electrodes in micro-scale MFCs (µMFCs). Several double-chamber µMFCs with 10.4 µL anode and cathode chambers were fabricated using silicon-microelectromechanical systems (MEMS) fabrication technology. µMFC systems assembled with modified gold anodes were operated under anaerobic conditions with the continuous feeding of anolyte and catholyte to compare the effect of different thiol molecules on the biofilm formation of Shewanella oneidensis MR-1. Performances were evaluated using polarization curves, Electrochemical Impedance Spectroscopy (EIS), and Scanning Electron Microcopy (SEM). The results showed that µMFCs modified with thiol self-assembled monolayers (SAMs) (cysteamine and 11-MUA) resulted in more than a 50% reduction in start-up times due to better bacterial attachment on the anode surface. Both 11-MUA and cysteamine modifications resulted in dense biofilms, as observed in SEM images. The power output was found to be similar in cysteamine-modified and bare gold µMFCs. The power and current densities obtained in this study were comparable to those reported in similar studies in the literature.

Simulation and analysis of P(VDF-TrFE) cantilever-beams for low frequency applications

The work presented here describes a structural design of piezoelectric co-polymer P(VDF-TrFE) cantilever-beams for very low frequency applications; the design is based on silicon bulk-micromachining and micro-electromechanical systems technology. COMSOL simulation software has been used to study the mechanical and electrical behavior of cantilever-beams. The dimensions of the beams designed are: 3mm×0.6mm×5μm, 5mm×1mm×5μm and 10mm×3mm×5μm. The configuration of the cantilever-beam comprises of an active layer of piezoelectric P(VDF-TrFE) with chrome-gold interdigitated electrodes for electrical signal output generated due to vibration of piezoelectric beams. Simulation results show that the cantilever-beam of dimension 10mm×3mm×5μm has a resonant frequency of 42.68 Hz, indicating that P(VDF-TrFE) is a favorable piezoelectric material for low and very low frequency applications.

Expanding Bias-instability of MEMS Silicon Oscillating Accelerometer Utilizing AC Polarization and Self-Compensation.

This paper presents a MEMS (Micro-Electro-Mechanical System) Silicon Oscillating Accelerometer (SOA) with AC (alternating current) polarization to expand its bias-instability limited by the up-converted 1/f noise from front-end transimpedance amplifier (TIA). In contrast to the conventional DC (direct current) scheme, AC polarization breaks the trade-off between input transistor gate size and white noise floor of TIA, a relative low input loading capacitance can be implemented for low noise consideration. Besides, a self-compensation technique combining polarization source and reference in automatic-gain-control (AGC) is put forward. It cancels the 1/f noise and drift introduced by the polarization source itself, which applies to both DC and AC polarization cases. The experimental result indicates the proposed AC polarization and self-compensation strategy expand the bias-instability of studied SOA from 2.58 μg to 0.51 μg with a full scale of ± 30 g, a 155.6 dB dynamic range is realized in this work.

Millimeter-Wave Air-Filled Slot Antenna With Conical Beam Based on Bulk Silicon MEMS Technology

In this communication, a millimeter-wave (mm-wave) air-filled slot antenna with a conical beam at 60 GHz is proposed. The antenna is suitable to be integrated with various IC systems mainly because it is designed and fabricated based on bulk silicon microelectromechanical systems (MEMS) technology. The whole structure is composed of an antenna part and a waveguide to substrate integrated waveguide (SIW) converter. In the antenna part, four meander slots are arranged annularly and excited by a square air cavity. The conical radiation pattern benefits from the out-phase ${E}$ -field excited on slots. An operating bandwidth of 2.17% for S11 < −10 dB and a conical beam peak gain of 3.21 dBi are achieved. The proposed antenna shows its potentials in mm-wave indoor communication.

FR4-based electromagnetic scanning micro-grating integrated with an angle sensor for a low-cost NIR micro-spectrometer.

Aiming to implement a low-cost single-photodetector-based NIR micro-spectrometer, we present a novel flame retardant 4 (FR4)-driven micro-grating for spectral dispersion and spatial scanning. It consists of a silicon blazed grating directly bonded onto the FR4 actuator platform and an integrated differential electromagnetic angle sensor. Owing to the better shock and vibration reliability, larger aperture, shorter fabrication cycle, and much lower cost, it may be a potential alternative to conventional silicon microelectromechanical systems scanning micro-gratings. The micro-spectrometer prototype based on this device shows a spectral range of 800-1800nm, a spectral accuracy of ±1.3 nm, and a 10nm spectral resolution.

A Renewal Weakest-Link Model of Strength Distribution of Polycrystalline Silicon MEMS Structures

Abstract Experimental data have made it abundantly clear that the strength of polycrystalline silicon (poly-Si) microelectromechanical systems (MEMS) structures exhibits significant variability, which arises from the random distribution of the size and shape of sidewall defects created by the manufacturing process. Test data also indicated that the strength statistics of MEMS structures depends strongly on the structure size. Understanding the size effect on the strength distribution is of paramount importance if experimental data obtained using specimens of one size are to be used with confidence to predict the strength statistics of MEMS devices of other sizes. In this paper, we present a renewal weakest-link statistical model for the failure strength of poly-Si MEMS structures. The model takes into account the detailed statistical information of randomly distributed sidewall defects, including their geometry and spacing, in addition to the local random material strength. The large-size asymptotic behavior of the model is derived based on the stability postulate. Through the comparison with the measured strength distributions of MEMS specimens of different sizes, we show that the model is capable of capturing the size dependence of strength distribution. Based on the properties of simulated random stress field and random number of sidewall defects, a simplified method is developed for efficient computation of strength distribution of MEMS structures.