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A Thesis

entitled

Whisker Growth Induced by Gamma Radiation on Glass Coated with Sn Thin Films

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

Whisker Growth Induced by Radiation on Glass Coated with Sn Thin Films

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.

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.

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

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

14 c SEM images of Sample 1 at: 10 kGy ...... 23

15 Data from Table 1 displayed graphically ...... 24

16 Results at 10 kGy for Sample 1...... 25

17 Frequency counts for Sample 1 at 0, 5, and 10 kGy ...... 26

18 a SEM images of: sample before irradiation ...... 27

18 b SEM images of: after exposure to 10 kGy dose ...... 27

18 c SEM images of: longer metallic whiskers ...... 27

18 d SEM images of: very small whisker ...... 27

18 e SEM images of: three whiskers ...... 27

18 f SEM images of: control sample 60 days post-deposition ...... 27

19 a Data from Table 2 shown graphically, where (a) shows whisker density ...... 28

19 b Data from Table 2 shown graphically, where (a) shows whisker length ...... 28

20 a Frequency of: whisker density in Sample 2 ...... 29

20 b Frequency of: whisker length in Sample 2 ...... 29

21 a Frequency counts of whisker density for Sample 2: shown for 3 areas exposed ..30

21 b Frequency counts of whisker density for Sample 2: data for all areas combined ..30

22 Frequency of lengths for Sample 2 after 10 kGy and after 20 kGy ...... 31

23 Frequency of lengths for Sample 3 after 10 kGy and after 20 kGy ...... 33

24 a SEM images taken of Sample 3 after: 10 kGy ...... 34

24 b SEM images taken of Sample 3 after: 20 kGy ...... 34

24 c SEM images taken of Sample 3 after 20 kGy + 30 days on the shelf ...... 34

24 d SEM images taken of: control after 60 days ...... 34

List of Abbreviations

Cd ...... Cadmium DCC ...... Dana Cancer Center DI ...... De-ionized HDR ...... High Dose Rate MW ...... Metal Whiskers Pb ...... Lead RoHS ...... Restriction of Hazardous Substances SEM ...... Scanning Electron Microscope Sn ...... Tin TEC 15 ...... Pilkington TCO material TCO...... Transparent Conducting Oxide Zn ...... Zinc

List of Symbols

α ...... Acceleration factor Å ...... Angstrom γ ...... Gamma µm ...... Micrometers

Ci ...... Curie cm ...... Centimeter Gy ...... Gray kV ...... Kilovolts mA...... Milliamps mm ...... Millimeter N ...... Number of SEM images taken r ...... Radius R ...... Whisker Creation Rate RSPON ...... Spontaneous creation rate RSTIM ...... Stimulated creation rate s ...... Second tR ...... Irradiation time tS ...... Shelf time

xii

Chapter 1

Introduction and Literature Review

1.1 Whisker overview

Metal whiskers (MWs) represent hair-like protrusions observed on surfaces of many technologically important materials, such as tin (Sn), zinc (Zn), cadmium (Cd), and others. When growing across leads of electrical components, whiskers cause short circuits resulting in catastrophic device failures. Despite 70 year of research and cumulative loss exceeding billions of dollars to multiple industries, from airspace to automotive, there are still no reliable solutions to whisker mitigation problem. One of the standard approaches for Sn, for example, has been addition of lead (Pb) to soldering alloys. With adoption of the environmental Restriction of Hazardous Substances (RoHS) directive adopted in the European Union in 2003, (European Union, 2003) the problem has significantly aggravated through mandatory use of lead-free tin-based solders, consequently spread worldwide through production standards. Threat to multiple industries that rely on tin based soldering make the need for understanding, predicting, and mitigating metal whiskers even more urgent.

An additional complicating factor here is the absence of reliable accelerated life testing procedures. Typically, whisker nucleation and growth are rather unpredictable in 1

time and may take anywhere from weeks to years. Some tests limited in time, such as those suggested by Joint Electron Devices Engineering Council, (Brusse, et al, 2000;

Gaylon, et al, 2005) are perceived as too narrow and not revealing the future of the tested sample.

Research into the growth patterns of metallic whiskers began in the mid-1900’s.

In 1946, these single crystals, or whiskers, were found on electronic surfaces, causing a myriad of electrical failures ranging from channel-frequency filters to copper oxide rectifiers (Dunn, 1976). These malfunctions have cost billions of dollars, as well as creating reliability concerns in electronics. Due to the problems caused by whiskers in industries such as automotive and aerospace, researchers have continued to investigate this topic in the search to find effective ways to mitigate their growth.

FIG. 1. Zinc whiskers on a hot dipped galvanized steel pipe (“Metal Whisker Photo

Gallery”, 2012). 2

Interest in whiskers has increased due to the removal of lead from many products.

Lead mitigates the growth of whiskers and therefore alleviates many of the problems associated with their growth. Whiskers are typically linear or linear with kinks causing notable bends but can come in many shapes. Each whisker appears as a crystalline structure with striations lengthwise and diameters on the order of 2 to 5 microns, lengths ranging from 0.003 mm up to 10 mm resulting in aspect ratio (ratios of length to diameter) up to 104; they possesses the ability to carry currents up to 10 mA (Dunn,

1976).

FIG. 2. Top: Diameter comparison of a hair to a metal whisker (NASA, 2015). Bottom:

Whisker on Sample 1. Length: 68.11 µm. Diameter: 0.47 µm.

Several factors are commonly considered correlating with whisker propensity: mechanical stresses, stress gradients, humidity, temperature, surface contaminations, and grain sizes. However, the fundamental question of what is the physics underlying those correlations and what ultimately leads to whisker growth remains unanswered, as some of the nominally identical samples exhibit no whiskers while others show significant whisker infestations. One of the proposed mechanisms points towards mechanical stress

(Shvydka and Karpov, 2016) relaxing during whisker growth and thus providing the necessary driving force for whiskering. On the other hand, it was inferred that the stress gradient rather than stress itself is the driving force behind whisker development

(Sobiech, 2009). Local recrystallization regions (Boettinger, 2005) and intermetallic compounds (Sarobol, 2013) have been referred to as possible stress sources. 4

Unfortunately, all these approaches lack predictive power, providing no estimates for whisker growth rates and parameters.

In recent years, an alternative theory has suggested that electric fields could be the driving force of these growths, due either to surface imperfections (charge patches) or external fields (Karpov, 2014; Shvydka and Karpov, 2016). For example, impurities in a sample can create holes, which allow for a charge to appear naturally in the sample.

Stresses and oxidation show that increased whisker growth (Barsoum et al, 2004) can be attributed to electric fields as well, since the oxidation creates stress in the plating, which in turn increases the electrical potential on the surface (Filimonov, 2004). To test this theory several experiments involving application of electric field in either capacitive setup (Vasko et al., MRS Comm 2015), or through generation of charged defects in insulating substrates underneath metal film (Niraula, 2016; Vasko et al., JAP 2015) were conducted, successfully demonstrating the effect. In this latter approach, 6 MeV electron beams were used in order to induce an accelerated growth of whiskers by a factor of approximately 200, which was done by accumulating charge defects in the insulating substrates subjected to ionizing radiation (Vasko, et al, JAP 2015). This created an electric field perpendicular to the film surface, ultimately resulting in the aforementioned whisker acceleration factor. The same group has also demonstrated accelerated whisker growth on Zn-plated samples under the 10 kV electron beam of a Scanning Electron

Microscope (SEM) under condition of charge accumulation in un-grounded sample.

(Niraula, 2016) Similar observations of whisker growth under SEM beams were reported for other metals (Song, 1996; Mizuhara, 2002; Miyake, 2007).

While observations of high energy electron beams producing or greatly enhancing whiskering have been observed several times, to the best of our knowledge, there is only one short report of the effects of photon irradiation on whisker growth, published about

50 years ago (Killefer, 2017). Their authors irradiated a tin-plated sample with 50kVp x- rays for 16 hours. No whiskers were observed immediately after irradiation; the sample was inspected 2 month later, when the irradiated area was established to develop a dozen of whiskers. No whiskers were found over the shielded area. While the authors pointed out at the inability of the x-ray source used to produce atomic displacement in metal film, no alternative explanation was presented.

1.2 Interactions of high-energy photons with matter

For photons with energies in the range of 1keV to 1 MeV there are three mechanisms of interaction with matter: coherent (Rayleigh) scattering, photoelectric effect, and incoherent (Compton) scattering. Coherent scattering results in photon deviation at a small angle without energy transfer, and will not be considered.

In the process of photoelectric interaction, an incoming photon is fully absorbed by an atom, resulting in ejection of an electron from one of the inner atomic shells. The hole created in that shell is filled with electrons from the outer shells.

FIG. 3. Sketch of photoelectric effect. A γ-ray collides with electron in inner shell of atom. This electron is ejected, and one of the electrons from an outer shell falls down to take its place.

Discovered by Arthur H. Compton in 1923, Compton Scattering is when an incident photon collides with an electron, rebounding the electron at angle ϕ and the photon at angle θ, illustrated in the figure below.

FIG. 4. Sketch of Compton scattering. Photon collides with electron, resulting in one recoiled electron and a scattered photon.

In Compton scattering, when an x-ray or γ-ray collides with an electron, the electron is recoiled at some angle, while the resultant photon loses energy. This relationship follow as

ℎ � − � = (1 − ��) �� where λ is the incident photon, λ’ is the resultant photon, h is Planck’s Constant, � is the rest mass of an electron, c is the speed of light, and θ is the angle of the resultant photon.

FIG. 5. Schematic showing defects, or holes, created following Compton and photoelectric interaction in sample. *Not to scale.

By using a line source, such as a high dose rate (HDR) seed, it is possible to create regions that experience different electric fields strengths within the same delivery.

This is visualized with a line profile of the delivered dose. Based on this, is possible to predict how much growth will be seen in any given region. Areas receiving the maximum dose are expected to see the most growth, while the growth is anticipated to decay as the distance from the source increases.

Both photoelectric and Compton scattering interactions are capable of producing ionizations within irradiated sample, as shown in the figure above. A grounded sample with no defects would show an equal number of electrons and holes in the substance.

After being exposed to γ-rays that induce scatter, the number of holes would be greater

than the number of electrons due to ejection of electrons. These missing electrons leave holes behind, inducing a charge in the sample substrate.

1.3 Electrostatic Charging by Ionizing Radiation

The phenomenon of material charging in the field of ionizing radiation is well known and often considered detrimental (McIlwain, 1975); practically useful applications, however, include radiation assisted, particularly e-beam assisted lithography

(Andersen, 1979). When a thick substrate is irradiated with a high-energy electron or photon beam, a fraction of electrons (secondary electrons generated under photon beam irradiation) is stopped in the substrate (e.g., the average range of a 1 MeV electron is ~0.2 cm in glass). For a thick glassy substrate, the embedded electrons may take a long time to move to ground or recombine, resulting in local charging possibly sustained for prolonged time periods. The polarity of charge accumulated in general depends on the chemical content of impurity in a glass material, as does the amount of charge per area, leading to accumulation of different impurities (McIlwain, 1975).

The x-ray and g-ray induced charging can be significant, in spite of the neutrality of that radiation agent, due to knocking off the electrons from the substrates. The electric fields induced through substrate charging fall in the range of MV/cm (Ezz-Eldin, 1994) corresponding to those predicted as whisker accelerating (Karpov, 2014). In this work, we utilize the effect of glass substrate charging to create external electric filed in the direction perpendicular to the substrate surface, and acting on thin films of tin (Sn), deposited on substrates.

Chapter 2

Materials and Methods

2.1 Samples

Samples of Sn thin films were deposited on 3 mm thick soda-lime glass coated with transparent conducting oxide (TCO, specifically, SnO2:F with nominal 15

Ohm/square sheet resistance; TEC-15 glass from Pilkington). These types of substrates are routinely used in our labs for photovoltaic applications and are known to support good film adhesion with various deposition approaches.

The first sample was rf-sputtered at room temperature on ~3x6 cm2 substrate to have Sn film thickness in the range of ~200 nm. This sample was divided into three regions by scribing the glass such that the central region would receive 100% of the total dose and the two outer regions would receive 25% of the dose delivered. This test was a preliminary proof-of-concept trial, serving to verify that whisker growth enhancement can be achieved under high-energy photon irradiation. Non-uniform irradiation pattern

(line source going through the center of the sample) and presence of scribes was used to show whisker growth as a function of electric field, different in central vs. outer regions of the sample. No control was used in conjunction with this sample.

The second sample is a 3x6 cm2 piece of TEC 15 with Sn of thickness 250±25 nm evaporated at the rate of 2-4 Å/s. The TEC 15 substrate was rinsed with DI water, dried with nitrogen, then an ultra-sonication bath in acetone for 20-25 minutes, dried again with nitrogen, followed with an ultra-sonication bath in DI water for the same amount of time and finally dried once again before applying the mask and evaporation. A mask was used such that 6 metallic regions were formed over a continuous TCO layer. The two innermost regions received 100% of the prescribed dose, the second and fifth regions received 40%, and the outermost regions received 20% of the dose. The regions were created through vacuum evaporation of 99.999% pure Sn from tungsten boats onto the substrates. The fabrication steps for this sample follow the protocol described by Borra, et al (2016). A control sample of Sample 2 was created simultaneously, identical to the second sample but without the mask during deposition.

The third sample was deposited under identical conditions to Sample 2, but was deposited onto a 1 mm thick microscope slide with a mask with five defined regions. In order to keep conditions the same, the slide was placed on a stack of identical slides, such that the thickness is the same as the TEC 15 and TCO samples, 3 mm. The innermost region received 100% of the dose, the second and fourth regions 35%, and the outermost regions 10% of the delivered dose. The control sample of Sample 3 is identical to

Sample 3, but has no mask. All 3 sample configurations are shown in what follows.

2.2 Planning Treatment in BrachyVision

Values for percentage of the dose delivered to a given region were pre-planned in

BrachyVision, commercial treatment planning system, targeting glass substrate to receive 12

a prescribed dose level of 5, 10, or 20 kGy. The overall dose delivery pattern is nonuniform in the direction perpendicular to the line source, following the rule that intensity

1 fall-off as � for a cylindrical source (Jons, 1983), where r is the distance between the source and the point of interest.

FIG. 6. Line profile for a 6 cm sample centered over a source at 3 cm. Dose drops off at the expected rate of 1/r, where r is the radius from the cylindrical source.

The Teflon holder was scanned using the Phillips Gemini Big Bore TF CT scanner at University of Toledo Dana Cancer Center (DCC) and imported into

BrachyVision. In BrachyVision, the holder was defined as “body” and the sample was added as structures. Structures representing the regions on the surface are placed within the glass, since it the field is generated within the glass due to defects. Once all structures have been defined, the isodose curves are adjusted to the target prescription, and the dwell times are calculated for the current source.

The VariSource iX HDR afterloader at DCC contains an Ir-192 source; in our experiments the source activity of 10.395 Ci calibrated on January 5, 2017.

FIG. 7. Sketch of Ir-192 source used in VariSource iX HDR Afterloader. The central portion (polkadots) is the active Ir-192 of length 3.6 mm and width 0.65 mm. Exterior is the stainless steel capsule (diamonds) of dimensions 4.5 mm by 0.9 mm. Attached to the capsule is the stainless steel cable (diagonal lines) of diameter 0.7 mm.

Ir-192 is a poly-energetic source, decaying to Pt-192 with a half-life of 73.83 days, and emitting beta and gamma-particles. Beta-particles are absorbed within the source-encapsulating alloy. Gamma-rays have the average photon energy of approximately 380 keV (Borg and Rogers, 1999).

FIG 8. Ir-192 energy spectrum.

The Sn film is so thin that the photon attenuation is negligible, but the effective attenuation coefficient (µ) for the glass is 0.24 cm-1, leading to ~7% of attenuation for

3mm-thick glass substrates. The source is connected to a wire that is attached to the

VariSource iX afterloader. It runs through the catheter to the predetermined dwell 14

positions, rests at those locations, and then moves to the next position until it has completed all dwell positions, and delivered the pre-planned dose. Dwell positions are 5 mm apart along a straight-line opening, drilled through the center of the Teflon holder.

The source then returns into the VariSource iX safe that is shielded in order to prevent radiation leakage when not in use.

FIG. 9. Sketch of sample geometries for (a) Sample 1 (b) Sample 2 (c) Sample 3. Line source denoted in red, where dose falls off at 1/r. For each sample, a strip has corresponding isodose curves denoted in other colors indicating the percentage of the total dose delivered to each region. (Killefer, Borra, et. al, 2017).

The selection of a total 10 and 20 kGy final dose was made based on the research done by accelerating whisker growth via 6 MeV electron beam (Vasko, Warrell, et. al,

2015).

2.3 Experimental Set-Up

The setup for radiation was the same for each sample. A catheter of length 139.9 cm was run from channel 1 of the Varian VariSource iX afterloader to a Teflon holder.

The holder has a cylindrical hole machined with a diameter of 2 mm, located 1.5 mm below the surface. The sample was placed on top of this holder so that the central region would be centered across this hole. The catheter, run from the VariSource iX to the

Teflon holder and through the hole was secured with paper tape such that the catheter will not move and the source moves through the entire diameter of the Teflon holder.

FIG. 10. Left: Image illustrating the setup with the catheter emerging from the

VariSource iX afterloader and run through the Teflon holder. Sample 1 is placed on top, centered above the catheter, ready for treatment. Right: A view from above showing

Sample 2, tape marking the location where the sample is to be placed each time and tape securing the catheter inside of the Teflon holder.

2.4 Irradiation

The glass is orders of magnitude thicker than the thin metallic film; therefore, almost all interactions with gamma-rays take place within a substrate. Resulting secondary electrons get knocked out of the substrate, leading to the charge build-up. The glass becomes positively charged, therefore inducing an electric field in the direction perpendicular to its surface, and acting on the Sn film while under radiation.

Irradiation was completed by using the VariSource iX afterloader and utilizing the plan created in BrachyVision. The plan was imported into the computer at the treatment station, and initiated. Each plan had a calculated number of fractions to be delivered to reach 5 kGy, 10 kGy or 20 kGy due to the limitations in place from the manufacturer (the doses in our experiments are orders of magnitude higher than those typically delivered in clinical treatments). The time for a single fraction cannot exceed 3000 seconds; therefore numerous fractions had to be delivered. Each fraction was a copy of the original fraction, such that the delivery is identical each time. The typical time for a single dose ranges from 35-45 minutes, depending on the decaying source strength. Due to the nature of the source, HDR Afterloader, and environment of a busy medical facility, the samples were irradiated in multiple sessions over several days. This was necessary to deliver up to 20 kGy dose, with each kGy taking 2-4 hours to deliver. 17

FIG. 11. Plan from BrachVision for Sample 2 with isodose curves and dwell times shown. The two innermost regions get 100% (innermost bright green), the second and fifth receive 40% (peach), and the outermost regions get 20% (outermost green) of the delivered dose.

2.5 Imaging

Scanning Electron Microscopy (SEM) is the main imaging technique used in this study. Instead of using light like traditional optical microscopes, the SEM uses electrons to form an image. Developed in the 1950’s, they offer larger field of depth, higher resolution, and high precision in magnifications of the images (Schweitzer, n.d.).

FIG. 12. Schematic of typical SEM.

Typical SEM’s have an electron gun, which focuses the electrons through the anode and magnetic lens. The scanning coil, backscattered electron detector, and secondary electron detector report back information about the scattered electrons to the computer in order to construct an image. SEM’s like the Hitachi S-4800 use low voltages and a Snorkel type objective lens for high resolution with large samples. This is particularly useful for imaging of semiconductor devices and still maintains low dose techniques (Hitachi).

Each sample was imaged several times during the process: once prior to irradiation to create a baseline for each sample and region, a second time once half of the dose had been delivered, a third time once all of the planned dose had been delivered, and a final time after sitting in laboratory conditions for 30 days following irradiation.

Imaging was done on the Hitachi S-4800 SEM. Twenty images were taken in each region with an acceleration voltage of 5 kV, current of 10 µA, and resolution 2560x1920 in TIF format. The magnification for preliminary tests (Sample 1) was 200x, all later samples were imaged at 2000x due to the “freshness” of the samples which resulted in smaller whisker lengths, requiring higher magnification.

A clear difference between a fresh and old sample can be seen between Samples 2 and 3, and Sample 1. Sample 1 was deposited long before it was irradiated, mostly kept on shelf for several months, but occasionally exposed to temperature changes, and elevated humidity. Therefore, it had some whisker growth before beginning irradiations.

All of the other samples were created and then test samples were irradiated, stored in a controlled environment with consistent room temperature and humidity, until the experiments were completed. Control samples were created together with the radiation exposed samples, then placed in a controlled environment until images were taken to compare to the irradiated samples.

Images were evaluated using ImageJ software by counting the number of visible whiskers, measuring the lengths of the whiskers, and calculating the density of whiskers per area in any given region. The results of these factors established that there is a relationship between the electric field and whisker growth. To measure length, the SEM images are imported into ImageJ and the scale bar from the images is measured as a set 20

number of pixels. This is used to calibrate the scale of the image, such that any length measured on the image can be translated into the appropriate units, µm.

FIG. 13. ImageJ software interface, with length selection tool selected, so the whiskers shown may be measured.

The density is calculated by finding number of whiskers per imaged area. This was done in groups from the pre-established regions. Each region receives a predetermined amount of radiation, which is correlated to the expected growth of whiskers in the region. All of the quantitative data presented, such as whisker densities and whisker lengths, come from analysis of SEM images.

Chapter 3

Results and Discussions

3.1 Sample 1

Sample 1 was fabricated several months before the irradiation experiments started. This sample consisted of thin-film Sn rf-sputtered on 3mm thick TEC15 glass, and then scribed through the metal and TCO layers to separate into 3 regions.

FIG. 14. SEM images of Sample 1 at: (a) 0 kGy (b) 5 kGy (c) 10 kGy. 22

The first sample had been irradiated to the total dose of 10 kGy, with imaging performed at 0 kGy, 5 kGy, and 10 kGy. Prior to irradiation (0kGy), whiskers having lengths averaging

13.66 µm (averaged over the entire sample) were present due to the age of the sample; the SEM magnification factor for this sample was kept at 200. Additional whisker growth was observed at

5 kGy and more growth at 10 kGy. The average whisker receiving 100% of the dose grew from

13.30 µm at 0 kGy to 32.65 µm following 10 kGy. The outer regions receiving 25% of the dose grew up to 21.84 µm on average.

TABLE 1. Sample 1 MW densities and lengths based on percentage of dose received to each region.

Irradiated sample, 25% Irradiated sample, 100% Whisker density, Whisker length, Whisker density, Whisker length, Dose, kGy #/mm2 µm #/mm2 µm 0 3.63±1.02 13.94±0.62 3.36±0.95 13.30±1.01 5 4.60±0.97 27.60±1.90 11.49±1.52 24.59±1.51 10 15.39±2.66 21.84±0.89 47.65±7.18 32.65±1.67

FIG. 15. Data from Table 1 displayed graphically.

Prior to irradiation, the whiskers present in both the outer and central regions were equivalent. Following 5 kGy, the density in the 100% region was greater than double

that of the 25% region. Following 10 kGy, the density in the 100% region was more than triple that of the 25% region. Hence, the densities increased following radiation dose levels; locations where the radiation dose and consequently induced electric fields were stronger yielded the greatest increase.

Density, 1/mm2 0 5 15 25 35 45 55 65

70 60

m) 50

µ 40 30 20

Length ( 10 0 5 15 25 35 45 55 65 Position (mm)

FIG. 16. Results at 10 kGy for Sample 1. Average length of whiskers increases as the values approach the maximum electric field at 30-40 mm along the x-axis of the sample.

The blue dashed lines indicate where the sample was scribed, and the red is the position of the line source.

Sample 1 Lengths of MWs

350

300

250

200

150 0 kGy

100 5 kGy

Number of Observations 50 10 kGy

. . 5 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 - 15. 10 ------100 - 1 - 100+ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Length (um)

FIG. 17. Frequency counts for Sample 1 at 0, 5, and 10 kGy. Whiskers are more abundant and increase in length as dose increases.

It becomes clear that the most frequent lengths for 0 kGy fall in the range of 1-15

µm, 5-20 µm for the 5 kGy, and 10-25 µm for 10 kGy, showing the correlation between length and dose delivered to the sample.

3.2 Sample 2

Sample 2 was fresh, such that it was made then used immediately, and was held at controlled room temperature conditions. The sample showed significantly smaller whiskers due to its recent fabrication, and was imaged at a higher SEM magnification of

2000. Prior to irradiation, no whiskers were observed on the sample surface. After 10 kGy of dose was delivered, the sample started growing whiskers with the average whisker length of 2.57 µm, further increasing to the average of 4.98 µm following 20 kGy. Approximately 15 days passed between sample deposition and completing 10 kGy, 25

and another 15 days between 10 kGy and 20 kGy. Once irradiation was completed, the samples were imaged again after 30 days (20 kGy + 30 days shelf) to see long-term effects of irradiation.

Figure 18. SEM images of: (a) sample before irradiation; (b) after exposure to 10kGy dose; here three short whiskers are visible. (c) Longer metallic whiskers grew after

20kGy delivered dose; this picture shows two longer than average representatives. (d)

Very small whisker (highlighted in red box) imaged on control sample after 35 days of sample deposition; (e) Three whiskers are shown on the surface of the same sample as in

(c) after additional 30 days of shelf life (60 days post-deposition); (f) Control sample 60 days post-deposition with two whiskers visible in the imaged region.

TABLE 2. Sample 2 MW densities and lengths for both the irradiated and control samples.

Irradiated sample Control sample Irradiation/ Dose, Whisker Whisker length, Shelf Whisker Whisker length, shelf time kGy density, µm time density, µm #/mm2 #/mm2 30 hours 10 282.65±3 2.57±0.12 1.74 60 hours 20 377.86±2 4.98±0.42 35 days 101.16±32.69 0.75±0.02 5.35 60 hours + 20 696.22±4 6.5±0.47 60 days 339.18±36.12 1.99±0.12 30 days 0.54

FIG. 19. Data from Table 2 shown graphically, where (a) shows whisker density and (b) shows whisker length.

FIG. 20. Frequency of (a) whisker density and (b) whisker length in Sample 2.

The regions were set up such that the average dose level in the differing sections were 100, 40, or 20% of the total dose, however these regions were not scribed and were connected by continuous TCO layer. We found no significant difference in whisker density or lengths from these regions (see Figure 16a for whisker densities broken down by the corresponding dose levels), therefore all images were processed together for irradiated sample, resulting in 120 SEM images per irradiation condition (Figure 16b illustrates frequency count data for whisker densities). This could be due to the layer of

TCO on the surface of the glass, which will be discussed below.

Figure 21. Frequency counts of whisker densities for Sample 2 after irradiation to 10 and

20kGy dose (received at 100% dose level regions): a) shown for 3 areas exposed to 20,

40, and 100% dose levels; b) data for all areas combined.

FIG. 22. Frequency of lengths for Sample 2 after 10 kGy and after 20 kGy. Following 20 kGy, the whiskers were longer and more abundant.

Sample 2 showed that the whiskers grew longer and more abundantly when exposed to more ionizing radiation. As opposed to the observation on Sample 1, no difference between metallized regions exposed to different dose levels was found.

Additional quantitative result for this sample was obtained through calculation of the acceleration ratio, which is defined as

The effect of the electric field on whisker growth is quantified as the whisker creation rate.

The difference between Rstim, the stimulated creation rate from the applied external electric field, and Rspon, the spontaneous creation rate, when no electric field is applied.

Using the irradiation time tR=60hours required to achieve 20 kGy total dose for the irradiated sample, shelf time tS=35 days=840 hours for the control sample, and corresponding average whisker densities from Table 2, we arrive at the acceleration factor a = (378/60)/(101/840) = 52. Evaluating the acceleration factor in terms of relative increase in the average whisker length would result in even higher values for a ~ 100.

Additional post-irradiation whisker growth appears to follow the rate of the shelf sample.

3.3 Sample 3

Sample 3 and its control counterpart, fabricated together with Sample 2, had Sn thin film evaporated on bare 1mm-thick microscope slides without TCO. The irradiated sample had 5 distinct Sn strips, corresponding to regions received 100, 35, and 10% of the total dose, respectively (see Fig. 9c). For this sample 20 SEM images (at 2000 magnification) were collected per Sn strip after irradiation to 10 and 20kGy, with additional set of images collected after 20kGy + 30 days on the shelf post-irradiation.

Control sample was also imaged at these times.

TABLE 3. Sample 3 lengths and densities of MWs for irradiated and control sample.

Note that the control sample sat on the shelf for 30 days prior to beginning the 60 day count, totaling 90 days of shelf life to achieve nodules.

Irradiated sample Control sample Irradiation Whisker Whisker Dose, Whisker Whisker /shelf density, Shelf time density, kGy length, µm length, µm time #/mm2 #/mm2 30 hours 10 28.56±12.03 0.20±0.07 0 0 0 60 hours 20 271.35±29.06 2.15±0.35

60 hour + 20 382.03±32.82 5.55±0.60 60 days 54.93±20.37 0.17±0.06 30 days Due to the absence of continuous TCO layer, connecting Sn strips, the expectation

with this sample was to observe direct correlation between irradiation dose levels at the

strip location further from the source and whisker growth. This sample, however, did not

follow expectations, possibly due to the composition of the microscope slide, or maybe

sample mishandling, went unnoticed. The general trend for this sample was pretty

similar to the other two samples subjected to Ir-192 irradiation in significant acceleration

of whisker growth, while being relatively whisker resistant.

Sample 3 Lengths of MWs

70 60 50 40 30 Series1 20 Series2 10 Number of Obervances 0 0-5 5-10 10-15 15-20 20-25 25-30 30+ Lengths (um)

FIG. 23. Frequency of lengths for Sample 3 after 10 kGy and after 20 kGy. The sample

was whisker resistant at 10 kGy. Following 20 kGy, the whiskers were longer and more

abundant.

The control sample in this case showed no whiskers, and only a 6 nodules in the

40 images taken.

FIG. 24. SEM images of Sample 3 after (a) 10 kGy (b) 20 kGy (c) 20 kGy + 30 days on the shelf (d) control after 60 days.

The sample was resistant to whiskers for the first 10 kGy, showing little growth.

After exposing Sample 3 to 20 kGy, whiskers grew in regions 1-5. Most of these whiskers were in regions 1-3. Whiskers grew longer after sitting on the shelf for 30 days.

We can estimate acceleration ratio for this sample based on overall averaged data for whisker densities, following the procedure similar to that of Sample 2. With the irradiation time tR=60hours required to achieve 20 kGy total dose for the irradiated 33

sample, shelf time tS=90 days=2160 hours for the control sample (we use the time interval when whiskers were first observed), and corresponding average whisker densities from Table 3, we arrive at the acceleration factor a = (271/60)/(55/2160) = 177. Again, using whisker lengths values would result in even higher acceleration ratio of >300.

3.4 Discussions

The average energy of the source of the gamma rays in the Ir-192 source used is

0.38 MeV, with a maximum energy below 1 MeV. Since the average energy of the source is 0.38 MeV, the energy transfer is primarily via Compton effect, where the ratio

-6 of electron and atomic masses is ∼ 4 × 10 . The average energy received by a Compton electron is about 40% of the source photon energy, and is never higher than the maximum photon energy; as a result, the atom receives <1 eV on average and cannot receive more than 4 eV (Attix, 1986). This is far below the displacement threshold energy for Sn, estimated as 22±2 eV (Andersen, 1979). Other mechanisms of energy transfer are even less efficient. Consequently, the Ir-192 source used is not capable of producing atomic displacement in Sn film, is non-destructive to the sample and suitable for whisker propensity testing.

The following general trends were observed in this project:

1) Irradiation with Ir-192 source resulted in accelerated whisker growth in all studied

samples. The dose of 20 kGy seems to be necessary to achieve the measured

effect, detectable through manageable number of SEM images. Acceleration

ratios of 50 to 100 were observed in samples having control counterparts.

2) Due to inability of the source to produce atomic displacements in Sn film, we

attribute this effect to charging on glass substrates under irradiation, resulting in

electric field induced in the direction perpendicular to the film surface. This

electric field is conducive to whisker growth.

3) Connecting metallized areas of Sn with TCO underneath results in creating

equipotential surface and thus leveling out of the induced electric field. This

effect is responsible for absence of correlation between whisker growth and dose

levels observed for Sample 2. Electrically disconnecting metallized areas, as was

done for Sample 1, leads to distinctly different regions of electric field levels

acting on different regions of the sample.

Chapter 4

Conclusion

4.1 Conclusion

All studied samples of thin Sn films deposited on glass showed that gamma-ray radiation can induce accelerated whisker growth, evaluated through changes in both whisker densities and lengths. Since energies of photons emitted by Ir-192 source used in the study are too low to cause atomic displacement in Sn film, we attribute the effect to charging of insulating glass substrates, supporting the films. All of the irradiated samples showed darkening in the glass substrate, which is a common phenomenon of color centers formation in glass (Kreidl, 1995), confirming that the radiation caused changes in the substrates. Irradiation-induced charges resulted in electric field in the direction perpendicular to the film surface, thus realizing conditions recently predicted to facilitate whisker nucleation and growth (Karpov, 2014).

Sample 1, fabricated on continuous TCO covered glass, was scribed to create 3 electrically disconnected areas: a central area, located closest to the source and receiving the full dose, and two adjacent areas, located further from the source, and therefore subjected to only 25% dose levels on average. This sample showed growth correlating 36

with the distance from the source, where the longest whiskers were the closest to the source, getting shorter in the lower dose areas.

Sample 2, also prepared on continuous TCO with Sn film deposited in strips did not show the same trend of MW parameters correlating with distance to the source, despite separate strips being irradiated to dose levels of 100, 40, and 20% of the maximum dose. We believe this is due to the presence of one connective layer of TCO across all sections. This allowed for the tangential portion of the electric field to be leveled out (Karpov, 2017), in turn creating a relatively even electric field across the surface of Sample 2. Field-stimulated whisker acceleration ratio for this sample was found to be ~50 if calculated for increase in whisker densities compared to the control, and close to 100 based on comparative increase in whisker lengths.

Sample 3 fabricated on microscope slides also as a set of Sn strips turned out the most whisker resistant. The control sample resisted whisker growth, showing only a few nodules even at the completion of the project. The test sample grew less MWs than the previous two samples, and very few in regions 4 and 5, located on one side of the sample.

Despite this, the acceleration factor for whisker density is 177 and the acceleration factor for whisker length is ~300, showing that the electric field produced did significantly stimulated whisker growth.

These three samples showed that Sn whisker growth is accelerated under gamma- ray irradiation. Connecting regions with TCO allows for regions to be equipotential, leading to the same enhancement in whisker growth over the irradiated area, independent of the distance to the source. Based on the data on post-irradiation whisker growth for samples held 30 days on shelf, we speculate that the electric field acts mostly at the 37

nucleation stage by diminishing the whisker nucleation barrier. In the absence of further electric filed stimulation, the irradiated samples grow whiskers similar to their control counterparts.

This type of testing is promising as a form on non-destructive accelerated life testing tool for whisker propensity, with the potential to aid in industry’s whisker problem. Large acceleration factors obtained in this work in practice mean that whisker propensity can be tested 50 to 100 times faster, reducing testing times to manageable days/weeks instead of many months or years.

There is still further research to be done in this area. Future tests could include testing a partially shielded TCO sample, testing samples with differing island size to see if there is a size threshold for whisker growth, testing differing thicknesses of the Sn thin film, or testing different substrates.

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