Silica Nanoparticles-Containing Replica Resin
Molds for Step-and-Repeat UV Nanoimprinting
Shunya Ito, Takahiro Nakamura, and Masaru Nakagawa
Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
E-mail: masaru.nakagawa.c5@tohoku.ac.jp

Introduction: UV nanoimprint lithography (UV-NIL) has attracted much attention especiallyin the field of semiconductor and optical devices owing to the cost-effective process with highresolution. Synthetic quartz molds are widely used for this technology, while the cost of themolds increases the process cost because the molds are fabricated by electron beam lithography.
The use of replica resin molds is one of the solutions to reduce the process cost (Figure 1). The
replica resin molds require high transmittance of UV light and high reliability in step-andrepeat UV nanoimprinting. The improvement of mechanical strength of the replica resin molds
is necessary for the mold performances of stable critical dimension and lifetime during repeatedmolding and demolding processes because the replica resin molds show remarkably lowermechanical strength than synthetic quartz molds. In this study, we developed UV cured hybridmaterials made from silica nanoparticles and a photopolymerizable monomer from thestandpoint of mechanical strength, to demonstrate step-and-repeat UV nanoimprinting using
the replica mold with 45-nm-width line patterns.
Experimental: Methacrylate-terminated silica nanoparticles (mean diameter of 14 nm) anddiacrylate monomers of AC10 and MC10 were selected as shown in Figure 2. The UV cured
films with and without the silica nanoparticles were prepared by wire-bar coating method. The
mechanical strength of the films was evaluated by nanoindenter with a Berkovich indenter. The
replica molds with and without the silica nanoparticles were fabricated from a fluorinated resin
mold with 45-nm-width line patterns. The replica resin and silica nanoparticles-containing resin
molds modified with a fluorinated release layer were applied to step-and-repeat UV
nanoimprinting. The molds after the UV nanoimprinting were observed by field-emission
scanning electron microscopy (FE-SEM).
Results and Discussion: The addition of the silica nanoparticles of 60 wt% into AC10-based
resin improved mechanical strength of the cured films. The Young’s modulus of the film wasincreased from 1.6 GPa to 4.4 GPa, which was almost the same value as polyimide. The silica
nanoparticles-containing AC10 hybrid mold with 45-nm-width lines could be available forstep-and-repeat UV nanoimprinting in 128 cycles, while the AC10 resin mold showedmorphological changes in the lines after 76 cycles (Figure 3). We demonstrated that theimprovement of mechanical strength lengthened the lifetime of the replica molds with 45-nmwidth lines in step-and-repeat UV nanoimprinting.
Our group has investigated the viscosity of diacrylate monomers in nano-gaps between
silica surfaces,[1,2] and recently revealed that the viscosity of AC10 began to increase at a longer
surface-surface distance between acryloyloxy-terminated monolayers than that between
methacryloyloxy-terminated monolayers owing to homology of chemical structure causing
strong molecular interactions.[3] According to this insight, the bulk viscosity of solution
containing the monomer (AC10 or MC10) and the silica nanoparticles was investigated. The
addition of the silica nanoparticles increased the viscosity of both AC10 and MC10 solutions.
The AC10 solution showed fluidity at contents of the silica nanoparticles of no more than 60
wt%. In contrast, the MC10 solution lost its fluidity at the content of 15 wt%. This result
suggested that the strong interactions of MC10 with the silica nanoparticles caused the increasein viscosity at a lower content owing to homology of terminated functional groups.
Reference: [1] S. Ito, et al., ACS Appl. Mater. Interfaces 9, 6591 (2017). [2] S. Ito, et al.,Langmuir 34, 9366 (2018). [3] S. Ito, et al., Chem. Lett. accepted (2019).

silica mold (master)

resin mold (replica)$100,000$10/shot

10,000shots$0.001/shot

10,000 shots/replica mold

Figure 1. Schematic illustration of reducing process cost by use of replica resin molds.

(a) silica nanoparticle
methacrylate-terminatedsilica nanoparticles(mean diameter = 14 nm)(b) base monomer

AC10
1,10-decanediol diacrylateMC101,10-decanediol dimethacrylate

Figure 2. Materials for hybrid molds. (a) Methacrylate-terminated silica nanoparticles and (b) basemonomers of 1,10-decanediol diacrylate (AC10) and 1,10-decanediol dimethacrylate (MC10).

(a) silica nanoparticles-containing AC10(content of 60 wt%)

1 mm(b) AC10

1 mm

Figure 3. FE-SEM images of (a) silica nanoparticles (60 wt%)-containing AC10 hybrid mold and (b)AC10 resin mold after step-and-repeat UV nanoimprinting in 128 and 76 cycles, respectively.