Microelectromechanical Systems Fabrication Process Research Articles

Theoretical modeling and experimental investigation of in-phase resonant MEMS mirrors with cascaded structures

This paper proposes an efficient nonlinear one-dimensional (1D) compact mass-damper-spring model to predict the dynamic response of electrostatic resonant micro-electro-mechanical system (MEMS) mirrors with cascaded structures. The time-dependent damping moment due to viscous shear and pressure drag is computed using semi-empirical analytical equations for comb-drive structures and device frames. Nonlinear electrostatic force induced by the comb drives is efficiently acquired based on the hybrid method. The optimized device is fabricated using MEMS fabrication processes based on a 4-inch silicon-on-insulator wafer. The proposed compact model with the measured key parameters from the fabricated device shows excellent capability to accurately predict nonlinear dynamic responses of the fabricated device, including parametric excitation and hysteretic frequency response, with an average error of less than 5%. In particular, our 1D model is three orders of magnitude faster than the conventional finite element method model (0.8 s versus 1 h), enabling efficient system-level optimization of the critical design parameters. Based on the parametric study, electrode gap distance and torsion spring width are found to be two critical design parameters and dimensional analysis is conducted for design optimization with scan angle enhancing from 16.8° to 24° compared with the first design.

CMOS Compatible MEMS Multienvironmental Sensor Chip for Human Thermal Comfort Measurement in Smart Buildings

In this article, CMOS compatible microelectromechanical systems (MEMS) multienvironmental sensor chip (MESC) is reported. The chip is comprised of temperature, humidity, and air velocity sensors. The MESC has several advantages. First, it incorporates low-cost three-mask CMOS compatible MEMS fabrication process to realize temperature, humidity, and air velocity sensing in the environment. Second, the air velocity sensor on MESC utilizes dual pairs of detectors and is completely released from the substrate, which achieved low-power consumption. Finally, a minimum level of crosstalk effect is observed among the sensors fabricated on MESC. The characterization results indicated that the developed temperature sensor achieved a sensitivity of 20.09 mV/°C, the humidity sensor obtained an average sensitivity of 7.83 fF/%RH, and the air velocity sensor achieved a sensitivity of 312 mV/m/s in dual detectors mode. Furthermore, the crosstalk among sensors on the chip was studied, which suggested that the air velocity sensor can affect the chip (temperature and humidity sensors) temperature by 2 °C–7 °C. This crosstalk is relatively less than the reported multisensor chips (10 °C). The experimental results achieved by the developed MESC make it promising for human thermal comfort (HTC) measurement in smart buildings in the era of the Internet of Things (IoT).

Dynamic behaviours of double-ended tuning fork based comb-driven microelectromechanical resonators for modulating magnetic flux synchronously

Microelectromechanical system (MEMS) resonators have been widely used in the magneto-resistive (MR) sensor for modulating the magnetic flux to enhance the detection limit. However, the manufacturing tolerances in MEMS fabrication processes make it challenging to fabricate the identical resonators with the same vibration frequency, which greatly decreases the detection limit of the MR sensor. To synchronize the MEMS resonators and improve the performance of the MR sensor, the double-ended tuning fork (DETF) based comb-driven MEMS resonators is proposed in this paper, making the system operate at the out-of-phase mode to complete the synchronization. The dynamic behaviour of the resonators is investigated through theoretical analysis, numerical solution based on MATLAB code and Simulink, and experimental verification. The results show that the transverse capacitances in the comb will significantly affect the resonance frequency due to the second-order electrostatic spring constant. It is the first time to observe the phenomenon that the resonant frequency increases with the increase of the bias, and it can also decrease with increasing the bias through adjusting the initial space between the fixed finger and the moving mass, they are different from the model about spring softening and spring hardening. Besides, the proposed DETF based comb-driven resonators can suppress the in-phase and out-of-phase mode through adjusting the driving and sensing ports, and sensing method, meanwhile make the magnetic flux modulation fully synchronized, and maximize the modulation efficiency, and minimize the detection limit. These characteristics are appropriate for the MR sensor, even other devices that need to adjust the resonance frequency and vibration amplitude. Furthermore, the model and the design can also be extended to characteristic the single end tuning fork based MEMS resonator and other MEMS-based MR sensors.

Design, fabrication, and calibration of a micro-load cell for micro-resistojet development.

In this paper, a micro-load cell utilized for the thrust measurement of a micro-resistojet is described. To improve the degree of completion of the micro-resistojet during the development process, a concept of a micro-load cell, which enables sufficient performance evaluation under space environmental conditions, was proposed through minimization and simplification of a thrust measurement stand. The piezoresistive sensing method was applied to this load cell, which was composed of a membrane, strain gauges, and a built-in Wheatstone bridge. The membrane size was designed using large-deflection theory so that the load cell could exhibit linear characteristics within the measurement range. A polysilicon strain gauge with a high gauge factor was used for the piezoresistor. The strain gauge also had very low sensitivity to temperature, allowing accurate measurement of the membrane deflection caused by thrust without requiring an additional compensation circuit. Four strain gauges were placed to form a full-bridge circuit at the edge of the membrane. The load cell was then realized by a microelectromechanical system fabrication process. The fabricated load cell was calibrated using a three-axis precision moving stage and a commercial load cell. As a result, the load cell output signal was linear in the measurement range of 1-7 mN and the measured sensitivity of the sensor was 1.566 31 × 10-4 V/mN. The calculated nonlinearity was within 1% in the measurement range.

The Improvement of Performance through Minimizing Scallop Size in MEMS Based Micro Wind Turbine.

In this paper we report on the improvement of performance by minimizing scallop size through deep reactive-ion etching (DRIE) of rotors in micro-wind turbines based on micro-electro-mechanical systems (MEMS) technology. The surface profile of an MEMS rotor can be controlled by modifying the scallop size of the DRIE surface through changing the process recipe. The fabrication of a planar disk-type MEMS rotor through the MEMS fabrication process was carried out, and for the comparison of the improvements in the performance of each rotor, RPM testing and open circuit output voltage experiments of stators and permanent magnets were performed. We found that the smooth etching profile with a minimized scallop size formed using DRIE results in improved rotation properties in MEMS-based wind turbine rotors.

Research on Micro‐Electro‐Mechanical System‐Based Integrated Energy Harvester with Test Structures

Expansion of Internet of Things (IoT) in industrial fields requires a novel power supply. Research on a micro‐electro‐mechanical system (MEMS)‐based integrated energy harvester with test structures for power supply is proposed. The MEMS fabrication process is applied and amended to achieve a better performance. A metal heat sink is fabricated at the center of the harvester for thermoelectric gathering. The photoelectric harvester is in the back of the device to integrate with the thermoelectric harvester. The output voltage factor and output power factor of the thermoelectric harvester are measured to be 0.58 V cm−2 K−1 and 2.76 × 10−2 μW cm−2 K−2. The efficiencies when the photoelectric harvester works with top and bottom sides, respectively, are 5.45% and 0.27%. In addition, several parameter test structures are designed to explain how external and internal parameters affect the performance of the proposed device. The maximum output power factor is found to be related to the length and width of the thermocouple. This relationship provides a way to amend the performance of the proposed device in our further research.

A MEMS Fabrication Process with Thermal-Oxide Releasing Barriers and Polysilicon Sacrificial Layers for AlN Lamb-Wave Resonators to Achieve fs·Qm > 3.42 × 1012.

This paper presents a micro-electro-mechanical systems (MEMS) processing technology for Aluminum Nitride (AlN) Lamb-wave resonators (LWRs). Two LWRs with different frequencies of 402.1 MHz and 2.097 GHz by varying the top interdigitated (IDT) periods were designed and fabricated. To avoid the shortcomings of the uncontrollable etching of inactive areas during the releasing process and to improve the fabrication yield, a thermal oxide layer was employed below the platted polysilicon sacrificial layer, which could define the miniaturized release cavities well. In addition, the bottom Mo electrode that was manufactured had a gentle inclination angle, which could contribute to the growth of the high-quality AlN piezoelectric layer above the Mo layer and effectively prevent the device from breaking. The measured results show that the IDT-floating resonators with 12 μm and 2 μm electrode periods exhibit a motional quality factor (Qm) as high as 4382 and 1633. The series resonant frequency (fs)·Qm values can reach as high as 1.76 × 1012 and 3.42 × 1012, respectively. Furthermore, Al is more suitable as the top IDT material of the AlN LWRs than Au, and can contribute to achieving an excellent electrical performances due to the smaller density, smaller thermo-elastic damping (TED), and larger acoustic impedance difference between Al and AlN.

Overview of residual stress in MEMS structures: Its origin, measurement, and control

Micro-electro-mechanical system (MEMS) technology has radically changed the scale, performance, and cost of a wide variety of sensors and actuators by taking advantage of batch fabrication. The multidisciplinary nature of MEMS employs knowledge of diverse technical areas to realize improved and novel transducer systems. This also brings various associated challenges that are otherwise being ignored in a simple macro-dimensional system. MEMS devices typically comprise several deposited thick and thin films as well as bonded of dissimilar materials (like silicon, metal, glass, etc.). Residual stress is one of the most common outcomes during this integration/stacking of distinctly different materials for the fabrication of novel MEMS structures. The residual stress may significantly affect the performance and reliability of the fabricated devices. Thus, the evaluation and regulation of residual stress are one of the crucial aspects to assess the functioning of modern-day MEMS devices. This paper reviewed the origins of residual stress in MEMS fabrication processes. Different techniques involved in testing and characterization of the residual stresses are reviewed. Few important case studies are discussed to highlight the effect of residual stress (generated during various fabrication processes) on characteristics of different MEMS structures. The brief overview of the possible route to minimize the residual stresses is also presented.

Active atomic force microscope cantilevers with integrated device layer piezoresistive sensors

Active atomic force microscope cantilevers with on-chip actuation and sensing provide several advantages over passive cantilevers which rely on piezoacoustic base-excitation and optical beam deflection measurement. Active microcantilevers exhibit a clean frequency response, provide a path-way to miniturization and parallelization and avoid the need for optical alignment. However, active microcantilevers are presently limited by the feedthrough between actuators and sensors, and by the cost associated with custom microfabrication. In this work, we propose a hybrid cantilever design with integrated piezoelectric actuators and a piezoresistive sensor fabricated from the silicon device layer without requiring an additional doping step. As a result, the design can be fabricated using a commercial five-mask microelectromechanical systems fabrication process. The theoretical piezoresistor sensitivity is compared with finite element simulations and experimental results obtained from a prototype device. The proposed approach is demonstrated to be a promising alternative to conventional microcantilever actuation and deflection sensing.

Fabrication, Experiment, and Simulation of a Flexible Microvalve-Integrated Microarm for Microgrippers Using Electrorheological Fluid

Because of the power density advantages of fluid power systems, many researchers have developed microactuators using homogeneous electrorheological (ER) fluids (ERFs) for applications to various micromachines. An ER valve, as a critical component of the ER actuator, can control ERF flow by the apparent viscosity increase resulting from the applied electric field without any mechanical moving parts. Hence, it is adequate for the miniaturization of a fluidic microactuator. However, it is not easy to integrate rigid ER valves into soft microrobots. To overcome these limitations, we developed a novel elastic ER microarm using flexible ER valves (FERVs) in this study. Each microarm consists of an FERV, a movable chamber, and a displacement constraint element, so that it bends with the inner pressure controlled by the FERV. We proposed and developed a micro-electromechanical system fabrication process for the FERV, movable chamber, and displacement constraint element. By utilizing the proposed method, we successfully fabricate a FERV-integrated microarm. The characteristics of the FERV were experimentally clarified. In addition, the bending motion of the FERV-integrated microarm was demonstrated by experiments and verified by finite-element method simulation. This ER microarm was shown to be feasible for an ER microgripper composed of multiple microarms.

Enhancement of microcantilever beams fabrication and determination of their mechanical properties using nanoindentation

Microcantilever beams were fabricated through a microelectromechanical system fabrication process; this process was improved by using etched slots. The actual mechanical properties of the cantilever beams can be estimated using a nanoindenter. Various cantilever beam widths and length-to-width ratios were used for evaluation. Five structures of cantilever beams were fabricated and compared. ANSYS software was used to simulate cantilever beam deflections. The etched slots increased the cantilever beam deflections, and the increase in deflection was apparent when the etched slot was located near the base of the beams. Type 4 was a cantilever beam with an etched slot of length L /2 near the base of the beam. The area of the etched slots of type 4 was larger, and the etched slot of type 4 was near the base of the cantilever beam. Type 4 had the shortest etching time, highest fabrication success rate, and greatest deflection. The difference between the measured and simulated (without parameter modification) deflections could exceed 10%. Modifying the force acting point and Young's modulus in the simulations reduced the difference between the measured and simulated deflections to 0.5%. This demonstrates that actual experimental conditions considerably influence simulation results, and this can be beneficial to subsequent studies.

Four-Probe Bridges for In-Line Determination of Thickness and Sidewall Etch Angle of Surface Micromachined Thin Films

Geometrical parameters, such as the thickness and the sidewall etch angle of microelectromechanical systems (MEMS) thin films, are important information for device design and simulation, material property extraction, and quality control in a fabrication process line. This paper presents an in-line test microstructure for measuring geometrical parameters of surface micromachined thin films. The structure consists of four-probe bridges with continuous step structures and deposited at three different angles. The extraction method takes advantage of the resistances of the step structures to determine the thickness and the sidewall etch angle of the phosphosilicate glass (PSG) layer and the thickness of the polysilicon layer. The sheet resistance and the width of the test structure are required for the extraction method and can also be measured by using the test structure. Thicknesses of (2.080 ± 0.011) µm, (2.142 ± 0.012) µm, (1.614 ± 0.014) µm and (2.892 ± 0.012) µm are obtained for the Poly 1 layer, the Oxide 1 layer, the Poly 2 layer and the stacked layer of Oxide 1-Oxide 2, respectively. The sidewall etch angles for the Oxide 1 layer and the stacked layer of Oxide 1-Oxide 2 are obtained as (77.51 ± 0.61)° and (76.17 ± 0.91)°, respectively. In comparison to previously reported thickness measurement approaches, the proposed method is nondestructive, and makes use of four-point probe technique which is featured with electrical input and output configuration, simple operation, low cost, fast response, good repeatability and ease of integration. Therefore, this method is more suited to in-line monitoring the MEMS fabrication process.

A liquid MEMS inclinometer sensor with improved sensitivity

This paper proposes a liquid microelectromechanical system (MEMS) inclinometer sensor and reports its design, fabrication, and characterization. In the sensor, a liquid metal droplet moves inside an annular-shaped channel, driven by gravity. The position of the liquid metal is reflected by electrodes and the new tilting angle is obtained. A MEMS fabrication process has been developed, and characterisation of the sensor has been carried out. Experiments show that the sensor has a resolution of 3.625° and a wide measuring range of ±45°. In comparison with solid MEMS gyroscopes, the proposed liquid sensor has advantages of fewer manufacturing steps and low costs. Beside, its resilience against impact is outstanding. Its sensitivity and accuracy are more than sufficient for hand held electronics, which are the main targeted applications of the proposed liquid sensor.

Insect-inspired acoustic micro-sensors.

Micro-Electro Mechanical System (MEMS) microphones inspired by the remarkable phonotactic capability of Ormia ochracea offer the promise of microscale directional microphones with a greatly reduced need for post-processing of signals. Gravid O. ochracea females can locate their host cricket's 5 kHz mating calls to an accuracy of less than 2° despite having a distance of approximately 500 μm between the ears. MEMS devices base on the principles of operation of O. ochracea's hearing system have been well studied, however commercial implementation has proven challenging due to the system's reliance on carefully tailored ratios of stiffness and damping, which are difficult to realize in standard MEMS fabrication processes, necessitating a trade-off between wide-band operation and sensitivity. A survey of the variety of strategies that have been followed to address these inherent challenges is presented.

2-D MEMS Scanner for Handheld Multispectral Dual-Axis Confocal Microscopes

We describe a 2-D microelectromechanical systems (MEMS) scanner for a handheld multispectral confocal microscope for early detection and diagnosis of cervical cancer. This paper is an expansion of work originally reported in the Proceedings of IEEE International Conference on Optical MEMS and Nanophotonics 2012. The MEMS scanner and fabrication process has been designed to improve performance for the dual-axis confocal microscopy architecture. Dual-axis confocal microscopy achieves comparable transverse and depth resolution to high numerical aperture (NA) single-axis confocal microscopy but achieves longer working distance by utilizing low-NA optics. The MEMS scanner has an inner gimbal design with torsional flexures separated from the reflectors to reduce light loss and oxide-free outer axis torsional flexures by utilizing poly vias for electrical access to the inner gimbal electrodes. The devices are large-scale batch fabricated using a double layer silicon-on-insulator (SOI) process and predice postreleasing of dies. The scanner has electrostatic optical deflection angles of 2° for the single-sided inner axis at 75 V and ±3° for the outer axis at 120 V. The device has resonance frequencies of ~2.48 kHz and ~348 Hz for the inner and outer axis torsional modes, respectively. Reflectance and fluorescence images with 650 μm × 800 μm field of view are demonstrated at 15 frames/s. The transverse (axial) resolutions are 4.78 μm (6.2 μm), 5.77 μm (6.2 μm), and 7.45 μm (5.6 μm) for wavelengths of 561, 662, and 781 nm, respectively.

A quality quantitative method of silicon direct bonding based on wavelet image analysis

The rapid development of MEMS (micro-electro-mechanical systems) has received significant attention from researchers in various fields and subjects. In particular, the MEMS fabrication process is elaborate and, as such, has been the focus of extensive research inquiries. However, in MEMS fabrication, component bonding is difficult to achieve and requires a complex approach. Thus, improvements in bonding quality are relatively important objectives. A higher quality bond can only be achieved with improved measurement and testing capabilities. In particular, the traditional testing methods mainly include infrared testing, tensile testing, and strength testing, despite the fact that using these methods to measure bond quality often results in low efficiency or destructive analysis. Therefore, this paper focuses on the development of a precise, nondestructive visual testing method based on wavelet image analysis that is shown to be highly effective in practice. The process of wavelet image analysis includes wavelet image denoising, wavelet image enhancement, and contrast enhancement, and as an end result, can display an image with low background noise. In addition, because the wavelet analysis software was developed with MATLAB, it can reveal the bonding boundaries and bonding rates to precisely indicate the bond quality at all locations on the wafer. This work also presents a set of orthogonal experiments that consist of three prebonding factors, the prebonding temperature, the positive pressure value and the prebonding time, which are used to analyze the prebonding quality. This method was used to quantify the quality of silicon-to-silicon wafer bonding, yielding standard treatment quantities that could be practical for large-scale use.

Grasping Force Control for a Robotic Hand by Slip Detection Using Developed Micro Laser Doppler Velocimeter.

The purpose of this paper is to show the feasibility of grasping force control by feeding back signals of the developed micro-laser Doppler velocimeter (μ-LDV) and by discriminating whether a grasped object is slipping or not. LDV is well known as a high response surface velocity sensor which can measure various surfaces—such as metal, paper, film, and so on—thus suggesting the potential application of LDV as a slip sensor for grasping various objects. However, the use of LDV as a slip sensor has not yet been reported because the size of LDVs is too large to be installed on a robotic fingertip. We have solved the size problem and enabled the performance of a feasibility test with a few-millimeter-scale LDV referred to as micro-LDV (μ-LDV) by modifying the design which was adopted from MEMS (microelectromechanical systems) fabrication process. In this paper, by applying our developed μ-LDV as a slip sensor, we have successfully demonstrated grasping force control with three target objects—aluminum block, wood block, and white acrylic block—considering that various objects made of these materials can be found in homes and factories, without grasping force feedback. We provide proofs that LDV is a new promising candidate slip sensor for grasping force control to execute target grasping.

A study on an AC electroosmotic micropump using a square pole – Slit electrode array

The paper presents a novel AC electroosmotic micropump using a square pole – slit electrode array called the SS-ACEO-MP. The AC electroosmosis (ACEO) generates convection flows when AC voltage is applied to a liquid such as water with fixed electrodes. The ACEO flow is stable without electrolysis due to the low AC voltage; however, to realize a micropump for wide applications, a special mechanism for a unidirectional flow is required. For that purpose, conventional devices utilize asymmetric electrodes, induced-charge electroosmosis, and traveling wave voltage. For practical applications, high slip velocity due to ACEO is required. The proposed pump consists of a square pole – slit electrode array, and efficiently generates a total flow by the slip velocity mainly on the slit electrodes. The proposed pump features a simple pillar-shaped structure without moving parts, efficient flow generation, and easy improvements of the pressure and the flow rate due to the arrayed structure. First, 3D FEM (finite element method) simulation of the SS-ACEO-MPs was performed and the characteristics were clarified. The size parameters that maximize the effective slip velocity under no load were clarified. Second, the SS-ACEO-MPs were fabricated by a MEMS (micro electro mechanical systems) fabrication process that included electroforming. Third, experiments on the SS-ACEO-MPs were performed. The characteristics of the SS-ACEO-MPs were clarified. Finally, at the applied voltage of 26Vpp, the maximum effective slip velocity of 1.6mm/s was realized with a unit size of 0.2×0.2×0.05mm3.

On-Chip Dynamic Mode Atomic Force Microscopy: A Silicon-on-Insulator MEMS Approach

The atomic force microscope (AFM) is an invaluable scientific tool; however, its conventional implementation as a relatively costly macroscale system is a barrier to its more widespread use. A microelectromechanical systems (MEMS) approach to AFM design has the potential to significantly reduce the cost and complexity of the AFM, expanding its utility beyond current applications. This paper presents an on-chip AFM based on a silicon-on-insulator MEMS fabrication process. The device features integrated xy electrostatic actuators and electrothermal sensors as well as an AlN piezoelectric layer for out-of-plane actuation and integrated deflection sensing of a microcantilever. The three-degree-of-freedom design allows the probe scanner to obtain topographic tapping-mode AFM images with an imaging range of up to 8μm × 8μm in closed loop.

Two-axis polydimethylsiloxane-based electromagnetic microelectromechanical system scanning mirror for optical coherence tomography.

Compact size and fast imaging abilities are key requirements for the clinical implementation of an optical coherence tomography (OCT) system. Among the various small-sized technology, a microelectromechanical system (MEMS) scanning mirror is widely used in a miniaturized OCT system. However, the complexities of conventional MEMS fabrication processes and relatively high costs have restricted fast clinical translation and commercialization of the OCT systems. To resolve these problems, we developed a two-axis polydimethylsiloxane (PDMS)-based MEMS (2A-PDMS-MEMS) scanning mirror through simple processes with low costs. It had a small size of 15×15×15??mm3, was fast, and had a wide scanning range at a low voltage. The AC/DC responses were measured to evaluate the performance of the 2A-PDMS-MEMS scanning mirror. The maximum scanning angles were measured as ±16.6??deg and ±11.6??deg along the X and Y axes, respectively, and the corresponding field of view was 29.8??mm×20.5??mm with an optical focal length of 50 mm. The resonance frequencies were 82 and 57 Hz along the X and Y axes, respectively. Finally, in vivo B-scan and volumetric OCT images of human fingertips and palms were successfully acquired using the developed SD-OCT system based on the 2A-PDMS-MEMS scanning mirror.