Micro Electro Mechanical Systems

Silicon-based wet anisotropic etching is one of the most widely used techniques for the realization of various kinds of components for the fabrication of microelectromechanical systems (MEMS)-based devices. This technique uses liquid-based etchants to selectively etch the bulk silicon to fabricate the microstructures. Various geometric models or atomic-level models have been presented for silicon anisotropic etching simulations to optimize the anisotropic etching processes and improve the efficiency of MEMS design. This chapter develops a novel model for silicon anisotropic etching in alkaline solutions based on the surface atom configurations on different crystal planes. The surface atoms are divided into some categories according to the atom configurations, and the microscopic etch rates of surface atoms are related to the macroscopic etch rates of different silicon crystal planes. The microscopic activation energies for some typical surface atoms are further calculated by the microscopic etch rates for surface atoms, to simplify the fitting procedures. The model has been extended to a simulation systems based on a dynamic cellular automaton method, and a series of simulations have been performed using the simulation system for various etching conditions. The simulation results demonstrate to be in agreement with the experimental results. This indicates the effectiveness of the model, and this is useful for the research of anisotropic etching technology and the development of MEMS design.

As a widespread form of dry etching, deep reactive ion etching (DRIE) is a highly anisotropic etch process. It alternates switching the chemistry for etching and passivation cycles, typically leads to characteristic scalloping patterns on the sidewalls with high aspect ratios. Measurements of the etch depth per cycle l _d and undercut length per cycle l _u show a strong dependence of the undercut ratio l _u /l _d on the trench aspect ratio for a wide range of opening sizes. Although various simulation models have been proposed, the determination of the corresponding parameters from experimental data remains unsolved. We present the use of (i) the continuous cellular automaton (CCA), to simulate the process reliably in three dimensions; (ii) the particle swarm optimization (PSO) method, to determine suitable values for the atomistic CCA parameters directly from experimental data; and (iii) a GPU, parallel implementation of the CCA, to increase the computational efficiency of the simulations. The resultant, parameter-optimized CCA simulations show good agreement with the experiments. The approach has a large potential for the simulation of other MEMS processes.

SU-8 photoresist can be used to produce high aspect ratio and three-dimensional (3D) lithographic patterning based on standard contact lithography equipment due to its excellent coating, planarization, and processing properties and thus has become the favorite photoresist material for the fabrication of various microelectromechanical system (MEMS) structures and devices. However, as feature sizes get smaller and pattern complexity increases, particular difficulties arise and need to be carefully considered. The accuracy and precision, with which a feature on a mask can be reproduced throughout a thick resist structure, will depend on key parameters in the setup, the material properties of the SU-8 resist, and the thickness of the resist structure. Modeling and simulation studies may help improve our understanding and process design of the SU-8 lithography, thereby allowing rapid product and process development. In this chapter, the basic process and mechanism of UV lithography of the SU-8 are introduced briefly. Various models for the lithography, including the aerial image model, exposure model, postexposure bake model, and development model, are presented and discussed. Main algorithms for the etching surface advancement simulation, including the string, ray-tracing, cellular automaton, and fast-marching algorithms, are then compared and analyzed. Simulations of the UV lithography of the SU-8 are presented, and a series of experiments have been performed for SU-8 2000 series photoresists under UV source with 365 nm (2.6 mW/cm2) radiation. The simulation results demonstrate to be in agreement with the experimental results. This is useful to optimize the inclined UV lithography processes of SU-8 photoresists and to accurately design and control the dimensions of some MEMS microstructures.

Electrostatically actuated microplates have been widely used in various microsensors and actuators actuated by electrostatic force. Deep knowledge of the microplates under electrostatic force and other physical quantities is extremely important for the design and optimization of the microsensors and actuators, which can largely reduce the cost and time to develop the proposed devices compared with over and over fabrication and testing in laboratory. This chapter gives a detailed illustration on the modeling methods of electrostatically actuated microplates. Three types of modeling methods are mainly discussed, that is, finite element modeling, lumped electromechanical modeling, and distributed electromechanical modeling. For the finite element modeling method, the electromechanical elements used to model the electrostatic domain and the establishment methods of finite element electromechanical models are given for electrostatically actuated microplates. The lumped electromechanical modeling method models the electrostatically actuated microplate as one-dimensional spring-mass-capacitor system, which can qualitatively analyze the collapse voltage (or pull-in voltage) and the reason why the resonant frequency shifts under electrostatic force. For the distributed electromechanical modeling method, the electromechanical coupling models for circular and rectangular microplates under electrostatic force and hydrostatic pressure are established, and explicit theoretical expressions for collapse voltage, static deflection, and capacitance variation are proposed. In addition, the distributed modeling method also focuses on the dynamic behavior analysis, especially the resonant frequency analysis of the electrostatically actuated microplate.

Two macromodeling techniques for inertial sensors are discussed in this chapter. The first one is a parametric model order reduction (PMOR) method based on the implicit moment matching to accommodate the parameter variation. The second one is the trajectory piecewise-linear (TPWL) method which is developed for dealing with the strong nonlinearity. For each technique, its effectiveness is demonstrated by the applications to read devices characterization.

Recently, multilayered structures have been utilized in MEMS applications, including infrared focal plane arrays, radio-frequency (RF) components, micromachined mirrors, etc. It is well known that MEMS devices are highly dependent on material parameters such as Young’s modulus and residual stress of the multilayered films. These properties determine both the final shape and the functionality of released microstructures and should therefore be accurately evaluated. Young’s modulus and residual stress for single-layer films have been widely studied by the cantilever deflections, wafer curvatures, displacements of variously designed microstructures, buckling lengths, membrane deflections, resonance frequency, pull-in voltages, and double-clamped beam deflections. However, these methods are not easily extended to multilayered films. Thus it is significantly expected to directly measure both Young’s modulus and residual stress for multilayer films simultaneously. This chapter presents some methods to characterize the material properties of the composite films by electrostatic pull-in testing and the resonance frequency testing approaches adopting the composite double-clamped beam or the cantilever beam. The analytical models are presented and test structures with different lengths and widths are designed. In situ methods for simultaneously extracting material properties (Young’s modulus and residual stress) of each layer for the composite films are reported. The extracting methods have been confirmed by FEM simulations and experiments.

Thermophysical properties of MEMS materials, such as thermal conductivity, thermal diffusivity, and coefficient of thermal expansion (CTE), are one of the most important properties in MEMS technology. Steady-state thermal response and transient-state thermal response of MEMS devices depend on the thermal conductivity and the thermal diffusivity of device materials. Thermally driven microstructures, on the other hand, exploit the thermal expansion effect for their operation. It is necessary to characterize the thermophysical properties of MEMS materials for the design of MEMS devices.This chapter will present online test microstructures and measurement methods for the thermophysical properties of MEMS conducting beams. The background of the work is reviewed in section “Introduction.” In section “Online Test Microstructure of Thermal Conductivity,” test microstructures for thermal conductivity based on steady-state thermal analysis are developed. Section “Online Test Microstructure of Thermal Conductivity and Thermal Diffusivity” is dedicated to discussing transient-state thermal analysis and proposing a test microstructure for both thermal conductivity and thermal diffusivity. In sections “Online Test Microstructure of the Coefficient of Thermal Expansion by Rotating Technique” and “Online Test Microstructure of the Coefficient of Thermal Expansion by a Pull-In Approach,” the coefficient of thermal expansion is extracted by micro-rotating structures and double-clamped beams, respectively. The former takes advantage of thermal actuation, while the latter makes use of the electrostatic pull-in approach. All the test microstructures proposed in sections. “Online Test Microstructure of Thermal Conductivity,” “Online Test Microstructure of Thermal Conductivity and Thermal Diffusivity” and “Online Test Microstructure of the Coefficient of Thermal Expansion by Rotating Technique” are stimulated electrically and measured electrically. They can find applications in MEMS fabrication process line to provide direct quality control and obtain the data needed by MEMS designers.

Thin film mechanical property evaluation has become increasingly important for micro devices. To assure the reliability of the devices and to predict the lifetime, fracture properties of thin films need to be investigated. Microtensile testing is a powerful technique to characterize the mechanical and fracture properties of microscale thin films. But the specimen preparation always causes a difficulty. The bulge test, as a relatively simple, fast, and precise method, is extended to the determination of the fracture properties of thin films, such as bending stiffness and prestress of the membrane material, the Young’s modulus, and fracture strength of single layer film and bilayer films. An accurate model describing load-deflection response is applied on thin films made of silicon nitride, silicon carbide, and composite diaphragms of silicon nitride grown on top of thermal silicon oxide films. Fracture reference stresses were computed according to the Weibull model for brittle fracture by integrating the membrane stress over the edge, surface, and volume of the samples, corresponding respectively to the assumption of dominant edge, surface, and volume flaws. This method is very efficient and able to quantify fracture parameters of single and multilayer films.

This chapter gives a complete development process of a high- temperature silicon pressure sensor. Firstly, the piezoresistive effect was described for sensor chip used in high temperature application. Based on the SiO₂ isolation layer, the leakage current generating through p-n junction can be eliminated, which ensured the proper function of piezoresistors in high-temperature working condition. Secondly, the mechanics models for circular, rectangular, and island-structured diaphragms had been presented. Also a theoretical guide for designing and optimizing a sensor chip was presented in section “Mechanics Model of the Pressure Sensor Chip.” Thirdly, based on the mechanics models, section “Structure Designing, Lithography Mask Designing and Fabrication of the Sensor Chip” presented a designing principle of sensor chip to find a balance point between the sensitivity and dynamic response frequency, in order to take full use of elastic strain energy induced by structure deformation. Then, lithography masks for corresponding pattern structures were presented. Followed by the lithography mask designing, the fabrication process of sensor chip was presented. Fourthly, the packaging technology for sensor chip was presented in section “Packaging Structure for the Pressure Sensor Chip.” The packaging structure not only enabled the sensor chip to work properly in a harsh environment but also ensured the packaged sensor had a good performance in dynamic and sensitivity performance. Finally, in section “Conclusions,” the experimental calibration for the sensitivity, dynamic performance, and cross-sensitivity of developed high-temperature silicon pressure sensor were conducted to obtain a comprehensive performance evaluation.

Micromachined silicon resonant pressure sensors have been widely used in automotive industry, medical instrument, aerospace, and military fields due to their high accuracy, long-term stability, and quasi-digital output. This chapter begins with the introduction of the working principle of the resonant pressure sensors, illustrating key relationships between (1) intrinsic resonant frequency and structural parameters, (2) pressure under measurement and resonant frequency shift, and (3) device sensitivity and structural parameters. Then, two kinds of micromachined silicon resonant pressure sensors based on electromagnetic and electrostatic excitations are presented, respectively, where device design, simulation, fabrication, and packaging are discussed in details. Finally, self-temperature compensation approaches are introduced to improve the performance of the micromachined silicon resonant pressure sensors, which can therefore function in a wide temperature range.

MEMS vibratory gyroscopes are widely used in various fields such as consumer electronics, automotive industry, and navigation systems owing to their merits of low power, low cost, and small volume. A MEMS gyroscope is one of the most important inertial devices, which is researched all over the world.This chapter will introduce a MEMS vibratory gyroscope, aiming to make more people understand what it is and how it works. Firstly, structure design and fabrication, as well as its dynamics theory, are illustrated. Secondly, closed-loop control for the drive mode and mode-matching control are demonstrated. Finally, closed-loop control for the sense mode, followed by temperature compensation method, is presented in detail. In addition, theoretical deduction, simulation, and experimental tests are combined tightly to make the proposed techniques understand easily.

This chapter presents the fundamental theory, mechanical design, fabrication technique, detecting circuit, and characterization of a novel double differential capacitive torsional accelerometer. The accelerometer consists of a double differential sensing structure with four proof masses hanging on a common V-shaped torsional beam which mainly aims to improve the temperature robustness and long-term performance of the torsional accelerometer. This chapter supplies a new method for the accelerometer performances improvement and this method can also be used in the design of other sensors.

Development of micromachined inertial sensors has been widely addressed for many years. Most micromachined inertial sensors generally use a mechanical structure including a solid proof mass suspended on springs, which raises the complexity of structure and fabrication and particularly restricts the high shock resistance of the sensor. In this chapter, we introduce a kind of micromachined thermal gas inertial sensor by using thermally driven gaseous flow instead of solid proof mass. The sensor generally consists of one or several heaters and multiple thermistors, which detects the deflections of temperature profile induced by inertial quantities. The thermal inertial sensors, including thermal convective accelerometer and thermal gas gyroscope, have exhibited unique advantages of simple structure, low cost, and high shock resistance.

This chapter introduces the micromachined thermal wind sensor by MEMS technology. First, three working principles and operation modes are addressed. Then, several typical wind sensors including 1D and 2D devices are presented and the experimental results are given. In order to improve the sensitivity and reduce the power consumption, several methods, such as thinned substrate and low-thermal-conductivity substrate, are presented. Experiments demonstrate that the power consumption has been reduced to less than 20 mW by glass reflow technology. In practice, the MEMS wind sensor can be packaged using DCA package, FC package, and Au-Au bonding package. Despite this, the wind sensor is easy to be affected by the surrounding temperature. Related model is established and verified by the experiments.

Vacuum sensor is a type of pressure sensor and widely used in the vacuum measurement. In this chapter, besides the traditional vacuum sensors, the micro-vacuum sensors based on silicon micromachining technique are introduced. What is more, the CMOS-compatible Pirani and thermal ionization vacuum sensors are discussed in details. The CMOS-compatible process is important for sensor fabrication, including suspending structure, wet etching, dry etching, etc. The influence of sensor to the circuit should also be considered, such as the thermal influence. The sensor system can integrate the sensor with voltage reference, signal sampling circuit, A/D converter (ADC), central processing unit (CPU), and interface circuit. This technique will generate the vacuum sensor system with sensor and its control system monolithically, which is convenient for applications. The integrated vacuum sensor system has the advantages of small size, volume, and weight, which would have its potential applications, such as the space exploration, micro-packaging, etc.

Measurement of fluid mechanics is very important in various fields, and flow sensors have been widely applied to execute accurate and efficient measurements. Compared with other sensing principle, thermal flow sensors are based on convective heat transfer and take merits of simple structure and easy use and thus offer a practical solution for various fluidics applications. In this chapter, we describe mainly hot-film anemometer fabricated on polyimide substrate. Hot-film or hot-wire anemometer utilizes a thermal element that serves as both a joule heater and a temperature sensor. We introduce the principle of thermal flow sensing, design and fabrication of the micro hot-film flow sensor, the measurement methodology, and application cases by using the micro hot-film flow sensors.

In this chapter, DC current sensors applicable to two-wire appliances are presented. Firstly, background information of current sensors is presented showing the properties of different current measurement applications. Working principles of passive and actuating cantilever-based current sensors are then demonstrated theoretically, and the expressions of magnetic force and piezoelectric outputs are derived. Specifically, the design of partitioning piezoelectric thin film from one plate to n plates in series is discussed. Following are structural design and prototype fabrication process. With the achieved prototypes, two types of current measurements are carried respectively for verification and demonstration of the working principles. Finally, a conclusion is drawn and perspective of cantilever-based current sensors is given.

Detection of trace energetic chemical (TEC) vapors is a challenging task because of the extremely low vapor concentrations of most TECs. Microcalorimeters, which consist of a suspended microbridge with integrated heaters and thermistors, are emerging as a powerful tool for fast detection of TECs. By heating the TEC molecules adsorbed onto the microcalorimeters to deflagration using the heaters and measuring the induced thermal responses and the total heat using the thermistors, microcalorimeters can detect TEC vapors through differential scanning calorimetry mode or differential thermal analysis mode. Due to the large surface areas, the small heat mass, and the rapid heating rates, the microcalorimeters are able to detect TEC vapors with low detection limits and fast detection rates.

Microcantilevers, which are highly attractive for their small size, high sensitivity, and low cost, have been successfully used in label-free biological and chemical sensing applications during the past 20 years. In this chapter, a piezoresistive microcantilever-based biochemical sensor is introduced, in which a mechanical bending induced by a biochemical reaction or absorption on the surface of the microcantilever is changed into an electrical signal by integrated piezoresistors. Theory and design method are introduced firstly, and then fabrication technique and characteristics of the microcantilever sensors are described in details. Finally, we introduce some biochemical detection results measured with the piezoresistive microcantilever-based sensors.

Microelectromechanical system (MEMS) technology has been promoting development of sensors. Micro-heaters, as typical MEMS devices, have been extensively researched for high-performance gas sensors in the past two decades. This chapter presents the design, fabrication, and characterization of micro-heaters with low power consumption, well-controlled temperature distribution, high mechanical strength, and long-term stability. Catalytic gas sensors and semiconductor gas sensors will be introduced with novel design and high performance. The goal of this chapter is to provide the reader with a general overview of micro-heaters and broader ways of designing better micro-heater-based gas sensors.

In recent years, MEMS, short for microelectromechanical systems, has drawn increasingly attention for its advantage on mass sensing. MEMS is the integration of mechanical elements, sensors, and electronics on a common silicon substrate through microfabrication technology. These devices replace bulky counterparts with micron-scale equivalent that can produce in large quantities by fabrication process used in integrated circuits in photolithography. Meanwhile, the devices reduce cost, bulk, weight, and power consumption greatly while increasing performance, production volume, and functionality by orders of magnitude. In particular, cantilever-based sensors fabricated by MEMS technology display high sensitivity as well as resolution in mass sensing.This chapter reports two methods for picogram-order mass sensing by studying the response in frequency shifts and amplitude changes of a simplified cantilever array. Firstly, the mode localization phenomenon in the linear vibration region of a localized cantilever array is investigated for mass sensing with ultrahigh sensitivity. It is demonstrated by theoretical analysis and experimental verification that eigenstate shifts (amplitude shifts) are three to four times greater than corresponding ones in resonant frequency with a picogram-order mass perturbation. Secondly, the synchronization phenomenon in the nonlinear vibration region of a synchronized cantilever array is studied for mass sensing with high resolution. Synchronization via beam-shaped cantilever array and one U-shaped cantilever coupled with another beam shape cantilever has been researched for different application, respectively. Specifically, frequency multiplication and phase noise suppression of synchronized oscillation are demonstrated via the theoretical analysis and experimental verification of frequency locking and synchronized region for coupled cantilever array, which is beneficial to improve the quality factor and resolution of mass sensing.

Humidity sensors play a significant role in agriculture, industry, household, medicine, and so on. Traditional mechanical humidity sensors have been replaced by electrical humidity sensors since the revolution of electronic industry. A lot of efforts have been made to perfect the design and fabrication of the electrical humidity sensors.This chapter will discuss some researches of the electrical humidity sensors with the aid of micromachining technologies. Here, a humidity sensor is not an instrument. Instead, it is an electronic device which is fabricated on a wafer just like a MOS FET. However, unlike transistors, design and fabrication of micromachined humidity sensors involve the knowledge of electrics, chemistry, mechanics, and so on.We aim to give a general view on the micromachined humidity sensors. Several design and fabrication technologies will be reviewed in this chapter.

This chapter reports on the state of the art of piezoelectric micro-/nano-mechanical devices in frequency control and sensing applications. Recent studies on bulk acoustic wave (BAW) devices are introduced, including investigation of high-coupling materials and filter and oscillator designs. A novel class of frequency devices based on Lamb waves is also reviewed. Micro- and nano-mechanical sensors for various sensing applications and integrated module are outlined.

Water pollution is serious in many countries, and eutrophication and heavy metals are the main pollutants. Cost-effective, miniaturized and highly sensitive sensors for on-site detection and online monitoring of water quality have attracted more attentions. In this chapter, several microsensors and systems are presented for highly sensitive detection of total phosphorus (TP), total nitrogen (TN), and heavy metal ions (Cu2+, Pb2+, Zn2+, Hg2+), which are main indicators for eutrophication and heavy metals pollution. As the effective ways to improve the sensitivity and the limit of detection, several sensitivity-enrichment methods such as nano-materials modification on microelectrodes, and pretreatment processes such as thermally assisted ultraviolet digestion and ionic-liquid based preconcentration are also introduced. The electrochemical microsensors were fabricated by MEMS-based bulk fabrication process, and integrated with microfluidic units to realize a continuous monitoring of trace pollution targets. A buoy-based automated analytical system was developed by integrating central controller module, sequential injection module, and electrochemical analytical module together, and it was applied to online monitoring of water quality in fresh waters.

A method of optimizing the shape of the driving electrode for switching application is proposed. This method increases the driving force, and decreases the driving voltage of the in-plane electrostatic MEMS switches as well. In this method, optimized comb shape is adopted so that better performance is obtained without the deterioration of other aspects of performance: the area of the device and the requirement of the process are similar to the traditional in-plane devices. Compared with the traditional comb tooth electrode, the experimental results show that the pull-down voltage of the optimized driving electrode is 39% lower than that of the traditional comb tooth electrode, and the driving performance is improved obviously. At a line width of 6 μm, a MEMS switch with a critical pull-down voltage of 14 V is implemented.

This chapter presents MEMS actuators driven by Lorentz force. This kind of actuators linearly depends on the current perpendicular to the magnetic field, requires no magnetic materials, and has no magnetic hysteresis effect. These advantages result in the actuators of simple structure, linear motion, fast response, reasonable power consumption, and ideal for large stroke application. A novel actuator with folded beams structure is developed for a large lateral stroke. A displacement of more than 55 μm was achieved with a magnetic field of 0.14 T and the driving current of 8 mA. The actuator can generate a large displacement by a low driving voltage and can be easily integrated with CMOS circuits. Lorentz force is proportional to the magnetic field and the driving current, which results in an easy control of the lateral displacement by the driving current. The simple structure and fabrication process of the actuator lead to a high fabrication yield and good stability.

The inertial switch as a kind of passive electric device is also called shock sensor (G-sensor), acceleration switch, vibration threshold sensor, or G-switch. With the developments of semiconductor and integrated circuit technologies, especially, the MEMS-based inertial switches have been attracting much attention due to many advantages such as small size, lower costs, and large volume production. And they are widely used in many applications such as accessories, toys, the transportation of special goods, automotive electronics, remote monitoring (RMON), Internet of Things (IoT) fields, etc. In this chapter, the basic physical model and the working principle of MEMS-based inertial switch is presented firstly. Then the latest progress of the MEMS inertial switch is introduced. Subsequently, the MEMS inertial switch based on non-silicon surface micromachining technology is described in detail, including its design, simulation, fabrication, and characterization. In addition, the inertial microswitches with different sensitive directions are proposed and fabricated, including the triaxial inertial switch and omnidirectional sensitive ones. Finally, a simple application example of the fabricated MEMS inertial switch is also performed for potential vibration monitoring module and system applications.

This chapter presents the design, analysis, fabrication, and testing of a microelectromechanical systems (MEMS) rotary microgripper with self-locking function via a ratchet mechanism. The microgripper utilizes rotary actuators to solve the pull-in problem of microgrippers during large displacement manipulation and therefore avoids the widely used conversion systems which necessitate a high driving voltage. Furthermore, the ratchet mechanism enables long-time gripping without having to continuously apply the external excitation signal. Thus, the damage to the gripped micro-scale objects caused by the external excitation signals can be significantly reduced. The microgripper is fabricated by a silicon-on-insulator (SOI) dicing-free process to protect the delicate device structures from damage during fabrication. The microgripper has a discrete opening range and can handle micro-scale objects with a size of 20 μm, 40 μm, and 60 μm based on the current design parameters. Test results showed the gripper obtained a displacement of 100 μm with an applied voltage of 31.5 V. A pick-lock-release gripping experiment on a magnolia pollen cell is performed to form a triangle to prove the feasibility of the gripper in handling biological cells.

In this chapter, a valveless piezoelectric and magnetic micropump driven by travelling wave is presented. The micropump, fabricated with polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA), consists primarily of a saw-tooth microchannel, substrates, and two kinds of integrated actuator arrays (piezoelectric bimorph arrays and NdFeB permanent magnetic arrays). The travelling wave beneath the top wall of the elastic microchannel can be induced by the actuator arrays, and the liquid particles are then transported along with the travelling wave in the microchannel. The micropumps are designed and fabricated with different microchannels (the saw-tooth and the straight microchannels). Appropriate geometry of the saw-toothed microchannel was also studied for optimizing the performance of the micropump. Experimental characterization of the micropump has been performed in terms of the frequency response of the flow rate and back pressure. The results demonstrate that this micropump is capable of generating a stable flow rate in microfluidic systems.

RF MEMS switch is a new type of RF component developed by MEMS technology. Like the macro switches and relays, RF MEMS switches use a mechanical way to control the signal on and off. The difference is that, RF MEMS switches have very small volume and are used to process RF or microwave signals. Compared with the traditional solid-state semiconductor RF switches, RF MEMS switches have the advantages of low insertion loss, high isolation, low power consumption, and high linearity. Hence, RF MEMS switches have a wide range of application prospects in many fields, such as radar, satellite, base station, and portable radio communication equipment.In this chapter, the configuration principles of RF MEMS switches is first introduced, including four basic EM models and two basic movement styles. According to the two basic movement styles, the vertical movement and the lateral movement, several RF MEMS switches with different features are shown. Two different vertical actuating membrane bridge RF MEMS switches are obtained based on two different substrates. The first one has the advantage of compatibility with GaAs MMIC process, and the second one has the feature of flexibility. Furthermore, another two lateral actuating RF MEMS switches are demonstrated based on the SOG process. One is the push-pull type switch controlled by only one actuation signal, and the other is the three-state switch actuated by rhombic structures. By designing lateral actuating structures, some performances of the RF MEMS switches are improved.

This chapter mainly introduces microwave power sensors based on the microelectromechanical system (MEMS) technology, in order to achieve the power detection, gain control, and circuit protection. In RF and microwave frequencies, the MEMS power sensors have the advantages of miniaturization, low power, high sensitivity, and compatible with GaAs monolithic microwave integrated circuits (MMIC), etc. In structure, several kinds of the power sensors are described according to different application requirements. They utilize the form of coplanar waveguide transmission lines, with small structural dimensions (generally <1 mm2). In theory, they adopt conversion principles of microwave power-heat-electricity or microwave power-force-electricity. In fabrication, they are accomplished with the GaAs MMIC process. In measurement, experiments demonstrate the validity of the proposed design and model. These sensors meet the characteristics of high performance and low cost. They can be used as implant devices and embedded in microwave communication and radar systems, such as the self-detection of the transceiver module and the measurement of leakage power in microwave module circuits. The MEMS microwave power sensors can directly measure the power of below 500 mW. For a higher microwave power measurement, it is usually necessary to couple or extract a portion of the microwave power by some structures. These MEMS power sensors have the ability to extend the frequency and phase measurements of microwave signals, constituting frequency and phase detectors.

As the essential power control and adjustment component, microwave power attenuator has been widely applied in spectrum analyzer, network analyzer, receivers, and other microwave instrument systems. There has been significant interest in developing miniaturized microwave devices with small geometric dimension for commercial microwave test systems. Traditional attenuator with large size is difficult to integrate with IC due to its cumbersome mechanical relay or switches. Radio frequency microelectromechanical system (RF MEMS) switches present many advantages such as less insertion loss, higher isolation, better linearity, and lower power consumption and more importantly, its small size. It has absorbed great attention for the purpose of miniaturization of attenuator. Recent research shows step attenuator based on RF MEMS switches meet well the state-of-the-art requirements of miniaturized reconfigurable attenuation devices with high precision and broadband performance. In this chapter, we introduce a compact 3-bit step attenuator based on RF MEMS switches with 0 ~ 70 dB attenuation at 10 dB intervals up to 20GHz. The on-chip attenuator consists of 12 ohmic MEMS switches, 3 π-type resistive attenuation networks, and microwave compensate structures. To optimize the attenuation characteristics within the broadband, theoretical analysis and 3D modelling were performed. The device was obtained using MEMS process combined with polysilicon integrated circuit (IC) process.In section “Structure Design,” the structure of the step attenuator was proposed and three modules were designed independently including: (1) Resistive attenuation network. Two approaches are presented and discussed. (a) Polysilicon thin film resistor with symmetric topology is applied to realize the high-precision π-type resistive attenuation modules (10, 20, 40 dB). (b) Distributed TaN single thin film resistor (STFR) equivalent to π-type resistive network was designed with a more compact structure (2/3 compared to traditional network). It can realize 5 ~ 30 dB attenuation with less parasite effects. (2) RF MEMS switches. Toggling part is the determining factor to reduce the footprint of the entire attenuator. SP2T switch including 2 ohmic Au contact single-cantilever MEMS switches was customized for the attenuator. (3) CPW and microwave compensate structures. Precise design of the transmission path connecting each module is significant to suppress the inherent insertion loss and improve the matching performance. Right-angle bends and T-junctions were analyzed and designed. Based on the results of these three blocks, the overall attenuator was assembled and optimized with consideration of matching performance.In section “Fabrication,” we describe the fabrication method of RF MEMS attenuators. The 3-bit attenuator was manufactured with surface micromachining process and polysilicon IC techniques on 4 inch wafers and features 11 lithography steps. Key processes are emphasized including high-precision resistance and excellent ohmic contact between polysilicon and Au CPW was realized with experimental research. The photosensitive PI was used as sacrificial layer and the patterning process was simplified. The CPW and upper electrodes of the switch which were manufactured by low stress Au electroplating process have small roughness and without evident buckling after the sacrificial layer released. Based on these process studies, the 3-bit attenuator is successfully fabricated.In section “Characterization,” characterization of the obtained RF MEMS attenuator is presented. The measurement results show that the driven voltage of RF MEMS switch is 32 ~ 42 V and it can operate more than 3 × 108 times (cold switched mode). When toggling between the different transmission paths using MEMS switches, the 3-bit attenuator can realize target attenuation in range of 0 ~ 70 dB at 10 dB intervals up to 20 GHz. The accuracy and error of all the attenuation states is better than ±1.88 dB and 2.11 dB. The return loss of the 3-bit switched attenuator is better than 11.95 dB. Micromachined reconfigurable attenuator based on RF MEMS switch is demonstrated and can be further applied in particular microwave systems.

Surface acoustic wave (SAW) resonators and film bulk acoustic resonators (FBAR) are two of the most important passive resonators. After several decades of research and development, they are widely applied for wireless communications such as filters and duplexers, and broad sensing applications like pressure sensors, temperature sensors, gas sensors, biosensors, photoelectric sensors, and many others. In this chapter, we first introduce basic concepts and mainstream topics of SAW and FBAR devices. Then we give a detailed presentation of several methods to fabricate high-performance resonators including using multilayered structures and improved process. Flexible SAW and FBAR devices are also introduced. Several sensing applications are given afterwards. Finally, a summary and outlook is presented regarding the further improvement of device performance and new research fields.

This chapter introduces the research work of MEMS-based optical-readable thermal imaging technology. An uncooled infrared focal plane array based on bi-materials cantilever microstructures is presented.

In this chapter, MEMS direct methanol fuel cell, a clean portable energy that has been widely concerned in recent years, is introduced. Starting from the working principle and basic structure of DMFC, we elaborate the design, manufacture, encapsulation, and testing of DMFC based on our previous work. An anode three-dimensional steady-state physical field-coupling model was established as an example of simulation and theoretical analysis of DMFC. The design and processing of the current collector were then demonstrated. As the core of DMFC, the fabrication of membrane electrode assembly was focused on where the preparation and measurement of Pt-based catalyst were also covered. A novel assembly method was taken as an example to illustrate the encapsulation methods and requirements for DMFC. The final part described the testing system and methods of DMFC performance, including the measurement and characterization of the catalyst. The effect of different operating conditions on the performance of DMFC was discussed. In summary, the chapter gives a comprehensive view to those who are interested in MEMS DMFC.

Rapid advances in microelectronic devices (such as embedded sensors, medical implants, wireless communication nodes, and so on) have stimulated the development in ambient energy harvesting over the last decade. In this chapter, the fundamental theory and the typical microfabrication of MEMS piezoelectric vibration energy harvester were introduced. Firstly, the application background of the vibration energy harvester was analyzed. Then the working principle of the piezoelectric vibration energy harvester was introduced. After that, the theoretical model was deduced through Hamilton’s principle and Euler-Bernoulli beam theory. The optimization of the piezoelectric vibration energy harvester was performed, and the optimization results show that the electromechanical conversion efficiency is not more than 50% and the PVEH with low damping ratio does not always have high power output, which challenges previous literature suggestion that lower damping ratio tends to higher power output. Finally, the typical microfabrication of MEMS based piezoelectric energy harvesters was described and the challenges of the MEMS PVEH were illustrated.

Triboelectric nanogenerator (TENG) is a new energy technology for converting human kinetic and ambient mechanical energy into electricity. The principle of the TENG is based on triboelectrification and electrostatic induction, in which the induced triboelectric charges can generate a potential drop and drive electron flow by a mechanical force. Since invented in 2012, the TENG has made rapid research progress and demonstrated various potential applications.This chapter will first introduce the working principle and four fundamental modes of the TENG and emphatically present the different applications of the TENG, such as microscale power source for wearable and portable electronics, mega-scale power source by harvesting water wave energy, self-powered active sensors for internet of things, and triboelectric-voltage-controlled source for capacitive devices.The goal of this chapter is to provide the reader with general concept, principle and modes of the TENG as a new energy technology, and a broader overview of potential applications that can be utilized to harvest human kinetic and ambient mechanical energy.

Microelectrode arrays (MEAs) have been applied as chronical interface with the neural system and play important roles in neural prosthesis for various diseases, including sensory and motion injuries such as blind, deaf, and paralyzed, and also mental diseases like depression, Parkinson’s disease, and epilepsy. Thanks the inherent merits of microfabrication, MEAs show the advantages of low cost, mass production, high density, flexibility (optional), small footprint, integratability with ICs (integrated circuits), etc. In this chapter, we discuss about the general requirements and consideration of materials, design, and fabrication of MEAs in details. Then the various devices and their applications in the central nervous system and the peripheral nervous system are overviewed, respectively.

With the rapid development of MEMS fabrication technologies, versatile microelectrodes with different structures and functions have been designed and fabricated. The flexible MEMS microelectrodes exhibit multiaspect excellent characteristics compared to stiff microelectrodes based on silicon or SU-8, which comprising: lighter weight, smaller volume, better conforming to neural tissue, and lower fabrication cost.This chapter mainly reviewed key technologies on flexible MEMS microelectrodes for neural interface in recent years, including: design and fabrication technology, fluidic channels, μLEDs, and electrode-tissue interface modification technology for performance improvement. Furthermore, the future directions of flexible MEMS microelectrodes were described including transparent and stretchable microelectrodes with characteristics of multifunction, high-density, biodegradation, and next-generation electrode-tissue interface modifications facilitated electrode efficacy and implantation safety.The goal of this chapter is to provide the reader a broader overview of flexible MEMS technologies that can be applied together to solve problems in neural interface.

Neural electrodeschallengesA neural interface is a kind of device or system which is used to connect neural system with external equipment, through recording electroneurographic (EEG) signals, a neural interface monitors or stimulates and regulates neural activities. To reduce as much as possible the interference of the device in neural system, neural interfaces are usually fabricated with MEMs technology, which helps to minimize the dimensions of neural interfaces. Apart from meeting the dimension requirements, neural interfaces should also be biocompatible in terms of biochemical characteristics, electrical properties, and mechanical properties. These requirements or limits mean more challenges upon neural interface materials and processing technologies. In this chapter, we will take implantable neural microelectrode array as an example, and introduce the development of existing microfabrication technology including working principle, material selection, structure, and manufacturing process of neural microelectrode device. At last, we will sum up the problems and challenges microelectrode devices are facing.