Table of contents

The opportunity for the commercialization of microfluidic systems has surged over the recent decade, primarily for medical and the life science applications. This positive development has been spurred by an increasing number of integrated, highly functional lab-on-a-chip technologies from the research community. Toward commercialization, there is a dire need for economic manufacture which involves optimized cost for materials and structuring on the front-end as well as for a range of back-end processing steps such as surface modification, integration of functional elements, assembly and packaging. Front-end processing can readily resort to very well established polymer mass fabrication schemes, e.g. injection molding. Also assembly and packaging can often be adopted from commercially available processes. In this review, we survey the back-end processes of hybrid material integration and surface modification which often need to be tailored to the specifics of miniaturized polymeric microfluidic systems. On the one hand, the accurate control of these back-end processes proves to be the key to the technical function of the system and thus the value creation. On the other hand, the integration of functional materials constitutes a major cost factor.

In this paper, a novel capacitive power sensor based on the microelectromechanical systems (MEMS) cantilever beam at 8–12 GHz is proposed, fabricated and tested. The presented design can not only realize a cantilever beam instead of the conventional fixed–fixed beam, but also provide fine compatibility with the GaAs monolithic microwave integrated circuit (MMIC) process. When the displacement of the cantilever beam is very small compared with the initial height of the air gap, the capacitance change between the measuring electrode and the cantilever beam has an approximately linear dependence on the incident radio frequency (RF) power. Impedance compensating technology, by modifying the slot width of the coplanar waveguide transmission line, is adopted to minimize the effect of the cantilever beam on the power sensor; its validity is verified by the simulation of high frequency structure simulator software. The power sensor has been fabricated successfully by Au surface micromachining using polyimide as the sacrificial layer on the GaAs substrate. Optimization of the design with impedance compensating technology has resulted in a measured return loss of less than −25 dB and an insertion loss of around 0.1 dB at 8–12 GHz, which shows the slight effect of the cantilever beam on the microwave performance of this power sensor. The measured capacitance change starts from 0.7 fF to 1.3 fF when the incident RF power increases from 100 to 200 mW and an approximate linear dependence has been obtained. The measured sensitivities of the sensor are about 6.16, 6.27 and 6.03 aF mW⁻¹ at 8, 10 and 12 GHz, respectively.

A low-cost nano-gap interdigitated electrode array (IDA) on a polymer substrate has been developed to realize a disposable nano-biosensor for biochemical clinical analysis. Utilizing the common instruments for optical lithography, nano-scale features were fabricated on a thermoplastic polymer to produce an electrochemical nano-biosensor in a disposable format. The IDA was realized on a 3-inch cyclo-olefin copolymer wafer, which illustrates the utility of our fabrication technique as a large-area nanofabrication process for a polymer using low temperature processes. In order to demonstrate the use of the sensor for lab-on-a-chip applications, the developed IDA was integrated with a microfluidic channel and applied for the electrochemical detection of poly-aminophenol with 10⁻⁸ M detection limit. The results indicate the developed fabrication technique is suitable for the inexpensive mass fabrication of highly sensitive nano-biosensors for disposable applications.

This paper describes the fabrication and characterization of novel photodefined polymer-embedded vias for silicon interposers. The fabricated polymer-embedded vias can help obtain ∼3.8× reduction in via-to-via capacitance as well as a reduction in insertion loss compared to TSVs with a silicon dioxide liner. Polymer-embedded vias 100 μm in diameter, 270 μm tall and at 250 μm pitch were fabricated. Resistance and leakage measurements were performed for the fabricated polymer-embedded vias. The average value of the measured resistance for 20 polymer-embedded vias is 2.54 mΩ and the maximum measured via-to-via leakage current for 10 pairs of polymer-embedded vias is 80.8 pA for an applied voltage of 200 V.

Backside exposure lithography has been proven to be able to generate needle-like microstructures. The structure profile can be controlled by varying the aperture diameter on the photomask and the distance between the photomask and the photoresist. This distance is usually defined by the glass thickness of the glass in backside exposure lithography. However, in our experience, needle-like structures can be generated easily in some cases but not in others. In order to accurately predict the microstructure profile fabricated by backside exposure lithography, in this study, we built a complete three-dimensional Fresnel–Kirchhoff diffraction model and used a binary approach to simulate the curing threshold. We found that the microstructure profile is influenced by diffraction in both the near-field (Fresnel) and far-field regions (Fraunhofer). Diffraction depends on the design pattern on the photomask and the glass thickness. In many cases, it changes gradually from the near-field to the far-field. This is exactly the reason that our approach generates needle-like structures. Structures ranging from 50 to 450 µm in height were simulated by our model and had a high degree of consistency with the fabricated results. This research may provide potential guidelines for the prediction and the fabrication of needle-like structures for future applications.

Commercial digital multimirror devices offer a cheap and effective alternative to more expensive spatial light modulators for ablation via beam shaping. Here we present femtosecond laser ablation using the digital multimirror device from an Acer C20 Pico Digital Light Projector and show ablation of complex features with feature sizes ranging from sub-wavelength (400 nm) up to ∼30 µm. Simulations are presented that have been used to optimize and understand the experimentally observed resolution.

This paper reports three-dimensional (3-D) microfabricated toroidal inductors intended for power electronics applications. A key fabrication advance is the exploitation of thick metal encapsulation of polymer pillars to form a vertical via interconnections. The radial conductors of the toroidal inductor are formed by conventional plating-through-mold techniques, while the vertical windings (up to 650 µm in height) are formed by polymer cores with metal plated on their external surfaces. This encapsulated polymer approach not only significantly reduces the required plating time but also exploits the relative ease of fabricating high-aspect-ratio SU-8 pillars. To form the top radial conductors, non-photopatternable SU-8 is introduced as a thick sacrificial layer. Two toroidal inductor geometries were fabricated and tested. The first inductor had an inner diameter of 2 mm, an outer diameter of 6 mm, 25 turns and a vertical via height of 650 µm. The second inductor had an inner diameter of 4 mm, an outer diameter of 8 mm, 50 turns and a vertical via height of 650 µm. Both inductor geometries were successfully fabricated and characterized in the frequency range of 0.1−100 MHz. Characterization results of the 25- and 50-turn inductors showed an average inductance of 76 and 200 nH, a low frequency (0.1 MHz) resistance of 0.2 and 1 Ω and a quality factor of 35 and 24 at 100 MHz, respectively. Finite-element simulations of the inductors were performed and agreed with the measured results to within 8%. The turn-to-turn breakdown voltage was measured to be in excess of 800 V and currents as high as 0.5 A could be successfully carried by the inductor windings.

We investigated the use of thermally treated polydimethylsiloxane (PDMS) for chemically-resistant microchannels. When the PDMS underwent the thermal treatment at 300 °C, swelling was reduced and the surface of the PDMS microfluidic channel endured well in the extracting media such as dichloromethane. Furthermore, despite the small decrease in size after thermal treatment, both the channel shape and transparency were maintained without showing fluid leakage. The thermally treated PDMS had more hydrophilic properties compared to the untreated PDMS. A single step post-casting process described in this work does not require complex chemical treatments or introduction of foreign materials to the host PDMS substrate, thus expanding the application area of PDMS-based microfluidics.

We characterize and quantify the performance of ultrasonic particle aggregation and positioning in a 100-well microplate. We analyze the result when operating a planar ultrasonic ring transducer at different single actuation frequencies in the range 2.20–2.40 MHz, and compare with the result obtained from different schemes of frequency-modulated actuation. Compared to our previously used wedge transducer design, the ring transducer has a larger contact area facing the microplate, resulting in lower temperature increase for a given actuation voltage. Furthermore, we analyze the dynamics of acoustic streaming occurring simultaneously with the particle trapping in the wells of the microplate, and we define an adaptive ultrasonic actuation scheme for optimizing both efficiency and robustness of the method. The device is designed as a tool for ultrasound-mediated cell aggregation and positioning. This is a method for high-resolution optical characterization of time-dependent cellular processes at the level of single cells. In this paper, we demonstrate how to operate our device in order to optimize the scanning time of 3D confocal microscopy with the aim to perform high-resolution time-lapse imaging of cells or cell–cell interactions in a highly parallel manner.

We demonstrate the microassembly of PDMS (polydimethylsiloxane) microfluidics with integrated circuits made in complementary metal-oxide-semiconductor (CMOS) processes. CMOS-sized chips are flip chip bonded to a flexible polyimide printed circuit board (PCB) with commercially available solder paste patterned using a SU-8 epoxy. The average resistance of each flip chip bond is negligible and all connections are electrically isolated. PDMS is attached to the flexible polyimide PCB using a combination of oxygen plasma treatment and chemical bonding with 3-aminopropyltriethoxysilane. The total device has a burst pressure of 175 kPA which is limited by the strength of the flip chip attachment. This technique allows the sensor area of the die to act as the bottom of the microfluidic channel. The SU-8 provides a barrier between the pad ring (electrical interface) and the fluids; post-processing is not required on the CMOS die. This assembly method shows great promise for developing analytic systems which combine the strengths of microelectronics and microfluidics into one device.

Biologically inspired sensor-designs are investigated as a possible path to surpass the performance of more traditionally engineered designs. Inspired by crickets, artificial hair sensors have shown the ability to detect minute flow signals. This paper addresses developments in the design, fabrication, interfacing and characterization of biomimetic hair flow-sensors towards sensitive high-density arrays. Improvement of the electrode design of the hair sensors has resulted in a reduction of the smallest hair movements that can be measured. In comparison to the arrayed hairs-sensor design, the detection-limit was arguably improved at least twelve-fold, down to 1 mm s^–1 airflow amplitude at 250 Hz as measured in a bandwidth of 3 kHz. The directivity pattern closely resembles a figure-of-eight. These sensitive hair-sensors open possibilities for high-resolution spatio-temporal flow pattern observations.

This paper presents a micromachined stress-free through silicon via (TSV) backend process for AlGaN/GaN-on-Si (1 1 1) platform-based devices, which was processed by assisted back grinding, chemical mechanical polishing, deep reactive ion etching (DRIE) and copper (Cu) electroplating for the TSV. The metal-filled TSV structure was formed to enhance thermal conduction from the frontend terminal to the backend terminal, especially the source region of the AlGaN/GaN-on-Si (1 1 1) platform-based RF power devices. At the end of a stress-free TSV dry etching process, we have changed RF power 600 W to 300 W to minimize thermal stress of the fabricated TSV electrode pad structure of the AlGaN/GaN-on-Si platform-based devices. Additionally, we have sputtered a multi-metal layer and electroplated Cu metal to interconnect a topside electrode to TSV. To protect the thinned TSV electrode pad structure from water pressure in a sawing process, we have covered photoresist (AZ4330RS) of 3.3 µm thickness on the top area of the structures. We confirmed that the proposed TSV formation method assisted by low-power operation DRIE and protection of the thinned TSV surface by using the thick photoresist is very effective to create minimally stressed TSV structures in AlGaN/GaN-on-Si platform-based devices. The improvement was proved by the yield of dice without bursting the pad structures on a 4 inch AlGaN/GaN-on-Si wafer.

Microfluidic drug delivery systems consisting of a drug reservoir and microfluidic channels have shown the possibility of simple and robust modulation of drug release rate. However, the difficulty of loading a small quantity of drug into drug reservoirs at a micro-scale limited further development of such systems. Electrohydrodynamic (EHD) printing was employed to fill micro-reservoirs with controlled amount of drugs in the range of a few hundreds of picograms to tens of micrograms with spatial resolution of as small as 20 µm. Unlike most EHD systems, this system was configured in combination with an inverted microscope that allows in situ targeting of drug loading at micrometer scale accuracy. Methylene blue and rhodamine B were used as model drugs in distilled water, isopropanol and a polymer solution of a biodegradable polymer and dimethyl sulfoxide (DMSO). Also tetracycline-HCl/DI water was used as actual drug ink. The optimal parameters of EHD printing to load an extremely small quantity of drug into microscale drug reservoirs were investigated by changing pumping rates, the strength of an electric field and drug concentration. This targeted EHD technique was used to load drugs into the microreservoirs of PDMS microfluidic drug delivery devices and their drug release performance was demonstrated in vitro.

This study designs and implements a proximity sensor consisting of inductive and capacitive sensing units. These two sensing units are vertically monolithic integrated on a single chip using the micro-fabrication processes. In addition, low-temperature fabricated nanoporous anodic aluminum oxide (np-AAO) is employed as the dielectric layer to enhance the performance of capacitive sensing. The characteristics of the presented vertically monolithic integrated inductive and capacitive proximity sensor are as follows: (1) enlarged sensing distance of conductive objectives: capacitive sensing unit for short distance detection and inductive sensing unit for long distance detection, (2) non-conductive object can be detected by the capacitive sensing unit, (3) fringe effect capacitive sensing is enhanced by the spiral coil electrode and (4) np-AAO has good dielectric properties (the dielectric constant is 11.9 in this study) for capacitive sensing. In application, various materials (including metal, plastic and a human finger) have been successfully detected by the presented sensor. Preliminary results demonstrate that the typical fabricated proximity sensor has a sensing range of 0.5–5 mm for the metal rod. In comparison, the inductive and capacitive sensing units have the sensing ranges of 1.5–5 and 0.5–3 mm, respectively. Moreover, the non-conductive plastic rod can be detected by the capacitive sensing unit.

The technology and preliminary qualitative tests of silicon–glass microreactors with embedded pressure and temperature sensors are presented. The concept of microreactors for leading highly exothermic reactions, e.g. nitration of hydrocarbons, and design process-included computer-aided simulations are described in detail. The silicon–glass microreactor chip consisting of two micromixers (multistream micromixer), reaction channels, cooling/heating chambers has been proposed. The microreactor chip was equipped with a set of pressure and temperature sensors and packaged. Tests of mixing quality, pressure drops in channels, heat exchange efficiency and dynamic behavior of pressure and temperature sensors were documented. Finally, two applications were described.

This paper reports on the use of electrolyte-enabled distributed transducers in a polymer-based microfluidic device for the detection of distributed static and dynamic loads. The core of the device is a polymer rectangular microstructure integrated with electrolyte-enabled distributed transducers. Distributed loads acting on the polymer microstructure are converted to different deflections along the microstructure length, which are further translated to electrical resistance changes by electrolyte-enabled distributed transducers. Owing to the great simplicity of the device configuration, a standard polymer-based fabrication process is employed to fabricate this device. With custom-built electronic circuits and custom LabVIEW programs, fabricated devices filled with two different electrolytes, 0.1 M NaCl electrolyte and 1-ethyl-3-methylimidazolium dicyanamide electrolyte, are characterized, demonstrating the capability of detecting distributed static and dynamic loads with a single device. As a result, the polymer-based microfluidic device presented in this paper is promising for offering the capability of detecting distributed static and dynamic loads in biomedical/surgical, manufacturing and robotics applications.

A new concept of atomic force microscope (AFM) oscillating probes using electrostatic excitation and piezoresistive detection is presented. The probe is characterized by electrical methods in vacuum and by mechanical methods in air. A frequency-mixing measurement technique is developed to reduce the parasitic signal floor. The probe resonance frequencies are in the 1 MHz range and the quality factor is measured about 53 000 in vacuum and 3000 in air. The ring probe is mounted onto a commercial AFM set-up and topographic images of patterned sample surfaces are obtained. The force resolution deduced from the measurements is about 10 pN Hz^−0.5.

Cu-based, single- and double-layered, microchannel heat exchangers (MHEs) were fabricated and assembled. Comparative measurements on liquid flow characteristics and heat transfer performance were conducted on these devices. Results were compared at the individual microchannel level as well as at the device level. The present results demonstrate that double-layered MHEs exhibit similar heat transfer performance while suffering a much lower pressure drop penalty compared to single-layered MHEs. Another Cu-based, double-layered, liquid–liquid counter-flow MHE was fabricated, assembled and tested. Results show that a low-volume, multilayered, high-performance, liquid-to-liquid MHE is achievable following the manufacturing protocols of the present double-layered, liquid–liquid counter-flow MHE.

This article demonstrates the manufacturing of microstructures in a thick polymer using electrostatic-induced lithography. Unlike previous work reported elsewhere, it focuses on the fabrication of structures from meso- to micro-scale. The electrostatic-induced lithography technique is proven to work with not only dc voltage but also ac voltage. Microstructures including microchannels, sinusoidal surface profile microstructures, waveguide core, microlens array and binary Fresnel zone plate have been successfully fabricated. The aspect ratio obtained for some samples is up to 4.5:1. The whole fabrication process is fast, cost-effective in terms of the simple experimental setup and no photosensitive material is needed. This process is expected to find applications in microfluidics, photonics or micro-opto-electro-mechanical systems.

A single-polarity (n-type) piezoresistive sensing chip for three-dimensional (3D) stress sensing has been microfabricated and fully calibrated. The sensing chip, which is capable of extracting the six stress components, is prototyped using bulk microfabrication techniques on a (1 1 1) crystalline silicon. A full calibration procedure employing uniaxial, thermal and hydrostatic loading has been conducted. The calibration results confirm the feasibility of our proposed technique of using single-polarity (n-type) sensing elements to develop 3D stress sensors. Preliminary testing results are presented to demonstrate the operation of our developed sensing chip.

A novel assembly approach for the integration of metal structures into polymeric microfluidic systems is described. The presented production process is completely based on a single solid-state laser source, which is used to incorporate metal foils into a polymeric multi-layer stack by laser bonding and ablation processes. Chemical reagents or glues are not required. The polymer stack contains a flexible membrane which can be used for realizing microfluidic valves and pumps. The metal-to-polymer bond was investigated for different metal foils and plasma treatments, yielding a maximum peel strength of R_ps = 1.33 N mm⁻¹. A minimum structure size of 10 µm was determined by 3D microscopy of the laser cut line. As an example application, two different metal foils were used in combination to micromachine a standardized type-T thermocouple on a polymer substrate. An additional laser process was developed which allows metal-to-metal welding in close vicinity to the polymer substrate. With this process step, the reliability of the electrical contact could be increased to survive at least 400 PCR temperature cycles at very low contact resistances.

This study presents the technological process for producing a lenticular array on quartz. With scanning immersion lithography used to define the 3D lenticular array structure on a quartz substrate, inductively coupled plasma-reactive ion etching (ICP-RIE) is able to directly etch the structure. Based on the surface measurement, ICP-RIE can completely etch the surface structure onto the quartz substrate, and the surface roughness can reach 30 nm. Furthermore, after the optical measurement of stereoscopic images, the left and the right stereoscopic images can be successfully transmitted to the corresponding places without crosstalk and with a splitting efficiency close to 80%. As a result, this technology allows for the production of a lenticular structure with various changes on the quartz surface.

We report the realization of three-dimensional free-standing structures with high complexity on silicon substrates without using surface micromachining. The etching is feasible using the reactive ion etching method in which three gases are used in a two-step etching process. The passivation step is carried out by means of hydrogen, oxygen and sulfur-hexafluoride (SF₆) while the etching step uses SF₆ in a low-density capacitive-coupled radio frequency reactor. The overall etching process can be adjusted to arrive at three-dimensional structures with desired underetchings. The formation of partially and fully suspended structures is reported. Multi-level floated structures have been realized using this method.

This paper presents an experimentally verified analytical model of temperature-dependent yield effects on the curvatures of composite beam structures used in complementary metal–oxide semiconductor microelectromechanical systems (CMOS MEMS). The temperature-dependent effects on composite beam curvatures of a thermal process can be predicted by extracting key parameters from the measured curvatures of a limited number of CMOS MEMS composite-layer combinations. The effects due to thermal history in MEMS packaging, which change the characteristics of beam curvatures due to material yield, are further analyzed. The models are verified with measured results from beam structures fabricated by an application-specific integrated circuit-compatible 0.18 µm 1P6M CMOS MEMS process using a white light interferometer. These models can be applied in electronic design automation tools to provide good prediction of temperature-dependent properties related to CMOS MEMS beam curvature, such as sensing capacitance, for monolithic sensor system on chip design.

Thermal roll-to-roll imprint lithography (R2RIL) is a simple and low-cost process for the mass production of micro/nanopatterns. However, in that it relies on highly viscous thermoplastic resists, it is limited in its ability to imprint precise patterns at a high speed. Moreover, the concentrated imprint force applied in R2RIL can damage the resist material which is structurally vulnerable at high process temperatures. Therefore, it is important to understand the temperature- and time-dependent characteristics of the resist material as well as the imprinting mechanism when using thermal R2RIL. In this work, the effects of the process temperature and rolling speed on thermal R2RIL of polycarbonate (PC) films were investigated to improve the process efficiency. Micro-scale line patterns were successfully transferred onto PC films from nickel (Ni) mold stamps. Consequently, line patterns with widths in the range of 5–80 µm were achieved at a traveling speed of 28.6 mm s^–1 and process temperature of 150 °C, which is just above the glass transition temperature (T_g). In addition, the patterning performance was investigated for different temperatures, rolling speeds and pattern sizes. The imprinted pattern profiles were measured by an alpha-step surface profiler to investigate the patterning performance. The results show that a much better imprint performance was achieved at 150 °C, compared to the result at temperatures below T_g. The physical mechanisms of thermal R2RIL on a PC film were studied by a finite-element analysis and the patterning process was successfully demonstrated by a visco-plastic deformation model.

This paper presents a novel and cost-effective method for fabricating high-density multilayer metal–insulator–metal (MIM) integrated capacitors. To eliminate the usage of numerous photolithography steps when parallel stacking multiple capacitors layers, a unique process has been developed based on depositing the MIM layers onto a substrate with two protruding pillars, polishing down the pillars to expose the multilayer cross sections and then selectively etching the metal layers on each pillar to form the alternating capacitor plate electrodes. For demonstration purpose, only capacitors with two dielectric layers were fabricated, and the measurement results were verified by a compact analytical model together with finite element simulations. With 200 nm thick silicon nitride/oxide dielectric layers, a capacitance density of 0.6 fF µm⁻² was achieved, which can be easily increased by scaling down the layer thicknesses and/or stacking more layers. A low equivalent series resistance (ESR) of 300–700 mΩ was measured, and the self-resonance frequency was above measurement limits (>100 MHz). Further design optimization shows that the ESR can be reduced to below 80 mΩ, while the operation frequency extended to above 2.6 GHz.

We present a microfluidic-based injection system designed to achieve intracellular delivery of macromolecules by directing a picoliter jet of a solution toward the individual cells. After discussing the concept, we present design specification and criteria, elucidate performance and discuss results. The method has the potential to be quantitative and of high throughput, overcoming the limitations of current intracellular delivery protocols.

In this study, we propose a triaxial force measurement sensor probe with piezoresistors fabricated via sidewall doping using rapid thermal diffusion. The device was developed as a tool for measuring micronewton-level forces as vector quantities. The device consists of a 15 µm thick cantilever, two sensing beams and four wiring beams. The length and width of the cantilever are 1240 µm and 140 µm, respectively, with a beam span of 1200 µm and a width of 10–15 µm. The piezoresistors are formed at the root of the cantilever and the sidewalls of the two sensing beams. The sensor spring constants for each axis were measured at k_x = 1.5 N m⁻¹, k_y = 3.5 N m⁻¹ and k_z = 0.64 N m⁻¹. We confirmed that our device was capable of measuring triaxial forces with a minimum detectable force at the submicronewton level.

A micro-electro-mechanical systems (MEMS) vertical levitation comb drive actuator has been created for the measurement of piezoelectric coefficients in thin/thick films or piezoelectrically active micro-scale components of other MEMS devices. The device exerts a dynamic force of 33 μN at an applied voltage of 100 V. The charge developed on the piezoelectric test device is measured using a charge sensitive pre-amplifier and lock-in technique, enabling measurements down to 1×10⁻⁵ pC. The system was tested with ten different piezoelectric samples with coefficients in the range 70–1375 pC N⁻¹ and showed a good correlation (r = 0.9997) to measurements performed with macroscopic applied stresses, and piezoelectric impedance resonance techniques. The measurement of the direct piezoelectric effect in micro- and nano-scale piezo-materials has been made possible using MEMS processing technology. This new application of a MEMS metrology device has been developed and fully characterized in order to accurately evaluate the functional properties of piezoelectric materials at the scale required in micro- to nano-scale applications.

In this paper, a micromachined energy harvester employing a lead-free (K, Na)NbO₃ (KNN) thin film was reported. KNN is one of the lead-free piezoelectric materials. It is a promising alternative to Pb(Zr, Ti)O₃ (PZT). In a micromachined energy harvester, a 2 µm thick KNN film was deposited on a silicon cantilever integrated with a proof mass. The energy harvester achieved an output power of 731 nW and a normalized power density (NPD) of 2.29 μW (g⁻² mm⁻³) at the resonant frequency of 1509 Hz with the acceleration of 10 m s⁻². The harvested energy was enhanced considerably by applying bulk micromachining, and was comparable to the PZT-based energy harvesters.

This study describes the replication of nano/micro-scale features using a Pd₄₀Ni₄₀P₂₀ bulk metallic glass (BMG) mold prepared using a femtosecond laser and nanoimprinting process. The use of the beam shaper feature of the femtosecond laser enabled the rapid fabrication of periodic nanostripes over an area of ∼5 × 4 mm² on the BMG mold following a single pulse of irradiation. The ablation pitch of the nanostructure irradiated with 100 mW of femtosecond laser power was determined to be 175.8 nm. The imprinting results demonstrate the applicability of Pd-based BMG in the replication of mold features ranging from 100 µm to 90 nm. Additionally, Pd-based BMG can itself be used as a mold to transfer features onto Au-based BMG and polydimethylsiloxane, where the results could be used to ascertain the workability of BMG for molding in a nano/micro-imprint process.

We report on the fabrication, mechanical characterization and electrostatic actuation of dielectric STO(0 0 1) thin film microcantilevers (MCs). Finite element analysis (FEA) is used for mechanical analysis and for calculating the distribution and the magnitude of the dielectric forces on the actual devices. The actuation of insulating oxide microstructures is of potential interest in the field of ferroelectric/multiferroic materials as well as for developing novel detecting schemes on dielectric oxides.

This paper describes fatigue characteristics of a polycrystalline silicon thin-film membrane under different humidity evaluated by out-of-plane resonant vibration. The membrane, without the surface of sidewalls by patterning of photolithography and etching process, was applied to evaluate fatigue characteristics precisely against the changes in the surrounding humidity owing to narrower deviation in the fatigue lifetime. The membrane has 16 mm square-shaped multilayered films consisting of a 250 or 500 nm thick polysilicon film on silicon dioxide and silicon nitride underlying layers. A circular weight of 12 mm in diameter was placed at the center of the membrane to control the resonant frequency. Stress on the polysilicon film was generated by deforming the membrane oscillating the weight in the out-of-plane direction. The polysilicon film was fractured by fatigue damage accumulation under cyclic stress. The lifetime of the polysilicon membrane extended with lower relative humidity, especially at 5%RH. The results of the fatigue tests were well formulated with Weibull's statistics and Paris' law. The dependence of fatigue characteristics on humidity has been quantitatively revealed for the first time. The crack growth rate indicated by the fatigue index decreased with the reduction in humidity, whereas the deviation of strength represented by the Weibull modulus was nearly constant against humidity.

In recent years, microstructures have been widely applied in many key optical elements and bio-elements. The effective and efficient fabrication of optical elements and bio-elements with superior performance has become an essential challenge. This requires very accurate shape replication of microstructures. The plate-to-plate hot embossing process is the most likely method of mass production for the replication of double-sided micro/nano structures with high precision and quality. However, the traditional uniform heating hot embossing process as the free boundary of open die forging leads to variation. In this research, three techniques are implemented; the conventional uniform heating technique, the non-uniform pressure compensating technique and the fixed boundary hot embossing technique. The temperature distribution of the hot-plates of the fixed boundary hot embossing technique are designed to keep the temperature in the center part higher than the outer part on the surface of the substrates. This phenomenon changes free boundary in conventional uniform heating into fixed boundary. The results demonstrate the potential of the fixed boundary hot embossing technique for the fabrication of large-area high brightness LGPs with double-sided microstructures. The results are also helpful for enhancing the performance of optical elements and bio-elements fabricated using the fixed boundary hot embossing technique.

Capillary forces in microfluidics provide a simple yet elegant means to direct liquids through flow channel networks. The ability to manipulate the flow in a truly automated manner has proven more problematic. The majority of valves require some form of flow control devices, which are manually, mechanically or electrically driven. Most demonstrated capillary systems have been manufactured by photolithography, which, despite its high precision and repeatability, can be labour intensive, requires a clean room environment and the use of fixed photomasks, limiting thereby the agility of the manufacturing process to readily examine alternative designs. In this paper, we describe a robust and rapid CO₂ laser manufacturing process and demonstrate a range of capillary-driven microfluidic valve structures embedded within a microfluidic network. The manufacturing process described allows for advanced control and manipulation of fluids such that flow can be halted, triggered and delayed based on simple geometrical alterations to a given microchannel. The rapid prototyping methodology has been employed with PMMA substrates and a complete device has been created, ready for use, within 2–3 h. We believe that this agile manufacturing process can be applied to produce a range of complex autonomous fluidic platforms and allows subsequent designs to be rapidly explored.

We present a novel valving mechanism to implement plasma extraction from whole blood on a centrifugal microfluidic 'lab-on-a-disc' platform. The new scheme is based on pressure-induced deflection of a liquid membrane which gates the centrifugally driven flow through a microfluidic structure. Compared to conventional concepts such as capillary burst valves, siphons or sacrificial materials, the valving structure presented here is represented by a compact, small-footprint design which obviates the need for (local) surface functionalization or hybrid materials integration, thus significantly reducing the complexity (and hence cost) of manufacture. As a pilot study of this new centrifugal flow control element, we demonstrate here the efficient separation of metered plasma from whole blood. While the flow is stopped, blood is separated into plasma and its cellular constituents by centrifugally induced sedimentation. After completion, the flow resumes by elevating the spinning frequency, providing up to 80% of the original plasma content to an overflow chamber within a short, 2 min interval. The amount of residual cells in the plasma amounts to less than 20 cells μl⁻¹.

In this paper, we report our efforts toward building a microelectromechanical system-based micropump. The micropump is driven by a wettability gradient and used to transport discrete drops. The gradient in wettability is distributed axisymmetrically, with hydrophobicity of the micropump surface decreasing radially toward the center. Both physical and chemical properties of the surface are altered to obtain the wettability gradient needed for driving the drops. The surface of the micropump is, first, patterned with pre-designed micro-features that define the roughness of the surface and, then, coated with a low-energy interface film. Results show that drops deposited on the surface of the micropump move, in a directional way, along the wettability gradient. The average velocity of the deposited drops is 5 mm s⁻¹. Measured contact angles decrease gradually from 157.0° to 124.2° toward the center of the micropump surface. Maximum driving force exerted by the solid surface on the drops is 12.82 µN. The average size of the drops transported on the surface of the micropump is 2 µL.

This work presents an SU-8 surface micromachining process using amorphous silicon as a sacrificial material, which also incorporates two metal layers for electrical excitation. SU-8 is a photo-patternable polymer that is used as a structural layer for MEMS and microfluidic applications due to its mechanical properties, biocompatibility and low cost. Amorphous silicon is used as a sacrificial layer in MEMS applications because it can be deposited in large thicknesses, and can be released in a dry method using XeF₂, which alleviates release-based stiction problems related to MEMS applications. In this work, an SU-8 MEMS process was developed using α-Si as a sacrificial layer. Two conductive metal electrodes were integrated in this process to allow out-of-plane electrostatic actuation for applications like MEMS switches and variable capacitors. In order to facilitate more flexibility for MEMS designers, the process can fabricate dimples that can be conductive or nonconductive. Additionally, this SU-8 process can fabricate SU-8 MEMS structures of a single layer of two different thicknesses. Process parameters were optimized for two sets of thicknesses: thin (5–10 µm) and thick (130 µm). The process was tested fabricating MEMS switches, capacitors and thermal actuators.

A simple methodology to prepare sub-100 nm resist nanopatterns with a high aspect ratio for the transfer of device nanofeatures is demonstrated. The novel method is based on a two- or multi-step developing process with the incorporation of an ∼4 nm thick metal film to protect the fine resist nanopatterns in the developer solution. Using this approach, sub-100 nm resist nanopatterns of different shapes were readily fabricated using the positive- and negative-tone electron-beam resists. Subsequently, fine device nanostructures could be readily converted from these fine resist nanopatterns with a high aspect ratio.

Vertically aligned silicon micropillar arrays have been created by deep reactive ion etching (DRIE) and used for a number of microfabricated devices including microfluidic devices, micropreconcentrators and photovoltaic cells. This paper delineates an experimental design performed on the Bosch process of DRIE of micropillar arrays. The arrays are fabricated with direct-write optical lithography without photomask, and the effects of DRIE process parameters, including etch cycle time, passivation cycle time, platen power and coil power on profile angle, scallop depth and scallop peak-to-peak distance are studied by statistical design of experiments. Scanning electron microscope images are used for measuring the resultant profile angles and characterizing the scalloping effect on the pillar sidewalls. The experimental results indicate the effects of the determining factors, etch cycle time, passivation cycle time and platen power, on the micropillar profile angles and scallop depths. An optimized DRIE process recipe for creating nearly 90° and smooth surface (invisible scalloping) has been obtained as a result of the statistical design of experiments.

In this paper, a simple process is suggested to estimate the interfacial toughness of the material system 'aluminum film/soft PDMS substrate'. The specimen, i.e. the aluminum film deposited on the soft polydimethylsiloxane (PDMS) substrate, is subject to a tensile load, and delaminating and buckling of aluminum film are observed in the perpendicular direction to the tensile strain. With the aid of the buckling blisters, the interfacial toughness of the material system is estimated. Large deformation is considered during the buckling of the thin film, and the interfacial toughness is deduced from a fracture theory. Besides, the evolution from one single blister to three blisters and then four blisters is observed in situ under microscope. This simplified method has potential applications to flexible electronics in which interfacial toughness of the metal film/soft substrate must be well controlled.

This work presents the experimental analysis of heat transfer in a straight polydimethylsiloxane microchannel with cross-sectional width/depth ratio of 8.47 using the temperature-sensitive paint (TSP) technique. The TSP technique is capable of acquiring temperature profiles for both the surface and the fluid with a detailed resolution of 144 µm per pixel through the microchannel under the constant wall temperature condition. The acquired temperature data have been analyzed to study the heat transfer and Nusselt numbers in both thermal developing and fully developed regions. The estimation of thermal entrance length from the experimental results shows good agreement with the empirical equation of the parallel-plate system, while the Nusselt numbers in the fully developed region deviate. This deviation is attributed to the small scale of the channel, which amplifies the effects of the electric double layer and the surface boundary condition. These results demonstrate the great potential of the TSP technique in providing detailed and valuable information for heat transfer investigation inside microdevices.

In this work, the bonding strength of microchips fabricated by thiol-ene free-radical polymerization was characterized in detail by varying the monomeric thiol/allyl composition from the stoichiometric ratio (1:1) up to 100% excess of thiol (2:1) or allyl (1:2) functional groups. Four different thiol-ene to thiol-ene bonding combinations were tested by bonding: (i) two stoichiometric layers, (ii) two layers bearing complementary excess of thiols and allyls, (iii) two layers both bearing excess of thiols, or (iv) two layers both bearing excess of allyls. The results showed that the stiffness of the cross-linked polymer plays the most crucial role regarding the bonding strength. The most rigid polymer layers were obtained by using the stoichiometric composition or an excess of allyls, and thus, the bonding combinations (i) and (iv) withstood the highest pressures (up to the cut-off value of 6 bar). On the other hand, excess of thiol monomers yielded more elastic polymer layers and thus decreased the pressure tolerance for bonding combinations (ii) and (iii). By using monomers with more thiol groups (e.g. tetrathiol versus trithiol), a higher cross-linking ratio, and thus, greater stiffness was obtained. Surface characterization by infrared spectroscopy confirmed that the changes in the monomeric thiol/allyl composition were also reflected in the surface chemistry. The flexibility of being able to bond different types of thiol-enes together allows for tuning of the surface chemistry to yield the desired properties for each application. Here, a capillary electrophoresis separation is performed to demonstrate the attractive properties of stoichiometric thiol-ene microchips.