Fabrication of microchannels in negative resist

Microchannels are very important components of the micro-fluidic systems. A novel method for fabrication of microchannels is presented in this paper. The microstructures for fluidic application is constructed by UV or X-ray lithography in positive resist and closed in crosslinked negative resist. This process can be used to fabricate high precise, complex microchannels due to the high precise microstructures of positive photoresist and the complex three-dimensional lithography. The optical transparent microchannels are suitable for optical measurements of the fluid flow and the profile of microchannels.     

Fabrication of antiscatter grids and collimators for X-ray and gamma-ray imaging by lithography and electroforming

Creatv MicroTech has developed unique fabrication techniques to make high precision, high-aspect-ratio metal microstructures to custom specifications. A lithography based fabrication method permits precise fabrication of various microstructures. Collimators and antiscatter grids with continuous, smooth, thin, parallel or focused septa have been fabricated using deep X-ray and optical lithography, combined with metal electroforming. Microfabrication of high-aspect-ratio structures, especially of relatively large areas, presents many challenges: specialized mask design and X-ray mask fabrication; resist preparation, optimal exposure parameters, post-exposure processing, electroforming, polishing, and final assembly. Here, we present microstructures of various designs that we fabricated and describe the challenges that had to be overcome.     

A low-temperature IC-compatible process for fabricatingsurface-micromachined metallic microchannels

In this paper, a low-temperature integrated-circuit (IC)-compatible process for fabricating metallic microchannels is described. Arrays of 1-100 metallic microchannels have been fabricated on silicon and glass substrates. The process can be extended to many planar substrate materials including polymers and ceramics. The microchannels are formed using microelectro-formed metals. The microchannels demonstrated in this paper use nickel as the structural material and gold as the surface coating on the inside walls of the microchannels. The inner dimensions of the individual microchannels fabricated to date range from 30 μm to 1.5 mm in width, 0.5 mm to several centimeters in length, and 5-100 μm in thickness. The wall thickness ranges from 5 to 50 μm. The microchannel fabrication technology enables the fabrication of surface microchannels with a relatively large cross-sectional area. The metallic microchannels can be fabricated to extend from the substrate edge. Interfacing schemes are given for attaching external pressure feeds     

Fabrication of grids and collimators using SU-8 as a mold

The fabrication process of ultradeep (aspect ratio greater than 25) microchannels in SU-8 photoresist using deep X-ray lithography is described. We have demonstrated that with single-layer coatings, 1-mm-deep trenches in continuous resist layer can be achieved reproducibly. Microchannels with vertical walls and with walls tilted up to 20° from vertical have been fabricated. Electroforming of the channels resulted in metal grids and collimators for various applications. A thickness enhancement method for fabrication of very tall structures is also described.We would like to thank Dr. Francesco De Carlo for support on the beamline, and Judi Yaeger and Ruben Khachatryan for the experimental assistance. The work was supported by NIH SBIR Grants: R44 CA76752 and R43 CA91762, and by the U.S. Department of Energy, under Contract No. W-31–109-ENG-38.     

Mixing characteristics in microchannels with biomimetic superhydrophobic (Lotus leaf replica) walls

We demonstrate here the mixing characteristics in microchannels with a biomimetic superhydrophobic (lotus leaf replica) wall. The lotus leaf replica is fabricated using a frugal, yet efficient, double-step soft lithography method. In microchannels with a lotus leaf replica wall, the unidirectional laminar flow pertaining to the low hydrodynamics regime changes into an erratic flow field beyond a critical flow rate. We show here that such lotus leaf replica-induced erratic flow, even for low Reynolds number, can be used for enhanced mixing at the microscale. The enhancement in the mixing is quantified by the reduction in the mixing length in the microchannels with the biomimetic lotus leaf replica wall as compared to identical microchannels with flat walls. We believe that the simple cost-effective methodology for enhancing mixing in microchannels, as demonstrated here, can be integrated into lab-on-a-chip devices, which may be beneficial for applications requiring microscale mixing like DNA sequencing, enzyme reaction, and medical diagnostics.     

One-step maskless grayscale lithography for the fabrication of 3-dimensional structures in SU-8

We propose a novel and simplified method to fabricate complex 3-dimensional structures in SU-8 photoresist using maskless grayscale lithography. The proposed method uses a Digital Micro-mirror Device (DMD^®) to modulate the light intensity across a single SU-8 photoresist layer. Top and back-side exposure are implemented in the fabrication of original structures such as cantilevers, covered channels with embedded features and arrays of microneedles. The fabrication of similar structures in SU-8 with other techniques often requires complex physical masks or the patterning of several stacked layers. The effects of critical process parameters such as software mask design, exposure and developing conditions on the quality of 3-D structures are discussed. A number of applications using bridges, cantilevers and micromixers fabricated using this methodology are explored.     

阵列型微拓扑结构基底的制备及其对神经细胞电生物物理特性的影响

以紫外光光刻、硅蚀刻及软光刻技术制备了微柱阵列型细胞培养基底.实验发现,在4μm的结构高度下,当微柱特征尺寸大于或等于4 μm时,该法能制备结构规整清晰的聚二甲基硅氧烷微柱阵列型基底.特征尺寸为2μm的基底已经接近该法的极限加工能力,所加工的微柱阵列出现倒伏或缺失.采用一种简单的倾斜角法可以制备一种聚苯乙烯微球致密阵列...     

Influence of developer temperature on the shape of structures fabricated by deep X-ray lithography

This paper describes the fabrication of poly(methyl methacrylate) (PMMA) microstructures with three-dimensional (3-D) sloped sidewalls using synchrotron-radiated (SR) deep X-ray lithography (DXRL). Here, the developer temperature was varied to produce variations in the inclination angle of the sloped sidewalls. We found that the PMMA sidewall inclination angle and height were controlled by the dosage, development time, and development temperature. When the development temperature was low, the inclination angle was nearly 0°, regardless of dosage amounts or exposure time. When the development temperature was high, microstructures with sloped sidewalls were fabricated; as the dosage amount and development time increased, the inclination angle increased. The ability to control the PMMA sidewall inclination angle suggests the application of this technique to microstructure fabrication technologies, such as 3-D microelectromechanical system (MEMS) device components, in which the inclination angle becomes the draft angle for moulding processes.     

Improvement of capillary electrophoresis property for microchannels fabricated by deep X-ray lithography

We fabricated the electrophoresis microchips using the UV polymerization technique. We employed plastic substrates that were suitable for rapid prototyping instead of glass and quartz. A thick UV negative photo resist was used to form molds and poly-dimethylsilozane (PDMS) was polymerized by a thermal curing process on the mold to obtain replica microchips. Electroosmotic flow (EOF) was measured to evaluate the surface. Characteristic differences between UV-fabricated and SR-fabricated microchips were evaluated by electro osmotic flow (EOF) measurement. It was observed that microchannels fabricated by SR lithography show constant peak heights and FWHMs. We also investigated the effect of the change of the channel width along the EOF direction. It is demonstrated that broadening width channel significantly restricts the sample diffusion towards the EOF direction and leads to the high resolusion separation on the PDMS microchips. Thus the advantage of the application of SR lithography to the mold fabrication is also demonstrated.     

Prototype of a novel micro-machined cytometer and its 3D hydrodynamic focusing properties

This paper presents a micro-machined cytometric device which can achieve a three-dimensional (3D) hydrodynamic focusing only through a novel but simple microfluidic structure, with the uniqueness that the depth of the microchannels is non-uniform. By using a SU-8 soft lithography containing two exposures, the PDMS device prototype is fabricated, and tested for its performance through fluorescent optical experiments. At the same time, a two-fluid model to describe the micro-flow transport and interaction behaviors is also established, based on volume of fluid (VOF) method for multi-phase flow. It is found that the experiment and the simulation results have good consistencies. Based on this, the influences of a few geometry parameters on device 3D focusing performance, which is evaluated by the focused width as well as the corresponding height, are further explored by numerical simulations. The results indicate that good 3D focusing could be obtained at relatively not high sheath-sample velocity ratios, mainly due to the introducing of the unique depth difference. The work of this paper, not only validates the design conception of the proposed novel structure convincingly, but also enhances our understanding of 3D hydrodynamic focusing in the design of cytometers, as well as similar microfluidic devices.     

Design and fabrication of 3-dimensional helical structures in polydimethylsiloxane for flow control applications

Soft lithography in 2-dimensional (2-D) was developed for polymer MEMS applications about two decades back. The technique was highly useful for replication of microstructure molds using a soft polymeric material called PDMS (polydimethylsiloxane). From its inception the process has been widely applied to microfluidics, biochips, hybrid biomedical microdevices etc. However, it was limited to only surface microstructures and 3-Dimensional (3-D) soft lithography although performed by some research groups involved some very precise and expensive techniques like stereolithography etc. The exploration of soft lithography in three dimensions by using a replication technique with copper wires with micron size diameters was performed by our group relatively recently (Singh et al. in International conference on MEMS, IIT Madras, Chennai, 2009). In this work we have used the 3-D replication and molding technique to develop concentric solenoid patterns around micro-channels in the bulk of PDMS. The solenoidal paths of various pitches ranging from 0.4 to 1.2 mm have been replicated in PDMS using an innovatively designed fixture. The solenoids have been structurally characterized using an inverted fluorescence microscope (Nikon 80i) for dimensional parameters like pitch, length etc. Further, the solenoidal path designs have been simulated, optimized and fabricated around a central channel of 80 μ diameter and we have observed the repeatability of this fabrication process multiple times. The purpose of this architecture is to initiate valving action wherein fluid movement in the central channel can be restricted by filling the surrounding solenoidal track with compressed air at high pressure so that it can squeeze the centrally located micro-channel carrying the liquid. This valving structure may find a lot of applications in lab on chip devices, PCR biochips, biomedical micro-devices etc.     

Fabrication method of two-level polymeric microstructure with the help of deep X-ray lithography

Two- or multi-level microstructures are getting more important in several applications such as multi-component micro optical elements and various microfluidic systems. In the present study, a simple and efficient method is newly proposed for a fabrication of the two-level polymeric microstructures. Making a mother two-level microstructure consists of two processes: (1) the hot embossing process for a fabrication of microstructures on a PMMA substrate, and (2) the deep X-ray lithography using the hot embossed substrate for a high aspect ratio microstructure fabrication, resulting in a high aspect ratio microstructure containing smaller microstructures on its surface. Making use of so fabricated two-level microstructures as a mother structure, one could achieve a mass replication of the same microstructures via injection molding process with a metallic mold insert obtained by a nickel electroforming onto the mother microstructure. In order to demonstrate the proposed method, a polymeric high aspect ratio microstructure having smaller square microstructures on its top surface was fabricated. The fabricated two-level microstructure shows fine vertical sidewalls, which is a characteristic feature of the deep X-ray lithography. In addition, a metallic mold insert for a mass replication was fabricated by a nickel electroforming process.     

Rapid prototyping of shrinkable BOPS-based microfluidic devices

Biaxially oriented polystyrene (BOPS) is a commercialized packaging material, which has the advantages of biocompatibility, non-toxic, transparency, light-weight and cost-effective. Due to the stress accumulated from both directions in plane during the fabrication process, when BOPS was reheated above the glass transition temperature, an isotropic shrinkage will occur. This study proposed a low-cost and rapid prototyping method for the fabrication of BOPS-based microfluidics device. Both laser ablation and micro-milling were used for the fabrication of microchannels on the surface of the BOPS sheet, after thermal induced shrinkage, microchannels with finer microstructure could be achieved. For the sealing of fabricated microchannels on BOPS, two approaches were made using a layer of BOPS or a layer of polyester adhesive film. The thermal induced shrinkage and bonding strength were carefully studied in this study. Several microfluidic devices, including a droplet generator and a diffusion mixer were also fabricated for demonstration. The proposed fabrication method for BOPS-based microfluidics is simple, rapid, cost-effective and without the requirement of cleanroom facility, with help of thermal induced shrinkage, finer structure with high resolution could be achieved with conventional lab tools.     

Soft lithography based on photolithography and two-photon polymerization

Over the past decades, soft lithography has greatly facilitated the development of microfluidics due to its simplicity and cost-effectiveness. Besides, numerous fabrication techniques such as multi-layer photolithography, stereolithography and other methods have been developed to fabricate moulds with complex 3D structures nowadays. But these methods are usually not beneficial for microfluidic applications either because of low resolution or sophisticated fabrication procedures. Besides, high-resolution methods such as two-photon lithography, electron-beam lithography, and focused ion beam are often restricted by fabrication speed and total fabricated volume. Nonetheless, the region of interest in typical microfluidic devices is usually very small while the rest of the structure does not require complex 3D fabrication methods. Herein, conventional photolithography and two-photon polymerization are combined for the first time to form a simple hybrid approach in fabricating master moulds for soft lithography. It not only benefits from convenience of photolithography, but also gives rise to complex 3D structures with high resolution based on two-photon polymerization. In this paper, various tests have been conducted to further study its performance, and a passive micromixer has been created as a demonstration for microfluidic applications.     

Fabrication and characterization of hydrogel-based microvalves

Several microvalves utilizing stimuli-responsive hydrogel materials have been developed. The hydrogel components are fabricated inside microchannels using a liquid phase polymerization process. In-channel processing greatly simplifies device construction, assembly, and operation since the functional components are fabricated in situ and can perform both sensing and actuation functions. Two in situ photopolymerization techniques, "laminar stream mode" and "mask mode," have been explored. Three two-dimensional (2-D) valves were fabricated and tested (response time, pressure drop, maximum differential pressure). In addition, a hydrogel/PDMS three-dimensional (3-D) hybrid valve that physically separates the sensing and regulated streams was demonstrated. Analytical modeling was performed on the 3-D valve. Hydrogel-based microvalves have a number of advantages over conventional microvalves, including relatively simple fabrication, no external power requirement, no integrated electronics, large displacement (185 μm), and large force generation (22 mN)     

Arrays of monocrystalline silicon micromirrors fabricated using CMOS compatible transfer bonding

In this paper, we present CMOS compatible fabrication of monocrystalline silicon micromirror arrays using membrane transfer bonding. To fabricate the micromirrors, a thin monocrystalline silicon device layer is transferred from a standard silicon-on-insulator (SOI) wafer to a target wafer (e.g., a CMOS wafer) using low-temperature adhesive wafer bonding. In this way, very flat, uniform and low-stress micromirror membranes made of monocrystalline silicon can be directly fabricated on top of CMOS circuits. The mirror fabrication does not contain any bond alignment between the wafers, thus, the mirror dimensions and alignment accuracies are only limited by the photolithographic steps. Micromirror arrays with 4/spl times/4 pixels and a pitch size of 16 /spl mu/m/spl times/16 /spl mu/m have been fabricated. The monocrystalline silicon micromirrors are 0.34 /spl mu/m thick and have feature sizes as small as 0.6 /spl mu/m. The distance between the addressing electrodes and the mirror membranes is 0.8 /spl mu/m. Torsional micromirror arrays are used as spatial light modulators, and have potential applications in projection display systems, pattern generators for maskless lithography systems, optical spectroscopy, and optical communication systems. In principle, the membrane transfer bonding technique can be applied for integration of CMOS circuits with any type of transducer that consists of membranes and that benefits from the use of high temperature annealed or monocrystalline materials. These types of devices include thermal infrared detectors, RF-MEMS devices, tuneable vertical cavity surface emitting lasers (VCSEL) and other optical transducers.     

A new fabrication method for borosilicate glass capillary tubes with lateral inlets and outlets

In this paper a new fabrication method for borosilicate glass capillary tubes is presented. As the interest in miniaturized total chemical analysis systems (μ-TAS) is increasing, the need for fluidic paths is growing and thus the study of microchannels, microtubes and microcolumns is an important topic of the microfluidic area. The capillary tubes presented here are fabricated by structuring and bonding three borosilicate glass wafers (7740 Corning Pyrex sR). Microchannels with lateral inlets and outlets have been successfully realized and well-defined size and shape have been obtained. Several capillary tubes with widths from 340 to 940 μm have been realized as well as different section shapes, which can be circular, elliptic or quasi-rectangular. The main fabrication steps and first characterizations are reported.     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Integrated structure of PMMA microchannels for DNA separation by microchip capillary electrophoresis

We proposed and fabricated an integrated structure of microchannels consists of three different functional PMMA layers for post-genome analysis, gene diagnosis, and screenings of useful materials for pharmaceutical. This integrated structure with 96 microchip capillary electrophoresis units in one chip is characterized as the simple structure with low cost and new aspects of the serial unit bio-chemical operation from DNA amplification to their analysis using microchip capillary electrophoresis. The design of the structure was performed using computational fluid dynamics, heat transmission, and electrophoresis simulation. To improve DNA separation resolution, microchannel with narrow width at the corner was adapted. The deep X-ray lithography process using synchrotron radiation “New SUBARU”, nano-imprint, and fusion bonding without bonding adhesive was applied for the fabrication of the integrated structure of microchannels. It was demonstrated that the proposed integrated structure of microchannels results in a good performance of the on-chip DNA amplification and separation in a small MCE unit area of 9 mm × 9 mm.     

Multi-scale,multi-depth lithography using optical fibers for microfluidic applications

This paper proposes and demonstrates a method for multi-scale, multi-depth three-dimensional (3D) lithography. In this method, 3D molds for replicating microchannels are fabricated by passing a non-focused laser beam through an optical fiber, whose tip is immersed in a droplet of photopolymer. Line width is adjustable from 1 to 980 µm using eight kinds of optical fibers with different core diameters. The height of line drawing can be controlled by adjusting the distance between the tip of the optical fiber and a substrate. The surface roughness (R_a, R_z) of a single line and plane was evaluated. The method was employed to fabricate a 3D mold of a microchannel containing tandem chambers, which was then successfully replicated in PDMS. Multi-scale, multi-depth 3D lithography can provide a simple, flexible tool for producing PDMS microfluidic devices.