Application Note
Sample Protection prior to FIB Processing
related instruments: Leica EM ACE600
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Each Atom Really Counts:
Protect Your Samples Prior to FIB Processing
Focused ion beam technology has
become an indispensable tool for site-
specific TEM sample preparation. It
allows to extract electron transparent
specimens with nanometer precision
using a focused Ga+-ion beam. The
application of a Pt-based protection layer
is often a first and critical step in order to
protect the surface from incurring FIB
induced damage. Such protection layers
are mainly deposited by means of IBID
(ion-beam induced deposition) and EBID
(electron-beam induced deposition).
Although the latter CVD process is known
to be less harmful, damage to the surface
and superficial layers of the specimen
can never be eliminated.
An efficient way to protect any sample is
to deposit an interstitial carbon
protection layer with a uniform density
and thickness. Conventional carbon rod
evaporation will impart a considerable
amount of heat on the sample causing
damage. Adaptive carbon thread
evaporation, on the other hand, will form
a strong and conductive layer while
maintaining the pristine characteristics
of the sample. This new evaporation
mode which was already well received
by the scientific community is
characterized by its superior flexibility
and high reproducibility. It enables its
users to deposit a uniform and conductive
carbon layer with minimal impact on the
sample. Moreover, its low sputter rate
and absolute absence of grain structures
allows an optimal protection and a
decrease of curtaining effects
respectively. Thıs application note
discusses protective coatings and shows
the most optimal route to protect your
samples and rule out any doubts
concerning the fine structure of the
surface up to the atomic level.
Figure 1. Examples of FIB-induced damage in TEM lamellas.
(A) Inter-atomic mixing and etching. (B) Severe curtaining
artefacts due to combination of granular EBID layer and
low-kV milling during final thinning of the lamella. (C)
High-Z EBID coating adjacent to region of interest
hampering EELS data acquisition.
Permanent marker deposition was suggested as a
mean to protect samples against focused ion
beam damage by simply drawing a line on the
sample using a black-colored permanent marker.
Although very fast and cheap, such layers are not
reproducible and thick layers often obscure a clear
visualization of the sample surface prior to FIB
milling. Hence, they hamper a fast localization of
the correct region of interest. Moreover, it is often
unclear how these layers will react once exposed
to high probe currents often used during atomic
resolution STEM imaging or prolonged EELS data
acquisitions. The resin used in the permanent
marker ink can outgas and be a potential source of
contamination, especially when performing
atomic resolution imaging or spectroscopy. The
applied layers are also rather polymeric and are
considerably less conductive.
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Figure 2. Carbon coating as an interstitial layer to prevent FIB milling artefacts and damage. Examples of FIB-induced damage in TEM lamellas. (A)
ACE600 stage after deposition of 35 nm of amorphous carbon. (B) Overview image (HAADF-STEM) of a mounted FIB lamella. (C) Overview image
(HAADF-STEM) to demonstrate the uniform thickness of the carbon protection layer. (D and E) Higher magnification images (HAADF-STEM) of the
sample, showing superior preservation of the surface atoms and layers. (Courtesy images Ricardo Evoagil, Jo Verbeeck, EMAT, University of
Antwerp; FIB sample prepared by Stijn Van den Broeck)
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LNT Application Note - SAMPLE PROTECTION PRIOR TO FIB PROCESSING
As an alternative, amorphous carbon coating can be used as a protective
layer. Although carbon can be deposited by a range of different deposition
modes, this application poses some stringent demands on the quality of
the layer. In order to be suited for this application, the carbon layer
should be: (1) of a uniform density, (2) conductive, (3) have a low impact
on the sample during deposition and (4) be free of any particulate matter.
Adaptive carbon thread is an efficient way to deposit an amorphous
carbon layer with minimal impact on the sample. Here, a carbon thread
is placed along 5 electrical terminals allowing to evaporate 4 carbon
thread segments completely. The adaptive process ensures a constant
power output by measuring the resistance after each pulse and adapting
the next power output accordingly. This process not only ensures a
complete use of each thread segment without release of debris but also
highly contributes to a superior carbon layer free of granular structures
and density variations.
Because the process results in a low energy evaporant, atom intermixing
does not occur at the surface of the sample. Figure 2A shows a SEM stub
after a 35 nm carbon coating. Consecutively, EBID and IBID Pt layers
were deposited locally after selecting the region of interest. Figure 2B
shows the final TEM lamella mounted on an omniprobe grid. Figure 2C
shows a uniform thickness of the carbon layer along the cross section.
Here, the substrate and thin film are protected by the carbon layer onto
which EBID and IBID layers were deposited. As a consequence, the
surface atoms and layers are protected from FIB milling artefacts. The
amorphous carbon protection layer can also be used to fine tune probe
aberrations or serve as a reference layer for analytical TEM.
To illustrate the superior performance of the adaptive carbon thread
coating in terms of diffuse and uniform coating, an irregular sample was
protected with carbon (figure 3A). A uniform thickness can still be
obtained and a full protection is guaranteed along the surface of the
sample up to atomic scale. The diffuse contrast on top of the first atomic
layer in image 3C is due to atmospheric contamination. A larger amount
of contaminants, however, can outgas during FIB imaging and can locally
cause the carbon to bubble up. In such cases a cleaning step should be
performed before FIB processing. This can be accomplished by glow
discharge or plasma cleaning.
To conclude, adaptive carbon thread evaporation produces amorphous
protection layers that are extremely suited against FIB milling damage
and artefacts. Its lower sputter rate also protects the sample during FIB
processing and ensures a complete protection of the surface and
subsurface layers up to atomic scale.
Figure 3. Adaptive carbon thread coating for irregular surfaces. (A) Overview
image of a uniform coating. (B) Detail of image A. (C) Image showing
surface atoms. (Courtesy images Ricardo Evoagil, Jo Verbeeck, EMAT,
University of Antwerp; FIB sample prepared by Stijn Van den Broeck)
Images acquired by Dr. Ricardo Evoagil and Prof.Dr. Jo Verbeeck, EMAT,
University of Antwerp. EMAT is part of the ESTEEM2 consortium (www.
esteem2.eu)
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Leica EM ICE Application Note ULTRA THIN CARBON FILMS ∙06/16 ∙ Copyright © by Leica Mikrosysteme GmbH, Vienna, Austria, 2015. Subject to modifications. LEICA and the Leica Logo are registered trademarks of Leica Microsystems IR GmbH.
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