Mechanical Gradient Cues for Guided Cell Motility and Control of Cell Behavior on Uniform Substrates

A novel method for the fabrication and the use of simple uniform poly(dimethylsiloxane) PDMS substrates for controlling cell motility by a mechanical gradient is reported. The substrate is fabricated in PDMS using soft lithography and consists of a soft membrane suspended on top of a patterned PDMS substrate. The difference in the gradient stiffness is related to the underlying pattern. It is shown experimentally that these uniform substrates can modulate the response of cell motility, thus enabling patterning on the surfaces with precise cell motility. Because of the uniformity of the substrate, cells can spread equally and a directional movement to stiffer regions is clearly observed. Varying the geometry underlying the membrane, cell patterning and movement can be quantitatively characterized. This procedure is capable of controlling cell motility with high fidelity over large substrate areas. The most significant advance embodied in this method is that it offers the use of mechanical features to control cell adhesion and not topographical or chemical variations, which has not been reported so far. This modulation of the response of cell motility will be useful for the design and fabrication of advanced planar and 3D biological assemblies suitable for applications in the field of biotechnology and for tissue‐engineering purposes.     

Fabrication of Stable Metallic Patterns Embedded in Poly(dimethylsiloxane) and Model Applications in Non‐Planar Electronic and Lab‐on‐a‐Chip Device Patterning

This article describes the fabrication of durable metallic patterns that are embedded in poly(dimethylsiloxane) (PDMS) and demonstrates their use in several representative applications. The method involves the transfer and subsequent embedding of micrometer‐scale gold (and other thin‐film material) patterns into PDMS via adhesion chemistries mediated by silane coupling agents. We demonstrate the process as a suitable method for patterning stable functional metallization structures on PDMS, ones with limiting feature sizes less than 5 μm, and their subsequent utilization as structures suitable for use in applications ranging from soft‐lithographic patterning, non‐planar electronics, and microfluidic (lab‐on‐a‐chip, LOC) analytical systems. We demonstrate specifically that metal patterns embedded in both planar and spherically curved PDMS substrates can be used as compliant contact photomasks for conventional photolithographic processes. The non‐planar photomask fabricated with this technique has the same surface shape as the substrate, and thus facilitates the registration of structures in multilevel devices. This quality was specifically tested in a model demonstration in which an array of one hundred metal oxide semiconductor field‐effect transistor (MOSFET) devices was fabricated on a spherically curved Si single‐crystalline lens. The most significant opportunities for the processes reported here, however, appear to reside in applications in analytical chemistry that exploit devices fabricated using the methods of soft lithography. Toward this end, we demonstrate durably bonded metal patterns on PDMS that are appropriate for use in microfluidic, microanalytical, and microelectromechanical systems. We describe a multilayer metal‐electrode fabrication scheme (multilaminate metal–insulator–metal (MIM) structures that substantially enhance performance and stability) and use it to enable the construction of PDMS LOC devices using electrochemical detection. A polymer‐based microelectrochemical analytical system, one incorporating an electrode array for cyclic voltammetry and a microfluidic system for the electrophoretic separation of dopamine and catechol with amperometric detection, is demonstrated.     

Cover Picture: Fabrication of Stable Metallic Patterns Embedded in Poly(dimethylsiloxane) and Model Applications in Non‐Planar Electronic and Lab‐on‐a‐Chip Device Patterning (Adv. Funct. Mater. 4/2005)

A composite image is shown that highlights examples of device architectures that either incorporate or exploit polymer‐embedded metallic microstructures. In work reported by Nuzzo and co‐workers on p. 557, new applications of soft lithography, in conjunction with advanced forms of multilayer metallization, are used to construct these exceptionally durable structures. They are suitable for use in non‐planar lithographic patterning, and as device components finding applications ranging from microelectronics to Lab‐on‐a‐Chip analytical systems. This article describes the fabrication of durable metallic patterns that are embedded in poly(dimethylsiloxane) (PDMS) and demonstrates their use in several representative applications. The method involves the transfer and subsequent embedding of micrometer‐scale gold (and other thin‐film material) patterns into PDMS via adhesion chemistries mediated by silane coupling agents. We demonstrate the process as a suitable method for patterning stable functional metallization structures on PDMS, ones with limiting feature sizes less than 5 μm, and their subsequent utilization as structures suitable for use in applications ranging from soft‐lithographic patterning, non‐planar electronics, and microfluidic (lab‐on‐a‐chip, LOC) analytical systems. We demonstrate specifically that metal patterns embedded in both planar and spherically curved PDMS substrates can be used as compliant contact photomasks for conventional photolithographic processes. The non‐planar photomask fabricated with this technique has the same surface shape as the substrate, and thus facilitates the registration of structures in multilevel devices. This quality was specifically tested in a model demonstration in which an array of one hundred metal oxide semiconductor field‐effect transistor (MOSFET) devices was fabricated on a spherically curved Si single‐crystalline lens. The most significant opportunities for the processes reported here, however, appear to reside in applications in analytical chemistry that exploit devices fabricated using the methods of soft lithography. Toward this end, we demonstrate durably bonded metal patterns on PDMS that are appropriate for use in microfluidic, microanalytical, and microelectromechanical systems. We describe a multilayer metal‐electrode fabrication scheme (multilaminate metal–insulator–metal (MIM) structures that substantially enhance performance and stability) and use it to enable the construction of PDMS LOC devices using electrochemical detection. A polymer‐based microelectrochemical analytical system, one incorporating an electrode array for cyclic voltammetry and a microfluidic system for the electrophoretic separation of dopamine and catechol with amperometric detection, is demonstrated.     

Soft UV-nanoimprint lithography on non-planar surfaces

We report on a simple and effective process that allows direct UV-imprinting of micro- and nanostructures on non-planar surfaces, even at sharp edges such as step surfaces. The key for the process is the use of a thin flexible polymer stamp, which was fabricated by spin-coating poly(dimethylsiloxane) (PDMS) on a pre-patterned Si or poly(methyl methacrylate) (PMMA) master and releasing the thin PDMS layer after curing. The thin PDMS stamp was used to conformally mold a UV resist layer coated on various non-planar substrates with different radii of curvature. With this method, we have successfully demonstrated micro- and nanopatterns down to 63 nm on curved surfaces as well as sharp step-like structures. The process so developed will improve the versatility and applicability of molding technologies in many applications that require patterning non-planar substrates, considering that most molding technologies allow for patterning only on planar substrates or surfaces with large curvature radii.     

Size‐Scalable and High‐Density Liquid‐Metal‐Based Soft Electronic Passive Components and Circuits Using Soft Lithography

The use of conducting liquids with high electrical conductivity, such as eutectic gallium–indium (EGaIn), has great potential in electronics applications requiring stretchability and deformability beyond conventional flexible electronics relying on solid conductors. An advanced liquid metal thin‐line patterning process based on soft lithography and a compatible vertical integration technique are presented that enable size‐scalable and high‐density EGaIn‐based, soft microelectronic components and circuits. The advanced liquid metal thin‐line patterning process based on poly(dimethylsiloxane) (PDMS) substrates and soft lithography techniques allows for simultaneous patterning of uniform and residue‐free EGaIn lines with line width from single micrometers to several millimeters at room temperature and under ambient pressure. Using this fabrication technique, passive electronic components and circuits are investigated under elastic deformations using numerical and experimental approaches. In addition, soft through‐PDMS vias with high aspect ratio are demonstrated for multilayer interconnections in 2.5D and 3D integration approaches. To highlight the system‐level potential of the patterning technique, a chemical sensor based on an integrated LC resonance circuit with a microfluidic‐tunable interdigitated capacitor and a planar spiral inductor is fabricated and characterized. Finally, to show the flexibility and stretchability of the resulting electronics, circuits with embedded light emitting diodes (LEDs) are investigated under bending, twisting, and stretching deformations.     

Anodized Aluminum Oxide/Polydimethylsiloxane Hybrid Mold for Roll‐to‐Roll Nanoimprinting

Two types (hard and soft) of the molds are widely used in nanoimprint lithography for a high throughput over a large area, and high‐resolution parallel patterning. Although hard molds have proven excellent resolutions and can be used at high temperatures, cracks often occur in the mold in addition to the requirement of high imprinting pressure. On the other hand, though soft molds can operate at lower pressures, they give poor pattern resolution. Here, a novel hybrid mold of anodized aluminum oxide (AAO) template attached to a flexible polydimethylsiloxane (PDMS) plate is introduced. Due to the flexible nature of PDMS, various polymer nanostructures are obtained on flat and curved substrates without crack formation on the AAO mold surface. Furthermore, the hybrid mold is successfully used for roll‐to‐roll imprinting for the fabrication of high density array of various shaped polymeric nanostructures over a large area.     

Modeling of in-plane distortions due to variations in absorber stress

The effort to achieve sub-0.25 μm X-ray lithography depends, in part, on the ability to maintain strict fabrication control leading to low distortion X-ray masks. This paper presents finite element (FE) models developed to identify sources of pattern in-plane distortions (IPD) during mask fabrication. In particular, mask fabrication processes inducing both uniform and non-uniform absorber stresses and the resulting distortions due pattern transferring through these stressed layers have been investigated.     

Detachment Lithography of Photosensitive Polymers: A Route to Fabricating Three‐Dimensional Structures

A technique to create arrays of micrometer‐sized patterns of photosensitive polymers on the surface of elastomeric stamps and to transfer these patterns to planar and nonplanar substrates is presented. The photosensitive polymers are initially patterned through detachment lithography (DL), which utilizes the difference in adhesion forces to induce the mechanical failure in the film along the edges of the protruded parts of the mold. A polydimethylsiloxane (PDMS) stamp with a kinetically and thermally adjustable adhesion and conformal contact can transfer the detached patterns to etched or curved substrates, as well as planar ones. These printed patterns remain photochemically active for further modification via photolithography, and/or can serve as resists for subsequent etching or deposition, such that photolithography can be used on highly nonconformal and nonplanar surfaces. Various 3D structures fabricated using the process have potential applications in MEMS (micro‐electromechanical systems) sensors/actuators, optical devices, and microfluidics.     

Fabrication of Organic Thin‐Film Transistors on Three‐Dimensional Substrates Using Free‐Standing Polymeric Masks Based on Soft Lithography

Here, a novel fabrication technique for integrated organic devices on substrates with complex structure is presented. For this work, free‐standing polymeric masks with stencil‐patterns are fabricated using an ultra‐violet (UV) curable polyurethaneacrylate (PUA) mixture, and used as shadow masks for thermal evaporation. High flexibility and adhesive properties of the free‐standing PUA masks ensure conformal contact with various materials such as glass, silicon (Si), and polymer, and thus can also be utilized as patterning masks for solution‐based deposition methods, such as spin‐coating and drop‐casting. Based on this technique, a number of integrated organic transistors are fabricated simultaneously on a cylindrical glass bottle with high curvature, as well as on a flat silicon wafer. It is anticipated that these results will be applied to the development of various integrated organic devices on complex‐structured substrates, which can lead to further applications.     

Functional Patterning of Biopolymer Thin Films Using Enzymes and Lithographic Methods

Two different lithographic techniques for the patterning of thin biopolymer films are developed. The first method is based on using a microstructured elastomeric mold for the structuring of thin films of regenerated cellulose. The thin films are manufactured by spin‐coating of trimethylsilyl cellulose (TMSC) and subsequent regeneration. The microchannels formed by the mold and the cellulose film are filled with a cellulase solution by capillary action. In the areas exposed to the enzyme solution, the cellulose film is digested, whereas the area in contact with the mold is protected from the enzymatic activity. Optical thickness measurements, atomic force microscopy and fluorescent staining confirm a successful patterning of cellulose on several substrates by this method. The second method is based on the structured regeneration of thin TMSC films. TMSC surfaces are protected with metal masks and exposed to vapors of hydrochloric acid. These treatments result in hydrophilic cellulose structures surrounded by hydrophobic TMSC with differing physicochemical properties. Treatments of the obtained structures with cellulases allow the selective removal of pure cellulose, whereas a TMSC pattern remains on the surface. These TMSC can be regenerated back to pure cellulose by treatments with vapors of hydrochloric acid. The developed methods allow the effective fabrication of micropatterned biopolymer thin films suitable for further functionalization and application in, e.g., bioanalytical devices. This is shown by the immobilization and detection of single‐stranded DNA on structured cellulose surfaces. Owing to the versatility of both patterning approaches the methods can be further extended to other combinations of substrates and enzymes.     

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。     

Polydimethylsiloxane Composites for Optical Ultrasound Generation and Multimodality Imaging

Polydimethylsiloxane (PDMS) is widely used in biomedical science and can form composites that have broad applicability. One promising application where PDMS composites offer several advantages is optical ultrasound generation via the photoacoustic effect. Here, methods to create these PDMS composites are reviewed and classified. It is highlighted how the composites can be applied to a range of substrates, from micrometer‐scale, temperature‐sensitive optical fibers to centimeter‐scale curved and planar surfaces. The resulting composites have enabled all‐optical ultrasound imaging of biological tissues both ex vivo and in vivo, with high spatial resolution and with clinically relevant contrast. In addition, the first 3D all‐optical pulse‐echo ultrasound imaging of ex vivo human tissue, using a PDMS‐multiwalled carbon nanotube composite and a fiber‐optic ultrasound receiver, is presented. Gold nanoparticle‐PDMS and crystal violet‐PDMS composites with prominent absorption at one wavelength range for pulse‐echo ultrasound imaging and transmission at a second wavelength range for photoacoustic imaging are also presented. Using these devices, images of diseased human vascular tissue with both structural and molecular contrast are obtained. With a broader perspective, literature on recent advances in PDMS microfabrication from different fields is highlighted, and methods for incorporating them into new generations of optical ultrasound generators are suggested.     

Soft Graphoepitaxy for Large Area Directed Self‐Assembly of Polystyrene‐block‐Poly(dimethylsiloxane) Block Copolymer on Nanopatterned POSS Substrates Fabricated by Nanoimprint Lithography

Polyhedral oligomeric silsequioxane (POSS) derivatives have been successfully employed as substrates for graphoepitaxial directed self‐assembly (DSA) of block copolymers (BCPs). Tailored POSS materials of tuned surface chemistry are subject to nanoimprint lithography (NIL) resulting in topographically patterned substrates with dimensions commensurate with the BCP block length. A cylinder forming polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) BCP is synthesized by sequential living anionic polymerization of styrene and hexamethylcyclotrisiloxane. The patterned POSS materials provide a surface chemistry and topography for DSA of this BCP and after solvent annealing the BCP shows well‐ordered microphase segregation. The orientation of the PDMS cylinders to the substrate plane could be controlled within the trench walls by the choice of the POSS materials. The BCP patterns are successfully used as on‐chip etch mask to transfer the pattern to underlying silicon substrate. This soft graphoepitaxy method shows highly promising results as a means to generate lithographic quality patterns by nonconventional methods and could be applied to both hard and soft substrates. The methodology might have application in several fields including device and interconnect fabrication, nanoimprint lithography stamp production, nanofluidic devices, lab‐on‐chip, or in other technologies requiring simple nanodimensional patterns.     

Multifaceted and Nanobored Particle Arrays Sculpted Using Colloidal Lithography

A novel method of fabricating multifaceted and nanobored particle arrays via colloidal lithography using colloidal‐crystal layers as masks for anisotropic reactive‐ion etching (RIE) is reported. The shape of the sculpted particles is dependent on the crystal orientation relative to the etchant flow, the number of colloidal layers, the RIE conditions, and the matrix (or mask) structure in colloidal lithography. Arrays of non‐spherical particles with sculpted shapes, which to date could not otherwise be produced, are fabricated using a tilted anisotropic RIE process and the layer‐by‐layer growth of a colloidal mask. These non‐spherical particles and their ordered arrays can be used for antireflection surfaces, biosensors, and nanopatterning masks, as well as non‐spherical building blocks for novel colloidal crystals. In addition, polymeric particles with patterned holes of controlled depths obtained by the present method can be applied to the fabrication of functional composite particles.     

Transferable Crack‐Free Colloidal Crystals on an Elastomeric Matrix with Surface Relief

Crack‐free three‐dimensional (3D) colloidal silica crystals are fabricated on an elastomeric polydimethylsiloxane (PDMS) stamp via the lift‐up method. A surface relief structure is fabricated on the PDMS substrate to enable the formation of colloidal crystal assemblies that cannot be achieved on a plane PDMS substrate owing to the hydrophobic nature of its surface. Four samples of uniform silica particles having different sizes are prepared for colloidal crystal assembly on PDMS substrates with various relief patterns. This strategy not only provides a means for the assembly of crack‐free colloidal crystals on a soft hydrophobic surface via the lift‐up method but enables the transfer of the crack‐free colloidal crystals onto a curved surface.     

A Versatile Approach for Direct Patterning of Liquid Metal Using Magnetic Field

Patterning of liquid metal (LM) is usually an integral step toward its practical applications. However, the high surface tension along with surface oxide makes direct patterning of LM very challenging. Existing LM patterning techniques are designed for limited types of planar substrates, which require multiple‐step operation, delicate molds and masks, and expensive equipment. In this work, a simple, versatile, and equipment‐free approach for direct patterning of LM on various substrates using magnetic field is reported. To achieve this, magnetic microparticles are dispersed into LM by stirring. When a moving magnetic field is applied to the LM droplet, the aggregated magnetic microparticles deform the droplet to a continuous line. In addition, this approach is also applicable to supermetallophobic substrates since the applied magnetic field significantly enhances the contact between LM and substrate. Moreover, remote manipulation of the magnetic microparticles allows direct patterning of LM on nonplanar surfaces, even in a narrow and near closed space, which is impossible for the existing techniques. A few applications are also demonstrated using the proposed technique for flexible electronics and wearable sensors.     

Application of X-rays to nanolithography

The development of a successful fabrication process for electron devices with dimensions in the sub-100-nm domain will require a form of a high-resolution and high-volume patterning. In this paper we discuss the extensibility of X-ray lithography to this domain in terms of the resolution of the technique, considering in detail the effect of diffraction and photoelectrons. We show that optimized masks and exposure systems can deliver with relative ease patterning in the 70-50-nm region, while phase-shifting techniques can extend the resolution to sub-40 nm. High volume is provided by the use of the mask. The challenge remains in the fabrication of the IX mask, and in the achievement of the necessary placement accuracy     

Soft Imprinting: Creating Highly Ordered Porous Anodic Alumina Templates on Substrates for Nanofabrication

We demonstrate a “soft‐imprinting” method for the fabrication of highly ordered porous anodic alumina (HOPAA) templates on different substrates (such as Si, glass slides, and flexible polyimide films) over large areas (> 1.5 cm²). In this process, Ar plasma etching is employed to soft imprint an evaporated Al film on the substrates using a free‐standing HOPAA template as a mask, thus creating ordered nanoindentations on the Al surface. The ordered nanoindentations in turn guide the subsequent anodization of Al to generate HOPAA templates on the substrates (HOPAA–substrates), which inherit the pattern of the free‐standing HOPAA mask. This soft‐imprinting technique is also applicable to the fabrication of HOPAA on flexible polymer films. To demonstrate the potential uses of the HOPAA–substrates in nanofabrication, highly ordered Au nanowire arrays are fabricated on a Si substrate and TiO₂ nanotube arrays are prepared on a glass substrate via solution‐ and vapor‐based fabrication processes, respectively.     

Rigorous diffraction simulations of topographic wafer stacks in double patterning

Double patterning is regarded as a potential candidate to achieve the 32 nm node in semiconductor manufacturing. A key problem for a standard litho-etch–litho-etch (LELE) double patterning process is to evaluate and tackle the impact of the wafer topography resulting from the hardmask pattern on the second lithography step. In this paper, we apply rigorous electromagnetic field (EMF) solvers to investigate the wafer topography effects. At first, the studied 3D mask is split into two masks. The topography resulting from the exposure with the first split mask is described by a patterned hardmask. Based on that, the bottom antireflective coating (BARC) thickness of the second wafer stack is optimized. Alternatively, a two beam interference and the full diffraction spectrum of the second mask are used as the illumination of the wafer stacks, respectively. Finally, simulated 3D resist profiles for different BARC thicknesses are shown. The importance of wafer topography impact, the optimization of topographic wafer stacks, and the possible solutions to compensate for the impact of the wafer topography are discussed.