New fluoropolymer materials

We report the synthesis of two classes of fluoropolymers that could impact several key lithographic techniques; one has potential applications in next generation photolithography (193 nm, 157 nm, and immersion lithography) and the other in lithographic techniques which are emerging as viable alternatives to photolithography for future applications (i.e., soft lithography).     

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.     

Photolithographic surface micromachining of polydimethylsiloxane (PDMS)

A major technical hurdle in microfluidics is the difficulty in achieving high fidelity lithographic patterning on polydimethylsiloxane (PDMS). Here, we report a simple yet highly precise and repeatable PDMS surface micromachining method using direct photolithography followed by reactive ion etching (RIE). Our method to achieve surface patterning of PDMS applied an O(2) plasma treatment to PDMS to activate its surface to overcome the challenge of poor photoresist adhesion on PDMS for photolithography. Our photolithographic PDMS surface micromachining technique is compatible with conventional soft lithography techniques and other silicon-based surface and bulk micromachining methods. To illustrate the general application of our method, we demonstrated fabrication of large microfiltration membranes and free-standing beam structures in PDMS.     

Soft lithography using acryloxy perfluoropolyether composite stamps

This paper describes composite patterning elements that use a commercially available acryloxy perfluoropolyether (a-PFPE) in various soft lithographic techniques, including microcontact printing, nanotransfer printing, phase-shift optical lithography, proximity field nanopatterning, molecular scale soft nanoimprinting, and solvent assisted micromolding. The a-PFPE material, which is similar to a methacryloxy PFPE (PFPE-DMA) reported recently, offers a combination of high modulus (10.5 MPa), low surface energy (18.5 mNm(-1)), chemical inertness, and resistance to solvent induced swelling that make it useful for producing high fidelity patterns with these soft lithographic methods. The results are comparable to, and in some cases even better than, those obtained with the more widely explored material, high modulus poly(dimethylsiloxane) (h-PDMS).     

Masterless soft lithography: patterning UV/ozone-induced adhesion on poly(dimethylsiloxane) surfaces

A novel microreactor-based photomask capable of effecting high resolution, large area patterning of UV/ozone (UVO) treatments of poly(dimethylsiloxane) (PDMS) surfaces is described. This tool forms the basis of two new soft lithographic patterning techniques that significantly extend the design rules of decal transfer lithography (DTL). The first technique, photodefined cohesive mechanical failure, fuses the design rules of photolithography with the contact-based adhesive transfer of PDMS in DTL. In a second powerful variation, the UVO masks described in this work enable a masterless soft lithographic patterning process. This latter method, UVO-patterned adhesive transfer, allows the direct transfer of PDMS-based polymer microstructures from a slab of polymer to silicon and other material surfaces. Both methods exploit the improved process qualities that result from the use of a deuterium discharge lamp to affect the UVO treatment to pattern complex, large area PDMS patterns with limiting feature sizes extending well below 1 microm (> or = 0.3 microm). The use of these structures as resists is demonstrated for the patterning of metal thin films. A time-of-flight secondary ion mass spectroscopy study of the process provides new insights into the mechanisms that contribute to the chemistry responsible for the interfacial adhesion of DTL transfers.     

A photocurable poly(dimethylsiloxane) chemistry designed for soft lithographic molding and printing in the nanometer regime

Patterning techniques that rely on high-resolution elastomeric elements such as stamps, molds, and conformable photomasks are operationally simple methods for nanofabrication that may find applications in areas such as molecular and organic electronics. The resolution of these "soft" lithographic procedures is often limited by the mechanical properties of the elastomers. We introduce here a chemically modified poly(dimethylsiloxane) material that is designed and optimized specifically for soft lithography, particularly in the nanometer regime. We demonstrate its use for nanopatterning tasks that are challenging with the commercially available elastomers that have been used in the past.     

Soft lithography microfabrication of functionalized thermoplastics by solvent casting

A soft lithographic method is described for casting functional thermoplastic devices with microscale features without the need for specialized tools or equipment. In the thermoplastic soft lithography process, termed solvent casting, low temperature supersaturated solutions of thermoplastic are poured over solvent permeable PDMS molds which allow omnidirectional solvent removal as they template functional microstructures into the thermoplastic layers. Rapid gelation of supersaturated solutions enables the deposition of multiple patterned layers of varying composition, with self‐adhesion of the solvent‐laden thermoplastic ensuring intimate bonding between adjacent layers. This latter feature is further used in this work to realize sealed thermoplastic microfluidic devices with high fidelity replication of microchannel features with negligible channel deformation. The incorporation of functional dopants into patterned thermoplastic layers allows the fabrication of thermoplastic devices with embedded fluorescent sensors and integrated conductive elements. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1315–1323     

Deformation of elastomeric pyramid pen arrays in cantilever‐free scanning probe lithography

Cantilever‐free scanning probe lithography (SPL) employs soft elastomeric pen arrays to deliver material or energy to a surface to achieve a high‐resolution, high‐throughput, and low‐cost nanopatterning. In particular, microscale elastomeric pyramid pen arrays are often adopted as the cantilever‐free architecture owing to their distinct structural and mechanical properties. To better understand the mechanical behavior of the elastomeric pyramid pen array during the lithographic printing process, we numerically investigate the compression of an elastomeric pyramid array in a nonadhesive and frictionless contact with a rigid substrate. Simple scaling laws of the width of the contact surface with respect to the compression displacement and force are found and compared with previous models and experiments. By changing the interpyramid distance or the thickness of the base of the pyramid array, increasing deviations from the established scaling laws are observed and explained. Furthermore, we demonstrate that the unique morphology of a compressed pyramid primarily determines the unusual shape of the features fabricated by a specific cantilever‐free SPL technique. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 731–738     

Ionic liquid microdroplets as versatile lithographic molds for sculpting curved topographies on soft materials surfaces

Soft lithography comprises a set of approaches for shaping the surface of soft materials such as PDMS on the microscopic scales. These procedures usually begin with the development of templates/masters normally generated by electron or photolithography techniques. However, the richness in available shapes is limited, usually producing shapes containing sharp parts. Innovation is called for to develop reliable approaches capable of imparting well-defined 3D curved shapes to these solids, a topology that is somehow unnatural for solid surfaces. Here we report on the use of tiny drops of room-temperature ionic liquid, organic liquids that have attracted increasing amounts of attention in recent years because of their unique chemical properties) as a versatile platform for imprinting PDMS with tunable 3D curved geometry, which is out of reach of conventional lithographic techniques and ranges from almost flat depressions to almost closed cavities on the millimeter to micrometer scale. The concept exploits a peculiar combination of physical properties displayed by ionic liquids as their null volatility and their polarity, together with some unique properties of liquid surfaces as their virtually null surface roughness. Proof-of-concept experiments show their application as chemical microreactors and ultrasmooth optical lenses. This all-liquid method is simple, low-cost, versatile, maskless, tension-free, and easily scalable, so we envision a community-wide application in numerous modern physical, chemical, biological, and engineering settings.     

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.     

New Developments in Soft Lithography

Abstract

The burgeoning area of soft lithography is reviewed with special emphasis on developments within the past three years. Applications in electronics have driven such developments, but more recently, other kinds of device structures and 3D prototyping have also found application, in part, through soft lithography. Microcontact printing (μCP), “lift off” μCP nano transfer printing (nTP), micromolding in capillaries (MIMIC), solvent assisted micromolding (SAMIM), replica molding (REM), and microtransfer molding are the main soft lithography schemes discussed.     

Photopatternable silicon elastomers with enhanced mechanical properties for high-fidelity nanoresolution soft lithography

Soft lithography has been widely used in stamping and printing processes for microfabrication as a low cost alternative to photolithography. However, conventional poly(dimethyl)siloxane (PDMS) stamp materials have limitations, especially in the submicrometer range, due to their low physical toughness and requirements for thermocuring. A new version of functional stamp materials with adjustable physical toughness has been developed for advanced soft lithography. We thus demonstrate here its photopatternability and nanoresolution soft lithography, which have proven to be difficult using commercial stamp materials.     

Generation of submicrometer structures by photolithography using arrays of spherical microlenses

This paper describes the fabrication of arrays of spherical microlenses by self-assembly of microspheres and the use of these arrays for nearfield photolithography to generate repetitive microstructures in photoresist. We used these arrays of microspheres to fabricate two types of elastomeric membranes: (i) membranes that have microspheres embedded in their surface and (ii) membranes that have hemispherical wells in their surface. Both types of membranes act as amplitude masks that pattern the intensity of illumination in the near field incident on the photoresist. Microspheres in the first type of membrane act as convergent lenses that generate recessed microstructures in positive photoresist. Hemispherical wells in the second type of membrane act as divergent lenses that produce protrusive microstructures in positive photoresist. This method can generate dense, regular arrays of microstructures with a variety of profiles--circular or hexagonal holes, circular posts, hollow posts, and cones--depending on the sizes and refractive indices of the spherical lenses and the distance between the lenses and the photoresist. This technique provides a simple route to large areas (>4 cm2) of repetitive, simple microstructures.     

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.     

Hard X-rays and soft-matter: processing of sol–gel films from a top down route

A current trend of applied research in the field of nanomaterials is the integration of bottom up and top down fabrication methods. Sol–gel chemistry is widely applied to obtain different functional materials from a bottom up route, especially in the case of thin films. To fabricate devices based on sol–gel films, which include nanocomposites and mesoporous ordered materials, application of lithography technologies is mandatory. Among the different lithographic approaches, photolithography is widely used by companies using micro-fabrication processes. In this context, photolithography is a typical top down method that requires to be integrated as much as possible with deposition of thin films from a liquid phase. Recently we have developed a new integrated fabrication method which uses high energy photons, such as hard X-rays, which typically have energies between 2.5 and 12 keV, for the manipulation and production of a large variety of functional materials and devices. In the present review a short overview of such achievements is presented.     

Molding of three-dimensional microstructures of gels

This Communication describes the use of patterned elastomeric stamps to mold, release, and stack hydrogels into three-dimensional microstructures. Molding of gels against stamps derivatized by a hexa(ethylene glycol)-terminated self-assembled monolayer or by an adsorbed monolayer of bovine serum albumin allowed the application of several soft lithographic techniques (replica molding, microtransfer molding, and micromolding in capillaries) to the microfabrication of gels. We describe procedures to generate coplanar or bilayered composites of gels.     

Microscale features and surface chemical functionality patterned by electron beam lithography: a novel route to poly(dimethylsiloxane) (PDMS) stamp fabrication

Poly(dimethylsiloxane) (PDMS) has become a ubiquitous material for microcontact printing, yet there are few methods available to pattern a completed PDMS stamp in a single step. It is shown here that electron beam lithography (EBL) is effective in writing patterns directly onto cured PDMS stamps, thus overcoming the need for multiple patterning steps. Not only does this method allow the modification of an existing lithographic pattern, but new 3D features such as cones, pits, and channels can also be fabricated. EBL can also be used to fabricate PDMS masks for photolithography whereby 1:1 pattern transfer into a photoresist is achieved. Additionally, direct EBL writing of surface chemical features has been achieved using a PDMS stamp coated with a self-assembled monolayer. An electrostatic mechanism appears to be operative in the EBL patterning process, as supported by calculations, thermogravimetric analysis, time-of-flight secondary ion mass spectroscopy, optical and atomic force microscopy, and chemical functionalization assays.     

Guided growth of nanoscale conducting polymer structures on surface-functionalized nanopatterns

We describe an electrochemical method of directly growing conducting polymer nanostructures between metal electrodes with the geometry controlled by hydrophilic/hydrophobic patterns. The surface patterning can be achieved by a large number of lithographic methods such as AFM, electron-beam, elastomeric microprinting, and photolithography and is compatible with industrial semiconductor fabrication processes. Conducting polymer structures so formed have good alignment compared to bulk synthesis and are grown in place between electrodes. Polypyrrole field effect transistors have been produced using this method. Electrical measurements show conductivity strongly dependent on the presence of anionic dopant species during growth. Devices grown with a high concentration of dopant show metallic behavior, while those with less doping behave as p-type semiconductors.     

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.     

Surface Molding of Microscale Hydrogels with Microactuation Functionality

This work describes the fabrication of numerous hydrogel microstructures (μ‐gels) via a process called “surface molding.” Chemically patterned elastomeric‐assembly substrates were used to organize and manipulate the geometry of liquid prepolymer microdroplets, which, following photo‐initiated crosslinking, maintained the desired morphology. By adjusting the state of strain during the crosslinking process, a continua of structures could be created using one pattern. These arrays of μ‐gels have stimuli‐responsive properties that are directly applicable to actuation where the basis shape and array geometry of the μ‐gels can be used to rationally generate microactuators with programmed motions. As a method, “surface molding,” represents a powerful addition to the soft‐lithographic toolset that can be readily applied to the simultaneous synthesis of large numbers of geometrically and functionally distinct polymeric microstructures.