We studied the exposure behavior of low molecular weight polystyrene as a negative tone electron beam lithography (EBL) resist, with the goal of finding the ultimate achievable resolution. It demonstrated fairly well-defined patterning of a 20-nm period line array and a 15-nm period dot array, which are the densest patterns ever achieved using organic EBL resists. Such dense patterns can be achieved both at 20 and 5 keV beam energies using different developers. In addition to its ultra-high resolution capability, polystyrene is a simple and low-cost resist with easy process control and practically unlimited shelf life. It is also considerably more resistant to dry etching than PMMA. With a low sensitivity, it would find applications where negative resist is desired and throughput is not a major concern.

An electron beam resist is typically applied by spin-coating, which cannot be reliably applied on nonplanar, irregular, or fragile substrates. Here we demonstrate that the popular negative electron beam resist polystyrene can be coated by thermal evaporation. A high resolution of 30 nm half-pitch was achieved using the evaporated resist. As a proof of concept of patterning on irregular surfaces, we fabricated nanostructures on the AFM cantilever and the optical fiber. Although an ice (H2O) resist has also been recently demonstrated as being capable of nanopatterning on irregular and fragile substrates, it requires specially designed accessories mounted inside a SEM chamber, whereas our process works with any thermal evaporator and is thus simpler and much more accessible. Nanofabrication on nonplanar surfaces may find applications in fields such as (AFM) tip-enhanced Raman spectroscopy for chemical analysis and lab-on-fiber technology.

Polydimethylsiloxane (PDMS) is the most popular and versatile material for soft lithography due to its flexibility and easy fabrication by molding process. However, for nanoscale patterns, it is challenging to fill uncured PDMS into the holes or trenches on the master mold that is coated with a silane anti-adhesion layer needed for clean demolding. PDMS filling was previously found to be facilitated by diluting it with toluene or hexane, which was attributed to the great reduction of viscosity for diluted PDMS. Here, we suggest that the reason behind the improved filling for diluted PDMS is that the diluent solvent increases in situ the surface energy of the silane-treated mold and thus the wetting of PDMS to the mold surface. We treated the master mold surface (that was already coated with a silane anti-adhesion monolayer) with toluene or hexane, and found that the filling by undiluted PMDS into the nanoscale holes on the master mold was improved despite the high viscosity of the undiluted PDMS. A simple estimation based on capillary filing into a channel also gives a filling time on the millisecond scale, which implies that the viscosity of PMDS should not be the limiting factor. We achieved a hole filling down to sub-200-nm diameter that is smaller than those of the previous studies using regular Sylgard PDMS (not hard PDMS, Dow Corning Corporation, Midland, MI, USA). However, we are not able to explain using a simple argument based on wetting property why smaller, e.g., sub-100-nm holes, cannot be filled, for which we suggested a few possible factors for its explanation.

ABSTRACT:We demonstrate the implementation of a hybrid solar cell that comprises a surface nanostructured ntype Si and poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate). The Si surface before deposition of the organic layer was nanostructured by using CsCl self-assembled nanoparticles as a hard mask and dry etching to form radial junction architectures and enhance light diffusion and absorption. Apart from the textured Si surface, processing parameters such as from metal-electrode shadow ratio, spincoating rate, and surfactant addition were properly adjusted to improve overall cell performance. Our hybrid solar cells achieve the best performance under optimized cell parameters with a power conversion efficiency of 8.84% and short-circuit current density of 30.5 mA/cm 2 . This combined technique provides a simple, scalable, and cost-effective process for fabricating hybrid solar cells.

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