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Chemically etched fiber tips for near-field
optical microscopy: a process for smoother tips
Patrick Lambelet, Abdeljalil Sayah, Michael Pfeffer, Claude Philipona, and
Fabienne Marquis-Weible
An improved method for producing fiber tips for scanning near-field optical microscopy is presented.
The improvement consists of chemically etching quartz optical fibers through their acrylate jacket. This
new method is compared with the previous one in which bare fibers were etched. With the new process
the meniscus formed by the acid along the fiber does not move during etching, leading to a much smoother
surface of the tip cone. Subsequent metallization is thus improved, resulting in better coverage of the
tip with an aluminum opaque layer. Our results show that leakage can be avoided along the cone, and
light transmission through the tip is spatially limited to an optical aperture of a 100-nm dimension.
© 1998 Optical Society of America
OCIS codes: 050.1220, 060.2370, 180.5810, 240.6700, 350.5730.
In scanning near-field optical microscopy ~SNOM! a
sharp probe tip of submicrometer dimension picks up
optical information in the near field by sending, collecting, or diffracting light at the surface of a sample.1
The near field that is the source of subwavelength
optical information is influenced as much by the
shape and optical properties of the sample surface as
it is by the tip itself.2 Reliable results thus critically
depend on the ability to work with well-defined tips
whose geometrical and optical properties are characterized and controlled.
Most of the SNOM probes used today are based on
tapered optical fibers. Tips are obtained either by
heating and pulling3,4 or by chemical etching5,6,7 in
aqueous solution of fluorhydric acid ~HF!, producing a
conical tip. To yield a subwavelength optical probe,
the conical tip is coated with an aluminum layer,
leaving a nanometer-size aperture at the tip apex.
The ideal tip is characterized by high optical transmission through a single hole with dimension of a few
tens of nanometers. The optical resolution is of the
When this research was performed, the authors were with the
Institut d’Optique Appliquée, Ecole Polytechnique Fédérale, 1015
Lausanne, Switzerland. A. Sayah is now with the Institut de
Microsystèmes, Ecole Polytechnique Fédérale, 1015 Lausanne,
Switzerland. The e-mail address for P. Lambelet is patrick.
[email protected].
Received 20 April 1998; revised manuscript received 3 August
1998.
0003-6935y98y317289-04$15.00y0
© 1998 Optical Society of America
order of the size of the aperture.8 Furthermore,
sharp tips characterized by a small radius of curvature are more appropriate, since they allow the tip to
come quite close to the sample surface when this
surface is not perfectly flat.
The heating and pulling process produces long tips
~1 mm!, with smooth surfaces, allowing good and uniform metallization with aluminum and leading to
well-defined apertures. Compromises between optical transmission and aperture diameter lead, for that
type of tip, to typical transmissions of 1028–1024 for
aperture sizes between 30 and 100 nm.4 The rather
low transmission is due to the strong attenuation of
the light along the taper as well as to the very small
cone angle of the tip ~typically a few degrees!.
Chemical etching, on the other hand, allows one to
produce fiber tips with much shorter cones ~;200
mm! and thus much larger cone angles. This leads
to higher transmission, with the light being guided in
the intact core of the fiber down to a few micrometers
from the tip apex. Typical transmissions of 1023
have been reported.9 The main drawback of chemical etching is the roughness of the surface obtained
after etching with HF, which decreases the quality of
the aluminum coating.10 Such tips often show leakage of light along the conical taper. In this paper we
propose a new etching technique that allows one to
produce much smoother surfaces and thus tips of
higher quality. Applying this new technique, we
have been able to obtain tips with a smooth metal
coating and no leakage along the taper. The high
transmission ratio of chemically etched tips can then
1 November 1998 y Vol. 37, No. 31 y APPLIED OPTICS
7289
Fig. 1. An optical fiber is dipped, with its acrylate jacket, in
aqueous hydrofluorydric acid covered by oil as a protective layer.
The acid diffuses through the acrylate and etches the quartz fiber.
After 35 min a sharp tip is formed.
be combined with some of the advantages of pulled
tips, such as smooth cone surfaces and opaque metal
coatings.
The new mechanism for fabrication of the tips is
based on the chemical etching of a glass fiber through
its acrylate jacket, as opposed to the standard chemical etching that is usually performed on bare fibers
after removal of the jacket. The etching setup is
shown in Fig. 1. An optical fiber, monomode at 633
nm ~FS-SN-3221 from 3M!, is dipped with its acrylate
jacket into an aqueous 40% HF solution. The HF
solution is covered by oil to protect the fiber against
acid vapor and is heated to 60 °C to accelerate the
procedure. The acid does not dissolve the acrylate
jacket but rather diffuses through it to etch the
quartz fiber. After 35 min the fiber is removed and
rinsed successively with water, trichlorethylene, and
acetone. At this point a tip has been formed inside
the acrylate jacket. To remove the jacket, an incision is made a few millimeters above the tip, and the
jacket, softened by acetone, can then be pulled by
being seized in front of the tip. The jacket is thus
removed without damage to the fiber tip after the tip
is formed.
This method is similar to a previous technique used
to make fiber tips by chemical etching.7 However,
although the mechanism of chemical etching of the
glass is the same, etching through the jacket leads to
a different tip formation process. When etching is
performed on a bare fiber, the meniscus formed by the
acid along the fiber is determined by the acid– oil–
quartz interface. As the fiber is etched, its diameter
decreases and following the laws of superficial tension; the meniscus height decreases until the tip is
fully formed. In the present method acid does not
etch the jacket but rather diffuses through it to etch
the glass. The meniscus height is determined by the
acid– oil–acrylate interface and remains constant
during tip formation. The formation of the tip is
thus essentially due to a diffusion process. As the
ions of acid react with the quartz, new ions have to
diffuse from the liquid to the surface of the quartz.
In the upper part of the meniscus the layer of liquid
is very thin, and thus the ions saturate more rapidly,
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APPLIED OPTICS y Vol. 37, No. 31 y 1 November 1998
Fig. 2. SNOM tips obtained by chemical etching of ~a! a fiber with
its acrylate jacket, ~b! a bare fiber; ~c! and ~d!, corresponding axial
views after metallization.
slowing down the etching rate. In the deeper liquid
the concentration of ions close to the fiber remains
higher owing to more efficient diffusion from the
large reservoir. This explains why a tip is formed,
although the height of the meniscus does not change.
Note that the diffusion through the acrylate is very
efficient. With the same acid concentration and
temperature, forming a tip across the acrylate jacket
takes only 5 min longer than on bare fibers.
In Fig. 2 scanning electron microscopy with a field
emission electron microscope ~JEOL, F 6300! is used
to give an image of a tip obtained by this new technique @Fig. 2~a!# and to compare it with a tip produced
under the same conditions but by etching a bare fiber
@Fig. 2~b!#. These figures show that the surface of
the tip etched through the jacket is very smooth ~comparable to pulled fibers! and does not suffer from the
surface irregularities clearly visible in Fig. 2~b!,
which are usually encountered in chemically etched
tips. Here the radius of curvature of the tip apex is
15 nm in Fig. 2~a! and 30 nm in Fig. 2~b!. This
improvement of the quality of the surface shows that
the movement of the meniscus along the tip when a
bare fiber is etched is the main effect responsible for
the irregularities observed on the surface. Keeping
the jacket on the fiber stabilizes the meniscus and
produces a smoother surface, with etching being governed essentially by diffusion. Although the global
shape of the tip, viewed on a larger scale, is symmetric, asymmetries can occur in the last few micrometers of the tip in the form of elongated depressions
along the axial direction. This effect, as can be seen
in Figs. 2~a! and 2~b!, however, is less pronounced in
fibers etched through the jacket.
The fiber tips are metallized with a 100-nm-thick
layer of aluminum ~deposited at 10 nmys, at a pressure P , 5 3 1026 mbar!. Axial views of the tip apex
after metallization are displayed in Figs. 2~c! and
2~d!. A smoother surface is observed on the tip
etched through the jacket, although aluminum grains
Fig. 3. Angular light distribution from a tip chemically etched ~a!
with the acrylate jacket, ~b! without the acrylate jacket. The scale
is in numerical apertures, NA.
are still present. The aperture is not clearly defined
for either type of tip. We attribute the difficulty of
obtaining clear circular apertures in the metal layer
to the very small radius of curvature ~typically 15
nm! characterizing the quartz tip of etched fibers.
In contrast, pulled tips are characterized by a flat
end, for which much better defined apertures have
been observed. We measured a transmission of
log~T! 5 23.3 6 0.8 ~26 samples! for the tips etched
through the jacket and log~T! 5 22.8 6 0.8 ~13
samples! for the tips made from bare fibers. This
means that the transmission varies between 8 3
1025 , T , 3 3 1022 for tips made with the new
etching process and 2 3 1024 , T , 1022 for the
other kind of tip. The higher transmission of the
latter tips is attributed to leakage of light along the
cone resulting from the greater roughness of the
surface. To check the effect of roughness, we completely closed the aperture at the tip apex by successively evaporating the aluminum from the side
and from the front of the tip. The transmission of
the tips etched with the jacket decreased to our
detection limit of Tclosed 5 1027, but the transmission of the other tips decreased only by a factor of 10
and was still Tclosed 5 7 3 1024. This result shows
the importance of the quality of the surface of the
quartz tip for guaranteeing efficient metallization
without leakage of light along the cone of the tip.
Note that quantitative information on the transmission of the tip is not sufficient to determine its
quality; information on leakage along the cone and
aperture size are important parameters as well.
To further analyze light leakage along the cone, we
measured the angular emission of both types of tip in
the far field. Figure 3 displays this far-field profile
for tips with a 100-nm-thick coating of aluminum,
obtained with a water-immersion microscope objective ~numerical aperture NA 5 1.2! by placing the tip
apex at the object focal plane of the objective and
imaging its back pupil on a CCD camera. Figure
3~a! corresponds to the fiber etched with the jacket
and shows a uniform and wide light distribution,
which is the fingerprint of a single, small ~,300 nm!
aperture. The low-contrast features appearing on
this image are due to leaking light along the cone but
with a total power much lower than the light emitted
by the small aperture. On the other hand, Fig. 3~b!,
Fig. 4. Near-field line scan of a 1:1 chromium grating of period
372 nm. Solid curve, topography; dotted curve, near-field optical
transmission. Optical resolution, Dx 5 100 6 10 nm.
which corresponds to etching a bare fiber, shows a
structure containing many speckles, which is due essentially to the light leaking along the cone of the tip.
Note that this far-field pattern is highly sensitive to
leaking light because the interference contrast of the
leaking light with the light coming from the tip depends on their relative total power, not on their actual intensity at the tip surface. Fifty percent of the
fibers etched with the acrylate jacket show a far-field
distribution similar to Fig. 3~a!, but none of the fibers
etched without the jacket can produce such a uniform
profile. These results show that the problem of light
leakage encountered until now with most etched fibers can be solved by etching through the jacket.
The resulting smoother surface of the tip allows one
to obtain better aluminum coverage without leakage.
This observation also shows that the difficulty encountered until now in obtaining a good coating on
chemically etched tips in comparison to that on
pulled tips is due not to a different chemical state of
the surface but to a higher surface roughness.
The problem of light leakage along the cone of the
tip is not necessarily critical for imaging in the nearfield, since this leaking light has a poor lateral confinement and just adds a background to the near-field
image. But for applications based on photochemical
reactions11 this leaking light must absolutely be
avoided, since it induces a reaction far from the targeted positions. For such applications it is critical to
have only one optical aperture on the SNOM tip.
To test the performance of the tip in a near-field
optical measurement, a transmission SNOM image of
a 1:1 chromium grating ~period 372 nm, chromium
thickness 21 nm! deposited on glass is taken with a
tip etched through the acrylate jacket. Figure 4
shows a cut through such an image. The profile of
the light transmission shows a contrast of h 5 0.28.
Measured on the same grating with four tips of each
type, the contrast in the optical profile is improved by
a factor of 2 ~hjacket 5 0.17 6 0.1 versus hbare 5 0.08 6
1 November 1998 y Vol. 37, No. 31 y APPLIED OPTICS
7291
0.07! for tips etched through the jacket. This difference can be explained by the more prominent background signal resulting from light leakage along
imperfectly coated tips obtained by etching bare fibers. From the slope of the sides of the grating, in
Fig. 4, a resolution of Dx 5 100 6 10 nm is measured
~10%–90% of the intensity!.
In conclusion, we have presented a modified etching technique for producing SNOM tips. The fiber is
dipped with its acrylate jacket in an HF solution
covered by a protective oil layer. In this way the
formation of the tip is governed only by diffusion and
results in smoother tips compared with those on
chemically etched bare fibers. This smoother surface, comparable with the surface of a pulled fiber,
allows one to obtain a high-quality aluminum coating
on the tip, leading to minimum light leakage along
the cone.
The authors are grateful to B. Senior ~Centre Interdépartmental de Microscopie Electronique, Ecole
Polytechnique Fédéral Lausanne! for taking the electron micrographs and to M. Gale ~Centre Suisse
d’Electronique et de Microtechnologie SA, Zürich! for
fabricating the test grating. Financial support from
the Swiss National Fund for Scientific Research is
acknowledged.
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APPLIED OPTICS y Vol. 37, No. 31 y 1 November 1998
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