Soft lithography meets self-organization: Some new developments in meso-patterning

This is a brief review of our recent and ongoing work on simple, rapid, room temperature, pressure-less and large area (∼ cm²) imprinting techniques for high fidelity meso-patterning of different types of polymer films. Examples include soft solid polymer films and surfaces like cross-linked polydimethylsiloxane (PDMS) and polyacrylamide (PAA) based hydrogels, thermoplastics like polystyrene (PS), polymethylmethacrylate (PMMA) etc both on planar and curved surfaces. These techniques address two key issues in imprinting: (i) attainment of large area conformal contact with the stamp, especially on curved surfaces, and (ii) ease of stamp detachment without damage to the imprinted structures. The key element of the method is the use of thin and flexible patterned foils that readily and rapidly come into complete conformal contact with soft polymer surfaces because of adhesive interfacial interactions. The conformal contact is established at all length scales by bending of the foil at scales larger than the feature size, in conjunction with the spontaneous deformations of the film surface on the scale of the features. Complex two-dimensional patterns could also be formed even by using a simple one-dimensional master by multiple imprinting. The technique can be particularly useful for the bulk nano applications requiring routine fabrication of templates, for example, in the study of confined chemistry phenomena, nanofluidics, bio-MEMS, micro-imprinting, optical coatings and controlled dewetting.     

A linear theory for soft ferromagnetic elastic solids

A general theory of magnetoelasticity is developed for soft ferromagnetic materials of multidomain structure, for which the hysteretic loss and exchange effect may be neglected. The general equations are linearized by assuming infinitesimal strains, linear constitutive equations, and that all magnetic variables (magnetic intensity, induction and magnetization) in the deformed body may be divided into two parts: a rigid body state and a perturbation state. The former are the same as those in the magnetostatics for a rigid body and the latter which are the added corrections due to deformations, are coupled with strains and stresses in a set of linear differential equations and boundary conditions. Two versions of the linear theory are given for materials with isotropic, cubic or uniaxial symmetry. One of them is applied to investigate the buckling of an elastic, isotropic plate in a transversally applied uniform magnetic field. The calculated results agree with previous observations.     

A comparison between bottom contact and top contact all organic field effect transistors assembled by soft lithography

All-organic Field Effect Transistors (FETs) on plastic were fabricated by means of an innovative, simple and inexpensive technique. A thin Mylar® foil acts both as substrate and gate dielectric. We used pentacene, deposited by thermal sublimation, as semiconducting layer while contacts were fabricated with poly(ethylene-dioxythiophene)/polystyrene sulfonate (PEDOT/PSS) by means of soft lithography. On the opposite side of the foil a thin PEDOT/PSS film, acting as gate electrode, was spin coated. It is worth noting that this technique allows the realization of bottom contact and top contact devices on the same substrate and with the same semiconducting layer. Furthermore, assisted by Scanning Probe Microscopy investigations, we investigated how the device structure influences its electrical properties in terms of hole mobility, Series Contact Resistance and parasitic capacitance effects. The comparison between top-contact and bottom-contact devices shows interesting marked differences that can be mainly attributed to a different quality of PEDOT/PSS-semiconductor interface. The flexibility of the obtained structure and the easy scalability of the technological process open the way for economic production of high-resolution organic devices.     

Collective buckling of an elastic beam array on an elastic substrate for applications in soft lithography

We analyze the collective buckling of an array of vertical elastic beams with their lower ends built into an elastic substrate. The beams interact between themselves through the deformation of the elastic substrate. The present analysis is more sophisticated than previous ones on rigid beams on an elastic substrate in that the beams are regarded as elastic rather than rigid. From the linear theory for elastic beam buckling and the linear theory of elasticity, an eigenvalue problem is formulated and solved. Calculations show that the deformability of the beams lowers the critical height of the beams, but it does not affect the buckling pattern much. Our work also suggests that the collective buckling is dominated by the interaction of neighboring beams through the deformation of the substrate rather than whether the beams are rigid or elastic. The results are useful for the better understanding, design and application of the nanostructures produced by soft lithography.     

Fabrication of mid-infrared frequency-selective surfaces by soft lithography

We describe the fabrication of large areas (4 cm(2)) of metallic structures or aperture elements that have ~100-350-nm linewidths and act as frequency-selective surfaces. These structures are fabricated with a type of soft lithography-near-field contact-mode photolithography-that uses a thin elastomeric mask having topography on its surface and is in conformal contact with a layer of photoresist. The mask acts as an optical element to create minima in the intensity of light delivered to the photoresist. Depending on the type of photoresist used, lines of, or trenches in, photoresist are formed on the substrate by exposure, development, and lift-off. These surfaces act as bandpass or bandgap filters in the infrared.     

A generalized comninou contact model for interface cracks in anisotropic elastic solids

An open question of interest to the mechanics of interface fracture is how to generalize the Comninou contact model for interface cracks in isotropic solids to the general anisotropic case. Part of the difficulty lies in that the peculiar oscillatory behavior can not be fully eliminated by Comninou's original assumption of pure pressure contact between the crack surfaces. In this paper, we propose a model that strictly enforces the non-oscillatory condition by allowing the crack face contact force to have a shear component normal to the direction of slip, which is somewhat reminiscent of frictionless slip between a pair of grooved surfaces. Based on that model, complex variable representations are adopted to determine the complete series expansion for the crack-tip field. The solutions are incorporated into a hybrid finite element procedure to develop a special element for closed interfacial crack tips obeying the generalized contact model. Numerical examples involving a partially closed crack between a pair of misoriented cubic crystals are given to illustrate how the special crack-tip element helps in determining the stress intensity factors as well as the contact zone geometry.     

Programmable chemical gradient patterns by soft grayscale lithography

A method for fabricating chemical gradients on planar and nonplanar substrates using grayscale lithography is reported. Compliant grayscale amplitude masks are fabricated using a vacuum-assisted microfluidic filling protocol that employs dilutions of a carbon-black-containing polydimethylsiloxane emulsion (bPDMS) within traditional clear PDMS (cPDMS) to create planar, fully self-supporting mask elements. The mask is then placed over a surface functionalized with a hydrophobic coumarin-based photocleavable monolayer, which exposes a polar group upon irradiation. The mask serves to modulate the intensity of incident UV light, thereby controlling the density of molecules cleaved. The resulting molecular-level grayscale patterns are characterized by condensation microscopy and imaging mode time-of-flight secondary-ion mass spectrometry (ToF-SIMS). Due to the inherent flexibility of this technique, the photofuse as well as the gradient patterns can be designed for a wide range of applications; in this paper two proof-of-concept demonstrations are shown. The first utilizes the ability to control the resulting contact angle of the surface for the fabrication of a passive pressure-sensitive microfluidic gating system. The second is a model surface modification process that utilizes the functional groups deprotected during the photocleavage to pattern the deposition of moieties with complementary chemistry. The spatial layout, resolution, and concentration of these covalently linked molecules follow the gradient pattern created by the grayscale mask during exposure. The programmable chemical gradient fabrication scheme presented in this work allows explicit engineering of both surface properties that dictate nonspecific interactions (surface energy, charge, etc.) and functional chemistry necessary for covalent bonding.     

Self-organized TiO2 nanotubes with controlled dimensions by anodic oxidation

The effect of ammonium fluoride (NH₄F) concentration on the dimensions (length, diameter, and wall thickness) of the self-organized nanotube arrays has been investigated. Results show that varying the concentration of NH₄F exerts a strong effect on changing the dimensions of the as-grown nanotube arrays. The length of the nanotube arrays increases gradually by increasing the concentration up to a maximum length at a concentration of 1.00 wt%, after which the length decreases slightly with the increase in NH₄F concentration. It was also observed that the diameter and wall thickness of the nanotube arrays vary with the change in concentration of NH₄F, where the diameter was found to alter between 80 and 140 nm, and the wall thickness decreases by increasing the NH₄F concentration. These results indicate that it is possible to entirely control the dimensions of the nanotube arrays, by tailoring the concentration of NH₄F besides the anodization time and voltage.     

The effect of aspect ratio on adhesion and stiffness for soft elastic fibres

The effect of aspect ratio on the pull-off stress and stiffness of soft elastic fibres is studied using elasticity and numerical analysis. The adhesive interface between a soft fibre and a smooth rigid surface is modelled using the Dugdale–Barenblatt model. Numerical simulations show that, while pull-off stress increases with decreasing aspect ratio, fibres get stiffer. Also, for sufficiently low aspect ratio fibres, failure occurs via the growth of internal cracks and pull-off stress approaches the intrinsic adhesive strength. Experiments carried out with various aspect ratio polyurethane elastomer fibres are consistent with the numerical simulations.     

Micropatterning of polymer-embedded metal nanoparticles by an ion beam contact lithography

A convenient and effective method to pattern polymer-embedded metal nanoparticles by ion irradiation has been developed. The thin Pluronic films containing silver nitrate as a precursor of silver nanoparticles were irradiated through a pattern mask with accelerated proton (H+) ions. It was found from the UV-Vis measurement that the formation of silver nanoparticles in the Pluronic matrix was dependant on the amount of silver nitrate. The 50 microm line (pitch 150 microm) patterns of the Pluronic containing silver nanoparticles were obtained with the thin film irradiated to 1 x 10(16) ions/cm2. The heat treatment effect on the morphology of the patterns was investigated by using a scanning electron microscope with an energy dispersive X-ray spectrometer. The results confirmed that the silver nanoparticles were successfully embedded in the Pluronic patterns and the patterns were changed into large silver particles by a heat treatment above 350 degrees C.     

Thin films of two functional oxides patterned laterally by soft lithography

Thin films of two laterally patterned functional oxides of uniform thickness were obtained in a two-step soft-lithographic micromolding process. CoFe(2)O(4)/ZnO and CoFe(2)O(4)/BaTiO(3) dual-phase patterns were fabricated. The films showed good replication of the pattern that was defined in the first patterning step. X-ray diffraction showed that the films consisted of two distinct phases, and magnetic force microscopy showed that the compounds were laterally separated, the separation pattern being the same as that of the initial soft-lithographic process. The films exhibited slight height variations near the edges of the phases, which were introduced in the first deposition step and were not fully compensated in the second deposition step. The films are sufficiently smooth to allow fabrication of multilayer structures.     

Effective elastic moduli of porous solids

The principles of continuum mechanics can be extended to porous solids only if the effective moduli are known. Although the effective bulk modulus has already been determined by approximating the geometry of a porous solid to be a hollow sphere, bounds could only be established for the other moduli. This problem of indeterminacy of the moduli is solved in this study using a particular model from the variation of the effective Poisson's ratio. In addition to this, the results are extended for the hollow sphere to real geometry by introducing a porositydependent factor. These results are compared with experimental data and the agreement is found to be good. As the effective Poisson's ratio cannot be determined accurately using experiments, the derived equation is verified using finite element analysis.     

Size and shape dependent steady-state pull-off force in molecular adhesion between soft elastic materials

This paper aims to study the size- and shape-dependent steady-state pull-off force in molecular adhesion between two soft elastic materials. The adhesion consists of a patch of non-covalent bonds formed between ligand and receptor molecules on opposing adhesion surfaces. Classical contact mechanics is used to model the deformation of elastic materials while Bell’s model is adopted to describe stochastic breaking/reforming of molecular bonds. A coupled elastic-stochastic model is established to show that there exists a critical adhesion size, which leads to a critical stress concentration index after proper normalization, beyond which stress concentration near the contact edge causes crack like failure of the adhesion patch governed by Griffith’s criterion and below which the pull-off traction is saturated at a constant strength governed by Bell’s model of molecular adhesion. In addition to size effect, optimal adhesion can also be achieved by designing the shape of the contact surfaces, although it is sensitive to small variations in shape at large adhesion size or stress concentration index. A robust, shape-insensitive high-strength adhesion state becomes possible when the adhesion size or the stress concentration index is sufficiently small.     

Interaction of two solids in contact

The interaction between a machined surface and an abrasive material was analyzed in the framework of concepts of electron exchange between their atoms. Using as examples several combinations of two solids in contact, it was shown that the extent of interaction between them is lowest when the electron exchange does not lead to the formation of energetically stable states by the interacting atoms and when the majority of nonlocalized electrons remains in a free state and forms a specific electron lubricant which prevents interaction. The results of this analysis were verified on the example of the selection of abrasive materials for grinding titanium alloys.     

High-fidelity replication of Dammann gratings using soft lithography

We report the experimental results of using the soft lithography method for replication of Dammann gratings. By using an elastomeric stamp, uniform grating structures were transferred to the UV-curable polymer. To evaluate the quality of the replication, diffraction images and light intensity were measured. Compared with the master devices, the replicas of Dammann gratings show a slight deviation in both surface relief profile and optical performance. Experimental results demonstrated that high-fidelity replication of Dammann gratings is realized by using soft lithography with low cost and high throughput.     

Improved performance of polarization-stable VCSELs by monolithic sub-wavelength gratings produced by soft nano-imprint lithography

We present a novel method for fabricating polarization-stable oxide-confined single-mode GaAs based vertical cavity surface emitting lasers (VCSELs) emitting at 850 nm using a new soft-lithography nano-imprint technique. A monolithic surface grating is etched in the output mirror of the laser cavity using a directly imprinted silica-based sol-gel imprint resist as an etch mask. The opto-electronic performance of these devices is compared to VCSELs fabricated by state-of-the-art electron-beam lithography. The lasers made using the soft nano-imprint technique show single-mode TM lasing at a threshold and laser slope similar to that of devices made by e-beam lithography. The soft nano-imprint technique also enables the fabrication of gratings with sub-wavelength pitch, which avoids diffraction losses in the laser cavity. The resulting single-mode VCSEL devices exhibit 29% enhanced efficiency compared to devices equipped with diffractive gratings.