Supplementary Materials
Double Oxide Deposition and Etching Nanolithography for
Wafer-scale Nanopatterning with High-Aspect-Ratio using
Photolithography
Jungho Seo1, Hanchul Cho1, Ju-Kyung Lee1, Jinyoung Lee1, Ahmed Busnaina1, and
HeaYeon Lee1,2
1
Department of Mechanical and Industrial Engineering, College of Engineering, Northeastern
University, Boston, MA 02115, U.S.A
2
Division of Quantum Phases and Devices, Department of Physics, Konkuk University, Seoul143701, South Korea
Tel) +1-617-373-8192
Fax) +1-617-373-3266
E-mail) he.lee@neu.edu
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S1. Anisotropic Plasma Etching Process
The DODE lithography method uses plasma etching to avoid isotropic wet etching. The
anisotropy of the etching can be modulated through adjustment of the parameters of the
plasma to deliver the plasma radicals vertically onto the surface. The mean free path of the
plasma radicals can be modified by the plasma etching conditions.1-3 A low mean free path
produces an isotropic etching profile with a lateral etch rate that is approximately equal to the
downward etch rate. Therefore, optimized etching conditions are an important factor in
obtaining high-mean-free-path plasma radicals for etching the nanotrench. We investigated
the relationship between the nanosize of the pattern and the plasma etching conditions by
varying the pressure, gas flow rate, and RF power. Our ICP system consists of platen bias
which is applied to parallel plate and coil bias for a high-density plasma source of ions. We
defined the anisotropy by following formula:
A 1
RL
RV
where, A is anisotropy,
RL is lateral etch rate, and RV is vertical etch rate.
1. Effect of Pressure
The effect of pressure was investigated, as shown in Fig. S1. The aspect ratio and etch rate
decreased as the process pressure was increased (Fig. S1a). The other plasma etching
conditions were held constant as follows: platen power (250 W), coil power (350 W), O2 gas
flow rate (0 sccm), SF6 gas flow rate (6 sccm), Ar gas flow rate (4 sccm), and 5 min of
etching time. The process was studied at pressures of 5 mTorr (Fig. S1b), 10 mTorr (Fig. S1c),
and 15 mTorr (Fig. S1d). Pressure is an important factor in determining the direction of
motion of the plasma radicals.4 The mean free path depends on the pressure. The collision
probability is proportional to the pressure, so that the radicals collide with the sidewalls at
high pressures. Thus, a low-pressure plasma is suited to vertical anisotropic etching.
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FIG. S1. Effect of pressure on plasma anisotropic etching: (a) 5 mTorr, (b) 10 mTorr, and (c) 15 mTorr .
2. Effect of the Oxygen Gas Flow Rate
The nanoscale SiO2 trench was etched using SF6/Ar/O2 gas mixtures in an ICP system. The
effect of the O2 gas flow rate was studied using different flow rates of 0 sccm (Fig. S2b), 2
sccm (Fig. S2c), and 4 sccm (Fig. S2d) with a fixed SF6 (6 sccm) + Ar (4 sccm) flow rate. An
increase in the O2 gas flow rate decreased the SiO2 etch rate and the anisotropic etch ratio, as
shown in Fig. S2a. A SF6-based gas mixture is typically used to etch SiO2, where sidewall
etching is prevented by the deposition of a fluorocarbon polymer on the sidewall. The O2 gas
plays role to decompose the deposition of the fluorocarbon polymer, which decreases the
anisotropic etch ratio.5 The etch profile is difficult to maintain as the size of the trench is
reduced to the nanoscale. Thus, a fluorocarbon gas is needed for an anisotropic etch profile.
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FIG. S2. Effect of O2 gas flow rate on plasma anisotropic etching: (a) 0 sccm, (b) 2 sccm, and (c) 4 sccm.
3. Effect of RF Power
An increase in the platen power increased both the SiO2 etch rate and the anisotropic etch
ratio linearly (Fig. S3a). The platen power tests corresponded to 150 W (Fig. S3b), 250 W
(Fig. S3c), and 350 W (Fig. S3d) at a coil power of 350 W and with a gas mixture of SF6 (6
sccm) / Ar (4 sccm). The gas flow rates were held fixed while a pressure of 5 mTorr was
maintained. A vertical etching profile is related to the direction of motion of incident radicals,
which can be implemented by the anisotropic motion of ions.6 The platen power was applied
to the substrate plate to create directional electric fields near the substrate to heighten the
anisotropy of the etching profile. The anisotropic etching profile was obtained when the
incident ions were delivered over a small angle into the nanotrench through an increase in the
platen power.
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FIG. S3. Effect of platne power on plasma anisotropic etching: (a) 150 W, (b) 250 W, and (c) 350 W.
S2. DODE (Double Oxide Deposition and Etching) Lithography Process
Fig. S4a shows the changes in the linewidth of microstructure due to the deposition of the 2nd
SiO2 layer. The 1.5-μm-thick second SiO2 layer was deposited onto the 500-nm-thick first
SiO2 layer of the microstructure. The 725-nm-thick oxide was deposited laterally on both
sides of the sidewall to obtain a 50-nm nanopattern. Moreover, 1 μm of residual SiO2
remained on the bottom of the nanopattern. The residual SiO2 was removed by anisotropic dry
etching, as shown in Fig. S4b. Consequently, SiO2-based nanostructures were successfully
fabricated with high aspect ratios that were greater than 20:1.
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FIG. S4. DODE lithography process for high-aspect-ratio nanostructure fabrication: (a) after oxide deposition on the
microstructure and (b) after dry etching of the residual SiO2.
S3. Nanolithography on 3-inch Au Wafe Substrate
DODE lithography method can be applied to different substrates such as glass, plastic and metal. Fig.
S5 shows the nanopatterning on 3-inch Au wafer by DODE process.
FIG. S5. Nanopatterning on a 3-inch Au wafer by DODE lithography. Various patterns of microstrucrues were transferred
from photomask through photolighography process. And nanopatterns were obtained by DODE process .
S4. Time Cost Comparison of DODE nanolithography with E-beam Lithography
The electron beam lithography is a commercialized lithography process for nano-scale patterning.
However, the key disadvantage of this method is low-throughput due to its low-speed process time.
The process time of the e-beam lithography is proportional to exposure area and is given by the
following formula:7
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T
D A
I
where T is the process time, I is the beam current, D is the dose and A is the exposed area. Therefore,
the e-beam lithography is not suitable for high-volume manufacturing.
Meanwhile, the DODE
lithography is not proportional to exposure area but process steps because the nanopatterns were
created from micropatterns which were transferred from mask through optical photolithography
process. The optical lithography is much faster than e-beam lithography for micron-size patterning for
wafer-scale manufacturing.
The DODE lithography process has additional thin film deposition and etching processes for nanoscale patterning as shown in Fig. S6a. The approximate process time for nanopatterning on a Si wafer
is about 1 hour. In our e-beam lithography system, the beam current is 38.7 pA for 10 μm aperture size
and the area dose amount is 524.8 μC/cm2. So the exposure area of the e-beam system is about 265.5
μcm2 during the 1 hour of writing time. To cover the 175 cm2 surface area of a 75 mm of Si wafer, the
minimum process time would extend to 2 × 109 s, about 75 years.
Consequently, high-throughput manufacturing is possible for DODE lithography method in terms of
wafer-scale lithography because it is scalable and batch process is possible as shown in Fig. S6b.
Moreover, the e-beam lithography is not compatible with plasma etching process for high aspect-ratio
nanostructure manufacturing because the e-beam resists are very thin layer. Therefore, the DODE
technique is promising lithography for large-scale and high-aspect ratio nanostructure fabrication.
FIG. S6. Comparison of DODE lithography with e-beam lithography for time cost of process: (a) process steps of DODE and
e-beam lithography for SiO2 nanopatterning on Si wafer and (b) process time as a function of exposure are.
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