Microelectromechanical Systems Structures Research Articles

Design of a MEMS speaker driver

Microelectromechanical systems (MEMS) have already found widespread use in highly integrated audio applications. Currently, they are used for microphones where their small size has enabled the use of multiple microphones in a single device to improve noise cancellation. Nowaday, even loudspeakers realized as MEMS structures are possible, which brings up new challenges in circuit design that are the focus of this paper.Starting with an overview of current research activities of MEMS speakers, their advantages and disadvantages, as well as the peculiarities of the driver design are discussed. Compared to electrodynamic speakers, MEMS speakers have a different physical structure which needs to be accounted for when designing the driving circuitry. Possible driver concepts are presented and compared with special consideration of their efficiency and audio quality. Existing concepts are shown and new concepts are proposed.

MOS Integrated MEMS Structure for Bio-sensing

Biosensors are used to identify analytes such as DNA molecules, proteins or micro-organisms. These devices couple a biological recognition element (the probe) with a physical transducer to generate an observable signal. The work presented here shows a cantilever embedded MOSFET (Metal oxide semiconductor field effect transistor) structure for biosensing. The main features of the structure are piezoelectric actuation/sensing and MOSFET based readout making it suitable for parallel array operation in conjunction with phase coding schemes. The integration of MEMS (Micro electro mechanical systems) structure with CMOS (Complementary MOS) fabrication process also leads to realizability of the system on a chip. The proposed structure has detection capability in sub-picogram range. A CMOS compatible fabrication process flow has also been suggested in this paper along with two different phase coding schemes for noise cancellation.

Вплив шорсткості поверхні комутуючих елементів НВЧ перетворювачів на ємність системи

In this article the intensive development of radio engineering devices and communication systems is described and analyzed. The characteristics of such devices have been improved due to the use of microelectromechanical systems (MEMS), which uses various types of switching elements based on MEMS. Application of converters based on MEMS has allowed providing low input energy losses, power management, cost of production and high reliability of such devices. It is proved that one of the most important characteristics of such radio engineering devices and communication systems is the roughness of the surface of the switching elements (keys), to which the specific requirements for the mechanical processing of the switching surface, as well as the thickness of the applied layer of the dielectric on the surface of the commuting elements are put forward. It is proved that with increasing roughness of the surface there is a decrease in the capacity in the lower position of the membrane. Therefore, in order to obtain the optimal ratio of capacities, it is necessary to ensure that the surfaces of the microsystem are roughened as little as possible. It is determined that the main properties of dielectric active media are mechanical and electrical interconnected ones. In the article the dependence between the roughness of elastic commuting elements and the capacitance that arises between the elastic element and the dielectric basis is considered. The proposed results make it possible to understand the necessity of implementing elastic elements with a high degree of surface finish to ensure uninterrupted control of the system when receiving incoming TV signals. Using the proposed methodology for calculating MEMS designs with electromagnetic active media, a model has been designed and manufactured as an experimental setup for studying a controlled C polarization converter on MEMS structures.

Micromachined microbeams made from porous silicon for dynamic and static mode sensing

Abstract Through a controlled variation of the applied current during porous silicon formation, newly developed processes enable previously unattainable structural integrity of all-mesoporous silicon microelectromechanical systems (MEMS) structures. Such structures are desirable for applications such as sensing where the large surface area and low Young’s modulus of the high porosity layer enable ultra-high sensitivity detection of adsorbed species. In this work, micromachined all-mesoporous silicon microbeams were released, allowing both the dynamic and static sensing modes to be studied using such porous structures. Resonant frequencies (50–250 kHz) of released doubly clamped porous silicon microbeams were measured, allowing mechanical properties to be extracted. Static mode sensing of vapour at the 1100 ppm level was also performed, with the released porous silicon cantilevers showing a significant 6.5 μm (3.7% of a 175 μm beam length) and repeatable deflection after exposure.

PVDF-TrFE Electroactive Polymer Mechanical-to-Electrical Energy Harvesting Experimental Bimorph Structure

Research of electrostrictive polymers has generated new opportunities for harvesting energy from the surrounding environment and converting it into usable electrical energy. Electroactive polymer (EAP) research is one of the new opportunities for harvesting energy from the natural environment and converting it into usable electrical energy. Piezoelectric ceramic based energy harvesting devices tend to be unsuitable for low-frequency mechanical excitations such as human movement. Organic polymers are typically softer and more flexible therefore translated electrical energy output is considerably higher under the same mechanical force. In addition, cantilever geometry is one of the most used structures in piezoelectric energy harvesters, especially for mechanical energy harvesting from vibrations. In order to further lower the resonance frequency of the cantilever microstructure, a proof mass can be attached to the free end of the cantilever. Mechanical analysis of an experimental bimorph structure was provided and led to key design rules for post-processing steps to control the performance of the energy harvester. In this work, methods of materials processing and the mechanical to electrical conversion of vibrational energy into usable energy were investigated. Materials such as polyvinyledenedifluoridetetra-fluoroethylene P(VDF-TrFE) copolymer films (1um thick or less) were evaluated and presented a large relative permittivity and greater piezoelectric β-phase without stretching. Further investigations will be used to identify suitable micro-electromechanical systems (MEMs) structures given specific types of low-frequency mechanical excitations (10-100Hz).

Improved properties of the MEMS-type ion-sorption micropump

In this paper, the authors demonstrate how to improve the properties of a recently elaborated microelectromechanical system (MEMS) ion-sorption micropump in order to generate high and ultrahigh vacuum inside MEMS and miniature nanoelectronics devices. This research goal has been achieved by applying mechanisms leading to an increase of the efficiency of a gas ionization process. One of the cathodes of the micropump was covered with different nanomaterials, which have either good field emission properties (carbon nanotubes) or high secondary emission coefficient (nanoporous silicon and magnesium oxide nanopowder). In this way, the number of primary or secondary electrons which collide with gas particles was multiplied. For all of the modified structures a significant increase of a discharge current inside the micropump was observed. In the case of electrodes covered with MgO, it increased about 1000 times. The use of these cathodes may allow to obtain ultrahigh vacuum inside hermetically sealed MEMS structure.

(Invited) How to Integrate MEMS on Foundry-Fabricated CMOS Backplanes

A large variety of new applications for Internet of Things (IoT), Industrie 4.0 and other consumer products are pushing volume and manufacturing of microelectromechanical systems (MEMS) into a new phase. Due to massive use of these sensors and actuators in mobile and industrial applications smaller systems combined with low power consumption and higher versatility are required. MEMS are transducers that sense or control physical, chemical or optical quantities creating devices for applications in the area of 3D-motion tracking, pressure sensing, light shaping or detection of irradiation. The combination of MEMS with an application specific integrated circuit (ASIC) based on CMOS technology enables the entire system to interact with outside world. These ASICs are providing required features such as analog-to-digital conversion, amplification, filtering, information processing and storage as well as communication to the outside world. There are different solutions available to combine MEMS with customized CMOS circuits. One choice known as system-in-package (SiP) is the manufacturing of MEMS and CMOS on separate wafers and its subsequent integration in a multi-chip module using interposer/ rewiring technologies. Key advatanges of this technique are high flexibility, high modularity and a complete decoupled manufacturing of CMOS and MEMS. This enables a rapid development and leads to low development costs. And, there is no impact of different sizes between CMOS and MEMS chip. In order to enhance the integration density, to drive system feature size down, to suppress potential parasitic capacitances and to reduce power consumption an integrated manufacturing on the same substrate is a must and known as system-on-chip (SoC) solution. On one hand this approach can be still realized by a multi-wafer processing and a final integration using bonding technologies. This technology is often named as heterogeneous integration. Key advantage of multi-wafer technology is the possibility to use high-performance MEMS materials such as monocrystalline silicon as actuation/sensing material and its integration on CMOS wafers. Disadvantages are the required alignment accuracies when using a metallic bond or limitation for integration densities when using via-last approach. A more consistent way is a complete monolithic integration of MEMS on CMOS substrates. Especially when large transducer arrays are required (e.g. bolometers, micro-mirror arrays, CMUT arrays) a monolithic integration of MEMS on CMOS is the best solution. All advantages are now on hand – high integration densities, low parasitic capacitances and an effective usage of CMOS area. Fraunhofer IPMS applies surface and bulk micromachining technologies for the integration of MEMS on customized CMOS backplanes. To reduce development costs and time-to-market IPMS combines standard CMOS processes offered from CMOS foundries with a subsequent integration of MEMS part in our MEMS fab. Due to the application specific design of the CMOS backplane this concept allows a perfect match between ASIC and MEMS functionalities. In a typical project flow CMOS and MEMS will be manufactured and tested in parallel to shorten development time. After ASIC design is finished it will be tested using Multi- Project- Wafer run (MPW) in a CMOS foundry. Using these MPW runs a complete characterization of CMOS functionality can be realized in an early project state at low costs. Parallel to CMOS testing an entire process development of MEMS part can be done on passive devices in our MEMS fab. If CMOS and MEMS characterization is completed the MEMS part will be monolithically integrated on these customized CMOS backplanes. But there are still some challenges for the integration of MEMS on foundry fabricated CMOS backplanes, e.g.: Modifications of last CMOS/ interconnect/ ILD layers to realize an appropriate interface to MEMS Clarification of PCM (process control monitoring) test environment, depth of test structures and possible arising contamination issues In-chip surface topology after interconnect processing to define necessary actions to achieve required planarity for further processing Tuning mix-and-match lithography to achieve high overlay accuracies (Picture 1: Reached overlay accuracy between last 0,18µm CMOS and first MEMS layer of about 30- 40nm after correction step using i-line stepper) Required chip design features when using sacrificial layer technology IPMS offers a wide range of surface and bulk micromachining technologies which are particularly suitable for the fabrication of sensors and actuators on pre-fabricated CMOS wafers using monolithic integration. Especially surface micromachining using inorganic sacrificial layer technology allows the realization of rather complex MEMS structures on CMOS backplanes. Based on examples such as spatial light modulators (SLM) and capacitive micromachined ultrasonic transducers (CMUT) we will present solutions for the integration of surface micromachined MEMS on customized CMOS substrates. Figure 1

Dual MEMS Resonator Structure for Temperature Sensor Applications

This paper reports an acoustic microelectromechanical system (MEMS) resonator structure that features dual-resonant response at 180 and 500 MHz. The MEMS structure uses aluminum nitride as acoustic layer and electrodes with dual design that provide dual-resonance behavior. Each resonant mode operates at the first symmetrical Lamb-wave mode (S0). Due to the large frequency separation between modes, device exhibits differentiated temperature coefficient of frequency) for each mode, which makes this structure suitable for thermometric beat frequency sensing. Reported devices are thus capable to multiply the 20-ppm/°C thermal sensitivity of the individual sensors by one order of magnitude, up to −334 ppm/°C for the thermometric beat frequency sensor.

High-Performance Low-Range Differential Pressure Sensors Formed With a Thin-Film Under Bulk Micromachining Technology

In the microelectromechanical systems (MEMS) field, low-range (e.g., kPa or sub-kPa) differential pressure sensors have remained difficult for decades. Generally, there are two unsolved problems that hinder volume production and wide applications in consumer-electronics. First, high sensitivity required very thin and large sensing diaphragm normally fabricated with low yield, and thus, high selling price. Second, high sensitivity required large deflection of the diaphragm easily causeing severe output of non-linearity that degrades the detecting precision. This paper reports a so-called thin-film under bulk (TUB) MEMS technique to high-yield fabricate high-performance low-range pressure sensors for smart central air-conditioning systems. With the single-wafer-based TUB structure, pressure-induced stress is highly concentrated in a bulk-silicon beam-island structure for piezoresistive readout, where a very thin but uniform poly-silicon diaphragm can be formed beneath the bulk-silicon structure to sustain the pressure. The beam-island reinforced structure brings about decreased deflection and highly linear output. Micro-columns are also formed under the bulk-silicon islands as over-range protection stoppers that are important for low-range pressure sensor. After the description about the 1.2 mm $\times 1.2$ mm tiny-sized sensor-chip design and its single-wafer fabrication process, a test is conducted that results in 22-mV output for 1.2-kPa measure range and very good linearity of ±0.05% (FS). About $100\times $ over-range protection capability of the sensor is also achieved. The novel high-yield TUB process can be used for low-cost production of high-performance pressure sensors for the applications of new-generation smart central air-conditional systems and spirometers, etc, and the bulk/surface combined TUB micromachining technique promises to be widely used for the formation of various complex MEMS structures. [2016-0309]

Investigating time-resolved response of micro thermal conductivity sensor under various modes of operation

This article provides measurements of thermal conductivity carried out on a microelectromechanical system (MEMS) structure for the application of selective gas sensing. MEMS thermal conductivity based gas sensors work by measuring the interaction of a heated polysilicon bridge with an ambient gas and sensing temperature changes. The tiny MEMS element provides an ability to measure noble gases and is especially applicable to binary mixtures. In this work we conducted experiments on the KWJ Engineering nano-powered thermal conductivity detector (TCD) elements to investigate the sensitivity of the sensor transient responses to gas composition. Measurements are made at constant power, constant resistance, and constant energy. By introducing feedback to maintain constant sensor temperature, the relative change of gas thermal conductivity with respect to temperature can be estimated and provide information about the concentration as well as insight into the composition of the gas mixture.

Low-voltage shock-mitigated micro-electromechanical systems structure

We report a low-voltage, yet effective, micro-electromechanical systems (MEMS) structure capable of mitigating external mechanical disturbances, such as a physical shock. External shock onto MEMS devices can be catastrophic as a conventional single membrane may travel beyond stable oscillatory distances under shock and become irreparably damaged. However, the simple addition of a second membrane on top of the single membrane drastically reduces oscillatory distances by electrostatically holding the bottom membrane within stable oscillation. The added elements, in conjunction with a fine-control algorithm, mitigate the impact of a mechanical shock onto the MEMS device. From experimental findings, it is found that the dual-membrane structure effectively reduces the travel distance of the bottom membrane by 41.5%, upon deploying merely 0.565 V onto the additional membrane. The dynamic implementation of the shock mitigation method, using an on-board accelerometer as a trigger, delivered in-situ mitigation of shock on a dual-membrane MEMS structure.

Analysis of the bending stiffness and adhesion effect in RF-MEMS structures

Microelectromechanical system (MEMS) is a special branch with a wide range of applications in sensing, switching and actuating devices. Designing the reliable MEMS for thin free-standing structures like as bridges and cantilevers requires understanding of the tribomechanical properties of the materials and structures.The effect of geometrical dimensions (cross-section dimensions and length) on mechanical and tribological behavior of free-standing MEMS structures made of electroplated gold was analyzed in this paper. Special attention was given to the dependences between stiffness and cantilever length and the dependences between bending stress and variable travel range of actuated load. The force position was moved from the beams free-end toward to the anchor. The tests were performed at room temperature (22°C) and relative humidity RH of 40% with a noise- and vibration-isolated and environment-controlled XE-70 AFM from Park Systems using the contact mode. Each measurement was repeated many times in order to improve the accuracy of the experimental results. The stiffness of a microcantilever varies if the position of the acting force is changed. The experimental results obtained were in good correlation with those obtained analytically.

A review of the most important failure, reliability and nonlinearity aspects in the development of microelectromechanical systems (MEMS)

PurposeThe purpose of this paper is to provide an overview of the major reliability issues of microelectromechanical systems (MEMS) under mechanical and environmental loading conditions. Furthermore, a comprehensive study on the nonlinear behavior of silicon MEMS devices is presented and different aspects of this phenomenon are discussed.Design/methodology/approachRegarding the reliability investigations, the most important failure aspects affecting the proper operation of the MEMS components with focus on those caused by environmental and mechanical loads are reviewed. These studies include failures due to fatigue loads, mechanical vibration, mechanical shock, humidity, temperature and particulate contamination. In addition, the influence of squeeze film air damping on the dynamic response of MEMS devices is briefly discussed. A further subject of this paper is discussion of studies on the nonlinearity of silicon MEMS. For this purpose, after a description of the basic principles of nonlinearity, the consequences of nonlinear phenomena such as frequency shift, hysteresis and harmonic generation and their effects on the device performance are reviewed. Special attention is paid to the mode coupling effect between the resonant modes as a result of energy transfer because of the nonlinearity of silicon. For a better understanding of these effects, the nonlinear behavior of silicon is demonstrated by using the example of Si cantilever beams.FindingsIt is shown that environmental and mechanical loads can influence on proper operation of the MEMS components and lead to early fracture. In addition, it is demonstrated that nonlinearity modifies dynamic response and leads to new phenomena such as frequency shift and mode coupling. Finally, some ideas are given as possible future areas of research works.Originality/valueThis is a review paper and aimed to review the latest manuscripts published in the field of reliability and nonlinearity of the MEMS structures.

Thermomechanical MEMS membranes for fiber optic temperature sensing

In this study, the design, fabrication, and characterization results of square and circular cross-sectioned, bimaterial microelectromechanical system (MEMS) membranes are presented. The MEMS structures, having width of 250 $\mu $m to 1.5 mm and 1-4 mm$^{2}$ die area, were designed towards integration on a single-mode fiber tip for temperature-sensing. Embedded diffraction grating underneath each membrane allows for interferometric detection of thermomechanical response through backside laser illumination. The displacement of MEMS membranes was monitored under direct heating, revealing 50--1800 nm/$^{\circ}$C thermomechanical sensitivity range. Furthermore, atomic force microscopy-based stiffness measurement on the MEMS membranes revealed a spring constant within 2--30 N/m range. Temperature measurement range and speed of the MEMS sensors were thoroughly tabulated based on experimental findings, as well as finite-element simulations and analytical calculations. Finally, proof-of-concept testing of a selected device was accomplished through air-coupling of the MEMS structure with a gradient index-collimated fiber, revealing $

A New Method for Real-Time Measuring the Temperature-Dependent Dielectric Constant of the Silicone Oil

This paper presents a new method for real-time measuring the temperature-dependent relative dielectric constant and the thermal expansion coefficient of the silicone oil by using a low-cost microelectromechanical systems (MEMS) structure. The relative dielectric constant of the silicone oil can be obtained by measuring the capacitance of an MEMS temperature sensing structure, and the silicone oil is used as the temperature sensing material of the capacitive structure. A processing circuit on a printed circuit board is designed to complete the measurement. A theoretical model is constructed to describe the behavior of the relative dielectric constant and the thermal expansion coefficient over a certain range of temperatures. Preliminary experimental results show that the variation of the relative dielectric constant and the thermal expansion coefficient versus temperature could be detected real timely and simply based on this method.

A first passage based model for probabilistic fracture of polycrystalline silicon MEMS structures

Experiments have shown that the failure loads of Microelectromechanical Systems (MEMS) devices usually exhibit a considerable level of variability, which is believed to be caused by the random material strength and the geometry-induced random stress field. Understanding the strength statistics of MEMS devices is of paramount importance for the device design guarding against a tolerable failure risk. In this study, we develop a continuum-based probabilistic model for polycrystalline silicon (poly-Si) MEMS structures within the framework of first passage analysis. The failure of poly-Si MEMS structures is considered to be triggered by fracture initiation from the sidewalls governed by a nonlocal failure criterion. The model takes into account an autocorrelated random field of material tensile strength. The nonlocal random stress field is obtained by stochastic finite element simulations based on the information of the uncertainties of the sidewall geometry. The model is formulated within the contexts of both stationary and non-stationary stochastic processes for MEMS structures of various geometries and under different loading configurations. It is shown that the model agrees well with the experimentally measured strength distributions of uniaxial tensile poly-Si MEMS specimens of different gauge lengths. The model is further used to predict the strength distribution of poly-Si MEMS beams under three-point bending, and the result is compared with the Monte Carlo simulation. The present model predicts strong size effects on both the strength distribution and the mean structural strength. It is shown that the mean size effect curve consists of three power-law asymptotes in the small, intermediate, and large-size regimes. By matching these three asymptotes, an approximate size effect equation is proposed. The present model is shown to be a generalization of the classical weakest-link statistical model, and it provides a physical interpretation of the material length scale used in the weakest-link model.

Fabrication of an infrared emitter using a generic integration platform based on wire bonding

This paper reports a novel approach for the fabrication of infrared (IR) emitters for non-dispersive IR gas sensing. The proposed concept enables the integration of superior resistive heater materials with microelectromechanical system structures. In this study, non-bondable filaments made of nickel chromium are attached to mechanical attachment structures using a fully automated state-of-the-art wire bonder. Formation of electrical contact between the integrated filaments and the electrical contact pattern on the substrate is performed using conventional gold stud bumping technology. The placement accuracy of the integrated filaments is evaluated using white-light interferometry, while the contact formation using stud bumping to embed the filaments is investigated using focused ion beam milled cross-sections. A proof-of-concept IR emitter has been successfully operated and heated up to in continuous mode for 3 h.

E-beam Evaporated Polysilicon for Lead Zirconate Titanate-Based Micro-Actuators

This letter reports the application of ultra-high vacuum E-beam evaporated polysilicon (UHVEEPoly) film for lead zirconate titanate (PZT)-based piezoelectric micro-electro-mechanical systems (MEMSs) for the first time. The UHVEEPoly film is employed as a passive structural layer in unimorph piezoelectric cantilever micro-actuators to demonstrate its applicability in PZT-based MEMS. Two sets of PZT micro-cantilever actuators have been fabricated and characterized in this letter. The first set has 4- $\mu \text{m}$ -thick UHVEEPoly as a structural layer while the second set is made of $4~\mu \text{m}$ thick single crystalline silicon structural layer obtained from an active layer of a silicon-on-insulator wafer. The static and dynamic behaviors of the two sets of actuators including their initial deflection profiles have been measured and compared. Results confirmed that the two sets of actuators exhibit similar characteristics, indicating the unique suitability of the UHVEEPoly films for realizing PZT-based MEMS that enables a cost effective process and formation of versatile MEMS structures.

Metal additive manufacturing for microelectromechanical systems: Titanium alloy (Ti-6Al-4V)-based nanopositioning flexure fabricated by electron beam melting

Three dimensional printing (3D printing) or additive manufacturing is a promising approach for construction of small-sized, complex structures in microelectromechanical systems (MEMS). This paper reports the design, fabrication and testing of an XY nanopositioning flexure made up of Titanium alloy (Ti-6Al-4V) created for the first time through electron beam melting (EBM), also known as electron beam additive manufacturing (EBAM). Titanium alloys present attractive characteristics including high biocompatibility, distinct strength and corrosion resistance. However, it has been difficult to machine titanium alloys through conventional processes. The use of additive manufacturing has enabled us to build a multi-dimensional nanopositioning flexure with amplified mechanical displacement and improved bandwidth contained in a compact structure.We first characterized mechanical properties of EBM-printed Ti-6Al-4V cantilevers and compared the results with those of bulk metal cantilevers. Due to the porous surfaces, the printed cantilevers acted as a softer material with an averaged Young’s modulus of 41GPa when considering only the outermost dimensions. By introducing inner widths of 0.51–0.53mm for the CAD-designed beam width of 0.7mm, we calculated a Young’s modulus of 90–120GPa, which is comparable to 108–120GPa reported in literature for bulk Ti-6Al-4V. With the completion of the initial characterization, fabrication of the flexure was then undergone and successfully carried out. Mechanical levers printed within the flexure amplified an actuation from a piezoelectric actuator by a factor of six to displace a positioning platform supported by the network of parallel supporting beams. The maximum displacement of 47.4μm was obtained at the driving voltage of 150V. The resonant frequencies measured for the x and y axes were almost identical 1854Hz and 1858Hz, respectively. A digital PID controller enabled laser-based dynamic positioning of the stage. For triangular sweeps at 16Hz and 122Hz, the positioning error was within 200nm and 500nm with time delays of 0.85ms and 2.48ms, respectively.

Reliability analysis of 3D heterogeneous microsystem module by simplified finite element model

Analyzing the structural reliability of 3D heterogeneous microsystem modules is an important step in their development. The finite element models of such modules are simplified by simulating the complicated structure of MEMS (microelectromechanical systems) devices integrated into a single interposer. In this study, thermal stress and cycling analyses for different finite element models of 3D heterogeneous microsystem modules are investigated. The results of the thermal stress analysis reveal the values of the maximum von Mises stress in the finite element models, at the interface between the interposer and the microgyroscope, and in the microgyroscope spring. They also illustrate the advantages and disadvantages of the different fabrication models. Module reliability assessments are also obtained through a thermal cycling analysis, the results of which show that properly simplified models designed to reduce computation time benefit the reliability analysis. This study provides useful suggestions for manufacturing and reliability assessments of 3D heterogeneous microsystem modules embedded using the through-silicon via technique.