White Paper
Study of Helium Ion Beam Exposed Nanostructures
by in situ AFM with ZEISS MERLIN
White Paper
Study of Helium Ion Beam Exposed Nanostructures
by in situ AFM with ZEISS MERLIN
Authors: Dr. Nils Anspach, Dr. Frank Hitzel
DME Nanotechnologie GmbH, Braunschweig
Dr. Endré Majorovits, Dr. Fabián Pérez-Willard,
Carl Zeiss Microscopy
Date:
July 2013
The AFM Option for the ZEISS MERLIN series [1] combines an innovative high end atomic force microscope
(AFM) with a scanning electron microscope (SEM). In situ high-resolution AFM measurements in the
SEM become possible. The unique AFM and SEM combination opens up new possibilities for the characterization
of nanostructures. With the AFM accurate quantitative 3D topographical data in the sub-nanometer scale can be
acquired. In addition, information about mechanical, electrical, and magnetic properties as well as chemical surface
potential of nanostructures is accessible. The combination with the SEM allows to set these analyses in relation to
macroscopic dimensions in the sample. In this white paper an experiment is described, which demonstrates the
power of the AFM/SEM combination for the analysis of helium ion beam exposed nanostructures.
3D Topographical Analysis of Nanopatterns
with the AFM tip positioned in the vicinity of the patterned
Ring patterns were generated in the body of a commercial-
area or region of interest (ROI). The AFM tip can be directed
ly available silicon AFM cantilever by means of direct helium
to the ROI in a simple and fast manner. This is done by firstly
ion beam lithography in a ZEISS ORION NanoFab[2].
centering the ROI in the SEM image and secondly, starting
Different ion beam doses ranging from 0.004 nC/µm² to
the AFM automatic tip approach routine. This process typi-
1 nC/µm² were used to generate ring patterns. The ring
cally takes less than a minute. After the tip approach the
patterns have a nominal diameter of 750 nm and a nominal
AFM measurement can be started.
line width of 80 nm.
In Figure 2 the top image shows AFM topographical data
For the study of the fabricated nanostructures, the sample
from the central area of the ROI: The ring on the right pro-
was transferred to a ZEISS MERLIN equipped with the AFM
trudes out of the sample surface and was written with a
Option. Fig. 1 shows a SEM image of the cantilever sample
dose of 1 nC / µm². The rings in the left part of the image
a
b
Figure 1: a) SEM image of the patterned AFM cantilever (center of the image, tip pointing upwards). In the upper right part of the image a second cantilever
(tip pointing downwards) can be seen, which is the one used for the AFM measurement. The AFM tip patterned area is located in the middle of the image.
b) AFM tip in the vicinity of the ROI. The white arrow denotes the patterned area.
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White Paper
a
b
Figure 2 (a) AFM topography data of the central region of the nanopatterned sample area. (b) 3D surface plot of the same sample area.
A directional illumination was added to the image to enhance the patterns on the left side.
a
b
Figure 3: a) and b) show the dimensional analysis of rings written with 1 nC / μm² and 0.1 nC / μm² helium ion dose, respectively (see text).
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White Paper
a
b
Figure 4 (b) AFM phase image at the edge of the nanopatterned area. An array of 2 x 6 rings is clearly visible in the right of the image. In a simultaneously acquired
topographical image (a) the low dose rings are not detectable, because the roughness of the surface exceeds by far the z profile of the rings.
appear as depressions. They were structured at the much
Similar results have been reported before for helium ion beam
lower dose of 0.1 nC / µm². The bottom image shows a
exposed nanostructures on SOI wafers in Boden et al. [3].
3D surface plot of the same sample area.
The experiments described in the following two sections,
The AFM Option for the ZEISS MERLIN series allows quantita-
demonstrate that helium implantation has an important
tive measurements not only of the lateral dimensions x and y,
impact in the physical and electrical properties of silicon.
but also of height z. The section tool of the AFM’s software,
facilitates measurement of width and height of the nano-
Phase Imaging
structures. The section tool was used to generate the Height
The ring structures generated at ion doses of 0.004 to
over Length profile plots shown in Figure 3.
0.02 nC/µm² are only some hundreds of picometers deep.
These low dose rings are hardly visible when scanning larger
Figure 3a illustrates the dimensional analysis of a ring struc-
areas in the AFM topography mode described in the previous
ture written with 1 nC / μm². The ring protrudes out of the
section. This is because the overall surface roughness exceeds
sample plane. Its height and width are around 40 nm and
by far the ring depth. For cases like this, a different image
100 nm, respectively. Additionally, it can be observed that
acquisition mode, phase imaging, turns out to be extremely
the area inside the ring is slightly elevated in comparison
useful to enhance contrast in the AFM.
to the surface area outside the ring structure.
Phase imaging generates contrast based on variations in
Figure 3b shows the dimensional analysis of a ring with
composition, adhesion, friction, viscoelasticity, as well as
0.1 nC / μm² dose. In this case the ring appears as a shallow
other factors. Local variations of these properties translate
trench with a depth of around 470 pm, i.e. 0.47 nm. The ring
into a different interaction with the probing AFM tip. Thus,
width is slightly less than 70 nm. When the helium ion beam
phase imaging makes it possible to detect such features
strikes the silicon sample, two effects can be expected: a) some
which show none, or very little, surface topography as
material removal, and at the same time b) some helium implanta-
compared to the rest of the sample surface.
tion in the silicon. For the low helium ion beam dose, 0.1 nC / μm²,
the first effect leads to a depression where the silicon surface
Figure 4 shows an AFM phase image of a 10 µm x 10 µm field.
was exposed to the ion beam. As the helium dose increases the
A regular array of rings can be seen clearly on the right of the
second effect, the helium implantation, becomes more impor-
image. The strong phase contrast is a clear indication that, even
tant leading to a local swelling of the silicon [3]. Thus, the rings
at the low dose of 0.004nC/µm², exposure to helium ions modi-
exposed with 1 nC/µm² no longer appear as trenches, but as
fies the physical properties of the silicon substrate.
elevated areas.
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White Paper
Kelvin Probe Microscopy
ing provides evidence that helium exposure has influenced
Besides topographical and phase imaging modes the AFM
the electrical properties of the exposed silicon. With increas-
Option for the ZEISS MERLIN series offers all other imaging
ing dose the work function difference increases.
modes known from high-end stand-alone AFM instruments.
For example, Kelvin Probe Force Microscopy (KPFM). In this
Summary
An array of ring nanostructures produced with the helium
ion beam of ZEISS ORION NanoFab was studied using ZEISS
MERLIN with the AFM option.
Under SEM observation the AFM tip can be placed on the
region of interest in a fast and safe manner. The workflow
can be accelerated dramatically, even if the sample, a silicon
cantilever tip, shows extreme topography.
AFM 3D topographical images of the ring nanostructures
a
deliver quantitative information on the ring profiles in x, y,
and z down to the sub nm scale. For a helium dose smaller
than 0.02 nC/µm² the ring patterns are not visible because
they disappear in the surface roughness but could be visualized using Phase imaging. For a higher dose of e.g. 1 nC/µm²
exposed areas protrude from the sample surface, possibly
due to lattice expansion through increased helium ion implantation. Phase imaging and Kelvin Probe Force Microscopy of the nanostructures were performed to study variations
in physical and electrical properties of the helium beam exposed areas. A direct dependency of work function and helium exposure dose could be shown.
b
Figure 5 a) KPFM image of nanostructured rings. b) Voltage as a function of
distance along the blue arrow in a). The work function difference decreases
with decreasing helium ion beam dose (0.02, 0.018, 0.016 and 0.014 nC/µm2).
The presented experimental results demonstrate the strength
of the AFM/SEM combination provided by this product for
the fast AFM in-situ characterization and quantitative analysis of nanometer scale patterns in the SEM.
mode, material contrast and surface potential differences
in materials on the nm scale can be imaged.
Recently, Saive et al. [4] have shown the effect of surface
structuring with a Gallium focused ion beam (FIB) on the
potential distribution across the channel of an organic field
effect transistor by Kelvin Probe Microscopy with an early
prototype of the AFM Option for the ZEISS MERLIN series.
KPFM was used to characterize further the ring nanostructures. Some results are shown in Fig. 5. The rings show a
References:
[1] The AFM option is also available for other ZEISS electron microscopes
on request.
[2] For more information on ZEISS ORION NanoFab please refer to
www.zeiss.com/nanofab
[3] S.A. Boden, Z. Moktadir, F.M. Alkhalil, H. Mizuta, H.N. Rutt, and D.M.
Bagnall, Milling of extremely thin silicon-on-insulator using the helium ion microscope, Abstract and Talk, EMC 2012 (2012).
[4] R. Saive, L. Mueller, E. Mankel, W. Kowalsky, and M. Kroeger, Doping of
strong KPFM, i.e. work function, contrast of approximately
TIPS-pentacene via Focused Ion Beam (FIB) exposure, Organic Electronics,
200 mV as compared to the non-exposed silicon. This find-
Vol. 14, 1570-1576 (2013).
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Carl Zeiss Microscopy GmbH
07745 Jena, Germany
Materials
[email protected]
www.zeiss.com/microscopy
EN_42_011_093 | CZ 07-2013 | Design, scope of delivery and technical progress subject to change without notice. | © Carl Zeiss Microscopy GmbH