Micro-electroforming metallic bipolar electrodes for mini-DMFC stacks

This paper describes the development of metallic bipolar plate fabrication using micro-electroforming process for mini-DMFC (direct methanol fuel cell) stacks. Ultraviolet (UV) lithography was used to define micro-fluidic channels using a photomask and exposure process. Micro-fluidic channels mold with 300 μm thick and 500 μm wide were firstly fabricated in a negative photoresist onto a stainless steel plate. Copper micro-electroforming was used to replicate the micro-fluidic channels mold. Following by sputtering silver (Ag) with 1.2 μm thick, the metallic bipolar plates were completed. The silver layer is used for corrosive resistance. The completed mini-DMFC stack is a 3.5 × 3.5 cm² fuel cell stack including a 1.5 × 1.5 cm² MEA (membrane electrode assembly). Several MEAs were assembly into mini-DMFC stacks using the completed metallic bipolar plates. All test results showed the metallic bipolar plates suitable for mini-DMFC stacks. The maximum output power density is 9.3 mW/cm² and current density is 100 mA/cm² when using 8 vol.% methanol as fuel and operated at temperature 30°C. The output power result is similar to other reports by using conventional graphite bipolar plates. However, conventional graphite bipolar plates have certain difficulty to be machined to such micro-fluidic channels. The proposed micro-electroforming metallic bipolar plates are feasible to miniaturize DMFC stacks for further portable 3C applications.     

Fabrication of photoplastic high-aspect ratio microparts and micromolds using SU-8 UV resist

 An innovative method for fabrication and rapid prototyping of high-aspect ratio micromechanical components in photoresist is discussed. The photoresist is an epoxy-negative-tone resist, called SU-8, which can be structured to more than 2 mm in thickness by UV exposure. Small gears of 530 μm in diameter and 200 μm in thickness have been realized in this photoplastic and their functionality has been demonstrated. In addition a process called MIMOTEC^TM (MIcroMOlds TEChnology) has been established for the fabrication of metallic micromolds. MIMOTEC^TM is based on the use of the SU-8 spun on high thicknesses and electrodeposition of nickel. Thermoplastic microcomponents have been injected and mounted in watches. Received: 25 August 1997/Accepted: 23 October 1997     

A highly uniform lamination micromixer with wedge shaped inlet channels for time resolved infrared spectroscopy

We present a horizontal multi-lamination micromixer with specially wedge shaped vertical fluid inlets for fast and highly uniform fluid mixing in the low millisecond range. The four-layer laminar flow is created by a fluidic distribution network, reducing the amount of fluid connectors to the macroscopic world to two. All the geometries of the channel inlets and the distribution network were optimized for low flow rates and hence for low sample consumption using CFD simulations. The device materials applied feature low absorption in the mid-infrared (wavelength 3--10 μm) allowing to use this device for time resolved infrared spectroscopy. The micromixer itself can be built by silicon micromachining techniques without the need of complicated fabrication steps. Due to a transparent calcium fluoride cover optical measurements are possible as well which were used to characterize the device. Mixing times in the range of 1 ms with optical color measurements of aqueous solutions and with time resolved infrared measurement of the proton exchange reaction of H₂O and D₂O are achieved.     

A sub-micron metallic electrothermal gripper

We report the design, fabrication, and characterization of a multiple bent beam, sub-micron metallic electrothermal gripper. A bottom electroplating mold for electrodes was patterned using electron beam lithography in an SU-8, followed by nickel electroplating. A top electroplating mold for a sub-micron metallic gripper with high aspect ratio bent beams (thickness of 1 μm, width of 350 nm) was prepared using electron beam lithography in a polymethyl methacrylate (PMMA), followed by nickel electroplating and dry release of the top and bottom molds. The sub-micron gripper was characterized using a nanomanipulator system installed in a dual column scanning electron microscopy/focused ion beam system. The ability of the jaw to close up to 1.39 μm displacement with high precision and reliability has been reproducibly observed at an applied current of 28 mA, corresponding to the maximum power consumption of 11.2 mW. Finite element modeling displacement results performed using ANSYS for effective bent beam widths of 370 nm showed a good agreement with the measured displacement results. The sub-micron gripper demonstrated herein will enable the reproducible manipulations with nano-scale resolution displacement and could provide an effective means of interface between nano-scale objects and the micro/macro scale robotic systems.     

Manufacturing costs for microsystems/MEMS using high aspect ratio microfabrication techniques

The emphasis on high aspect ratio micromachining techniques for microsystems/MEMS has been mainly to achieve novel devices with, for example, high sensing or actuation performance. Often these utilize deep structures (100–1,000 μm) with vertical wall layers but with relatively modest spatial resolution (1–10 μm). As these techniques move from research to industrial manufacture, the capital cost of the equipment and the cost of device manufacture become important, particularly where more than one micromachining technique can meet the performance requirements. This paper investigates the layer-processing costs associated with the principal high aspect ratio micromachining techniques used in microsystems/MEMS fabrication, particularly silicon surface micromachining, wet bulk etching, wafer bonding, Deep Reactive Ion Etching, excimer laser micromachining, UV LIGA and X-ray LIGA. A cost model (MEMSCOST) has been developed which takes the financial, operational and machine-dependent parameters of the different manufacturing techniques as inputs and calculates the layer-processing costs at the wafer and chip level as a function of demand volume. The associated operational and investment costs are also calculated. Cost reductions through increases in the wafer size and decreases in chip area are investigated, and the importance of packaging costs demonstrated.     

Micromachining of passive planar micromixer on poly (methyl methacrylate) substrate with excimer laser ablation

Excimer laser ablation technique was introduced into this work to fabricate a passive planar micromixer on the PMMA substrate. T-junction shaped and width-changed S-shaped microchannels were both designed in this micromixer to enhance mixing effect. The mixing experiment of distilled water and Rhodamine B with injection flow rate of 500 and 1,500 μm/s validates the mixing effectivity of this micromixer, and indicates the feasibility of excimer laser ablation in the microfabrication of μ-TAS device.     

Roll manufacturing of flexible microfluidic devices in thin PMMA and COC foils by embossing and lamination

Microfluidics on foil is gaining momentum due to a number of advantages of employing thin films combined with the capability of cost-effective high-volume manufacturing of devices. In this work, ultra-thin, flexible Y-microreactors with microchannels of 100 μm width and 30 μm depth were fabricated in thermoplastic polymer foils. The fluidic pattern was hot roll embossed in 125 μm thick poly-methyl-methacrylate (PMMA) and 130 μm thick cyclic-olefin-copolymer (COC) films using a dry-etched microstructured silicon wafer as a flat embossing tool in a laminator. The sealing of the channels was performed with two different techniques, one based on lamination of SU8 dry film resist (DFR) and the other one based on spin-coated poly-dimethylsiloxane (PDMS). Testing of the interconnected microreactor was carried out using two dye colorant solutions to demonstrate mixing.     

A roller embossing process for rapid fabrication of microlens arrays on glass substrates

This paper reports an innovative technique for rapid fabrication of polymeric microlens arrays based on UV roller embossing process. In this method, a thin flat mold is fabricated by electroforming of nickel against a microlens master. The thin Ni mold with microlens cavities is then wrapped onto cylinder to form the roller. During rolling operation, the roller pressing and dragging the UV-curable photopolymer layer on the glass substrate through the rolling zone, the microlens array is formed. At the same time, the microlens array is cured by the UV light radiation while traveling through the rolling zone. The technique can be developed to an effective roll-to-roll process at room temperature and with low pressure. In this study, a roller embossing facility with UV exposure capacity has been designed, constructed and tested. Under the proper processing conditions, the 100×100 arrays of polymeric microlens, with a diameter of 100 μm, a pitch of 200 μm and a sag height of 21 μm can be successfully fabricated.     

Prototyping of microfluidic systems using a commercial thermoplastic elastomer

This article describes the fabrication of microfluidic networks (μFNs) from a commercially available (styrene)–(ethylene/butylene)–(styrene) (SEBS) block copolymer (BCP). The unique combination of hard and elastomeric properties provided by this material promotes high-throughput replication of fluidic structures using thermoforming technologies, while retaining the advantage of quick and easy assembly via conformal contact, as commonly achieved for devices fabricated from poly(dimethylsiloxane (PDMS). We employ Versaflex CL30, which is optically transparent, available at low cost (e.g., $2.50/Lbs), and likely to be compatible with a broad range of biological species. We demonstrate excellent fidelity in replication of fluidic structures using hot embossing lithography in conjunction with a photolithographically prepared Si/SU-8 master mold. Moreover, we introduce rapid prototyping of high-quality structures using an approach that we call soft thermoplastic lithography (STPL). Thanks to the rheological characteristics of the SEBS copolymer, STPL enables thermoforming on a heated master at temperatures around 170°C. Using this approach, replication can be completed within a very short period of time (e.g., less than 3 min) without the need of resorting to pressure- or vacuum-assisted instrumentation. Serving as a proof-of-concept, we devise a μFN that is suitable for the formation of miniaturized arrays comprising fluorescently labeled oligonucleotides and proteins on hard plastic substrates. Resultant spots are characterized by high fluorescent contrast, excellent edge definition, and uniform distribution of probes within the modified areas.     

Fabrication of three-dimensional hemispherical structures using photolithography

A photolithography technique using SU-8 and PDMS was developed to fabricate three-dimensional hemispherical structures. This technique utilized a mask-aligner and normal binary coded photomasks to generate hemispherical pits on SU-8, followed by PDMS molding to obtain an array of dome-shaped structures. Using this technique, a microfluidic device was fabricated with a patterning area that consisted of an array of 5 μm wells and dome-shaped structures with 10 μm diameter and 6 μm height. Encoded microbeads, 6 μm in size, were immobilized and patterned in the microfluidic device under flow conditions and a DNA hybridization experiment was performed to demonstrate the incorporation of encoded beads that would enable a high level of multiplexing in bioassays.     

Design and fabrication of PZT microcantilevers with freestanding structure

For developing freestanding piezoelectric microcantilevers with low resonant frequency, some critical mechanical considerations, especially cantilever bending, were given in this study. Two strategies, using piezoelectric thick films and adding a stress compensation layer, were calculationally analyzed for mitigating the cantilever bending, and then was applied for the fabrication of PZT freestanding microcantilevers. (100) oriented PZT thick films with the thickness of 6.93 μm were grown on the Pt/SiO₂/Si substrate by chemical solution deposition (CSD), and the SiO₂ layer with the thickness of 1.0 μm was kept under the PZT layer as a stress compensation layer of the freestanding microcantilevers. The freestanding microcantilevers fabricated with the micromachining process possessed the resonant frequency of 466.1 Hz, and demonstrated no obvious cantilever bending.     

Design and robustness analysis of structurally decoupled 3-DoF MEMS gyroscope in the presence of worst-case process tolerances

This paper presents design of a structurally decoupled 3-DoF non-resonant MEMS gyroscope with increased robustness and gain. The proposed design utilizes dynamic amplification in 2-DoF drive mode oscillator to achieve large gain. The device performance is verified through behavioral model simulations considering the fabrication limitations of standard electroplated nickel micromachining process, MetalMUMPs, of 20 μm structural layer thickness. A wide operational bandwidth of 1.74 kHz with dynamically amplified again of 0.2 μm is achieved at low actuation voltages. A design sensitivity and Monte Carlo analysis is carried out to show the robustness of the design, without any feedback control, within the fabrication process tolerances. Moreover to verify the device performance under the application of angular velocity, a rate table characterization is carried out which resulted in sense mass displacement of 30 nm corresponding to the rotation induced Coriolis force at actuation voltage of 20 V_acV_{ac} and 30 V_dcV_{dc} with angular rotation of 50 rad/s. Behavioral model simulations proved to be an cost-effective and time-saving alternative to the traditional iterative fabrications and physical level simulations.     

Design, fabrication and testing of a high performance silicon piezoresistive Z-axis accelerometer with proof mass-edge-aligned-flexures

This paper presents design, fabrication and testing of a quad beam silicon piezoresistive Z-axis accelerometer with very low cross-axis sensitivity. The accelerometer device proposed in the present work consists of a thick proof mass supported by four thin beams (also called as flexures) that are connected to an outer supporting rim. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high performance applications. In the present study, low cross-axis sensitivity is achieved by improving the device stability by placing the four flexures in line with the proof mass edges. Various modules of a finite element method based software called CoventorWare^™ was used for design optimization. Based on the simulation results, a flexure thickness of 30 μm and a diffused resistor doping concentration of 5 × 10¹⁸ atoms/cm³ were fixed to achieve a high prime-axis sensitivity of 122 μV/Vg, low cross-axis sensitivity of 27 ppm and a relatively higher bandwidth of 2.89 kHz. The designed accelerometer was realized by a complementary metal oxide semiconductor compatible bulk micromachining process using a dual doped tetra methyl ammonium hydroxide etching solution. The fabricated accelerometer devices were tested up to 13 g static acceleration using a rate table. Test results of fabricated devices with 30 μm flexure thickness show an average prime axis sensitivity of 111 μV/Vg with very low cross-axis sensitivities of 0.652 and 0.688 μV/Vg along X-axis and Y-axis, respectively.     

Fabrication and testing of embedded microvalves within PMMA microfluidic devices

The integration of a PDMS membrane within orthogonally placed PMMA microfluidic channels enables the pneumatic actuation of valves within bonded PMMA–PDMS–PMMA multilayer devices. Here, surface functionalization of PMMA substrates via acid catalyzed hydrolysis and air plasma corona treatment were investigated as possible techniques to permanently bond PMMA microfluidic channels to PDMS surfaces. FTIR and water contact angle analysis of functionalized PMMA substrates showed that air plasma corona treatment was most effective in inducing PMMA hydrophilicity. Subsequent fluidic tests showed that air plasma modified and bonded PMMA multilayer devices could withstand fluid leakage at an operational flow rate of 9 μl/min. The pneumatic actuation of the embedded PDMS membrane was observed through optical microscopy and an electrical resistance based technique. PDMS membrane actuation occurred at pneumatic pressures of as low as 10 kPa and complete valving occurred at 14 kPa for ~100 μm by 100 μm channel cross-sections.     

In-situ fabrication of polymer microsieves for ��TAS by slanted angle holography

We present a simple, versatile method for the in-situ fabrication of membranes inside a microfluidic channel during a chip manufacturing process using only two extra slanted angle holographic exposure steps. This method combines the strengths of both inclined UV exposure and holographic lithography to produce micrometer-sized three-dimensional sieving structures. Using a common chip material, the photoresist material SU-8, together with this method, a leak-free membrane-channel connection is obtained. The resulting membranes are monodisperse, with a very well-defined pore geometry (i.e., microsieves with a pore diameter between 500 nm and 10 μm) that is easily controllable with the holographic set-up. The selectivity of in-situ fabricated microsieves with a pore diameter of 2 μm will be demonstrated using polystyrene beads of 1 and 3 μm.     

Novel U-shape gold nanoparticles-modified optical fiber for localized plasmon resonance chemical sensing

A novel fiber-optic localized plasma resonance (FO-LPR) sensor composed of a U-shape optical fiber was proposed and demonstrated in this study. The U-shape optical fiber was fabricated by a femtosecond laser micromachining system. The dimensions of the U-shape zone were 100 μm in depth measured from the surface of the polymer jacket layer, 80 μm in width in the jacket layer, 60 μm in width in the cladding layer. The total length is 5 mm. After laser annealing treatment, the average surface roughness was 205.8 nm as determined by Atom Force Microscope (AFM). The exposed surface of the U-shape fiber was modified with self-assembled gold nanoparticles to produce the FO-LPR sensor. The response of the sensor shows that the signal increases linearly with increasing refractive index. The sensor resolution of the sensor was determined to be 1.06 × 10⁻³ RIU.     

Development of precision transfer technology of atmospheric hot embossing by ultrasonic vibration

We succeeded to transfer a precise micro-pattern combining with an ultrasonic vibration in an atmospheric hot embossing on the almost same condition as a vacuum hot embossing. This paper reports the effect of the ultrasonic vibration that was verified experimentally. In the conventional method, a metallic mold and a plastic sheet are heated more than the glass transition temperature of the plastic, and the softened plastic is flowed into the pattern only by applying a load. On the other hand, a longitudinal ultrasonic vibration is added in the molding process of an ultrasonic-vibration hot embossing. The synergy effect of the load and the ultrasonic vibration enables flowing of the plastic into a more precise pattern of the metallic mold. The longitudinal wave generated by an ultrasonic vibration system of the frequency 15 kHz and output 900 W. A pattern of the Ni mold used in the experiment was a pyramid hole in which a peak was cut and sidewalls were rounded. Entrance lengths of pyramids were from 100 to 530 μm and its all of the depth were 260 μm. A polycarbonate was chosen with a replication material. Compared with the condition that the ultrasonic vibration was not used, a contact force and a contact time could be reduced to about 1/3 and 1/12, respectively.     

Mechanical punching of 15 μm size hole

This study investigates the size limit of a hole produced by the conventional punching process. In the micron size hole punching, there are two main technical obstacles that complicate the miniaturization steps. One is the fabrication of the micro punch tools with high dimensional accuracy and the other is the accurate alignment of the tools within the die clearance of 1∼2 μm. In this study, we tried to mechanically punch a 15 μm size hole. For this, we fabricated and alignedthe punch tools. Micro punch tools made of tungsten carbide were fabricated by micro-electrical discharge machining (micro-EDM). The diameter of punch tip was 15 μm, and that of the die hole, 17 μm. With the developed micro punching system, tools were aligned, and then, 15 μm size holes were made on 13-μm-thick brass and stainless steel foil, respectively.     

Miniaturized variable-focus lens fabrication using liquid filling technique

This paper describes a simple method for fabricating a variable-focus lens using polydimethylsiloxane (PDMS) filling with liquid to produce a variable-focus lens. A 2-mm diameter lens was designed in this experiment, expected to reach a focal length in the range of 3 ∼ 12 mm. The theoretical value between the liquid volume and the lens contact angle at different focal lengths were simulated and measured. The pumped-in liquid volumes ranged from 200 to 1,400 μl. The contact angles ranged from 14.25° to 49.02°. Variable focal length was produced by changing the PDMS film deformation using different micro-fluidic volumes. The focal length produce in the experiment was from 4 ∼ 10 mm. The proposed method successfully fabricated a variable-focus lens. Bonding PDMS only once using no expensive instrument such as oxygen plasma was accomplished. The final objective is to insert the variable focus lens into portable optical imagery products.     

Simple and low-cost fabrication of PDMS microfluidic round channels by surface-wetting parameters optimization

Convenient for both biologists and MEMS designers, Polydimethylsiloxane (PDMS) polymer is intensively investigated for its biocompatibility, transparency, high resistance under plasma treatment, flexibility and resistance to high temperature. However, for microfluidic applications, the fabrication of PDMS circular channels is difficult to achieve except by wire moulding. In this article, we present a simple, fast and low-cost fabrication method which can be applied out of clean-room environment. It is based on the deposition of alginic acid sodium salt aqueous solution, enabling the formation of a liquid cylinder on the most hydrophilic part of a hydrophilic/hydrophobic patterned surface. We experimentally studied the interaction between liquid rivulets and surfaces presenting a contrast of wettability and/or a stepwise texture. Subsequent moulding of the half-cylinder of liquid produces round PDMS microfluidic channels. The optimal parameters for hydrophilic/hydrophobic patterns have then been applied to produce the roundest possible channels. The realisation of both straight channels 300–500 μm wide, 1 cm long and 75° tangent chord angle at best, and Y-shaped channels with the same dimensions and 55° TCA is demonstrated.