Alborz Amirsadeghi

Complete plastic nanofluidic devices for DNA analysis via direct imprinting with polymer stamps

Development of all polymer-based nanofluidic devices using replication technologies, which is a prerequisite for providing devices for a larger user base, is hampered by undesired substrate deformation associated with the replication of multi-scale structures. Therefore, most nanofluidic devices have been fabricated in glass-like substrates or in a polymer resist layer coated on a substrate. This letter presents a rapid, high fidelity direct imprinting process to build polymer nanofluidic devices in a single step. Undesired substrate deformation during imprinting was significantly reduced through the use of a polymer stamp made from a UV-curable resin. The integrity of the enclosed all polymer-based nanofluidic system was verified by a fluorescein filling experiment and translocation/stretching of λ-DNA molecules through the nanochannels. It was also found that the funnel-like design of the nanochannel inlet significantly improved the entrance of DNA molecules into nanochannels compared to an abrupt nanochannel/microfluidic network interface.

A Simulation Study on the Effect of Cross-Linking Agent Concentration for Defect Tolerant Demolding in UV Nanoimprint Lithography

The chemistry and composition of UV-sensitive resists are key factors determining the stress in the molded resist structure in UV nanoimprint lithography (UV-NIL) and thus the success of the process. The stress in the molded structure is mainly generated due to shrinkage of the resist in the UV curing step and also adhesion and friction at the stamp/resist interface in the subsequent demolding step. Thus, understanding of the stress generated in these steps is critical to the improvement of the process as well as the development of new UV resists. In this paper the effect of resist composition on the stress generation was studied by numerical simulations of the curing and demolding steps in UV-NIL. Parameters required for the simulation, such as resist shrinkage, Youngs modulus, fracture strength, friction coefficient, crack initiation stress, and debonding energy, were determined experimentally for different resist compositions. As the cross-linking agent concentration increases the fracture strength also improves. In addition, as more cross-linking agent is added to the resist composition, both shrinkage stress due to the curing and also adhesion at the stamp/resist interface increase resulting in a larger maximum local stress experienced by the resist on demolding. By normalizing the overall maximum local stress by the fracture stress of the resist, we found that there is an optimum for the cross-linking agent concentration that leads to the most successful imprinting. Our finding is also corroborated by qualitative experimentations performed for UV-NIL with various resist compositions.

Polymerization shrinkage stress measurement for a UV-curable resist in nanoimprint lithography

A simple method was developed to obtain the polymerization shrinkage stress exerted on the sidewalls of resist/stamp interface in ultraviolet nanoimprint lithography. This method is based on the measurements of demolding force which is the sum of adhesion and friction forces. The mean polymerization shrinkage stress on the sidewalls can readily be decoupled from overall demolding force by independently measuring the friction coefficient, adhesion force and geometries of stamp structures. The polymerization shrinkage stress on the sidewalls is overall larger than adhesion and increases by adding more cross-linking agent to the resist composition. This indicates that in addition to lowering the adhesion at the resist/stamp interface, development of resists with low degrees of shrinkage during UV curing is critical to reducing demolding force. It was also found that the shrinkage stress depends not only on the resist composition but also the stamp structure. A pillar structured stamp leads to a larger stress than a stamp with gratings with identical depth and width.

Deformation behavior in 3D molding: experimental and simulation studies

Three-dimensional (3D) molding is a simple and effective technique using a modified hot embossing process to produce large area, hierarchical 3D micro/nanostructures in polymer substrates. However, the use of a thin intermediate polydimethylsiloxane (PDMS) stamp inevitably causes dimensional changes in the 3D molded channel, with respect to those in the brass mold protrusion and the intermediate PDMS stamp structures. Here we investigate the deformation behavior of the 3D molded poly(methyl methacrylate) (PMMA) substrate and the intermediate PDMS stamp in 3D molding through both experimentation and numerical simulation. Depending on the height, period and aspect ratio of the brass mold protrusions and the thickness of the intermediate PDMS stamp, strain contours of the intermediate PDMS stamp layer along the periphery of the 3D molded channels are varying, which leads to a nonuniform elongation of the imprinted structures in the 3D molded channel. Increasing the height and decreasing the period of brass mold protrusions leads to higher total strain of the intermediate PDMS stamp. It was found that for high aspect ratio brass mold protrusions the maximum strain of the intermediate layer occurs in the bottom center of the 3D channels. However, with decreasing aspect ratio of the brass mold protrusion the highest elongation occurs at the bottom corners of the channel causing less elongation of the intermediate PDMS stamp and imprinted structures on the bottom surface of the 3D channel. These experimental results are in good agreement with the results obtained from the numerical simulation performed with a simple 2D model.

The role of hydrophobic silane coating on Si stamps in nanoimprint lithography

Hydrophobic silane coatings have been successfully applied to the surface of Si stamps to improve demolding in nanoimprint lithography (NIL). However, the role of the silane coating has only been studied either indirectly, by measuring adhesion or friction coefficients for Si and substrate surfaces without patterns, or collectively, by measuring the overall demolding force that does not differentiate contributions of friction dissipation, stored elastic energy, and adhesion. Here, for the first time, we present experimental evidence on the role of the silane coating in improving demolding in UV-NIL by using different silane coatings. The silane coatings were characterized by x-ray photoelectron spectroscopy, water contact angle, and friction force measurements. Then, the work of demolding was systematically measured for different silane coatings using stamps with the same micropattern but different pattern depths. Comparison of the results to the theoretical model developed for fiber-matrix debonding energy by Sutcu and Hillig [Acta Metall. Mater. 38(12), 2653-2662] indicated that with a hydrophobic silane coating, the main parameter contributing to overall demolding work shifts from adhesion to stored elastic energy and frictional dissipation as surface adhesion keeps decreasing. The results confirm that the main role of the silane coating in reducing the demolding is to reduce surface adhesion rather than friction at the stamp/substrate interface.

Polymerization Shrinkage and Adhesion in UV-Nanoimprint Lithography

Ultraviolet nanoimprint lithography (UV-NIL) is a molding-based nanofabrication technique utilizing the change in viscosity of a UV resist upon UV irradiation and is one of the most promising candidates for mass-production lithography [1]. The biggest challenge, however, in UV-NIL is demolding, a process to remove the stamp from the molded substrate. Numerous efforts have been made to make the demolding process easier with the main goal being the reduction in adhesion at the stamp/resist interface. However, it is not only the adhesion at the stamp/resist interface that produces stress during demolding. The force applied to the sidewalls of stamp structures by resist shrinkage occurring during UV curing also contributes to demolding force. Experimental methods such as bilayer beam deflection [2] have been employed to measure stress generated in a thin resist layer during UV curing. However, the stress measured by these methods does not represent the real UV-NIL process in which the resist is locally confined within the stamp structures. To the best of our knowledge, there has been no attempt to experimentally determine the stress by polymerization shrinkage in the actual UV-NIL system.Copyright