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Technical Note
High Resolution Imaging at Low Acceleration Voltages
and Low Beam Currents with ZEISS MERLIN
Technical Note
High Resolution Imaging at Low Acceleration Voltages
and Low Beam Currents with ZEISS MERLIN
Authors: Dr. Jörg Stodolka, Dr. Heiner Jaksch,
Jeff Marshman, Jijin Yang , Jean-Claude Menard, Dr. Michael Albiez
Carl Zeiss Microscopy
Date: June 2010
Introduction
Instrumentation
MERLIN combines high resolution imaging, optimized at all
MERLIN combines a double condenser with the well-proven
energies, with unprecedented ease of use. The continuously
GEMINI technology of ZEISS. The GEMINI column offers good
adjustable current up to several hundred nA allows time-
resolution at low acceleration voltages. Even at low energies
saving image acquisition and high-speed analytics. In situ
the beam is kept stable with the help of a beam booster that
cleaning and local charge compensation add up to a system
accelerates the electrons in the column above 10 kV prior to
which is a highly versatile and user friendly FE-SEM, offering
deceleration on the sample surface. The twin lens of GEMINI
endless possibilities for imaging and analysis of specimens.
consists of an electrostatic and a magnetic part, limits the
magnetic field to the column and makes undisturbed investigations of magnetic samples possible. It is also used as a
detection lens, separating secondary electrons from backscatter electrons. The latter are directly detected by an on-axis
backscatter detector on an energy-specific basis. Without any
additional adjustment, it can be toggled between the various
Double condenser
detectors. The implementation of an additional condenser
(Fig.1) into the GEMINIII column offers the unique opportunity to continuously change the current from 4 pA up to
Inlens EsB detector
300 nA (depending on configuration and acceleration voltage)
Inlens SE detector
without having to change any apertures. It was specifically
Beam booster
developed for offering high resolution possibilities at all curMagnetic lens
rents. While the upper condenser sets the current, the lower
condenser accounts for optimum resolution by optimizing the
aperture angle. For a larger depth of field the system can be
Scan coils
Electrr ostatic lens
AsB detector
Specimen
switched to a different imaging mode and the convergence
angle is decreased.
Figure 1
Outline of the ZEISS GEMINI II column, unique to MERLIN FE-SEM.
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Technical Note
Imaging Results
The following examples show the huge range of low energy and / or low current investigations that are possible with
MERLIN without losing resolution. In the simplest case imaging of conductive samples at low energies offers the opportunity to be extremely sensitive to surface information. Details
can be revealed that normally would be blurred due to larger
penetration depths of the electrons at higher voltages. Fig. 2
reveals grains on the gold islands of a gold-on-carbon sample (due to back sputtering of material) – a specimen that is
normally used for resolution measurements. The image was
acquired at 400 V and 7 pA only. Such surface detail disappears with an increase of beam energy.
100 nm
Figure 2
Details of grains on gold islands of a gold-on-carbon sample, increased surface
detail at low voltages: 400 V, 7 pA, in lens detector.
Carbon nanotubes are an important object of recent research.
Although the diameters of the up to several centimeters long
cylindrical structures are in the nanometer range these tubes
are the strongest and stiffest material yet discovered. This
toughness makes them ideal candidates for materials science
with a wide range of applications such as in the clothing
industry and even in architecture. In biology they have been
used to transport RNA into cells while their electrical
properties have led to the development of ultracapacitors and
transitors. The imaging of carbon nanotubes can be done
at all energies but only at low energies (1 kV and below) the
tubes do not appear translucent and the cylindrical wall can
be investigated. Fig. 3 shows an image acquired at an energy
of only 30 V. Even at extremely low voltages the surface of
the specimen shows fine details.
10 µm
Figure 3
Carbon nanotubes investigated at 30 V, 250 pA, inlens detector: the bright
areas are thin layers of polymer. Detailed imaging is possible even at lowest
The thin layer of silica seen in Fig. 4 is another example for
energies.
high surface sensitivity at low acceleration voltages. Silica
is primarily used in the production of glass and glass fibers.
Additionally, layers of Silica (SiO2) are used as highly stable
electric insulators on silicon wafers. At higher energies the
fine surface details would be lost. While the silica does not
charge up at higher currents, it does so at higher acceleration
voltages. However, for some samples a reduction of current
is crucial to avoid charging.
2 µm
Figure 4
Investigation of a silica layer at 30 V, 250 pA, Everhart-Thornley detector,
highest topographical detail at lowest voltages.
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Technical Note
Fig. 5 shows the image of alumina spheres that more recently
seem to become the new standard sample for testing the
resolution capability of SEMs at low voltages. At energies
below 500 eV and currents of about 50 pA the charging of
these non-conductors (industrially used as grinding powder or
carrier material in catalysts) can be sufficiently suppressed
to show even the finest surface detail that was not accessible
before.
20 nm
Figure 5
Finest surface detail on Al2O3 spheres at low voltage and low beam current:
450 V, 60 pA, inlens detector.
In Fig. 6 small gold particles can be seen on the surface of
titanosilicate, a material used as a heterogene catalyst for
oxidations. Due to the highly charging nature of the sample
this information is only available at low currents. Another
catalyst is Gold-Palladium nanoparticles commonly used for
the production of hydrogen peroxide.
100 nm
Figure 6
Titanosilicate: small gold particles on crystal form of silicon dioxide:
1 kV, 80 pA, inlens detector. Fine surface detail at low current.
Fig. 7 shows an analysis of these particles in alumina as a
carrier material. The use of the ZEISS specific on-axis energy selective backscatter detector (EsB) proves that material
contrast images do not need high currents to be impressive.
With MERLIN, materials science is no longer limited to certain
ranges of parameters. Apart from academics, this versatility
may also be of crucial importance for service laboratories.
20 nm
Figure 7
AgPd catalyst in Al2O3 carrier, material information even at low current and
low voltage conditions: 800 V, 80 pA, EsB detector.
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Technical Note
In the semiconductor industry failure analysis is of crucial
importance in order to optimize the performance of a device
or to identify problems in its production. For integrated
circuits the chip has to be delayered by means of reactive ion
etching and/or lapping and is then inspected layer by layer.
Investigating the layers with an SEM offers the challenge of
imaging low-k dielectrics that are used to reduce the time
constant of the chip. These dielectrics are non-conductive and
beam-sensitive. Hence, low beam currents have to be used in
order to avoid charging (Fig. 8-10). Additionally, the surface
of a layer containing low k dielectrics can be investigated
without encountering any significant beam damage or
delamination of metal lines at low voltages and low beam
currents (Fig. 11).
20 nm
Figure 9
Detail of the same semiconductor device, 2 kV, 57 pA: even at high
magnifications no charging is observed due to low beam current.
100 nm
100 nm
Figure 8
Figure 10
Image of a polished semiconductor device at metal 1 /via level, 2 kV, 57 pA,
Semiconductor device polished to the gate oxide level, 1 kV, 20 pA:
charging is prevented by using low beam current.
prevention of charging and beam damage.
100 nm
Figure 11
Semiconductor device polished to the contact level just before exposing the
polysilicon level, 300 V, 70 pA: high surface sensitivity without charging and
beam damage.
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Technical Note
With low voltage/current options at hand, the staining of
samples in life sciences is no longer needed. Investigations of
tissue slices are becoming more time-efficient and even reveal
more detail: Fig. 12-14 show the fine nanostructures of the
collagen fibrils and the matrix of proteoglycans and other
proteins in bovine cornea and sclera, which have never been
seen with an SEM before.
The cornea and sclera together form the outer fibrous shell of
the eye globe and withstand both internal and external forces
to maintain the shape of the eyeball. While the cornea is
transparent, the sclera is not. Both the cornea and sclera are
mainly composed of collagen fibrils surrounded by a matrix
containing proteoglycans and other proteins. High resolution
SEM imaging of biological samples is often challenged by
100 nm
Figure 12
Bovine scleral collagen fibrils with D-periodicity and matrix with nanostructure
charging and beam damage. Strategies for dealing with these
of ~5 nm in diameter (bright color), 500 V, 30 pA: details that were never seen
two issues involve the use of low acceleration voltage
with an SEM before.
and probe current. However, low kV and low probe current
introduce other problems during imaging. For example, low
kV impairs resolution and causes faster contamination buildup
at high magnification. The signal-to-noise ratio is another
concern when using low probe current. With the high
resolution capabilities of MERLIN at low energies and beam
currents these strategies can be pursued without encountering the aforementioned issues. The uncoated and unstained
samples provide information that was never
accessible before.
400 nm
Summary
Figure 13
Bovine corneal collagen fibrils showing a characteristic D-periodicity, 500 V,
MERLIN offers high resolution imaging even at low acceleration
30 pA.
voltages and low beam currents. Charging and beam damage of
specimens can be prevented. The GEMINI II column – unique to
MERLIN SEM – ensures optimum resolution under all conditions, making MERLIN a versatile tool for imaging and analysis
with unmatched ease of use.
100 nm
Figure 14
Bovine corneal collagen fibrils with D-periodicity which are imbedded in a 3D
cross-bridge network containing nanostructures of ~5 nm in diameter,
500 V, 30 pA: highly detailed images at low kV / low beam current conditions.
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Carl Zeiss Microscopy GmbH
07745 Jena, Germany
BioSciences or Materials
[email protected]
www.zeiss.com/microscopy
EN_42_011_098 | CZ-08/2013 | Design, scope of delivery and technical progress subject to change without notice. | © Carl Zeiss Microscopy GmbH