Direct printing of polymer microstructures on flat and spherical surfaces using a letterpress technique

We have developed a letterpress technique capable of printing polymer films with micrometer scale feature sizes onto flat or spherically shaped nonporous substrates. This printing technique deposits polymer only in desired regions thereby eliminating subsequent developing and subtraction steps. Flat or curved printing plates, which are fabricated from either rigid or deformable materials, are used to transfer thin molten polymer films onto flat target substrates. By deforming the printing plates into a spherical shape, it is also possible to print patterned films onto the concave side of a spherically deformed target substrate. These printed films serve as good resists for both wet chemical etching and reactive ion etching. Interferometric measurements of the polymer film thickness are used to probe physical mechanisms affecting printing instabilities, pattern fidelity, and edge resolution. Our experimental study indicates that this letterpress technique may prove suitable for high-throughput device fabrication involving large-area microelectronics

Photoresist-free printing of amorphous silicon thin-film transistors

Conventional fabrication of amorphous silicon thin-film transistors (a-Si TFTs) requires patterning numerous photoresist layers, a subtractive process that is time consuming and expensive. This letter describes transistor fabrication by a photoresist-free approach in which polymer etch masks are letterpress printed from flexible polyimide stamps. Pattern registration is achieved through optical alignment since the printed masks are thin and optically transparent. This modified fabrication scheme produces transistor performance equivalent to conventionally fabricated a-Si TFTs. The ability to directly print etch masks onto nonhomogeneous substrates brings one step closer the realization of flexible, large-area, macroelectronic fabrication

Microfluidic detection and analysis by integration of thermocapillary actuation with a thin-film optical waveguide

We demonstrate a nonintrusive optical method for microfluidic detection and analysis based on evanescent wave sensing. The device consists of a planar thin-film waveguide integrated with a microfluidic chip for directed surface flow. Microliter droplets are electronically transported and positioned over the waveguide surface by thermocapillary actuation. The attenuated intensity of propagating modes is used to detect droplet location, to monitor dye concentration in aqueous solutions, and to measure reaction rates with increasing surface temperature for a chromogenic biochemical assay. This study illustrates a few of the capabilities possible by direct integration of optical sensing with surface-directed fluidic devices

Generation of high-resolution surface temperature distributions

We have performed numerical calculations to study the generation of arbitrary temperature profiles with high spatial resolution on the surface of a solid. The characteristics of steady-state distributions and time-dependent heating and cooling cycles are examined, as well as their dependence on material properties and device geometry. Ideally, low-power consumption and fast response times are desirable. The simulations show that the achievable spatial resolution is on the order of the substrate thickness and that the response time t+ depends on the width of the individual heating elements. Moreover, the rise time t+ can be significantly shortened by deposition of a thermal insulation layer, which also reduces the power consumption and increases lateral resolution

Physical mechanisms governing pattern fidelity in microscale offset printing

We have studied the offset printing of liquid polymers curable by exposure to ultraviolet light onto flat and unpatterned silicon and glass substrates. The interplay of capillary, viscous, and adhesion forces dominates the dynamics of ink transfer at small feature sizes and low capillary number. For smooth and nonporous substrates, pattern fidelity can be compromised because the ink contact lines are free to migrate across the substrate during plate separation. Using a combination of experiments and equilibrium simulations, we have identified the physical mechanisms controlling ink transfer and pattern fidelity. In considering the resolution limit of this technique, it appears that the dynamics of ink flow and redistribution during transfer do not explicitly depend on the absolute feature size, but only on the aspect ratio of film thickness to feature size. Direct printing holds promise as a high-throughput fabrication method for large area electronics

Morphology of liquid microstructures on chemically patterned surfaces

We study the equilibrium conformations of liquid microstructures on flat but chemically heterogeneous substrates using energy minimization computations. The surface patterns, which establish regions of different surface energy, induce deformations of the liquid–solid contact line. Depending on the geometry, these deformations either promote or impede capillary breakup and bulge formation. The contact angles of the liquid on the hydrophilic and hydrophobic regions, as well as the pattern geometry and volume of liquid deposited, strongly affect the equilibrium shapes. Moreover, due to the small scale of the liquid features, the presence of chemical or topological surface defects significantly influence the final liquid shapes. Preliminary experiments with arrays of parallel hydrophilic strips produce shapes resembling the simulated forms. These encouraging results provide a basis for the development of high resolution lithography by direct wet printing

Using convective flow splitting for the direct printing of fine copper lines

Liquid ribbons of solutions of copper hexanoate in a volatile solvent were drawn on a glass slide using either fine glass capillaries or an ink jet printer. After solvent evaporation, the solute was observed to segregate into multiple pairs of stripes much narrower than the initial ribbon diameter. These stripes were then converted to pure copper by annealing. Surface profiles indicate that the thickness, width, and number of lines formed are strongly dependent on the solution viscosity and volume per unit length deposited. From flow visualization studies and surface profiling, we have found that evaporative cooling produces Bénard–Marangoni convection patterns which accrete the solute along two key boundaries of the flow, namely the three phase contact line and the outer edge of a stagnant region about the ribbon apex. These findings suggest that optimization of the deposition and evaporation process can be used to "write" fine metallic lines from a wider liquid precursor

Effect of contact angle hysteresis on thermocapillary droplet actuation

Open microfluidic devices based on actuation techniques such as electrowetting, dielectrophoresis, or thermocapillary stresses require controlled motion of small liquid droplets on the surface of glass or silicon substrates. In this article we explore the physical mechanisms affecting thermocapillary migration of droplets generated by surface temperature gradients on the supporting substrate. Using a combination of experiment and modeling, we investigate the behavior of the threshold force required for droplet mobilization and the speed after depinning as a function of the droplet size, the applied thermal gradient and the liquid material parameters. The experimental results are well described by a hydrodynamic model based on earlier work by Ford and Nadim. The model describes the steady motion of a two-dimensional droplet driven by thermocapillary stresses including contact angle hysteresis. The results of this study highlight the critical role of chemical or mechanical hysteresis and the need to reduce this retentive force for minimizing power requirements in microfluidic devices