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Introduction of Focused Ion Beam (Spring 2013)
Ningyuan Wang, 3550808
EEE-5425
Introduction to Nanotechnology
Spring 2013

Abstract—The Focused Ion Beam (FIB) is a tool that using the
focused Gallium ion beam directed towards the samples, and
upon interaction it generates signals that are used to create high
magnification images of the sample by mapping that signal to the
beam position. The ions contain many times more energy and
heavier than electrons on the surface of sample so when they
impact a material they sputter away atoms from the surface. Thus,
the FIB system can use as an etching tool. Also, a chemistry in gas
form can be injected close to the surface and allow material
deposition. An example of using all above three applications is
doing the micro-sampling for Transmission Electron Microscope.
Index Terms—Focused Ion Beam, FIB, Liquid Metal Ion
Source, Gallium, Applications of FIB, FIB imaging, etching,
deposition, Micro-sampling for TEM
I. INTRODUCTION
T
HE Focused Ion Beam (FIB) system is a significant
method of the Nanotechnology. Nowadays, the FIB is the
only method that actually operating in Nano-scale. The
minimum size of the metal deposition is typically around 15nm,
and for etching, the size can be even a little bit small than
deposition. The following sections will talk about some details
about the FIB system, which including the history, instrument,
and applications.
TABLE I
DETAILS OF FOCUSED ION BEAM
FIB
Particle
Beam
Penetration
depth
Average
electrons
Type
Elementary charge
Particle size
mass
size
In polymer at 30 kV
In polymer at 2 kV
In iron at 30 kV
In iron at 2 kV
Secondary electron
Ga+ Ion
+1
0.2nm
1.210-25kg
nm range
60 nm
12 nm
20 nm
4 nm
100-200
This paper is submitted on March 16th, 2013.
Ningyuan Wang is with the Florida International University, Miami, FL
33199 USA (e-mail: [email protected]).
II. HISTORY OF FOCUSED ION BEAM
When the Focused Ion Beam (FIB) system first built, the
technology using is not the Liquid Metal Ion Source (LMIS)
which we use today. In 1975, the first FIB system based on field
emission technology developed by Levi-Setti [1] and based on
gas field ionization sources by Orloff [2] and Swanson. Also, in
the same year, Krohn and Ringo [3] produced first high
brightness ion source, which we still us today in the Focused
Ion Beam system: Liquid Metal Ion Source. Then, three years
later, in 1978, the first FIB based on an LMIS was built by
Seliger et al [4].
III. INSTRUMENT
The basic FIB instrument consists of a vacuum system and
chamber, a liquid metal ion source, an ion column, a sample
stage, detectors, gas delivery system, and a computer to run the
complete instrument as shown schematically in Figure 1. The
typical FIB system may have three vacuum pumping regions,
one for the source and ion column, one for the sample and
detectors, and a third for sample exchange. The liquid metal ion
source in the ion column which can move vertically provides
the finely focused ion beam that makes possible high lateral
resolution removal of material. The detector is used to catch the
sputtering electrons and ions for the imaging. Five axis
motorized eucentric stage motion allows rapid sputtering at
various angles to the specimen. The ion beam interaction with
organo-metallic species facilitates site specific deposition of
metallic.
2
There are several metallic elements or alloy sources that can
be used in a LMIS. Gallium (Ga) is currently the most
commonly used LMIS for commercial FIB instruments for a
number of reasons: (i) its low melting point minimizes any
reaction or inter diffusion between the liquid and the tungsten
needle substrate, (ii) its low volatility at the melting point
conserves the supply of metal and yields a long source life, (iii)
its low surface free energy promotes viscous behavior on the
(usually W) substrate, (iv) its low vapor pressure allows
Gallium to be used in its pure form instead of in the form of an
alloy source and yields a long lifetime since the liquid will not
evaporate, (v) it has excellent mechanical, electrical, and
vacuum properties, (vi) its emission characteristics enable high
angular intensity with a small energy spread, and (vii) A high
brightness is obtained due to the surface potential, the flow
properties of the Gallium, the sharpness of the tip, and the
construction of the gun which results in both ionization and
field emission.[5]
TABLE II
CHEMISTRY PROPERTIES OF GALLIUM
Gallium
Melting point
Boiling point
Energy of first ionization
Figure 1. (a) A schematic diagram of a basic FIB system. (b) A single beam
FEI 200TEM
A. Liquid Ion Source
The LMIS has the ability to provide a source of ions of
approximately 5 nm in diameter. Figure 2a shows a schematic
diagram of a typical LMIS which contains a tungsten needle
attached to a reservoir that holds the metal source material.
Figure 2. (a) A schematic diagram of a liquid metal ion source (LMIS) and (b)
an actual commercial Ga LMIS (courtesy of FEI Company).
1310 K
1448 K
1620 K
1838 K
2125 K
2518 K
302.9146 K, 29.7646 , 85.5763
2477 K, 2204 , 3999
578.6kJ·m-1
Vapor pressure
1 Pa
10 Pa
100 Pa
1 kPa
10 kPa
100 kPa
B. Ga+ Ion Emission
Ga+ ion emission occurs via the following two steps as
shown in Figure 3:
--First, the heated Gallium flows and wets a tungsten
needle having a tip radius of ~2-5um. Once heated, the Gallium
may remain molten at ambient conditions for weeks due to its
super-cooling properties. An electric field applied to the end of
the wetted tip causes the liquid Ga+ to form a point source on
the order of 2-5 nm in diameter in the shape of a “Taylor cone.”
The conical shape forms as a result of the electrostatic and
surface tension force balance that is set up due to the applied
electric field.
--Second, once force balance is achieved, the cone tip is
small enough such that the extraction voltage can pull Gallium
from the tungsten tip and efficiently ionize it by field
evaporation of the metal at the end of the Taylor cone.
3
IV. APPLICATIONS OF FOCUSED ION BEAM
Figure 3 Ga+ ion emission
C. The Ion Column
Once the Ga+ ions are extracted from the LMIS, they are
accelerated through a potential down the ion column. Figure 4
and Figure 5 are the construction of ion column.
Typically, the ion column has two lenses. The condenser lens
(lens 1) is the probe forming lens and the objective lens (lens 2)
is used to focus the beam of ions at the sample surface. A set of
apertures of various diameters also help in defining the probe
size and provides a range of ion currents that may be used for
different applications. In the middle, is the beam blanking. The
beam blanking is used to prevent unwanted erosion of the
sample by deflecting the beam away from the center of the
column.
Figure 4 Construction of typical ion column
Figure 5 Real FIB ion column
The basic applications of Focused Ion Beam system can be
divided into three aspects: etching, deposition and imaging.
They all depend on the reaction on the surface of sample when
the ion beam impinges it.
The ability to etch, image, and deposit material using a
focused ion beam (FIB) instrument depends critically on the
nature of the ion beam, solid interactions. Figure 6 shows a
schematic diagram illustrating some of the possible ion beam or
material interactions that can result from ion bombardment of a
solid.
Figure 6 Schematic diagram of the sputtering process and ion-solid interactions
A. Etching
Etching takes place as a result of physical sputtering of the
target. An understanding of sputtering requires consideration of
the interaction between an ion beam and the target. Sputtering
occurs as the result of a series of elastic collisions where
momentum is transferred from the incident ions to the target
atoms within a collision cascade region. A surface atom may be
ejected as a sputtered particle if it receives a component of
kinetic energy that is sufficient to overcome the surface binding
energy of the target material. The secondary electrons and
secondary ions are collected and detected to form images.
However, there is also limitation about the FIB etching. FIBs
are most often used to create features of high aspect ratio.
Sputtered material and backsputtered ions may therefore
deposit on surfaces that are in close proximity to the active
milling site. Thus, surface degradation due to redeposition of
sputtered material must also be considered during FIB etching.
Figures 7 is SEM images of three trenches observed at a 70°
tilt.
Figure 7 Rectangular FIB trenches in (100) Si milled at normal incidence by
4
applying: (a) single fluence, (b) double fluence, and (c) triple fluence of Ga+
ions at 25 keV.
Therefore, controlling or at least predicting the manner in
which redeposition of sputtered material will occur can be
significant for the successful and rapid production of high
quality specimens by FIB techniques.
B. Deposition
FIB material deposition is currently commonly employed for
deposition of conductors and insulators for IC circuit edit and
for deposition of material to mask or protect the sample surface
during FIB micromachining for cross section analysis, for
Transmission Electron Microscope (TEM) sample preparation
and other micromachining applications.
Fig. 8 shows the spatial relationship of the gas source, the
focused Ga+ ion beam, the sample surface, and the volatilized
and sputtered species.
All deposited films contain not only the desired metals but
also incorporate impurities from the incompletely decomposed
precursor and contain Ga from the focused ion beam. The
percentage of contamination can vary significantly depending
on the deposition conditions, but in all cases, the amount of
gallium and carbon is a significant amount of the total.
TABLE IV
COMMON DEPOSITION GASES AND SOURCE TEMPERATURE
Metal
Composition (metal:C:Ga:O)
Resistivity(µohm-cm)
W
75:10:10:5
150-225
Pt
45:24:28:3
70-700
C. 3D-Nanofabrication
The three-dimensional nanofabrication technology, also can
be seen as a special type of FIB deposition, has been jointly
developed by Himeji Institute of Technology, NEC, and SII
(Matsui, Kaito and Fujita, 2000). [6] Using the FIB-CVD
technique allows the formation of 3D structures that cannot be
made with existing technologies.
Figure 9 shows the 3D fabrication of a spring coil structure.
The spring coil was formed by rotating the ion beam at a 13s
cycle. The diameter of the coil is 600 nm, while the diameter of
the wire is 80 nm.
Figure 8 Schematic drawing of deposition/controlled material removal
process. The enhanced etch process is shown. If adsorbed gas decomposes
to non-volatile products, then deposition will take place.
For FIB induced material deposition to occur, a precursor
must have two properties. The precursor must have a sufficient
sticking probability to stick to a surface of interest in sufficient
quantity, and it must, when bombarded by an energetic ion
beam, decompose more rapidly than it is sputtered away by the
ion beam.
Of the many precursors that have been successfully used for
FIB deposition, those whose FIB induced decomposition
results in metal deposition (e.g. W, Pt, Au, Al, and Cu) are the
most commonly used. The microstructure, composition and
resistivity of the deposited metals varies. FIB deposited Pt and
W films are amorphous, but FIB deposition of Cu and Au
produces polycrystalline films.[5] W and Pt are by far the most
commonly deposited metals and are routinely and mostly
interchangeably used for semiconductor circuit edits and as a
surface protection for SEM or TEM specimen preparation.
TABLE III
COMMON DEPOSITION GASES AND SOURCE TEMPERATURE
Element
W
Pt
Al
SiO2
SiO2
C
Precursor gas
Tungsten Hexcarbonyl, W(CO)6
Methylcyclopentadienyl platinum
trimethyl, (CH3)3(CH3C5H4)Pt
Trimethylamine Alane, (TMAA),
(CH3)3NAlH3
O2 and tetraethoxysilane (TEOS)
O2 and tetraemethoxysilane
(TMOS), Si(OCH3)4
Phenanthrene
55
30
(melting point)
25 , 1 torr vapor
pressure
Figure 9 3D fabrication of a spring coil.
The example above that the FIB-CVD technique allows for
forming three-dimensional structures at a fixed position, which
could not be achieved using existing techniques. The SIM2000
series scanning ion microscope has a function of making FIB
processing data automatically based upon the 3-D CAD data.
This function allows users to form three-dimensional
nanostructures very easily.
An example of this function is shown in the Figures 9 and 10.
First, the user makes a CAD file of any three-dimensional
structure as shown in the Figure 9. This CAD data is divided in
the Z-axis direction into several pieces of data to make a series
of FIB processing data automatically. Then this series of data is
loaded to the SMI2000 series scanning ion microscope and
deposition begins. The SMI2000 uses the CAD data to
automatically form the three-dimensional nanostructure as
shown in the Figure 10.
5
2)
Figure 9 3D CAD drawing of a feature.
3)
beam for imaging the system operates as high resolution
field emission SEM.
The second imaging mode uses the ion beam while the
electron beam is blanked. The FIB imaging mode is used
for grain analysis, voltage contrast imaging, and defining
of milling areas. It also reveal chemical differences, and
are especially useful in corrosion studies, as secondary ion
yields of metals can increase by three orders of magnitude
in the presence of oxygen, clearly revealing the presence
of corrosion.
The last imaging mode is the so called CrossBeam
operation mode: Both beams are turned on and while the
ion beam is milling a defined area, the SEM is used to
image the milling process at high resolution in real time.
This enables the operator to control the milling process on
a nanometer scale and to perform extremely accurate
cross sections and device modifications.
Figure 10 3D FIB fabrication performed automatically from the CAD drawing
in figure 9.
D. FIB Imaging
The FIB instrument is similar to a scanning electron
microscope (SEM), except that the beam that is rastered over
the sample is an ion beam rather than an electron beam.
Secondary electrons are generated by the interaction of the ion
beam with the sample surface and can be used to obtain
high-spatial-resolution images. Most modern FIB instruments
supplement the FIB column with an additional SEM column so
that the instrument becomes a versatile “dual-beam” platform,
as shown in Figure 11 for imaging, material removal, and
deposition at length scales of a few nanometers to hundreds of
microns.
Figure 12 Secondary electron imaging (left) and secondary ion imaging
(right).
V. MICRO-SAMPLING FOR TEM
The FIB micro-sampling technique is developed to remove a
portion of the micro-sample with a thickness of about 0.1µm
for TEM observation. The process of lifting out the
micro-sample, mounting the micro-sample on a special carrier,
and thinning of the micro-sample by FIB milling are performed
in the FIB system continuously. All of the preparing steps are
accomplished under vacuum in the same FIB system. The
procedure for the FIB micro-sampling is schematically
illustrated in Figure 13.
Figure 11 Schematic illustration of a dual-beam FIB–SEM instrument.
Expanded view
The CrossBeam system can operate at three different
imaging modes.
1) By blanking the ion beam and only using the electron
Figure 13 Schematic illustration of the FIB micro-sampling technique.
6
First, the region of interest is covered with a protective layer
of W or Pt by FIB assisted deposition. Then the sample is deep
trenched by FIB milling to remove a portion of sample
(micro-sample) from the region of interest (Figure 13 a). Next,
the micro-sample is separated from the bulk sample and lifted
out using a micromanipulator (Figure 13b). The micro-sample
is then mounted onto an edge of a micro-sample carrier (Figure
13c). Finally, the mounted micro-sample is secured by an FIB
assisted metal deposition (Figure 13d). The micro-sample
carrier is mounted on an FIB/TEM(STEM) compatible
specimen holder so that thinning of the micro-sample can be
performed just after securing it to the micro-sample carrier. [7]
A series of FIB images showing the procedure to prepare a
micro-sample is shown in Figure 14.
localized and characterized by TEM observation. After the plan
view TEM observation, the micro-sample is transferred back to
the FIB system for in order to prepare a cross sectional TEM
specimen. In the FIB system, the carrier is rotated so the
incident ion beam is perpendicular to the micro-sample (Fig.
15f). Then, the micro-sample is separated from the carrier by
FIB milling, subsequently transferred to the surface of a flat
substrate material (Fig. 15g-h), and secured with FIB assisted
metal deposition (Fig. 15i). Finally, the site to be characterized
is thinned for cross sectional TEM observation (Fig. 15j). [8]
Figure 15 Procedure for the micro-sampling of a specific site for cross sectional
and plan view TEM observation
Figure 14 Series of FIB images showing the procedure for the FIB
micro-sample preparation.
First, the area to be examined is localized (Figure 4a). Then,
metal (W or Pt) is deposited as the protection layer over the
region of interest (Figure 4b). The thickness of the deposition
layer is normally 0.5 to 1µm. Next, the front side of the region
of interest is deep trenched by FIB milling (Figure 4c). After
that, the back side and both right and left sides of the region of
interest are deep trenched leaving a micro-bridge at the upper
left corner of the sample (Figure 4d). The sample is then tilted
60° and the bottom of the sample is milled to separate the
bottom portion of the sample from the bulk sample (Figure 4e).
The bulk sample is then tilted back to normal to the ion beam
(Figure 4f), and a micromanipulator probe is bonded to the
micro-sample with W deposition. The size of the deposited W
bonding layer is about 2µm2µm and the thickness of the layer
is approximately 0.5µm. After the probe bonding, the
micro-bridge is cut off with FIB milling to completely separate
the micro-sample from the bulk sample (Figure 4g). Then, the
micromanipulator probe is elevated to lift out the micro-sample
(Figure 4h). The milling and bonding operations are performed
using a Ga+ ion beam at an accelerating voltage of 30kV. The
beam currents and the beam diameters of the Ga+ ion beam
used in these operations ranged from 10-20 nA and 400- 600
nm, respectively.
Figure 15 shows the procedure to prepare the specimen for
both cross sectional and plan view TEM observation of the
same site. First, a micro-sample is prepared from the specific
site for plan view TEM observation (Fig. 15a-e). The site is
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[8]
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Ningyuan
Wang
was
born
in
Shijiazhuang, Hebei Province, China in
1989. She received B.S. degrees in
electrical engineering from Florida
International University, Miami FL, USA
and Hebei University of Technology,
Tianjin, China in 2012.
From 2010 to 2012, she is an
7
undergraduate student in Florida International University. Her
work several project with partner during the study. Including, a
power project which using three different power supplies, wind,
solar and battery, to accomplish a car runaway-resistance
system. And a guiding robot based on RFID technology and
Kinect to detective the path.