Table of contents

This paper reports on a mechanically tri-stable switch mechanism based on laterally moving electrostatic curved-electrode actuators. The switch is configured in a 'true' single-pole-double-throw configuration (SPDT), i.e. a single-switch mechanism allows for the input signal to be switched between two output ports. The switch has three stable states: (1) input to first output; (2) switch off; (3) input to second output. Because of a latching mechanism, these states are mechanically stable, i.e. they are maintained without applying external actuation energy. The fabrication of the switches is done by a single photolithographical step and deep etching of a silicon-on-glass wafer which is subsequently coated with sputtered gold. The switch design features active opening, and the contact force is created passively by the deflected cantilevers. The curved-electrode actuators are utilized close to their end position where they develop their maximum force to guarantee a very large opening force which makes the switch less susceptible for contact stiction. The actuation voltages for different designs and functions of the switches are between 30 and 85 V.

Six different source chemicals (organosilanes) were successfully used for deposition of self-assembled monolayers (SAMs) onto silicon substrates by a vapor phase process. Five different fluorocarbon coatings and one hydrocarbon coating were deposited. The thermal stability of the coatings was studied in detail with respect to degradation as a function of temperature, and for the fluorocarbon coatings also the degradation rate at 400 °C. For fluorocarbon coatings deposited from FDTS a useful lifetime of approximately 90 min at 400 °C was found allowing the coating to survive high temperature MEMS packaging operations, while fluorocarbon coatings deposited from FOTS, FOMDS, FOTES and FOMMS were less stable. The hydrocarbon coating deposited from OTS degrades already at approximately 200 °C. The thermal stability of the SAM coatings was found to be significantly reduced if aggregations from the deposition process are present on the coatings.

This study presents a new pneumatic micropump featuring three membrane-enclosed air chambers with different volumes, such that serially connected actuation of these membranes can generate fluid movement. When compressed air fills the chambers, the membranes are pushed downward sequentially, resulting in the liquid in the underlying fluid channels being pumped forward peristaltically. Since the chambers are filled up sequentially with compressed air, from the smallest to largest chamber, this time delay generates a peristaltic motion in the membranes and forces the liquids to flow only along one direction. The pneumatic micropump is made of polydimethylsiloxane (PDMS) using soft lithography techniques. When compared with other pneumatic micropumps that usually require at least three electromagnetic valves (EMV), this new micropump can be operated by using a single EMV. Experimental results show that the micropump provides good performance even at low flow rates. The back pressure of the pneumatic micropump is measured at a fixed peak frequency to demonstrate the functionality of the micropump. The optimum operating conditions and geometric parameters of the micropump are systematically explored. A maximum flow of 108 µl min⁻¹ is obtained at a driving frequency of 10 Hz and an air pressure of 25 psi (172.4 kPa) when a membrane with a thickness of 80 µm and a microchannel with a width of 500 µm are tested. The development of these micropumps could be crucial for automatic miniature biomedical and chemical analysis systems.

To monitor the evaporation kinetics of drops from solid surfaces, and to investigate the interaction between liquids and solids, microscopic drops of liquids were deposited onto atomic force microscope cantilevers. Due to the surface tension of the liquid, the Laplace pressure inside the drop, and the change of the interfacial stress at the solid–liquid boundary, the cantilever bends and is deflected by typically a few hundred nanometers. We used liquids with different vapour pressures and surface tensions, in order to vary the evaporation time and also the magnitude of the surface forces exerted by the drops. For fast evaporating drops the cantilever bending along the longitudinal axis was measured versus time. In the case of non-evaporating drops the overall bending was recorded with optical methods. We developed a FEM model for cantilever bending as an improvement to a previously presented analytical model. FEM simulations are confirmed by experimental results.

In this paper, a fully wafer-level packaged RF MEMS switch has been demonstrated, which has low operation voltage, using a piezoelectric actuator. The piezoelectric actuator was designed to operate at low actuation voltage for application to advanced mobile handsets. The dc contact type RF switch was packaged using the wafer-level bonding process. The CPW transmission lines and piezoelectric actuators have been fabricated on separate wafers and assembled together by the wafer-level eutectic bonding process. A gold and tin composite was used for eutectic bonding at a low temperature of 300 °C. Via holes interconnecting the electrical contact pads through the wafer were filled completely with electroplated copper. The fully wafer-level packaged RF MEMS switch showed an insertion loss of 0.63 dB and an isolation of 26.4 dB at 5 GHz. The actuation voltage of the switch was 5 V. The resonant frequency of the piezoelectric actuator was 38.4 kHz and the spring constant of the actuator was calculated to be 9.6 N m⁻¹. The size of the packaged SPST (single-pole single-through) switch was 1.2 mm × 1.2 mm including the packaging sealing rim. The effect of the proposed package structure on the RF performance was characterized with a device having CPW through lines and vertical feed lines excluding the RF switches. The measured packaging loss was 0.2 dB and the return loss was 33.6 dB at 5 GHz.

A reactive ion etching (RIE) process of Pyrex glass with a high etch rate is presented in this work, using standard lithography processes. This process enables us to create deep cavities with vertical side walls and with relatively high accuracy in Pyrex, with minimal additional process steps. An optimization of the etch process conducted using the design-of-experiment method to increase the etch rate results in an analytical model to describe the etch rate of the Pyrex. The etch rate which was optimized in this work was about 3500 A min⁻¹ with a verticality of about 85°. Additionally, the implementation of deep cavities in Pyrex, in order to create a notch-free deep silicon etch, is presented. This is done by engineering the working material (i.e., silicon-on-glass wafer) by using Pyrex dry etch and by adding a thin metallic layer between the silicon and the Pyrex cavities.

Uncapped, hollow polymeric microstructures were fabricated on a silicon substrate using electric field induced stretching and detachment. Initially, square or cylinder microposts were generated using a solvent-assisted capillary molding technique, and a featureless electrode mask was positioned on the top of the microstructure with spacers maintaining an air gap (∼20 µm). Upon exposure to an external electric field (1.0–3.0 V µm⁻¹), the hollow microstructures were destabilized and stretched by the well-known electrohydrodynamic instability, resulting in contact of the top polymer surface with the mask. Subsequently, detachment of the capping layer occurred upon removal of the mask due to larger adhesion forces at the polymer/mask interface than cohesion forces of the polymer. These hollow microstructures were tested to capture the budding yeast, Saccharomyces cerevisiae, for shear protection.

We present a procedure for inducing chaotic mixing based on a non-periodic patterning of the walls making use of the Weierstrass fractal function to generate the locations for the grooves. We show the numerical analysis of flow in three different geometries generated with the Weierstrass function and compare the results with a fourth geometry, quite similar to the staggered herringbone mixer (SHM) of Stroock et al (2002 Science295 647), for which the patterning is periodic. We evaluate the Lyapunov exponents for massless and non-interacting particles advected by the flow and traced along the channels. We also compute the entropy of mixing for binary mixtures. Finally, we compute generalized (fractal) dimensions associated with the interface of the two fluids. The results show consistently substantial enhancement in mixing efficiency for two of the Weierstrass channels compared to the SHM.

Two-step activation of paper batteries has been successfully demonstrated to provide quick activation and to supply high power to credit card-sized biosystems on a plastic chip. A stack of a magnesium layer (an anode), a fluid guide (absorbent paper), a highly doped filter paper with copper chloride (a cathode) and a copper layer as a current collector is laminated between two transparent plastic films into a high power biofluid- and water-activated battery. The battery is activated by two-step activation: (1) after placing a drop of biofluid/water-based solution on the fluid inlet, the surface tension first drives the fluid to soak the fluid guide; (2) the fluid in the fluid guide then penetrates into the heavily doped filter paper with copper chloride to start the battery reaction. The fabricated half credit card-sized battery was activated by saliva, urine and tap water and delivered a maximum voltage of 1.56 V within 10 s after activation and a maximum power of 15.6 mW. When 10 kΩ and 1 KΩ loads are used, the service time with water, urine and saliva is measured as more than 2 h. An in-series battery of 3 V has been successfully tested to power two LEDs (light emitting diodes) and an electric driving circuit. As such, this high power paper battery could be integrated with on-demand credit card-sized biosystems such as healthcare test kits, biochips, lab-on-a-chip, DNA chips, protein chips or even test chips for water quality checking or chemical checking.

This paper describes the fabrication method of an all SU-8 microfluidic device with built-in 3D fine micromesh structures. 3D micromesh structures were seamlessly integrated into the SU-8 sealed microchannel. To eliminate gap formation and filling of the microchannel, the built-in micromeshes in the microchannel were formed by photolithography after bonding the SU-8 top-cover layer and the SU-8 bottom substrate. The lift-off method, using lift-off resist as a sacrificial layer, was utilized to release the all SU-8 microfluidic chips. Monolithic SU-8 structures realize uniform physical and chemical surface properties required in microfluidic devices for practical use. As an application, fragmentation of a water droplet in an organic carrier formed by a two-phase flow was demonstrated.

A sacrificial layer etching process in joint channels is studied in this paper. If the etching process proceeds from a wide channel to a narrow one, the etching front presents a straight line during the whole process according to the experiments. If the etching process proceeds from a narrow channel connected with a wide channel, it is an arc rather than a straight line when the etching front reaches the wide channel. Some other interesting phenomena are also observed during etching the joint structure. For a narrow–wide channel, the etching rate decreases suddenly near the joint area, while for a wide–narrow channel, the etching rate increases quickly near the joint area and decreases gradually after that. A new model is proposed to explain these phenomena, which is proved to match the experimental data well.

This paper proposes an analytical model for calculating the squeeze-film air damping of a rectangular torsion mirror at finite normalized tilting angles. The general Reynolds equation is first modified to a nonlinear equation for the condition. Based on the nonlinear equation, the damping pressure, the damping torque and the coefficient of the damping torque are derived as functions of the tilting angle and the aspect ratio of the mirror plate. To show the relation clearly and for the ease of application, the coefficient of the damping torque is given in curves in addition to complicated analytical expressions. The results show that the damping torque coefficient is a highly nonlinear function of the tilting angle and basically a linear function of the aspect ratio of the mirror. The coupling between the two factors is appreciable but not very strong. When the tilting angle is reduced to zero, the result of this paper agrees perfectly well with those of previous papers, which have been verified by experiments and/or numerical calculation. The analytical results are effective for normalized tilting angles up to 0.7.

This paper proposes a design concept and fabrication method of a planar three-dimensional (3D) microfluidic flow-focusing device (MFFD) that can produce monodisperse single/double emulsions in a closed/open microfluidic system. The device consists of three layers of SU-8 resist structures to form coaxial embedded orifices at the center of the microchannel with dimensions ranging from 50 µm to 200 µm by means of the black photoresist shadow method. Two or three immiscible fluids can be focused through the coaxial orifices, producing monodispersed droplets with a coefficient of variance (CV) of less than 4.1%. At the orifice, the inner liquid thread stays confined to the central axis of the microchannel, surrounded by the continuous phase. As the dispensed phase (inner fluid thread) does not wet channel walls, our proposed 3D MFFD can produce single emulsions for both water-in-oil (W/O) and oil-in-water (O/W) droplets utilizing the same device. The droplet diameter ranges from 50 µm to 300 µm. Also, double emulsions containing one to several internal droplets were successfully produced in the closed channel configuration. In addition, we demonstrated for the first time the feasibility of forming W/O droplets and polymer particles in an open channel configuration by withdrawing the fluid from the outlet channel. W/O droplets and polymer particles, smaller than 10 µm and 40 µm, respectively, were successfully produced. In contrast to the closed channel configuration where the droplet size decreases with an increasing flow rate, in an open channel configuration, the droplet size increases with an increasing withdrawal rate. The unique fabrication of the monolithic 3D MFFD device utilizing SU-8 resist overcomes problems regarding orifice sizes/shapes, alignment and assembly for current axisymmetric flow-focusing devices (AFFD) based on capillary microtubes, and provides flexibility for the future development of an integrated miniaturized lab-on-a-chip microsystem.

Actuation or sensing in microdevices is often achieved through electro-mechanical coupling. In practice though, the electro-mechanical system is complicated by the effects influenced by the gas surrounding the system. The gas damping may be of the same order of magnitude as the electric and mechanical forces, and thus it needs to be accounted for in the design of the devices. Notably in microsensor design, controlling the amount of damping is crucial in achieving the desired measurement accuracy and sensitivity. A certain amount of damping is required to filter out high frequency oscillations, but too heavy damping reduces the sensitivity of the device. In this paper we present a modelling method based on the finite element method to simulate the behaviour of a planar gas-damped microdevice under electrostatic loading. The transient model takes into account the true nonlinear behaviour of the damping and includes effects from non-uniform gap height. The computational cost of the simulations has been significantly reduced by various reduced-order and reduced-dimensional methods utilized in the model development. The method is used to simulate an accelerometer prototype under voltage ramp loading, up to the pull-in. The results of the simulation are compared to capacitance measurements of the real device. The method is also suitable for other types of planar microdevices, such as pressure sensors, micromirrors or microswitches.

A new model for the bending of a Bernoulli–Euler beam is developed using a modified couple stress theory. A variational formulation based on the principle of minimum total potential energy is employed. The new model contains an internal material length scale parameter and can capture the size effect, unlike the classical Bernoulli–Euler beam model. The former reduces to the latter in the absence of the material length scale parameter. As a direct application of the new model, a cantilever beam problem is solved. It is found that the bending rigidity of the cantilever beam predicted by the newly developed model is larger than that predicted by the classical beam model. The difference between the deflections predicted by the two models is very significant when the beam thickness is small, but is diminishing with the increase of the beam thickness. A comparison shows that the predicted size effect agrees fairly well with that observed experimentally.

A single-crystal silicon (SCS) micromirror array (MMA) was designed, fabricated and characterized to achieve stable UV light reflectors for biochip fabrication applications such as peptide synthesis systems. The MMA was optimized for biochip applications, with a simple fabrication process (only three photomasks), a large mirror size (210 × 210 µm²) and appropriate separation (60 µm). All the mirrors in the array (16 × 16) were reliable because silicon was used as the structural material for mechanical and optical parts. The mirror surface roughness was very low, being less than 3.93 nm. The turn-on response time was 104 µs and turn-off time was about 800 µs, including settling time. The measured pull-in voltage for the 256 mirrors had an average of 97.0 V and a standard deviation of 2.1 V. A UV reflection test using standard photoresist, AZ1512, showed that the exposure contrast ratio was about 125. From these characterizations, the MMA was shown to have good performance for a useful light modulator for maskless UV lithography, especially in biochip applications.

Demonstrated and investigated here is a method to seal microfluidic systems by soldering. As a particularly difficult case of growing importance, the sealing of openings contaminated with paraffin wax was studied. Solder paste, screen printed on a metallized silicon substrate, was melted locally through application of 6.5–10 V to a 5 Ω copper film resistor for a few seconds and was found able to drive an intermediate layer of paraffin away and seal a 0.2 mm diameter circular via by wetting a surrounding copper pad. Although verified to be robust, the process did result in failing seals on excessive heating because of consumption of the pads. Correctly performed, the technique provided a seal at least withstanding a pressure of 8 bar for 8 h at 85 °C.

We have designed, fabricated and characterized large displacement distributed-force polymer actuators driven only by the surface tension of water. The devices were inspired by the hygroscopic spore dispersal mechanism in fern sporangia. Microdevices were fabricated through a single mask process using a commercial photo-patternable silicone polymer to mimic the mechanical characteristics of plant cellulose. An analytical model for predicting the microactuator behavior was developed using the principle of virtual work, and a variety of designs were simulated and compared to the empirical data. Fabricated devices experienced tip deflections of more than 3.5 mm and angular rotations of more than 330° due to the surface tension of water. The devices generated forces per unit length of 5.75 mN m⁻¹ to 67.75 mN m⁻¹. We show initial results indicating that the transient water-driven deflections can be manipulated to generate devices that self-assemble into stable configurations. Our model shows that devices should scale well into the submicron regime. Lastly, the actuation mechanism presented may provide a robust method for embedding geometry-programmable and environment-scavenged force generation into common materials.

This paper presents a whole benzocyclobutene (BCB) film encapsulated 0-level packaging using a wafer level BCB bonding technique. The wafer-scale membrane transfer technique using silicon carrier wafers was used to make BCB membrane caps for encapsulation placed above the device wafers. The BCB multiple coating process using CYCLOTENE 4026-46 was developed to make an encapsulation cap. The average height of the BCB cap was 40 µm with a little curvature on the membrane for the dimension of 2 mm × 3 mm. The RF characteristics using coplanar waveguide (CPW) lines were measured to evaluate the effect of BCB film packaging and the results were compared with those of pyrex #7740 glass packaged CPW lines. It shows that the insertion loss (S₁₂) of BCB-packaged CPW lines is better than that of pyrex glass packaged CPW lines at high frequencies. The insertion loss change of CPW lines by BCB film packaging is below 0.01 dB up to 90 GHz, while the glass-packaged CPW lines show about 0.1 dB deviation from 20 GHz to 110 GHz.

We present here the use of polydimethylsiloxane (PDMS) membranes as a new soft polymer substrate (ε_r ≈ 2.67 at 77 GHz) for the realization of ultra-flexible millimeter-wave printed antennas thanks to the extremely low Young's modulus (E_PDMS < 2 MPa). Ultimately this peculiar property enables one to design wide-angle mechanically beam-steering antennas and flexible conformal antennas. The experimental characterization of PDMS material in V- and W-bands highlights high loss tangent values (tanδ ≈ 0.04 at 77 GHz). Thus micromachining techniques have been developed to reduce dielectric losses for antenna applications at millimeter waves. Here the antenna performance is demonstrated in the 60 GHz band by considering a single microstrip patch antenna supported by a PDMS membrane over an air-filled cavity. After a brief description of the design approach using the method of moments (MoM) and the finite-difference time-domain (FDTD) technique, the technological processes are described in detail. The input impedance and radiation patterns of the prototype are in good agreement with numerical simulations. The radiation efficiency of the micromachined antenna is equal to 60% and is in the same order as that obtained with conventional polymer bulk substrates such as Duroids. These results confirm the validity of the new technological process and assembly procedure, and demonstrate that PDMS membranes can be used to realize low-loss planar membrane-supported millimeter-wave printed circuits and radiating structures.

We demonstrate a non-contact pumping mechanism for the manipulation of aqueous solutions within microfluidic devices. The method utilizes multi-layer soft lithography techniques to integrate a thin polydimethylsiloxane (PDMS) membrane that acts as a diffusion medium for regulated air pressure and a vacuum. Pressurized microchannels filter air through the PDMS membrane due to its high gas permeability causing a pressure difference in the liquid channel and generating flow. Likewise, a vacuum can be applied to pull air through the membrane allowing the filling of dead-end channels and the removal of bubbles. Flow rates vary according to applied pressure/vacuum, membrane thickness and diffusion area. A gas permeation pump is an inexpensive alternative to other micropumps. The pump is easily integrated with highly arrayed multi-channel/chamber applications for micro-total analysis systems, fluid metering and dispensing, and drug delivery. Flow rates of 200 nl min⁻¹ have been achieved using this technique. Successful localized fluid turning at intersections, fluid metering and filling of dead-end chambers were also demonstrated.

The formation of micro-scale droplets in liquids is crucial for many applications. This paper reports a new microfluidic chip capable of generating tunable micro-droplets in liquids by using the combination of two microfluidic techniques, microfluidic flow focusing and a controllable moving-wall chopper. The microfluidic chip can generate droplets with tunable sizes. Dispersed phase sample flow is first hydrodynamically focused into a narrow stream by using neighboring sheath flows containing continuous-phase samples. A new chopping microstructure called 'controllable moving walls', which is a pair of side chambers orthogonally placed next to the sample flow channel, is used to generate micro-droplets. The moving-wall structures can be deformed by external air pressure to cut the pre-focused stream into segments. By controlling the air injection frequency and air pressure of the side chambers, tunable micro-droplets with a relatively uniform diameter can be formed accordingly. Experimental data show that a maximum deformation of 62.5 µm could be achieved at a pressure of 30 psi for a moving wall with a cross section of 100 µm × 50 µm. By controlling the flow rate between the dispersed and continuous phases with a range from 2 to 20, micro-droplets with a relatively uniform size ranging from 10 µm to 120 µm can be successfully generated. The micro-droplets have a much more uniform size compared to previous studies. This new microfluidic device can be promising for a variety of applications such as emulsification, nano-medicine and droplet-based microfluidics.

The inhomogeneous ac electric fields used for particle manipulation in dielectrophoresis(DEP)-based microdevices not only produce forces on the particle, but also generate volume forces in the liquid by producing gradients in conductivity and permittivity due to local heating. The forces on the liquid give rise to fluid motion, which is referred to as electrothermal flow. This paper presents a numerical study on the electrothermally induced fluid flow on the dielectrophoretic microelectrode array. The fluid movement is numerically solved by coupling electrical, thermal and mechanical equations. A number of parameters including frequency, electrode structure, conductivity of the fluid and external heating that influence the fluid flow patterns are investigated. Particle behavior under the effects of electrothermal flow is studied. The viscous drag force on the particles arising from the electrothermal fluid flow becomes apparent as the particle size is reduced to the sub-micrometer scale. In certain circumstances, the drag force may be of the same order as or much greater than the DEP force. Under the effect of the electrothermal fluid flow, small particles may exhibit movements differing from that in the common DEP environment. These results provide significant suggestions for the manipulation of nanoparticles using ac electric fields under the normal DEP conditions.

Laser-induced backside wet etching (LIBWE) is an effective method for crack-free etching of transparent materials such as glass and quartz. Traditionally, LIBWE is performed using ultraviolet (UV) laser sources. However, this study describes the use of an economic Q-switched 532 nm green laser in the LIBWE microfabrication of sodalime glass substrates. Using a common organic dye (Rose Bengal) as the photoetchant, crack-free microstructures with a minimum feature size of 18 µm are obtained. The typical etch rate is approximately 10 to 70 nm/pulse and the maximum attainable depth is found to be approximately 65 µm. The etch threshold is 5.7 J cm⁻². The surface quality of the micro-trenches produced by the visible LIBWE source is comparable to that obtained in the traditional UV LIBWE process. Microtrenching in sodalime is demonstrated to show the feasibility of microfluidic chip development using visible LIBWE.

This paper reports on the changes of PDMS interface characteristics due to the long-term influence of aqueous alkaline solutions, which are frequently used fluids in biocatalytic reactions. Soft lithographic techniques were used to produce polymeric microfluidic systems containing a fluidic layer with multiple cavities for biocatalytic reactions and a pneumatic control layer. The surface energy, the surface roughness and the absorption of liquids on PDMS are analysed as they are important factors affecting the microfluidic current. The results obtained can be used to provide design guidelines for adapting PDMS-based microfluidic devices in long-term bio catalysis reactions.

This paper studies the energy conversion efficiency for a rectified piezoelectric power harvester. An analytical model is proposed, and an expression of efficiency is derived under steady-state operation. In addition, the relationship among the conversion efficiency, electrically induced damping and ac–dc power output is established explicitly. It is shown that the optimization criteria are different depending on the relative strength of the coupling. For the weak electromechanical coupling system, the optimal power transfer is attained when the efficiency and induced damping achieve their maximum values. This result is consistent with that observed in the recent literature. However, a new finding shows that they are not simultaneously maximized in the strongly coupled electromechanical system.

Numerical simulation, micro-fabrication and flow visualization were performed for a micro planar serpentine channel to reveal the mixing and separation characteristics of two fluid streams with or without density variation. When the densities of the injected fluids are equal, the induced vortices laminate the interface and considerably increase the interfacial area in a spiral manner. It compensates the negative effect of the short residence period of fast flow for mixing. When the densities of the injected fluids slightly differ, the denser fluid distributes in regions of the outer corner of turning. Separation is the effect resulting from a density difference and a velocity difference in a flow field that can be promoted on flowing in a serpentine channel with a relatively rapid bulk flow. The design of a channel incorporating an alternation of cross-sectional areas improves the velocity difference. This hypothesis is verified by both numerical simulation and experimental observation. A green dye (density 2030 kg m⁻³) is separated from deionized water (density 1000 kg m⁻³) and double interfaces are formed significantly under conditions of flow in a micro planar serpentine channel at Reynolds number 16.

This paper investigates the dynamic behavior of a microbeam-based electrostatic microactuator. The cross-section of the microbeam under consideration varies along its length. A mathematical model, accounting for the system nonlinearities due to mid-plane stretching and electrostatic forcing, is adopted and used to examine the microbeam dynamics. The differential quadrature method (DQM) and finite difference method (FDM) are used to discretize the partial–differential–integral equation and generate frequency-response curves for various microstructure geometries and different voltages. We show that the use of the DQM, with a few grid points, in conjunction with the FDM applied to the space derivatives and time derivatives, respectively, yields excellent convergence of the dynamic solutions. The stability of these solutions is examined using Floquet theory. Results are presented to display the dynamics and the effect of variable geometry on the frequency-response curves of the microstructure. We first demonstrate convergence of the DQM–FDM discretized dynamics model as the number of grid points is varied from 5 to 13, while the number of time steps in one time period is fixed at 100. The proposed DQM–FDM discretized dynamic model is then compared to recently reported models. We show that the shape of the frequency-response curves of the microbeam, excited near its first natural frequency, is very sensitive to the approximations employed in the construction of the model. Finally, we examine the effect of varying the gap size and the microbeam thickness and width on its frequency-response curves for hardening-type and softening-type behaviors.

For a long time wet bulk-micromachining has been an easy and cost-effective method for fabricating silicon micro-sensors. Anisotropic wet etching is the key processing step for the fabrication of microstructures. Among different silicon etchants, TMAH based etchants are becoming popular because of their low toxicity and CMOS compatibility. The etch rate of wet anisotropic etching of silicon depends on the crystal plane orientation, type of etchant and their concentrations. In anisotropic etching, convex corners are attacked; therefore, a proper compensating structure design is often required when fabricating microstructures with sharp corners (convex corners). In the present work, two ⟨1 0 0⟩ bar compensation structures have been used for convex corner compensation with 25% wt TMAH–water solution at 90 ± 1 °C temperature. Generalized empirical formulae are also presented for these compensation structures for TMAH–water solution. Both the ⟨1 0 0⟩ bar structures provide perfect convex corners but the ⟨1 0 0⟩ wide bar (structure 2) is more space efficient than the ⟨1 0 0⟩ thin bar (structure 1) and it requires nearly 30% less groove width.

There is strong experimental evidence for the existence of strange modes of failure of microelectromechanical systems (MEMS) devices under mechanical shock and impact. Such failures have not been explained with conventional models of MEMS. These failures are characterized by overlaps between moving microstructures and stationary electrodes, which cause electrical shorts. This work presents modeling and simulation of MEMS devices under the combination of shock loads and electrostatic actuation, which sheds light on the influence of these forces on the pull-in instability. Our results indicate that the reported strange failures can be attributed to early dynamic pull-in instability. The results show that the combination of a shock load and an electrostatic actuation makes the instability threshold much lower than the threshold predicted, considering the effect of shock alone or electrostatic actuation alone. In this work, a single-degree-of-freedom model is utilized to investigate the effect of the shock–electrostatic interaction on the response of MEMS devices. Then, a reduced-order model is used to demonstrate the effect of this interaction on MEMS devices employing cantilever and clamped–clamped microbeams. The results of the reduced-order model are verified by comparing with finite-element predictions. It is shown that the shock–electrostatic interaction can be used to design smart MEMS switches triggered at a predetermined level of shock and acceleration.

A friction meter with consideration of contact surface shape is proposed for the evaluation of the static and dynamic friction coefficients on the sidewalls of micromachined structures. In order to validate the proposed friction measurement method, a friction meter for sidewalls was designed employing simple beam springs with holding and driving comb actuators fabricated using a silicon deep reactive ion etching process. In experiments to assess the meter, a shuttle was placed at a certain position by the driving actuator, and a symmetric normal holding force was subsequently applied to the sidewalls of the shuttle. After increasing the driving voltage with a ramp slope, the sliding distance was measured so as to determine the static and dynamic friction coefficients with consideration of the spring nonlinearity. To characterize the suggested friction meter, experiments were performed to investigate the effects of the normal force and the contact surface shape on friction coefficients by varying the contact widths and the number of contact points. The results indicate that the friction coefficients increased with the normal holding force, whereas the contact surface shape did not show a noticeable effect on the friction coefficients.

This paper presents a 2-axis optical scanner fabricated by a MEMS process for use in optical coherence tomography (OCT) imaging. It comprises planar coils placed above a single permanent magnet; it is driven by an electromagnetic force in the static or low frequency (∼10 Hz) mode, which is required for OCT systems. Two coils are fabricated on a movable plate for X-axis rotation with a mirror, and four coils are fabricated on a movable frame for Y-axis rotation; each coil has a double-layer structure. Two types of torsion beams are designed and tested. The maximum optical scanning angle of the optical scanner with meandering torsion beams was ±8° for a drive current of ±4.6 mA and ±10.3 mA in the X- and Y-axes, respectively. The resonant frequencies of the optical scanner were 106 Hz and 80.5 Hz in the X- and Y-axes, respectively; the frequency characteristics indicated that the scanner can be driven from dc to several Hz. The fabricated optical scanner was applied to an OCT system and cross-sectional images of an onion were obtained by scanning along both axes.

A dielectric, chip-scale MEMS packaging method is discussed. The packaging method uses wafer-to-wafer bonding of micromachined glass wafers with a reflowed, glass, sealing ring. The glass wafers are micromachined and have metal and silicon structures patterned on them with metal and fluidic feedthroughs. A variety of getters and sealing designs are disclosed to vary the pressure of the microcavity by many orders of magnitude from under 1 mTorr up to 1 atm (760 000 mTorr), enabling either vacuum or damped packaging of the device elements on the same chip. The final singulated, all-glass, chip-scale package can have electrical, optical/IR and fluidic interfaces. Applications for resonators, switches, optical sensors and displays are discussed.

Transdermal drug delivery is generally limited by the extraordinary barrier properties of the stratum corneum, the outer 10–15 µm layer of skin. A conventional needle inserted across this barrier and into deeper tissues could effectively deliver drugs. However, it would lead to infection and cause pain, thereby reducing patient compliance. In order to administer a frequent injection of insulin and other therapeutic agents more efficiently, integrated arrays with very short microneedles were recently proposed as very good candidates for painless injection or extraction. A variety of microneedle designs have thus been made available by employing the fabrication tools of the microelectronics industry and using materials such as silicon, metals, polymers and glass with feature sizes ranging from sub-micron to nanometers. At the same time, experiments were also made to test the capability of the microneedles to inject drugs into tissues. However, due to the difficulty encountered in measurement, a detailed understanding of the spatial and transient drug delivery process still remains unclear up to now. To better grasp the mechanisms involved, quantitative theoretical models were developed in this paper to simultaneously characterize the flow and drug transport, and numerical solutions were performed to predict the kinetics of dispersed drugs injected into the skin from a microneedle array. Calculations indicated that increasing the initial injection velocity and accelerating the blood circulation in skin tissue with high porosity are helpful to enhance the transdermal drug delivery. This study provides the first quantitative simulation of fluid injection through a microneedle array and drug species transport inside the skin. The modeling strategy can also possibly be extended to deal with a wider range of clinical issues such as targeted nanoparticle delivery for therapeutics or molecular imaging.

A micro-thermal flow sensor is developed using thin-film thermocouples as temperature sensors. A micro-thermal flow sensor consists of a heater and thin-film thermocouples which are deposited on a quartz wafer using stainless steel masks. Thin-film thermocouples are made of standard K-type thermocouple materials. The mass flow rate is measured by detecting the temperature difference of the thin-film thermocouples located in the upstream and downstream sections relative to a heater. The performance of the micro-thermal flow sensor is experimentally evaluated. In addition, a numerical model is presented and verified by experimental results. The effects of mass flow rate, input power, and position of temperature sensors on the performance of the micro-thermal flow sensor are experimentally investigated. At low values, the mass flow rate varies linearly with the temperature difference. The linearity of the micro-thermal flow sensor is shown to be independent of the input power. Finally, the position of the temperature sensors is shown to affect both the sensitivity and the linearity of the micro-thermal flow sensor.

Thermal stress-induced damage in multilayered films formed on substrates and cantilever beams is a major reliability issue for the fabrication and application of micro sensors and actuators. Using closed-form predictive solutions for thermal stresses in multilayered systems, specific results are calculated for the thermal stresses in PZT/Pt/Ti/SiO₂/Si₃N₄/SiO₂ film layers on Si substrates and PZT/Pt/Ti/SiO₂ film layers on Si₃N₄ cantilever beams. When the thickness of the film layer is negligible compared to the substrate, thermal stresses in each film layer are controlled by the thermomechanical mismatch between the individual film layer and the substrate, and the modification of thermal stresses in each film layer by the presence of other film layers is insignificant. On the other hand, when the thickness of the film layer is not negligible compared to the cantilever beam, thermal stresses in each film layer can be controlled by adjusting the properties and thickness of each layer. The closed-form solutions provide guidelines for designing multilayered systems with improved reliability.

This note describes a method for the parallel self-assembly of out-of-plane surface-micromachined structures that uses non-uniform residual stresses, inherent in many surface-micromachining processes. The residual stresses are used to achieve a one-time only actuation capable of lifting and assembling raised structures. Theory is provided to calculate the deflection and stiffness of bi-layer cantilevers. Devices for amplifying the vertical deflection are demonstrated and used to assemble large arrays of devices.

A new fabrication technique based on etching is employed to convert a copper foil into a porous structure with an array of micron size pores. The motivation stems from the need to develop a more efficient and controllable gas diffusion medium for fuel cell applications. The influence of mask shape, mask width and etching time was investigated experimentally. A correlation to predict trench width with etching time was derived; normalizing by mask width allows one to collapse the data. The etching rates to obtain micro-scale features, which are of the order of 1–2 µm min^–1, are mainly dominated by the mask width due to mass-transport resistance. It is possible to control the pore dimensions, porosity and pore size distributions with this technique.

Spark-assisted chemical engraving (SACE) is an unconventional micro-machining technology particularly suited for glass processing, taking advantage of electrochemical discharges. This technology distinguishes itself by its simplicity and flexibility, the possibility of machining high aspect ratio structures, the excellent surface qualities and the non-utilization of expensive clean room facilities. As the process is a serial one, the material removal rate becomes an important issue. It is shown that, by adequate tool vibration, it is possible to improve the mean material removal rate in gravity-feed drilling by a factor of 2. Micro-holes in glass with a depth of 300 µm are drilled in less than 10 s.