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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  

 

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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

Table of Figures

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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

       

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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.

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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

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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

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−11s. 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

 

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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  

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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.  

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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  

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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  

€

γ SY   Atomic  Sputtering  Yield  

Atoms/Incident  Ion  

€

εSB   Surface  Binding  Energy  

keV  

€

γ SY = −0.002εSB6 + 0.0063εSB

5 − 0.0751εSB4 + 0.3994εSB

3 − 0.7492εSB2 −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  

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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  

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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  

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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  

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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.      

€

γ SY = −0.002εSB6 + 0.0063εSB

5 − 0.0751εSB4 + 0.3994εSB

3 − 0.7492εSB2 −1.0696εSB + 6.1123

  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/beam [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%20Tutorial%20Fundamentals%20II.pdf [16] Mitchel, A. Melting, casting and forging problems in titanium alloys. Vancouver: ELSEVIER.

  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.

      4    

Appendix Appendix  1  -­‐  Sputtering  Yield  Fluctuation  Data  

 

No of ions

Sputtering Yield

H C O Tot 10 2.4 0.6 0.4 3.4 20 2.05 0.35 0.3 2.7 30 2.47 0.4 0.33 3.2 40 2.13 0.375 0.275 2.8 60 2.15 0.35 0.3667 2.9 80 2.14 0.3125 0.4 2.9

100 2.08 0.35 0.44 2.9 120 1.93 0.33 0.4167 2.7 140 1.79 0.3 0.3857 2.5 160 1.81 0.3188 0.4 2.5 180 1.8 0.3278 0.4333 2.6 200 1.75 0.32 0.425 2.5 240 1.73 0.325 0.4458 2.5 280 1.69 0.3321 0.4286 2.5 320 1.73 0.3344 0.4438 2.5 360 1.68 0.3278 0.4556 2.5 400 1.62 0.3125 0.445 2.4 450 1.61 0.3022 0.4333 2.3 500 1.61 0.306 0.424 2.3 550 1.63 0.3127 0.4273 2.4 600 1.58 0.3167 0.42 2.3 650 1.58 0.32 0.4369 2.3 700 1.59 0.3271 0.4386 2.4 750 1.57 0.3227 0.4413 2.3 800 1.58 0.3325 0.4425 2.4 850 1.59 0.3412 0.4388 2.4 900 1.58 0.3389 0.4367 2.4 950 1.56 0.3474 0.4453 2.4

1000 1.58 0.347 0.439 2.4 1100 1.59 0.3364 0.4309 2.4 1250 1.6 0.3368 0.4304 2.4 1300 1400 1500 1.62 0.342 0.4367 2.4 1600 1700 1.64 0.3476 0.4365 2.4 1800 1900 2000 1.64 0.344 0.4295 2.4 2200 2400 1.68 0.3517 0.44 2.5 2600 2800 3000 1.64 0.3497 0.43 2.4 3250 3500 3750 4000 1.62 0.3465 0.4208 2.4 4500 5000 1.6 0.347 0.4156 2.4

 

  17  

 Appendix 2 - Sputtering yield vs Surface Binding Energy Graph (Raw Data)

  SBE 0 degree incidence angle SY

Hydrogen Carbon Oxygen Avg Hydrogen Carbon Oxygen Avg (total)

0.01 0.01 0.01 0.01 3.110 2.080 0.827 6.017

0.25 0.25 0.25 0.25 2.990 2.000 0.797 5.787

0.5 0.5 0.5 0.5 2.840 1.900 0.760 5.500

0.75 0.75 0.75 0.75 2.660 1.810 0.711 5.181

1 1 1 1 2.450 1.670 0.659 4.779

1.25 1.25 1.25 1.25 2.150 1.430 0.583 4.163

1.5 1.5 1.5 1.5 1.940 1.260 0.522 3.722

1.75 1.75 1.75 1.75 1.770 1.130 0.464 3.364

2 2 2 2 1.610 1.030 0.420 3.060

2.25 2.25 2.25 2.25 1.500 0.932 0.384 2.816

2.5 2.5 2.5 2.5 1.400 0.861 0.358 2.619

2.75 2.75 2.75 2.75 1.310 0.796 0.330 2.436

3 3 3 3 1.230 0.753 0.308 2.291

3.25 3.25 3.25 3.25 1.180 0.706 0.290 2.176

3.5 3.5 3.5 3.5 1.120 0.669 0.271 2.061

3.75 3.75 3.75 3.75 1.080 0.634 0.259 1.973

4 4 4 4 1.030 0.601 0.250 1.881

4.25 4.25 4.25 4.25 0.995 0.568 0.234 1.797

4.5 4.5 4.5 4.5 0.960 0.544 0.225 1.729

4.75 4.75 4.75 4.75 0.928 0.518 0.218 1.664

5 5 5 5 0.896 0.497 0.211 1.604

5.25 5.25 5.25 5.25 0.870 0.471 0.202 1.542

5.5 5.5 5.5 5.5 0.850 0.458 0.195 1.503

5.75 5.75 5.75 5.75 0.820 0.440 0.188 1.448

6 6 6 6 0.796 0.418 0.183 1.396

6.25 6.25 6.25 6.25 0.775 0.405 0.177 1.356

6.5 6.5 6.5 6.5 0.757 0.390 0.168 1.315

6.75 6.75 6.75 6.75 0.733 0.376 0.163 1.272

7 7 7 7 0.715 0.366 0.158 1.239

7.25 7.25 7.25 7.25 0.698 0.355 0.152 1.205

7.5 7.5 7.5 7.5 0.686 0.345 0.148 1.179

7.75 7.75 7.75 7.75 0.672 0.338 0.144 1.153

8 8 8 8 0.657 0.328 0.141 1.125

8.25 8.25 8.25 8.25 0.642 0.319 0.138 1.099

8.5 8.5 8.5 8.5 0.629 0.313 0.133 1.075

8.75 8.75 8.75 8.75 0.614 0.303 0.129 1.046

9 9 9 9 0.600 0.297 0.126 1.022

9.25 9.25 9.25 9.25 0.584 0.290 0.121 0.995

9.5 9.5 9.5 9.5 0.575 0.281 0.117 0.974

9.75 9.75 9.75 9.75 0.565 0.275 0.115 0.954

10 10 10 10 0.555 0.266 0.111 0.932