Microelectromechanical Systems Applications Research Articles

Numerical and experimental investigations to analyze the micro-hole drilling process in spark-assisted chemical engraving (SACE)

With the increased utilization of glass in micro-electromechanical systems (MEMS) applications, need is generated to produce micro-features on glass with excellent surface quality, i.e., better hole circularity and less thermal cracks. Spark-assisted chemical engraving (SACE) has proven to be an emerging and reliable technology for micro-machining of glass that includes local joule heating of the work material for its removal. However, some problems need to be covered in the micro-hole drilling process such as low machining depth, low aspect ratio, high thermal cracks, and an increase in machining time with the increase in depth. Moreover, few finite element modeling (FEM) studies have been reported to analyze the material removal aspects in SACE. This present study developed the FEM-based thermal model to analyze the material removal rate (MRR) and investigated the different machining conditions to improve the micro-hole drilling process in SACE. Improvement in depth and aspect ratio can be achieved by using a different tool’s shape which variably enhances the flow of electrolytes and thereby increases the formation rate of gas film. Pointed and cylindrical tools were used for micro-drilling operations. MRR, hole entrance diameter, machining depth, aspect ratio, thermal cracks, and machining time were observed under different machining conditions. FEM study revealed that MRR improves with the increase in both the applied voltage and electrolyte concentration. The MRR was predicted by utilizing the plots of temperature distributions and was found in accordance with the experimental results of MRR. The experimental study concluded that the pointed tool produced micro-holes with a higher machining depth and aspect ratio at an improved machining rate, while the cylindrical tool produced micro-holes with a smaller number of thermal cracks and better circularity.

Structural design, analysis and DOE of MEMS-based capacitive accelerometer for automotive airbag application

Accelerometers are one of the simplest and most important applications of Micro-Electromechanical Systems (MEMS). This paper deals with the design, analysis and Design of Experiments (DOE) of a MEMS-based capacitive accelerometer for automotive applications. The key design parameters to be considered while designing an accelerometer are the sensitivity and the bandwidth of operation. The present work focuses on the design and improvement of an accelerometer to ensure that it operates for a larger bandwidth without compromising on the sensitivity. The structural parameters, overall dimensions and size of the accelerometer are first identified. Subsequently, a parametric analysis is performed using $${2}^{k}$$ Design of Experiments for various g values from 5 to 50 g totalling 40 runs. Depending on the level of influence, these structural parameters are then modified for an improved accelerometer design. Later, a sensitivity analysis is performed. It was seen that the improved design performed better than the rest. A Modal analysis was also performed to determine the resonant/natural frequencies and the mode shapes. It was found that the presented design is safe to operate. This method of approach would help in arriving at a better accelerometer for the desired application.

The piezoresistive mobility modeling for cubic and hexagonal silicon carbide crystals

The piezoresistive effect is characterized by the change in the resistivity of a material relative to mechanical forces exerted on it. Such materials can be used as pressure sensors and are among the most important components for micro-electro mechanical system applications. To date, most research on the piezoresistive effect has been directed toward cubic crystalline materials such as Si; however, the prospective non-cubic materials, such as SiC, are known to have exciting and promising properties. SiC exhibits high-temperature robustness and is chemically stable. It is expected that these properties can be applied to a variety of applications. These materials fall in the category of hexagonal crystalline systems, and it is difficult to evaluate the piezoresistive properties of such materials. In this study, we discuss the piezoresistive mobility model that corresponds to both the cubic and the hexagonal crystalline systems. This mobility model is derived from the empirical fitting of the Gauge Factor (GF) values using the longitudinal and the transverse piezoresistive coefficients and the material-unique fitting parameters. Our proposed method has been implemented in the original device simulator and has been evaluated with respect to both Si and SiC materials. This report shows the well-matched GF values and suggests that the proposed piezoresistive effect model can be implemented in device simulation modeling.

Novel fabrication of fixed suspended silicon nitride structure for MEMS devices with dry etching

A method of fabricating suspended LPCVD and PECVD silicon nitride structure is demonstrated for a wide range of MEMS (Micro-Electro-Mechanical Systems) applications. Low stress LPCVD and PECVD silicon nitride film of 1 μm thickness were selected for the structure separately. Optical Lithographic parameters, viz, photoresist (PR) thickness, PR variety and baking parameters were optimized to obtain the PR suitable for selective etching of silicon and silicon nitride. Parameters of the dry etching process were also optimized to achieve anisotropic etching of silicon nitride and isotropic etching of Si to release the silicon nitride beam. The silicon nitride structures, thus released, were characterized using Scanning Electron Microscope (SEM) and Laser Doppler Vibrometer (LDV). Finite Element Method (FEM) analysis was carried out using COMSOL, to compare with the experimental modes of vibration investigated using the Laser Doppler Vibrometer (LDV). Thus we demonstrate that the first mode at 30 kHz was indeed the optimum match.

Fabrication, calibration, and preliminary testing of microcantilever-based piezoresistive sensor for BioMEMS applications.

In this study, the authors demonstrate the fabrication, calibration, and testing of a piezoresistive microcantilever-based sensor for biomedical microelectromechanical system (BioMEMS) application. To use any sensor in BioMEMS application requires surface modification to capture the targeted biomolecules. The surface alteration comprises self-assembled monolayer (SAM) formation on gold (Au)/chromium (Cr) thin films. So, the Au/Cr coating is essential for most of the BioMEMS applications. The fabricated sensor uses the piezoresistive technique to capture the targeted biomolecules with the SAM/Au/Cr layer on top of the silicon dioxide layer. The stiffness (k) of the cantilever-based biosensor is a crucial design parameter for the low-pressure range and also influence the sensitivity of the microelectromechanical system-based sensor. Based on the calibration data, the average stiffness of the fabricated microcantilever with and without Au/Cr thin film is 141.39 and 70.53 mN/m, respectively, which is well below the maximum preferred range of stiffness for BioMEMS applications. The fabricated sensor is ultra-sensitive and selective towards Hg2+ ions in the presence of other heavy metal ions (HMIs) and good enough to achieve a lower limit of detection 0.75 ng/ml (3.73 pM/ml).

Electrodeposition of Au-Cu Alloys and the Micro-Mechanical Properties

Electrodeposited gold is used in movable micro-components of micro-electro-mechanical systems (MEMS) devices to allow further enhancement in the sensitivity and miniaturization of the device [1]. Mechanical strengths of gold are low among materials commonly used as structure materials in electronic devices. Electrodeposition method is advantageous in strengthening metallic materials because of the ability to refine the average grain size to nano-scale, which the mechanical strength increases along with a reduction in the grain size based on the grain boundary strengthening mechanism or the Hall-Petch relationship. Alloying of metallic materials is also an effective strategy to achieve the strengthening according to the solid solution strengthening mechanism.On the other hand, mechanical properties of metallic materials exhibit the sample size effect when dimensions of the specimen are reduced to micro or sub-micro scale. Therefore, mechanical property characterization of materials for MEMS applications should be conducted using specimens having dimensions in micro-scale. Various micro-mechanical testing methods have been developed in our group to characterize micro-specimens, such as micro-compression test [2,3], micro-tensile test [4], and micro-bending test [5]. Among them, micro-bending test is suggested to be the most suitable method to evaluate materials for applications in MEMS when compared with micro-compression and micro-tensile tests since cantilever-like structures are often applied in MEMS components, such as the micro-spring in a MEMS accelerometer.In this study, Au-Cu alloys are prepared by electrodeposition, and micro-mechanical properties of the electrodeposited Au-Cu alloys are characterized by a micro-bending test. Young’s modulus is an important information in design of movable micro-components. A non-destructive Young’s modulus evaluation method, resonance frequency measurement, is conducted using the Au-Cu alloy micro-cantilevers. Also, long-term vibration tests are conducted to reveal structure stability of the Au-Cu alloy micro-cantilevers.The Au-Cu electrolyte used in this work was a commercially available electrolyte provided by MATEX Co. Japan, which contained 17.3 g/L of X3Au(SO3)2 (X = Na, K), 1.26 g/L of CuSO4, and ethylenediaminetetraacetic acid as the additive with pH at 7.5. The electrodeposition was carried out at 50 °C, and the current density was varied from 0.1 to 8 mA/cm2. Specimens used in the micro-bending test were micro-cantilevers fabricated from the Au-Cu alloy films by focused ion beam. Dimensions of the micro-cantilevers fabricated were 10 µm × 10 µm × 50 µm. The micro-bending tests were conducted using a test machine equipped with a spherical diamond indenter at a constant displacement rate of 0.05 µm/s.For the Young’s modulus and long-term structure stability evaluations, the micro-cantilevers were prepared by electrodeposition combined with lithography process. The long-term vibration test was carried out under conditions of the cycle number in a range from 103 to 106, the frequency of 10.0 Hz, and the acceleration of 1.0 G (1 G = 9.8 m/s2). The structure stability was determined by evaluating height profiles of top surface of the micro-cantilevers before and after the vibration test by a 3D optical microscope.The Au-Cu alloy film electrodeposited at 5 mA/cm2 had the finest average grain size among films prepared in this work, which the grain size was 4.76 nm. Bending test of a micro-cantilever prepared from this film shows a high yield stress at 1826 MPa. Gold, copper, and Au-Cu alloys are known to be ductile materials, but brittle fracture was observed during the bending test when the current density used in prepared the film was higher than 5 mA/cm2.From the resonance frequency method, Young’s moduli of the Au-Cu alloy micro-cantilevers were ranged from 68.0 to 79.5 GPa, which were similar to the value of bulk-size specimens and indicated no sample size effect on the Young’s modulus. Regarding the long-term structure stability, after 106 cycles of the vibration test, all the micro-cantilever were still intact, and the height profiles did not change much after the vibration. Results obtained in this study confirmed high mechanical strength and structure stability in the electroplated Au-Cu alloys and demonstrated the advantage in applications as movable components in MEMS devices.[1] D. Yamane et. al. Appl. Phys. Lett. 104 (2014) 074102.[2] C.Y. Chen et. al. Electrochem. Commun. 51 (2016) 51-54.[3] H. Tang et. al. J. Electrochem. Soc. 164 (2017) 244-247[4] Y. Kihara et. al. Mater. Lett. 153 (2015) 36-39[5] K. Asano et. al. ECS J. Solid State Sci. Technol. 8 (2019) P412-P415.

Processing-Structure-Property Relations for Radiation Tolerance of Relaxor-Ferroelectric Thin Films.

This work investigates the role of microstructure on the radiation tolerance of relaxor-ferroelectric, lead magnesium niobate-lead titanate, thin films for piezoelectric microelectromechanical system (MEMS) applications. Thin films comprised of 0.7Pb[Mg1/3Nb2/3]O3-0.3PbTiO3 were fabricated via chemical solution deposition on platinized silicon wafers. Processing parameters, i.e., pyrolysis and annealing temperatures and durations, were varied to change the microstructure of the films. The functional response of the films was characterized before and after exposure to gamma radiation [up to 10 Mrad(Si)]. Within the total ionization dose studied, all films showed a <5% change in dielectric response and polarization and <15% change in piezoelectric response, after irradiation. While all films showed substantial radiation tolerance, those with large columnar grains showed the highest dielectric and piezoelectric response and, therefore, might offer the best approach for enabling piezoelectric MEMS devices for applications in radiative environments.

Robust super-twisting sliding mode controller with state observer for position tracking of ionic polymer–metal composite actuators

To date, a novel class of smart materials, known as ionic polymer–metal composites, have been intensively studied because of their huge potential applications in robotics, microelectromechanical systems, biomedical devices, and artificial muscles. The position tracking control of the ionic polymer–metal composite actuators is one of the challenging tasks due to the highly nonlinear, uncertain, and unmodeled dynamics. These inaccurate and unmodeled dynamics cause the unwanted disturbances which reduce the control performance. Therefore, a robust feedback controller for position tracking of the ionic polymer–metal composite actuators is necessary. To address this issue, this article presents a super-twisting sliding mode controller combined with integral-chain differentiator as state observer. The main advantage of the proposed method is its potential to estimate the states of the system continuously. As a result, the controller not only eliminates the parameter uncertainties and external disturbance but also it can overcome the chattering problem. The results confirm the feasibility and efficiency of the proposed method both in simulation and experiment.

Coexisting modes and bifurcation structure in a pair of coupled detuned third order oscillators

The bifurcation structure of a system of two non-linear coupled oscillators is considered here. This system is motivated by coupled micro-oscillators proposed for microelectromechanical systems (MEMS) applications. The model starts from previous work in which the authors considered the bifurcation structure in a pair of identical coupled, oscillators, each of which is modelled by a third-order nonlinear system. In the present paper, the case is considered where the two oscillators are non-identical i.e. detuned from each other. The series of bifurcations obtained is described in detail. It is seen that the symmetry of the bifurcation structure present in the identical oscillators is lost in the detuned oscillators. A very surprising homoclinic bifurcation is seen in one region of the parameter space, and this is examined in more detail through consideration of a simplified system.

A systematic review of magnetron sputtering of AlN thin films for extreme condition sensing

In this article, a systematic overview of the published works on magnetron sputtering of AlN thin films for harsh condition applications is presented. Harsh environments may be defined as the operating conditions exhibiting extreme shock loads, high temperature, high pressure/forces and corrosive media. Some of the harsh environments include petrochemical plants, automobile/engine, marine, aerospace, in-situ health monitoring, prosthetics, etc. In these applications, sensors are necessary to monitor and control process parameters such as pressure, temperature, fluid flow, chemical contents, PH and among others. As such, the sensor materials should be able to withstand the high and cyclic conditions within such operations. Aluminum nitride (AlN) is a ceramic material, which finds application in micro-electromechanical systems (MEMS) such as acoustic sensors, energy harvesters, transducers and resonators. These applications are motivated by its (AlN) high piezoelectric effect, high surface acoustic wave velocity, excellent dielectric permittivity, high thermal stability, wide band gap, chemical inertness, and so forth. As such, AlN films have been deposited on various non-metallic substrates such as Si, sapphire and polymers and on metallic substrates such as Ti6Al4V, diamond, Al, Cr and 304L stainless steel for different sensing applications. It is deduced that studies involving characterization of RF magnetron sputtered AlN thin films in harsh conditions are still scarce. The review serves as a resource for researchers and industry for development of sensors and transducers for harsh condition applications.

Uncertainty quantification of MEMS devices with correlated random parameters

The concern about process deviations rises because that the performance uncertainty they cause are strengthened with the miniaturization and complication of Microelectromechanical System (MEMS) devices. To predict the statistic behavior of devices, Monte Carlo method is widely used, but it is limited by the low efficiency. The recently emerged generalized polynomial chaos expansion method, though highly efficient, cannot solve uncertainty quantification problems with correlated deviations, which is common in MEMS applications. In this paper, a Gaussian mixture model (GMM) and Nataf transformation based polynomial chaos method is proposed. The distribution of correlated process deviations is estimated using GMM, and modified Nataf transformation is applied to convert the correlated random vectors of GMM into mutually independent ones. Then polynomial chaos expansion and stochastic collection can be implemented. The effectiveness of our proposed method is demonstrated by the simulation results of V-beam thermal actuator, and its computation speed is faster compared with the Monte Carlo technique without loss of accuracy. This method can be served as an efficient analysis technique for MEMS devices which are sensitive to correlated process deviations.

Stimulus Responsive 3D Assembly for Spatially Resolved Bifunctional Sensors.

3D electronic/optoelectronic devices have shown great potentials for various applications due to their unique properties inherited not only from functional materials, but also from 3D architectures. Although a variety of fabrication methods including mechanically guided assembly have been reported, the resulting 3D devices show no stimuli-responsive functions or are not free standing, thereby limiting their applications. Herein, the stimulus responsive assembly of complex 3D structures driven by temperature-responsive hydrogels is demonstrated for applications in 3D multifunctional sensors. The assembly driving force, compressive buckling, arises from the volume shrinkage of the responsive hydrogel substrates when they are heated above the lower critical solution temperature. Driven by the compressive buckling force, the 2D-formed membrane materials, which are pre-defined and selectively bonded to the substrates, are then assembled to 3D structures. They include "tent," "tower," "two-floor pavilion," "dome," "basket," and "nested-cages" with delicate geometries. Moreover, the demonstrated 3D bifunctional sensors based on laser induced graphene show capability of spatially resolved tactile sensing and temperature sensing. These multifunctional 3D sensors would open new applications in soft robotics, bioelectronics, micro-electromechanical systems, and others.

Studies on ferroelectric and nanomechanical response of single-layered PZT thick film for energy harvester applications

We have made an attempt to fabricate free standing warpage free PZT thick films of 100 µm thickness by tape casting method. The poly crystalline nature and nanoscale polarization switching of free standing thick film were confirmed using X-ray diffraction technique and piezo force microscopy, respectively. The piezoelectric coefficient (d33) of 100 µm free-standing Pb(Zr0.56Ti0.44)O3 [PZT] thick film was calculated using double beam laser interferometer and found to be 400 pm/V. From the results, it indicates that free-standing PZT thick films are suitable candidate for miniaturization of harvesting device by directly bonding them on microsystem components for microelectromechanical systems (MEMS) applications.

Morphological Transformation between Flat and Tube Structures by Coordinated Motions of Soft Pneumatic Microactuators

Microactuators are the most distinctive and challenging microdevices among micro electromechanical systems (MEMS) relative to microsensors or electronic circuits. Soft and flexible microactuators have been achieved by introducing polymers as structural materials in addition to conventional materials. Expanding the application of MEMS to the biomedical field requires particular features, such as softness and small devices. It is important to address small and fragile biological objects while satisfying the demand for minimally invasive medicine. Both MEMS and biomedical applications require three-dimensional microstructures for higher-order functions. In general, microactuators are limited to simple motions such as bending. Our group has developed an openable artificial small intestinal tract system. An array of pneumatic balloon actuators (PBAs) transforms a flat structure into a tube structure representing the small intestine. Coordination of the bending motions of the PBAs enables the formation of a three-dimensional tube structure. Each PBA is 400 μm × 1800 μm × 100 μm. The diameter of the tube structure is 1 mm. Additional higher-order functions of the artificial small intestine, such as peristaltic motion, are currently of interest for us. This paper describes the morphological transformation of a soft microstructure and further potential possibilities of coordinated motions of soft microactuators.

A Hybrid Two-Axis Force Sensor for the Mesoscopic Structural Superlubricity Studies.

Structural superlubricity (SSL) is a state of nearly zero friction and zero wear between two directly contacted solid surfaces. Recently, SSL was achieved in mesoscale and thus opened the SSL technology which promises great applications in Micro-electromechanical Systems (MEMS), sensors, storage technologies, etc. However, load issues in current mesoscale SSL studies are still not clear. The great challenge is to simultaneously measure both the ultralow shear forces and the much larger normal forces, although the widely used frictional force microscopes (FFM) and micro tribometers can satisfy the shear forces and normal forces requirements, respectively. Here we propose a hybrid two-axis force sensor that can well fill the blank between the capabilities of FFM and micro tribometers for the mesoscopic SSL studies. The proposed sensor can afford 1mN normal load with 10 nN lateral resolution. Moreover, the probe of the sensor is designed at the edge of the structure for the convenience of real-time optical observation. Calibrations and preliminary experiments are conducted to validate the performance of the design.

Vibration and sensitivity analysis of piezoelectric microcantilever as a self-sensing sensor

Piezoelectric microcantilevers (MCs) have extensive applications in microelectromechanical systems. One of the applications of piezoelectric MCs is in self-sensing sensors. These sensors are highly popular due to their high accuracy, quick response, and environmental compatibility. Since the output current of piezoelectric layer is used as the sensing parameter in piezoelectric MCs, sensor optimization requires the maximum output current for each specific vibration. This paper uses dynamic piezoelectric MC analysis in different operating environments (air and liquid) to determine the factors influencing the output current of a piezoelectric layer. To obtain the differential equation of vibration, the hydrodynamic force applied to the piezoelectric MC by using the sphere string model. The equation was obtained via the Euler-Bernoulli beam theory and the Lagrange equation. The differential equation of the movement would yield both the MC deformation and the piezoelectric layer current. Using the Sobol statistical method for sensitivity analysis, the effect of each geometric parameter of the piezoelectric MC on the output current was studied to find the optimal geometry for the maximum output current. Results show that the output power of the piezoelectric layer is reduced in a liquid environment. Moreover, increasing the density of the liquid further exacerbates the drop in the output power. Therefore, in cases where the MC is to be used in a liquid environment (such as in biosensors), it is best to use a low-density liquid. The results also show that the electric current produced in the piezoelectric layer increases by reducing the length of the MC.

Application of Micro Electro-Mechanical Sensors (MEMS) Devices with Wifi Connectivity and Cloud Data Solution for Industrial Noise and Vibration Measurements

This paper introduces the application of Microelectromechanical Systems (MEMS) for industrial noise and vibration measurements. Sound measurement was done using a compact sound level meter with type 1 Micro-Electro-Mechanical Systems (MEMS) microphone while the vibration measurement used a compact vibration measuring device with 3-axial accelerometer, which is also a MEMS type sensor. The MEMS Sensors being used in both measuring devices provide advantages in terms of features and specifications such as weight reduction, compact in size and economically affordable for various applications. Besides MEMS Sensors, the devices are equipped with wireless connectivity to a local server via a WIFI router. This eliminates the usage of wired connection in data handling. Furthermore, Cloud connectivity is an add-on feature to the wireless capabilities of both devices. Using the Cloud features enable threshold alert alarm, data monitoring and collection to be done regardless of time and location of devices, provided that the internet connectivity is available to the user and the devices.

Enhanced-absorption template method for preparation of double-shell NiO hollow nanospheres with controllable particle size for nanothermite application

Nanothermite, due to its high energy density, short ignition delay time, and self-sustained exothermic reaction, is attracting wide attention. In this study, an ammonium hydroxide assisted enhanced-absorption carbon microsphere template method is adopted to prepare double-shell NiO hollow nanospheres with controllable particle size, which further are evaluated as the oxidizers in nanothermite. A possible evolutionary process to form the double-shell NiO is described according to the transmission electron microscopy images at different calcination temperature. The thermal and combustion performances of Al/NiO nanothermite are studied by differential scanning calorimetry, electric ignition and constant-volume pressure cell tests. Results show that as the particle size of NiO hollow nanospheres decreases, the nanothermites show lower activation energy, higher heat release, stronger light intensity, shorter ignition delay time and higher pressurization rate. Based on the optical signal, the combustion process is mainly affected by the mass diffusion distance, depending on the gap between the inner and outer layer. These results indicate that Al/NiO nanothermite has excellent ignition and combustion properties, which are very promising for practical applications in microelectromechanical system (MEMS).

Actuation and sensor integrated self-powered cantilever system based on TENG technology

Actuation and sensors for driving and monitoring of cantilever beam system are of most significant in the field of micro-electro-mechanical systems (MEMS). However, the AC voltage driving module and piezoelectric materials used in cantilever highly raise the system complexity and costs. Here, we propose a motion-triggered cantilever beam (MC) accompanied by an integrated real-time oscillation-monitoring sensor (ROS) designed based on the principle of triboelectric nanogenerator (TENG). Utilizing the periodically built and neutralized high DC voltage during the operation of the freestanding mode TENG, the low-frequency mechanical motion (down to 5 up to 200 mm/s) can be converted into high-frequency oscillation up to 100 Hz. Different from AC trigger method that requires excitation frequency matching the intrinsic frequency of the cantilever, DC actuation strategy is universal to all kinds of cantilever systems. Additionally, based on the single electrode mode TENG, the integrated self-powered ROS shows ultra-high sensitivity (223 mV/mm compared with 72 mV/mm for laser micro-motion meter). Finally, a self-powered micro-weighing system based on MC and ROS is demonstrated. This work exhibits notable advantages of using TENG technology for both power supply and sensing in MEMS applications.

Optimization of Titanium Nitride Film for High Power RF MEMS Applications

In this paper, TiN film has been deposited and optimized at room temperature for high power radio-frequency microelectromechanical system (RF-MEMS) applications. Being hard, titanium nitride is used in the contact area. The contact material should have low resistance and high hardness. TiN thin films were deposited by DC magnetron reactive sputtering using a four inch high purity titanium target in a nitrogen (N2) environment. X-ray diffraction (XRD) analysis is used to confirm crystal structure and purity of TiN film. The effect of various N2 pressure on resistivity and hardness of TiN thin film is investigated. The resistivity of the film decreases and hardness increases with N2 pressure.