Stress-Free Titanium-based Thin Films for Inner Ear ...

Linköpings universitetSE–581 83 Linköping+46 13 28 10 00 , www.liu.se

Linköping University | The Department of Physics, Chemistry and BiologyMaster’s thesis, 30 ECTS | Applied Physics

2020 | LIU-IFM/LITH-EX-A--20/3821--SE

Stress-Free Titanium-based ThinFilms for Inner Ear Microphones– The last missing part of a technology for totally implantablehearing aid implantsSpänningsfri och tunn titanfilm till en hörapparat för innerörat

Dina Ehsan

Supervisor : Naureen GhafoorExaminer : Jens Birch

http://www.liu.se

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Abstract

An implantable hearing aid device is being developed by a project group which is part of anEU initiative. This device contains a diaphragm consisting of a submicron thick freestandingtitanium film, which should be free of internal stresses. Stress is the force exerted per unitcross-sectional area of the film and it can impair the functionality and performance of thedevice. The stress that evolves in a thin film during deposition at a substrate is compressiveor/and tensile and affects the bending that occurs of the substrate due to the lateral forceapplied to the substrate by the stressed film.

The goal of this diploma work was to contribute to the understanding of in situ stress evolu-tion in a micron thick titanium film and thereby by tuning different physical parameters toobtain minimal residual stress in the films after growth. Titanium films were deposited onsilicon sistrates using DC magnetron sputtering. The stress in the material varied, by tuningdifferent physical parameters such as working pressure, power, distance between magnetronand the sample and substrate bias. For this thesis, firstly two different series were done;one where the changing parameter was the distance between the sample and the magnetronand one where it was the working pressure. Later a last series were done to see what effectthe bias has on the stress. A multi-beam optical sensor system (MOS) was used to measurethe stress in real-time during deposition. X-ray diffraction (XRD) was later used to makepost-deposition stress measurements to verify the stress obtained from the MOS. However,the MOS shows the stress evolution in real time and XRD shows a ’final’ average value thatcan be compared with the stress obtained from MOS-data when the deposition is finished.

The results showed that the stress goes from compressive to tensile as the working pressureand the distance between the magnetron and the sample increases. There are other factors,such as the temperature/heating in the main chamber, base pressure of the main chamber,cleaning of the sample and also where the argon gas is let in to the process chamber (in thisproject called the main chamber (MC)), that influences the results. This will in turn influencethe repeatability of the data/measurements, since these effects can affect the process of nucle-ation and coalescence. The stress evolution can change if a bias is applied during the initialstage of the deposition process when the film has still not grown thick. This is could be dueto the bias not having much of an impact on the stress evolution when the film is thicker andthereby more porous.

Populärvetenskaplig sammanfattning

En implanterbar hörapparat skall utvecklas av en projektgrupp, som är en del av ett EU-projekt. Apparaten innehåller ett membran som består av en fristående titanfilm som är cirkaen mikrometer tunn och denna film ska vara fri från inre spänningar. Spänning är kraften perenhet tvärsnittsarea av den växande filmen och det kan påverka materialets funktionalitetoch prestanda. Spänningen som bildas i titanfilmen kan antingen vara kompressiv spänningeller dragspänning. Detta kan leda till en böjning av substratet som filmen växer på somföljd av de laterala krafter som appliceras av den växande filmen.

Målet med detta examensarbete var att bidra med förståelse av hur spänning kan bildas itunna filmer och sedan genom att ändra på fysikaliska parametrar skapa en spänningsfrititanfilm på ett kiselsubstrat genom att använda DC magnetron sputtring. De fysikaliskaparametrarna som kan påverka spänning som då bildas i ett material är argontryck i kam-maren, magnetronens effekt, avstånd mellan prov och magnetron samt elektrisk spänningsom appliceras på substratet. Det var huvudsakligen två varierande fysikaliska parametrarsom undersöktes i detta arbete; argontrycket och avståndet mellan magnetron och provet.Det gjordes två olika serier där påverkan av dessa parametrar utreddes och sedan gjordesytterligare en serie för att undersöka hur elektrisk spänning påverkar spänningen. En ”multi-beam optical sensor” (MOS) användes för att mäta hur filmens spänning utvecklas underfilmtillväxt. Sedan användes röntgendiffraktion (X-ray diffraction (XRD)) för att verifiera deslutgiltiga spänningsvärdena vid filmtillväxtens slut från MOS-data.

Resultatet visar att spänningen går från kompressiv till dragspänning när både argontrycketoch avståndet mellan magnetronen och provet ökar. Det finns även andra faktorer som tem-peraturändringar och bastrycket i kammaren, rengöringsprocessen av substraten samt vartargongasen släpps in i sputtringskammaren, som kan ha en påverkan på resultatet. Dettagör det väldigt svårt att få repeterbarhet i data och mätningar, eftersom dessa faktorer kanha olika påverkan på nukleations- och sammanväxningsprocessen av atomerna när spänninguppstår. Det är dock möjligt att minimera spänning i filmtillväxten genom att applicera enelektrisk spänning på substratet, dock skall detta göras i början av filmtillväxtsprocessen.Detta är på grund av att tjockleken på titanfilmen kan påverka den elektriska spänningensinverkan.

Acknowledgments

Foremost, I would like to express my sincere gratitude to my supervisor Naureen Ghafoorfor giving me the opportunity to be part of this project, helping me throughout this work andgiving valuable guidance. Her motivation has inspired me and she has taught me how to doresearch in a field that before this diploma work seemed too complicated, but now has mademe more interested in research. Thank you for answering my never ending questions.

I would also like to thank Per Sandström and Samiran Bairagi for their endless help andsupport every time I encountered problems with the systems I used. I will be forever gratefulthat, even though you were not part of this project and had hundred of other things to do,you were always around to help. This diploma work would nearly not have been possible todo without your help that encouraged me at times where things seemed impossible.

I would like to thank my examiner Jens Birch for his insightful comments and positive energy,Sjoerd Broekhuisen for his help regarding XRR-measurements and Babak Bakhit for teachingme how to carry out stress measurements with XRD. I also thank the EUREKA project group;SwissNeutronics, the University of Zurich and Cochlear Technology Centre in Belgium fora great collaboration. Your contribution has been valuable for me to carry out my diplomawork.

Last but absolutely not the least, my forever and never ending gratitude to my parents andbrother who have been my pillar of strength and source of energy. Thank you for your love,prayers, caring and sacrifices. Thank you for preparing me for my future and supporting mewhile I chase my dreams. Thank you for being my hope.

I would also like to dedicate a special thank you to my friends for being my cheerleadersand giving me mental support throughout the years. Your encouragement has helped mecountless of times.

Contents

1 Introduction 11.1 Aim of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Delimitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 The Use of Related Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Theory 32.1 Project Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 DC Magnetron Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Stress Measurements with Multi-Beam Optical Sensor (MOS) . . . . . . . . . . 52.4 Stress - Analysis and Mathematical Background . . . . . . . . . . . . . . . . . . 6

2.4.1 Tensile and Compressive Stress . . . . . . . . . . . . . . . . . . . . . . . . 62.4.1.1 Origin of Tensile and Compressive Stress . . . . . . . . . . . . . 72.4.1.2 Parameters Affecting the Stress . . . . . . . . . . . . . . . . . . 7

2.4.2 Stress Measurements with MOS . . . . . . . . . . . . . . . . . . . . . . . 82.4.3 Important Aspects of the Stoney Equation . . . . . . . . . . . . . . . . . 9

2.5 X-Ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.5.1 X-ray Reflectivity (XRR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.5.2 Wafer Curvature Measurements for Film Stress . . . . . . . . . . . . . . 122.5.3 Transmission Electron Microscopy (TEM) . . . . . . . . . . . . . . . . . . 13

2.5.3.1 Bright respectively Dark Field Mode . . . . . . . . . . . . . . . 142.5.3.2 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Method 153.1 The Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2 Sample Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.3 Deposition and Stress Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 153.4 TEM - Preparation and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.5 XRD - Preparation and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.5.1 Reflectivity Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 183.5.2 Wafer Curvature Measurements for Film Stress . . . . . . . . . . . . . . 19

4 Related Work 204.1 Varying Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.2 Analysis of the Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.3 Conclusions from the Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 23

5 Result 245.1 The Stress Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.1.1 Pressure Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.1.2 Distance Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

5.2 Other Relevant Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.2.1 Effect of Base Pressure in MC . . . . . . . . . . . . . . . . . . . . . . . . . 285.2.2 Effect of Heater Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 295.2.3 Effect of Argon Gas Location . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.3 XRD - Wafer Curvature Measurements . . . . . . . . . . . . . . . . . . . . . . . . 305.4 The Effect of Bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6 Discussion 346.1 The Pressure and the Distance Series . . . . . . . . . . . . . . . . . . . . . . . . . 346.2 Other Relevant Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356.3 XRD - Curvature Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 366.4 The Bias Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366.5 Instrumental/Equipment Trouble . . . . . . . . . . . . . . . . . . . . . . . . . . . 376.6 Improvements for Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

7 Conclusion 39

Bibliography 40

A Deposition Rates and XRR Data 42

B Additional Results 43B.1 Stress*thickness Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43B.2 Data for XRD Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Chapter 1

Introduction

There is a large application range for the use of thin films and it is mostly used to modify orenhance the properties of a surface. The structure of thin films; morphology, grain size andboundaries, has an effect on the mechanical, optical and electrical properties of the material.This, in turn, will affect the way the material performs and for which applications it can beused for. Therefore, it is crucial to understand which physical parameters and processes canhave an impact on the function and synthesis processes of the thin films [1].

By changing the physical parameters for the deposition process, it is possible to controlproperties of the thin film [2]. There are different types of deposition techniques that couldbe used to create thin films, but the outcome can be very much alike. Working pressure, sub-strate bias voltage, source power or temperature are a few physical parameters controllingthe evolution of microstructure in thin films [3, 4] and it has mostly to do with the energy ofthe deposited atoms [5]. Deposited atoms will undergo certain stages when they meet eachother, including nucleation, growth, and coalescence, where grain boundaries are formed[6, 2].

During and post-deposition, an internal stress can exist in the thin films. The origin of stressin thin films is associated with the evolution of structure and how energetic the depositedatoms are [5] when they arrive to the surface. Stress can cause the material to fail by eithercracking or decohesion [7] which affects its functionality and performance. The stress can beeither tensile or compressive [8].

Titanium is a material that is widely used in microelectromechanical systems (MEMS) be-cause of its physical properties. It has good conductivity, is thermally stable, has highhardness and do not have many crystallographic imperfections [4]. In MEMS applicationsTi can be used for cochlear implants [9]. Thin films that are used for MEMS must be stressfree or have minimal stress since stress can lead to the device to malfunction [4]. By doingin situ stress measurements during deposition and varying physical parameters affecting thedeposition process, it is possible to control and minimise residual stress in thin films. Multibeam optical sensor (MOS) systems can be used to measure the stress during deposition andthe plotted data will give information about the stress evolution in the material.

In this thesis thin films of titanium will be deposited onto a thin silicone substrate using DCmagnetron sputtering. By varying different physical parameters, it is possible to see how thestress in the material will evolve. The goal is to create a stress-free titanium based thin filmfor an implantable cochlear implant. This thesis contributes to research for a product that canlead to good health and well being of people and thereby also contributes to sustainability.

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1.1 Aim of the Thesis

The aim of this diploma work is to

• Contribute to the understanding of how in situ stress is evolved in a micron thickfilm and how different parameters controlling the deposition can affect the evolutionof stress.

• Obtain minimal residual stress in a titanium based thin film by varying the physical pa-rameters working pressure and the position of the magnetron when doing deposition.

• Combine different physical parameters and relate it to the theory in order to do themeasurements and learn the process of doing research.

This will be done by measuring stress in the films during growth and varying the physicalparameters to receive a stress free titanium film that is 1 µm thick. In situ real-time stressmeasurements will be done followed by post-deposition stress measurements and the datawill be analysed to contribute to the understanding of stress development

1.2 Delimitations

In order to simplify the project, the changing parameters will be working pressure and theposition of the magnetron from the sample. If there will be time left until the diploma workcomes to an end, also the effect of substrate bias will be investigated. For this thesis, the focusis only on the deposition of titanium and not other materials. The post-deposition stressmeasurements will be done on some of the samples and this is only to verify the values ofstress obtained from the real-time measurements.

1.2.1 The Use of Related Work

Chapter 4 will contain related work done for this project before this diploma work wasstarted. Images and data will be provided in order to obtain a better understanding of thisproject, but also why those changing parameters were chosen. However, in that chapter onlyparts of that related work highly connected to the measurements and results in this diplomawork will be presented.

1.3 Outline

The thesis will as an introduction to the work contain the theory needed to understand theresults and the related work, which is Chapter 2. The theory will mainly focus on the equip-ment used to conduct the measurements, the mathematical background of stress and howthat is related to the equipment used and theoretical background of stress which will focuson what types of stress there are and how they evolve. Chapter 3 describes the method usedto conduct the experiments/measurements, but also the physical parameters varied to seethe effect on the stress. In Chapter 4, the related work will be presented. Chapter 5 is wherethe result from the experiments are shown and described. In the following two chapters thediscussion and the conclusion made from this project will be presented. The first appendixgives data from the XRR-measurements and the second appendix includes some additionalresults to those who are presented in chapter 5.

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Chapter 2

Theory

In the sections below, the theory necessary for the understanding of this project is described.Some project information will be presented at first, followed by the analysis and the math-ematical background of stress. Lastly some information regarding the techniques used toconduct this project is described briefly in order to understand the methods described in thenext chapter.

2.1 Project Background

The University of Zurich and Cochlear Technology Centre in Belgium is developing an im-plantable hearing aid implant. One part of this device consists of a submicron thick titaniumdiaphragms which has a 0.5 mm diameter. The purpose of the diaphragms is to act as asound receptor and pick up sound energy from fluid of the inner ear and transfer it to gas,from which a microphone can transform the energy to an electrical signal. Hence, the di-aphragms works as a sensor which has an impact on how well the microphone is performing.

In order to integrate the diaphragms in the hearing aid implant, titanium is sputtered onto apolymer that is decomposable. The reason why titanium is used is because it is a biocompat-ible material. The diaphragms should be connected to the structure in a way that minimalresidual stress, hermeticity and robustness is obtained.

The main focus of this project is to grow thin films of titanium onto silicon, which is thesubstrate, and during growth measure the stress. Later, parameters controlling the stressshould be tuned to obtain a stress-free titanium film and a membrane that is a micron thick.

2.2 DC Magnetron Sputtering

DC magnetron sputtering is a PVD technique that is used for the deposition of thin films ontoa substrate. This deposition method provide properties such as high hardness, resistance tocorrosion, lower friction or other desired electrical/optical properties, which makes it often agood option for deposition of thin films. It can also provide deposition of metastable phasessince materials can be created outside the thermodynamic equilibrium range [10].

The sputtering process starts by inserting the substrate into the main chamber and then evac-uating the air with a vacuum pump in order to decrease the pressure. The chamber will thenbe filled with a desired gas which will be ionized and create a plasma and later the ions willbe accelerated towards the target. The target consists of the material that is going to be de-posited onto the substrate. When the gas-ions hits the target through collision, atoms willeject and deposit onto the substrate. The working gas used for this project is argon whichproduces Ar+, which is a positively charged ion. The reason why Ar+ ions are accelerated

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towards the target is because a negative voltage is applied on the target. This principle isdescribed by figure 2.1. Note that Ar+ is an inert gas, hence it will not interfere with thedeposited material

Figure 2.1: This schematic figure shows the principle of DC magnetron sputtering. Sputtering oper-ates under vacuum conditions where target particles is sputtered onto the substrate when hit by iongas.

The ultra high vacuum (UHV) magnetron sputtering machine used in this lab is from Mantis.Figure 2.2 shows a schematic of the system.

Figure 2.2: A schematic of the DC magnetron sputtering machine used for this project. The mainparts of the system are named in the figure.

It was possible to change the position of the substrate holder and the magnetron (Z-shift),the tilt of the magnetron and the substrate holder and the rotation of the the substrate. Thesettings used for the depositions are explained in chapter 3.

The procedure of sample loading and unloading will be described in the following chapter.However, it is important to know some theory behind some of the steps. When unloading a

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sample, the load-lock (LL) chamber needs to be vented, which is to relieve the low pressurein order to open the LL-door. After a sample is loaded in the LL, the LL needs to be pumped,which is to decrease the pressure to obtain vacuum. This is important since there is a valvebetween the main chamber (MC) and the LL. This valve needs to be opened when the sampleis being transferred into the MC with the transfer-arm (which holds the sample holder witha clamp). In this process, the pressure in the MC will maintain constant since the vacuuminside the MC is high (low pressure). The optimal condition for when to open the valveis when the pressure in the LL is maximum one order of magnitude larger than than thepressure in the MC. The sample needs to be cleaned before being put into the system and theprocedure for this is also explained in the next chapter.

As mentioned earlier, the target used for this project is titanium (75 mm in diameter and3 mm thick), which will be deposited onto a thin silicon substrate. Titanium has a goodcorrosion resistance, which makes it an excellent metallic biomaterial to use for implants.How corrosive an implant is has an impact on how it performs and its durability, which inturn affects the biocompatibility of the medical device [11].

2.3 Stress Measurements with Multi-Beam Optical Sensor (MOS)

In order to obtain desired properties such as optical, electronic or mechanical, it is crucial tounderstand and control the stress in thin films during deposition. By controlling the stressduring the deposition process, one is able to avoid the unwanted properties that could leadto the device performing poorly [7].

Stress in thin films can be measured by observing the change in the substrate curvature byusing a laser system during deposition. The stress in the film induces a curvature in thesubstrate. In this project, a multi-beam optical sensor (MOS) from the brand k-Space Asso-ciates Inc. was used. With this device it was possible to real-time monitor the thin film stressby sending in an array of parallel laser beams through a collimator and monitor how thesubstrate curvature changes with a CCD camera [7]. This method of measuring stress is non-destructive [5]. Figure 2.3 shows how the laser system works and how 2 dimensional spotsare created, which will help in the curvature measurements.

Figure 2.3: This figure shows how the detection of the laser works and how the 2D array spots aregenerated from the laser [12].

The optics that are used to control the laser are in a laser optics housing, with one lens andone mirror. As can be seen from the figure above, the lens is used to generate multiple paral-lel laser beams from one single laser. These parallel beams shines onto the substrate during

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deposition as an array of spots which in turn is reflected onto the mirror that is positionedin such way that the reflected laser beams from the substrate is caught by the CCD camera.This creates a 2D array of spots (see figure 2.3) with a spacing ∆d between the spots. Duringdeposition, ∆d changes in two orthogonal directions which generates 2D curvature data.Hence this is used to determine curvature changes [12]. In the case of growing a film onthe substrate, the curvature change is caused by internal stress in the film attached to thesubstrate. How the data is plotted and the mathematics behind this principle can be read insubsection 2.4.2.

The MOS-system is supplied with a laser diode and a temperature controller. The laser in-tensity can be controlled via the electronics connected to the laser optics housing box and viathe software. The temperature controller controls the temperature of the laser and operatesautomatically whenever the laser power is on.

2.4 Stress - Analysis and Mathematical Background

Stress (σ) is the force per unit cross-sectional area ([N/m2]) of the film [8]. Stress in thinfilms can lead to the film not functioning as it should. It has an effect on the performance,reliability, durability [5] and the physical properties, such as sound velocity, band structureor the thermal properties, [1] of the device itself.

How the stress evolves in a film depends on the microstructural and the surface conditions ofthe film. This is strongly tied to the deposition process when the film grows. Stress could beextrinsic, which means that the stress appears when an external force is applied during filmgrowth or if there is a thermal mismatch between the film and the substrate. However, in thisthesis the main focus is the intrinsic stress which happens when the thin film is growing but isconstrained compared to its in-plane dimension since it is bonded to a substrate. Structureswill evolve in this process which could be grain growth, due to impurity, morphologicalevolution, formation of point defects etc [13].

2.4.1 Tensile and Compressive Stress

As mentioned earlier, the stress in a thin film can be determined by measuring the forceapplied on a certain area. A film is said to be undergoing a tensile stress when the surfaceof the substrate is contracting in a way that the radius of the curvature R is positive, R>0.However, when the stress induces a contraction of the film in such way that the R is negative,R<0, the film is under a compressive stress. This means that the curvature of the substratecan determine what type of stress is induced [8]. A schematic figure of this phenomena isshown below.

Figure 2.4: The difference between compressive and tensile stress, in both bending and how the radiusof curvature differs.

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When a material undergoes compressive stress, the size of it will reduce (compression). How-ever for tensile stress, the material will elongate. This can be seen in figure 2.4, where the ar-rows point to the film for the compressive stress and the arrows pointing away for the tensilestress. Hence, compressive stress can make a material peel off, buckle or blister and tensilestress can lead to the material to crack [5].

2.4.1.1 Origin of Tensile and Compressive Stress

When a film is growing on a substrate, it undergoes different stages in internal stress. Itmeans that the stress evolution in thin films are not only compressive or tensile, it is a mixbetween both.

During the first step of film growth, target atoms will arrive to what is then a flat substratewith no stress. These atoms will diffuse on the surface of the substrate and encounter withother atoms [2] and start to form clusters. These clusters are called nucleation islands and arethe origin of compressive stress due to a surface tension being created [6].

When these islands grow, coalescence will occur. Islands large enough will interact andgrow together, which means that a grain boundary is created [6]. When the boundaries growcloser, the free energy of the thin film will decrease and this will yield a tensile stress [2]. Thisprocess is explained in figure 2.5 below.

Figure 2.5: In step 1, the nucleation islands are created by atoms encountering with each other whichcreates a compressive stress. The islands will grow large and closer to each other and form grainboundaries, which is seen in step 2. This creates a tensile stress. Compare to figure 2.4.

2.4.1.2 Parameters Affecting the Stress

There are some kinetic processes that happen during deposition that can have an impacton the stress. Deposition methods such as evaporation or electrodeposition are called non-energetic processes where the atoms being deposited have a lower kinetic energy. In thisproject, DC magnetron sputtering is used as a deposition method and that is an energeticdeposition process. The atoms being deposited onto the substrate have high thermal energycompared to non-energetic processes [5].

The working pressure has an impact on the stress. Thin films tend to go from tensile tocompressive stress when the argon pressure decreases. At low pressures of argon (typicallyaround 1 mTorr), compressive stress is shown in the thin film. Tensile stress occurs when thepressure is increased. When the pressure is increased, there will be more collision betweenenergetic sputtered atoms and the gas ions, hence they will lose more of their kinetic energy.For lower pressures, deposited atoms will not collide with as many gas ions, which meansminimal kinetic energy will be lost during the path to the substrate surface. When arriv-ing to the surface, with more energy (than the case for higher pressures), they can inducedisplacement of atoms on the substrate surface. The displaced atom can relocate to a more

7

favourable site, but it can also create point defects. When point defects occur due to theinvolvement of energetic particles, it will lead to compressive stress developing which is aresult of atomic peening [5]. In deposition processes, the atomic peening effect describes howenergetic particles bombards the growing film and the effects it might have. This is related tothe working pressure in the chamber as indicated [14].

By applying a bias voltage to the substrate during deposition will increase compressive stressor decrease tensile stress. By applying a voltage, there is an electric field created betweenthe substrate and the anode, which makes the argon ions to accelerate towards the substrate.The bombardment of the gas ions will give higher energy to the target ions when a voltageis applied and as for the case for the pressure, target ions with high energy deposited on thesurface can cause displacement of the other atoms and make the surface ’move in’. This willcreate a higher compressive stress in the thin film than it would for a film deposited onto asubstrate without a bias voltage [3].

Stress is also dependent on the magnetron power and substrate temperature [4].

2.4.2 Stress Measurements with MOS

When the film grows on the substrate during deposition, a force is applied on the substratecausing it to bend, as can be seen in figure 2.4. Stoney derived an equation that determinedthe relationship between the curvature of the substrate [5] and the stress. The equation iscalled ”The Stoney formula” and is presented below:

1R

=6

Yh2s

h f σ (2.1)

where h f is the thickness of the film, hs is the thickness of the substrate and Y is the biaxialmodulus of the substrate [5], which in this case is silicon and has a value of 180.6 GPa. Theproduct σh f is called the stress-thickness product, where the unit is force/length [N/m]. Afilm that is stressed applies a force on the substrate it is growing on as well as creating abending. When a film is much thinner than the substrate, the bending of the substrate doesnot affect the film [13].

As mentioned in section 2.3, the stress evolution can be measured in real-time during filmgrowth. The parallel laser beams generated from the MOS creates a pattern of laser spots onthe film (see figure 2.3) and detects the changes of surface curvature. When the substrate isbending the spacing ∆d between the spots will change. The mean differential spacing ∆d/dis the mean value of the difference between the original spots and the change of their distance.The relationship between the curvature 1/R and ∆d/d is defined in equation 2.2 below as:

1R

=cosα

2L¨

∆dd

(2.2)

where α is the angle of incidence (where the laser hits the substrate) and L is the distancebetween the substrate and the CCD camera [8]. This equation notes the mean vertical differ-ential spot spacing.

By combining equation 2.1 and 2.2, the stress σ of the film in Stoney’s formula can be ex-pressed in terms of the mean differential spacing ∆d. Equation 2.3 below shows this relation-ship:

σ ¨ h f =∆dd

¨Yh2

s ¨ cosα

12L(2.3)

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The equation for the mean horizontal differential spot spacing is given below:

1R

=1

2L ¨ cosα¨

∆dd

(2.4)

By combining equation 2.1 and 2.4, the stress σ of the film in Stoney’s formula can be ex-pressed as the following equation:

σ ¨ h f =∆dd

¨Yh2

s12L ¨ cosα

(2.5)

It is the CCD-camera that will measure how the laser spots are aligned. The informationreceived will be plotted as the average horizontal or the average vertical mean differentialspot spacing as a function of time. It is because the average of all differential spot spacingvalues are taken into account [8]. In this thesis, only the average vertical mean differentialspacing is taken into consideration. However, a difference between how the stress looks likewhen comparing data from the vertical and the horizontal differential spot spacing will beprovided in section 5.

The plotted data is however converted to the stress-thickness (σh f ) product, which can bereceived from equation 2.3 when using the mean differential spot spacing data, as a functionof thickness. It can also be plotted only as σ as a function of thickness. How the data is plottedas a function of thickness is described in section 2.5.

2.4.3 Important Aspects of the Stoney Equation

There are some important aspects to take into consideration when measuring the stress inthin films using the Stoney Equation and these are listed below:

1. Equation 2.1 is true if the substrate is flat before deposition. To obtain the stress in thefilm, a change in the curvature is necessary. When determining the stress of the filmafter deposition, the curvature must be measured before and after deposition. This ishighly important when doing post-depostition stress measurements with XRD, whichwill be explained in section 2.5. For real-time measurements during deposition withMOS, the pre-deposition curvature is not needed, because a reference value is used[13].

2. Thin substrates should be used when making depositions since the curvature is in-versely proportional to the square of hs. This has to do with the sensitivity of the mea-surements. When producing thick films, it is however recommended to use thick sub-strates. This is mainly to avoid the fact that the substrate can deflect in a way that itmay break during deposition. When making a thin film approximation which indicatesthat h f < < hs, the layers will contribute to the curvature independent of each other andadditively. If the film has stress in it itself, it does not affect the layers that are belowit. The thin film approximation is needed for Stoney’s formula but it can also becomeinaccurate. This happens when the ratio between the film and the substrate becomesnoticeable [13]. The ratio between the film and the substrate h f /hs should not be largerthan 10%. When the Stoney equation fails, some modifications of the original formulaneeds to be done [15]. For this project, hs was approximately 155 µm and h f was neverlarger than 1 µm, hence fulfilling the condition.

3. It is important to know the thickness of the film when measuring the stress with curva-tures. The thickness and the stress will both develops during the deposition. Hence therate of the deposition is necessary to measure the stress evolution. This is explained inprevious subsection [13].

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4. When doing stress measurements after deposition (with XRD), the stress-thicknessproduct σh f should be replaced with integrated stress that measures the stress through-out the material. This is due to the fact that there are stress variations in the mate-rial and this is not determined only by curvature measurements after deposition. Forstress-measurements during deposition, the curvature can be affected by multiple fac-tors. These factors could be the material that arrives from vapour, relaxation processin the film (dependent on the depth) and the stress from the surface. The stress of thesurface is mostly because of the crystallographic orientation or the morphology [13].

5. The Stoney equation is also highly dependent on the so called isotropy transverse ar-gument. This implies that the substrate, related to the film, has transversal isotropicelastic property [15]. In this project, single crystalline Si(100) are used which also haveanisotropic properties, hence fulfilling the condition. The thickness and the stress of thefilm should also be uniform and this is highly dependent on the deposition rate andprocess [15].

6. The mean stress can be measured, which is the case for this thesis. As mentioned earlier,σh f is divided by h f [8].

2.5 X-Ray Diffraction (XRD)

Crystalline materials can create 3D diffraction pattern when they are exposed to a beam ofincident X-rays. XRD is an analytical method used for the characterisation of crystallinematerials. The information received could be about the structure, phase, crystallographicorientation, grain size, defects etc. This method is nondestructive [16].

For this project, XRD is mainly used for two different task; post-deposition stress mea-surements (wafer curvature) as a way to verify the stress obtained from MOS when doingreal-time stress measurements, but also for the determination of the deposition rate whichis a way to determine the thickness. The latter is because we want the data to be plotted asstress vs thickness.

The principle behind this technique is the use of an X-ray diffractometer that has an X-raysource, a sample stage and a detector. The source is a cathode tube that generates X-rayswhen a filament is heated where electrons are produced. These electrons accelerates to atarget-material when a voltage is applied and a characteristic X-ray spectrum is generatedwhen the electrons with enough energy can knock out the electrons from the inner shells ofthe material. Monochromatic X-rays are filtered from this spectra and directed towards thesample, as the detector is rotated and records diffracted X-rays [17]. When the incident X-rays interact with the film, constructive interference that satisfies Bragg’s law which showsthe relationship between the wavelength of the X-rays to the angle of diffraction and thespacing between the lattices, see equation below [16].

nλ = 2dsinθ (2.6)

where n is an integer, λ is the characteristic X-ray wavelength, θ is the angle of diffractionand d is the spacing between the diffracting crystal planes. Diffracted X-rays are detected bya detector, when the detector scan the sample in a pre-determined range of 2θ-angles. Thedata plotted will be diffraction peaks at certain angles and then converted into d-spacingwhich is unique for each compound.

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2.5.1 X-ray Reflectivity (XRR)

A technique that can be used to study/characterise thin film surfaces is X-ray reflectivity.Suppose that there is a homogenous substrate, where on top of it a homogenous film isdeposited. The incident X-ray beam will go through two interfaces where the the first oneis the air/film and the second one is the film/substrate. The beam is partly reflected at eachinterface while some of the beam continues through the material. X-rays can be seen as wavesand the difference in paths between the x-rays reflected at the first interface and the secondinterface results in a phase shift, that is dependent on the incident angle [18]. The surface hasto be smooth in order for the reflected rays to not deviate from Fresnel’s reflectivity law. Ifa smooth thin film is present on a smooth substrate, the film thickness can be deduced fromthe angular spacings of oscillations in the reflected X-ray intensity. XRR is mostly used tomeasure the thickness of thin films [19].

By measuring the thickness, the rate of the deposition can be calculated by dividing thethickness with the deposition time. This ratio will later be multiplied with the time for eachdata point received from MOS in order to receive the data as a function of thickness. Thethickness obtained from the XRR-measurements will be used in equation 2.3 to convert thevertical mean differential spot spacing data to stress. Thereby for this project, XRR was usedto measure the thickness of the thin films.

When we receive data from the XRR, the plotted data will be a footprint followed by fringes.The distance between two fringe-maximas is proportional to the thickness of the film [18].When analysing the data, a software is used to fit the data to a curve that is simulated. Whenfitting the plot received from measurements, parameters such as material density, layer thick-nesses and roughness of the interface are used. Since a simulation is used to fit the data, it isnot an exact method. It however estimates the thickness roughly [20].When doing XRR (and XRD), different scans are done which are dependent on certain anglesand heights. Optics and scan parameters are important in order for the X-rays to be reflectedon the sample and reliable data to be obtained. The parameters taken into considerationwhen doing XRR for this project are shown in the figure below.

Figure 2.6: This figure explains how the angles are related to the sample and the incident and reflectedbeam. It also shows the sample height Z.

ω is the angle between the incident beam and the sample surface, 2θ is the angle between theincident and the reflected beam detected by the detector and Z is the sample height. ω hashalf the value of 2θ, ω = 2θ/2. The different scans are explained below:

• The 2θ scan is done in order to check the zero position and the intensity of the directbeam. A scan is made to see at which angle highest intensity of the detected x-rays arethe highest, the position of the detector will be set to that angle.

11

• The Z-scan is the second scan done and this is in order to set the sample height, seefigure 2.6. If the sample is not at the correct height, suppose that it is too far down,there is a risk that the incident X-rays will miss the sample and thereby not be detectedby the detector, which will lead to no or very little signal will be obtained.

• When mounting the sample or when setting the sample height Z, the sample may tiltand not lay flat on the sample holder. This is why ω scan is done, in order to find thesample offset of the substrate plane with respect to the surface of the sample. 2θ will beset to a fixed value and the X-ray source will irradiate the sample in a ω-range.

• From the 2θ-ω scan, the data for which the thickness is determined from is obtained,where diffraction from planes parallel to the sample surface is collected. For this mea-surement, both 2θ and ω are varied, with ω being half the value of 2θ. The sampleoffsets will be set to zero for this scan [18].

In order to meet the analytical requirements, there are some optics that should be chosenwhen doing an XRR-measurement. When the X-ray beam hits the sample, the beam may betoo wide, which means it can hit other areas that are not part of the sample. What can be doneis to use some sort of collimator in order to narrow the beam and for XRD-measurementsin general, slits can be used. The dimension of the chosen slits should be similar to thedimensions of the sample. If the slit is a divergence slit, it will be put in between the incidentbeam and the sample to match with the dimensions of the diffraction and the sample. If itis a receiving slit, it is put between the sample and the detector to improve resolution andeliminates scattered rays. A mask can also be used to correct the beam size so it matchesthe dimensions of the sample. The slits and the mask will provide maximum illuminationon the sample. Filters can also be used to selectively reduce intensity of beams with certainwavelengths. For XRR-measurements the beam is lined-focused to illuminate more of thesample [21].

For the 2θ, ω, Z and 2θ-ω scans, some parameters such as scan range (°), step size and timeper step (s) need to be determined. This will set a time for each measurement. The scanparameters together with the optics used to make the XRR-measurements are described insection 3.

2.5.2 Wafer Curvature Measurements for Film Stress

XRD can be used to measure the curvature of a substrate wafer post-deposition. The curva-ture of the substrate is 1/R in Stoney’s equation 2.1 and therefore this method is a way todetermine stress in thin films. When a film that is growing on a substrate is strained, thesubstrate will bend with a radius of curvature R. The substrate wafer curvature can be mea-sured from ω-shifts of a substrate Bragg-peak when shifting the measurement position X onthe sample, see figure 2.7. This can be done by doing high resolution rocking curve measure-ments (plot X-ray intensity as a function of ω) with XRD. The information that is obtainedfrom the rocking curves are the direction in which the substrate lattice planes are oriented.The figure below shows a schematic of the wafer curvature measurement. Note that ∆ω =ω2-ω1.

12

Figure 2.7: This figure is an illustration of a strained film attached to a substrate (the curve), with aradius R. ω is shifted at different positions X on the sample. It also shows a simplified schematic of thedata received, which is the intensity as a function of ω that is shifted for each position X on the sample.

Figure 2.7 also shows a simplified schematic of how the plot obtained from the measurementlooks like, where ω is shifted with the measurement position on the sample X. X will ap-proximately be the arc length of the curvature if assuming R > > X. Then X = ∆ω*R, whichmeans that if plotting ω as a function of X, 1/R is the slope of the curve. The stress of the thinfilm can thereby be calculated by using Stoney’s equation, see equation 2.1. For more ”exact”measurements, 1/R should be determined for the substrate both before and after deposition.Their difference will be equal to the right side of equation 2.1.

2.5.3 Transmission Electron Microscopy (TEM)

In order to analyse the samples, but to also learn how to use analytical techniques in researchtransmission electron microscopy (TEM) was used to examine materials on nanoscale level,since regular light microscopy is not an option. TEM is a powerful tool to study materialswith dimensions smaller than 100 nm. It is possible to receive information regarding themorphology, crystal structure and fine features in the examined material [22]. An electronbeam (energy > 100 kV) with a short wavelength is used, which provides images of thespecimen with high resolution [23]. A schematic figure of transmission electron microscopyis shown below.

Figure 2.8: The electron beam interacts with atoms in the sample and transmit through it, The trans-mitted electrons are detected by a detector where information regarding morphology and/or crystallo-graphic orientation will be received.

13

The sample is put onto the TEM-grid where the position is controlled via a mechanical arm.The microscope consists of metal apertures and electromagnetic lenses (condensers and ob-jectives) to focus the electron beam coming from an electron gun. Electrons are negativelycharged and behave like waves and later deflects when electric/magnetic field is applied.Only electrons with certain energy can pass through. The electrons that are transmitted isfocused on the sample, which later provides information regarding the sample. When theelectrons pass through the specimen, electromagnetic lenses focus, magnify and project themonto a screen as an image [22]. The image provided is in 2D.

2.5.3.1 Bright respectively Dark Field Mode

By changing the objective aperture to certain focal spots, it is possible to decide if either trans-mitted or diffracted electrons can pass through the aperture. If transmitted electrons passesthrough, a so called bright-field image is obtained where the dark parts of the image indi-cates thicker areas and heavy atoms. When the aperture blocks the transmitted electrons, thediffracted ones pass through. If the diffraction conditions are satisfied, information regardingthe crystallinity is shown bright in the dark-field mode [22].

2.5.3.2 Sample Preparation

When studying samples with TEM, they must be electron transparent, which means thatthey must be thin enough for the electrons to transmit through the material. The thicknessof the sample should be around 100 nm, which makes it easy for electrons to go through.The sample preparation starts by first cutting the sample with a diamond wheel saw in fiveto six pieces that are 2x1 mm in dimension. By observing the pieces under the microscope,the best two are chosen. The two pieces are flipped onto their sides with the film sidesfacing each other and then they are put into a TEM-grid and glued. The grid is glued onto aglass-plate and polished on both its sides with a series of diamond films where the grain sizeis decreased gradually, until a thickness of 50-60 µm is reaches. The sample is later put into aso called ion milling machine which makes it thinner by shooting ions on it. This is in orderto make it electron transparent.

It is important to note that for this thesis work, TEM was only used once to examine a samplemade prior to the start of this work. Hence, the results for that will be provided in chapter 4and not 5.

14

Chapter 3

Method

In this chapter, the execution of the procedures done to conduct this project is described. Theinformation and theory behind the machines is explained in the previous chapter, hence thischapter only focuses on the method. The settings for the machines are explained either in thetext or in a table. The figures attached are used to simplify the understanding the principlesfor the reader.

3.1 The Substrate

As mentioned in section 2.2, silicon (Si(100)) substrates was used to make depositions on.The thickness of these substrates vary between 150-155 µm. A new flat substrate is used forevery deposition. The thin film deposited onto the substrate is around 620-650 nm for most ofsamples and this is for the plotted data to be comparable. The thicknesses for the bias seriesmight be larger than 650 nm.

3.2 Sample Cleaning

Surfaces usually contain contaminants that needs to be removed prior to sputtering. In orderto remove particles and dust from the sample, there was a cleaning procedure done beforethe sample was mounted on the sample holder and was put on the rack of the transfer-arm. There was three different solvents used for cleaning; trichloroethylene, acetone andisopropanol.

The first step was to transfer the sample into a beaker and fill it with trichloroethylene toa level higher where the sample surface is covered with solvent. The beaker was put intoan ultrasonic bath for three minutes. An ultrasonic bath is a small tank that is filled withwater. An ultrasonic energy is applied which will make the water ”vibrate”, hence free thesurface from particles [24]. The beaker was removed from the bath after three minutes andthe liquid was poured into a jar with waste-trichloroethylene. Same procedure as done withthe trichloroethylene was done with first the acetone and then isopropanol respectively, butfor five minutes each. The reason why the liquids was used in that order is because acetoneremoves trichloroethylene and isopropanol removes acetone.

3.3 Deposition and Stress Measurement

All the magnetrons were lowered to their maximum level to prevent any collisions whenloading the sample holder. The LL was vented and the clamp that fastens the sample holderwas opened to release it. The sample holder was transferred to a fume hood where thesample was kept in a beaker with isopropanol. The sample was blown with N2 and was put

15

onto the sample holder with its cleanest side facing down. The sample holder was reloadedin the LL and the clamp was fastened. Since the substrates that were used were thin, noclampers were used to fasten it in place on the sample holder in order to avoid the samplebreaking and letting the sample to bend freely during deposition. The sample holder waspushed in almost 10 cm into the LL away from the valve opening with the transfer since thesubstrate has a light weight and has no clamps securing it. When the system was pumped,the substrate did not break or disappear in the chamber during the opening/closing of turboor roughing pump valves. When the system was pumped and the pressure in the LL haddecreased to a level where it was at least one order of magnitude larger than the pressure inthe MC, the LL-MC gate valve was opened. The sample was transferred into the MC and theclamp that holds the sample holder was opened. The transfer arm was removed from theMC and the LL-MC gate valve was closed.

When the sample was loaded, the temperature controller and the laser were turned on. Theintensity of the laser was set to maximum via the software and the CCD-camera and theoptics were adjusted until a symmetric 2D array of spots could be seen, look at figure below.

Figure 3.1: A symmetric 2D array of laser spots prior to deposition.

For lower pressures, the optics were adjusted so the spots were closer to each other andfor higher pressures the optics were adjusted so the spots where far away from each other(reason explained in section 6). When the laser setup was done, argon was opened to enterthe main chamber. Before turning on the magnetron, 30 seconds of data was collected as areference for the stress-measurements in the MOS-software. After, the magnetron and themeasurement of the spots was turned on at the same time. The figure below shows how itlooks like in the MC during deposition.

Figure 3.2: During deposition in the main chamber.

16

In order to understand which parameters/mechanisms control the stress, the stress of the filmwas measured under different conditions. The positions of the magnetron varied between 15,30, 50 and 70 mm, where at 15 mm the magnetron was the closest to the sample. For eachposition, depositions were made for 1.5, 2.5, 3.5 and 4.5 mTorr. This was in order to see whicheffect the position of the magnetron and the pressures have on the stress evolution, look attable 3.1 below.

Table 3.1: These are the parameters that were varied during the depositions. Look at table A.1 for theirrates.

70 mm 50 mm 30 mm 15 mm1.5 mTorr 1.5 mTorr 1.5 mTorr 1.5 mTorr2.5 mTorr 2.5 mTorr 2.5 mTorr 2.5 mTorr3.5 mTorr 3.5 mTorr 3.5 mTorr 3.5 mTorr4.5 mTorr 4.5 mTorr 4.5 mTorr 4.5 mTorr

Note that 70 mm, 50 mm, 30 mm and 15 mm are not the distances between the magnetronand the sample. These are the ’terms’/numbers used by the software. The distance betweenthe magnetron and the sample when mentioning 70 mm is approximately 20 cm, for 50 mmit is 18 cm, for 30 mm it is 16 cm and for 15 mm it is 14.5 cm. In this thesis, the softwarenumbers will be used.

When making the depositions, the power was kept constant at 300 W for all depositions.The sample stage was for all depositions kept at 142 mm position and pointed towardsthe transfer arm for successful loading and unloading of samples. No substrate heating orrotation of the sample stage were done. The angle of the magnetron was kept at -2.5° for allmeasurements. Table A.1 gives information regarding the rates.

First a series of rate depositions were done, where each deposition lasted for 10 minutes (seetable A.1) and no stress measurements were done for these. The rate depositions were donein order to calculate the rates of the depositions. The rates were calculated by measuringthe thickness of the film (with XRR - explained in section 3.5) and dividing it with the de-position time. For the stress-measurement depositions, a thickness of circa 620-650 µm wassupposed to be reached. Therefore, rate depositions were done for when the magnetron wereat different positions and the desired thickness was divided by the rate. Then, the time forthe depositions where stress was measured could be calculated. The rates were also usedto convert the plotted data from a function of time to a function of thickness. When usingthe MOS for the stress measurements, the distance between the sample and the CCD-camerawas kept constant at 102 cm and the angle of incidence was 45°.

When the deposition was done, the magnetron was turned off, and the measurement of themean differential spot spacing went on for another 5-10 minutes, with the argon running,to let it stabilise. When the measurement was done, the argon was turned off and after afew minutes where the pressure had stabilised, the LL-MC gate valve was opened and thesample was removed from the MC. When the gate-valve had closed, the LL was vented.

In order to see what effect the a substrate bias has on the stress evolution and if it is possibleto minimise the residual stress, a series where different biases were applied during the depo-sition process was made. This series was done for when the setting parameters were 30 mmand 2.5 mTorr (see table 3.1). For the first series, the argon gas was opened in the MC andthe biases applied were 100 V and 130 V. For the second series, the argon was opened at thetarget and the biases applied were 0 V, 100 V, 150 V and 200 V.

17

3.4 TEM - Preparation and Analysis

The routine for the sample preparation is explained in section 2.5.3.2. The sample is firstpolished very roughly with a silicone film of rough grains. The diamond films that are usedto polish the sample until it reaches a thickness of 50 µm, have the grain sizes 30, 15, 6, 3and 1 µm respectively. The polish with the largest grain size was used first and then it wasgradually decreased. When the desired thickness was obtained, the ion milling machine wasused to make it thinner. When the sample was transparent enough, it was examined with theTEM following the principle described in the theory. Both dark and bright field images weretaken in order to obtain information about the sample.

In this thesis, TEM was used for one sample (which was made prior to this diploma work).The result for that is provided in chapter 4

3.5 XRD - Preparation and Analysis

The XRD was used for two different purposes in this diploma work; reflectivity measure-ments in order to measure the thickness of the thin film and thereby the deposition rate andfor post-deposition stress measurements to verify the stress from the laser measurements. Inthe subsections below, the method for both are explained.

3.5.1 Reflectivity Measurements

For the reflectivity measurements, the incident beam optics was parallel beam X-ray mirrorfor Cu radiation, a fixed beam mask of 10 mm and a slit of 1/32 (divergence slit). For thediffracted beam path the detector used was PIXcel-3D, a collimator slit of 0.27 mm (beamattenuator) and a large beta filter (Ni) was used. These optics were also set in the opticssetup in the software. The generator was set to 40 kV and 40 mA. All the positions in theinstrument settings tab was set to 0° or 0 mm and the sample was mounted as flat as possibleon the sample stage. Since the sample was very thin (fragile), no tape was used to set it inplace.

The first measurement was a 2θ-scan in order to check the zero position of the directed beamintensity. A manual scan was done with the following setting:

• Scan Mode Range: 1°

• Step Size: 0.010°

• Time per Step: 0.5 s

When the scan was done, ’Peak Mode’ was chosen by right mouse clicking and the cursorwas moved to the peak position. When the right position of the detector was found, thesample offset for 2θ was set to 0.0° in order to correct the zero position.

The next scan was a Z-scan and this was done to set the sample height. In the instrumentsettings, Z was set to 9.5 mm in order to match the thickness of the sample. The settings forthe manual scan was:

• Scan Mode Range: 1 mm

• Step Size: 0.010 mm


18

The Z-value in the instrument tab may need to be changed after this scan to a value less orhigher than 9.5 mm. When the scan is done, the graph obtained has high intensity values atlow Z and low intensity values at higher Z. The cursor was moved to half of the maximumintensity received. After this, an ω-scan was done in order to set the horizontal tilt. Thesetting for this measurement was as previous ones. When the scan was done, the cursorwas moved to the peak with highest intensity. The Z-scan followed by the ω-scan should berepeated until there are no changes in both the positions, since as one of them are changed,the other will also change. When the last ω-scan is done, the sample offset for ω is set to zero.After this measurement, the scan where the thickness will be determined from, the 2θ-ω scanwas done with the following setting:

• Scan Mode Range: 2°

• Step Size: 0.005°


With a software created in MATLAB, the plotted data was downloaded and the peaks of thefringes were analysed in order to measure the thickness of the film and thereby the depositionrate.

3.5.2 Wafer Curvature Measurements for Film Stress

For the wafer curvature measurements, XRD was used. The optics that were used to carryout the measurements were capillary X-Ray lens for the incident beam optics and a tripleaxis with open detector for the diffraction optics. For the incident beam optics a 2 mm maskwas also used in order to confine the X-rays with a size of 2 mm by 2 mm. The reason whythese optics were chosen was because of their sensitivity.

The same principle as described for previous subsection for the different scan where followed,but for this measurement also a χ scan was made which is for the tilt in the other directionas ω. For the stress measurements, a χ-ω and an ω-program was made, for nine differentpositions on the sample (-8 mm, -6 mm, -4 mm, ..., 8 mm) which is called X. A χ-ω programwas made in the software to optimise ω (receive high intensity of ω scans) and after that ωscans were done for the nine different positions. This measurement was done for the ninedifferent positions of the sample and for two different samples. One sample was where fromthe MOS-measurements, a more tensile stress could be observed and for the other sample amore compressive stress could be observed.

19

Chapter 4

Related Work

In this chapter, the work previously done in this project regarding stress measurements, vary-ing parameters and the analytical techniques will be presented. This will mostly include re-sults obtained from those measurements that was the main starting point to this diplomawork. The method to conduct the experiments are similar to those who have been presentedin chapter 3.

4.1 Varying Parameters

In this diploma work, the changing parameters are mainly the working pressure and theposition of the magnetron from the sample. The power is however kept constant at 300 W.In previous work, the changing parameters were the power and the working pressure, butthe distance between the magnetron and the sample was kept constant. The magnetron wasat 70 mm position. What was investigated was at which powers and pressures minimumresidual stress is obtained, but also when the quality of the thin films are the best.

The figure below shows some the deposition rates as a function of working pressure for threedifferent powers; 200 W, 300 W and 400 W. It can be seen that for higher powers, the deposi-tion rate is also higher. The powers varied between 1.5 mTorr - 10 mTorr for each series, lookat figure 4.1.

Figure 4.1: The difference in deposition rates between different powers for a series of working pressures.The powers chosen were 200 W, 300 W and 400 W, as indicated by the figure.

20

Figure 4.2 shows how the stress*thickness varies when the pressure is kept constant at 2.5mTorr and the power is varied at 200 W, 300 W and 400 W. Figure

Figure 4.2: Difference between stress*thickness, for varying powers; 200 W, 300 W and 400 W. Thepressure is kept constant at 2.5 mTorr. For the rates of these depositions, look at 2.5 mTorr in figure4.1.

The graphs below are given as stress as a function of thickness. The deposition for 1.5/2.5mTorr, see orange plot in figure 4.3, the pressures were changed between 1.5 mTorr and 2.5mTorr every 25 nm.

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Figure 4.3: The stress as a function of thickness for different pressures 1.5 mTorr, 1.5/2.5 mTorr,2mTorr, 2.5 mTorr and 3 mTorr. The power was kept constant at 300 W.

Figure 4.4: The stress as a function of thickness for different pressures 3 mTorr, 5 mTorr and 8 mTorr.The power was kept constant at 300 W.

4.2 Analysis of the Samples

The residual stress was at a minimum for when the pressure was varied between 1.5 mTorrand 2.5 mTorr, at a power of 300 W (see figure 4.3. In order to see the film on a nanoscalelevel and examine its features such as morphology and crystal structure, TEM was done. The

22

principles follows the routine explained in section 2.5.3.2. The figure below shows the brightfield image of the sample.

Figure 4.5: A bright field image of the sample where the working pressure is shifted between 1.5 and2.5 mTorr every 25 nm.

4.3 Conclusions from the Related Work

The conclusion that were drawn from the related work is that a lower deposition rate willinfluence the quality of the film. For higher pressures, a more tensile stress is created inthe film, which will also yield more voids and defects and make it easier for the film tofracture. Higher pressures will also give lower deposition rates, as indicated by figure ??. Forthis purpose, the range of pressure that was chosen to work in for this diploma work wasbetween 1.5-4.5 mTorr and the power was always at a constant level of 300 W. The TEM-datashows, when the working pressure is shifted between 1.5/2.5 mTorr, the density of the filmis high but as the film grows thicker, voids (defects) start to form. This was the main reasonwhy it was chosen to make a series where the distance of the magnetron was moved closerto the sample. As the distance between the magnetron and the sample decreases, accordingto theory, there will be more energetic bombardment of particles which can make the filmgrow denser. Figure 4.3 also shows that for the 1.5/2.5 mTorr deposition, that the stress willbe compressive from a beginning but as it starts going towards the tensile, it will be hard toreduce it towards lower values. It is also visible in figure 4.5 that for the first layers, thereare minimum voids/defects in the structure (where the pressure probably is 1.5 mTorr), butthe structure seem to change once the pressure is changed to 2.5 mTorr as voids start to form.Hence tensile stress might be hard to reverse.

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Chapter 5

Result

5.1 The Stress Measurements

The figures in the subsections below show the stress evolution for the pressure and the dis-tance series, look at table 3.1 for the settings. The result is discussed in chapter 6.

5.1.1 Pressure Series

Below, the figures from the pressure series; 1.5 mTorr (figure 5.1), 2.5 mTorr (figure 5.2), 3.5mTorr (figure 5.3) and 4.5 mTorr (figure 5.4) is presented.

Figure 5.1 shows how the stress evolves for different distances at a pressure of 1.5 mTorr. Thedistances are indicated both in table 3.1 and in the figure itself. Both the solid and dashedblue graphs shows the stress evolution in the thin film, using the same setting.

Figure 5.1: The stress evolution as a function of thickness for four distances; 15 mm, 30 mm, 50 mmand 70 mm. The pressure is kept constant at 1.5 mTorr for these measurements. There are two graphsfor the distance 50 mm provided.

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Both the solid and the dashed green line in figure 5.2 shows the stress evolution using thesame settings.

Figure 5.2: The stress evolution as a function of thickness for four distances; 15 mm, 30 mm, 50 mmand 70 mm. The pressure is kept constant at 2.5 mTorr for these measurements. There are two differentgraphs for the distance 30 mm provided.

Figure 5.3: The stress evolution as a function of thickness for four distances; 15 mm, 30 mm, 50 mmand 70 mm. The pressure is kept constant at 3.5 mTorr for these measurements.

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Figure 5.4: The stress evolution as a function of thickness for four distances; 15 mm, 30 mm, 50 mmand 70 mm. The pressure is kept constant at 4.5 mTorr for these measurements.

5.1.2 Distance Series

Below, the figures from the distance series; 15 mm (figure 5.5), 30 mm (figure 5.6), 50 mm(figure 5.7) and 70 mm (figure 5.8) is presented. For each figure, the distance is kept fixed andthe pressure is varied (1.5 mTorr, 2.5 mTorr, 3.5 mTorr and 4.5 mTorr), look at table 3.1.

Figure 5.5: Stress evolution as a function of thickness for four different pressures, 1.5 mTorr, 2.5mTorr, 3.5 mTorr and 4.5 mTorr, at a constant magnetron position of 15 mm. The noisy data for the1.5 mTorr graph is due to the bad quality of the laser intensity.

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Figure 5.6: Stress evolution as a function of thickness for four different pressures, 1.5 mTorr, 2.5mTorr, 3.5 mTorr and 4.5 mTorr, at a constant magnetron position of 30 mm.

Note that for figure 5.7, there are two different plots for 1.5 mTorr. The both depositions weredone with the same settings.


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5.2 Other Relevant Results

The result in the subsections below were not mainly part of what was intended for investi-gation. These are however interesting findings which are relevant to the work. Note that thedepositions for each subsection are done with the same sample to target distance and sput-tering gas pressure. The only difference is either the base pressure, the temperature of theheater in the MC and and the opening of argon gas inlet, either directly in the main chamberor closer to the target. The figures provided below are plots of the stress as a function ofthickness. In Appendix B, the stress*thickness plots are provided.

5.2.1 Effect of Base Pressure in MC

Figure 5.9 below shows what effect a good and a bad base pressure in the MC prior to despo-sition could have on the stress evolution. The depositions are made with the same settings,which is when the position of the magnetron was at 70 mm and the pressure of the argon was1.5 mTorr. The only difference between these depositions is the base pressure, which for onedeposition (blue graph) was high (9.9E-7 mTorr) and for the other (red graph) was low (3.2E-9mTorr). High pressure indicates low vacuum and low pressure indicates high vacuum.

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Figure 5.9: These plots shows how the stress can evolve under the same conditions where the onlydifference is the pressure in the main chamber prior to the deposition. For the blue graph, the pressurewas high and for the red graph the pressure was low. The values are indicated by the legend on theupper right corner. See also figure B.1.

5.2.2 Effect of Heater Temperature

When doing a longer deposition or many after each other, the temperature shown on ther-mocouple which is placed behind the substrate heater gradually increases. Figure 5.10 showshow the temperature recorded before starting the deposition can influence the stress evolu-tion. The temperature is stable around 18°C-30°C, but it can increase up to 60°C-70°C afterfour or five depositions in a row. The blue graph is for a deposition where the temperatureis 18.9°C and the red graph is for a deposition where the temperature was 58.0°C prior tothe deposition. For both depositions, the magnetron is at a position of 30 mm and the argonpressure is 2.5 mTorr. The only difference is the temperature of the heater. The pressure inthe MC prior to the depositions is 4.4E-8 mTorr for the blue graph and 5.8E-8 mTorr for thered.

Figure 5.10: Temperature shown on thermocouple which is placed behind substrate heater recordedbefore starting the deposition. This figure shows the impact it might have on the stress evolution. Seealso figure B.2.

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5.2.3 Effect of Argon Gas Location

For the measurements that are presented in figure 5.11, the magnetron was at a 30 mm po-sition and the argon pressure was 2.5 mTorr. However, for one of the measurements (redplot), the argon gas was opened at the target and for the other measurements, the argon wasopened in the main chamber (blue plot).

Figure 5.11: The difference of stress evolution between the measurements where the argon is openedat the target and when it is opened in the main chamber. 30 mm, 2.5 mTorr. See also figure B.3.

5.3 XRD - Wafer Curvature Measurements

Figure 5.12 shows the optimised omega scans for each position. The plot to left is at a positionif -8 mm and the plot to the right is at a position of 8 mm. The peak positions (ω-value) wasplotted as a function of X. The figure below shows the data for the sample with a more tensilestress (see also figure 5.14). The same principle was applied on the sample with compressivestress. See the data in table B.1 and B.2 in appendix B.

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Figure 5.12: For each plot, the peak position at an angle ω was plotted as a function of sample positionfor nine different positions (-8 mm, -6 mm, ..., 8 mm). The plot to the left is -8 mm and the one to theright is at 8 mm.

Using figure 5.12 and table B.1 and B.2, and plotting the data in MATLAB, the followingfigure will be obtained. The slope of the curve (1/R in equation 2.1) is 0.1848. Using equation2.1, the stress obtained will be 135 MPa. For the sample where the stress is more compressive,the slope of the curve is -0.1055 and the stress obtained is -121 MPa. The slope of the plots isindicated by figure 5.13.

Figure 5.13: The slope of the curves 1/R, given by ω as a function of X. Using Stoney’s equation 2.1,the stress can be measured.

Figure 5.14 shows how the stress is evolved in real-time stress measurements. These sampleswere the ones examined with the XRD. The stress value at the end of the deposition forthe sample where the stress is indicated by the red curve is approximately 36 MPa. As canbe seen, the stress evolves towards a tensile. For the blue curve, where the stress is morecompressive, the final stress value for that sample is approximately -127 MPa.

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Figure 5.14: The XRD curvature measurements were done for these two plots that shows real-timestress measurement with MOS. The red plot has a final stress value of 36 MPa and the blue plot hasa stress of -127 MPa.

5.4 The Effect of Bias

Figure 5.15 and 5.16 both shows stress*thickness plots when the magnetron was at a 30 mmdistance and the working pressure was 2.5 mTorr. The only difference between these seriesis that for figure 5.15, the argon gas was opened in the MC and for figure 5.16 the argon wasopened at the target (see section 6.5 for reason). The plots are stress*thickness due to thedeposition rates missing because of time limitation.

For figure 5.15, the argon was opened at 600 s. For figure 5.16, the time when the argon wasopened is indicated by the legend in the graph.

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Figure 5.15: The applied biases indicated by the figure. For these plots the argon was opened in theMC. The bias was applied at 600 s for both depositions.

Figure 5.16: The applied biases indicated by the figure. For these plots the argon was opened in at thetarget.

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Chapter 6

Discussion

6.1 The Pressure and the Distance Series

What was indicated by the related work in chapter 4 was that as the pressure is increasing,the turning point of where the stress is the most compressive, to it growing more tensile willincrease and go towards tensile (see figure 4.3 and 4.4). It can also be seen that the stress isfollowing a trend. This trend can also be seen in the distance series in chapter 5, see figure5.5, 5.6, 5.7 and 5.8. For all the magnetron positions, as the pressure is increased the transitionfrom compressive to tensile stress occurs at a stress level closer to tensile, where for the 1.5mTorr it is the most compressive and for the 4.5 mTorr it is more tensile. When increasingthe distance between the magnetron and the sample, the stress values (and the transitionfrom compressive to tensile stress) are different during the initial stage, but as the film grows(see figure 5.8), the stress levels for all pressures at that distance is the same at 120 MPa.For lower distances (see figure 5.5), the stress difference at 500 nm is almost 300 MPa. Itwas indicated by the theory in subsection 2.4.1.2 that a lower pressure will yield more highenergetic bombardment of target particles since they do not lose much of their kinetic energycolliding with many gas ions. This in turn will give a more compressive stress, which canalso be seen in the plots in all four distance series.

According to the theory, when bringing the magnetron closer to the sample should also yieldmore high energetic bombardment of target particles and making the stress more compres-sive. However, looking at the results in the pressure series in figure 5.1, 5.2, 5.3 and 5.4, thisis not the case for all. For all the series, except for 3.5 mTorr (figure 5.3), the position of 50mm seem to be out of place and not following the trend with where the turning point of thestress is. It can also be seen that as increasing the pressure to 3.5 mTorr and 4.5 mTorr, thetransition from compressive to tensile stress may be different during the initial stage of filmgrowth but the stress around 500 nm is very much alike where both are 120 MPa. There onlyseem to be a difference in stress at 500 nm for the 1.5 mTorr series, where a stress difference ofalmost 300 MPa can be seen from 1.5 mTorr to 4.5 mTorr (see figure 5.1). A smaller differencecan be seen for 2.5 mTorr series (see figure 5.2). Thereby, increasing the distance between themagnetron and the sample and increasing the working pressure, does not have an effect onthe stress value towards the ending of the film growth since the stress levels are alike.

As indicated by figure 5.7, 5.1 and 5.2, even if the films are deposited with the same settings,they do not look alike or have the same final stress and therefore not always repeatable.This could be due to many things such as heater temperature or base pressure in the mainchamber, which is further discussed in section 6.2.

Higher pressures and when the distance from the magnetron to the sample is larger will bothgive a film with higher tensile stress, which is indicated by all the results. Higher pressures

34

also yield a lower deposition rate, which during the film growth is that atoms reach the sub-strate at a lower rate. Due to gas scattering, arriving atoms will not bring any extra energy tothe surface and therefore a porous film will be expected. Since higher pressures and longerdistances creates porous films and higher tensile stress, it is not something that should beused. For lower pressures however, because of the higher energetic particles, the film couldbe denser. This is due to the energy of the incoming atoms (imagine them ”pushing in” theplace they arrive at) making the atoms pack more densely. Because of the density, after doingthe pressure and the distance series it was decided that it would be more favourable to workwith lower pressures and a distance where the magnetron is closer to the sample when thebias was going to be applied.

Comparing the result from the different series, it is difficult to say which setting (pressureand distance from magnetron to sample) gave the minimal residual stress because of the lackof repeatability. In section 6.5 the difficulties and problems with working with the magnetronwill be explained. These difficulties made it hard to make depositions at lower pressures(such as 1.5 mTorr and towards the end of the diploma work at 2.5 mTorr). For this reason,the optimum conditions chosen to investigate was at 2.5 mTorr and when the magnetron wasat a distance of 30 mm. For this setting, a new series of measurement was performed where abias voltage was applied in order to see what effect it has on the stress and if it can minimiseit.

6.2 Other Relevant Results

The results that are included in section 5.2, were observations that unfortunately were dis-covered towards the ending of this diploma work. Hence, there was not so much time left tofurther investigate how much effect these might have on the results. It can also be seen fromthe distance/pressure series that the results not always are repeatable and this could be dueto the following aspects discussed below.

In figure B.1, it can be seen that when the base pressure in the MC prior to deposition is high(low vacuum), 9.9E-7 mTorr, there is a lot of noise in the data and the stress level is higher atalmost 150 MPa. When the base pressure however is two orders of magnitude lower (highvacuum), 3.2E-9 mTorr, the stress evolution seem to be more stable and with less noise. Thenoise in the turning point of the red plot is due to the poor laser intensity, read section 6.5.This could be due to that when making depositions, it is important that there is high vacuumin the MC in order to minimise ”interference” of other background gases or contaminants.Since there is a large difference in base pressure between the two depositions, that mighthave an influence on the stress and the deposition process, as can be seen. The depositionwhere its stress data is given by the blue plot (the higher base pressure), was made afterthere was some maintenance done on the system. For that the MC was vented and thatcould lead to contaminants and other particles being present in the chamber. It was alsodiscovered very late in the project that it is very important to clean sputter before doing anymeasurements. To clean sputter is simply to put in a sample and sputter for 10-20 minutes toremove any oxidation present on the target and to minimise the contaminants in the chamber.

Figure 5.10 shows the effect of the heater temperature. However, these changes can also bedue to temperature changes that occur during deposition from the radiation of the target.When the magnetron has been used for a longer time, or when many depositions have beenmade after one and another, it can get warmer in temperature. The power has been 300 Wfor all depositions done in this project and the sample size is 2x2 cm, which gives a powerdensity of 75 W/cm2. The samples also have a thickness of 155 µm and this can lead to thembeing heated up fast. The difference of the initial growth and the so called ”turning point”

35

could also be influenced by condensation. When putting in a new sample in a system thathas previously been running for a few hours, the substrate will have a colder temperaturethan its environment. This makes it easier for contaminants and other gas particles to stickto it.

As can be seen in figure 5.11, there is a slight difference between the stress evolution whenthe argon is opened in the MC and when it is opened at the target (see also section below ofwhy it was chosen to be opened at the target at the end of the project). However, it is difficultto say if this difference is because of the difference of argon location, when there has been alack of repeatability in the data. If the argon is opened at the target, it should give a higherconcentration of particle bombardment towards the substrate. According to theory, if there ismore high energetic bombardment of target particles, the compressive stress should increase(lower turning point). This does not fit together with the trend seen in figure 5.11. In figureB.3 it be seen that the slope of the stress*thickness data is lower for the red curve than it is forthe blue curve, indicating that the stress should also be lower. It is however difficult to sayanything about the stress at 1 µm since the deposition for the blue curve is only for half ofthe time than the red.

Another reason why it is difficult to have repeatable data could be because of the cleannessof the sample. As at least the initial part of stress evolution for all depositions is compressive(see figures in the results), it might be more sensitive to the surface. The nucleation processis what ”creates” the compressive stress and if there are particles and contaminants presenton the surface, that might lead to the data behaving different for different depositions, evenif the deposition settings are identical.

6.3 XRD - Curvature Measurements

The two samples that were chosen were samples which one in the MOS stress measurementshad shown to have a tensile stress at the end and the other had shown to have a compressive,see figure 5.14. The XRD measurements showed clearly that the slope of the curves 1/Rwas negative for the compressive and positive for the tensile, see figure 5.13. This was anindication that the MOS and the XRD are somewhat in agreement, even if the values of thestresses are not very alike. For the compressive stress, the values do not differ a lot, but forthe tensile there is a difference of 100 MPa. One reason for this change could be that the ratedeposition for the red plot in figure 5.14 was never calculated from an XRR measurement dueto time limitation, but instead estimated to a value that made sense in accordance with tableA.1. Another reason of the changes of stress between the different methods could be thattitanium reacts easily with oxygen. This means that when the sample is taken out from theLL, a layer of oxide will form on the top which also could affect the stress. It can however beconcluded that XRD is an alternative way to calculate the average stress in the material, evenif it does not show how it evolves as MOS does.

6.4 The Bias Series

As mentioned in the results, the reason why the stress for figure 5.15 and 5.16 is not plotted isbecause of the deposition rates not being measured due to time limitation. However, there aresome conclusions that can be drawn from the stress*thickness plots. The difference betweenthe depositions is the location of the opening of the argon, when the bias was applied in thedeposition process and the deposition time. For figure 5.15, there is a clear relaxation in thecurves when the bias is applied and the larger bias gives a more relaxed curve. For figure5.16, there was a small relaxation when the bias was applied, but after a while the stress

36

continues to grow with the same rate as it was before. It is difficult to say if the reason for thiswas the location of where the argon was opened. The behaviour might however be due to atwhich stage/time of the deposition when the bias is applied. When it is applied during thebeginning (figure 5.15), the voltage might have a larger impact on the stress evolution. As thefilm grows thicker (figure 5.16) it also becomes more porous and the bias might not have asgood of an effect. The idea was to apply the bias just before the stress turns zero to relax theevolution and keep it close to zero. However, if the plan is to grow thicker films, it might bea solution to apply the bias in the beginning.

6.5 Instrumental/Equipment Trouble

The reason why it was chosen to work with argon opened at the target towards the end ofthe project is because the magnetron shut off during deposition when the argon was openedin the MC. There might have been an electrical short circuit created in the magnetron due toparticles/flakes falling between the anode and the cathode. The deposition process for whenthe argon was opened in the MC was even harder when lower working pressures (1.5 and2.5 mTorr) and a smaller distance between magnetron and sample (15 and 30 mm) was used.This made it hard to make depositions at those settings. Changing the location of the argonopening makes it harder to compare the newer results to older ones done in this thesis work.

In some of the plots (the black curves in the distance series), there are some noise in thedata. This is due to the poor laser quality that was used when measuring the differential spotspacing during depositions. The quality of the laser got worse as more depositions were donesince the windows of the main chamber (from where the laser hit the sample and the samplereflected it onto a mirror outside the chamber via another window) got also deposited. Thiscreated something that looked like a mirror and made the registration of the laser spots bythe camera very poor. As the windows were clean, the laser intensity also became betterwhich provided data that did not have any noise.

6.6 Improvements for Future Work

Since it was shown that the argon location at the target was a better alternative than openingit in the MC, smaller distances and lower pressures can now be used to minimise the stresseven further. An alternative could be to work with 15 and 30 mm for 1.5 mTorr instead.

It was also shown that applying a bias can indeed reduce the stress, as predicted by theorysince the target ions have larger kinetic energy. However, a solution is to not wait for longbefore turning the bias on and do it in the beginning of the deposition process as can be seenin figure 5.15.

Since there is difficult to obtain repeatability in the data it is very important to do depositionsunder the same conditions. If one series of measurements is going to be done:

• The samples used for depositions should be cleaned at the same time and in the samemanner.

• The base pressure in the MC should be stable and be more or less the same prior toall depositions. The base pressure should then have the same order of magnitude pre-deposition.

• The heater temperature in the main chamber should be similar prior to every depositionand therefore it is not favourable to do the entire series in the same day as it mightincrease the temperature in the MC.

37

• Clean sputter the system for approximately 15 minutes prior to your deposition.

• It is also important to inspect the films on a nanoscale level to see the crystal structureand to see if defects or voids are present. TEM is then a powerful technique to use sinceit also can show how dense the film is.

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Chapter 7

Conclusion

When using higher pressures and a larger distance between the magnetron and the sample,the stress evolves faster towards tensile and has higher values at around 120 MPa. To obtainminimal residual stress, it is then more favourable to use lower pressures and smaller dis-tances as higher energetic bombardment of target particles will make the stress grow morecompressive and the stress evolution towards higher values occurs later in the depositionprocess. This will give lower stress values towards the end, <40 MPa. The stress evolvestowards the same value when higher pressures and larger distances between magnetron andthe sample are used. Applying a bias will relax the stress evolution, if the bias is applied dur-ing the initial stage of the deposition. This could be used to minimise the residual stress in thefilm. Due to lack of repeatability in the data it is crucial for future work that depositions aremade under the same conditions. The effect of base pressure in the MC, temperature changesin the chamber but also bad sample cleaning and contaminants in the MC can all have aneffect on the nucleation and coalescence process as the film grows.

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Appendix A

Deposition Rates and XRR Data

The table below shows the rate measurements. The thickness of a film obtained with the XRRwas divided with the time for the deposition in order to calculate the rate.

Table A.1: The rates of the depositions given by thickness measurements with XRR and divided bytheir respective deposition time. The unit of the rates are nm/s.

15 mm 30 mm 50 mm 70 mm1.5 mTorr 0.269 0.191 0.161 0.1322.5 mTorr 0.254 0.19 0.172 0.1353.5 mTorr 0.243 0.1896 0.18 0.1354.5 mTorr 0.235 0.18 0.18 0.134

The figure below shows the fringes that were analysed with a software program to estimatethe thicknesses of the film, see section 2.5.1 for theory. This figure however is related to thework done previous to this master thesis.

Figure A.1: The thickness of a film can be measured by analysing the fringes of the plot. Theory of thisis provided in section 2.5.1.

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Appendix B

Additional Results

B.1 Stress*thickness Plots

Figure B.1: Stress*thickness evolution for the measurements where there is a indifference in the basepressure. 70 mm, 1.5 mTorr. Look at figure 5.9 for stress data.

Figure B.2: Stress*thickness evolution for the measurements where there is indifference in the temper-ature in the MC. 30 mm, 2.5 mTorr. Look at figure 5.10 for stress data.

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Figure B.3: Stress*thickness evolution for the measurements where argon is opened at the target andfor when it is opened in the main chamber. 30 mm, 2.5 mTorr. Look at figure 5.11 for stress data.

B.2 Data for XRD Plots

The data from the following tables were used to plot the graphs in figure 5.13, in order toevaluate the value of the slope 1/R.

Table B.1: This data is used to plot the blue curve in figure 5.13.

X [mm] ω [rad]6 0.60184 0.60202 0.60230 0.6024-2 0.6026-4 0.6029-6 0.6031

Table B.2: This data is used to plot the red curve in figure 5.13.

X [mm] ω [rad]8 0.60486 0.60434 0.60402 0.60360 0.6033-2 0.6029-4 0.6025-6 0.6021-8 0.6018

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