MEMS Structures Research Articles

A Room Temperature ZnO-NPs/MEMS Ammonia Gas Sensor.

This study uses ultrasonic grinding to grind ZnO powder to 10–20-nanometer nanoparticles (NPs), and these are integrated with a MEMS structure to form a ZnO-NPs/MEMS gas sensor. Measuring 1 ppm NH3 gas and operating at room temperature, the sensor response for the ZnO-NPs/MEMS gas sensor is around 39.7%, but the origin-ZnO powder/MEMS gas sensor is fairly unresponsive. For seven consecutive cycles, the ZnO-NPs/MEMS gas sensor has an average sensor response of about 40% and an inaccuracy of <±2%. In the selectivity of the gas, the ZnO-NPs/MEMS gas sensor has a higher response to NH3 than to CO, CO2, H2, or SO2 gases because ZnO nanoparticles have a greater surface area and more surface defects, so they adsorb more oxygen molecules and water molecules. These react with NH3 gas to increase the sensor response.

Electron Radiation Effects of Grain-Boundary Evolution on Polycrystalline Silicon in MEMS.

A specimen observed with a transmission electron microscope (TEM) was processed by focused ion beam (FIB) from a surface-micromachined polycrystalline silicon MEMS structure. Electron irradiation and in situ observation were performed on a selected grain boundary in the specimen. The grain boundary was observed and located by using lattice-oriented selective TEM photography. An evolution progress of amorphization of small silicon grain within the grain boundary and recrystallization of amorphous silicon were observed. A silicon grain turned into several smaller bar grains within the grain boundary. The mechanism of grain-boundary evolution inducing a change of conductivity of polycrystalline silicon has been revealed. The conductivity of polycrystalline silicon influenced by electron irradiation could be attributed to the change of grain boundary.

Investigation of Wafer-Level Fabricated Permanent Micromagnets for MEMS.

Monolithic integration of permanent micromagnets into MEMS structures offers many advantages in magnetic MEMS applications. A novel technique called PowderMEMS, based on the agglomeration of micron-sized powders by atomic layer deposition (ALD), has been used to fabricate permanent micromagnets on 8-inch wafers. In this paper, we report the fabrication and magnetic characterization of PowderMEMS micromagnets prepared from two different NdFeB powder particle sizes. A remanence of 423 mT and intrinsic coercivity of 924 mT is achieved at the low ALD process temperature of 75 °C, making this process compatible with MEMS technology. The magnetic reversible mechanism in the micromagnets is discussed with the help of the Wohlfarth equation. To ensure the operability of such integrated micromagnets in different application environments, we conducted a set of experiments to systematically investigate the thermal and corrosive stability. NdFeB micromagnets with larger powder particle size (d50 = 25 µm) exhibit high thermal stability in air. Furthermore, the corrosion stability of the micromagnets is significantly improved by an additional silicon oxide passivation layer deposited by plasma-enhanced chemical vapor deposition (PECVD). The presented results demonstrate the durability of PowderMEMS micromagnets, enabling their application in various fields, e.g., microfluidics, sensors, actuators, and microelectronics.

Modeling and Measurement of Thermal-Mechanical-Stress-Creep Effect for RF MEMS Switch Up to 200 °C.

High-temperature processes, such as packaging and annealing, are challenges for Radio-Frequency Micro-Electro-Mechanical-Systems (RF MEMS) structures, which could lead to device failure. Coefficient of thermal expansion (CTE) mismatch and the material’s creep effect affect the fabrication and performance of the MEMS, especially experiencing the high temperature. In this paper, the Thermal–Mechanical-Stress-Creep (TMSC) effect during thermal processes from room temperature (RT) to 200 °C is modeled and measured, in which an Au-cantilever-based RF MEMS switch is selected as a typical device example. A novel Isolation-Test Method (ITM) is used to measure precise TMSC variation. This method can achieve resolutions of sub-nanometer (0.5 nm) and attofarad (1 aF). There are three stages in the thermal processes, including temperature ramping up, temperature dwelling, and temperature ramping down. In different stages, the thermal–mechanical stress in anchor and cantilever, the grain growth of gold, and the thermal creep compete with each other, which result in the falling down and curling up of the cantilever. These influencing factors are decoupled and discussed in different stages. The focused ion beam (FIB) is used to characterize the change of the gold grain. This study shows the possibility of predicting the deformation of MEMS structures during different high-temperature processes. This model can be extended for material selection and package temperature design of MEMS cantilever in the further studies.

Pulse frequency effect on the NiTi plasmas characteristics and thin film properties and fabrication of NiTi micro-comb structures

Nickel-titanium (NiTi) is a thermal shape memory alloy (SMA) widely used in the actuator domain in miniaturized systems. With a titanium nitride (TiN) capping layer, NiTi exhibits better mechanical properties and enhances biocompatibility in biomedical applications. In addition, TiN can be efficiently heated to a higher temperature than induce the phase transformation in NiTi MEMS structures. This could be easily achieved by fabricating a TiN heater pattern over or alongside the NiTi MEMS structures. The subject of this work is the NiTi thin films deposited utilizing the pulsed DC magnetron sputter (PDCMS) deposition process at various pulse frequencies (50–350 kHz). The TiN capping layer was also grown on the Si/NiTi stack by the same process. The NiTi micro-comb and TiN micro-heater MEMS structures were fabricated using bulk laser micromachining. Freestanding MEMS structures of NiTi and TiN thin films were realized by etching the bottom silicon with an etch rate of 600 nm/Sec using chemical wet etch by tetramethylammonium hydroxide TMAH.

Research progress of tunneling magnetoresistance sensor

Sensors play an important role in Internet of Things (IoT) industry and account for a rapidly growing market share. Among them, the magnetic sensor based on tunneling magnetoresistance (TMR) effect possesses great potential applications in the fields of biomedical, navigation, positioning, current detection, and non-destructive testing due to its extremely high sensitivity, small device size and low power consumption. In this paper, we focus on the development of TMR sensor technology routes, covering a series of research advances from a sensor transducer to three-dimensional magnetic field detection, and then to the applications. Firstly, we recall the development history of TMR sensors, explain its working principle, and discuss the method to improve the output linearity of single magnetic tunnel junction. Next, we state the Wheatstone-bridge structure, which can inhibit temperature drift in detail and review several methods of fabricating the full bridge of TMR sensors. Furthermore, for the market demand of three-dimensional magnetic field detection, we summarize the methods of designing and fabricating three-dimensional sensing structure of the TMR sensor. At the same time, we list several optimization schemes of TMR sensor performance in terms of sensitivity and noise level. Finally, we discuss two types of emerging applications of TMR sensors in recent years. The TMR sensors can also be used in intelligence healthcare due to their ultra-high sensitivity. In addition, devices from the combination of spin materials and MEMS structure have attracted wide attention, especially, because of the large commercial market of microphones, spin-MEMS microphones utilized TMR techniques will be the next research hotspot in this interdisciplinary field.

The preparation of CNT/PMMA composite film on SiO2 substrates

Abstract Microelectromechanical system (MEMS) has been highly valued since the size of the MEMS structure is miniaturized, the spacing between the components is in the nanometer range, and the behaviors of friction and adhesion greatly affect the reliability of MEMS. An atomic force microscope (AFM) was used to observe the surface morphology of carbon nanotube (CNT)/polymethyl methacrylate (PMMA) film, and X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical state of typical elements on the surface of the film. The friction and wear properties of composite nano-films under macroscopic conditions were systematically studied. The research results show that the composite nano-film has excellent anti-friction and anti-wear properties.

Silicon MEMS Nanomechanical Membrane Flexure Sensor With Integrated High Gauge Factor ITO

Introduction of structural modification and novel transduction materials to nanomechanical cantilever (NMC) sensor can bring considerable improvement in sensor performance leading to ultra-sensitive nanomechanical sensor platforms. This work reports development of an optimized and highly sensitive silicon MEMS Nanomechanical Membrane-Flexure (NMF) piezoresistive surface stress sensor. This MEMS structure consists of a circular adsorbate membrane suspended by four inverse trapezoidal flexures. The electromechanical transduction of the sensor is performed by integrating a high gauge factor low temperature sputter deposited Indium Tin Oxide (ITO) thin film as piezoresistor. The ITO thin film was experimentally characterized for extracting its electrical, mechanical, and electromechanical properties to assess its candidature in integrating as strain sensing element with nanomechanical sensors. The gauge factor of ITO thin film was measured using a high precision four-point bending fixture experimental setup. An ultra-thin ITO film deposited at room temperature with no oxygen flow and post annealing treatments exhibited a high negative gauge factor of −430, making it an excellent candidate in nanomechanical sensor platform. The incorporation of sputter deposited ITO thin film as piezoresistive layer obviated the need for doping in silicon which further reduced the fabrication process complexity for NMF sensor. The NMF sensor fabrication following SOI based silicon MEMS process along with device characterization have also been carried out as part of this work. The optimized design of piezoresistive NMF sensor exhibited an improved surface stress sensitivity of nearly 15 times higher than conventional cantilever sensor and thereby opening new possibilities in bio-chemical and environmental sensing applications. [2021-0127]

Model order reduction based on direct normal form: application to large finite element MEMS structures featuring internal resonance

Dimensionality reduction in mechanical vibratory systems poses challenges for distributed structures including geometric nonlinearities, mainly because of the lack of invariance of the linear subspaces. A reduction method based on direct normal form computation for large finite element (FE) models is here detailed. The main advantage resides in operating directly from the physical space, hence avoiding the computation of the complete eigenfunctions spectrum. Explicit solutions are given, thus enabling a fully non-intrusive version of the reduction method. The reduced dynamics is obtained from the normal form of the geometrically nonlinear mechanical problem, free of non-resonant monomials, and truncated to the selected master coordinates, thus making a direct link with the parametrisation of invariant manifolds. The method is fully expressed with a complex-valued formalism by detailing the homological equations in a systematic manner, and the link with real-valued expressions is established. A special emphasis is put on the treatment of second-order internal resonances and the specific case of a 1:2 resonance is made explicit. Finally, applications to large-scale models of micro-electro-mechanical structures featuring 1:2 and 1:3 resonances are reported, along with considerations on computational efficiency.

Transferability of weakest link model parameters of polysilicon from indentation fracture to failure of MEMS structures

Polysilicon weakest link model parameters obtained from indentation fracture are used to predict failure in MEMS structures. Plain and notched polysilicon micro-cantilevers were fabricated using surface micro-machining and bending tests were carried out till the structure failed catastrophically. To estimate the failure probability of fabricated samples, numerical studies were carried out on models closer to the actual experimental condition, by estimating and incorporating fabrication details such as undercut and roughness. Initially, the undercut inherent to fabrication process was estimated on plain cantilevers by comparing the elastic behavior with experiments. The geometry of open cavities in the sidewall, formed as a consequence of etching, were characterized using SEM micrographs. The stress field obtained from 3D FEM analysis of the notched geometries, including these geometric details, showed great sensitivity to the positioning of cavities in the sidewall relative to the top surface. Applying weakest link theory based local criteria, the probability distribution of failure load of tested structures was generated using weakest link parameters from indentation fracture. Failure loads, thus predicted, were validated with experimental data.

(Invited) Development of Pulse-Heated Diaphragm Type MEMS Gas Sensors for Indoor Air Quality Measurement

IntroductionAmid growing concern about indoor air quality (IAQ), MOS type gas sensors for the detection of volatile organic compounds (VOC) are widely used in household appliances such as air purifiers for on-demand control of the appliances depending on VOC concentration in surrounding air. There are also potential demands for gas sensors to be used for mobile applications, with which people can monitor environmental conditions in dwelling or workplaces. However, detection accuracy, power consumption and cost of conventional MOS type gas sensors have not yet reached the level generally required for VOC concentration monitoring by mobile applications. To solve these issues, we have developed a new diaphragm type MEMS gas sensor, and studied their appropriate pulsed heating conditions. This optimized MEMS sensor chip was integrated with an ASIC with built-in temperature and humidity sensors in a SMD package, and its practical performance and mechanical durability were evaluated.Sensor structuresTwo types of MEMS structures were used for this study as shown in Fig.1. Air-bridge type MEMS structure [1] [2] which is being produced on a commercial basis has a square-shaped micro hot plate (MHP) with dimensions of 0.1 x 0.1 mm. Materials of the sensor electrodes and heater are Platinum, and the gas sensing material is Palladium doped Tin dioxide, which is turned into a paste using terpineol as a solvent. The gas sensing material was applied onto the MHP by micro dispenser, and then was calcined at 450°C for 3 hours in an electric furnace. The newly developed diaphragm type MEMS structure has a round shape MHP with 270m diameter which can avoid stress concentration of SiN/SiO2 membrane. Thickness of each layer stacked on the MHP was also optimized to achieve uniform dispersion of internal stress. The same sensing material as above mentioned was applied onto the MHP of the diaphragm type MEMS by screen printing method so that the gas sensing layers are formed in one printing pass on a wafer that produces 74K chips of MHP.MethodIn order to determine the appropriate pulsed heating conditions and thickness of the sensing material, sensor resistances in air (Ra) and in various concentrations of gases (Rg) were measured by using the air-bridge type MEMS sensor. The sensing material was intermittently heated for 100 msec. at several different temperature conditions in a range from 300 ~ 450 °C, and at different heating intervals ranging from 1 ~ 60 seconds. The thickness of gas sensing layer was varied in a range from 20 ~ 50 micro meters. For the diaphragm type MEMS sensor, the sensing material was heated at 430°C for 100 msec. at 1 second intervals.Results and ConclusionsSensing characteristics of the air-bridge type MEMS sensor to toluene and ethanol were investigated at different working temperature as shown in Fig. 2. Response to toluene increased as the working temperature decreased within the range between 350°C and 450°C, suggesting the capability for detecting low concentration toluene below 1 ppm. On the other hand, Response to ethanol within the same temperature range remained largely the same. Toluene and ethanol sensitivities both drastically dropped at 300°C. There were significant differences in toluene response depending on the heating intervals as shown in Fig.3. Response to toluene became smaller as the heating interval was longer. This result implies that a reaction of the gas sensing material with gas was inhibited by water vapor which had adsorbed on the surface of the sensing element during the heater OFF period. As for the effect of sensing material thickness on sensitivity, ethanol sensitivity was relatively more affected by the thickness than toluene. The sensor with the thinnest sensing layer showed the highest ethanol response, especially at a short heating interval.Taking these results into consideration, driving conditions of the diaphragm type MEMS sensor were determined as described above. Its sensing characteristics to various gases were examined as shown in Fig. 4. This sensor showed a particularly large response to m-xylene and hydrogen sulfide which are both among the major indoor air pollutants, and very small response to hydrogen and methane, both of which would work as an interference gas in IAQ measurement if present in ambient air. It was confirmed in parallel that the diaphragm type MEMS sensor with the determined driving conditions has a sufficient mechanical durability under the heater ON/OFF cycle test equivalent to 10 years operation, and the drop test which is required for mobile applications. The diaphragm type MEMS sensor produced by high-efficiency screen printing method was expected to have a capability as a practical solution for mobile IAQ monitoring. In addition, user-friendliness of the sensor was much improved by integrating with ASIC in comparison with conventional type single sensor.

An All-Silicon Process Platform for Wafer-Level Vacuum Packaged MEMS Devices

Thispaper introduces a novel, inherently simple, and all-silicon wafer-level fabrication and hermetic packaging method developed for MEMS devices. The proposed method uses two separate SOI wafers to form highly-doped through-silicon vias (TSVs) and suspended MEMS structures, respectively. These SOI wafers are then bonded by Au-Si eutectic bonding at 400 °C, achieving hermetic sealing and signal transfer without requiring any complex via or trench refill process steps. The package vacuum is measured using encapsulated MEMS resonators to be as low as 15 mTorr with the help of successfully activated thin-film getters. The combined fabrication and packaging yield is around %89 and chips still maintain a package pressure below 100 mTorr after more than 6 years. The packages show an extremely high strong bonding strength (>40 MPa) and are proved to remain hermetic after temperature cycling (25 °C-85 °C) and harsh temperature shock (5 min@300 °C) tests. The all-silicon MEMS resonators fabricated and packaged using the proposed method project up to a 2.3× enhancement in the bias instability and ~ 4× in the temperature sensitivity of frequency output compared to an identical MEMS resonator fabricated using the silicon-on-glass (SOG) technology.

Acoustics on small scales

We present a modelling strategy based on the finite element method to describe flexible, piezoelectric structures surrounded by a compressible fluid, including viscosity. Non-conforming interfaces based on the Mortar method are used to couple the different physical domains. Finally, we present an application example of a piezoelectrically actuated MEMS structure to illustrate the modeling procedure and the impact of viscous effects.

Sacrificial grid release technology: a versatile release concept for MEMS structures

Micro-electro-mechanical-systems (MEMS) structures with different in-plane dimensions often need to be released simultaneously from the bulk of the wafer and a single dry etching or wet etching technique cannot fulfill all release requirements. In this paper we present a universally applicable solution to release MEMS structures with different surface areas in a controlled and uniform way, which combines isotropic etching of a sacrificial silicon support structure by xenon difluoride with a predefined etch surface made by deep reactive ion etching. Two applications of this Sacrificial Grid Release Technology are presented, in which MEMS devices are released in silicon-on-insulator wafers. The demonstrated applications involve the release of microstructures with in-plane dimensions ranging from tens of micrometers to a few millimeters. The sacrificial silicon structure provides mechanical support which allows freedom in process flow design for fragile MEMS structures. The release technique can also be used to separate the chips from the wafer.

Measurement of internal stress in thin films using MEMS structures

Measurement of internal stress in thin films using MEMS structures

MEMS for detection of environmental pollutants: A review pertains to sensors over a couple of decades in 21st century

Abstract Micro Electro Mechanical Systems has gained significant attention in recent years for the trace level detection of Environmental pollution. Due to miniaturized size and expectance of superior performance, MEMS design and modeling plays a crucial role in sensor fabrication. MEMS sensors fabrication is a challenging task and thus requires a precise balance between novel ideas tempered with process integration. Unless of which low quality sensors may get fabricated resulting in poor performance, wastage of time and increased cost of production. To anticipate these losses, one should have clear idea of fabrication processes, difficulties involved in practical realization and optimized fabrication recipes. One should always have handful of information about feasibility of MEMS structures before actual fabrication. This review outlines various reported aspects of MEMS sensors, such as sensing performance, power and its design optimization using software simulation, packaging and testing. This work has reported advancements in the process flow for fabrication of MEMS sensors over a couple of decades. Although there exist many reviews for MEMS, but all of them would stands for particular domain. For example some reviews cover only material used and suggest probable combination of materials for future sensing. Some reviews justify the structures and geometries of MEMS and some of them just studied the packaging and testing of MEMS. i.e. reviews done on MEMS sensors so far were in discrete form. Literature that brings complete information of MEMS sensors altogether can be rarely seen. This work intends to bridge this gap and provide complete, comprehensive and in-depth ideas about the design of a sensor, its software simulation, selection of application specific sensing materials, fabrication and up to the final level device testing optimized in all aspects for future study.

Highly sensitive Co3O4 nanoparticles/MEMS NO2 gas sensor with the adsorption of the Au nanoparticles

This study produces Co3O4 nanoparticles (NPs) using ultrasonic wave grinding technology. The experimental results show that the average diameter of Co3O4-NPs is about 6 nm. Au nanoparticles are adsorbed onto the surface of Co3O4-NPs and integrated with a MEMS structure to form a Au/Co3O4-NPs/MEMS NO2 gas sensor. The diameter of Au nanoparticle is approximately 1 nm. For a 100-ppb NO2 gas concentration, an Au/Co3O4-NPs/MEMS gas sensor has the greatest sensor response and has a higher sensitivity to NO2 than other gases (SO2, NH3, CO, CO2) when NO2 gas is introduced at 10 ppm and at the optimal operating temperature of 136 ℃.

Dynamics analysis of timoshenko perforated microbeams under moving loads

This paper aims to present a modified continuum mathematical model capable on investigation of dynamic behavior and response of perforated microbeam under the effect of moving mass/load for the first time. A size-dependent finite element model with non-classical shape function is exploited to solve the mathematical model and obtain the dynamic response of perforated Timoshenko microbeams under moving loads. To that end, first, equivalent material and geometrical parameters for perforated beam are developed, based on the regular squared perforation configuration. Second, both the stiffness and mass property matrices including the microstructure effect based on modified couple stress theory and Timoshenko first-order shear beam theory are derived for two-node finite element using new shape function. After that, the interaction between the load and beam is modeed and unified with the equation of motion of the beam incorporating mass inertia effects of moving load. The developed procedure is validated and compared. Effects of perforation parameters, moving load velocities, inertia of mass, and the microstructure size parameter on the dynamic response of perforated microbeam structures have been investigated in a wide context. The achieved results are helpful for the design and production of MEMS structures such as frequency filters, resonators, relay switches, accelerometers and mass flow sensors, with perforation.

On the Finite-Element Analysis of Resonance MEMS Structures based on Acoustic Lamb Waves

This paper considers issues of modeling ultra-high frequency MEMS resonators based on acoustic Lamb waves. In addition, the analysis of factors that determine whether it is possible to increase resonators working frequency and quality factor is carried out. Influence of resonator excitation scheme and acoustic waveguide thickness for a range of piezoelectric materials (i.e. AlN, ZnO, GaN) on phase velocity for acoustic Lamb wave zero modes is investigated. As a result, we've got estimations determining dependence of resonators electromechanical coupling coefficient on their geometry.

An efficient macro model for CMOS-MEMS thermal wind speed sensor

In this paper, we propose a thermal wind speed sensor Verilog-A model with thermal-fluid-electric hybrid characteristics. Because the structure of the thermal wind sensor and the interface circuit chip are designed separately, which cannot effectively carry out MEMS-IC (integrated circuit) co-simulation, the overall performance of the sensor and interface circuit chip is prone to large deviation. At the same time, the separation of MEMS structure and IC design process leads to long product development cycle and high cost. This paper proposes an improved simulation model by analyzing the behavior characteristics of the thermal wind speed sensor. The simulation model consists of 11 thermistors, 3 capacitors, a voltage-controlled voltage source, and a voltage-controlled current source. The Taylor series expansion of the differential equation of heat conduction is carried out, and then the thermistor and capacitor networks are established according to the corresponding relationship between the thermal parameters and the electrical parameters, including the thermal conduction unit, the convection unit and the heat capacity in the chip. The model fully considers the effects of various physical effects and parasitic effects, and is implemented using the hardware description language Verilog-A. The sensor and interface circuit can be simulated simultaneously to optimize the circuit design. It is beneficial to the integration of sensors and interface circuit, reducing the number of tape-outs, increasing the success rate, and reducing costs. The consistency between the simulation results and the experimental curves verified the correctness of the macro model. The behavior characteristics of constant power (CP) and constant temperature heating were described. In CP mode, the output voltage error is less than ± 6.5%, the chip temperature error is less than 4.0%. The presented modeling approach can be the basis for the development of complete MEMS-IC design technology library of thermal wind speed sensor, aiming at optimum designs of integrated thermal wind speed sensor.