Electron. Mater. Lett., Vol. 10, No. 2 (2014), pp. 351-355

Fabrication of Nano-Structures on Glass Substrate by Modified Nano-Imprint Patterning with a Plasma-Induced Surface-Oxidized Cr Mask

So Hee Lee,1 Su Yeon Lee,

1 Seong Eui Lee,

1 Heon Lee,

2,* and Hee Chul Lee1,*

1Department of Advanced Materials Engineering, Korea Polytechnic University, Korea

2Division of Materials Science and Engineering, Korea University, Korea

(received date: 24 July 2013 / accepted date: 28 August 2013 / published date: 10 March 2014)

In this study, we introduce a process for fabrication of nano-sized structural arrays on glass using modified nano-imprint patterning. A PVC (polyvinyl chloride) stamp was prepared by hot embossing, and a Cr-oxide-patternetch-mask was used. The etch-mask was formed by oxidizing the surface of exposed Cr region by oxygenplasma treatment at room temperature. The fabrication of the etch-mask was conducted by immersing the locallyoxidized Cr pattern in resin remover and Cr-etchant. The residual UV resin and un-oxidized Cr pattern wereselectively removed, resulting in the obvious array of Cr-oxide etch-mask-pattern. The array of glass nano-structures was formed by reactive ion etching (RIE) using CF4 and Ar gas discharge. After removing the Cr-oxide mask, the final nano-structure had a height of 40 nm and a diameter of 170 nm, which was slightly lessthan the diameter of the original master-mold. The plasma treatment gave rise to a rough glass surface with root-mean-square (RMS) roughness of 29.25 nm, while that of bare glass was 0.66 nm. A high optical transmittancedue to reduction in reflectance was observed at the plasma-treated rough surface, as well as for the array of nano-structures. The highest measured optical transmittance was 97.2% at a wavelength of 550 nm; an increase ofabout 7.2% compared to bare glass.

Keywords: nano-imprint lithography, nano-structures, glass patterning, optical transmittance, plasma oxidation

1. INTRODUCTION

Recently, glass substrates are in great demand for industrial

applications such as solar cells, flat panel display, and touch

sensor panels. For such optical devices, efficient use of light

through functional patterning is greatly needed. However,

fine patterning of glass is difficult, and up-to-date glass-

patterning methods include random chemical etching and

direct laser writing. In chemical etching, it is hard to form

structures with arbitrary micro- to nano-dimensions,[1-3] while

laser writing has the problems of not only slow production

yield, but also thermal deformation of the glass substrate

during the process.[4] Accordingly, in this study, fabrication

of nano-structures has been carried out by nano-imprint

lithography (NIL) owing to its high throughput and dimensional

accuracy.[5-9] Also, NIL has the potential to generate sub-

10 nm fine patterns.[10] For the etch-masks used for nano-

imprinting, metal films are preferred to oxide ones because

they are easy to form on substrates. However, metal masks

suffer from low etch selectivity. Oxide masks require long

deposition times as well as high temperatures which could

cause bending of the glass substrates.

In the present work, we have adopted a modified nano-

imprint patterning technique that includes successive plasma

surface oxidation of pre-deposited metal masks at room

temperature. Additionally, plasma treatment using CF4 and

Ar was introduced in order to obtain better optical trans-

mittance of the glass substrate after creation of the array of

nano-patterns.

2. EXPERIMENTAL PROCEDURE

2.1 Fabrication of a PVC stamp

Figure 1 depicts our whole procedure for nano-sized

patterns on glass. Hot embossing was used to fabricate a

polyvinyl chloride (PVC) thermo-plastic polymer stamp for

UV nano-imprint patterning. As shown in Fig. 1(a), hot

embossing was implemented at 120°C, which is above the

glass temperature of PVC. With a pressure of 20 bar, the

pattern of the master mold could be replicated to the surface

of the PVC substrate. For fine pattern transfer, the master

mold was treated with releasing agent for easy detachment

from the embossed polymer after imprinting.[11] Otherwise,

the polymer sticks to the grooves of the master mold and

causes severe distortion of the transferred patterns. The

monolayer of releasing material [(heptadcafluoro-1,1,2,2-

tetra-hydrodecyl) tricholoro-silane (CF3(CF2)5(CH2)2SiCl3)]

was formed on the mold surface by self-assembly in the

DOI: 10.1007/s13391-013-3230-z

*Corresponding author: heon.lee@gmail.com*Corresponding author: eechul@kpu.ac.kr©KIM and Springer

352 S. H. Lee et al.

Electron. Mater. Lett. Vol. 10, No. 2 (2014)

solution phase.[12,13]

2.2 Fabrication and characterization of nano-patterns

on glass substrates

The procedure for UV nano-imprinting lithography is

shown in Fig. 1(b). First, a 50-nm-thick Cr metal layer was

sputter-deposited on borosilicate glass (Schott Boro 33). The

nano-imprinting was performed with a UV-curable monomer

solution compressed to a hydraulic pressure of 20 bar;

between the mold and substrate. When the liquid resin was

exposed to UV light (25 mW/cm2), the photo-initiator molecule

dissociated into fragments which initiated the polymerization

of the monomers. The UV-curable-imprinting resin used in

this study contained the base monomer (benzylmetacrylate,

C11H12O2) mixed with various agents (i.e., viscosity modifier,

anti-sticking agent and UV photo-initiator) to modify its

properties.[14]

The glass substrates were randomly plasma-treated using a

reactive-ion-etching (RIE) reactor (Plasmart) as shown in

Fig. 1(c). The reactor was a parallel-plate system powered by

a 13.56 MHz RF generator; coupled through an automatic

tuning network. O2 RIE was continued for 30 sec, with a

pressure of 140 mTorr and a power of 25 W, for removal of

the residual layer formed during nano-imprinting, and for the

oxidation of the exposed Cr region. Then the sample was

ultrasonically treated in optimized resin remover for 30 min.

During the treatment with the Cr-etchant solution, the Cr-

metal regions were cleanly removed, while the surface-

oxidized CrOx pattern remained on the glass due to high etch

selectivity. The glass substrate was selectively etched by the

RIE process using CF4 and Ar discharge gasses and the

newly-formed CrOx pattern as the etch-mask.

Finally, the nano-pattern on the glass was completed by

removing the CrOx mask patterns during long immersion in

Cr-etchant solution.

Planar and cross-sectional images of the imprinted patterns,

and the processed nano-structures were observed using a

scanning electron microscope (SEM, Jeol Nova nano 200).

The optical transmittance of glass with nano-patterns was

measured in the wavelength region from 300 to 800 nm

using a UV-VIS spectrophotometer (Scinco). The surface

roughness and surface morphology were analyzed with an

atomic force microscope (AFM, PSI).

3. RESULTS AND DISCUSSION

In Fig. 2(a), SEM photomicrographs show the PVC-stamp

surface formed by the hot embossing process. The pitch of

the master stamp patterns was 400 nm and the diameter of

the holes was 200 nm. As the hot embossed pattern is

reversed from the master stamp, the fabricated PVC stamp

has a pillar structure. The well-distributed cylindrical structure

of the PVC stamp has diameters ranging from 170 to 190 nm,

implying that transfer from the master stamp was successful.

The planar and cross-sectional SEM photographs of the resin

patterns imprinted by UV nano-imprint lithography are

presented in Fig. 2(b). The resin shows a hole-pattern which

is identical to the master stamp as reverse duplication took

place twice. The diameter of the holes ranged around

200 nm. With 20 bar of imprinting pressure, a residual layer

with a thickness of 20 nm was demonstrated.

Figure 3 shows the cross-sectional SEM photographs of

imprinted resin patterns with oxygen plasma exposure time

varying from 5 to 40 sec. The oxygen plasma treatment was

carried out with an RF power of 25 W and a pressure of

140 mTorr. A 20-nm-thick residual layer was easily removed

within 5 sec by oxygen plasma treatment at low RF power.

Fig. 1. Fabrication procedure of: (a) PVC stamp by hot embossing,(b) resin pattern by UV nano-imprint lithography, and (c) nano-struc-ture on glass substrate.

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Electron. Mater. Lett. Vol. 10, No. 2 (2014)

After the residual layer was removed, the local Cr patterns

were in contact with the plasma ambient containing reactive

oxygen radicals. When Cr metal is exposed to the oxygen

plasma treatment, an oxidation reaction occurs and causes

the selected Cr-oxide pattern to be formed at the surface. The

plasma oxidation reaction at the surface can be expressed as:

Cr + O* → CrOx (1)

where O* indicates reactive oxygen radicals.

The imprinted resin was little changed within an O2

plasma exposure time of 25 sec, but after an exposure time

of 40 sec, the resin pattern was severely deformed and the

original shape had disappeared. Accordingly, in order to

enable plasma-surface-oxidation of the exposed Cr region

without excessive deformation of the imprinted resin patterns,

oxidation exposure time was fixed at 30 sec.

It is very difficult to remove the hardened resin created by

UV hardening and plasma exposure. In order to obtain an

erasing method, elevated temperature and ultrasonic treatment

was attempted of hardened resin into mixed solutions of

resin-remover and acetone. Figure 4 shows SEM photographs

of patterned resins after treatment in various ratios of resin-

remover and acetone. The mix-ratios of resin-remover and

acetone varied from 10/0 to 7/3. In a solution of pure resin-

remover, a considerable amount of hardened resin remained

on the glass substrate in spite of elevated temperature or

ultrasonic treatment. When 20% acetone was added to the

solution, the remaining resin almost disappeared. A perfectly

clean surface without any residual resin could be obtained

with ultrasonic treatment as shown in Fig. 4(h). When the

acetone content was increased to 30%, some resin remnants

were observed on the substrate, as shown in Figs. 4(c), (f),

and (i).

Fig. 2. Nano-sized pattern images of: (a) PVC stamp by hot emboss-ing, and (b) resin on glass by UV nano-imprint lithography.

Fig. 3. Cross-sectional SEM images of imprinted resins at various O2

plasma exposure times.

Fig. 4. SEM photographs of patterned resin after treatment in variousmixture-ratios of resin remover and acetone at room temperature (a-c), elevated temperature (d-f), and ultrasonic vibration (g-i); RR isresin remover.

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Electron. Mater. Lett. Vol. 10, No. 2 (2014)

To construct glass nano-structures, the remaining Cr-oxide

patterns were used as etch-masks. Tetrafluoromethane (CF4)

and Ar were used as discharge gasses for the plasma etching

of glass nano-structures. CF4 gas was used to promote

chemical reactions with the glass, resulting in volatile SiF4

evaporation. The reaction can be expressed by:

SiO2 + 4F* → SiF4↑ + O2↑ (2)

where F* indicates reactive fluorine radicals.

The addition of Ar caused dissociation of the carbon-

fluorine polymer through physical bombardment.

Figure 5(a) shows SEM photographs of plasma-etched

glass through a Cr-oxide mask. As the etching of the glass

progressed, the nano-scale Cr-oxide mask-patterns were

seen to protrude from the substrate; and the surface of the

plasma-etched glass was observed to be rougher than that of

bare glass. After removing the Cr-oxide mask, final glass

nano-structures in the form of cylinders were revealed (Fig.

5(b)). Each structure has a diameter of about 170 nm and a

height of about 40 nm. The diameter of the structures was

slightly (about 15%) less than that of the master stamp.

Figure 6 shows the optical transmittance of plasma-treated

and plasma-etched, nano-structured glasses. The transmittance

of bare glass was plotted for comparison. Compared to the

bare glass, an increase of transmittance was observed for

glass that was plasma-etched using CF4 and Ar discharge

gasses. The transmittance of plasma-treated glass was about

93.5% at the standard wavelength of 550 nm, while that of

bare glass was approximately 90%. The increase of trans-

mittance can be related to variations in surface morphology

leading to light trapping. As shown in Fig. 7, the root-mean-

square (RMS) roughness of plasma-etched glass; measured

with an atomic force microscope (AFM), was 29.25 nm,

while that of bare glass was only 0.66 nm. This result

implies a remarkable modification of the glass surface by an

etching process that included Ar-ion-flux-bombardment and

fluorine radical-reaction.[15] Figure 6(c) shows the optical

transmittance spectrum of glass given nano-structure by

plasma etching. The nano-structured glass shows the highest

transmittance of 97.2% at the wavelength of 550 nm. This

high transmittance may be due to the combined effect of

nano-sized structures and plasma-etched surfaces. The rough

glass surface left by plasma etching can be thought of as

nano-scale texturing that causes light trapping.[15] Also,

nano-sized geometric structures corresponding to the range

of visible wavelengths can generate additional light coupling

Fig. 5. SEM photographs of: (a) plasma-etched glass through a Cr-oxide mask, and (b) nano-sized glass structure after removal of Croxide mask.

Fig. 6. Optical transmittance spectra of: (a) bare, (b) only plasma-treated, and (c) plasma-etched, nano-structured glass in wavelengthsranging from 300 to 800 nm.

Fig. 7. AFM surface images of: (a) bare, and (b) plasma-treatedglass.

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effect to minimize reflection loss.[16,17]

4. CONCLUSIONS

In conclusion, the modified nano-imprint lithography

using localized, plasma-surface oxidation offers a very

competitive process for glass patterning of nano-structures.

Through plasma-surface oxidation followed by deposition of

a Cr-metal mask, we were able to overcome both low etch-

selectivity of a metal mask on glass, and the bending of the

glass substrate during deposition of an oxide mask. Using

ultrasonic vibration along with an optimized mixture-ratio of

resin remover and acetone, hardened resin could be thoroughly

removed from the glass substrate. The nano-structured glass

including plasma-etched surfaces showed the highest

transmittance of 97.2% at the wavelength of 550 nm. The

high transmittance may be related to light trapping or to a

light-coupling effect caused by the combination of nano-

structure and plasma-etched surface-texturing.

The demonstrated method for fabrication of nano-

structures on glass substrates is very promising for use as

nano-scale electronic or optical devices, as well as for

hydrophilic-hydrophobic control for application to biological

devices.

ACKNOWLEDGEMENTS

This work was supported by the Korean National Research

Foundation (NRF) grant (CAFDC/Seong Eui Lee/No. 2007-

0056090), funded by the Korea government (MSIP).

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