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A Thesis Entitled Whisker Growth Induced by Gamma Radiation On A Thesis entitled Whisker Growth Induced by Gamma Radiation on Glass Coated with Sn Thin Films by Morgan Killefer Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Physics and Astronomy _________________________________________ Dr. Diana Shvydka, Committee Chair _________________________________________ Dr. Victor Karpov, Committee Chair _________________________________________ Dr. Richard E Irving, Committee Member _________________________________________ Dr. Amanda Bryant-Friedrich, Dean College of Graduate Studies The University of Toledo August 2017 Copyright 2017, Morgan Killefer This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of Whisker Growth Induced by Radiation on Glass Coated with Sn Thin Films by Morgan Killefer Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Physics and Astronomy The University of Toledo August, 2017 Metal whiskers (MWs) represent hair-like protrusions on surfaces of many technologically important materials, such as Sn, Zn, Cd, Ag, and others. When grown across leads of electrical components, whiskers cause short circuits resulting in catastrophic device failures. Despite cumulative loss to industry, mostly through reliability issues, exceeding billions of dollars, MWs related research over the past 70 years, brought more questions than answers. Moreover, the absence of reliable accelerated life testing procedures makes it especially difficult to evaluate whisker propensity with tests limited in time. A recently developed theory about electric fields being the cause of MW growth holds a promise of shedding light on their fundamental nature. Its main statement is that nucleation and growth of MWs happen in response to local electric fields acting on metal films. We adopted an approach of generating electric fields through charged defects created in insulating glass substrates supporting Sn metal films. These defects are produced under ionizing radiation of gamma-rays. Use of ionizing radiation for generation of electric fields may be preferable to a simpler capacitor-type setup, which iii requires a second electrode, often leading to shorts due to whisker growth through the capacitor air gap. We observed accelerated MW growth upon exposure of Sn metal film samples deposited on glass to Ir-192 gamma-ray source. The source, having its highest photon energy below 1MeV, is not able to produce structural changes in the material, making the substrate charging the only effect responsible for stimulation of MW growth. Qualitatively, we observed that after applying up to 20 kGy radiation dose to Sn thin film coated glass over a course of approximately 60 days, both the whisker densities and lengths increased significantly compared to control samples. Using a parameter of acceleration ratio, characterizing whisker growth rate in radiation exposed vs. control sample, we are able to offer a quantitative assessment of whisker growth enhancement. Our observations offer insights into whisker physics and a possibility of development of non-destructive accelerated test desperately needed in multiple industrial applications. iv I dedicate this work to my family, who love me unconditionally and have supported me in my every endeavor. Thank you. Acknowledgements I would like to thank a number of people for the support and assistance throughout my time at the University of Toledo. Firstly, I would like to thank my advisor, Dr. Diana Shvydka, for all of her encouragement and insight. She is a magnificent mentor and teacher. A warm thank you to the members of my committee, Dr. Victor Karpov and Dr. Richard Irving. Your time and help in completing my Master’s is appreciated. Finally, a thank you to the whisker group. Your assistance was immensely valuable to this research. v Table of Contents Abstract .............................................................................................................................. iii Acknowledgements ..............................................................................................................v Table of Contents ............................................................................................................... vi List of Tables ................................................................................................................. viii List of Figures .................................................................................................................... ix List of Abbreviations ......................................................................................................... xi List of Symbols ................................................................................................................. xii 1 Introduction and Literature Review .........................................................................1 1.1 Whisker Overview ............................................................................................1 1.2 Photoelectric Effect ...........................................................................................6 1.3 Ionization through the Compton Effect ..........................................................10 2 Materials and Methods ...........................................................................................11 2.1 Samples.... .......................................................................................................11 2.2 Treatment Planning in BrachyVision ..............................................................12 2.3 Experimental Set-Up ........................................................................................16 2.4 Irradiation. .......................................................................................................17 2.5 Imaging.... .......................................................................................................18 3 Results and Discussions .........................................................................................22 3.1 Sample 1... .......................................................................................................22 vi 3.2 Sample 2.... ......................................................................................................26 3.3 Sample 3.... ......................................................................................................32 3.4 Discussions .....................................................................................................35 4 Conclusions.... ........................................................................................................37 4.1 Conclusions .....................................................................................................37 References ..........................................................................................................................40 vii List of Tables 1 Sample 1 MW densities and lengths based on percentage of dose received .........23 2 Sample 2 MW densities and lengths for both the irradiated and control ...............28 3 Sample 3 lengths and densities of MWs for irradiated and control sample ...........33 viii List of Figures 1 Zinc whiskers on a hot dipped galvanized steel pipe ...............................................2 2 Top Diameter comparison of a hair to a metal whisker ..................................................3 2 Bot Whisker on Sample 1 ...............................................................................................4 3 Sketch of photoelectric effect ..................................................................................7 4 Sketch of Compton scattering ..................................................................................8 5 Schematic showing defects, or holes, created following Compton interaction .......9 6 Line profile for a 6 cm sample centered over a source at 3 cm .............................13 7 Sketch of Ir-192 source in VariSource ix HDR afterloader ...................................14 8 Ir-192 energy spectrum ..........................................................................................14 9 a Sketch of sample geometries for: Sample 1 ...........................................................15 9 b Sketch of sample geometries: Sample 2 ................................................................15 9 c Sketch of sample geometries: Sample 3 ................................................................15 10 L Image ill. the setup with the catheter from the VariSource iX emerging ..............16 10 R A view from above showing Sample 2 ..................................................................16 11 Plan from BrachVision for Sample 2 with isodose curves and dwell times ..........18 12 Schematic of typical SEM .....................................................................................19 13 ImageJ software interface ......................................................................................21 14 a SEM images of Sample 1 at: 0 kGy .......................................................................22 14 b SEM images of Sample 1 at: 5 kGy .......................................................................22 ix 14 c SEM images of Sample 1 at: 10 kGy .....................................................................23 15 Data from Table 1 displayed graphically ...............................................................24
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