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680 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com
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Oxidation Sharpening, Template Stripping, and Passivation of Ultra-Sharp Metallic Pyramids and Wedges
Hyungsoon Im and Sang-Hyun Oh *
Sharp metallic tips and wedges are capable of squeezing and
manipulating optical energy beyond the diffraction limit by
harnessing surface plasmon polaritons—collective charge
density oscillations of conduction electrons. [ 1–6 ] Plasmonic
nanofocusing using sharp metallic tips can be combined
with scanning probe microscopy and enables optical spec-
troscopy to be performed with nanoscale resolution. [ 3,7 ] For
metallic tips used in scanning tunneling microscopy, the fore-
most atom of the tip primarily determines tunneling current
regardless of the surrounding surface morphology of the
tip. [ 7 ] However, plasmonic nanofocusing demands more strin-
gent control of the tip quality, including its sharpness, surface
roughness, taper angle, and tip-to-tip reproducibility. This
is because surface plasmon waves are tightly bound to the
metal interface, which makes their propagation highly sensi-
tive to surface roughness, contamination, and the geometry
of the tip.
Various techniques have been developed to engineer
metallic tips for plasmonic nanofocusing and near-fi eld scan-
ning optical microscopy (NSOM), including electrochem-
ical etching, focused ion beam (FIB) milling, metallization
of optical fi bers, attachment of a metal nanoparticle to the
end of a pulled fi ber, and template stripping. [ 3,5,7–9 ] Among
these methods, template stripping provides a practical route
to mass-produce sharp and ultrasmooth metallic tips. [ 9–13 ]
The key advantage of template stripping is that one can lev-
erage mature silicon processing technologies—for example
plasma etching, crystal-orientation-dependent wet etching,
and ion-beam milling—to create smooth patterns in a crystal-
line Si wafer, which are then transferred to the metal, rather
than directly patterning polycrystalline metal fi lms that are
typically rough and hard to plasma-etch. Noble metals can
easily wet the Si surface and faithfully replicate the smooth
patterns of the Si molds, yet they have a poor adhesion to
oxidized Si surfaces, allowing the patterned metals to be
peeled off later. Template stripping has been combined with
various techniques such as FIB lithography, anisotropic wet
etching, nanosphere lithography, plasma etching, and nano-
imprinting. [ 11,14,15 ] For the fabrication of pyramids via tem-
plate stripping, one combines standard lithography and
crystal-orientation-dependent wet etching of Si to create
smooth pyramidal pits and then replicates those structures
in deposited metal fi lms as protruding pyramids. By changing
the shape of the etch window, it is also possible to create
metallic wedges for plasmonic waveguiding. [ 6,16,17 ]
While reproducible template-stripping fabrication of
metallic pyramids has been demonstrated, it is still desir-
able to make the tips sharper to improve the resolution for
NSOM and to enhance the fi eld intensity for plasmonic nano-
focusing. Moreover, while the apex angle of template-stripped
pyramids, 70.52 ° , is set by the angle between opposing {111}
planes in a Si (100) wafer, [ 13 ] for some applications it is useful
to reduce the taper angle to tailor the plasmonic fi eld con-
fi nement and achieve more adiabatic nanofocusing. [ 4 ] How-
ever, the mass production of ultra-sharp metallic tips with a
small taper angle remains a practical challenge and the deli-
cate structures are susceptible to damage. [ 18 ] In particular, the
ability to sharpen the tip further without losing its mechan-
ical and chemical integrity is important for near-fi eld imaging
applications.
Here we utilize thermal oxidation of patterned Si tem-
plates to tune the taper angle of template-stripped metal
pyramids and wedges while also further sharpening their tips.
More than 80 million pyramids with a radius of curvature as
small as 5 nm can be produced in parallel over a single 4-inch
wafer. We also show that it is important to protect these
sharp metallic tips against degradation, which can be done by
adding a thin protective layer.
Figure 1 a shows the process fl ow for the fabrication of
metallic tips using oxidation-sharpened Si templates. To fab-
ricate a Si template, a 4-inch Si (100) wafer is coated with a
SiO 2 hard mask, and then a dense array of rectangular and
circular windows are patterned via standard photolithography
and plasma etching. After crystal-orientation-dependent wet
etching of the Si in a potassium hydroxide (KOH) solution,
rectangular and circular etch-windows create elongated
V-groove trenches and inverted pyramids, respectively. DOI: 10.1002/smll.201301475
Plasmonic Nanofocusing
Dr. H. Im, [+]Prof. S.-H. OhDepartment of Electrical and Computer Engineering University of Minnesota Minneapolis , 200 Union St. S.E. , Minneapolis , MN 55455 , USA E-mail: sang@umn.edu
[+]Present address: Center for Systems Biology, Massachusetts General Hospital, 185 Cambridge Street, Boston, MN 02114, USA
small 2014, 10, No. 4, 680–684
Oxidation Sharpening, Template Stripping, and Passivation of Ultra-Sharp Metallic Pyramids and Wedges
681www.small-journal.com© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Next, to reduce the taper angle and further sharpen the
tips, we leverage a well-established Si processing method,
namely, oxidation sharpening. Oxidation sharpening has
been used to make sharp Si probes for atomic force micros-
copy, [ 19 ] fi eld emitters, [ 20 ] Si wedges, and V-grooves. [ 21 ] It has
been theoretically and experimentally shown that an oxida-
tion-induced mechanical stress in concave-shaped Si patterns
can reduce oxidation rates at the corners and create sharp
cusps. [ 22,23 ] While it is not possible to perform high-temper-
ature oxidation directly on noble metals, template stripping
can transfer oxidation-sharpened Si patterns into metals, and
furthermore, invert the geometry by turning trenches into
protruding metallic tips. The reusability of the Si template
(>30 times) is another key benefi t to this scheme. [ 11 ] While
other methods have been developed to produce metallic
pyramids or wedges by depositing metals onto Si trenches, in
most cases the patterned template needs to be dissolved to
release the patterned metals. [ 17,24 ]
Here oxidation sharpening was performed by growing
thermal oxide on the patterned Si templates at 1000 or
1100 ° C via dry or wet oxidation depending on the oxide
thickness. Then, a Au or Ag fi lm was deposited on the oxi-
dized Si template using an electron-beam evaporator using
a slow deposition rate of 0.1 Å/s for the fi rst 20 nm, followed
by faster deposition to reach the fi nal thickness of 200 nm.
During the metal evaporation process, the Si template was
placed straight above the metal source at normal incidence
to ensure that metal atoms can get into the sharp corners of
the oxidized Si template. The metal fi lm is then peeled off
of the Si template using an adhesive backing layer such as
thermally-curable epoxy (EPO-TEK) or UV-curable optical
adhesive (Norland Products, NOA 61). Cross-sectional
scanning electron micrographs (SEMs) of Si V-groove tem-
plates with and without thermal wet oxidation at 1100 ° C to
grow a 450-nm-thick oxide on the {111} planes are shown in
Figures 1 b and 1 c, respectively. The apex becomes sharper
with the thermal oxidation and can be completely fi lled with
the metal. Figures 1 d and 1 e show template-stripped Au
wedges that were sharpened via wet oxidation at 1100 ° C to
grow a 300-nm-thick oxide layer on the {111} Si crystal planes.
The cusped apex of the template-stripped Au wedge is ultra-
sharp. Furthermore, the surfaces of the template-stripped
wedges retain the smoothness of the Si template, which can
reduce optical and electronic losses and thereby improve the
surface plasmon resonances. Smooth surfaces can also reduce
unwanted random hotspots if these metallic pyramids are
used for Raman spectroscopy. [ 12 ]
Figures 2 a–d show SEMs of Au pyramids template-
stripped off of Si templates that have been sharpened with
various thermal oxidation times. Dry oxidation times of 30,
Figure 1. (a) Process fl ow for fabricating sharpened pyramids and wedges via thermal oxidation of patterned Si templates. Crystal-orientation-dependent wet etching of Si in a potassium hydroxide (KOH) solution creates V-groove trenches and inverted pyramids. Subsequent thermal oxidation induces mechanical stress at concave-shaped Si patterns at the bottom of the template, which reduces local oxidation rates and creates sharper and narrower cusps. A fi lm of Au or Ag is deposited on the template and covered by an adhesive backing layer (e.g. optical adhesive). Then the metallic fi lm adhered to the backing layer is peeled off of the template, revealing protruding pyramids and wedges with smooth top surfaces. (b) A scanning electron micrograph (SEM) of a 200 nm-thick metallic fi lm deposited on the Si template without thermal oxidation. (c) SEM of a 200 nm-thick metallic fi lm deposited on the Si template with thermal wet oxidation at 1100 ° C for 27 min. It can be seen that the Au fi lm fi lls the sharp corner completely. (d) SEM of the fabricated Au wedges with sharpened edges. (e) Zoomed-in SEM of a Au wedge with ultra-smooth surfaces.
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H. Im and S.-H. Oh
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communications
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
60, 300, and 570 min at 1000 ° C led to SiO 2 layers with thick-
nesses of 20, 39, 140, 220 nm on the {100} plane and 30, 54,
170, and 253 nm on the {111} planes, respectively. The tip
radius decreases from 24 nm (after 30 min oxidation) down
to 8 nm (after 570 min oxidation), and the taper angle of the
tip decreases from 65 ° to 32 ° . Also, the four edges of each
pyramid become sharper after oxidation by the same mech-
anism. The base width of the pyramid is determined by the
size of the KOH etch window, while the tip size and taper
angle can be adjusted independently by the thermal oxida-
tion process.
Because these metallic nanostructures are made only
through batch processing steps—photolithography, plasma
etching, wet etching, thermal oxidation, and metal evapora-
tion—millions of pyramids can be fabricated at a low cost.
Figure 2 e shows a photograph of a 4-inch Si wafer containing
a total of 80 million pyramids with base widths of 1.5 μ m,
demonstrating the high-throughput nature of our process.
Figure 2 f shows a bird’s-eye view SEM of an array of 1.5 μ m
metallic pyramids at a density of 1.3 × 10 7 pyramids per cm 2 .
All of these pyramids can be peeled off simultaneously using
an adhesive backing layer. Alternatively, individual pyramids
can be picked out on-demand and attached at the end of a
scanning probe. [ 9 ]
Several groups have reported that sharp metallic tips
are susceptible to various degradation mechanisms such as
heat-induced damages upon laser illumination, unwanted
surface migration of metal atoms, mechanical wear during
scanning, contamination, corrosion, and oxidation. [ 7,25–27 ]
Au and Ag are the most widely used metals for plasmonic
applications. Ag is less expensive than Au, and exhibits lower
optical losses at visible and near-infrared frequencies. Unlike
Au, however, Ag can readily oxidize in the ambient environ-
ment and forms corrosion fi lms consisting of Ag 2 S, [ 28 ] which
can degrade the sharpness and optical performance of Ag
tips. Both Au and Ag tips are mechanically soft and thus can
suffer from blunting upon extended surface scanning, neces-
sitating a robust protection mechanism. We observed that
within two weeks of exposure in the air, the radius of curva-
ture of the sharpest Au tip (5 nm tip radius) was signifi cantly
degraded to 19 nm, as shown in Figure 3 a.
Previous work demonstrated that a thin layer of Al 2 O 3
can enhance the stability of metallic nanostructures. [ 14,26,29–31 ]
Indeed Al 2 O 3 is known to be an excellent barrier for gas
Figure 2. SEMs of template-stripped Au pyramids from Si templates sharpened with varying dry oxidation times: (a) 30 min, (b) 60 min, (c) 300 min, and (d) 570 min at 1000 ° C. The tip radius and taper angle are (a) 24 nm, 65 ° (b) 16 nm, 62 ° (c) 10 nm, 47 ° (d) 8 nm, 32 ° , respectively. (e) A photograph of a 4-inch Si template covered with a 200-nm-thick Ag fi lm. The template is fabricated by standard photolithography, and each square array (2 mm × 2 mm) contains 1.3 × 10 5 pyramids per mm 2 . (f) SEM of a template-stripped Au pyramid array containing 1.3 × 10 5 pyramids per mm 2 .
small 2014, 10, No. 4, 680–684
Oxidation Sharpening, Template Stripping, and Passivation of Ultra-Sharp Metallic Pyramids and Wedges
683www.small-journal.com© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
diffusion and has been used as a passivation layer to prevent
surface oxidation in various microelectronic devices. [ 32 ] In
addition, the Al 2 O 3 layer can improve the mechanical strength
of a metal surface by adding a hard protective coating [ 27 ]
while also improving its thermal stability. [ 33 ] Atomic layer
deposition (ALD) [ 32 ] —a conformal layer-by-layer deposi-
tion technique based on self-saturating surface chemistry—
is a particularly desirable method for depositing the Al 2 O 3
passivation layer on template-stripped metallic tips. First,
the fi lm thickness is determined precisely by the number of
ALD cycles with a deposition rate of between 1–2 Å/cycle
depending on deposition conditions. Also, the ALD process
for Al 2 O 3 can be performed at a low temperature (50 ° C),
which prevents unwanted oxidation of the Ag surfaces and
thermal stress to the tips as well as the underlying optical
adhesive during the deposition process. Figure 3 a shows
SEMs of Au pyramid tips with and without
a 2 nm Al 2 O 3 passivation layer. The Al 2 O 3
layer was deposited through 12 cycles of
ALD at 50 ° C right after template strip-
ping. Both the coated and uncoated sam-
ples were imaged and then kept in petri
dishes for two weeks at which time they
were imaged again and the sharpness of
the tips were compared. After two weeks,
Au tips without the Al 2 O 3 layer were sig-
nifi cantly degraded and the average radius
of curvature was increased from 8.7 nm to
16 nm (Figure 3 b). In contrast, the radius
of curvature of Al 2 O 3 -coated Au tips
changed from 10 nm to only 12 nm during
the same period, demonstrating the effec-
tiveness of the Al 2 O 3 layer in enhancing
the stability of metal tips. The increased
initial tip radius of the coated sample is
from the 2-nm-thick Al 2 O 3 layer.
In conclusion, we have demonstrated
a simple fabrication method for template-
stripping ultra-smooth metallic pyramids
with sharpened tips. Thermal oxidation
of anisotropically etched Si templates can
tune the taper angle of the tip between
70.52 ° and 32 ° , while also sharpening the
tip radius down to 5 nm. Because the Si
template is patterned through photo-
lithography and anisotropic wet etching,
more than 80 million pyramids can be
made in parallel from a reusable Si tem-
plate. Furthermore, sharpened edges of
the pyramids and wedges can sustain
tightly confi ned wedge plasmons, which
can lead to interesting possibilities for
plasmonic waveguiding. [ 6 ] We also show
that an Al 2 O 3 coating that is as thin as
2 nm can prolong the stability of Au tips
and can also protect Ag tips from blunting,
corrosion, and oxidation. Therefore,
low-cost mass production of ultra-sharp
metallic pyramids and wedge waveguides
would benefi t a wide range of applications including NSOM,
tip-enhanced Raman spectroscopy (TERS), [ 34,35 ] heat-
assisted magnetic recording, [ 36 ] optical trapping, [ 1 ] nonlinear
spectroscopy, [ 3 ] and nanophotonic circuitry. [ 6 ]
Experimental Section
Tip Fabrication : A 100 nm-thick SiO 2 layer was grown on a Si (100) wafer using wet oxidation at 1000 ° C. This oxide layer serves as a masking layer during subsequent Si etching. The etching area was defi ned by conventional photolithography with a posi-tive photoresist, SPR-955. Briefl y, the photoresist was spin-coated at 4000 r.p.m. for 30 s and baked on a hot plate at 105 ° C for 1 min. The resist was exposed by an i-line Canon stepper (Canon 2500 i3) and developed in CD-26 for 90 s. The underlying oxide
Figure 3. (a) Zoomed-in SEMs of Au pyramids. The sharp metallic tip with a radius curvature of 5 nm is degraded after two weeks while the tips coated by a 2 nm-thick Al 2 O 3 layer remain sharp. (b) The average radius of metallic tips is 8.7 ± 3.7 nm right after template stripping, but degrades to 16.4 ± 3.2 nm after two weeks (n = 4). When Al 2 O 3 coats the tip, the average initial tip size becomes about 10 ± 1.0 nm due to the presence of Al 2 O 3 coating, but it increases only to 12 ± 1.2 nm after two weeks (n = 8).
small 2014, 10, No. 4, 680–684
H. Im and S.-H. Oh
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communications
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
layer was then etched by reactive ion etching (STS model 320). The photoresist was removed by acetone and further cleaned by a piranha solution (1:1 mixture of sulfuric acid and hydrogen peroxide). Then, the Si wafer covered with the patterned oxide layer was placed in a 20% potassium hydroxide (KOH) solution and etched at 60 ° C. After KOH etching, the oxide mask layer was stripped in a buffered oxide etching (BOE) solution. After standard RCA cleaning steps, fresh thermal oxide was grown on the pat-terned Si wafer at 1000 or 1100 ° C. After thermal oxidation, a 200-nm-thick Au or Ag fi lm was deposited on the oxidized Si tem-plates using electron-beam evaporation (CHA, SEC600). Subse-quently, a thermally curable or UV-curable optical adhesive was applied, and the metal fi lm now adhered on the backing layer was peeled off of the Si template. After peeling, the metal tips were immediately coated with a 2-nm-thick Al 2 O 3 layer using ALD at 50 ° C. The deposition rate of Al 2 O 3 at 50 ° C is ∼ 1.8 Å per cycle.
Acknowledgements
This work was supported by grants from the U.S. Department of Defense (DARPA Young Faculty Award; N66001–11–1–4152), Offi ce of Naval Research Young Investigator Award (N00014–11–1–0645), National Science Foundation CAREER Award (DBI 1054191), and Seagate Technologies. We thank Timothy Johnson for helpful comments.
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Received: May 14, 2013Revised: August 2, 2013Published online: October 2, 2013
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