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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

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Oxidation Sharpening, Template Stripping, and Passivation of Ultra-Sharp Metallic Pyramids and Wedges

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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).

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H. Im and S.-H. Oh

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© 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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