Conventional Microelectromechanical Systems Research Articles

(Invited) SAE-MEMS Technology for Electrostatic Vibrational Energy Harvesters

Energy harvesting [1] is expected to be a self-powering technology for small wireless sensor nodes in the next generation Internet-of-Things (IoT). In particular, small vibrational energy harvesters (VEHs) based on micro-electro-mechanical systems (MEMS) technology, which are battery-free and can generate electricity even at night or in the dark, are being investigated worldwide for next-generation power sources for small wireless IoT nodes [2,3]. Electrostatic MEMS VEHs, which use semi-permanent charge-holding dielectrics called "electrets," are useful for power generation using environmental vibrations because they have a higher output power density at lower frequencies than other methods [4]. Further miniaturization, higher performance, and higher productivity will be required to use electret-type MEMS VEHs as power sources for compact wireless IoT nodes and other devices. Although the integration of MEMS VEHs with power management circuits enables miniaturization, higher performance, and improved productivity, the monolithic integration of electret-type MEMS VEHs and electronic circuits is an unexplored area in conventional technology. Conventional electrostatic MEMS VEHs involve corona discharge, electron beam irradiation, X-ray irradiation, or high-temperature treatment as electret charge processing, which imposes restrictions on the design and manufacturing of MEMS structures and the integration of MEMS and electronic circuits. Recently, self-assembled electrets (SAEs), which are electrets that do not require any charging process, have been developed using materials for organic electroluminescent devices [5]. The surface charge density of SAEs is equivalent to those of previously reported electrets for VEHs, and thus SAE is expected to improve the performance of VEH and simplify the manufacturing process. However, there have been no reports of MEMS devices using SAEs.Our proposed method is the first technology to form SAEs inside MEMS structures using only a vacuum evaporation process at room temperature, and we have succeeded in actual vibration power generation [6]. This technology can be integrated into existing semiconductor processes because the electret formation requires only room-temperature deposition, making it possible to monolithically integrate electret-type MEMS VEHs and electronic circuits on the same substrate. In addition, since the SAE surface potential is proportional to the film thickness, the amount of electricity generated can be further increased by increasing the thickness of the SAE film. Therefore, this technology is expected to accelerate the miniaturization, performance, and productivity improvement of MEMS environmental vibration power generation devices. Akinaga, “Recent advances and future prospects in energy harvesting technologies,” Jpn. J. Appl. Phys. 59 (2020) 110201.D. Mitcheson, E. M. Yeatman, G. K. Rao, A. S. Holmes, and T. C. Green, “Energy Harvesting from Human and Machine Motion for Wireless Electronic Devices” Proc. IEEE 96 (2008) 1457–1486.Suzuki, “Recent progress in MEMS electret generator for energy harvesting,” IEEJ Trans. Elec. Electron. Eng. 6 (2011) 101-111.Toshiyoshi, S. Ju, H. Honma, C.-H. Ji, and H. Fujita, “MEMS vibrational energy harvesters,” Sci. Technol. Adv. Mater. 20 (2019) 124-143.Tanaka, N. Matsuura, and H. Ishii, “Self-Assembled Electret for Vibration-Based Power Generator,” Sci. Rep. 10 (2020) 6648.Yamane, H. Kayaguchi, K. Kawashima, H. Ishii, and Y. Tanaka, “MEMS post-processed self-assembled electret for vibratory energy harvesters,” Appl. Phys. Lett. 119 (2021) 254102.

Investigating capacitive force sensors with 3D printed flexible structures as dielectric layers

The utilization of a three dimension (3D) printing for the fabrication of micro-electromechanical system (MEMS) sensors presents various advantages including low cost, customization, simplicity, and speed. This study involved conducting mechanical and electrical analyses on three types of dielectric layer structures using the finite element method. Capacitive force sensors were produced through 3D printing using fused deposition modeling (FDM). The sensitivity and stiffness of the sensors were determined by measuring their capacitance and displacement changes under force using a self-made measurement system. The experimental results indicate that pre-heating the dielectric layer can reduce the hysteresis effects. In terms of more complex structures, stringing phenomena during the printing process can compromise the reliability of the sensor. A suitable dielectric layer structure can be designed based on the measurement range and sensitivity of the sensor. The results of this study demonstrate the potential for additive manufacturing to replace conventional MEMS processes in sensor production. With the advancement of FDM printing technology, it is increasingly feasible to apply 3D printing technology to sensor manufacturing in the future.

Extending atomic layer deposition for use in next-generation piezoMEMS: Review and perspective

The objective of this work is to describe the current state of the rapidly evolving field of 3D piezoelectric microelectromechanical systems (piezoMEMS), and where it needs to go to fully leverage the potential performance benefits offered by atomic layer deposition (ALD). We define 3D piezoMEMS as the application of piezoelectric ALD films to 3D, high aspect-ratio, mechanically pliable structures. Since there are so few existing reports of 3D piezoMEMS, a literature review of ALD films applied to conventional microelectromechanical system (MEMS) devices is given. ALD processes for piezoelectric thin films are reviewed in the context of relevant applications such as transducers and actuators. Examples include aluminum nitride, hafnium zirconate, doped-hafnia, lead zirconate-titanate, lead hafnate, and lead hafnate-titanate. New concepts for ALD-enabled 3D piezoMEMS actuators are presented with supporting theoretical calculations that show that chip-scale mechanical work densities could be improved by ≫10× compared to conventional planar piezoMEMS. 3D fabrication methods are also discussed, while the future needs of atomic layer processing are highlighted.

Micro 3D printing of a functional MEMS accelerometer

Microelectromechanical system (MEMS) devices, such as accelerometers, are widely used across industries, including the automotive, consumer electronics, and medical industries. MEMS are efficiently produced at very high volumes using large-scale semiconductor manufacturing techniques. However, these techniques are not viable for the cost-efficient manufacturing of specialized MEMS devices at low- and medium-scale volumes. Thus, applications that require custom-designed MEMS devices for markets with low- and medium-scale volumes of below 5000–10,000 components per year are extremely difficult to address efficiently. The 3D printing of MEMS devices could enable the efficient realization and production of MEMS devices at these low- and medium-scale volumes. However, current micro-3D printing technologies have limited capabilities for printing functional MEMS. Herein, we demonstrate a functional 3D-printed MEMS accelerometer using 3D printing by two-photon polymerization in combination with the deposition of a strain gauge transducer by metal evaporation. We characterized the responsivity, resonance frequency, and stability over time of the MEMS accelerometer. Our results demonstrate that the 3D printing of functional MEMS is a viable approach that could enable the efficient realization of a variety of custom-designed MEMS devices, addressing new application areas that are difficult or impossible to address using conventional MEMS manufacturing.

On the nature of wetting transition on high-aspect-ratio pNIPAAm micropillar structures

Wetting transition (Cassie state to Wenzel state or reversible) plays an important role in advanced liquid manipulation systems and digital microfluidics. Although there are several methods for achieving the functional surfaces, research on a surface capable of controlling on-demand wetting transition more precisely using temperature with robust structures is necessary. Herein, we report the fabrication of a surface with temperature-driven wettability control. We fabricated for the first time high-aspect-ratio micropillar structures using a temperature-responsive polymer, poly(N-isopropylacrylamide) (pNIPAAm), by a conventional microelectromechanical systems process followed by a soft-lithography process. A water droplet placed on the fabricated functional surface changes from the Cassie wetting state to the Wenzel wetting state owing to the synergistic effect of the high-aspect-ratio micropillar structures and the material properties of temperature-dependent swelling of pNIPAAm. Furthermore, the wetting transition time can be controlled by varying the concentration of the crosslinker and ambient temperature.

Quality factor control of mechanical resonators using variable phononic bandgap on periodic microstructures

The quality factor (Q-factor) is an important parameter for mechanical resonant sensors, and the optimal values depend on its application. Therefore, Q-factor control is essential for microelectromechanical systems (MEMS). Conventional methods have some restrictions, such as additional and complicated equipment or nanoscale dimensions; thus, structural methods are one of the reasonable solutions for simplifying the system. In this study, we demonstrate Q-factor control using a variable phononic bandgap by changing the length of the periodic microstructure. For this, silicon microstructure is used because it has both periodicity and a spring structure. The bandgap change is experimentally confirmed by measuring the Q-factors of mechanical resonators with different resonant frequencies. The bandgap range varies depending on the extended structure length, followed by a change in the Q-factor value. In addition, the effects of the periodic structure on the Q-factor enhancement and the influence of stress on the structural length were evaluated. Although microstructures can improve the Q-factors irrespective of periodicity; the result of the periodic microstructure is found to be efficient. The proposed method is feasible as the novel Q-factor control technique has good compatibility with conventional MEMS.

Tilting micromirror platform based on liquid dielectrophoresis

This study presents an electrically actuated tilting micro platform based on liquid dielectrophoresis with three axes movement using three droplets situated 120° apart from each other. The interdigitated electrodes produce a non-uniform electric field that generates a body force. The dielectrophoretic mechanism is responsive within at least 30 ms, and it eliminates the solid-solid contact. The tilting platform enabled an angular coverage up to 0.9° (±0.02°), with a maximum displacement of 120 µm. The tilting micromirror platform has beam steering characteristics suitable for various optical applications. The actuating platform sensor is a cost-effective and simple alternative method to study liquid dielectrophoresis without measuring the droplet contact angle. Furthermore, the unique configuration without any solid-solid contact offers a potential improvement for applications in optics, actuators, and other conventional microelectromechanical systems.

Design and Application of MEMS-Based Hall Sensor Array for Magnetic Field Mapping

A magnetic field measurement system based on an array of Hall sensors is proposed. The sensors are fabricated using conventional microelectromechanical systems (MEMS) techniques and consist of a P-type silicon substrate, a silicon dioxide isolation layer, a phosphide-doped cross-shaped detection zone, and gold signal leads. When placed within a magnetic field, the interaction between the local magnetic field produced by the working current and the external magnetic field generates a measurable Hall voltage from which the strength of the external magnetic field is then derived. Four Hall sensors are fabricated incorporating cross-shaped detection zones with an identical aspect ratio (2.625) but different sizes (S, M, L, and XL). For a given working current, the sensitivities and response times of the four devices are found to be almost the same. However, the offset voltage increases with the increasing size of the detection zone. A 3 × 3 array of sensors is assembled into a 3D-printed frame and used to determine the magnetic field distributions of a single magnet and a group of three magnets, respectively. The results show that the constructed 2D magnetic field contour maps accurately reproduce both the locations of the individual magnets and the distributions of the magnetic fields around them.

Kirigami Engineering-Nanoscale Structures Exhibiting a Range of Controllable 3D Configurations.

Kirigami structures provide a promising approach to transform flat films into 3D complex structures that are difficult to achieve by conventional fabrication approaches. By designing the cutting geometry, it is shown that distinct buckling-induced out-of-plane configurations can be obtained, separated by a sharp transition characterized by a critical geometric dimension of the structures. In situ electron microscopy experiments reveal the effect of the ratio between the in-plane cut size and film thickness on out-of-plane configurations. Moreover, geometrically nonlinear finite element analyses (FEA) accurately predict the out-of-plane modes measured experimentally, their transition as a function of cut geometry, and provide the stress-strain response of the kirigami structures. The combined computational-experimental approach and results reported here represent a step forward in the characterization of thin films experiencing buckling-induced out-of-plane shape transformations and provide a path to control 3D configurations of micro- and nanoscale buckling-induced kirigami structures. The out-of-plane configurations promise great utility in the creation of micro- and nanoscale systems that can harness such structural behavior, such as optical scanning micromirrors, novel actuators, and nanorobotics. This work is of particular significance as the kirigami dimensions approach the sub-micrometer scale which is challenging to achieve with conventional micro-electromechanical system technologies.

A Robot Boat Powered by Liquid Metal Engines

AbstractUnlike conventional microelectromechanical systems (MEMS), machines based on gallium liquid metal alloys rely on Marangoni flows to induce motion without mechanical moving parts. Despite this advantage, currently developed liquid metal enabled robotic systems still need the locomotion of liquid metal droplets, thereby generating weak actuating forces and limiting their manoeuvrability. Here, a liquid metal engine (LME) is created to work as a jet thruster to continuously power a robot boat without the need of locomotion. In addition, the engine is electrically driven by surface tension without any mechanical moving parts. The structural design and the parameters of operation of the LME are investigated and optimized. Finally, the manipulation of the LME using a motor control unit is demonstrated for driving an untethered robot boat to realize complex locomotion with the precise control over the direction and speed. Thus, this LME could offer new opportunities for developing high‐performance liquid metal machines to power robotic systems.

Fabrication and Characterization of Roll-to-Roll-Coated Cantilever-Structured Touch Sensors.

It is common in the field of printed electronics that polydimethylsiloxane (PDMS) be used as a dielectric layer for capacitive sensors because of its high elasticity and restoration force. However, capacitive sensors with the PDMS dielectric layer have a lower sensitivity than those with an air-gap structure that has been fabricated by the conventional micro-electromechanical system (MEMS) process. This paper presents a productive method for fabricating air-gap structures for touch sensors by roll-to-roll slot-die coating. The air-gap is formed by coating and removing a sacrificial layer. Cantilever-structured capacitive touch sensors with an air-gap are fabricated as follows: First, the bottom electrode, the dielectric layer, and the poly(vinyl alcohol) (PVA) sacrificial layer are roll-to-roll slot-die-coated on a flexible substrate. In addition, the spacer layer is spin-coated. On the sacrificial and spacer layers, the top electrode and structural layer are formed by spin-coating. Then, the air-gap and cantilever structure are made by removing the sacrificial layer in water. The cantilever-structured sensor samples are examined in terms of sensitivity, hysteresis, and repeatability. In particular, the electrical performance of the samples is compared to those with the PDMS dielectric layer. Experimental results show that the cantilever-structured sensor samples have significantly higher sensitivity compared to those with the PDMS dielectric layer.

TiO2/airgap-based polarizing beam splitter: design, simulation, and fabrication based on MEMS technologies

In this paper, we introduce a new thin-film polarizing beam splitter (PBS) based on air as a layer with a low refractive index (L) and titanium dioxide (TiO2) as a material with a high refractive index (H). A cube-type PBS presents an active area of approximately 1.0 × 1.0 mm2, and the airgap layer in conjunction with TiO2 thin-films provides an extremely high refractive index contrast (1.2). The PBS cube is projected onto a borosilicate glass substrate and the multiple layers obtained through TFCalcTM simulations, expressed in multiples of a quarter-wavelength optical thickness, are 1.6H L 1.1H 1.5L 1.1H L 1.6H. The projected structure ensures the separation of the polarizing states of light (transmission of the p-polarization and reflection of the s-polarization) from the visible to near infrared spectral range, over a bandwidth higher than 170 nm. This is the minimal working bandwidth obtained for visible range (centered at 550 nm), considering a polarization degree higher than 0.95. The projected PBS is fabricated based on conventional microelectromechanical systems (MEMS) technologies, namely, thin-film deposition, annealing, and etching processes. Finally, the fabricated PBS was experimentally characterized, with the measured transmittance of the fabricated structure reaching close to the expected TFCalcTM simulation result. This characterization is the final validation of the structure since, theoretically, reflection is the complement of transmission in a dielectric thin-film multilayer.

Reliable and Robust Fabrication Rules for Springtail-Inspired Superomniphobic Surfaces.

We report a reliable and robust method for the fabrication of bioinspired superomniphobic surfaces with precise concave-cap-shaped micropillar arrays. This method includes silicon-based conventional microelectromechanical systems (MEMS) and polymer replication processes. We have elucidated two critical cases of fabrication rules for precise micromachining of a negative-shaped bioinspired silicon master. The fabricated polymeric structure replicated from the semipermanent silicon master based on the design rules exhibited high structural fidelity and robustness. Finally, we validated the superomniphobic properties, structural durability, and long-term stability of the fabricated bioinspired surfaces.

Infrasound microphones using microfabricated piezoresistive silicon diaphragms

We report on the development of a new digital infrasound measurement system with integrated digitizing electronics. The system was designed to meet an input-referred noise floor specification of 95 μPa/√Hz over the frequency range of 0.01 Hz to 100 Hz. The infrasound sensor uses a microfabricated silicon diaphragm with four integrated piezoresistors connected in a full-bridge configuration. Fabricating the sensing element on a silicon wafer using conventional micro-electromechanical systems (MEMS) processing techniques is a convenient approach for realizing sensitive diaphragms with integrated piezoresistors, and, in turn, sensing elements that intrinsically respond to DC. This approach is thought to be suitable for large infrasound arrays. Integrating a secondary transducer into the package provides self-test capabilities, along with a geophone for correlating infrasound signals with seismic vibrations. Sensor self-noise and sensitivity measurements are presented, along with a discussion of system design considerations, including design of the electronics and packaging.

Nonlinear Multi-Mode Wideband Piezoelectric MEMS Vibration Energy Harvester

The conventional micro-electromechanical system (MEMS) energy harvesters can only generate voltage disadvantageously in a narrow bandwidth at higher frequencies. In this paper, we propose a piezoelectric MEMS harvester with the capability of vibrating in multi-degrees-of-freedom, whose operational bandwidth is enhanced by taking the advantage of both the multimodal and nonlinear mechanisms. The proposed harvester has a symmetric structure with a doubly-clamped configuration enclosing three proof masses in distinct locations. Thanks to the uniform mass distribution, the energy harvesting efficiency can be considerably enhanced. To determine the optimum geometry for the preferred nonlinear behavior, we propose an automated design and optimization methodology based on the genetic algorithm (GA). By using the micromachining process, our optimized harvester with a total volume of 4.1 mm3 has been fabricated and measured. The prototype measurements demonstrate that our proposed piezoelectric MEMS harvester is able to generate voltage with a frequency bandwidth of 59 Hz, ranging from 227 to 286 Hz. When the device operates at its second-mode resonant frequency, the nonlinear behavior can be obtained with an extremely small magnitude of the base excitation (i.e., 0.2 m/s2). Its normalized power density (NPD) of 595.12 ( $\mu \text{W}\cdot \mathrm {cm}^{-3}\cdot \text{m}^{-2}\cdot \text{s}^{4}$ ) is found to be superior to any previously reported wideband aluminum nitride (AlN) piezoelectric MEMS harvesters in the literature.

FR4-Based Electromagnetic Scanning Micromirror Integrated with Angle Sensor.

This paper presents a flame retardant 4 (FR4)-based electromagnetic scanning micromirror, which aims to overcome the limitations of conventional microelectromechanical systems (MEMS) micromirrors for the large-aperture and low-frequency scanning applications. This micromirror is fabricated through a commercial printed circuit board (PCB) technology at a low cost and with a short process cycle, before an aluminum-coated silicon mirror plate with a large aperture is bonded on the FR4 platform to provide a high surface quality. In particular, an electromagnetic angle sensor is integrated to monitor the motion of the micromirror in real time. A prototype has been assembled and tested. The results show that the micromirror can reach the optical scan angle of 11.2° with a low driving voltage of only 425 mV at resonance (361.8 Hz). At the same time, the signal of the integrated angle sensor also shows good signal-to-noise ratio, linearity and sensitivity. Finally, the reliability of the FR4 based micro-mirror has been tested. The prototype successfully passes both shock and vibration tests. Furthermore, the results of the long-term mechanical cycling test (50 million cycles) suggest that the maximum variations of resonant frequency and scan angle are less than 0.3% and 6%, respectively. Therefore, this simple and robust micromirror has great potential in being useful in a number of optical microsystems, especially when large-aperture or low-frequency is required.

Design, modeling, and fabrication of crab-shape capacitive microphone using silicon-on-isolator wafer

This paper presents design, modeling, and fabrication of a crab-shape microphone using silicon-on-isolator (SOI) wafer. SOI wafer is used to prevent the additional deposition of sacrificial and diaphragm layers. The holes have been made on diaphragm to prevent back plate etching. Dry etching is used for removing the sacrificial layer, because wet etching causes adhesion between the diaphragm and the back plate. Crab legs around the perforated diaphragm allow for improving the microphone performance and reducing the mechanical stiffness and air damping of the microphone. In this structure, the supply voltage is decreased due to the uniform deflection of the diaphragm due to the designed low-K (spring constant) structure. An analytical model of the structure for description of microphone behavior is presented. The proposed method for estimating the basic parameters of the microphone is based on the calculation of the spring constant using the energy method. The microphone is fabricated using only one mask to pattern the crab-shape diaphragm, resulting in a low-cost and easy fabrication process. The diaphragm size is 0.3 mm×0.3 mm, which is smaller than the conventional microelectromechanical systems capacitive microphone. The results show that the analytical equations have a good agreement with measurement results. The device has the pull-in voltage of 14.3 V, a resonant frequency of 90 kHz, an open-circuit sensitivity of 1.33 mV/Pa under bias voltage of 5 V. Comparing with previous works, this microphone has several advantages: SOI wafer decreases the fabrication process steps, the microphone is smaller than the previous works, and crab-shape diaphragm improves the microphone performances.

Vibration of mechanically-assembled 3D microstructures formed by compressive buckling

Micro-electromechanical systems (MEMS) that rely on structural vibrations have many important applications, ranging from oscillators and actuators, to energy harvesters and vehicles for measurement of mechanical properties. Conventional MEMS, however, mostly utilize two-dimensional (2D) vibrational modes, thereby imposing certain limitations that are not present in 3D designs (e.g., multi-directional energy harvesting). 3D vibrational micro-platforms assembled through the techniques of controlled compressive buckling are promising because of their complex 3D architectures and the ability to tune their vibrational behavior (e.g., natural frequencies and modes) by reversibly changing their dimensions by deforming their soft, elastomeric substrates. A clear understanding of such strain-dependent vibration behavior is essential for their practical applications. Here, we present a study on the linear and nonlinear vibration of such 3D mesostructures through analytical modeling, finite element analysis (FEA) and experiment. An analytical solution is obtained for the vibration mode and linear natural frequency of a buckled ribbon, indicating a mode change as the static deflection amplitude increases. The model also yields a scaling law for linear natural frequency that can be extended to general, complex 3D geometries, as validated by FEA and experiment. In the regime of nonlinear vibration, FEA suggests that an increase of amplitude of external loading represents an effective means to enhance the bandwidth. The results also uncover a reduced nonlinearity of vibration as the static deflection amplitude of the 3D structures increases. The developed analytical model can be used in the development of new 3D vibrational micro-platforms, for example, to enable simultaneous measurement of diverse mechanical properties (density, modulus, viscosity etc.) of thin films and biomaterials.

Design and FEM simulation for a novel resonant silicon MEMS gyroscope with temperature compensation function

A kind of resonant silicon micro-electro-mechanical systems (MEMS) gyroscope with self-temperature compensation function is designed to overcome the problem of conventional MEMS gyroscope, whose measuring precision is greatly affected by the temperature effect. Sensitive element of the system is composed of four symmetrically distributed double ended tuning fork (DETFs). They are of the same dimensions. Two of the DETFs which are closing to the proof mass are used to sense the force effect and temperature effect while the other two DETFs are just used to sense the temperature effect. By comparing these two different conditions, the self-temperature compensation model can be deduced. In order to verify the feasibility of the design, finite element method (FEM) is used to compute the performance of system. It is concluded that in the range of −20 to 80 °C, the maximum relative error of the resonant frequency variation is reduced from 16.3 to 3.1%. Simulating result indicates this scheme of design is effective to overcome the temperature effect of this kind of gyroscope.

Fano resonance based ultra high-contrast electromagnetic switch

We experimentally achieve highly asymmetrical enhanced-Q Fano resonances in metallic electromagnetic structures fabricated on conductive planes. We show that the complete destructive interference mechanism of the dark and bright resonant modes generated by a pair of electromagnetically coupled open-ended conductive arms can lead to the asymmetric resonance characterized by a near-unity transparency window followed by a deep scattering band. With the incorporation of a variable capacitor between the coupled metallic strips, the dynamic tunability of the resonant modes is achieved, which can be exploited in high isolation switches and modulators in the GHz spectrum. The switching contrast of over 50 dB achieved through Fano resonance is much higher considering its compact size (i.e., the transmission path is much smaller than the wavelength λ/30). The dispersion based tunable Fano switch offers several advantages over conventional microelectromechanical system and CMOS based switches.