Control of Sub-50 nm Thickness ofUV-Cured Thin Films by Print-and-Imprint Method
Masaru Nakagawa, Kento Ochiai, Akiko Onuma, Kazue Suto,
Takahiro Nakamura, and Shunya Ito
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: The process of ultraviolet nanoimprint lithography (UV-NIL) involving
several dry etching procedures has one of crucial issues. The issue is to level a whole residuallayer thickness of an imprinted resist layer as thin as possible. Some displacement techniqueof UV-curable liquid droplets is necessary for achieving the leveling of residual layer
thicknesses independently of differences in area density of pattern recesses on a mold surface.
For the purpose, ink-jet printing to dispense low-viscosity UV-curable liquid is adopted in jet
and flash imprint lithography (J-FIL). Alternatively, we have proposed laser-drilled screen
printing [1] and study its applicability to high-viscosity UV-curable liquids. The residuallayer thickness would be determined simply by a total volume of UV-curable liquid displacedon a substrate surface, under the condition that an imprinted area is constant in UV
nanoimprinting. However, an undefined imprinted area is generally formed after imprint in
the case that the external dimension of the mold is larger than the area of displaced liquid
droplets (Figure 1). Even if the mold with a mesa structure is used, the liquid exudes out of
the mesa region. The undefined imprinted area and the liquid exudation cause difficulty tocontrol residual layer thickness. In this study, we selectively modified mold and substratesurfaces with an oleophobic fluorinated self-assembled monolayer (F-SAM) to investigate
whether the presence of F-SAM enabled to prepare a certain area of UV-cured thin films andto control the residual layer thickness lower than 50 nm by tuning total volumes of UVcurable liquid.
Experimental: Surfaces of fused silica superstrates with a 10×10 mm2 mesa structure andsilicon wafer substrates were wholly modified with tridecafluoro-1,1,2,2tetrahydrooctyltrimethoxyilane (FAS13) [2]. Each 8.4×8.4 mm2 area of the superstrate mesa
region and the substrate were cleaned by UV/O3 mask exposure. The selectively cleaned
surface was modified with an adhesive layer of 3-(acryloxyoxy)propyltrimethoxysilane(ACL) for the substrate and with a release layer of chlorodimethyl(3,3,3trifluoropropyl)silane (FAS3-Cl) for the superstrate. Polyimide (PI) laser-drilled screen masks[3] with 10-µm-diameter through holes of certain pitches were used. Print-and-imprint
method with fluorescent UV-curable liquid (NL-SU1FL) [4] was carried out as illustrated inFigure 2.
Results and Discussion: PI masks with three types of pitches of 80, 125, and 175 mm wereprepared. The three-type pitches corresponded to total numbers of 11,025, 4,489, and 2,304
liquid droplets, which were designed for the residual layer thickness of 50, 20, and 10 nm.
The superstrate with a mesa modified with FAS13 and FAS3 was contacted with the substrate
as shown in Figure 2 under an applied force of lower than 30 N. Figure 3 shows reflectionmode optical microscope images of cured resin films with designed thicknesses of 50 nm [Fig.
3(a)], 20 nm [Fig. 3(b)], and 10 nm [Fig. 3(c)]. The shapes of cured thin films were almost
consistent with those of chemical registration which were performed for the superstrate andsubstrates. The chemical registration had a suppressive effect on unintended spread of theUV-curable resin between the superstrate and substrates. The actual average thicknesses
determined by surface profiling were 41.6, 20.4, and 7.5 nm.
Reference: [1] WO.2016031237.A1. [2] ACS Appl. Mater. Interfaces 9, 6591 (2017). [3] J.
Vac. Sci. Technol. B 35, 06G301 (2017). [4] Bull. Chem. Soc. Jpn. 91, 178 (2018).

(a)designed area

superstrate (mold)press

substrate
fluorescent liquid droplets(b)

undefined cured film
caused by liquid leakage

(c)

2 mm

2 mm

Figure 1. (a, b) Schematic illustrations of (a) cross-sectional view and (b) top-view of liquidspreading on contact of a (mold) superstrate with a substrate on which droplets of UV-curable liquid
were screen-printed with a polyimide laser-drilled mask. (c) Actual fluorescence microscope imagesof fluorescent liquid droplets on a substrate and fluorescent UV-curable liquid placed between a moldand substrate.
superstrateABAACAABAACAsubstrate
FFFFFFFFFFFFOOSiOF

A: FAS13FFSiOClOOFSiOO

B: FAS3-Cl

C: ACL

Figure 2. Concept of chemical registration to suppress liquid leakage on contact of a superstrate witha substrate. The superstrate surface has an external region of FAS13 and an internal region of FAS3Cl, while the substrate had an external region of FAS13 and an internal region of ACL.
(a)

(b)

(c)

Figure 3. Reflection-mode optical microscope images of UV-cured thin films with a thickness of (a)42, (b) 20, and (c) 8 nm which were prepared by tuning the total number of liquid droplets
screen-printed onto substrates in print-and-imprint.