Abstract
We measured the surface forces generated between fused silica surfaces in a low-viscosity oleophilic diacrylate monomer for reliably repeated ultraviolet (UV) nanoimprinting, and studied the influence of water in monomer liquids on the forces. Fused silica surfaces, with a static contact angle of 52.6 ± 1.7° for water, owing to the low degree of hydroxylation, hardly showed reproducible surface forces with repeated scan cycles, comprising approach and separation, even in an identical liquid monomer medium with both of low and high water content. The monomer liquid with a high water content of approximately 420 ppm showed a greater tendency to increase the surface forces at longer surface-surface distances compared with the monomer liquid with a low water content of approximately 60 ppm. On the other hand, silica surfaces with a water contact angle of < 5° after exposure to vacuum UV (VUV) light under a reduced air pressure showed reproducible profiles of surfaces forces using the monomer with a low water concentration of approximately 60 ppm for repeated surface forces scan cycles even in separately prepared silica surfaces, whilst they showed less reproducible profiles in the liquids with high water content of 430 ppm. These results suggested that water possibly adsorbed on the hydrophilic and hydrophobic silica surfaces in the monomer liquid of the high water concentration influenced the repeatability of the surface forces profiles.
Highlights
Nano-patterning technologies have been reaching the sub-15 nm and single-digit-nanometer scales especially in the field of semiconductor memory devices
The fused silica surfaces showed a contact angle of 52.6 ± 1.7◦ for water. This contact angle indicated that the fused silica surface mainly consisted of siloxane (Si-O-Si) groups according to the reports showing the relationship of the siloxane and silanol density with water contact angle.[30,31]
We focused on the empirical reproducibility in surface forces measurements for performing stably repeated UV nanoimprinting with silica molds
Summary
We have recently reported the surface forces between unmodified or modified silica surfaces in a simple low-viscosity diacrylate monomer for sub-15 nm UV nanoimprinting.[21] Modified silica surfaces with a chemisorbed monolayer could be prepared by chemical vapor surface modification with chlorodimethyl(3,3,3-trifluoropropyl)silane (FAS3-Cl) or tridecafluoro-1,1,2,2tetrahydrooctyltrimethoxysilane (FAS13). These monolayers were often used as an antisticking layer in UV nanoimprinting to decrease the surface free energy of silica molds.[22,23,24] The hydrophobic diacrylate monomer of 1,10-decanediol diacrylate (AC10) with a bulk viscosity of 10 mPa·s at 25 ◦C was selected because of its low viscosity, which is suitable for the jet and flash imprint lithography (JFIL) process.[4,25] We revealed that the surface modification with the fluorinated monolayers decreased the surface forces between silica surfaces in AC10 owing to the decreased interactions between the outermost oleophobic surface and the oleophilic liquid monomer. The reproducibility of the surface forces detected with SFA was discussed in terms of the water content in the monomer liquid and the hydrophilicity of the silica surfaces
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