Microelectromechanical Systems Applications Research Articles

MEMS reservoir computing system with stiffness modulation for multi-scene data processing at the edge

Reservoir computing (RC) is a bio-inspired neural network structure which can be implemented in hardware with ease. It has been applied across various fields such as memristors, and electrochemical reactions, among which the micro-electro-mechanical systems (MEMS) is supposed to be the closest to sensing and computing integration. While previous MEMS RCs have demonstrated their potential as reservoirs, the amplitude modulation mode was found to be inadequate for computing directly upon sensing. To achieve this objective, this paper introduces a novel MEMS reservoir computing system based on stiffness modulation, where natural signals directly influence the system stiffness as input. Under this innovative concept, information can be processed locally without the need for advanced data collection and pre-processing. We present an integrated RC system characterized by small volume and low power consumption, eliminating complicated setups in traditional MEMS RC for data discretization and transduction. Both simulation and experiment were conducted on our accelerometer. We performed nonlinearity tuning for the resonator and optimized the post-processing algorithm by introducing a digital mask operator. Consequently, our MEMS RC is capable of both classification and forecasting, surpassing the capabilities of our previous non-delay-based architecture. Our method successfully processed word classification, with a 99.8% accuracy, and chaos forecasting, with a 0.0305 normalized mean square error (NMSE), demonstrating its adaptability for multi-scene data processing. This work is essential as it presents a novel MEMS RC with stiffness modulation, offering a simplified, efficient approach to integrate sensing and computing. Our approach has initiated edge computing, enabling emergent applications in MEMS for local computations.

Fabrication of silicon sharp nanocones using dry etch with periodic oxygen plasma shrinking and wet etch

Silicon (Si) nanocones have a wide range of applications in microelectromechanical systems and nanoelectromechanical systems. There is an increasing demand for precise control over the size and shape of nanocones. This paper proposed a novel method combining Si dry etch with periodic oxygen plasma shrinking, wet etch, and oxidation sharpening to achieve well-defined sharp Si nanocones. First, the standard Bosch process was employed to create the base part of nanocones. Second, two alternating steps of etching with sulfur hexafluoride/octafluorocyclobutane plasma and photoresist shrinkage with oxygen plasma were used to form the cone-shaped structures on top of the cylindrical bases. Third, to obtain a sharp tip, wet etching was carried out in either potassium hydroxide or a nitric acid/hydrofluoric (HF) acid mixture. To further sharpen the Si tips, thermal oxidation and HF dipping were conducted and the apex of nanocones can be down to 20 nm. This technique provides a cost-effective way to manufacture nanocones for various applications.

Microcontact printing fabrication of diamond MEMS cantilevers on silicon substrate

Diamond shows great potentials as an ultimate material candidate for application in microelectromechanical systems (MEMS). In this work, we report microcontact printing (μCP) transfer as a facile technique to fabricate diamond MEMS cantilever arrays. We started from the Deryaguin–Landau–Verwey–Overbeek (DLVO) principles and formulated a colloidal “ink” that contains nanodiamonds (NDs) seeds well dispersed in polymethyl methacrylate/acetone. The NDs seeds were then transfer printed onto a silicon (100) substrate by using a flexible polydimethylsiloxane (PDMS) stamp with predefined pattern. The densely packed NDs pattern (∼109 cm−2) enables facile and selected-area growth of diamond with microwave plasma chemical vapor deposition (MPCVD) technique. A diamond film with minimal graphite contaminants and low residual stresses was obtained after growth at 1118 K for 3 h. The diamond cantilevers were released from the substrate by wet chemical etching using KOH (40 % wt.) and isopropyl alcohol (10 % vol.). An examination of the vibrational kinetics indicates that the diamond film has a high Young's modulus (∼795 GPa) and the cantilevers exhibit consistently distributed resonant frequencies and high quality factors in ambient air. The method opens the door for large-scale, cost-effective production of uniform diamond MEMS devices for sensor and quantum applications.

A high aspect ratio surface micromachined accelerometer based on a SiC-CNT composite material

Silicon carbide (SiC) is recognized as an excellent material for microelectromechanical systems (MEMS), especially those operating in challenging environments, such as high temperature, high radiation, and corrosive environments. However, SiC bulk micromachining is still a challenge, which hinders the development of complex SiC MEMS. To address this problem, we present the use of a carbon nanotube (CNT) array coated with amorphous SiC (a-SiC) as an alternative composite material to enable high aspect ratio (HAR) surface micromachining. By using a prepatterned catalyst layer, a HAR CNT array can be grown as a structural template and then densified by uniformly filling the CNT bundle with LPCVD a-SiC. The electrical properties of the resulting SiC-CNT composite were characterized, and the results indicated that the electrical resistivity was dominated by the CNTs. To demonstrate the use of this composite in MEMS applications, a capacitive accelerometer was designed, fabricated, and measured. The fabrication results showed that the composite is fully compatible with the manufacturing of surface micromachining devices. The Young’s modulus of the composite was extracted from the measured spring constant, and the results show a great improvement in the mechanical properties of the CNTs after coating with a-SiC. The accelerometer was electrically characterized, and its functionality was confirmed using a mechanical shaker.

Novel Route for Enhancing Piezoelectricity of Ferroelectric Films: Controlling Nontrivial Polarization States in Pb(Zr, Ti)O3 Monodomain Superlattice Structure.

Artificial superlattice films made of Pb(Zr0.4Ti0.6)O3 and Pb(Zr0.6Ti0.4)O3 were investigated for their polarization states and piezoelectric properties theoretically and experimentally in this study. The developed theory predicts nontrivial polarization along neither [001] nor [111] directions in (111)-epitaxial monodomain superlattice films with uniform compressive strain. Such films were achieved via pulsed laser deposition. When the layer thickness is reduced to 3 nm, d33 becomes 128 ± 3.8 pm/V at 100 kV/cm and 71.3 ± 2.83 pm/V at 600 kV/cm, comparable to that of (111)-oriented Pb(Zr0.4Ti0.6)O3 or Pb(Zr0.6Ti0.4)O3 bulks and clearly exceeding that of the typical clamped films. The measurement agrees with the theoretical analysis, which reveals that the enhanced piezoelectricity is due to rotation of the nontrivial polarization. Furthermore, the theoretical study predicts an even larger d33 exceeding 300 pm/V for specific parameters in superlattice films with uniform tensile strain, which is promising for applications of microelectromechanical systems.

Enhancing Manufacturability of SU-8 Piezoelectric Composite Films for Microsystem Applications.

Piezoelectric thin films are extensively used as sensing or actuating layers in various micro-electromechanical systems (MEMS) applications. However, most piezoelectrics are stiff ceramics, and current polymer piezoelectrics are not compatible with microfabrication due to their low Curie Temperature. Recent polymer-composite piezoelectrics have gained interest but can be difficult to pattern. Photodefinable piezoelectric films could resolve these challenges by reducing the manufacturability steps by eliminating the etching process. But they typically have poor resolution and thickness properties. This study explores methods of enhancing the manufacturability of piezoelectric composite films by optimizing the process parameters and synthesis of SU-8 piezo-composite materials. Piezoelectric ceramic powders (barium titanate (BTO) and lead zirconate titanate (PZT)) were integrated into SU-8, a negative epoxy-based photoresist, to produce high-resolution composites in a non-cleanroom environment. I-line (365 nm) light was used to enhance resolution compared to broadband lithography. Two variations of SU-8 were prepared by thinning down SU-8 3050 and SU-8 3005. Different weight percentages of the piezoelectric powders were investigated: 5, 10, 15 and 20 wt.% along with varied photolithography processing parameters. The composites' transmittance properties were characterized using UV-Vis spectroscopy and the films' crystallinity was determined using X-ray diffraction (XRD). The 0-3 SU-8/piezo composites demonstrated resolutions < 2 μm while maintaining bulk piezoelectric coefficients d33 > 5 pm V-1. The films were developed with thicknesses >10 μm. Stacked layers were achieved and demonstrated significantly higher d33 properties.

MONITORING OF OIL ANALYSIS THROUGH SENSORS

Inserting the Industry 4.0 universe into companies is necessary to guarantee their competitiveness and continuity in the market. And, one of the areas in which industry 4.0 and its technologies are most prominent is maintenance, as the use of intelligent mechanisms are capable of promoting the reliability of systems functioning, predicting failures and anticipating problems and breakdowns in equipment. , thus contributing to increased performance and reduced aggregate costs. The present study then starts from the idea of using Lab-on-chip technology for the hydraulic fluid and lubricant monitoring system and aims to verify the application of Microelectromechanical Systems (MEMS) in maintenance. From the literary review, it was possible to verify that studies relating the use of microsensors for monitoring lubricants are still scarce and from this, applied research was suggested for this purpose, developing a lab-on-chip that be capable of replacing complex and high-cost laboratory analyses.

MEMS Huygens Clock Based on Synchronized Micromechanical Resonators

With the continuous miniaturization of electronic devices, microelectromechanical system (MEMS) oscillators that can be combined with integrated circuits have attracted increasing attention. This study reports a MEMS Huygens clock based on the synchronization principle, comprising two synchronized MEMS oscillators and a frequency compensation system. The MEMS Huygens clock improved short-time stability, improving the Allan deviation by a factor of 3.73 from 19.3 ppb to 5.17 ppb at 1 s. A frequency compensation system based on the MEMS oscillator’s temperature-frequency characteristics was developed to compensate for the frequency shift of the MEMS Huygens clock by controlling the resonator current. This effectively improved the long-term stability of the oscillator, with the Allan deviation improving by 1.6343 × 105 times to 30.9 ppt at 6000 s. The power consumption for compensating both oscillators simultaneously is only 2.85 mW∙°C−1. Our comprehensive solution scheme provides a novel and precise engineering solution for achieving high-precision MEMS oscillators and extends synchronization applications in MEMS.

Magnetron Sputtered Lead Titanates Thin Films for Pyroelectric Applications: Part 1: Epitaxial Growth, Material Characterization.

Pyroelectric materials, are those materials with the property that in the absence of any externally applied electric field, develop a built-in spontaneous polarization in their unit cell structure. They are regarded as ideal detector elements for infrared applications because they can provide fast response time and uniform sensitivity at room temperature over all wavelengths. Crystals of the perovskite Lead Titanate (PbTiO3) family show pyroelectric characteristics and undergo structural phase transitions. They have a high Curie temperature (the temperature at which the material changes from the ferroelectric (polar) to the paraelectric (nonpolar) phase), high pyroelectric coefficient, high spontaneous polarization, low dielectric constant, and constitute important component materials not only useful for infrared detection, but also with vast applications in electronic, optic, and Micro-electromechanical systems (MEMS) devices. However, the preparation of large perfect, and pure single crystals of PbTiO3 is challenging. Additionally, difficulties arise in the application of such bulk crystals in terms of connection to processing circuits, large size, and high voltages required for their operation. A number of thin film fabrication techniques have been proposed to overcome these inadequacies, among which, magnetron sputtering has demonstrated many potentials. By addressing these aspects, the review article aims to contribute to the understanding of the challenges in the field of pyroelectric materials, highlight potential solutions, and showcase the advancements and potentials of pyroelectric perovskite series including PbZrTiO3 (PZT), PbxCa1-x (PZN-PT), etc. for which PbTiO3 is the end member. The review is presented in two parts. Part 1 focuses on material aspects, including preparation methods using magnetron sputtering and material characterization. We take a tutorial approach to discuss the progress made in epitaxial growth of lead titanate-based ceramics prepared by magnetron sputtering and examine how processing conditions may affect the crystalline quality of the growing film by linking to the properties of the substrate/buffer layer, growth substrate temperature, and the oxygen partial pressure in the gas mixture. Careful control and optimization of these parameters are crucial for achieving high-quality thin films with desired structural and morphological characteristics.

Simulation, synthesis, and analysis of strontium-doped ZnO nanostructures for optoelectronics and energy-harvesting devices

The demand for clean and sustainable alternative energy resources is linearly increasing day by day due to the prevailing electricity crisis. Small-scale energy harvesting is considered a sustainable way to generate clean energy. Advanced energy solar cells, mainly dye-sensitized solar cells use solar energy and convert it into electrical energy. Similarly, MEMS-based piezoelectric materials are used to convert mechanical energy into electrical energy. For these applications, zinc oxide is considered one of the most suitable materials with high conductive, tunable band gap, and piezoelectric properties. However, altering these properties can be carried out by the addition of metal and other materials. Various research work has been carried out to study the addition of conductive metal as a dopant to alter the properties of zinc oxide. In this study, Strontium has been doped in ZnO to form a nanostructure for application in DSSC and microelectromechanical systems (MEMS) energy harvesters. Analysis has been conducted using the simulation and fabrication method. The results show that the doping and the pore size of the substrate (Anodic Aluminum oxide membrane) largely affect the output voltage and current. The difference between the simulated and experimental results was less than 1%, which shows the accuracy of the simulation. Tuning of the band gap can be observed by the addition of Sr in the ZnO nanostructure. For microelectromechanical systems energy harvesters, Sr-doped ZnO nanostructures deposited on anodic aluminum oxide show 7.10 mV of voltage and 1.11 uA of current output. The addition of Sr doping in ZnO shows the improvement in the generated current and voltage for the energy harvester and the improvement in overall power conversion efficiency for dye-sensitized solar cells. MEMS-based energy harvesting devices and low-cost advanced solar cells are promising to improve the efficiency of energy generation at a small scale.

Enhancing supercapacitor performance through design optimization of laser-induced graphene and MWCNT coatings for flexible and portable energy storage

The field of supercapacitors consistently focuses on research and challenges to improve energy efficiency, capacitance, flexibility, and stability. Low-cost laser-induced graphene (LIG) offers a promising alternative to commercially available graphene for next-generation wearable and portable devices, thanks to its remarkable specific surface area, excellent mechanical flexibility, and exceptional electrical properties. We report on the development of LIG-based flexible supercapacitors with optimized geometries, which demonstrate high capacitance and energy density while maintaining flexibility and stability. Three-dimensional porous graphene films were synthesized, and devices with optimized parameters were fabricated and tested. One type of device utilized LIG, while two other types were fabricated on LIG by coating multi-walled carbon nanotubes (MWCNT) at varying concentrations. Characterization techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and voltammetry, were employed to analyze the fabricated devices. AFM analysis revealed a surface roughness of 2.03 µm for LIG due to laser treatment. SEM images displayed compact, dense, and porous surface morphology. XRD analysis confirmed the presence of graphene and graphene oxide, which was further supported by energy-dispersive X-ray spectroscopy (EDX) data. Raman spectroscopy indicated that the fabricated samples exhibited distinct D and G bands at 1362 cm–1 and 1579 cm–1, respectively. Cyclic voltammetry (CV) results showed that LIG's capacitance, power density, and energy density were 6.09 mF cm–2, 0.199 mW cm–2, and 3.38 µWh cm–2, respectively, at a current density of 0.2 mA cm–2. The LIG-MWCNT coated electrode exhibited a higher energy density of 6.05 µWh cm–2 and an areal-specific capacitance of 51.975 mF cm–2 compared to the LIG-based devices. The fabricated device has potential applications in smart electronics, nanorobotics, microelectromechanical systems (MEMS), and wearable and portable electronics.

Quality analysis of material jetted silicone material for soft robotics application

Silicone and silicone-based elastomers find their applications in micro-electromechanical systems (MEMS) and soft robotics. Mechanical properties of such materials upon processing them with additive manufacturing are challenging and need significant improvement for enhanced performance in biomedical applications. With extrusion-based additive manufacturing, there is an inherent porosity inside the printed volume which negatively influences the quality and mechanical properties of printed parts. In this study, material jetting a drop-on-demand additive manufacturing technique is used to 3D print silicone samples of shore hardness 60A. These samples are analysed for tensile strength, surface roughness, porosity, and pores characterization for assessing the quality and performance of silicone 60A and the results suggest that silicone 60A material used in material jetting printing process is a potential material for low force application in soft robotics.

Investigating the Effect of an Elliptical Bluff Body on the Behavior of a Galloping Piezoelectric Energy Harvester

The extraction of energy from naturally oscillating objects has recently garnered considerable attention from researchers as a robust and efficient method. This study specifically focuses on investigating the performance of a galloping piezoelectric micro energy harvester (GPEH) designed for self-powered microelectromechanical systems (MEMS). The proposed micro energy harvester comprises a cantilever beam composed of two layers, one being silicon and the other being a piezoelectric material (PZT-5A). The harvester is equipped with an elliptical tip cylinder, and the entire system is modeled using lumped parameters. To simulate the response of the system, the size-dependent coupled governing equations are numerically solved, enabling the extraction of the dynamic behavior of the energy harvester. Furthermore, computational fluid dynamics (CFD) simulations are employed to model the effect of the flow field on the oscillations of the beam. Different aspect ratios (AR) of the elliptical cylinder are taken into account in the simulations. The study examines the impact of the aspect ratio and mass of the elliptical tip cylinder on the harvested power of the system. The results demonstrate a notable decrease in the extracted power density for AR = 1 and 2 compared to higher aspect ratios. In the case of AR = 5, the device exhibits an onset wind speed of 7 m/s. However, for AR = 10, the onset wind speed occurs at a lower wind velocity of 5.5 m/s, resulting in a 66% increase in extracted power compared to AR = 5. Additionally, the results reveal that increasing the normalized mass from 10 to 60 results in a 60% and 70% increase in the output power for AR = 5 and AR = 10, respectively. This study offers valuable insights into the design and optimization of galloping piezoelectric micro energy harvesters, aiming to enhance their performance for MEMS applications.

Enhancement of cooling performance in MEMS by modifying 3D packaging structure: A design, integration, analysis and test study

The heat dissipation of three-dimensional (3D) integrated Micro-Electro-Mechanical Systems (MEMS) has garnered considerable attention in scientific research and electronic applications in recent years. As power demands increase, device sizes decrease, and energy consumption is limited, the use of liquid and forced air cooling techniques is diminishing. In light of the growing emphasis on achieving carbon neutrality, there is renewed interest in passive cooling methods, which present a new imperative for environmentally-friendly thermal management of chips in the field of green energy. While the concept of designing thermal paths to mitigate heat in chips is not novel, it has become increasingly important due to the rapid accumulation of heat, reduction in chip volume, and the need to minimize energy consumption and carbon emissions. Consequently, the implementation of appropriate cooling structures within the available space of the chips has emerged as an appealing approach to enhance passive cooling. In order to tackle this challenge, four cooling structures, namely embedding network structure, overlaying extension platform, overlaying micro-pillar array, and overlaying surface microstructures, have been employed in the packaging process to augment the passive cooling capabilities of the chips. In this study, a 3D packaging model is developed to analyze the thermal characteristics of MEMS under the influence of power, TSV voltage, and heat source area using the finite element method. Additionally, the cooling structures are optimized based on the cooling performance analysis, taking into account the self-remaining space within the system and the heat source area factor. The application of four different cooling structures in 3D MEMS leads to enhanced cooling performance. In the event that liquid cooling and forced air cooling are not utilized, the system experiences a decrease in maximum temperature by 3.66 K, resulting in a corresponding decrease in temperature difference by 43.79 %. These reductions significantly contribute to the mitigation of thermal deformation and stress in MEMS, thereby enhancing the thermal reliability of the system. To verify the accuracy of the thermal simulation, a thermal test is conducted on the embedded network structure under simulated conditions. The thermal simulation, which has been validated through thermal testing, indicates that the implementation of four cooling structures within the self-contained area of MEMS has a notable effect on reducing temperature. This finding holds promise for its potential application in 3D MEMS.

4H-Silicon Carbide as an Acoustic Material for MEMS.

This paper discusses the potential of 4H-SiC as a superior acoustic material for MEMS, particularly for high-performance resonator and extreme environments applications. Through a comparison of the crystalline structure along with the mechanical, acoustic, electrical, and thermal properties of 4H with respect to other SiC polytypes and silicon, it is shown that 4H-SiC possesses salient properties for MEMS applications, including its transverse isotropy and small phonon scattering dissipation. The utility and implementation of bonded SiC on insulator (4H-SiCOI) substrates as an emerging MEMS technology platform are presented. Additionally, this paper reports on the temperature-dependent mechanical properties of 4H-SiC, including the temperature coefficient of frequency and quality factor for Lame mode resonators. Finally, the 4H-SiC MEMS fabrication including its deep reactive ion etching is discussed. This paper provides valuable insights into the potential of 4H-SiC as a mechano-acoustic material and provides a foundation for future research in the field.

Numerical Analysis of Viscosity and Surface Tension on Microdroplet Dynamics in Microelectromechanical Systems Applications

Numerical Analysis of Viscosity and Surface Tension on Microdroplet Dynamics in Microelectromechanical Systems Applications

Mathematical modelling of the partial differential equations in microelectromechanical systems (MEMS) and its applications

This paper presents a model of a doubly clamped electrically actuated microbeam, which is a commonly used structure in microelectromechanical systems (MEMS). The model is based on Euler–Bernoulli beam theory and includes the effect of electrostatic forces on the beam’s deflection. The electric field is modeled using the parallel plate capacitor model, and the deflection of the beam is calculated using the Galerkin method. The behavior of a microbeam subjected to the van der Waals force, which is a weak intermolecular force that arises from the interaction between the beam and a nearby surface. The microbeam is modeled using the Euler–Bernoulli beam theory, and the van der Waals force is modeled using the Lennard–Jones potential. At the last we study the model of MEMS based on multi-walled carbon nanotubes (MWCNTs). MWCNTs have unique mechanical, thermal, and electrical properties, which make them ideal for use in MEMS applications. The approximate solution of the developed models is found by using homotopy perturbation based Aboodh transformation (HPATM). HPATM is a mathematical method used to solve nonlinear equations by converting them into linear forms. This approach involves introducing a small parameter and applying perturbation theory to obtain a solution in a series form. The method’s accuracy is defined based on the existing literature because its solution matched the variation iteration-based Laplace method. Also, we compared its results with the finite difference method. The validity of the stability analysis is further established by examining the stability in the vicinity of the fixed points. Sketches are made of the phase portraits close to the equilibrium points.

Electrohydrodynamic Redox Printing: A Swiss Army Knife of Metals Microfabrication?

The dissolution and deposition of metals in solvents is widely known and used in a large range of applications, from electroplating to electro-machining. More recently, it has also been used for additive manufacturing (AM) at the sub-micron scale, since electroplated metals offer as-deposited high-quality metals (in terms of porosity and contamination) compared to other small scale AM techniques [1,2]. These intricate, high-quality metal structures have a range of applications in microelectromechanical systems, photonics or micro-robotics. However, the nature of the deposition process requires a conductive substrate to provide the electrons necessary for the localized reduction of metal ions. This requirement renders the direct fabrication of microelectronic devices using electrochemical small scale AM a challenge, as often deposition on electrically insulating substrates is required.Here, we present our work on electrohydrodynamic redox printing (EHD-RP), an electrochemical small scale AM technique, and our efforts towards deposition on various substrates. We’ll first introduce EHD-RP with a focus on our recent work on the deposition from aqueous metal salt solutions onto conductive substrates. We’ll then discuss strategies to expand the concept of EHD-RP towards deposition on non-conductive substrates and how electrically floating structures can be fabricated. These strategies render EHD-RP a versatile technique for metals microfabrication.[1] M. Nydegger, A. Pruška, H. Galinski, R. Zenobi, A. Reiser and R. Spolenak. Additive Manufacturing of Zn with Submicron Resolution and its Conversion into Zn/ZnO core-shell structures. Nanoscale, 2022[2] L. Hirt, A. Reiser, R. Spolenak and T. Zambelli. Additive Manufacturing of Metal Structures at the Micrometer Scale. Adv. Mater., 29(17), 2017

Atomic layer deposited SnO2/ZnO composite thin film sensors with enhanced hydrogen sensing performance

Hydrogen gas (H2) has long been regarded as a clean and renewable energy carrier alternative to the fossil fuel, therefore the development of highly sensitive and reliable hydrogen sensor is imperative for the large-scale implementation of green energy hydrogen. In this work, SnO2, ZnO thin film and SnO2/ZnO composite thin film H2 sensors were fabricated on SiO2/Si substrates by atomic layer deposition (ALD). The effect of ALD cycles and heat treatment temperatures on the sensing properties of the thin film sensors was investigated deeply. It is found that the 100-cycle SnO2/150-cycle ZnO composite film shows the best H2-sensing performance such as large response of 93 to 30 ppm H2 at 200 ℃, fast response/recovery time of 50 s/29 s, and low detection limit of 250 ppb. Moreover, the response of SnO2/ZnO composite film sensor is 10 times and 2 times higher than that of the pure ZnO and SnO2 thin film ones at working temperature of 200℃, respectively. A possible mechanism for performance improvement of the SnO2/ZnO composite film H2 sensor is proposed, attributed to the synergistic effect of the SnO2-ZnO heterojunction and the oxygen vacancies in the SnO2 film. ALD may be a feasible strategy to construct Si-based highly sensitive hydrogen sensors for micro-electromechanical system application.

Achievement of High Perpendicular Anisotropy and Modification of Heat Treatment Peeling in Micron-Thickness Nd-Fe-B Films Used for Magnetic MEMS.

Thick Nd-Fe-B permanent magnetic films with good perpendicular anisotropy have important applications in magnetic microelectromechanical systems (MEMSs). However, when the thickness of the Nd-Fe-B film reaches the micron level, the magnetic anisotropy and texture of NdFeB film will become worse, and it is also prone to peeling during heat treatment, which seriously limits their applications. In this paper, Si(100)/Ta(100 nm)/NdxFe91-xB9(x = 14.5, 16.4, 18.2)/Ta (100 nm) films with thicknesses of 2-10 μm are prepared by magnetron sputtering. It is found that gradient annealing (GN) could help improve the magnetic anisotropy and texture of the micron-thickness film. When the Nd-Fe-B film thickness increases from 2 μm to 9 μm, its magnetic anisotropy and texture do not deteriorate. For the 9 μm Nd-Fe-B film, a high coercivity of 20.26 kOe and high magnetic anisotropy (remanence ratio Mr/Ms = 0.91) are achieved. An in-depth analysis of the elemental composition of the film along the thickness direction is conducted, and the presence of Nd aggregation layers at the interface between the Nd-Fe-B and the Ta layers is confirmed. The influence of thicknesses of the Ta buffer layer on the peeling of Nd-Fe-B micron-thickness films after high-temperature annealing is investigated, and it is found that increasing the thickness of the Ta buffer layer could effectively inhibit the peeling of Nd-Fe-B films. Our finding provides an effective way to modify the heat treatment peeling of Nd-Fe-B films. Our results are important for the development of Nd-Fe-B micron-scale films with high perpendicular anisotropy for applications in magnetic MEMS.