Table of contents

A modified gel-casting technique was used to fabricate a 1–3 piezoelectric ceramic/polymer composite substrate formed by irregular-shaped pillar arrays of small dimensions and kerfs. This technique involves the polymerization of aqueous piezoelectric (PZT) suspensions with added water-soluble epoxy resin and polyamine-based hardener that lead to high strength, high density and resilient ceramic bodies. Soft micromoulding was used to shape the ceramic segments, and micropillars with lateral features down to 4 µm and height-to-width aspect ratios of ∼10 were achieved. The composite exhibited a clear thickness resonance mode at approximately 70 MHz and a k_eff ∼ 0.51, demonstrating that the ceramic micropillars possess good electrical properties. Furthermore, gel-casting allows the fabrication of ceramic structures with non-conventional shapes; hence, device design is not limited by the standard fabrication methods. This is of particular benefit for high-frequency transducers where the critical design dimensions are reduced.

Micro-patterns, typically fabricated by microelectromechanical systems technologies, have been applied to two-dimensional (2D) environments for tissue engineering applications. Nano-stereolithography, a unique solid freeform technology, is now available to apply micron-sized patterns to three-dimensional (3D) scaffolds in a direct process. Many studies have reported that the micro-patterns, which are smaller than cell sizes, have effects on cell behavior. Thus, we considered that a scaffold incorporating micro-patterns might be more appropriate for tissue engineering applications than non-patterned scaffolds. In this study, we fabricated 3D scaffolds with micro-patterns (micro-pillar and micro-ridge types) on each layer using an NSTL system. In an in vitro study using pre-osteoblast cells, we observed the effects of micro-patterns on cellular behaviors, such as proliferation, adhesion and osteogenic differentiation. The scaffolds with micro-patterns showed significantly improved cell adhesion ability versus a scaffold with no patterning. We also observed that the expression of osteogenic markers, such as ALP and Runx2, increased significantly in scaffolds with micro-pillar and micro-ridge patterns compared with non-patterned scaffolds. Thus, it could be a promising strategy for effective tissue engineering applications to add such micro-patterns on 3D scaffolds.

The evolution of the interfacial profile for one complete droplet formation process in a flow-focusing device was investigated. Two critical curvature values that represent the liquid thread detachment from walls and the onset of pinch-off were discovered. The rate of curvature evolution depends on the flow-rate ratio between the two immiscible fluids and was modulated by acoustic actuation. Three distinct evolution regions which represent, respectively, the periods before and after the liquid thread detachment from walls and after the onset of pinch-off till final breakup are shown. In the case under acoustic actuation, the droplet formation process is accelerated; both the time till the liquid thread detachment from walls and the onset of pinch-off are speeded up. Phase-averaged two-phase micro-particle-image-velocimetry (µPIV) was applied to extract the oscillating flow fields in the two immiscible phases. Periodic contracting and expanding streaming flow fields in the dispersed phase and steady streaming flow fields in the continuous phase were obtained. The steady streaming flow in the continuous phase provides an additional driving force which helps to drive the continuous phase into the gutters. This additional flow facilitates the onset of breakup and thus smaller droplets are formed upon acoustic actuation.

A method of buried cavity fabrication in low loss DP951 and new DP9K7 LTCC (low-temperature co-fired ceramic) materials is described in this paper. Laser micromachining and method based on sacrificial volume material (SVM) are studied. Cavities are fabricated in LTCC materials using two different SVMs—cetyl alcohol and carbon tape. The influence of laser system parameters on cutting quality of the LTCC materials is studied. Moreover, thermal properties of the LTCCs and used SVMs are analyzed using combined thermo-gravimetric analysis, differential thermal analysis and differential thermo-gravimetry. Geometries of the LTCC test structures fabricated using different SVMs are analyzed using a scanning electron microscope and x-ray tomography. Energy dispersive spectroscopy and surface wettability measurements are used to analyze changes in LTCC materials atomic composition after co-firing with SVMs.

A novel film bulk acoustic resonator (FBAR) with two resonant frequencies which have opposite reactions to temperature changes has been designed. The two resonant modes respond differently to changes in temperature and pressure, with the frequency shift being linearly correlated with temperature and pressure changes. By utilizing the FBAR's sealed back trench as a cavity, an on-chip single FBAR sensor suitable for measuring pressure and temperature simultaneously is proposed and demonstrated. The experimental results show that the pressure coefficient of frequency for the lower frequency peak of the FBAR sensors is approximately −17.4 ppm kPa⁻¹, while that for the second peak is approximately −6.1 ppm kPa⁻¹, both of them being much more sensitive than other existing pressure sensors. This dual mode on-chip pressure sensor is simple in structure and operation, can be fabricated at very low cost, and yet requires no specific package, therefore has great potential for applications.

A radio frequency micro-electro-mechanical system (RF-MEMS) phase shifter based on the distributed MEMS transmission line (DMTL) concept towards maximum achievable phase shift with low actuation voltage with good figure of merit (FOM) is presented in this paper. This surface micro-machined analog DMTL phase shifter demonstrates low power consumption for implementation in a Ka-band transmit/receive (T/R) module. The push–pull-type switch has been designed and optimized with an analytical method and validated with simulation, which is the fundamental building block of the design of a true-time-delay phase shifter. Change in phase has been designed and optimized in push and pull states with reference to the up-state performance of the phase shifter. The working principle of this push–pull-type DMTL phase shifter has been comprehensively worked out. A thorough detail of the design and performance analysis of the phase shifter has been carried out with various structural parameters using commercially available simulation tools with reference to a change in phase shift and has been verified using a system level simulation. The phase shifter is fabricated on the alumina substrate, using a suspended gold bridge membrane with a surface micromachining process. Asymmetric behaviour of push–pull bridge configuration has been noted and a corresponding effect on mechanical, electrical and RF performances has been extensively investigated. It is demonstrated 114° dB⁻¹ FOM over 0–40 GHz band, which is the highest achievable FOM from a unit cell on an alumina substrate reported so far. A complete phase shifter contributes to a continuous differential phase shift of 0°–360° over 0–40 GHz band with a minimum actuation voltage of 8.1 V which is the highest achievable phase shift with the lowest actuation voltage as per till date on the alumina substrate with good repeatability and return loss better than 11.5 dB over 0–40 GHz band.

The fabrication of nanostructured substrates with precisely controlled geometries and arrangements plays an important role in studies of surface-enhanced Raman scattering (SERS). Here, we present two processes based on electron-beam lithography to fabricate gold nanostructures for SERS. One process involves making use of metal lift-off and the other involves the use of the plasma etching. These two processes allow the successful fabrication of gold nanostructures with various kinds of geometrical shapes and different periodic arrangements. 4-mercaptopyridine (4-MPy) and Rhodamine 6G (R6G) molecules are used to probe SERS signals on the nanostructures. The SERS investigations on the nanostructured substrates demonstrate that the gold nanostructured substrates have resulted in large SERS enhancement, which is highly dependent on the geometrical shapes and arrangements of the gold nanostructures.

The design, fabrication and detailed characterization of a fully electroplated, magnetostatic low-cost MEMS scanning mirror are presented. By electroplating bright nickel on a sacrificial substrate, robust soft-magnetic micromirrors may be fabricated. The technology is simpler and cheaper than the standard process using bulk silicon micromachining of silicon-on-insulator wafers for fabricating magnetostatic scanners. The presented Ni mirrors exhibit deflection angles of ±7° at resonance for small external magnetic fields of 0.23 mT. Such magnetic fields are easily generated by miniaturized solenoids, making integration, for instance, into endoscopic systems possible.

We present a method that makes it possible to deposit and bond micro-elements on an underlying device. In this manner, micro-constructions can be built by the addition of micro-bricks. The elements are fabricated on a flexible substrate covered with a release coating and are subsequently transferred and bonded onto the target device in a collective process. The process works on complex geometries and can accommodate surface defects, while maintaining a tightly controlled geometry. Application to the fabrication of liquid crystal micro-cells is presented.

The limitations of conventional glass etching methods are a bottleneck in the further development of glass as a substrate, which is critically important in micro- and nanoelectromechanical systems. In this paper, we describe a new wet etching method for Pyrex glass using a metal etchant. An aluminum pattern is deposited on the Pyrex and then subjected to thermal and electrostatically induced interdiffusion to form an aluminum-rich layer of controllable thickness in the glass, which can then be etched with an aluminum etchant. Because the underlying glass acts as an etch stop, no etching mask is required and the etching depth is effectively controlled by the electric field strength. This method can be used for fabricating glass nanochannels and nanodepth structures, and also in processes incompatible with hydrogen fluoride.

Miniature concave hollows, made by wet etching silicon through a circular mask, can be used as mirror substrates for building optical micro-cavities on a chip. In this paper, we investigate how inductively coupled plasma (ICP) polishing improves both shape and roughness of the mirror substrates. We characterize the evolution of the surfaces during the ICP polishing using white-light optical profilometry and atomic force microscopy. A surface roughness of 1 nm is reached, which reduces to 0.5 nm after coating with a high reflectivity dielectric. With such smooth mirrors, the optical cavity finesse is now limited by the shape of the underlying mirror.

In this paper, we present for the first time a systematic investigation of the role of the surface tension in anisotropic wet etching of silicon, focusing on the sidewall angle. We show that in the KOH and NaOH solutions with high surface tension a reliable fabrication of vertical sidewalls is possible. Etching experiments on (1 0 0)-Si and surface tension measurements via bubble pressure tensiometry were conducted using inorganic etchants in a wide range of concentrations. Finally, the surface tension of the etching solutions was identified as an important quantity that determined the etching behavior, while concentration dependences were eliminated by controlling the surface tension using temperature.

Most recent works on miniature tasks are concentrated on developing tools to take advantage of the visual servoing feedback to control the ultra-small interaction forces. This paper spans an extensive platform for automatic controlling of boundary interaction forces with high precision in the level of micro/nano-Newton with extensive micro/nanoengineering applications such as the microsurgery. To this end, a comprehensive piezoresistive microcantilever (PMC) model considering the shear deformation and rotary inertia effects treating as the distributed-parameters model along with the Hertzian contact force is presented. The purpose of considering the Hertzian contact force model is to investigate the dynamic response of the interaction force between the microcantilever's tip and the specimen. Afterward, a control platform is introduced to automatically manipulate the PMC to follow an ideal micro/nano-interaction force. By using the integrated PMC with the micromanipulator and a digital signal processor, an intuitive programming code is written to incorporate the micromanipulator and the controller in a real-time framework. To calibrate and verify the induced voltage in the PMC, a self-sensing experiment on the piezoelectric microcantilever is carried out to warrant the calibration procedure. Some experiments are established to affirm the validity of the proposed control for the autonomous real-time tasks on the boundary interaction force control. Unlike the conventional research studies, the measured force here contributes as the feedback source in contrast to the vision feedback while force sensors possess more precision, productivity and small size. This technique has several potential applications listed but not limited to the micro/nanomanipulation, developing artificial biological systems (e.g., fabricating hydrogel for the scaffold), and medicine such as microsurgery. As a result, using the proposed platform, we are able to manipulate and control the boundary interaction forces precisely at the level of few μN and the presented theoretical and experimental results to study the mechanical responses of the interaction force are in a good agreement.

This paper presents a thermal peripheral blood flowmeter where a force sensor is integrated to compensate the blood flow measurement. Since blood flow is highly sensitive to the contact force between the sensor and skin, previous blood flowmeters needed to be fixed on the skin with a constant contact force. We integrate a force sensor with a thermal blood flowmeter to measure both blood flow and contact force simultaneously for force-compensated blood flow measurement. The blood flowmeter presented here is composed of a resistance temperature detector and a piezoresistive force sensor and was fabricated by surface and bulk micromachining techniques. In the experimental measurement, the blood flow linearly decreased with the contact force at the rate of 31.7% N^–1. By using the measured compensation coefficient, the device showed a constant blood flow with the maximum difference of 6.4% over the contact force variation of 1–3 N, and otherwise showed the maximum difference of 75.0%. The present device is suitable for applications with portable biomedical instrumentation or air-conditioning systems for the estimation of human thermoregulation status.

In the field of experimental fluid mechanics the measurement of unsteady, distributed wall shear stress has proved historically challenging. Recently, sensors based on an array of flexible micro-pillars have shown promise in carrying out such measurements. Similar sensors find use in other applications such as cellular mechanics. This work presents a manufacturing technique that can manufacture micro-pillar arrays of high aspect ratio. An electric discharge machine (EDM) is used to manufacture a micro-drilling tool. This micro-drilling tool is used to form holes in a wax sheet which acts as the mold for the micro-pillar array. Silicone rubber is cast in these molds to yield a micro-pillar array. Using this technique, micro-pillar arrays with a maximum aspect ratio of about 10 have been manufactured. Manufacturing issues encountered, steps to alleviate them and the potential of the process to manufacture similar micro-pillar arrays in a time-efficient manner are also discussed.

A fabrication method based on negative tone lift off EBL is developed for constructing nano-structures at end faces of multimode optical fibers. With this new approach, precise and robust nano-structures with high spatial resolutions can be fabricated with minimum damage to the optical fiber face during the fabrication process. Based on this approach, high numerical aperture micro Fresnel zone plates (MZP) with focal lengths ∼3 µm were fabricated on the face of an optical fiber. The focusing characteristics of the fabricated MZP showed good consistency with the numerical simulations at the specified wavelength (∼405 nm).

We report the nanofabrication of a sulfur dioxide (SO₂) sensor with a tripolar on-chip microelectrode utilizing a film of single-walled carbon nanotubes (SWCNTs) as the field ionization cathode, where the ion flow current and the partial discharge current generated by the field ionization process of gaseous molecules can be gauged to gas species and concentration. The variation of the sensitivity is less than 4% for all of the tested devices, and the sensor has selectivity against gases such as He, NO₂, CO, H₂, SO₂ and O₂. Further, the sensor response presents well-defined and reproducible linear behavior with regard to concentration in the range investigated and a detection limitation of <∼0.5 ppm for SO₂. More importantly, a tripolar on-chip microelectrode with SWCNTs as a cathode exhibits an impressive performance with respect to stability and anti-oxidation behavior, which are significantly better than had been possible before in the traditional bipolar sensor under explicit circumstances at room temperature.

In this paper we present, for the first time, a piezoresistive bulk mode microelectromechanical systems (MEMS) resonator that employs a passive differential input to suppress parasitic feedthrough. This novel compact approach lies in contrast to conventional methods using a dummy resonator. Our approach makes use of additional non-active (passive) wire bonds to match the parasitics of the device under test. We show here that a 37.75 dB decrease in feedthrough level is achieved while the measured resonant peak height increases correspondingly from 0.22 to 9.32 dB as a result of applying the novel passive differential scheme. Furthermore, a semi-analytical model is developed to predict the piezoresistive frequency response of the device. From the close fit between the presented model and the measured data, we show that our model is able to predict the response of our device.

Recently, there have been several reports on the observation of cavitation in microfluidics and in micropumps. Though cavitation is a common occurrence in micropumping, this is one of the least understood of all micropumping phenomena, and very limited progress has been made to study the behavior of cavitation in micropumps. Hence, a dedicated study on cavitation in micropumps and its effects on the performance of the micropump would be very useful. This work presents an experimental study on the behavior of cavitation in valveless micropump. The mechanism of cavitation occurrence in valveless micropumps has been explained by applying macroscale pumping principles to suit micropumping. The different stages of micropump cavitation have been defined through suitably conducted experiments and the results have been presented.

A novel electromagnetic energy harvester (EH) with multiple vibration modes has been developed and characterized using three-dimensional (3D) excitation at different frequencies. The device consists of a movable circular-mass patterned with three sets of double-layer aluminum (Al) coils, a circular-ring system incorporating a magnet and a supporting beam. The 3D dynamic behavior and performance analysis of the device shows that the first vibration mode of 1285 Hz is an out-of-plane motion, while the second and third modes of 1470 and 1550 Hz, respectively, are in-plane at angles of 60° (240°) and 150° (330°) to the horizontal (x-) axis. For an excitation acceleration of 1 g, the maximum power density achieved are 0.444, 0.242 and 0.125 µW cm⁻³ at vibration modes of I, II and III, respectively. The experimental results are in good agreement with the simulation and indicate a good potential in the development of a 3D EH device.

The nano-patterning characteristics of GaAs is investigated using a pulse applied atomic force microscope (AFM). Very short range voltage pulses of micro to nano-seconds' duration are applied to a conductive diamond-coated silicon (Si) tip in contact mode, to regulate the created feature size. The effects of pulse conditions such as pulse voltage, duration, frequency, offset voltage, anodization time, and applied tip pressure on nano-dot generation are characterized, based on the experiments. An interesting phenomenon, nano-pit creation instead of nano-dot creation, is observed when the applied pulse duration is less than 100 μs. Pulse frequency and offset voltage are also involved in nano-pit generation. The electrical spark discharge between the tip and the GaAs's surface is the most probable cause of the nano-pit creation and its generation mechanism is explained by considering the relevant pulse parameters. Nano-pits over 15 nm in depth are acquired on the GaAs substrate by adjusting the pulse conditions. This research facilitates the fabrication of more complex nano-structures on semiconductor materials since nano-dots and nano-pits could be easily made without any additional post-processes.

In this paper, a high performance wafer-level vacuum packaging technology based on GSG triple-layer sealing structure for encapsulating large mass inertial MEMS devices fabricated by silicon-on-glass bulk micromachining technology is presented. Roughness controlling strategy of bonding surfaces was proposed and described in detail. Silicon substrate was thinned and polished by CMP after the first bonding with the glass substrate and was then bonded with the glass micro-cap. Zr thin film was embedded into the concave of the micro-cap by a shadow-mask technique. The glass substrate was thinned to about 100 µm, wet etched through and metalized for realizing vertical feedthrough. During the fabrication, all patterning processes were operated carefully so as to reduce extrusive fragments to as little as possible. In addition, a high-performance micro-Pirani vacuum gauge was integrated into the package for monitoring the pressure and the leak rate further. The result shows that the pressure in the package is about 120 Pa and has no obvious change for more than one year indicating 10⁻¹³ stdcc s⁻¹ leak rate.

We present a study of the thermal and mechanical properties of the negative photoresist KMPR and the influence of processing conditions on those properties. The process parameters chosen all relate to the cross-linking level of the photoresist: the UV exposure dose, the baking temperature and the bake length. The stability of KMPR at high temperatures was also examined. The glass transition temperature was measured using dynamic mechanical analysis, with a maximum measured value of 128 °C achieved in our tests. Relating the glass transition temperature to the cross-linking level of the material, exposure doses higher than 2 J cm⁻² were shown to have a negligible effect on the cross-linking (for 80 μm thick films). Using thermogravitmetric analysis, KMPR has been shown to lose significant mass when heated above 200 °C. Young's modulus of KMPR was measured to be between 2.0 GPa for samples hard-baked at 100 °C and 2.7 GPa for samples baked at 150 and 200 °C. Creep behavior for KMPR held under strain was studied for samples prepared under a range of processing temperatures. Finally the thermally-induced cross-linking of unexposed KMPR was studied, with samples post-exposure baked at 150 °C, or 120 °C for at least an hour, cross-linking sufficiently to prevent development.

We have realized lateral contact switches using electroless deposition of Au/Cu on Si with all the necessary features including source, gate and drain. The metallization approach is simple and effective, allowing for Au/Au contact and inducing near-zero curvature even in long (up to 1 mm length) and slender (5 µm × 5 µm) cantilever beams (spring constant is 0.02 N m^–1). The switching time for these long beams is approximately 80 µs under ambient conditions. The lifetime of the fabricated switches is a function of current levels applied between the source and the drain (approximately 3000 cycles with a current load of 0.6 mA). The capacitance change between the on- and off-states of the switches is about 3 fF.

Soft mold imprinting has been widely used to improve quality and uniformity in large area nano-imprint processes because of its flexibility. However, the aspect ratio of soft mold imprinting is limited because of its physical properties. In this paper, a multi-mask layers transfer process was demonstrated to realize a high aspect ratio in the soft mold imprint process. In this process, a thin hard mask was used as an intermediate mask for a high aspect ratio mask layer fabrication. Based on this, a 60 nm line width gratings mask, whose aspect ratio is higher than 4, was fabricated; we also realized a uniform photonic crystal (PC) pattern on large and rough gallium nitride (GaN) epitaxial wafers.

This paper reports the successful fabrication of flexible replicas with polymeric nanopillars using a process that combines nanosphere lithography, dry deep etching, soft-lithography, nanomolding and hydrophobic modification. The polymeric nanopillars with various sizes and three different periodicities have been implemented for systematic investigations on the interfacial properties on those surfaces. Such a flexible polymeric surface exhibited the maximum static contact angle of 166.8° at the nanopillars with a diameter of 60 nm, height of 710 nm and periodicity of 300 nm. The optimum aspect ratio should be less than 7 to avoid defects and collapses among those polymeric nanopillars during nanomolding. Metastable contact at the transition state indeed occurred on the parts of the intrinsic nanopillars, the experimental results of which also matched well to the classical theory of critical contact angle. Using hydrophobic modifications, metastable contacts among those polymeric nanopillars have further been eliminated. The polymeric nanopillars reported here were verified as having very strong adhesion as well as superhydrophobicity because such nanopillars made microdroplets hang firmly on the vertical surfaces of those designed replicas.

The design, fabrication and characterization of a recently developed test platform for the characterization of nanoscale properties of thin films are presented. Platforms are comprised of a microfabricated cascaded thermal actuator system and test specimen. The cascaded thermal actuator system is capable of providing tens of microns of displacement and tens of milli-Newton forces simultaneously while applying a relatively low temperature gradient across the test specimen. The dimensions of the platform make its use possible in both the scanning/transmission electron microscope environments and on a probe station under an optical microscope. Digital image correlation was used to obtain similar accuracy (∼10 nm) for displacement measurements in both a SEM and under an optical microscope. Proof of concept experiments were performed on freestanding 250 nm thick Pt thin films.

In industrial applications of a micromechanical silicon resonator as a physical sensor, a high-quality factor Q and a low-temperature coefficient of Q (TCQ) are required for high sensitivity in a wide temperature range. Although the newly developed thin film encapsulation technique enables a beam to operate with low viscous damping in a vacuum cavity, the Q of a flexural vibration mode is limited by thermo-elastic damping (TED). We proposed a torsional beam resonator which features both a high Q and a low TCQ because theoretically the torsional vibration mode does not suffer from TED. From experiments, Q of 267 000 and TCQ of 1.4 for the 20 MHz torsional vibration mode were observed which were superior to those of the flexural mode. The pressure of the residual gas in the cavity of only 20 pl volume, which is one of the energy loss factors limiting the Q, was successfully estimated to be 1–14 Pa. Finally, the possibilities of improving the Q and the difference of the measured TCQ from a theoretical value were discussed.

This paper presents the design and experimental validation of dc-dynamic biasing for > 50× switching time improvement in severely underdamped fringing-field electrostatic MEMS actuators. The electrostatic fringing-field actuator is used to demonstrate the concept due to its robust device design and inherently low damping conditions. In order to accurately quantify the gap height versus voltage characteristics, a heuristic model is developed. The difference between the heuristic model and numerical simulation is less than 5.6% for typical MEMS geometries. MEMS fixed–fixed beams are fabricated and measured for experimental validation. Good agreement is observed between the calculated and measured results. For a given voltage, the measured and calculated displacements are typically within 10%. Lastly, the derived model is used to design a dc-dynamic bias waveform to improve the switching time of the underdamped MEMS actuators. With dynamic biasing, the measured up-to-down and down-to-up switching time of the actuator is ∼35 μs. On the other hand, coventional step biasing results in a switching time of ∼2 ms for both up-to-down and down-to-up states.

Resonant sensing has been used to detect various physical or chemical quantities, however many reported sensing systems are still bulky and expensive as they rely on external instruments to conduct measurements. For practical applications, it is often desirable to have a low-cost miniaturized sensor chip with multiple sensing devices. This paper presents the first attempt to implement a capacitive resonant sensor array in a complementary metal oxide semiconductor process. The damping loss of all sensors is compensated by placing them in an oscillator loop that contains a phase-locked loop (PLL) driving circuit. The inclusion of a PLL is beneficial for sustaining the oscillation of various resonant frequencies due to the manufacturing tolerance and detected quantity within the PLL capture range. The fabricated micromechanical resonators have a resonant frequency near 200 kHz. To evaluate the sensor performance, a parylene-D thin film was deposited on microstructures and the corresponding frequency shift of 12 devices was on average 33.87 kHz. The resolution for mass detection was estimated to be 23 pg.

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