Surface Energy Influence On Ion Sputtering Process

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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 its development in the late 1970s and early 1980s1. 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 30C, 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

    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 -