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Physical and Chemical Properties of Ambient Temperature Sputtered Silicon Carbide Films

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Physical and Chemical Properties of Ambient Temperature Sputtered Silicon Carbide Films PHYSICAL AND CHEMICAL PROPERTIES OF AMBIENT TEMPERATURE SPUTTERED SILICON CARBIDE FILMS by DANIEL THOMAS SHELBERG Submitted in partial fulfillment of the requirements For the degree of Master of Science: Engineering Thesis Adviser: Dr. Chung-Chiun Liu Department of Chemical Engineering CASE WESTERN RESERVE UNIVERSITY May, 2010 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis/dissertation of ______________________________________________________ candidate for the ________________________________degree *. (signed)_______________________________________________ (chair of the committee) ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ (date) _______________________ *We also certify that written approval has been obtained for any proprietary material contained therein. Copyright © 2010 by Daniel T. Shelberg All right reserved Table of Contents List of Tables………………………………………………………………… 2 List of Figures……………………………………………………………...... 3 Acknowledgements………………………………………………………… 5 List of Abbreviations……………………………………………………….. 6 Glossary………………………………………………………………………. 7 Abstract………………………………………………………………..…….... 8 Chapter 1: Introduction...…………………………………………………... 9 Chapter 2: Theoretical Background……………………………………… 11 Chapter 3: Deposition Conditions……………………………………….. 15 Chapter 4: Chemical Properties 4.1 Composition and Bonding…………………………………….. 16 4.2 Chemical Resistance…………………………………………… 24 Chapter 5: Physical Properties 5.1 Hardness and Young’s Modulus..…………………………….. 36 5.2 Flexibility and Film Adhesion.………………………………… 40 5.3 Resistance to Moisture Diffusion.……………………………. 45 Chapter 6: Conclusions and Recommendations………………………. 56 Appendix……………………………………………………………………… 57 References…………………………………………………………………… 58 Page 1 List of Tables Table Page Table 1. Diffusion coefficients for Kapton 53 and SiC A.1 Trace elements in sputter target 56 A.2 Nanoindentation data 56 Page 2 List of Figures Figure Page Figure 1. Sputtering process illustration 12 Figure 2. Typical XPS of SiC film shows a carbon rich film with 16 oxidation at the surface. Figure 3. XPS of Si 2p peak in SiC film at the surface and after a 1 18 minute sputter. A chemical shift in this peak can be seen between the two layers. Figure 4. XPS of C 1s peak in SiC film at the surface and after a 1 18 minute sputter. A chemical shift in this peak can be seen between the two layers. Figure 5. Sputter yield data for Si and C from nuclear tables. This 19 does not reveal why the films in this study are 3:2 carbon to silicon. Figure 6. Depth profile of SiC / Pt interface shows the thickness of this 21 interface. Figure 7. Uncoated and SiC coated USB flash device used in 24 saltwater corrosion study. Figure 8. Initial saltwater corrosion results show heavy corrosion of 26 one contact point for an uncoated device. Figure 9. Second saltwater test results, main corrosion points on both 28 devices. Figure 10. Image of potassium hydroxide (KOH) etch damage of SiC 30 film on silicon wafers. Figure 11. SEM secondary electron image of 400W etched SiC film 32 on a silicon wafer. Figure 12. SEM backscattered image of 400W etched SiC film on a 33 silicon wafer. Figure 13. XPS of etched 400W sample, carbon peaks show a large 34 chemical shift, and potassium peaks from residual etchant. Figure 14. Loading and indentation curves for nanoindentation of SiC 36 on silicon wafer. Page 3 Figure 15. Image of SiC film on Kapton, which shows SiC causes the 40 Kapton to curl into itself. Figure 16. Diagram for bending adhesion test. 40 Figure 17. SEM images for bending test around 20 AWG wires. Only 41 compressive stress results in cracking. Figure 18. SEM image for bending test around 28 AWG wires. Only 41 compressive stress results in cracking. Figure 19. SEM image for complete bending in tensile and 42 compressive direction. Figure 20. Moisture diffusion setup. The difference in relative humidity 44 is the driving force and is measured over time. Figure 21. Moisture diffusion test chambers. 45 Figure 22. Moisture diffusion through Kapton results in a diffusion 50 coefficient on the order of 10 Figure 23. Moisture diffusion through SiC coated Kapton results in a 52 diffusion coefficient on the order of 10 Page 4 Acknowledgements I would like to thank Dr. Chung Chiun Liu for his support and advice throughout my research. Additionally I would like to thank David Greer and Shubin Yu of the Electronics Design Center for sputtering samples, and Laurie Dudik of the Electronics Design Center for engineering support. I would also like to thank Wayne Jennings of the Swagelock Center for Surface Analysis for help in performing XPS. Page 5 List of Abbreviations and Symbols A – constant in power law relation 2 A(hc) – contact area as a function of contact depth (nm ) A1 – integration constant AFM – atomic force microscopy a – constant used in similarity transform b – constant used in similarity transform C – concentration of water (mol/L) – initial concentration of water on dry side(mol/L) – concentration of water on wet side(mol/L) – diffusion coefficient of water through a medium (m2 sec-1) - prefactor for Arrhenius diffusion (m2 sec-1) 2 -1 – diffusion coefficient of water through Kapton (m sec ) 2 -1 - diffusion coefficient of water through SiC (m sec ) - transform variable (seca mb) E – Young’s modulus Er – reduced modulus EPMA – electron probe microanalysis Θ - concentration difference ratio F – force (µN) Fmax – maximum force (µN) h – displacement (nm) H – hardness (GPa) hc – contact depth (nm) hf – constant in power law relation (nm) hmax – maximum displacement k – Boltzmann constant 8.617 10 ⁄ KOH – potassium hydroxide LED – light emitting diode m - constant in power law relation MOSFET – metal oxide semiconductor field effect transistor ν – Poisson’s ratio PECVD – plasma enhanced chemical vapor deposition PVC – polyvinyl chloride r - generation rate of water (mol L-1 sec-1) S – contact stiffness t – time (sec) T – temperature (K) SEM – scanning electron microscopy SiC – silicon carbide USB – universal serial bus V – humidity sensor voltage (V) – initial humidity sensor voltage on dry side(V) – humidity sensor voltage on wet side SiC/Kapton interface (V) – humidity sensor voltage on dry side Kapton/SiC interface (V) – humidity sensor voltage on dry side SiC/Air interface (V) – humidity sensor voltage on wet side(mol/L) XPS – x-ray photoelectron spectroscopy z – distance along the cylindrical z-axis Page 6 Glossary Atomic Force Microscopy (AFM): Surface imaging technique that uses an atomically sharp tip to probe a sample’s surface. AFMs are sometimes fitted with a diamond probe that can be used to make indentations or scratches in the sample. Chemical Shift: The shift of a peak’s intensity relative to a reference state due to the nature of the bonded atoms. Compressive Stress: Stress in which the material is under load and deforms toward its center. Electron Probe Microanalysis (EPMA): Surface analysis technique that involves using a focused electron beam to generate element characteristic x-rays from a target. Quantification is possible but depends on x-ray yield of target and quality of detector. Kapton: A polyimide manufactured by DuPont that is often used in flexible circuitry. Nanoindentation: Technique that uses an atomically sharp diamond tip to load the target with a specified amount of force and measure indentation depth. From this data the hardness and Young’s modulus can be calculated. Relative Humidity: The ratio of the amount of the current water vapor partial pressure in the air to saturated partial pressure at a specified temperature. Scanning Electron Microscopy (SEM): A surface imaging technique that uses secondary or backscattered electrons from a focused electron beam to image a target. Secondary electrons are dependent more on topography and backscattered electrons are dependent on nuclear density. Sputtering: The bombardment of a target material with ions which in turn releases target atoms. Tensile Stress: Stress in which the material is under a force and is deformed away from its center. X-ray Photoelectron Spectroscopy (XPS): A surface analysis technique that involves bombarding a target with x-rays which produces photoelectrons and Auger electrons. The binding energies of the photoelectrons are evaluated which are element and bond characteristic. Sputter depth profiling is often available but is sample destructive. Page 7 Physical and Chemical Properties of Ambient Temperature Sputtered Silicon Carbide Films Abstract by DANIEL THOMAS SHELBERG Silicon carbide is known for its hardness, chemical resistance, and moisture barrier properties. This study demonstrates the effectiveness of silicon carbide films deposited at ambient temperatures. Nanometer scale films showed excellent moisture resistance due to a small diffusion coefficient. They demonstrated high hardness which indicates favorable wear resistance. Chemical resistance was found to be particularly good at room temperature, and higher temperature tests revealed the possibility of engineering better films. The films have been shown to be extremely flexible, smooth, and pinhole free. Therefore, nanometer scale silicon carbide films have exceptionally good properties for use as a protective coating in electronic devices. Page 8 Chapter 1: Introduction Silicon carbide is known for its hardness, moisture
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