Showing papers in "Journal of Micromechanics and Microengineering in 2006"

Bernoulli–Euler beam model based on a modified couple stress theory

Abstract: 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 review of microvalves

Abstract: This review gives a brief overview of microvalves, and focuses on the actuation mechanisms and their applications. One of the stumbling blocks for successful miniaturization and commercialization of fully integrated microfluidic systems was the development of reliable microvalves. Applications of the microvalves include flow regulation, on/off switching and sealing of liquids, gases or vacuums. Microvalves have been developed in the form of active or passive microvalves employing mechanical, non-mechanical and external systems. Even though great progress has been made during the last 20 years, there is plenty of room for further improving the performance of existing microvalves.

Efficiency of energy conversion for a piezoelectric power harvesting system

Abstract: 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.

Characterization of electrowetting actuation on addressable single-side coplanar electrodes

Abstract: This paper studies and characterizes electrowetting-on-dielectric (EWOD) actuations on coplanar electrodes with an electrode-free cover plate or no cover plate. By arranging driving and reference electrodes on one plate, such an EWOD configuration can accommodate more sensing mechanisms from above and thus allows increased flexibility for system development. Various coplanar electrodes are tested for contact angle changes by EWOD with a focus on the effect of the percentage gap between electrodes and are found to be in good agreement with a simple analytical model. The droplet-moving devices demonstrate the successful moving, cutting and merging of droplets (~0.1 µl) in a parallel-plate configuration, i.e., between the driving plate with coplanar electrodes and the passive plate with no electrode. EWOD actuation in single plate configuration, i.e. no cover plate, is also demonstrated, adding additional flexibility for the system design.

Fabrication of multi-layer SU-8 microstructures

Abstract: The fabrication of multi-level SU-8 microstructures using multiple coating and exposure steps and a single developing step has been achieved for up to six layers of SU-8. Alternating layers of SU-8 2010 (thin) and SU-8 2100 (thick) photoresist films were spin coated, followed by soft-bake, ultraviolet (UV) exposure and post-exposure bake steps. The multiple SU-8 layers were simultaneously developed to create patterned microstructures with overall thicknesses of up to 500 µm and minimum lateral feature size of 10 µm. The use of a single developing step facilitated fabrication of complex multi-level SU-8 microstructures that might be difficult, or even impossible, to achieve by sequential processing of multiple SU-8 layers that are individually coated, baked, exposed and developed.

A MEMS acoustic energy harvester

Abstract: This paper presents the development of a micromachined acoustic energy harvester for aeroacoustic applications. The acoustic energy harvester employs a silicon-micromachined circular, piezoelectric composite diaphragm for electroacoustic transduction. Lumped element modeling, design, fabrication and characterization of a micromachined acoustic energy harvester prototype are presented. Experimental results indicate a maximum output power density of 0.34 µW cm−2 at 149 dB (ref. 20 µPa) and suggest a potential output power density, for this design, of 250 µW cm−2 with an improved fabrication process.

The hydrodynamic focusing effect inside rectangular microchannels

Abstract: This paper presents a theoretical and experimental investigation into the hydrodynamic focusing effect in rectangular microchannels. Two theoretical models for two-dimensional hydrodynamic focusing are proposed. The first model predicts the width of the focused stream in symmetric hydrodynamic focusing in microchannels of various aspect ratios. The second model predicts the location and the width of the focused stream in asymmetric hydrodynamic focusing in microchannels with a low or high aspect ratio. In both models, the theoretical results are shown to be in good agreement with the experimental data. Hence, the models provide a useful means of performing a theoretical analysis of flow control in microfluidic devices using hydrodynamic focusing effects. The ability of the proposed models to control the focused stream within a micro flow cytometer is verified in a series of experimental trials performed using polystyrene microparticles with a diameter of 20 µm. The experimental data show that the width of the focused stream can be reduced to the same order of magnitude as that of the particle size. Furthermore, it is shown that the microparticles can be successfully hydrodynamically focused and switched to the desired outlet port of the cytometer. Hence, the models presented in this study provide sufficient control to support cell/particle counting and sorting applications.

A thermal actuator for nanoscale in situ microscopy testing: design and characterization

Abstract: This paper addresses the design and optimization of thermal actuators employed in a novel MEMS-based material testing system. The testing system is designed to measure the mechanical properties of a variety of materials/structures from thin films to one-dimensional structures, e.g. carbon nanotubes (CNTs) and nanowires (NWs). It includes a thermal actuator and a capacitive load sensor with a specimen in-between. The thermal actuator consists of a number of V-shaped beams anchored at both ends. It is capable of generating tens of milli-Newton force and a few micrometers displacement depending on the beams' angle and their number. Analytical expressions of the actuator thermomechanical response are derived and discussed. From these expressions, a number of design criteria are drawn and used to optimize the device response. The analytical predictions are compared with both finite element multiphysics analysis (FEA) and experiments. To demonstrate the actuator performance, polysilicon freestanding specimens cofabricated with the testing system are tested.

A physical model to predict stiction in MEMS

Abstract: One of the most important reliability problems in micro-electromechanical systems (MEMSs) is stiction, the adhesion of contacting surfaces due to surface forces. After reviewing the known physical theory, and the measurement method commonly used to investigate stiction, we present a model that can be used to investigate the sensitivity of MEMS to stiction. It quantitatively predicts the surface interaction energy of surfaces in contact. Included in the model is the roughness of the contacting surfaces and the environmental conditions (humidity and temperature). This is done by describing the surface interaction energy as a function of the distance between the surfaces. This distance is not a unique number, but rather a distribution of distances. It is shown that, if we know this distribution, we can calculate the surface interaction energy. The model is suitable for the prediction of forces due to capillary condensation and molecular interactions.

A novel fabrication method of flexible and monolithic 3D microfluidic structures using lamination of SU-8 films

Abstract: The fabrication of three-dimensional (3D) microfluidic networks entirely made of SU-8 with integrated electrodes is reported. The described technology allows the fabrication of uncrosslinked SU-8 dry film on a polyester (PET) sheet and its subsequent lamination to form closed microstructures. Unlike other reported methods, transferred layers are patterned following the bonding step allowing a more accurate and simple alignment between levels than techniques using already patterned layers. Dry release of the complete polymer microstructure was demonstrated. Flexible microfluidic chips were obtained. This technique uses simple tools and no wafer bonder is used but lamination techniques which are more collective processes. Limitations in the method for layers thicker than 50 µm have been observed and are discussed. Hydraulic flow experiments have been performed to study the deformation of the cover layer which could influence adjacent flow in a three-dimensional configuration. Important deformations have been observed for layers 10 µm thick and an average pressure greater than 100 kPa. No deformations have been noted for layers with thicknesses greater than 35 µm and for average pressures up to 200 kPa. No failures occurred within the range of the experimental set-up, i.e. up to 300 kPa.

Micromachined microbial and photosynthetic fuel cells

Abstract: This paper presents two types of fuel cells: a miniature microbial fuel cell (µMFC) and a miniature photosynthetic electrochemical cell (µPEC). A bulk micromachining process is used to fabricate the fuel cells, and the prototype has an active proton exchange membrane area of 1 cm2. Two different micro-organisms are used as biocatalysts in the anode: (1) Saccharomyces cerevisiae (baker's yeast) is used to catalyze glucose and (2) Phylum Cyanophyta (blue-green algae) is used to produce electrons by a photosynthetic reaction under light. In the dark, the µPEC continues to generate power using the glucose produced under light. In the cathode, potassium ferricyanide is used to accept electrons and electric power is produced by the overall redox reactions. The bio-electrical responses of µMFCs and µPECs are characterized with the open-circuit potential measured at an average value of 300–500 mV. Under a 10 ohm load, the power density is measured as 2.3 nW cm−2 and 0.04 nW cm−2 for µMFCs and µPECs, respectively.

A femtosecond laser-induced periodical surface structure on crystalline silicon

Abstract: A laser-induced periodic surface structure (LIPSS) has attracted research interest for its promising potential in micromachining for microelectronics and microelectromechanical systems. A femtosecond laser-induced periodical surface structure was investigated for polished crystalline silicon. The observed structure is similar to the classical ripples that are characterized by long, nearly parallel lines extending over the entire irradiated area on the metal and silicon surface after continuous or pulsed laser irradiation. The spacing of the ripples nearly equals the irradiation wavelength. The depth of these ripples increases nonlinearly with the fluence of irradiation. The orientation of these periodic structures is perpendicular to the vector of the electric field of the laser beam. It seemed that the pattern formed by a femtosecond laser complies well with conventional models. Unlike the patterns formed by a continuous or nanosecond pulsed laser, however, the spacing of the ripple formed by femtosecond pulses is not influenced by the incident angle of the laser beam. The formula used to predict the ripple spacing in the conventional model does not apply to the femtosecond laser induced ripple structure. A plausible explanation to this phenomenon is proposed. The effect of the pulse repetition rate was studied and it was found that a femtosecond laser oscillator generates the same periodic structure as the amplified laser system.

The nonlinear response of resonant microbeam systems with purely-parametric electrostatic actuation

Abstract: Electrostatically-actuated resonant microbeam devices have garnered significant attention due to their geometric simplicity and broad applicability. Recently, some of this focus has turned to comb-driven microresonators with purely-parametric excitation, as such systems not only exhibit the inherent benefits of MEMS devices, but also a general improvement in sensitivity, stopband attenuation and noise rejection. This work attempts to combine each of these areas by proposing a microbeam device which couples the inherent benefits of a resonator with purely-parametric excitation with the simple geometry of a microbeam. Theoretical analysis reveals that the proposed device exhibits desirable response characteristics, but also quite complex dynamics. Of particular note is the fact that the device's nonlinear frequency response is found to be qualitatively dependent on the system's ac excitation amplitude. While this flexibility can be desirable in certain contexts, it introduces additional design and operating limitations. While the principal focus of this work is the proposed system's nonlinear response, the work also contains details pertaining to model development and device design.

Low-pressure, high-temperature thermal bonding of polymeric microfluidic devices and their applications for electrophoretic separation

Abstract: A new method for thermally bonding poly(methyl methacrylate) (PMMA) substrates has been demonstrated. PMMA substrates are first engraved by CO2-laser micromachining to form microchannels. Both channel width and depth can be adjusted by varying the laser power and scanning speed. Channel depths from 50 ?m to 1500 ?m and widths from 150 ?m to 400 ?m are attained. CO2 laser is also used for drilling and dicing of the PMMA parts. Considering the thermal properties of PMMA, a novel thermal bonding process with high temperature and low bonding pressure has been developed for assembling PMMA sheets. A high bonding strength of 2.15 MPa is achieved. Subsequent inspection of the cross sections of several microdevices reveals that the dimensions of the channels are well preserved during the bonding process. Electroosmotic mobility of the ablated channel is measured to be 2.47 ? 10?4 cm2 V?1 s?1. The functionality of these thermally bonded microfluidic substrates is demonstrated by performing rapid and high-resolution electrophoretic separations of mixture of fluorescein and carboxyfluorescein as well as double-stranded DNA ladders (?X174-Hae III dsDNA digest). The performance of the CO2 laser ablated and thermally bonded PMMA devices compares favorably with those fabricated by other professional means.

Sharp beveled tip hollow microneedle arrays fabricated by LIGA and 3D soft lithography with polyvinyl alcohol

Abstract: This paper describes a fabrication process of hollow microneedle arrays with a sharp beveled tip for transdermal drug delivery. A master is fabricated through a double deep x-ray lithography process. First, a polymethylmethacrylate (PMMA) sheet is exposed to produce single PMMA parts with a sawtooth profile. The tip angle of each tooth determines the final tip angle of the microneedles. The PMMA parts are assembled and glued on a conductive substrate and then exposed through a second x-ray mask containing an array of hollow triangles as absorbing structures. A metal layer is then electrodeposited around the needles in order to form the future base of the array. A polyvinyl alcohol (PVA) solution is cast on top of the master to form a negative mold of the microneedle array after a low temperature curing and peel-off steps. A liquid PMMA solution is cast on top of the PVA negative mold and after the full PMMA polymerization the PVA is dissolved in water. This fabrication method can be performed in a non-clean room environment and requires little instrumentation. It is therefore compatible with a low-cost mass-fabrication scheme.

A PDMS-based gas permeation pump for on-chip fluid handling in microfluidic devices

Abstract: 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.

Micromachined polymer electrolyte membrane and direct methanol fuel cells- : a review

Abstract: This review reports recent progress of the development of micromachined membrane-based fuel cells. The review first discusses the scaling law applied to this type of fuel cell. Impacts of miniaturization on the performance of membrane-based fuel cells are highlighted. This review includes only the two most common micro fuel cell types: proton exchange membrane micro fuel cells (PEMµFC) and direct methanol micro fuel cells (DMµFC). Furthermore, we only consider fuel cells with the active area of a single cell less than 1 square inch. Since the working principles of these fuel cell types are well known, the review only focuses on the choice of material and the design consideration of the components in the miniature fuel cell. Next, we compare and discuss the performance of different micro fuel cells published recently in the literature. Finally, this review gives an outlook on possible future development of micro fuel cell research.

Creating micro-scale surface topology to achieve anisotropic wettability on an aluminum surface

Abstract: A technique for fabricating micropatterned aluminum surfaces with parallel grooves 30 ?m wide and tens of microns in depth is described. Standard photolithographic techniques are used to obtain this precise surface-feature patterning. Positive photoresists, S1813 and AZ4620, are selected to mask the surface, and a mixture of BCl3 and Cl2 gases is used to perform the etching. Experimental data show that a droplet placed on the micro-grooved aluminum surface using a micro-syringe exhibits an increased apparent contact angle, and for droplets condensed on these etched surfaces, more than a 50% reduction in the volume needed for the onset of droplet sliding is manifest. No chemical surface treatment is necessary to achieve this water repellency; it is accomplished solely by an anisotropic surface morphology that manipulates droplet geometry and creates and exploits discontinuities in the three-phase contact line. These micro-structured surfaces are proposed for use in a broad range of air-cooling applications, where the management of condensate and defrost liquid on the heat transfer surface is essential to the energy-efficient operation of the machine.

Active micro-mixers using surface acoustic waves on Y-cut 128° LiNbO3

Abstract: This study presents an active method for micro-mixers using surface acoustic waves (SAW) to rapidly mix co-fluent fluids. Mixing is challenging work in microfluidic systems due to their low-Reynolds-number flow conditions. SAW devices were fabricated on 128? Y-cut lithium niobate (LiNbO3). The micro-mixers are these piezoelectric actuators integrated with polydimethylsiloxane microchannels. The effects of the applied voltages on interdigitated transducers (IDTs) and two layouts, parallel- and transversal-type, of micro-mixers on the mixing performance were experimentally explored. The experimental results revealed that the parallel-type mixer achieved a higher mixing effect. Meanwhile, a higher applied voltage on the IDTs led to a significant improvement in the mixing performance of the active micro-mixer. Typical temperature effects associated with the applied voltages on the IDTs were also investigated. Finally, a digestion reaction between a protein (hemoglobin) and an enzyme (trypsin) was performed to verify the capability of the micro-mixers. The protein?enzyme mixture was qualitatively analyzed using mass spectrometry. Using these SAW-based mixers, the amount of digested peptides increased. Additionally, the protein?enzyme mixture was also quantitatively analyzed using high-performance liquid chromatography. Experimental data showed that the amount of digested peptides increased 21.1% using the active mixer. Therefore, the developed micro-mixers can be applied in microfluidic systems for improving mixing efficiency and thus enhancing the bio-reaction.

An energy-based model for electrowetting-induced droplet actuation

Abstract: Electrowetting (EW) induced droplet motion has been explored in the past decade in view of its promising applications in the field of microfluidics. This paper demonstrates the potential of energy-based analyses for modeling the performance of EW-based fluid actuation systems. Analyses based on system energy minimization offer simplified modeling tools to predict the overall performance of EW systems while circumventing the need to model the numerous complexities in the system. An analytical model is developed to estimate the actuation force on a droplet moving between two electrodes. The origins and contributions of various components of the actuation force are analyzed. The effects of dielectric parameters, electrode layout, droplet geometry and shape are discussed with the objective of maximizing the actuation force. The actuation force model is combined with semi-analytical models for predicting the forces opposing droplet motion to develop a model that predicts transient EW-induced droplet motion. Parametric results are obtained to evaluate the importance of operating voltage, fluid properties and droplet geometry on droplet motion.

A split and recombination micromixer fabricated in a PDMS three-dimensional structure

Abstract: In this paper we propose a new split and recombination (SAR) micromixer that is compatible with the microfabrication process of polydimethylsiloxane (PDMS). We evaluate the mixing efficiency of the fabricated SAR micromixer and find that it increases interfaces exponentially. Simulation using CFD-ACE+ shows a cross-sectional view of the flow and estimates the mixing efficiency of the SAR micromixer and the pressure drop for a unit of the SAR micromixer. A mixing experiment involving phenolphthalein and NaOH solutions shows that interfaces, represented as red lines, are increased by SAR mixing. The result of our mixing experiment involving blue dye and water is evaluated to determine the mixing efficiency by calculating the standard deviation (stdev) of the pixel intensity of the observed image. After the seventh unit of the SAR micromixer, solutions are mixed to 90% at Re 0.6. The number of units needed to reach a mixed state in which the stdev is lower than 0.05, a 90% mixed state, increases from 5 to 10 for a flow rate ranging from 0.1 µl min−1 (Re 0.012) to 1000 µl min−1 (Re 120) including numerical analysis results. The pressure drop increases proportionally from 2.8 Pa to 35 000 Pa when the flow rate increases from 0.1 µl min−1 (Re 0.012) to 1000 µl min−1 (Re 120) in the numerical analysis results.

Pneumatically driven peristaltic micropumps utilizing serpentine-shape channels

Abstract: This study presents a novel pneumatic micropump featuring a serpentine-shape (S-shape) microchannel. Fluid is driven through the device by the hydrodynamic pressure generated by the peristaltic action of membranes located at the intersections of the fluidic microchannel and the S-shape microchannel. The pneumatic micropump is fabricated in PDMS (polydimethylsiloxane) using MEMS (micro-electro-mechanical-systems)-based techniques. The micropump provides an improved pumping rate and is controlled using a single electromagnetic valve (EMV) switch. The experimental results reveal that the pumping rate can be increased by increasing the operational frequency of the EMV, the pressure of the externally supplied compressed air or the number of membranes. As the compressed air travels along the S-shape microchannel, it causes the membranes to deflect. The time-phased deflection of successive membranes along the microchannel length generates a peristaltic effect which drives the fluid along the microfluidic channel. The maximum attainable pumping rate is influenced by the time interval between the deflections of adjacent membranes, and is therefore affected by the geometric characteristics of the serpentine microchannel. The back pressure of the serpentine-shape micropump is measured at a fixed peak frequency to prove its ability to overcome the fluidic resistance. The optimum operating conditions and geometric parameters of the micropump are verified experimentally. It is found that the maximum pumping rate is 7.43 µl min−1 and is provided by a micropump with seven membranes actuated by 20 psi air pressure and 9 Hz operational frequency.

A degassing plate with hydrophobic bubble capture and distributed venting for microfluidic devices

Abstract: This paper introduces a microfluidic wall plate that allows the removal of gas bubbles from a gas/liquid mixture in a distributed fashion, i.e., throughout the flow path, eliminating the need for discrete separators common in macroscopic practice. Integrated into a microfluidic system at critical locations, such a degassing plate prevents the build up of gas bubbles in microchannels so as to maximize the effective reaction area, decrease the flow resistance and keep the chamber pressure under check. Furthermore, the plate surface is designed to capture the gas bubbles preferentially on designated venting sites, so that the rest of the surface can be dedicated to other functions, such as the catalyst or electrodes. The mechanism of bubble capture is explained by surface energy minimization, and two types of bubble sinks are proposed and verified. Once captured, the bubbles can be vented out through hydrophobic venting holes small enough (e.g. sub-micron) to block the liquid by surface tension. By chemically generating CO2 inside a small chamber (30 mm ? 50 mm ? 1.5 mm) sealed by the degassing plate, the process of bubble capture and removal is visually demonstrated. A porous polypropylene membrane with ~0.2 ?m diameter holes shows that gas can be removed with only several kPa of internal pressure while water stays free of leakage even under 2.4 ? 105 Pa (35 psi). Venting is effective in any gravitational orientation, paving the way for portable microfluidic devices.

A capacitive RF power sensor based on MEMS technology

Abstract: Wideband 100 kHz–4 GHz power sensors are presented, which are based on sensing the electrostatic force between an RF signal line and a suspended membrane. The electrostatic force, which is proportional to the square of the rms signal voltage and thus to the signal power, results in a displacement of the suspended membrane. This displacement is detected capacitively, allowing the sensing of the signal power with extremely low dissipative losses; therefore the sensor can be placed in a transmission line with negligible disturbance of the signal. Devices have been designed and fabricated successfully by aluminum surface micromachining using photoresist as the sacrificial layer. Optimization of the design with SONNET has resulted in measured return and insertion losses (S11 and S21) better than -30 dB and -0.15 dB, respectively, up to 4 GHz, and a sensitivity of 90 aF mW-1.

Polydimethylsiloxane membranes for millimeter-wave planar ultra flexible antennas

Abstract: We present here the use of polydimethylsiloxane (PDMS) membranes as a new soft polymer substrate (er ≈ 2.67 at 77 GHz) for the realization of ultra-flexible millimeter-wave printed antennas thanks to the extremely low Young's modulus (EPDMS < 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.

An all SU-8 microfluidic chip with built-in 3D fine microstructures

Abstract: 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.

High-mode resonant piezoresistive cantilever sensors for tens-femtogram resoluble mass sensing in air

Abstract: According to the demand for an ultrasensitive mass sensor for bio/chemical molecular detection, resonant cantilever sensors are developed for detection in an air environment. Both a piezoresistive bridge and a metal coil are integrated in the cantilever for signal sensing and Lorentz-force resonance excitation, respectively. Compared with conventional first flexure mode resonance, measurement results for the second mode resonance show an improved mass-sensing resolution from 0.17 pg to 0.06 pg due to the higher quality factor. For further improving the resolution, an optimized electromagnetic excitation method specifically for the second resonant mode is proposed and developed. The optimized method provides a two-point excitation that matches the second mode shape function of the cantilever deflection and excites the second mode more efficiently. Compared to a cantilever with a conventional excitation method, the optimally excited cantilever improves the quality factor from 307 to 857. Based on the experimental results for the optimally excited second mode resonant sensor, 29 × 10−15 g resolution for in-air mass sensing is achieved. The developed second mode resonant cantilever sensor with piezoresistive sensing element integrated on-chip is promising for use in high-performance portable biological/chemical sensing applications.

Rapid mold temperature variation for assisting the micro injection of high aspect ratio micro-feature parts using induction heating technology

Abstract: Hot embossing and injection molding are popular methods to duplicate micro features formed during polymer micro-fabrication of MEMS devices. However, both methods face challenges in filling the polymer melt completely into a micro-featured geometry of a high aspect ratio. In this study, electromagnetic induction heating combined with water cooling is used to achieve rapid mold surface temperature control during the micro-feature injection molding process. A CAE simulation was also developed through integration of both thermal and electromagnetic analysis modules of ANSYS, and its capability and accuracy were verified experimentally. Efficiency evaluations of induction heating and the uniformity of mold temperature control were conducted on a micro-featured mold. This mold was designed with a micro channel array of 30–50 µm in width and 120 and 600 µm in depth, corresponding to aspect ratios ranging from about 2.4 to 12. The accuracies of the micro channels in molded PMMA parts can be used to evaluate the effect of mold temperature on replication accuracy. It was found that rapid mold surface heating with temperature rising from 60 °C to between 100 °C and 140 °C by induction heating requires 2–3.5 s, while the mold temperature returns to 60 °C in about 70–110 s. The simulated mold surface temperature results are consistent with measured results. Achieving the same temperature variation by switching circulation coolants of different temperatures requires at least 7 min. The simulation also reveals that the electromagnetic wave can penetrate into the bottom of the micro channel and results in only about a 2 °C difference in temperature uniformity. For mold temperatures of 100 °C, 120 °C and 140 °C, the molded channel depths were 94.9 µm, 105.4 µm and 116.0 µm, respectively, when the ideal channel depth was 120 µm. When the channel depth is 600 µm, the mold temperature must exceed 120 °C, so that reasonable accuracy in micro-feature replication can be achieved. Our results to date indicate that the aspect ratio for molded PMMA micro channels can be as high as 12. Efficient mold temperature variation by induction heating to improve the replication accuracy in molding micro features is successfully illustrated.

Characterization of metal and metal alloy films as contact materials in MEMS switches

Abstract: This study presents a basic step toward the selection methodology of electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This involves the interrelationship between two important parameters, resistivity and hardness, since they provide the guidelines and assessment of contact resistance, wear, deformation and adhesion characteristics of MEMS switches. For this purpose, thin film alloys of three noble metals, platinum (Pt), rhodium (Rh) and ruthenium (Ru) with gold (Au), were investigated. The interrelationship between resistivity and hardness was established for three levels of alloying of these metals with gold. Thin films of gold (Au), platinum (Pt), ruthenium (Rh) and rhodium (Ru) were also characterized to obtain their baseline data for comparison. All films were deposited on silicon substrates. When Ru, Rh and Pt are alloyed with Au, their hardness generally decreases but resistivity increases. This decrease or increase was, in general, dependent upon the amount of alloying.

Pool boiling heat transfer on artificial micro-cavity surfaces in dielectric fluid FC-72

Abstract: The boiling performance and flow mechanism on artificial micro-cavity surfaces with different geometric parameters are presented in the present study. The test surfaces are manufactured on a 625 µm thick, 10 mm × 10 mm square silicon plate. The treated cavities are all cylinders with three diameters (200, 100 and 50 µm) and two depths (200 and 110 µm). The densities of the cavities were designed to be 33 × 33, 25 × 25 and 16 × 16 arrays with 100, 200 and 400 µm spacings, respectively. The characteristics of heat transfer for pool boiling of FC-72 on artificial micro-cavity surfaces were also examined. In this paper, visualization of the flow patterns was conducted to investigate the characteristics of the bubbles in the growth and departure process. The results indicated that boiling incipience and temperature excursion of silicon-based surfaces are more significant than those of metal-based surfaces reported in the literature. The effects of cavity density are stronger in the high heat flux region than in the low heat flux region because of the bubble/vapor coalescence near the heating surface. The heat transfer coefficient increases with heat flux and cavity density but a denser cavity will suppress the value of critical heat flux (CHF). Besides, in moderate and high heat flux regions, a larger cavity diameter surface shows earlier decay and a lower peak value of the heat transfer coefficient. The maximum value of CHF on the base area was 3 × 105 W m−2 (30 W m−2) for the test surface with a 33 × 33 cavity array, which is almost 2.5 times that of the plain silicon surface.