Table of contents

SU-8 has become the favourite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. However, as feature sizes get smaller and pattern complexity increases, particular difficulties and a number of material-related issues arise and need to be carefully considered. This review presents a detailed description of these effects and describes reported strategies and achieved SU-8 HAR and 3D structures up to August 2006.

Microinjection molding (µIM) appears to be one of the most efficient processes for the large-scale production of thermoplastic polymer microparts. The microinjection molding process is not just a scaling down of the conventional injection process; it requires a rethinking of each part of the process. This review proposes a comparative description of each step of the microinjection molding process (µIM) with conventional injection molding (IM). Micromolding machines have been developed since the 1990s and a comparison between the existing ones is made. The techniques used for the realization of mold inserts are presented, such as lithography process (LIGA), laser micromachining and micro electrical discharge machining (µEDM). Regarding the molding step, the variotherm equipment used for the temperature variation is presented and the problems to solve for each molding phase are listed. Throughout this review, the differences existing between µIM and conventional molding are highlighted.

The paper aims at developing a wafer-level testing method to examine Young's modulus and residual stress of structural materials of capacitive micro-devices by detecting the pull-in voltages of micro test beams made of the materials to be tested. We derive a formula that correlates the Young's modulus, residual stress and pull-in voltage of the micro test beams. The analytical model considers the fringing field capacitance, the distributed characteristics of the micro test beams and the electromechanical coupling effect. By the present method, one can extract Young's modulus and residual stress simultaneously by detecting the pull-in voltages of two test beams of different lengths. Three common structural materials used in micro-devices are demonstrated: mono-crystalline silicon, poly-silicon and aluminum. The extracted Young's moduli and residual stresses agree very well with the experimental measurement. The present method is expected to be applicable to the wafer-level testing in MEMS device manufacture and compatible with the wafer-level testing in IC industry since the test and the pickup signals are both electrical.

This paper presents a laser-micromachined polymeric membraneless fuel cell. The membraneless fuel cell, constructed with three polymethyl methacrylate (PMMA) layers, takes advantage of two laminar flows in a single micro channel to keep the fuel and oxidant streams separated yet in diffusional contact. Laser micromachining was employed to make the flow channel and electrode substrate based on PMMA. The anode and cathode electrodes were fabricated by wet-spraying catalyst inks onto the gold-coated PMMA substrate. The packed fuel cell has been electrochemically characterized by an electrochemical analyser. The membraneless fuel cell works stably with Reynolds numbers ranging from 7.65 to 30.6. At room temperature, the laminar-flow-based micro membraneless fuel cell can reach a maximum power density of 0.58 mW cm⁻² with 0.5 M HCOOH in 0.1 M H₂SO₄ solution as fuel and O₂ saturated 0.1 M H₂SO₄ solution as oxidant. When 0.01 M H₂O₂ in 0.1 M H₂SO₄ solution is used as oxidant, a maximum power density of 1.98 mW cm⁻² is obtained. The paper reports for the first time the use of hydrogen peroxide in sulfuric acid as the oxidant. The new oxidant composition allows a simple recycling process and better fuel utilization.

This paper presents a novel method of thickness measurement for microelectromechanical system (MEMS) structures using micro-Raman spectroscopy. When heating by a constant power laser, the local temperature rise of a microscale structure depends on the thickness and thermal characteristics of the structure. The thickness information can then be evaluated by the temperature induced Raman shift. Theoretical analysis and simulation of this method are performed. The small spot size of the laser in micro-Raman spectroscopy enables thickness measurement with a high spatial resolution. This measurement method is confirmed by measuring the thickness of a MEMS single-crystalline silicon (c-silicon) membrane. The measurement result also consists of that of scanning electron microscopy (SEM) for the same sample. It has the advantages of being a non-contact and nondestructive process, no preparation, and spatial mapping aspects. The proposed method is also feasible for materials with a temperature sensitive Raman signal.

This study reports a new microfluidic device capable of fine-tuned sample-flow focusing and generation of micro-droplets in liquids by controlling moving wall structures. Two microfluidic components including an 'active microchannel width controller' and a 'micro chopper' can be used to fine-tune the width of the hydrodynamically pre-focused stream and subsequently generate micro-droplets. In this study, a basic concept of a 'controllable moving wall' structure was addressed and applied as the active microchannel width controller and the micro chopper to generate the proposed function. Pneumatic side chambers were placed next to a main flow channel to construct the controllable moving wall structures. The deformation of the controllable moving wall structure can be generated by the pressurized air injected into the pneumatic side chambers. The proposed chip device was fabricated utilizing polymer material such as PDMS (polydimethylsiloxane) to provide the flexibility of the controllable moving wall deformation. The microfluidic chip device with dimensions of 2.5 cm in width and 3.0 cm in length can be fabricated using a simple fabrication process. Experimental data showed that the deformation of the controllable moving wall structure can be adjusted by applying different air pressures, so that the width of the main flow channel can be controlled accordingly. By utilizing the proposed mechanism, the pre-focused dispersed phase stream could be actively focused into a narrower stream, and well-controlled micro-droplets with smaller diameters could be generated. The stream width can be reduced from 30 µm to 9 µm and micro-droplets with a diameter of 76 µm could be generated by utilizing the proposed device. In addition, to generate micro-droplets within smaller diameters, uniform size distribution of the micro-droplets can be obtained. According to the experimental results, development of the microfluidic device could be promising for a variety of applications such as emulsification, nano-medicine and droplet-based microfluidics.

A manufacturing process of micro nickel/diamond abrasive pellet array lapping tools using a LIGA-like technology is reported here. The thickness of JSR THB-151N resist coated on an aluminum alloy substrate for micro lithography can reach up to 110 µm. During the lithography, different geometrical photomasks were used to create specific design patterns of the resist mold on the substrate. Micro roots, made by electrolytic machining on the substrate with guidance of the resist mold, can improve the adhesion of micro nickel abrasive pellets electroplated on the substrate. During the composite electroforming, the desired hardness of the nickel matrix inside the micro diamond abrasive pellets can be obtained by the addition of leveling and stress reducing agents. At moderate blade agitation and ultrasonic oscillation, higher concentration and more uniform dispersion of diamond powders deposited in the nickel matrix can be achieved. With these optimal experiment conditions of this fabrication process, the production of micro nickel/diamond abrasive pellet array lapping tools is demonstrated.

This paper presents a total internal reflection based chip which generates evanescent waves for highly sensitive fluorescent imaging. The chip is monolithically, massively cast in polydimethylsiloxane (PDMS) at a very low cost using a Si mold fabricated by Si anisotropic wet etching and deep reactive ion etching (DRIE). Our method integrates all miniaturized optical components, namely cylindrical microlens, prism and waveguide, into one monolithic PDMS chip; thus assembly is unnecessary, and misalignment is eliminated. The slide-format and monolithic chip can be used with both upright and inverted fluorescent microscopes with flexible sample delivery platforms. The flexibility of sample delivery platforms facilitates various surface treatment/immobilization techniques required in fluorescent imaging. Moreover, the fiberoptics coupling into the chip allows a broad choice of wavelengths and types of laser sources ranging from UV to IR. We have successfully demonstrated the capability of the chip in highly sensitive imaging of tetramethylrhodamine (TMR) fluorescent dye and immobilized fluorescent nanobeads. Our monolithic, miniaturized TIR-based chip could potentially serve as an evanescent excitation-based platform integrated into a micro-total analysis system (μ-TAS).

A novel UV-curable low-stress hyperbranched polymer (HBP) micromolding process is presented for the fast and low-temperature fabrication of hydrophilic microfluidic devices. Process, material and surface properties of the acrylated polyether HBP are also characterized and compared to those of polydimethylsiloxane (PDMS) and cyclic olefin copolymers (COC). The HBP dispensed on a PDMS master was cured at room temperature using a 3 min UV exposure at the intensity of 22.2 mW cm⁻². Thermal, mechanical and surface properties of the micromolded HBP structures have been characterized and resulted in a glass transition temperature of 55 °C, Young's modulus of 770 MPa and hydrophilic surface having a water contact angle of 54°. Micromolding of 33 µm thick HBP microstructures has been demonstrated. We achieved 14.5 µm wide vertical walls, 14.7 µm wide fluidic channels, 24.1 µm wide square pillars and 53.4 µm wide square holes. A microfluidic network device, composed of microfluidic channels and reservoirs, was fabricated and its microfluidic performance has been verified by a fluidic test.

This paper presents the design, kinematics, fabrication and characterization of a monolithic micro positioning two degree-of-freedom translational (XY) stage. The design of the proposed MEMS (micro-electro-mechanical system) stage is based on a parallel kinematics mechanism (PKM). The stage is fabricated on a silicon-on-insulator (SOI) substrate. The PKM design decouples the motion in the XY directions. The design restricts rotations in the XY plane while allowing for an increased motion range and produces linear kinematics in the operating region (or workspace) of the stage. The truss-like structure of the PKM also results in increased stiffness by reducing the mass of the stage. The stage is fabricated on a silicon-on-insulator (SOI) wafer using surface micromachining and a deep reactive ion etching (DRIE) process. Two sets of electrostatic linear comb drives are used to actuate the stage mechanism in the X and Y directions. The fabricated stage provides a motion range of more than 15 µm in each direction at a driving voltage of 45 V. The resonant frequency of the stage under ambient conditions is 960 Hz. A high Q factor (∼100) is achieved from this parallel kinematics mechanism design.

A novel platform is developed to study the mechanotransduction of cardiomyocytes. After the cardiomyocytes are seeded on 10 µm thin, dome-shaped membranes with different sizes, their morphology and beating frequency are monitored. While the cells seeded on a larger membrane (800 µm × 800 µm) grow normally, cells on a smaller membrane (200 µm × 200 µm) grow in isolated groups, showing a lumped morphology. The beating frequency of the cells on the smaller membrane was faster than on the larger membrane. This intriguing phenomenon may be explained by the mechanical stress determined by the area of the membrane that affects the opening of ion channels.

Aiming at topographical surface micromachining of metallic and in particular Ti surfaces, we have investigated the possibility of using robust SU-8 and more recent electrophoretic photoresist technology in combination with flexible, non-contact maskless UV and electron beam lithography for mask generation in a through-mask electrochemical micromachining process. The shape evolution during diffusion controlled isotropic metal dissolution under circular mask openings of different sizes is in good agreement with numerical simulations, the cavities approaching hemispherical shape before their diameter reaches three times the initial mask opening diameter. Taking advantage of an important size effect in current density in combination with the developed methods, it was for the first time possible to create topographical gradient structures in a single micromachining step, difficult or impossible to achieve with competing techniques. Via through-mask anodization a locally confined nano-topography could be created within Ti surfaces. We further present an original method capable of electrochemical micromachining of highly curved conductive surfaces through electrophoretically deposited photoresist patterned by maskless 3D-UV lithography, going beyond the limitations of existing approaches.

The shrinking critical dimensions of modern technology place heavy requirements on optimizing feature shapes at the micro and nano scale. Ion beams are increasingly used for technology development in the nano-scale world. In recent years, many approaches and research results have indicated that re-deposition of sputtered target atoms is the most serious problem in fabricating micro and nano devices. A simulation tool is essential to reduce unnecessary time and efforts spent in process development. In this paper, we present two-dimensional string-based simulation software for ion milling and focused ion beam direct fabrication, AMADEUS-2D (advanced modeling and design environment for sputter processes). We discuss the numerical model for considering sputtering and re-deposition fluxes. In addition, we investigate sputtering yield and sputtered atom distributions as obtained from several binary collision simulation codes. The newly developed simulation code is validated by comparison with experimental data on single-pixel hole milling, on the width and dose dependence of trench formation and on the effective sputtering yield as a function of scan speed.

This paper presents a novel process for fabricating out-of-plane microneedle sheets of biocompatible polymer using in-plane microneedles. This process comprises four steps: (1) fabrication of in-plane microneedles using inclined UV lithography and electroforming, (2) conversion of the in-plane microneedles to an out-of-plane microneedle array, (3) fabrication of a negative PDMS mold and (4) fabrication of out-of-plane microneedle sheets of biocompatible polymer by hot embossing. The in-plane microneedles are fabricated with a sharp tip for low insertion forces and are made long to ensure sufficient penetration depth. The in-plane microneedles are converted into an out-of-plane microneedle array to increase the needle density. The negative mold is fabricated for mass-production using a polymer molding technique. The final out-of-plane microneedle sheets are produced using polycarbonate for biocompatibility by employing the hot embossing process. The height of the fabricated needles ranges from 500 to 1500 µm, and the distance between the needles is 500 to 2000 µm. The radii of curvature are approximately 2 µm, while the tip angles are in the range of 39–56°. Most of the geometrical characteristics of the out-of-plane microneedles can be freely controlled for real life applications such as drug delivery, cosmetic delivery and mesotherapy. Since it is also possible to mass-produce the microneedles, this novel process holds sufficient potential for applications in industrial fields.

Electrochemical printing (EcP) is a maskless solid freeform microfabrication process capable of depositing metal and alloy patterns on macroscopically large conductive substrates. The maximum plating rate achievable with EcP is dictated by the mass transfer limiting current density (i_lim). The dependence of i_lim on the dimensionless fly-height (h/d), the nozzle Reynolds number (Re) and the Schmidt number (Sc) is well known for classical impinging jet systems that employ a well-supported electrolyte and electrodes of similar size to the impinging jet nozzle. Significant ion migration and a large cathode (relative to the nozzle) make EcP mass transfer unique when compared to a classical impinging jet electrode. In this paper, we report experimentally measured limiting currents between 2 and 25 A cm⁻². The corresponding simulated limiting currents fall between 1 and 20 A cm⁻², for 14 < Re < 290, 0.28 < h/d < 0.7 and Sc = 5507. Best fits of experiments and simulations show the limiting current depends on Re^0.31±0.03 for experiments and on Re^0.48±0.01 for simulations; both fall in the expected range of Re^1/3 and Re^1/2 for Sc ≫ 1. The fly-height dependence, however, is significantly stronger than a classical impinging jet electrode due to the role of ionic migration and the unique geometry. This work provides useful engineering tools to help implement EcP.

Packaging is an emerging technology for microsystem integration. The silicon-on-insulator (SOI) wafer has been extensively employed for micromachined devices for its reliable fabrication steps and robust structures. This research reports a packaging approach for silicon-on- insulator-micro-electro-mechanical system (SOI-MEMS) devices using through-wafer vias and anodic bonding technologies. Through-wafer vias are embedded inside the SOI wafers, and are realized using laser drilling and electroplating. These vias provide electrical signal paths to the MEMS device, while isolating MEMS devices from the outer environment. A high-strength hermetic sealing is then achieved after anodic bonding of the through-wafer-vias-embedded SOI wafer to a Pyrex 7740 glass. Moreover, the packaged SOI-MEMS chip is compatible with surface mount technology, and provides a superior way for 3D heterogeneous integration.

A silicon micromachined high-shock accelerometer with a bonded hinge structure is presented. The sensitivity of this accelerometer can be adjusted by changing the dimensions of the sensing beams, and the resonance frequency is mainly decided by the hinges. Based on simulation results of the finite element method, design parameters are obtained for a device with a resonance frequency of 573 kHz and large range of 200 000g. The shock accelerometer is fabricated by advanced silicon bulk micromachining technology, including deep-reactive ion etching and silicon–silicon bonding technology. The primary performance of the accelerometer is examined by a free dropping-bar system. The results of the shock tests show that the accelerometer has a sensitivity of 0.516 µV g⁻¹ for a 44 614g shock acceleration under the 5 V excitation.