Microcontact printing of biomolecular gratings from SU-8 masters duplicated by Thermal Soft UV NIL

Thermal Soft UV nanoimprint lithography (NIL) was performed to replicate nanostructures in SU-8 resist. The SU-8 resist was structured with a PDMS stamp molded against an original silicon master which comported gratings of lines (500 nm width/1 μm pitch). The patterns obtained in SU-8 were used in a second step as a template for PDMS molding of daughter stamps. Pattern transfer quality and dimension control were achieved on these second generation PDMS stamps using AFM measurements. As a final validation of the whole duplication processes, these second generation PDMS stamps were finally employed to perform μCP of streptavidin molecules on a glass slide activated by plasma O₂ treatment. AFM observation and fluorescence microscopy reveal that molecular patterns produced with SU8-molded PDMS stamps are not discernable from those obtained with a PDMS stamp directly molded on the original silicon master. Coupling Thermal Soft UV NIL and microcontact printing opens a new method for generating a large quantity of SU-8 templates on which functional PDMS stamps can be replicated in a reduced time. We thus propose a functional duplication process for soft-lithography implementation which may further reduce the cost of this technology for industrial development.     

Soft Electronics Manufacturing Using Microcontact Printing

This work describes a microcontact printing (µCP) process for reproducible manufacturing of liquid gallium alloy–based soft and stretchable electronics. One of the leading approaches to create soft and stretchable electronics involves embedding liquid metals (LM) into an elastomer matrix. Although the advantages of liquid metal–based electronics have been well established, their mainstream adoption and commercialization necessitates development of precise and scalable manufacturing methods. To address this need, a scalable µCP process is presented that uses surface‐functionalized, reusable rigid, or deformable stamps to transfer eutectic gallium–indium (EGaIn) patterns onto elastomer substrates. A novel approach is developed to create the surface‐functionalized stamps, enabling selective transfer of LM to desired locations on a substrate without residues or electrical shorts. To address the critical needs of precise and reproducible positioning, alignment, and stamping force application, a high‐precision automated µCP system is designed. After describing the approach, the precision of stamps is evaluated and EGaIn features (as small as 15 µm line width), as well as electrical functionality of printed circuits with and without deformation, are fabricated. The presented process addresses many of the limitations associated with the alternative fabrication processes, and thus provides an effective approach for scalable fabrication of LM‐based soft and stretchable microelectronics.     

Reversal μCP using hard stamps

In this work, we present a new method of Micro-Contact Printing (μCP) which we call reversal μCP using hard stamps which can be used for the fabrication of different structures like negative index materials, e.g., split ring resonators (SRRs), dots and squares made of gold. Typically soft stamps made of PDMS (polydimethylsiloxane) inked with thiols are used for μCP. The softness of the stamp material entails a lot of problems like deformation of the structures, sagging and pairing of the protruding features and reliability of the process. We use hard stamps which are spin coated with a thiol solution so that the thioles stay in the recessed areas of the stamp. In the following μCP process an EVG^®620 is used to bring the stamp and substrate into contact so that the thiols on the stamp diffuse and bind to the gold surface and serve as an etch mask for succeeding wet chemical etching. Using this method overcomes the disadvantage of a soft stamp material. Smallest feature sizes down to 100 nm are shown.     

角度定量化估算软光刻PDMS印章的平面变形扭曲

引进与平面图案变形扭曲大小相称的角度参数θ,提出了一种对软光刻技术中PDMS印章的角度平面扭曲进行定量评价的方法。聚二甲基硅氧烷(PDMS)平面印章的单个图案的平均扭曲角(绝对扭曲角θ1)和相邻图案间的扭曲角(相对扭曲角θ2)由角度参数θ的差值(θ)决定。经四种不同粘附固定处理的PDMS印章中进行角度定量估算比较,发现采用硅烷修饰的玻璃板基板制备的PDMS印章显示很强的粘附性和最小的平面变形扭曲,其绝对扭曲角度θ1为3.98×10-3,相对扭曲角θ2为1.22×10-3。研究结果表明,角度定量化估算可实现软光刻中图案平面变形扭曲的定量评价和比较,并通用于弹性印章基板筛分选择。     

Fabrication of ordered micro- and nano-scale patterns based on optical discs and nanoimprint

A simple method to fabricate one-dimensional(1-D) and two-dimensional(2-D) ordered micro- and nano-scale patterns is developed based on the original masters from optical discs, using nanoimprint technology and soft stamps. Polydimethylsiloxane(PDMS) was used to replicate the negative image of the 1-D grating pattern on the masters of CD-R, DVD-R and BD-R optical discs, respectively, and then the 1-D pattern on one of the PDMS stamps was transferred to a blank polycarbonate(PC) substrate by nanoimprint. The 2-D ordered patterns were fabricated by the second imprinting using another PDMS stamp. Different 2-D periodic patterns were obtained depending on the PDMS stamps and the angle between the two times of imprints. This method may provide a way for the fabrication of complex 2-D patterns using simple 1-D masters.     

Soft‐Contact Optical Lithography Using Transparent Elastomeric Stamps and Application to Nanopatterned Organic Light‐Emitting Devices

Conventional photolithography uses rigid photomasks of fused quartz and high‐purity silica glass plates covered with patterned microstructures of an opaque material. We introduce new, transparent, elastomeric molds (or stamps) of poly(dimethylsiloxane) (PDMS) that can be employed as photomasks to produce the same resist pattern as the pattern of the recessed (or non‐contact) regions of the stamps, in contrast to other reports in the literature^[1] of using PDMS masks to generate edge patterns. The exposure dose of the non‐contact regions with the photoresist through the PDMS is lower than that of the contact regions. Therefore, we employ a difference in the effective exposure dose between the contact and the non‐contact regions through the PDMS stamp to generate the same pattern as the PDMS photomask. The photomasking capability of the PDMS stamps, which is similar to rigid photomasks in conventional photolithography, widens the application boundaries of soft‐contact optical lithography and makes the photolithography process and equipment very simple. This soft‐contact optical lithography process can be widely used to perform photolithography on flexible substrates, avoiding metal or resist cracks, as it uses soft, conformable, intimate contact with the photoresist without any external pressure. To this end, we demonstrate soft‐contact optical lithography on a gold‐coated PDMS substrate and utilized the patterned Au/PDMS substrate with feature sizes into the nanometer regime as a top electrode in organic light‐emitting diodes that are formed by soft‐contact lamination.     

Direct Graphene Transfer and Its Application to Transfer Printing Using Mechanically Controlled,Large Area Graphene/Copper Freestanding Layer

Direct graphene transfer is an attractive candidate to prevent graphene damage, which is a critical problem of the conventional wet transfer method. Direct graphene transfer can fabricate the transferred graphene film with fewer defects by using a polymeric carrier. Here a unique direct transfer method is proposed using a 300 nm thick copper carrier as a suspended film and a transfer printing process by using the polydimethylsiloxane (PDMS) stamp under controlled peeling rate and modulus. Single and multilayer graphene are transferred to flat and curved PDMS target substrate directly. With the transfer printing process, the transfer yield of a trilayer graphene with 1000 µm s^?1 peeling rate is 68.6% of that with 1 µm s^?1 peeling rate. It is revealed that the graphene transfer yield is highly related to the storage modulus of the PDMS stamp: graphene transfer yield decreases when the storage modulus of the PDMS stamp is lower than a specific threshold value. The relationship between the graphene transfer yield and the interfacial shear strain of the PDMS stamp is studied by finite‐element method simulation and digital image correlation.     

Nanoimprint in PDMS on glass with two-level hybrid stamp

In this article we examine the use of two-level hybrid-material stamps and nanoimprint lithography (NIL) of poly(dimethylsiloxane) (PDMS) on glass substrates. A silicon/SU8 stamp manufacturing process has been developed, in order to combine nanometer and micrometer structures, thus avoiding complex deep etching processes. The stamp has been test printed in polymethyl methacrylate (PMMA) to demonstrate functionality. We describe polymer flow problems for imprinting large structures and identify optimized parameters, in accordance with previously published findings. The use of PDMS as imprint polymer was examined. Imprinting works well, however, large recovery after separation shrinks the micrometer channels substantially and renders the nanochannels useless. Glass substrates in combination with silicon stamps were used, evaluated and showed to work well at low temperature.     

Diffusion of thiols during microcontact printing with rigid stamps

In this work we investigated the diffusion of thiol molecules during microcontact printing (μCP) with rigid stamps – a process we call reversal μCP. We use such stamps instead of soft polydimethylsiloxane (PDMS) stamps to overcome the problems of sagging and pairing of structures and deformation. We varied thiol concentration in ethanol as well as different contact times. It shows that the lower the concentration the thinner the lines and the longer the contact time the thicker the lines until saturation is reached. Working in this saturation region overcomes the problem of the strong dependency of line width over contact time which is usually observed in microcontact printing and which restricts the reproducibility of the process itself. Gold lines with a width down to 80 nm have been achieved using this novel method.     

Photopatternable Conductive PDMS Materials for Microfabrication

Conductive photodefinable polydimethylsiloxane (PDMS) composites that provide both high electrical conductivity and photopatternability have been developed. The photosensitive composite materials, which consist of a photosensitive component, a conductive filler, and a PDMS pre‐polymer, can be used as a negative photoresist or a positive photoresist with an additional curing agent. A standard photolithographic approach has been used to fabricate conductive elastomeric microstructures. Feature sizes of 60 µm in the positive photoresist and 10 µm in the negative photoresist have been successfully achieved. Moreover, as the conductive filler, silver powders significantly improve the electrical conductivity of the PDMS polymer, but also provide enhanced mechanical and thermal properties as well as interesting biological properties. The combined electrical, mechanical, thermal, and biological properties along with photopatternability make the PDMS‐Ag composite an excellent processable and structural material for various microfabrication applications.     

Capillary Force Lithography: A Versatile Tool for Structured Biomaterials Interface Towards Cell and Tissue Engineering

This Feature Article aims to provide an in‐depth overview of the recently developed molding technologies termed capillary force lithography (CFL) that can be used to control the cellular microenvironment towards cell and tissue engineering. Patterned polymer films provide a fertile ground for controlling various aspects of the cellular microenvironment such as cell–substrate and cell–cell interactions at the micro‐ and nanoscale. Patterning thin polymer films by molding typically involves several physical forces such as capillary, hydrostatic, and dispersion forces. If these forces are precisely controlled, the polymer films can be molded into the features of a polymeric mold with high pattern fidelity and physical integrity. The patterns can be made either with the substrate surface clearly exposed or unexposed depending on the pattern size and material properties used in the patterning. The former (exposed substrate) can be used to adhere proteins or cells on pre‐defined locations of a substrate or within a microfluidic channel using an adhesion‐repelling polymer such as poly(ethylene glycol) (PEG)‐based polymer and hyaluronic acid (HA). Also, the patterns can be used to co‐culture different cells types with molding‐assisted layer‐by‐layer deposition. In comparison, the latter (unexposed substrate) can be used to control the biophysical surrounding of a cell with tailored mechanical properties of the material. The surface micropatterns can be used to engineer cellular and multi‐cellular architecture, resulting in changes of the cell shape and the cytoskeletal structures. Also, the nanoscale patterns can be used to affect various aspects of the cellular behavior, such as adhesion, proliferation, migration, and differentiation.     

Reactive Imprint Lithography: Combined Topographical Patterning and Chemical Surface Functionalization of Polystyrene‐block‐poly(tert‐butyl acrylate) Films

Here, reactive imprint lithography (RIL) is introduced as a new, one‐step lithographic tool for the fabrication of large‐area topographically patterned, chemically activated polymer platforms. Films of polystyrene‐block‐poly(tert‐butyl acrylate) (PS‐b‐PtBA) are imprinted with PDMS master stamps at temperatures above the corresponding glass transition and chemical deprotection temperatures to yield structured films with exposed carboxylic acid and anhydride groups. Faithful pattern transfer is confirmed by AFM analyses. Transmission‐mode FTIR spectra shows a conversion of over 95% of the tert‐butyl ester groups after RIL at 230 °C for 5 minutes and a significantly reduced conversion to anhydride compared to thermolysis of neat films with free surfaces in air or nitrogen. An enrichment of the surface layer in PS is detected by angle‐resolved X‐ray photoelectron spectroscopy (XPS). In order to demonstrate application potentials of the activated platforms, a 7 nm ± 1 nm thick NH₂‐terminated PEG layer (grafting density of 0.9 chains nm^?2) is covalently grafted to RIL‐activated substrates. This layer reduces the non‐specific adsorption (NSA) of bovine serum albumin by 95% to a residual mass coverage of 9.1 ± 2.9 ng cm^?2. As shown by these examples, RIL comprises an attractive complementary approach to produce bio‐reactive polymer surfaces with topographic patterns in a one‐step process.     

Novel growth of ZnO micro-rod arrays using hydrophobically micropatterned surfaces

Large-area ZnO micro-rod arrays were successfully achieved on a patterned glass substrate in an aqueous solution at mild growth conditions. Self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS, C₁₈H₃₇SiCl₃) were prepared on a glass substrate through micro-contact printing (μCP) technique. Site-selective ZnO growth was achieved by the patterned SAM-covered substrate. X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were used to analyze as-fabricated ZnO arrays. ZnO micro-rods have a uniform diameter of about 300 nm with an aspect ratio of 15. It is noteworthy that the alkyl end-groups of OTS strongly directed the nucleation and growth of ZnO, and ZnO arrays were crystallized on organic group-covered regions. By modifying the μCP process, patterns that differ from the original PDMS stamp could be obtained.     

Formation of Hierarchically Structured Thin Films

Here, we report the preparation of hierarchically structured polymer brushes with well‐defined geometries via multiple step microcontact printing (MS‐µCP) of inks containing different ratios of initiator‐terminated thiols and non‐reactive alkylthiols. Thick (and dense), polymer brushes grew from self‐assembled monolayers (SAMs) with high concentration of initiator‐terminated thiols, and these brushes exhibited high chemical etch‐resistance, compared to thin (and less dense), brushes grown from more dilute initiator‐terminated SAMs. Upon etching, patterned crosslinking polymer brush films decorated with thin layers of Au, could be lifted off the surface to form geometrically well‐defined free‐standing hierarchical films. These polymer brush films showed interesting buckling instabilities when compressed. Areas with different brush thicknesses and Au backing showed markedly different buckling behavior, leading to unusual patterns of wrinkles with different wavelengths and orientations toward the force field.     

Polyelectrolyte Brushes as Ink Nanoreservoirs for Microcontact Printing of Ionic Species with Poly(dimethyl siloxane) Stamps

In this work a new variation of microcontact printing is described, which is used to transfer chemical patterns onto different substrates. The approach is based on the use of conventional elastomeric stamps modified with polyelectrolyte brushes. It is demonstrated that, by using stamps modified with brushes acting as preconcentrating/sorbent nanolayers, it is possible to control the uptake of aqueous inks containing ionic species. This controlled uptake can be easily used for site‐selective delivery of the loaded species by means of soft lithography. The potential of this approach is demonstrated by creating patterned counterion domains in a flat polyelectrolyte brush and by promoting a site‐selective metallization through galvanic displacement reactions with reactive species.     

软印刷技术

软印刷技术是基于弹性体印章/模具来转移图形结构的微纳加工技术。详细介绍了软印刷技术中转移图形结构的多种方式,并探讨软印刷技术在微纳电子学、光学、传感器、生物等领域的广泛应用。对软印刷技术的弹性体印章/模具制备、聚二甲基硅氧烷的属性、理论研究等进行了探讨。     

Capillary Force Lithography: Fabrication of Functional Polymer Templates as Versatile Tools for Nanolithography

The implementation of high‐resolution polymer templates fabricated by capillary force lithography (CFL) is explored both in nanoimprint lithography (NIL) and in the wet‐etching of metals. Several different thermoplastic and UV‐curable polymers and types of substrates are incorporated into the general CFL procedure to meet the diverging requirements of these two applications. The mechanical stability of UV‐curable templates for imprinting in polymers, as examined by atomic force microscopy (AFM), and their anti‐adhesive properties are excellent for application in NIL. The conditions for curing the UV‐curable polymer are optimized in order to obtain high‐stability polymer templates. Gold patterns on silicon with a lateral resolution of 150 nm are fabricated by subsequent lift‐off in acetone. Similar patterns with a lateral resolution of 100 nm are fabricated using templates of thermoplastic polymers on gold layers on silicon as an etch mask. The transfer of stamp residues during CFL with these polymer templates is proven by X‐ray photoelectron spectroscopy (XPS) and AFM friction analysis. For poly(methylmethacrylate) (PMMA), the presence of large amounts of silicon‐containing residues is found to compromise the processability of the resulting template in subsequent O₂ reactive‐ion etching (RIE) treatment. The extent of silicon contamination is up to six times less for polystyrene (PS). At this level, the etch performance of the PS etch mask is not affected, as was the case for PMMA. Accurate downscaling of the lateral dimensions of the resulting metal patterns by several factors with respect to the dimensions of the PS etch mask is achieved by over‐etching of the gold. Overall, the results in this paper demonstrate the potential of CFL templates as tools for high‐resolution soft lithography.     

Simple Patterning via Adhesion between a Buffered‐Oxide Etchant‐Treated PDMS Stamp and a SiO2 Substrate

A very simple polydimethylsiloxane (PDMS) pattern‐transfer method is devised, called buffered‐oxide etchant (BOE) printing. The mechanism of pattern transfer is investigated, by considering the strong adhesion between the BOE‐treated PDMS and the SiO₂ substrate. PDMS patterns from a few micrometers to sub‐micrometer size are transferred to the SiO₂ substrate by just pressing a stamp that has been immersed in BOE solution for a few minutes. The patterned PDMS layers work as perfect physical and chemical passivation layers in the fabrication of metal electrodes and V₂O₅ nanowire channels, respectively. Interestingly, a second stamping of the BOE‐treated PDMS on the SiO₂ substrate pre‐patterned with metal as well as PDMS results in a selective transfer of the PDMS patterns only to the bare SiO₂. In this way, the fabrication of a device structure consisting of two Au electrodes and V₂O₅ nanowire network channels is possible; non‐ohmic semiconducting I–V characteristics, which can be modeled by serially connected percolation, are observed.     

利用PDMS纳米结构薄膜增强玻璃基板出光

以纳米压印技术为基础制备了具有纳米结构的聚二甲基硅氧烷(PDMS)薄膜.将纳米结构中间聚合物模板(IPS)薄膜覆盖在有PDMS溶液的玻璃基板上,真空加热后在玻璃基板上得到PDMS网格结构薄膜.这种方式得到的网格结构形状保持较好且厚度均匀无气泡,IPS薄膜不仅可以反复使用以减少Si母版的材料损耗,还可以缩短网格结构的制备...     

Fabrication of microlens arrays using UV micro-stamping with soft roller and gas-pressurized platform

The conformal contact between the roller stamper and the thin flexible substrate is important for precisive replication of microstructures. In this study, we have proposed a novel mechanism which employs a soft roller stamper and a gas-pressurized platform to fabricate UV-cured polymeric microlens arrays on a continuous flexible substrate. The new facilities constructed in this study comprise a polydimethylsiloxane (PDMS) stamping roller, a gas-pressurized platform and an UV light source underneath the platform. The soft roller was made by casting a pre-polymer of PDMS in a plastic master of microlens array. During the rolling micro-stamping process, the microlens array cavity on the soft roller is first filled with liquid UV-curable resin. The roller stamper then rolls and stamps over the moving transparent thin polymeric substrate which is located on the gas-pressurized platform. At the same time, the UV light irradiates beneath the platform and cures the resin in the rolling zone. Thus, the microlens array patterns are successfully fabricated. The dimensions of the microlens are 115.5 μm in diameter, of, a sag height of 7.88 μm in sag height, and 200 μm in pitch size. This method developed in this work clearly demonstrates its potential of using the soft mold and the gas-pressurized platform for continuous mass production of films with microstructural patterns.