Surface Energy Influence On Ion Sputtering Process
1.
4
Fourth Year Bachelors Project
For the Degree of
B.Eng in Mechanical Engineering
Journal Paper
Daniel Wallace Reilly
Surface Energy Influence in Ion Sputtering Process
1st
May 2015
Project Supervisor – Dr. Jining Sun
School of Engineering and Physical Sciences
Mechanical Engineering
2. 1
Table of Abbreviations
Table of Figures
Table of Tables
Abbreviation Meaning
FIB Focused ion beam
SEM Scanning electron microscope
Ga Gallium
Cu Copper
SRIM Stopping and ranges of ions in matter
TRIM Transport of ions in matter
SBE Surface binding energy
SY Sputtering yiled
LMIS Liquid metal ion source
Figure Number Name Page
1 FIB column (exploded view of parts) 4
2 Ion – solid interactions 5
3 Ion incident angle 5
4 Incident angle against sputtering yield 6
5 Effects of channeling 6
6 Micro-accelerometer post-processing 6
7 Atomic trajectories for determining simulated target thickness 7
8 TRIM main calculation input window 8
9 SY flux as No. of ions altered 8
10 Energy of atoms reaching target surface 8
11 Theoretical SY and SBE relationship 8
12 Zoomed image of polymer film 9
13 Custom designed and built stage 9
14 First cuts performed 9
15 Silver paint applied to polymer cut and stage 10
16 Initial cuts, low current beam rastering damage visible 10
17 Cross section of cuts 11
18 Custom stage with gold coating and silver paint applied 11
19 Custom stage placed inside FIB chamber 12
20 Cut dimensions after strain removed 12
21 Cross section of strain induced cuts 12
22&23 Volume/cross section approximations 12
Table Number Name Page
1 No.760 Formvar atomic properties 7
2 Equation symbols defined 9
3 Material removal rate of strained and un-strained sample cuts 13
3. 2
Table of Contents
TABLE OF ABBREVIATIONS 1
TABLE OF FIGURES 1
TABLE OF TABLES ERROR! BOOKMARK NOT DEFINED.
ABSTRACT 3
INTRODUCTION 4
LITERATURE REVIEW 4
WHAT IS A FOCUSED ION BEAM? 4
ION – SAMPLE INTERACTION 5
INCIDENT ANGLE INFLUENCE ON SPUTTERING YIELD 5
FIB MICRO AND NANOMILLING 6
OBJECTIVES 7
SRIM/TRIM 7
BRIEF OVERVIEW OF THE SOFTWARE 7
SIMULATION METHOD 7
EXPERIMENTS 9
MATERIAL 9
ISSUES ENCOUNTERED AND LESSONS LEARNT 9
EXPERIMENTAL METHOD (STANDARD STAGE) 10
STAGE PREPARATION 10
INITIAL CUTS 10
CROSS SECTIONING 10
EXPERIMENTAL METHOD (CUSTOM STAGE) 11
STAGE PREPARATION 11
VOLUME CALCULATIONS 12
RECTANGULAR CUTS 12
LINEAR CUTS 12
RESULTS AND DISCUSSION 13
CONCLUSIONS 13
FUTURE WORK 14
ACKNOWLEDGMENTS 14
REFERENCES 14
APPENDIX 16
4. 3
Surface Energy Influence in Ion Sputtering Process
Daniel Wallace Reilly
Supervisor: Dr. Jining Sun
B.Eng Mechanical Engineering
School of Engineering and Physical Sciences
Heriot Watt University
Riccarton
Edinburgh
EH14 4AS
Scotland
Abstract
The aim of this paper is to investigate theoretically and physically how altering the surface binding energy of a
material alters the rate in which atoms are sputtered during focused ion beam - FIB - milling. Although there are
techniques currently available for increasing the sputtering yield, this paper will investigate a method in which the
target material properties alone will be altered, instead of the ion beam properties. Method chosen for altering the
surface binding energy is to induce a strain on the sample material. A custom design stage was used to induce the strain.
Simulation software was used to predict the relationship. The strain imposed testing compared to the no-strain testing
gave a sputtering yield ratio of 1:1.105 respectively.
5.
4
Introduction
Focused Ion Beam (FIB) milling is used more
often than ever since it’s development in the late 1970’s
and early 1980’s1
. It is an extremely useful and accurate
method for various – extremely localised - imaging,
milling and deposition techniques.
One factor that has been widely investigated
with the FIB milling process in particular is the speed in
which material can be removed. Many of the techniques
currently utilised change the quality or integrity of the
structure being created and are therefore not always a
viable means of decreasing the operating time. The rate
of material removal on the scale (micro/nano) that this
method of milling operates in is measured by the
sputtering yield. Sputtering yield is defined technically
as; the mean number of sputtered target atoms per
incident ion2
. Although techniques are currently
available for increasing the sputtering yield, this paper
will investigate a method in which the target material
properties alone will be altered, instead of the ion beam
parameters.
It is a known fact that sputtering yield is very
sensitive to the surface binding energy of the target
material3
. Therefore, it would be reasonable to assume
that if this surface binding energy could be altered, the
rate at which material could be removed could also be
better controlled.
The method chosen to alter the surface binding energy
was to impose a strain on the target material. This was
in the hope that a clear correlation between the
sputtering yield and the applied strain would become
apparent. The combination of this knowledge and
understanding has created the idea to both theoretically -
through simulation - and experimentally investigate the
strain-induced method of increasing the surface binding
energy.
Literature Review
What is a Focused Ion Beam?
Focused or focusing – A term describing the effect
of narrowing or concentrating a wide spread of
matter or light into a much smaller area. A good
example of this is focusing sunlight through a
magnifying glass.
Ion – An atom or a group of atoms that have an
electric charge. Positive ions, or cations, are
formed by the loss of electrons. Negative ions,
or anions, are formed by the gain of electrons4
.
Beam – A narrow unidirectional flow of
electromagnetic radiation or particles4
.
The fundamental abilities of FIBs are; deposition,
sputtering and imaging on an extremely small scale –
the nano and micro scale. The reasoning behind the
choice of an ion source opposed to say photons or
electrons is simply: ions have a much larger mass and
therefore possess the potential for far greater energy
density when in the form of a beam6
. The process
begins in the ion source. The source is very different to
broad ion beams, which are generated from the likes of
plasma sources. This is due to its tiny source size, which
is in the range of 1nm-100nm, allowing for the beam to
be tightly focused, giving the beam a higher energy
density. Many applications and research with the use of
FIB consider the liquid metal ion source – LMIS – to be
the most appropriate for micro-machining and similar
techniques. The reasoning behind choosing LMIS over
another ion sources is that LMIS has the potential to
generate the brightest and most highly focused beam -
when connected to the appropriate optics7
. A liquid
metal ion source is a metal in the liquid state, heated
until it depletes, consequently emitting ions. The liquid
Figure 0 – FIB column (exploded view of parts)1
6. 5
metal ion source chosen for many industries and
applications is the Gallium-based blunt needle source.
This choice is due to the many benefits Gallium has in
relation to this application. It has a low melting
temperature of approximately 30°C, a relatively low
volatility and a low vapor pressure16
.
The emitted ions have to be focused and directed in a
controllable and consistent fashion for them to be
effective and useable. An ion column is used to contain
and control the flow, size and shape of the beam. As
seen in figure 1 the ions pass through various lenses,
apertures and octopoles before exiting the column and
ultimately impacting with the sample. The beam current
typically varies between 1pA and 10nA1
. The variable
aperture, seen in figure 1, is the part of the column that
controls the beam current. However, these become
damaged over time due to the bombardment of ions and
as a result are not always accurate to their specified
value.
Ion – Sample interaction
When an ion reaches and collides with the target or
sample surface, it loses the kinetic energy and
momentum it possessed very quickly. The main
products of the collision and transfer of energy are listed
below:
• Ion reflection and backscattering
• Electron emission
• Electromagnetic radiation
• Atomic sputtering and ion emission
• Sample damage
• Sample heating
Typically the ion comes to rest in the sample
material, which is commonly known as ion
implantation7
. The whole ion-solid interaction can be
classed under the title of ‘energy cascades’. The energy
cascade for an individual ion takes place in an
extremely small time frame of around 10
€
−11
s. It is
during this time that all of the aforementioned events
occur. In figure 2, a pictorial representation of the most
commonly accepted model for ion-solid interactions is
shown. There are two forms of interactions that take
place – inelastic and elastic. In inelastic interactions,
sometimes known as electronic energy loss, ion energy
is lost to the electrons in the sample material, which
results in ionization and emission of electrons and
electro-magnetic radiation from the sample. Elastic
interactions are called nuclear energy loss. The ions
kinetic energy is transferred to the stagnant atoms and
can result in damage if enough energy is present. This
damage is the movement of target atoms from their
original sites giving the possibility of sputtering7
. For
complete sputter, the atom must leave the target
material with a kinetic energy - normal to the surface -
greater than the surface binding energy. For the case of
5-30keV Ga impinging on most solids, the collision
cascade involves a series of independent binary
collisions. If the translational energy transferred to a
target atom during a collision exceeds a critical value
called the displacement energy, the atom will be
knocked from its original site7
.
Incident Angle Influence on Sputtering Yield
Discounting changing the properties of the ion
beam, the most widely used technique for increasing the
sputtering yield of a target material is by increasing the
angle of incidence – the angle between the path of the
beam and the target materials surface. Figure 3 and 4
show the definition of incidence angle in relation to the
ion and the target materials surface and the relationship
between SY and angle of incidence respectively.
Figure 2 – Ion-solid interactions7
Ga+
Figure 3 – Ion Incident Angle
θ
7. 6
The angle of incidence influences sputtering
yield due to the nature of the ions trajectory: the larger
the incident angle, the nearer the surface of the target
material the ions implantation and cascades occur.
However, there is a limit to the ideal angle of incidence
- ions begin to reflect as the angle is increased. The two
influencing parameters come to a point of maximum
sputtering yield at around 75°-80°. This effect has been
confirmed for 25-30keV Ga+ into a variety of
materials7
. Depending on the atomic structure, the
influence increasing the angle of incidence has on
sputtering yield can vary by an order of magnitude for
strongly channeling crystal orientations in materials
such as copper. This is due to the effects of channeling.
When channeling occurs, the ion reaches a greater depth
in the sample and therefore transfers its energy over a
greater depth of the material: due sputtering only
occurring for surface atoms, this has a detrimental effect
on the sputtering yield. Figure 5 displays the interaction
depth of primary Ga+ ions and the difference in
cascades when channeling is both present and absent9
.
Increasing the angle of incidence has one
major deficiency – it changes the nature and profile of
the cut. Due to the angle at which the ion beam is
colliding with said material, the result is angled
sidewalls, or skewed features. Therefore, whilst
increasing the incident angle increases sputtering yield,
it brings with it some complications, and as a
consequence it is not a viable option for many processes
where the only significant benefit is reduction in
machining times.
FIB Micro and Nanomilling
The most common use of the FIB system is for
milling. There are numerous applications where this
technique is utilised including; preparation of a range of
devices such as lenses on the ends of fibers11
; pseudo
spin valves12
; pillar micro-cavities13
and stacked
Josephson junctions14
. In essence, it is a fundamental
and widely utilised tool in the micro and nano industry.
It has opened the door to many opportunities for further
research in fields that require nano and micro structures
and has thus created the drive and desire to increase the
capabilities and performance of FIB so that it can
further change the shape and scope of many other fields
out with its own.
A good example application of FIB milling
that was seen to be of both practical and beneficial use,
other than pure research or interest, is its use in micro-
accelerometers15
. Figure 6 shows an accelerometer post-
processing with one final modification required – the
small readout gap that can be seen in figure 6 on the
right side of the image.
It requires a very high resolution cut at a
specific angle to create an accurate distance for
measuring the acceleration of whatever the device is
applied to. The flexibility and accuracy of FIB makes it
the perfect tool for this sort of application. The square
proof mass is suspended by two suspension beams, and
the narrow readout gap is situated in a small silicon
beam at the moving end of the proof mass. In order to
keep the FIB milling time as short as possible, a two-
stage process is utilised. A high current ion beam is
used initially to produce a coarse trench before using a
lower current – more accurate – FIB beam to finish the
process. The result is a 0.4µm wide gap at an angle of
45°. The milling time reported is around 2 minutes per
device1
. This is not an excessive time considering the
accuracy and scale on which the operation is
undertaken. However, if the time is related to the
material removed – it is incredibly slow. Additionally, if
there was a production requirement of say 300 (three
Figure 4 – Incident Angle against Sputtering Yield8
Figure 5 – Effects of Channeling10
Figure
6
–Micro-accelerometer
post-processing15
8. 7
hundred) accelerometers to be finished off by the FIB
process aforementioned – a small change in the time
taken to perform each cut could prove highly beneficial
financially.
Objectives
The objective of this experimental journal is to
investigate the effect surface binding energy has on
sputtering yield. The method chosen to alter the surface
binding energy was the introduction of strain into the
target material. By understanding the affect strain has
on the material removal rate (sputtering yield) we can
further understand the relationship between strain and
surface binding energy.
SRIM/TRIM software package was used to
produce a theoretical relationship between surface
binding energy and sputtering yield. The main
assumption made in the simulation part of the
investigation was that the surface binding energy is
directly influenced by strain. A theoretical graph was
produced, displaying the relationship between surface
binding energy and sputtering yield. The purpose of this
was to further understand and acknowledge the affect
altering the surface binding energy has on the sputtering
yield.
These theories and assumptions have to be
challenged experimentally for any real validation or
confirmation of the predictions made. Due to limited
funding, only two variations of sample strain will be
investigated, with strain and without strain. There will
be two cuts made on each sample; a rectangle cut and a
line cut. The volume removed will be calculated and
compared to one another.
SRIM/TRIM
Brief overview of the software
The Stopping and Ranges of Ions in Matter
software (SRIM-2008) is a collection of software
packages. They calculate many features of the transport
of ions in matter. TRIM (The Transport of Ions in
Matter) is the most comprehensive program included.
Numerous material and ion properties can be adjusted
within the software to affect the available output files.
Surface binding energy is the largest contributor to
changes in the sputtering yield, where ion energy and
incident angle are constant, and is thus the targeted
variable.
Each atom possesses its own surface binding
energy. The sputtering yield is calculated for each atom
individually. The individual sputtering yields are then
summed to provide a total representative sputtering
yield for the target. Although the software will not
produce results accurate enough to merely super impose
the experimental values on the same graph, it will give a
good indication towards the nature of the relationship. A
statement on the accuracy of the software, from the
software producers, is as follows:
“The sputtering yield is very sensitive to the surface
binding energy (SBE) which you input to the
calculation. Be aware that for real surfaces, this energy
changes under bombardment due to surface roughness
and damage, and also due to changes in the surface
stoichiometry for compounds. The sensitivity of
sputtering yield to surface binding energy may be
displayed during the calculation by using the plotting
menu. The plots of sputtering yield to SBE are accurate
to about 30%.”
Simulation Method
The Target material chosen to provide a trend
was the polymer No. 760 Formvar (PMMA). It is very
similar to the polymer, polyimide, which was used on
the experimental side. The atomic properties of the
simulation polymer are given in table 1.
Table
1
–
No.
760
Formvar
Atomic
Properties
Firstly, a target thickness had to be chosen for
the simulation. This was done by running a simulation,
viewing the ion and atomic trajectory data, then
reducing or increasing the target thickness to fit. A
screenshot of the ion trajectory for a few ions is shown
below in figure 7. This reduced the time taken to run
each simulation - whilst ensuring accuracy of results.
This is because reducing the thickness in which the
software and computer has to deal with improves
computational time. It also has no effect on the results
as all cascades are contained within the defined region.
The Ion trajectories (red lines) and atomic trajectories
(green, blue and turquoise lines) never touch or extend
past the defined target thickness.
Constituent
H
Hydrogen
C
Carbon
O
Oxygen
Atom stoichiometry 8 5 2
Weight (amu) 1.008 12.01 15.99
Surface binding energy 2 7.41 2
Figure
7
–
Atomic
trajectories
for
determining
simulated
target
thickness.
9. 8
Secondly, a value for the number of ions fired
at the target material for each simulation run also had to
be determined. The method chosen to do so started with
inputting all relevant values and properties into the main
software window. A screenshot of this input interface is
shown in figure 8 to clearly present all finalised inputs
chosen for the simulation stage.
The ‘Total Number of Ions’ was altered to the
value of ten (10) before running a simulation. This was
the first value for number of ions chosen to produce the
scatter graph shown below in figure 9. The calculated
sputtering yield was then recorded and inserted into the
spreadsheet to record and plot the data. The amount of
ions fired was then increased incrementally, while
retaining all other inputs for consistency, until the plot
of results settled to find a consistent number of ions for
more accurate and reliable data. Data retrieved is given
in appendix 1 under ‘Sputtering Yield Fluctuation
Data’. As can be seen from figure 9 below, the plot
displays a settled region at four thousand (4000)
incident ions and was thus the number of ions chosen to
produce an SY against SBE trend.
Figure
10
displays
a
graph
produced
by
the
software
itself,
during
one
simulation
run
with
a
total
of
four
thousand
(4000)
ions.
It
shows
a
general
representation
of
the
relationship
between
sputtering
yield
and
surface
binding
energy,
but
it
is
not
interactive
and
has
no
definitive
numerical
values.
The
area
on
the
left
hand
side
of
figure
10
within
the
‘not
sputtered’
region
is
all
the
atoms
that
have
reached
the
targets
surface,
but
have
less
energy
normal
to
the
targets
surface
than
is
required
for
them
to
be
sputtered.
It
is
clear
to
see
that
this
region
of
the
graph
holds
the
largest
quantity
of
atoms.
Changing
or
altering
the
surface
binding
energy
would
effectively
shift
the
‘Not
Sputtered’
line
to
the
left
and
therefore
increase
the
number
of
sputtered
atoms.
It
is
through
this
theory
that
the
idea
to
investigate
how
inducing
a
strain
on
the
target
material
would
affect
the
sputtering
yield
was
born.
Experiments
Results were then plotted for varying values of
surface binding energy. Natural sputtering yield was
used as the reference value. The surface binding energy
was then altered above and below the natural value over
a large range. This was to gain a simulated
understanding - both visual and numerical - of how the
sputtering yield would alter in relation to the surface
binding energy present in the individual atoms of the
PMMA polymer No. 760 Formvar. The final scatter
graph produced in figure 11 is a far more accurate and
interactive plot than that in figure 10 discussed prior. A
line of best fit can be produced to decipher a numerical
relationship between the independent (surface binding
energy) and dependent (sputtering yield) variables. The
best equation of fit is disclosed in equation 1 overleaf.
Figure
8
–
TRIM
main
calculation
input
window
Figure
9
–
SY
flux
as
No.
of
ions
altered
Figure
10
–
Energy
of
atoms
reaching
target
surface
Figure
11
–
Theoretical
SY
and
SBE
relationship
10. 9
Equation
1
–
SY
vs
SBE
Equation
of
best
fit
Table
2
–
Equation
symbols
defined
Experiments
N.B. This section is documentation of the process
undertaken to obtain all experimental results. The
method and steps taken will be covered as clearly and
concisely as possible.
Material
The chosen target material was a thin
polyimide film. It was chosen due to its similar atomic
properties to the polymer used in the simulation testing.
Below in figure 12 is an image of what the sample
material chosen looks like.
For completion of experimental results, there
was a requirement for two stages (a stage being a device
that holds the sample material for milling and insertion
into the FIB chamber). The first stage was a readily
available, standard, flat surface stage. The second stage
had to be capable of inducing and holding a constant
strain on the target polymer. The design that was chosen
and manufactured is shown below in figure 13.
The stage was manufactured from aluminium.
This was due to two main reasons. Firstly, to satisfy the
need to have a conductive material to dissipate and
conduct the production of charge as ions implant
themselves into the target material. Secondly,
aluminium is cheap and was readily available.
The stage is operated by wrapping a thin strip
of polymer around it as shown in the bottom left image
in figure 13, screwing in the end clamps, and then
turning the wheel. Turning the wheel lowers and raises
the flat polymer bed. Raising the bed past the point of
contact with the polymer results in a strain being
induced on the polymer. The reactant force on the
polymer bed due to the imposed strain locks the wheel
in place to avoid any slackening. Even very slight
slacking of the strain would be visible when being
observed through the SEM or FIB machines and is
therefore highly undesirable.
Issues Encountered and Lessons Learnt
Prior to the cuts used for results, initial cuts
were performed that had poor cut quality; this is to be
expected due to a lack of conductivity. The polymer was
attached to the stage prior to the gold coat being
applied, therefore leaving only the very edges as a
possible route for conduction of excess charge. The
result was a heavily skewed, un-uniform cut with a far
lower possibility of meaningful results. The first cuts
are shown below in figure 14.
The clear drift observable in figure 14 is most
noticeable in the line cut. On the left image, the line cut
highlighted by the red box has close to zero depth due to
the constant drift of the sample during each scan of the
FIB. This resulted in an incomprehensible set of results
that simply had to be discarded. However, it served as a
valuable lesson on the importance of good sample-stage
conductivity. Not only did it pose problems with the cut
quality, but due to the use of an SEM for post cut
imaging, if a charge is in the sample it heavily interferes
with the image quality of the SEM due to its use of
electrons for imaging.
Symbol
Symbolises
Units
€
γSY
Atomic
Sputtering
Yield
Atoms/Incident
Ion
€
εSB
Surface
Binding
Energy
keV
€
γSY = −0.002εSB
6
+0.0063εSB
5
− 0.0751εSB
4
+0.3994εSB
3
− 0.7492εSB
2
−1.0696εSB +6.1123
Figure
02
–
Zoomed
image
of
polymer
film
Polymer
bed
Figure
13
–
Custom
Designed
and
Built
Stage
Figure
14
–
First
Cuts
Performed
11. 10
Experimental Method (Standard Stage)
Stage Preparation
The first experiment conducted was with the
standard stage – no strain. The polymer was cut into a
small square, around the size of the stage. The size and
shape of the polymer is not crucial. The polymer was
then coated with gold in the vacuum sputter coating unit
and a small amount of silver paint was applied around
the perimeter to ensure the best possible conductivity
(shown in figure 15).
Initial Cuts
The FIB chamber was then vented for internal
access. Normal stage was inserted and tightened in
position before closing again for de-pressuring of the
chamber. The sample was located using the SEM and
altered to the optimal 10mm focal point. A smooth
surface of the polymer, close to the silver paint was
chosen for optimal conductivity. A tilt of 52° was then
applied to the stage for alignment with the FIB column.
Low current FIB imaging was used to locate the smooth
appropriate work area chosen to perform the cuts. The
machine settings for all cuts were as follows:
• Ion energy - 30keV
• Ion current – Set to 7nA (actual current from
meter – 5.5nA due to aperture damage)
• Ion source - Ga± (LMIS)
The cuts that were specified on the machines
interface were a simple rectangle and a simple line. The
rectangle was set to 5µm x 10µm with a 2µm depth.
Line cut was a 10µm line with the width purely
dependent on the beam and once again a 2µm depth.
Run time was predicted to be 1 (one) minute and thirty-
six (36) seconds for the rectangle and two (2) to three
(3) seconds for the line. These machining times are
relatively short, however they are very small and simple
designs so do not reflect any real applications out with
determining the machining rate (sputtering yield).
Once the cuts were completed, a thin deposit of
platinum was used for the line cut for better image
contrast when obtaining the results. This is due to the
image showing a contrast between platinum and the
polymer, rather than the polymer contrasted against free
space. A very fine needle is extruded from the outer
region of the chamber and addresses the sample. The
platinum is then deposited on the specified area. An
image of the two first cuts along with the platinum
deposit can be seen in figure 16.
It is clear to see from the bottom middle image
in figure 16 that even the low current FIB imaging scan
was capable of damaging the material. The dark square
region around the line cut is the result of this low
current scan. However it should not alter the results as
the damage should be consistent across the whole dark
section and therefore make no change to the volume
calculations as the cut and imaging are all performed
after the low current scan. The cuts themselves are of
relatively good resolution and don’t show too many
signs of unwanted behavior such as drift during the
milling process. This supports the assumption that the
drift in the first cuts was caused by the unwanted
presence of charge.
Cross Sectioning
Once the cuts were at the stage shown in figure
16 – including the line cut having the layer of platinum
deposited – sizing and cross sectioning of the structures
was the next step. For the rectangle cut, two cross
sections were performed. The reason for this was to
achieve a representation of the profile of the cut bed or
floor for a more accurate volume calculation. The cut
selected was a stepped cross section cut; this produced a
stepped cut that went below the deepest part of the
rectangle cut – revealing the form of the structure in a
way that better supported further analysis. A second cut
was also made in the same fashion, slightly farther
along the rectangle lengthways. Both cross sections are
shown in figure 17 with all relevant sizes included. The
cross section made for the line cut was a single cross
section. Again the stepped cross-section cut was used;
Figure
15
–
Silver
Paint
applied
to
polymer
cut
and
stage.
Figure
06
–
Initial
cuts.
Low
current
beam
rastering
damage
visible
12. 11
the platinum deposit helped greatly in producing an
imaging contrast to clearly detail the boundaries of the
structure.
Experimental Method (Custom Stage)
Stage Preparation
For the custom stage, the preparation and
method was slightly different due to the added
complexity of inducing a strain. First, the polymer was
cut into a thin, long, rectangular piece. It was wrapped
around the stage and fastened as described in the stage
design section. Prior to the gold coating being applied,
the stage was loosened in order to allow the gold coat to
cover both sides of the polymer. The stage was then
placed in the chamber as seen in left side image in
figure 18 for coating. Once the coat was applied, the
stage was tightened past the point of contact with the
polymer so that there was substantial tension in the
system and ultimately a strain present in the polymer.
Silver paint was then applied on the edges of the
polymer once again for optimal conduction as seen in
right side image in figure 18.
The stage was inserted into the FIB chamber
and tightened in position before closing and de-
pressurising the chamber once again (figure 19).
The SEM was activated and the stage was
positioned at the optimal focal point. The surface was
imaged using the SEM to scrutinise the presence of drift
in regards to the added possibility of unwanted motion
due to the stages capability of height adjustments.
Everything appeared completely stationary so once a
suitable location - near the silver paint - was located the
stage was tilted to 52° for use with the FIB column. All
machine parameters, cut sizes and cutting times were
consistent with the standard stage experiment. Upon
completing the cuts, the stage was removed from the
chamber to enable the strain to be removed. The wheel
was loosened until the polymer was sitting on the stage
with no tension. This was to ensure that all cut
dimensions were recorded for true size, not strained
size. Stage was re-inserted into the chamber for cross-
sectioning and final SEM imaging for dimensions. The
produced cuts – post strain removal – are shown in
figure 20. The cut quality is slightly less than that of the
standard stage cuts. This could be due to very slight
stage movement or vibration. The line cut was coated
with a thin deposition of platinum as before for imaging
purposes post cross sectioning.
Figure
17
–
Cross
section
of
cuts
Figure
18
–
Custom
stage
with
gold
coating
and
silver
paint
applied
Figure
19
–
Custom
stage
placed
inside
FIB
chamber
Figure
20–
Cut
dimensions
after
strain
removed
13. 12
Cross Sectioning
The exact same process was undertaken for
cross sectioning as with the standard stage. Line cut was
cross-sectioned once and rectangle cut was cross-
sectioned twice. The results of these sections are shown,
inclusive of dimension, in figure 21. The rectangle cut
yielded positive findings, with a highly calculable
volume. The line cut had very poor imaging potential,
possibly due to vibration. It was thought that the
vibration was occurring due to possible lack of contact
with the stage after slackening and removing the strain.
This presented issues when calculating the volume of
material removed.
Volume Calculations
Rectangular Cuts
In order to calculate the volume of material
removed in each cut, an approximation of the shape and
dimensions had to be made from the SEM images. For
calculating the volume of the rectangle cuts, the volume
was partitioned into three sections – one for each
segment post cross sectioning. Figure 22 displays the
shapes used for approximating the volume of material
removed from the strained and unstrained cuts
respectively. Time taken to perform cut and therefore
remove the material was one (1) minute and thirty-six
(36) seconds for both trials.
Linear Cuts
For the line cut volume approximations, a
different shape had to be chosen to represent the profile
of the cut. The bed depth was assumed to be constant as
only one cross section was taken for this reading. Figure
23 depicts the chosen shapes and hence the decided
cross section for the entirety of the cut. The area
calculated was then multiplied by the length of the line
cut. Length of cut was taken from the SEM images in
figures 16 and 20. Time taken to perform the line cut
was two (2) seconds for both trials.
Figure
21
–
Cross
section
of
strain
induced
cuts
Figure
22
–
Volume
approximations
Dimensions
in
mm
Figure
23
–
Line
cut
cross
section
approximation
14. 13
Results and Discussion
The result of the simulations performed, was a
graph depicting the numerical relationship between
surface binding energy and sputtering yield. It produced
the following numerical trend:
The experimental side of this journal produced
two sets of data for investigating and validating the
suggested relationship – one set from a non-strained
sample and one from a strained sample. The results are
given in table 3; this format provides a convenient
means of direct comparison. Material removal rate is
used instead of sputtering yield, as the number of ions
impinged on the sample stage and number of atoms
sputtered was unknown. For volume calculations, all
figures and area assumptions discussed in the
experimental method were used to yield the table 3’s
values.
Table
3
–
Material
removal
rate
of
strained
and
un-
strained
sample
cuts
Un-strained sample
Mill Type
Volume Removed
(
€
µm3)
Mill Time
(s)
Removal Rate
(
€
µm3/s)
Line Mill 2.28 2 1.14
Rectangle
Mill
156.9 96 1.63
Strained sample
Mill Type
Volume Removed
(
€
µm3)
Mill Time
(s)
Removal Rate
(
€
µm3/s)
Line Mill 2.41 2 1.21
Rectangle
Mill
177.6 96 1.85
The goal of this journal was to experimentally
test the theory that inducing a strain on a sample
polymer would result in an alteration of the materials
surface binding energy and hence increase the rate of
material removal – sputtering yield – using FIB milling.
The change in material removal rate when a strain was
induced on the sample polymer is clearly portrayed by
the above figures in table 3. It evidently has the
anticipated influence on the surface binding energy, thus
increasing the material removal rate. The limited
number of experiments and therefore data available for
analysis prevented the possibility of plotting the
findings to compare against the simulation results. This
was purely due to financial constraints. However, the
positive and undeniable increase in material removal
rate that the induced strain has, is a very promising
finding. The images used for approximating the volume
of material removed from the rectangle mills were of
very good resolution and greatly support the proposed
theory – compared to the line cut results. The line mill
images were not as clear but still provided an image
with enough clarity and quality to approximate the
volume of material removed and gave very similar ratio
in change compared to the better-imaged rectangular
mills – further enforcing the belief and trust in the
results found.
The strain induced was not measured due to it
being rather irrelevant when only comparing two results
– one with no strain and one with strain. The goal was
purely to see if inducing a strain altered the surface
binding energy, and in turn increased the material
removal rate or sputtering yield.
The ratio in which the induced strain changed
the material removal rate by combining the results for
both the rectangle and linear cuts is:
Strained removal rate : Un-strained removal rate
⇓
1.105 : 1.000
Conclusions
Increasing the sputtering yield of samples
during FIB milling has been a topic of much discussion
and research since the development of the FIB
technology. Many existing techniques are used to
improve both the speed and diversity of the milling
process. Altering the surface binding energy, which is
the most influential property of the sample material in
terms of changing the sputtering yield, is one that has
lacked significant research and development. With the
positive findings and clear change in material removal
rate inducing a strain has, it is a refreshing and possibly
soon-to-be applied method for further controlling the
rate in which atoms are released from sample materials.
The recorded change in material removal rate was
around 10.5%, which is quite a substantial
improvement. This could potentially decrease operating
times and reduce operating costs if implemented
correctly. The experiments lacked a broad range of
results due to financial limitations. This restricted the
quantity of experimental results that could be produced
and compared with simulation trends. However, as the
predicted outcome was met, it can only be taken as a
positive. Hopefully it is one further step forward in
harnessing the full potential of the very adaptable and
promising capabilities of the FIB process.
€
γSY = −0.002εSB
6
+ 0.0063εSB
5
− 0.0751εSB
4
+ 0.3994εSB
3
− 0.7492εSB
2
−1.0696εSB + 6.1123
15. 14
Future Work
Having only retrieved four experimental results
from just two samples, the natural progression from
here would be to do a far greater range of experimental
testing. The custom stage would be adjusted in even
increments and the strains would be measured. At each
increment the same cuts would be performed for a more
comprehensive and meaningful batch of results. Each
cut would be done multiple times, allowing the results
to be averaged in the hope of cancelling some
uncertainties. The next step would then be to analyse
and summarise the results of strain against material
removal rate in the hope that the trend matches that of
the simulation trend predicted using the SRIM/TRIM
monte carlo software package.
Acknowledgments
Dr. Jining Sun - I would like to send my
deepest appreciation to my supervisor Dr.Sun, not only
for his availability, guidance and invaluable knowledge
on the subject, but for his patience and understanding
through the whole life span of the project.
Mark Leonard – I would like to extend my
thanks to Mr. Leonard for his assistance in the
experimental part of my project, for his knowledge and
for his patience during this time.
References
[1] Reyntjens, S., & Puers, R. (2001). A review of
focused ion beam applications in microsystem
technology. Katholieke Universiteit Leuven. Heverlee:
Institute of Physics.
[2] Ziegler, J. F. (2013 йил N/A-N/A). Tutorial #2
– Target Mixing and Sputtering. Retrieved 2015 йил
6/3-March from PARTICLE INTERACTIONS WITH
MATTER:
http://www.srim.org/SRIM/Tutorials/SRIM%20Tutorial
%202%20-%20Mixing%20and%20Sputtering.pdf
[3] Tawara, H., & Yamamura, Y. (1996).
ENERGY DEPENDENCE OF ION-INDUCED
SPUTTERING YIELDS FROM MONATOMIC SOLIDS
AT NORMAL INCIDENCE. Okayama: Academic Press,
Inc.
[4] Farlex. (2015 йил 2-4). Dictionary/Thesaurus.
Retrieved 2015 йил 2-4 from The Free Dictionary:
http://www.thefreedictionary.com/ion
[5] Collins. (2015 йил 2-4). English Dictionary.
Retrieved 2015 йил 2-4 from Collins Dictionary:
http://www.collinsdictionary.com/dictionary/english/be
am
[6] Tseng, A. A. (2004). Recent developments in
micromilling using focused ion beam technology.
Arizona: INSTITUTE OF PHYSICS PUBLISHING.
[7] C.A.Volkert, & A.M.Minor. (2007). Focused
Ion Beam Microscopy and Micromachining. Warandale:
www/mrs.org/bulletin.
[8] Pyka, W. (2000). Feature scale modeling for
etching and deposition processes in semiconductor
manufacturing.
[9] SemiPark. (2008 йил 1-5). Ion Implantation.
Retrieved 2015 йил 10-4 from Semi Park:
http://www.semipark.co.kr/semidoc/waferfab/ion_impt2
.asp?tm=8&tms=4
[10] F.Schiappelli, Kumar, R., Prasciolu, M., Cojac,
D., & Cabrini, S. (2004). Efficient fiber-to-waveguide
coupling by a lens on the end of the optical fiber
fabricated by focused ion beam milling. Trieste:
Elsevier.
[11] C.W.Leung, Bell, C., Burnell, G., & Blamire,
M. G. (2005). Current-perpindicular-to-plane giant
magnetoresistance in submicron pseudo-spin-valve
devices. Cambridge: Cambridge.
[12] H.Lohmeyer, Sebald, K., Gutowski, J., Kroger,
R., Kruse, C., Hommel, D., et al. (2005). Reasonant
modes in monolithic nitride pillar microcavities.
Bremen: The European Physical Journal B.
[13] S.J.Kim, Hatano, T., Kim, G. S., Kim, H. Y.,
Nagado, M., & Inomata, K. (2004). Charecteristics of
two-stacked intrinsic Josephson junctions with a
submicron loop on a Bi2Sr2CaCu2O8+d(Bi-2212)
single crystal whisker. Cheju: Elvevier.
[14] J.H.Daniel, & Moore, D. F. (1999). A
microaccelerometer structure fabricated in silicon-on-
insulator using a focused ion beam process. Cambridge:
Elsevier.
[15] Möller, W. (2010 йил 1-1). Ion Implantation
and Irradiation. Retrieved 2015 йил 2-4 from Spirit:
http://www.spirit-
ion.eu/tl_files/spirit_ion/files/FZD_tutorial/SPIRIT%20
Tutorial%20Fundamentals%20II.pdf
[16] Mitchel, A. Melting, casting and forging
problems in titanium alloys. Vancouver: ELSEVIER.
16. 15
[17] Nellen, P. M., Langford, R. M., Gierak, J., &
Fu, Y. (2007). Focused Ion Beam Micro- and
Nanoengineering. Pennsylvania: MRS Bulletin.
[18] ROTOMETALS, I. (2010 йил January).
Material Safety Data Sheet. Material Safety Data Sheet
Gallium MSDS . San Laendro, California.
