Leica Microsystems EM ACE600 Application Note

Application Note

Sample Protection prior to FIB Processing

related instruments: Leica EM ACE600

Material

Research

Life Science

Research

Medical

Research

Industrial

Manufacturing

Natural

Resources

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.

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)

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)

Leica Microsystems EM ACE600 Application Note

Related papers

Other documents