Emergent Soft Lithographic Tools for the Fabrication of Functional Polymeric Microstructures

Polymeric microstructures (PMs) are useful to a broad range of technologies applicable to, for example, sensing, energy storage, and soft robotics. Due to the diverse application space of PMs, many techniques (e. g., photolithography, 3D printing, micromilling, etc.) have been developed to fabricate these structures. Stemming from their generality and unique capabilities, the tools encompassed by soft lithography (e. g., replica molding, microcontact printing, etc.), which use soft elastomeric materials as masters in the fabrication of PMs, are particularly relevant. By taking advantage of the characteristics of elastomeric masters, particularly their mechanical and chemical properties, soft lithography has enabled the use of non-planar substrates and relatively inexpensive equipment in the generation of many types of PMs, redefining existing communities and creating new ones. Traditionally, these elastomeric masters have been produced from relief patterns fabricated using photolithography; however, recent efforts have led to the emergence of new methods that make use of masters that are self-forming, dynamic in their geometric and chemical properties, 3D in architecture, and/or sacrificial (i. e., easily removed/released using phase changes). These “next generation” soft lithographic masters include self-assembled liquid droplets, microscale balloons, templates derived from natural materials, and hierarchically microstructured surfaces. The new methods of fabrication supported by these unique masters enable access to numerous varieties of PMs (e. g., those with hierarchical microstructures, overhanging features, and 3D architectures) that would not be possible following established methods of soft lithography. This review explores these emergent soft lithographic methods, addressing their operational principles and the application space they can impact.     

Microfluidic chip fabrication using hot embossing and thermal bonding of COP

The application of silicon mold inserts by micro‐hot embossing molding has been explored in microfluidic chip fabrication. For the mold insert, this study employed an SU‐8 photoresist to coat the silicon wafer. Ultraviolet light was then used to expose the pattern on the SU‐8 photoresist surface. This study replicates the microstructure of the silicon mold insert by micro‐hot embossing molding. Different processing parameters (embossing temperature, embossing pressure, embossing time, and de‐molding temperature) for the cycle‐olefin polymer (COP) film of microfluidic chips are evaluated. The results showed that the most important parameter for replication of molded microfluidic chip is embossing temperature. De‐molding temperature is the most important parameter for surface roughness of the molded microfluidic chip. The microchannel is bonded with a cover by thermal bonding processing to form the sealed microfluidic chip. The bonding temperature is the most important factor in the bonding strength of the sealed microfluidic chip. Copyright © 2009 John Wiley & Sons, Ltd.     

Micro- and nanopatterned star poly(ethylene glycol) (PEG) materials prepared by UV-based imprint lithography

A UV-based imprint lithography method is used for the direct surface structuring of hydrogel-based biomaterials, which are prepared from a family of tailor-made star poly(ethylene glycol) formulations. Bulk star poly(ethylene glycol) (PEG) hydrogels are fabricated by cross-linking acrylate-functionalized star PEG macromolecules. Cross-linking is achieved by radical reactions initiated by UV irradiation. This UV-curable star PEG formulation allows templating of mold structures to yield a stable, stand-alone, elastomeric replica of the mold. In particular, when a secondary, soft mold is used that consists of a perfluorinated elastomer with inherent excellent release properties, nanometer-sized features (down to 100 nm) can be imprinted without specialized equipment. The applied UV-based imprint lithography is a fast and simple technique to employ for the direct topographic structuring of bulk PEG-based biomaterials. The UV-based imprinting into the star PEG prepolymer by means of a perfluorinated, soft mold can be carried out on the bench top, while nanoscale resolution is demonstrated.     

Design, fabrication and characterization of monolithic embedded parylene microchannels in silicon substrate

This paper presents a novel channel fabrication technology of bulk-micromachined monolithic embedded polymer channels in silicon substrate. The fabrication process favorably obviates the need for sacrificial materials in surface-micromachined channels and wafer-bonding in conventional bulk-micromachined channels. Single-layer-deposited parylene C (poly-para-xylylene C) is selected as a structural material in the microfabricated channels/columns to conduct life science research. High pressure capacity can be obtained in these channels by the assistance of silicon substrate support to meet the needs of high-pressure loading conditions in microfluidic applications. The fabrication technology is completely compatible with further lithographic CMOS/MEMS processes, which enables the fabricated embedded structures to be totally integrated with on-chip micro/nano-sensors/actuators/structures for miniaturized lab-on-a-chip systems. An exemplary process was described to show the feasibility of combining bulk micromachining and surface micromachining techniques in process integration. Embedded channels in versatile cross-section profile designs have been fabricated and characterized to demonstrate their capabilities for various applications. A quasi-hemi-circular-shaped embedded parylene channel has been fabricated and verified to withstand inner pressure loadings higher than 1000 psi without failure for micro-high performance liquid chromatography (microHPLC) analysis. Fabrication of a high-aspect-ratio (internal channel height/internal channel width, greater than 20) quasi-rectangular-shaped embedded parylene channel has also been presented and characterized. Its implementation in a single-mask spiral parylene column longer than 1.1 m in a 3.3 mm x 3.3 mm square size on a chip has been demonstrated for prospective micro-gas chromatography (microGC) and high-density, high-efficiency separations. This proposed monolithic embedded channel technology can be extensively implemented to fabricate microchannels/columns in high-pressure microfluidics and high-performance/high-throughput chip-based micro total analysis systems (microTAS).     

Mechanism for the formation of isolated poly(p-xylylene) fibrous structures under shadowing growth

Because of the low sticking coefficient, conventional parylene deposition is known to achieve the conformal coating on corrugated or patterned surfaces. However, recently, it has been shown that in contrary to the conformal coating, extremely nonconformal and isolated fibrous parylene structures can be formed on surfaces if it is deposited at an oblique angle using a directional flux. We demonstrate that directional flux can create a high local vapor pressure facing the flux, while the reflection of monomers because of a small sticking coefficient would generate a background vapor pressure. The parylene oblique angle deposition is a combination of the shadowing growth and a much slower conformal coating process, which together give rise to the isolated fibrous structure.     

A Simple Method for Fabricating Patterned Curvilinear Microstructures in Poly(dimethylsiloxane) by Selective Wetting

The fabrication of patterned microstructures in poly(dimethylsiloxane) (PDMS) is a prerequisite for soft lithography. Herein, curvilinear surface relief microstructures in PDMS are fabricated through a simple three‐stage approach combining microcontact printing (μCP), selective surface wetting/dewetting and replica molding (REM). First, using an original PDMS stamp (first‐generation stamp) with linear relief features, a chemical pattern on gold substrate is generated by μCP using hexadecanethiol (HDT) as an ink. Then, by a dip‐coating process, an ordered polyethylene glycol (PEG) polymer‐dot array forms on the HDT‐patterned gold substrate. Finally, based on a REM process, the PEG‐dot array on gold substrate is used to fabricate a second‐generation PDMS stamp with microcavity array, and the second‐generation PDMS stamp is used to generate third‐generation PDMS stamp with microbump array. These fabricated new‐generation stamps are utilized in μCP and in micromolding in capillaries (MIMIC), allowing the generation of surface micropatterns which cannot be obtained using the original PDMS stamp. The method will be useful in producing new‐generation PDMS stamps, especially for those who want to use soft lithography in their studies but have no access to the microfabrication facilities.     

Toward Mass Producible Ordered Bulk Heterojunction Organic Photovoltaic Devices

A strategy to fabricate nanostructured poly(3‐hexylthiophene) (P3HT) films for organic photovoltaic (OPV) cells by a direct transfer method from a reusable soft replica mold is presented. The flexible polyfluoropolyether (PFPE) replica mold allows low‐pressure and low‐ temperature process condition for the successful transfer of nanostructured P3HT films onto PEDOT/PSS‐coated ITO substrates. To reduce the fabrication cost of masters in large area, we employed well‐ordered anodic aluminum oxide (AAO) as a template. Also, we provide a method to fabricate reversed nanostructures by exploiting the self‐replication of replica molds. The concept of the transfer method in low temperature with a flexible and reusable replica mold obtained from an AAO template will be a firm foundation for a low‐cost fabrication process of ordered OPVs.     

Microstencils for the patterning of nontraditional materials

A microfabrication technique that uses a photolithographically patterned film as a microstencil has been developed. This microstencil has a bilayer structure comprised of parylene and SU-8 films with thicknesses from 4 to 100 microm. The parylene layer enables the microstencil to be mechanically peeled from hydrophilic substrates. Since no chemicals are required to release the microstencil, this technique can be used to pattern chemically and biologically sensitive materials. The amount of material deposited can be automatically controlled by the height of the SU-8 structures or externally controlled by spin coating or other thin film deposition techniques. This patterning method is very versatile and has been used to pattern features as small as 25 by 25 microm on silicon, glass, and polymer substrates. As an initial demonstration, we have patterned wax, cells, proteins, sol, and CYTOP.     

Rapid prototyping for injection moulded integrated microfluidic devices and diffractive element arrays

This paper describes two fabrication procedures that makes it possible to design, fabricate and injection mold a microfluidic system with an on board coupling element or an optical array platform in less than four hours. Epoxy masters for the array and a single diffractive element were produced using conventional soft lithography techniques and a commercially available UV curable epoxy. The fabrication of the master for the integrated microfluidic device utilized the surface chemistry of polyester and its interaction with the anionic surfactant sodium dodecyl sulfate (SDS), to selectively inhibit the adhesion between the epoxy and the polyester film during the curing reaction. The transfer of a microfluidic design and the required coupling element (632 nm holographic grating) along the base of the channel was completed in a single step. The turnaround time from design to injection molded device whether a microchannel or array was 3.5 h.     

Fabrication of microfluidic systems in poly(dimethylsiloxane)

Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.     

PDMS微流体系统的加工制作

目前,微流体装置越来越多地应用到分析系统、生物医学、化学等基础研究领域。传统的微流体系统制作方法是对玻璃和硅片进行刻蚀。用软刻法制作PDMS(Poly(dimethylsiloxane)：聚二甲基硅氧烷)微流体装置比传统的制作方法更快速,成本更低廉,并且对于通道的密封也不需要玻璃或硅芯片键合密封等复杂工艺。这类软刻法的核心技术是快速原样制作法和复制压模技术。相对于微电子加工工艺,软刻法制作过程不需要超静环境,化学家和生物学家可在普通的实验室实现加工制作。本文介绍了PDMS微装置在分离和生物材料模式化等方面的应用。     

Fabrication of Micro‐ and Nanoscale Polymer Structures by Soft Lithography and Spin Dewetting

In this work, the process of spin dewetting of a polymer solution on a topographically patterned PDMS mold was used for fabrication of micro‐ and nanaoscale polymer structures. Spin coating was used to provide a fast and reproducible coating. This simple technique was capable of producing a wide range of polymer feature geometries from a single microfabricated mold. This experimental study looks at the effects of the original mold feature geometry as well as the polymer solution concentration on the resultant microstructures. Polystyrene and poly(propyl methacrylate) were used as model polymers. Features with film thickness ranging from <100 nm to >5 µm were obtained using this technique. The process was also extended to fabrication of nanoscale features.

     

Fabrication of photoprocessible azo polymer microwires through a soft lithographic approach

In this work, a soft lithographic approach has been developed to fabricate free-standing azo polymer microwires with unique photoprocessible characteristics. In the process, an epoxy-based azo polymer (BP-AZ-CA) was used to prepare both the soft lithographic masters and the microwires. The masters were prepared by photofabricating surface relief gratings on BP-AZ-CA thin films. Then the elastomeric stamps were prepared by replica molding of poly(dimethylsiloxane) prepolymer against the masters. With use of the stamps and a solution of BP-AZ-CA as "ink", the microwires were prepared by contact printing and wet etching. The microwires possessed a uniform sub-micrometer-scale transverse dimension and macroscopic longitudinal dimension. Those characteristic sizes depended on the adjustable features of the masters and stamps used in the process. The transverse dimension of the microwires could be altered after exposure to a linearly polarized Ar+ laser single beam with the polarization direction perpendicular to the longitudinal axes of the microwires. Upon irradiation of interfering p-polarized Ar+ laser beams, regular surface relief structures could be inscribed on the microwires along the longitudinal direction, which coincided with both the polarization direction of the laser beams and the grating vector direction of the interference pattern. The microwires with photoprocessible properties are potentially usable as sub-micrometer-scale materials in future miniaturized components and devices. The approach reported in this work can be further extended to the fabrication of nano-/microwires from other polymeric materials.     

Design of experiment for optimization of plasma-polymerized octafluorocyclobutane coating on very high aspect ratio silicon molds

As-fabricated deep reactive ion etched (DRIE) silicon mold with very high aspect ratio (>10) feature patterns is unsuitable for poly(dimethylsiloxane) (PDMS) replication because of the strong interaction between the Si surface and the replica and the corrugated mold sidewalls. The silicon mold can be conveniently passivated via plasma polymerization of octafluorocyclobutane (C4F8), which is also employed in the DRIE process itself, to enable the mold to be used repeatedly. To optimize the passivation conditions, we have undertaken a Box-Behnken experimental design on the basis of three passivation process parameters (plasma power, C4F8 flow rate, and deposition time). The measured responses were fluorinated film thickness, demolding status/success, demolding force, and fluorine/carbon ratio on the fifth replica surface. The optimal passivation process conditions were predicted to be an input power of 195 W, a C4F8 flow rate of 57 sccm, and a deposition time of 364 s; these were verified experimentally to have high accuracy. Demolding success requires medium-deposited film thickness (66-91 nm), and the thickness of the deposited films correlated strongly with deposition time. At moderate to high ranges, increased plasma power or gas flow rate promoted polymerization over reactive etching of the film. It was also found that small quantities of the fluorinated surface were transferred from the Si mold to the PDMS at each replication, entailing progressive wear of the fluorinated layer.     

Duplication of photoinduced azo polymer surface-relief gratings through a soft lithographic approach

In this work, a soft lithographic approach has been developed to duplicate photoinduced surface-relief-gratings (SRGs) of azo polymer films to generate the surface pattern replicas composed of different materials on various substrates. For this purpose, thin films of an epoxy-based azo polymer (BP-AZ-CA) were prepared by spin-coating, and SRGs with different structures were inscribed by exposing the films to interference patterns of Ar(+) laser beams at modest intensity (150 mW/cm(2)). Using the azo polymer films as masters, stamps of poly(dimethylsiloxane) (PDMS) were prepared by replica molding. The PDMS stamps were then used to transfer the solutions of poly(3-hexylthiophene) (P3HT), multiwalled carbon nanotube (MWNT), and BP-AZ-CA to different substrates by contact printing. Through this process, surface pattern replicas made of the functional materials were obtained. The pattern formation and quality depended on the factors such as the solution concentration, contacting time in the printing process, and printing pressure. Under the proper conditions, the printed patterns showed the same grating periods as the masters and the same relief depths as the stamps (replicas of the masters). This approach, showing some attractive characteristics such as the easiness of master preparation and the versatility of soft fabrication processes, can be applied to the fabrications of optical functional surfaces, sensors, and photonic devices.     

Vapor-phase self-assembled monolayer for improved mold release in nanoimprint lithography

Resist adhesion to the mold is one of the challenges for nanoimprint lithography. The main approach to overcoming it is to apply a self-assembled monolayer of an organosilane release agent to the mold surface, either in the solution phase or vapor phase. We compared the atomic force microscopy, ellipsometry, reflection-absorption infrared spectroscopy, and contact angle results collected from substrates treated by two different application processes and found that the vapor-phase process was superior. The vapor-treated substrates had fewer aggregates of the silane molecules on the surface, because the lower density of the agent in the vapor phase was not conducive to aggregation formation, and received a superior coating of the releasing agent, because the vapor was more effective than the solution in penetrating into the nanoscale gaps of the mold. A pattern transfer of 20 parallel nanowires with a line width of 40 nm at 100 nm pitch-size was performed faithfully with the vapor-treated mold without any resist adhesion.     

Initial stages of growth of poly(p-xylylene) coatings: AFM study

The morphology of poly(p-xylylene) ultrathin films prepared by vapor deposition polymerization on the surface of single-crystal silicon (100) and on the cleaved surface of mica at a substrate temperature of 20°C has been studied by atomic force microscopy. At the initial stage, the growth of the poly(p-xylylene) coating follows the island mechanism. Within the framework of pyramidal model of island growth, the mean diffusion length for monomer p-xylylene is calculated: For the single-crystal silicon, this parameter is 15 ± 3 nm; for the cleaved surface of mica, 9 ± 2 nm. The nature of the substrate and defects on its surface show a peculiar effect on the structure of the poly(p-xylylene) coating. Thus, at a low monomer flow, nucleation of polymer islands on the surface of silicon is predominantly homogeneous, whereas on the cleaved surface of mica, it is heterogeneous. A change in the monomer flow significantly affects the rate of nucleation of polymer islands.     

Three-dimensional interconnected microporous poly(dimethylsiloxane) microfluidic devices

This technical note presents a fabrication method and applications of three-dimensional (3D) interconnected microporous poly(dimethylsiloxane) (PDMS) microfluidic devices. Based on soft lithography, the microporous PDMS microfluidic devices were fabricated by molding a mixture of PDMS pre-polymer and sugar particles in a microstructured mold. After curing and demolding, the sugar particles were dissolved and washed away from the microstructured PDMS replica revealing 3D interconnected microporous structures. Other than introducing microporous structures into the PDMS replica, different sizes of sugar particles can be used to alter the surface wettability of the microporous PDMS replica. Oxygen plasma assisted bonding was used to enclose the microstructured microporous PDMS replica using a non-porous PDMS with inlet and outlet holes. A gas absorption reaction using carbon dioxide (CO(2)) gas acidified water was used to demonstrate the advantages and potential applications of the microporous PDMS microfluidic devices. We demonstrated that the acidification rate in the microporous PDMS microfluidic device was approximately 10 times faster than the non-porous PDMS microfluidic device under similar experimental conditions. The microporous PDMS microfluidic devices can also be used in cell culture applications where gas perfusion can improve cell survival and functions.     

Rapid fabrication of microchannels using microscale plasma activated templating (microPLAT) generated water molds

Poly(dimethylsiloxane) (PDMS) is a common material used in fabricating microfluidic devices. The predominant PDMS fabrication method, soft lithography, relies on photolithography for fabrication of micropatterned molds. In this technical note, we report an alternative molding technique using microscale PLasma Activated Templating (microPLAT). The use of photoresist in soft lithography is replaced by patterned water droplets created using microPLAT. When liquid PDMS encapsulates patterned water and then solidifies, the cavities occupied by water become structures such as microchannels. Using this method, device fabrication is less time consuming, more cost efficient and flexible, and ideal for rapid prototyping. An additional important feature of the water-molding process is that it yields structural profiles that are difficult to achieve using photolithography.     

Stability of microfabricated high aspect ratio structures in poly(dimethylsiloxane)

The stability of structures microfabricated in soft elastomeric polymers is an important concern in most applications that use these structures. Although relevant for several applications, the collapse to the ground of high aspect ratio structures (ground collapse) is still poorly understood. The stability of soft microfabricated high aspect ratio structures versus ground collapse was experimentally assessed, and a new model of ground collapse involving adhesion was developed. Sets of posts with diameters from 0.36 to 2.29 microm were fabricated in poly(dimethylsiloxane) and tested in air or immersed in water and ethanol to change the work of adhesion. The critical aspect ratio (the highest length-to-width ratio for which a post is not at risk of collapsing) was determined as a function of the diameter. The critical aspect ratio in air ranged from 2 to 4 and increased with the diameter. Work of adhesion was found to be determinant for and inversely correlated to stability. These results highlight the role played by adhesion and offer the possibility of improving stability by reducing the work of adhesion. The ground collapse model developed accounted for the main features of structure stability. The results indicate that ground collapse can be a limiting factor in the design of soft polymer structures.