Surface Micromachining Technology Research Articles

Microfabrication Technologies for Nanoinvasive and High-Resolution Magnetic Neuromodulation.

The increasing demand for precise neuromodulation necessitates advancements in techniques to achieve higher spatial resolution. Magnetic stimulation, offering low signal attenuation and minimal tissue damage, plays a significant role in neuromodulation. Conventional transcranial magnetic stimulation (TMS), though noninvasive, lacks the spatial resolution and neuron selectivity required for spatially precise neuromodulation. To address these limitations, the next generation of magnetic neurostimulation technologies aims to achieve submillimeter-resolution and selective neuromodulation with high temporal resolution. Invasive and nanoinvasive magnetic neurostimulation are two next-generation approaches: invasive methods use implantable microcoils, while nanoinvasive methods use magnetic nanoparticles (MNPs) to achieve high spatial and temporal resolution of magnetic neuromodulation. This review will introduce the working principles, technical details, coil designs, and potential future developments of these approaches from an engineering perspective. Furthermore, the review will discuss state-of-the-art microfabrication in depth due to its irreplaceable role in realizing next-generation magnetic neuromodulation. In addition to reviewing magnetic neuromodulation, this review will cover through-silicon vias (TSV), surface micromachining, photolithography, direct writing, and other fabrication technologies, supported by case studies, providing a framework for the integration of magnetic neuromodulation and microelectronics technologies.

Nanostructured Substrate-Mediated Bubble Degassing in Microfluidic Systems.

Microfluidic platforms have been widely used in a variety of fields owing to their numerous advantages. The prevention and prompt removal of air bubbles from microchannels are important to ensuring the optimal functioning of microfluidic devices. The entrapment of bubbles in the microchannels can result in flow instability and device performance disruption. Active and passive methods are the primary categories of degassing technologies. Active methods rely on external equipment, and passive methods operate autonomously without any external sources. This study proposed a passive degassing method that employs a nanoscale surface morphology integrated into the substrate of a microfluidic device. Nanostructures exhibit a microchannel geometry and are fabricated based on surface micromachining technology using silver ink and chemical etching. Consequently, the gas permeability is enhanced, resulting in effective degassing through the nanostructure. The performance of this degassing method was characterized under varying substrate permeabilities and input pressure conditions, and it was found that increased permeability facilitates the degassing performance. Furthermore, the applicability of our method was demonstrated by using a serpentine channel design prone to gas entrapment, particularly in the corner regions. The nanostructured substrate exhibited significantly improved degassing performance under the given pressure conditions in comparison to the glass substrate.

Fabrication and Characterization of a Flexible Thin-Film-Based Array of Microelectrodes for Corneal Electrical Stimulation.

The electric stimulation (ES) of the cornea is a novel therapeutic approach to the treatment of degenerative visual diseases. Currently, ES is delivered by placing a mono-element electrode on the surface of the cornea that uniformly stimulates the eye along the electrode site. It has been reported that a certain degree of correlation exists between the location of the stimulated retinal area and the position of the electrode. Therefore, in this study, we present the development of a sectioned surface electrode for selective electric stimulation of the human cornea. The proposed device consists of 16 independent microelectrodes, a reference electrode, and 18 contact pads. The microelectrodes have a size of 200 µm × 200 µm, are arranged in a 4 × 4 matrix, and cover a total stimulation area of 16 mm2. The proposed fabrication process, based on surface micromachining technology and flexible electronics, uses only three materials: polyimide, aluminum, and titanium, which allow us to obtain a simplified, ergonomic, and reproducible fabrication process. The fabricated prototype was validated to laboratory level by electrical and electrochemical tests, showing a relatively high electrical conductivity and average impedance from 712 kΩ to 1.4 MΩ at the clinically relevant frequency range (from 11 Hz to 30 Hz). Additionally, the biocompatibility of the electrode prototype was demonstrated by performing in vivo tests and by analyzing the polyimide films using Fourier transform infrared spectroscopy (FTIR). The resulting electrode prototype is robust, mechanically flexible, and biocompatible, with a high potential to be used for selective ES of the cornea.

Design and comprehensive analysis of a novel structure of ARCL-to-SCL GSG transition for millimeter wave applications

A comprehensive study of air core recta-coax line (ARCL) based on MEMS surface micromachining technology is presented in this paper. The ARCL-to-semi-rectangular coaxial transmission line (SCL) ground-signal-ground (GSG) transition is proposed and designed for probe testing and integration with MMIC chips. To obtain the characteristic impedance of SCL, an equivalent model and theoretical derivation process is presented. In order to consider the effects of processing on performance, electrical effects of dielectric support strap parameters and potential fabrication issues including offset layers and over etching are investigated. For verification, the back-to-back GSG transition is fabricated and measured which has a low insertion loss of 0.13dB up to 50GHz, and a return loss better than -23dB within 5-50GHz. As an example, a demonstration of ARCL with GSG transition for MMIC chips integration is demonstrated.

A Robust Micro Mechanical-Latch Shock Switch With Limited Deformation Space

Mechanical-latch mechanisms are often used in shock switches to turn the switches on by the desired inertial force in the specified sensing direction. However, while experiencing an unexpected large inertial force in opposite sensing direction, the switch might slip off the contact electrode and return back to its initial position to cause malfunction, so-called slippage effect. Here a robust micro mechanical-latch shock switch to resist the slippage effect is reported by limiting deformation space on one side of the latch. An analytical model is developed to determine the dimensional parameters. The proposed shock switch made of Ni is fabricated by the metal-based surface micromachining technology so that low contact resistance can be easily achieved. The experimental results show that the switch can be successfully latched after experiencing a downward shock of 13.11 G, close to the simulated threshold level of 12.14 G. Furthermore, even after applying over 2000 G opposite impact acceleration, the switch can remain latched due to the limited deformation space design, whereas the switch without the limited space design cannot. The contact resistance after latching is demonstrated around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$40\Omega $ </tex-math></inline-formula> , six orders less than the resistance before latching. These results verify the accuracy of the static model and the robustness of the proposed micro shock switch design to avoid undesired unlatching with low contact resistance at compact size.

Design and implementation of an acoustic-vibration capacitive MEMS microphone

A novel acoustic-vibration capacitive microelectromechanical system microphone is designed, fabricated, and implemented in this paper. The new microphone consists of a rigid diaphragm and mass blocks sensitive to low-frequency vibratory and sound signals. This sensor takes advantage of the semiconductor technology to design the capacitance sensor structure by surface micromachining technology, and the inertial mass blocks are shaped using the bulk silicon micromachining technology. The structure of the anti-stiction-dimple array is designed and deployed at the bottom of the diaphragm and the backplate to avoid the risk of sensor failure by vibration stiction. The bottom and top of the backplate are designed with an anti-humidity hydrophobic insulation protective layer, which avoids adsorption of moisture and attachment of foreign particles. The thickness of the mass blocks can be controlled by the combination of the dry and the wet micromachining method, which is sensitive to different frequency responses. This study can design and produce wafer level silicon with high consistency. The inertial mass proposed in this research can be achieved through a 6 in. wafer process with &amp;gt;80% consistency when the thickness of the mass is around 300 μm. The sensing frequency can be reduced to below the 4 kHz frequency bandwidth with enhanced sensitivity in the ±0.5 dB range. Typical characteristic results show that the open-circuit sensitivity of the microphone is 12.63 mV/Pa (37.97 dBV/Pa) at 1 kHz (with 94 dB as the reference sound level). The total harmonic distortion and acoustic overload point are 0.21% and 121.2 dB sound pressure level, respectively. The electronic stethoscope is a typical application of this research, which can collect the characteristics and frequency spectrum of low-frequency cardiac vibration signals.

Getting ready for beyond-5G, super-IoT and 6G at hardware passive components level: a multi-state RF-MEMS monolithic step attenuator analyzed up to 60 GHz

Looking at 2030, the landscape of technology will be dominated by paradigms like 6G, Super-IoT (Internet of Things) and Tactile Internet (TI). From the perspective of Hardware (HW) components technologies, the turning into reality of such scenarios will demand for a radical reconceptualization of devices, sub-systems and systems, probably modifying the concept of HW itself. Driven by the target of taking initial steps in the direction of such future applications, this work discusses a 4 bit RF power step attenuator entirely realized in RF-MEMS technology. Physical samples are fabricated in a surface micromachining technology and rely on electrostatically actuated cantilevered MEMS ohmic switches to select or short resistive loads placed along the RF line. Fabricated devices are tested and validated up to 30 GHz, while simulations are discussed up to 60 GHz for the full set of allowed configurations. Despite a few technology non-idealities, the network shows levels of attenuation with a flatness as good as 1 dB over 60 GHz frequency span. The measured and simulated data reported in this work offer important indications on how to improve the network concept, both at technology and design level.

Research Progress of MEMS Inertial Switches.

As a typical type of MEMS acceleration sensor, the inertial switch can alter its on-off state while the environmental accelerations satisfy threshold value. An exhaustive summary of the design concept, performance aspects, and fabrication methods of the micro electromechanical system (MEMS) inertial switch is provided. Different MEMS inertial switch studies were reviewed that emphasized acceleration directional and threshold sensitivity, contact characteristics, and their superiorities and disadvantages. Furthermore, the specific fabrication methods offer an applicability reference for the preparation process for the designed inertial switch, including non-silicon surface micromachining technology, standard silicon micromachining technology, and the special fabrication method for the liquid inertial switch. At the end, the main conclusions of the current challenges and prospects about MEMS inertial switches are drawn to assist with the development of research in the field of future engineering applications.

Miniaturized W‐band bandpass filter based on rectangular micro‐coaxial structure

This paper presents a miniaturized W-band bandpass filter based on rectangular micro-coaxial structure. The filter with third-order Chebyshev response is analyzed and designed. It uses half-wavelength resonators and realizes the coupling between resonant units by adding grounded cylinders. The center frequency is 97.5 GHz with 3-dB fractional bandwidth (FBW) of 7.5%. Moreover, the prototype is fabricated by the surface micromachining technology, which greatly reduces its volume. The measured results are close to the simulations in operating band and the minimum insertion loss is 1.9 dB.

Toward applications of near-field radiative heat transfer with micro-hotplates

Bringing bodies close together at sub-micron distances can drastically enhance radiative heat transfer, leading to heat fluxes greater than the blackbody limit set by Stefan–Boltzmann law. This effect, known as near-field radiative heat transfer (NFRHT), has wide implications for thermal management in microsystems, as well as technological applications such as direct heat to electricity conversion in thermophotovoltaic cells. Here, we demonstrate NFRHT from microfabricated hotplates made by surface micromachining of hbox {SiO}_2/hbox {SiN} thin films deposited on a sacrificial amorphous Si layer. The sacrificial layer is dry etched to form wide membranes ({100},upmu hbox {m} times {100},upmu hbox {m}) separated from the substrate by nanometric distances. Nickel traces allow both resistive heating and temperature measurement on the micro-hotplates. We report on two samples with measured gaps of {610},hbox {nm} and {280},hbox {nm}. The membranes can be heated up to {250},^{circ }hbox {C} under vacuum with no mechanical damage. At {120},^{circ }hbox {C} we observed a 6.4-fold enhancement of radiative heat transfer compared to far-field emission for the smallest gap and a 3.5-fold enhancement for the larger gap. Furthermore, the measured transmitted power exhibits an exponential dependence with respect to gap size, a clear signature of NFRHT. Calculations of photon transmission probabilities indicate that the observed increase in heat transfer can be attributed to near-field coupling by surface phonon-polaritons supported by the hbox {SiO}_2 films. The fabrication process presented here, relying solely on well-established surface micromachining technology, is a key step toward integration of NFRHT in industrial applications.

The Design, Simulation and Fabrication of an Omnidirectional Inertial Switch with Rectangular Suspension Spring.

An omnidirectional inertial switch with rectangular spring is proposed in this paper, and the prototype has been fabricated by surface micromachining technology. To evaluate the threshold consistency and stability of omnidirectional inertia switch, the stiffness of rectangular suspension springs is analyzed. The simulation result shows that the coupling stiffness of the rectangular spring suspension system in the non-sensitive direction is a little more than that in the sensitive direction, which indicated that the omnidirectional switching system’s stability is reinforced, attributed to the design of rectangular springs. The dynamic response simulation shows that the threshold of the omnidirectional inertial switch using the rectangular suspension spring has high consistency in the horizontal direction. The prototype of an inertial switch is fabricated and tested successfully. The testing results indicate even threshold distribution in the horizontal direction. The threshold acceleration of the designed inertial switch is about 58 g in the X direction and 37 g in the Z direction; the contact time is about 18 μs.

An intensive approach to optimize capacitive type RF MEMS shunt switch

Optimization of the RF MEMS switch is essential to enhance its performance characteristics at high frequency Millimeterwave applications. Many optimization techniques have been proposed based on design factors and output responses but not confined with the resonant frequency of RF transmission signal. In this paper, a novel optimization technique is proposed using Multiphysics FEM simulations based on electromagnetic and electromechanical studies. A novel Capacitive type of RF MEMS shunt switch is designed by using iterative meander technique and optimized at the resonant frequency of 41 ​GHz which can efficiently used for millimeter wave applications up to 60 ​GHz. The proposed optimization model which is a novel bottom – up approach consists of intermediate steps for optimization of various layers of switch from substrate to beam. The CPW transmission line with 60/100/60 as G/S/G and having 0.5 ​μm as thickness is considered to allow the frequencies up to 60 ​GHz. The width of the suspended Beam of switch is optimized to obtain up-state capacitance of 37 ​fF which allows the RF signal at 41 ​GHz. Parametric analysis has been carried out to obtain optimized thickness of each layer. Two types of switches other than Fixed – Fixed beam are designed using meandering technique to reduce pull in voltage. Among these the proposed Iterative meander switch shows very low pull in voltage of 1.4 ​V and also shows good return loss of −48.9 ​dB at 41 ​GHz and high isolation of −48.42 ​dB at 38 ​GHz. The proposed switches which are optimized at 41 ​GHz does not contains the perforations and suffers from serious stiction problems at downstate. Hence, perforations are introduced in switch membrane and fabricated using surface micromachining technology. The optimized iterative meander switch shows low pull – in voltage of 1.85 ​V which is closely approximated to the simulated value.

Design and Assembly of a Thin-Plate Mechatronic Atomizer by 3D Printing

Microfluidic structures and devices have been studied over decades for the transport of liquid through internal channels using versatile microfabrication schemes such as surface and bulk micromachining technologies. One challenge in consideration of the device design involves the breakthrough of microfluidic reservoir and channels being substantially limited in two-dimensional (2D) geometry. However, recent progress of the emerging 3D printing technologies has showed great potential to overcome this problem in a simple manner. This paper comprehensively reports an additive manufacturing of polylactic acid (PLA) layers to significantly improve the complexity in the formation of the 3D microfluidic structures as compared to conventional micro-manufacturing techniques. Moreover, a handheld mechatronic device with a small height of ~10 mm, assembled with a thin planar atomizer and a micro controller, was produced and demonstrated for generation of droplets (~6 μm in diameter). Both the analytical and experimental results indicated that the grids of channel microstructures were simply varied by different line widths (300–500 μm) and spacing (250–400 μm) 3D printed within the device, thereby providing the design capability for capillary flow. In this regard, a variety of complex micro devices fabricated via computer-aided design (CAD) and the 3D printing method could be applied for more applications than ever, such as microfluidic delivery of biomedical materials and health care devices of a small size.

Innovative Method Using Adhesive Force for Surface Micromachining of Carbon Nanowall.

The application of a carbon nanowall (CNW) via transfer is very demanding due to the unusual structure of vertically grown wall-shaped that easily collapses. In addition, direct growth on a device cannot obtain a precision-patterned shape because of the temperature limit of the photoresist (PR). Therefore, in this paper, we demonstrate a new CNW surface micromachining technology capable of direct growth. In order to reduce unexpected damage caused by chemical etching, a physical force was used to etch with the adhesive properties of CNWs that have low adhesion to silicon wafer. To prevent compositing with PR, the CNW was surface modified using oxygen plasma. Since there is a risk of surface-modified CNW (SMCNW) collapse in an ultrasonic treatment, which is a physical force, the CNW was coated with PR. After etching the SMCNW grown on PR uncoated area, PR was lifted off using an acetone solution. The effect on the SMCNW by the lift-off process was investigated. The surface, chemical, and structural properties of PR-removed SMCNW and pristine-SMCNW were compared and showed a minimal difference. Therefore, the CNW surface micromachining technique was considered successful.

Miniaturized langasite MEMS micro-cantilever beam structured resonator for high temperature gas sensing

In this paper, a miniaturized lanthanum gallium silicate (langasite, LGS, La3Ga5SiO14) micro-electromechanical systems (MEMS) micro-cantilever beam structured resonator is proposed for high temperature gas sensing applications. The proposed MEMS gas sensor is based on the thickness-shear vibration of the high temperature piezoelectric substrate, LGS, which is fabricated with bulk and surface micromachining technology. The fabrication process is optimized for better stability and integrity of the proposed LGS MEMS gas sensor. In order to improve the gas sensing sensitivity, the sample is further deposited with ZnO nanorods, where abundant gas molecules are expected to be adsorbed. Laboratory tests are subsequently carried out to validate the sensor’s response to various gas species, verify the adsorption mechanism, and investigate the sensitivity of the proposed gas sensor. Results show that compared to several resonator-based gas sensors, the proposed gas sensor displays a better sensitivity for CO2 sensing. The cross-sensitivity of ZnO nanorods between various gas species can be further addressed by principal component analysis technology, where important features can be extracted to distinguish between different gas species and/or concentrations. The proposed MEMS LGS resonator can be utilized for harsh environment applications, such as turbine-engine combustion chambers, nuclear power plants, and hazardous chemical industries, where high temperature can hamper most room temperature gas sensors. Combined with appropriate machine learning technology, the proposed gas sensor can be a good alternative to expensive, bulky gas chromatography devices.

Fabrication of 128 × 128 MEMS tunable F-P cavity optical filter array with surface micromachining

In this paper, a tunable Fabry-Perot (F-P) cavity filter with a large 128 × 128 array is successfully fabricated for an infrared (IR) hyperspectral imager via surface micromachining technology. A low-temperature (less than 200 °C) surface micromachining process is developed for manufacturing F-P tunable filter arrays, which has satisfactory compatibility. A suspended micro-bridge structure is created with a polyimide sacrificial layer. To obtain a large tuning range, long support arm structures are studied. The structure also has a compact array of folding arms. The filter can be tuned via an electrostatic force. The filter, which has an initial cavity length of 2.3 μm, has a displacement of 1.5 μm at an applied bias voltage of 8 V. The tuning wavelength of the filter is from 4.7 μm to 2.8 μm with an actuation voltage of less than 2 V. At the centre wavelength of 4 μm, the spectral transmittance of the filter is 73%. The technology for the fabrication of microelectro mechanical systems (MEMS) filter arrays as dispersive elements has potential applications in hyperspectral imaging.

MEMS-based Micro-scale Wind Turbines as Energy Harvesters of the Convective Airflows in Microelectronic Circuits

As an alternative to conventional batteries and other energy scavenging techniques, this paper introduces the idea of using micro-turbines to extract energy from wind forces at the microscale level and to supply power to battery-less microsystems. Fundamental research efforts on the design, fabrication, and test of micro-turbines with blade lengths of just 160 µm are presented in this paper along with analytical models and preliminary experimental results. The proof-of-concept prototypes presented herein were fabricated using a standard polysilicon surface micro-machining silicon technology (PolyMUMPs) and could effectively transform the kinetic energy of the available wind into a torque that might drive an electric generator or directly power supply a micro-mechanical system. Since conventional batteries do not scale-down well to the microscale, wind micro-turbines have the potential for becoming a practical alternative power source for microsystems, as well as for extending the operating range of devices running on batteries.

The Analysis of the Influence of Threshold on the Dynamic Contact Process of a Fabricated Vertically Driven MEMS Inertial Switch.

In this work, to evaluate the influence of the threshold on the dynamic contact process, five models (number 1, 2, 3, 4, 5) with different thresholds were proposed and fabricated with surface micromachining technology. The contact time and response time were used to characterize the dynamic contact performance. The dynamic contact processes of the inertial switches with gradually increasing thresholds were researched using analytical, simulation, and experimental methods. The basic working principle analysis of the inertial switch shows that the contact time of the inertial switch with a low-g value can be extended by using a simply supported beam as the fixed electrode, but the high-G inertial needs more elasticity for fixed electrode. The simulation results indicate that the response time and contact time decrease with the increment in the designed threshold. Prototypes were tested using a dropping hammer system, and the test result indicates that the contact time of the inertial switch with a fixed electrode of the simply supported beam is about 15 and 5 μs when the threshold is about 280 and 580 g, respectively. Meanwhile, the contact time can be extended to 100 μs for the inertial switch using a spring as the fixed electrode when the threshold is about 280 and 580 g. These test results not only prove that the spring fixed electrode can effectively extend the contact time, but also prove that the style of the fixed electrode is the deciding factor affecting the contact time of the high-G inertial switch.

Design, fabrication and characterization of high temperature thin film heat flux sensors

Thin film thermopile (TFTP) heat flux sensors have been developed in this paper. The property of TFTPs was modeled and analyzed by FEA COMSOL in order to optimize performance. Two types of TFTPs were fabricated by surface micromachining technology. For the TFTPs operating at temperature no more than 400 °C, spin coated polyimide was used as the thermal insulator. While for high temperature operation, the ion beam deposited silicon dioxide was chosen as the thermal insulator. Furthermore, thermally grown oxide (TGO) of aluminum layer inserted between metal and silicon dioxide improved TFTPs reliability at high temperature. The TFTPs was calibrated by a halogen tungsten lamp calibration system. The results showed that the output voltage of TFTPs responded linearly with the heat flux, which indicated that the TFTPs with optimized design were capable of providing surface heat flux measurement, and the TFTPs with silicon oxide as thermal insulator had the potential to work at 1000 °C.

Microfluidic based impedance biosensor for pathogens detection in food products.

A MEMS-based impedance biosensor was designed, fabricated, and tested to effectively detect the presence of bacterial cells including E. coli O157:H7 and Salmonellatyphimurium in raw chicken products using detection region made of multiple interdigitated electrode arrays. A positive dielectrophoresis based focusing electrode was used in order to focus and concentrate the bacterial cells at the centerline of the fluidic microchannel and direct them toward the detection microchannel. The biosensor was fabricated using surface micromachining technology on a glass substrate. The results demonstrate that the device can detect Salmonella with concentrations as low as 10 cells/mL in less than 1 h. The device sensitivity was improved by the addition of the focusing electrodes, which increased the signal response by a factor between 6 and 18 times higher than without the use of the focusing electrodes. The biosensor is selective and can detect other types of pathogen by changing the type of the antibody immobilized on the detection electrodes. The device was able to differentiate live from dead bacteria.