Download STIMULATED METAL WHISKER GROWTH by James McCulloch

STIMULATED METAL WHISKER GROWTH
by
James McCulloch
A senior thesis submitted to the faculty of
Brigham Young University - Idaho
in partial fulfillment of the requirements for the degree of
Bachelor of Science
Department of Physics
Brigham Young University - Idaho
March 2016
c 2016 James McCulloch
Copyright All Rights Reserved
BRIGHAM YOUNG UNIVERSITY - IDAHO
DEPARTMENT APPROVAL
of a senior thesis submitted by
James McCulloch
This thesis has been reviewed by the research committee, senior thesis coordinator, and department chair and has been found to be satisfactory.
Date
Dr. Jon Paul Johnson, Advisor
Date
Dr. Todd Lines, Senior Thesis Coordinator
Date
Dr. Evan Hansen, Committee Member
Date
Dr. Richard Hatt, Committee Member
Date
Dr. Stephen McNeil, Chair
ABSTRACT
STIMULATED METAL WHISKER GROWTH
James McCulloch
Department of Physics
Bachelor of Science
Metal whiskers are needlelike objects that can grow from grains on a metal surface. The purpose of this experiment was to stimulate metal whisker growth.
Charging pre-existing whiskers and creating an electric field is theorized to
stimulate their growth. To test this, five samples were used. Images were
taken of all samples. Then, four of the samples were irradiated in various accelerators. More images were then taken of the samples. Next, before and after
whisker densities were calculated from the images. A Monte Carlo N-Particle
code was developed to determine if whisker growth is related to the energy
deposited in the sample. It was concluded that stimulated whisker growth is
related to charging a sample and creating an electric field in the metal, not
depositing energy into it.
ACKNOWLEDGMENTS
I would like to express my gratitude to the National Science Foundation
(NSF) for providing the funding for my internship at the University of Toledo,
which was a life-changing experience.
I will always be grateful to Dr. Diana Shvydka for being my mentor during
my internship, assisting me with my project, and supervising my hours of clinic
time. Her time and advice has changed my life. I would also like to thank Dr.
Victor Karpov for initiating and allowing me to be part of the metal whiskers
project at the University of Toledo. Many thanks, as well, to Dr. Richard
Irving, Corey Grice, and Greg Warrell for all of their assistance and support
throughout my internship. And finally, I am grateful to Dipesh Niraula for
assisting me on the Scanning Electron Microscope.
Contents
Table of Contents
xi
List of Figures
xiii
1 Introduction
1.1 Experimental Design . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 MCNP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1
3
2 Procedure
2.1 Scanning Electron Microscope
2.1.1 SEM Grounded . . . .
2.1.2 SEM Ungrounded . . .
2.2 Medical Accelerator . . . . . .
2.3 Ion Accelerator . . . . . . . .
2.4 Control . . . . . . . . . . . .
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3 Results
3.1 Control Sample . . . . . .
3.2 SEM Grounded Sample . .
3.3 SEM Ungrounded Sample
3.4 Tin Beam Sample . . . . .
3.5 Medical Sample . . . . . .
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4 MCNP
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5 Conclusion
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Bibliography
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xi
List of Figures
1.1
Metal Whisker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.1
2.2
Metal Whiskers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sample Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
8
3.1
3.2
3.3
3.4
3.5
3.6
Collected Experimental Data . . . .
Control Sample Densities . . . . . .
SEM Grounded Sample Densities .
SEM Ungrounded Sample Densities
Tin Sample Densities . . . . . . . .
Medical Sample Densities . . . . .
4.1
4.2
4.3
4.4
4.5
MCNP
MCNP
MCNP
MCNP
MCNP
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11
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Visual Editor . . . . . . . . . . . .
Medical Sample Results . . . . . .
Ungrounded SEM Sample Results .
Layer Deposition . . . . . . . . . .
Zinc Oxide Results . . . . . . . . .
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xiii
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Chapter 1
Introduction
Metal whiskers are “Hair-like metal structures that erupt outward from a grain or
several grains on a metal surface.” [1] Whiskers were first noticed during World War
II. They were growing on cadmium parts inside of radios and causing them to short.
It was soon discovered that whiskers grew on many different types of metals - the
most prominent metals being cadmium, tin, and zinc. Since the discovery of metal
whiskers, there has been work done to find ways to prevent their growth. Lead helps
to prevent the growth of whiskers but it does not stop the growth entirely. Part of
the reason it is hard to prevent whisker growth is because the cause of their growth
is still unknown. In order to stop whiskers from growing, it must first be understood
what causes them to grow. It is theorized that electric fields or energy deposition
may be the cause. This experiment tests both theories.
1.1
Experimental Design
The purpose of the following experiment was to test whether or not whisker growth
can be stimulated by creating an electric field in a sample. This was done by exposing
1
2
Chapter 1 Introduction
Figure 1.1 A metal whisker imaged with the Scanning Electron Microscope.
samples with pre-existing whiskers to particle beams. NASA sent the University of
Toledo samples from a steel floor covered by zinc oxide, which had whiskers growing
on it. Using a particle beam on a floor piece causes an electric field to occur in the
sample. The steel acts as a conductor so charge does not accumulate on it. The zinc
oxide on the surface however, accumulates charge. By putting a charge on the zinc
oxide, an electric field is created. If the sample is not grounded, then the electric field
will not dissipate. Leaving the particle beam on the sample will cause the sample to
remain charged for the designated time.
The floor samples given by NASA were cut into smaller pieces with metal shears.
The whisker density on each piece was counted. Then the pieces were sent to their
various particle accelerators to be irradiated. After irradiation, they were sent back
to have their whisker densities counted a second time. If there was a shift in the
whisker densities, then the induced electric fields had an effect on whisker growth.
1.2 MCNP
3
There were three types of particle beams used in this experiment. A medical linac
(Varian True Beam), a Scanning Electron Microscope (SEM), and an Ion Accelerator.
A medical linac is an accelerator that is used to treat cancer patients. The SEM is
an accelerator because it has an electron beam that can be shot at a specific target.
The SEM irradiated two samples after an incident occurred in which a sample was
grounded when it wasnt supposed to be. Because of this, a second sample was run
through the SEM. This second sample was not grounded.
1.2
MCNP
MCNP stands for Monte Carlo N-Particle code. It is a software package for Monte
Carlo modeling of radiation transport. A code was written in MCNP to simulate the
irradiation of the medical sample and the ungrounded SEM sample. MCNP was used
to do this because it is designed to handle large numbers of particles bombarding
specific surfaces. The purpose in creating the code was to calculate the amount
of energy deposited in the system and determine if energy deposition and whisker
growth are related. If energy deposition is related to metal whisker growth, then the
hypothesis of the experiment will have been proven false and electric fields would not
be considered the cause of whisker growth. Energy deposition would be the cause of
whisker growth.
4
Chapter 1 Introduction
Chapter 2
Procedure
Before any sample was irradiated it was taken to the SEM (Scanning Electron Microscope). There, images were taken of the surface. These images were taken of random
locations on the sample surface. To do this the sample was attached to the SEM
sample stand with carbon tape. The sample stand is a small device made of metal
that can enter the SEM through an opening in its side. Then a piece of copper tape
was attached to the top of the sample and to the stand to ensure that the sample was
grounded. Grounding the sample would prevent charge build up. This is in line with
the theory that continuous electric fields cause whisker growth. Thereafter images
were taken of the sample. Each image was taken at the same magnification making
each image the same size. The number of whiskers in each image were counted. The
average number of whiskers per picture was then calculated. Logger Pro was used
to find the area of the pictures, and from this information the initial whisker density
was calculated. Afterwards, each sample was sent to its designated accelerator.
When counting the whiskers in each picture, only whiskers that were completely in
the picture were counted. In a previous experiment that dealt with whisker densities,
5
6
Chapter 2 Procedure
Figure 2.1 An image of one of the samples that was used to calculate
whisker density.
whiskers would be counted even if they were not entirely in the picture as long as the
base of the whisker was in the picture [1]. For the samples in this experiment, it was
difficult to determine where the base was. In order to prevent counting one whisker
multiple times, whiskers not completely in the picture were not counted.
2.1
Scanning Electron Microscope
The SEM had both a grounded and ungrounded sample. It was originally intended
to only have an ungrounded sample. An error occurred that resulted in a grounded
sample. In order to determine whether or not whisker growth is a result of electric
fields, an ungrounded sample was used as well. The grounded sample will show
whether or not energy deposition alone will cause whisker growth.
2.2 Medical Accelerator
2.1.1
7
SEM Grounded
This sample was attached to the SEM stand with carbon tape. The carbon tape
grounded the sample to the metal stand. Then the sample was put in the SEM and
exposed to the electron beam at 10.0 KeV and 10µA. This was done for five 1.5 hour
sessions and one 2 hour session for a total of 10 hours of exposure. The purpose of
this was to expose both samples for a long period of time to see if any whiskers grew.
2.1.2
SEM Ungrounded
The ungrounded sample was prepared by putting a piece of glass between the SEM
sample stand and the sample. The glass acted as an insulator and caused the sample
to remain ungrounded for the entire time it was irradiated. The glass also captured
electrons and became charged causing an electric field in the sample. Carbon tape
was used to connect the sample to the glass. Carbon tape was also used to attach the
glass to the SEM sample stand. The sample was exposed to the 10.0 KeV electron
beam at 10µA. This was done for two 3 hour sessions and one 4 hour session for a
total of 10 hours.
2.2
Medical Accelerator
The sample was taped to a piece of glass that had the same area as the sample. The
glass’s purpose was to catch electrons that made it through the sample. The glass
would then become charged and cause an electric field to be created in the sample.
The sample was kept in a clear plastic case during travel. The sample was then left
in the case while it was irradiated so that the electron beam would scatter over the
sample surface. The sample and case were then placed on the couch in order to be
irradiated. The couch would normally be the surface that a patient would receive
8
Chapter 2 Procedure
Figure 2.2 This is a virtual representation of the samples that were designed
for use in MCNP. The above image represents the ungrounded SEM sample
and the medical accelerator sample. The left part of the image is the sample
viewed from above, and the right side of the image is a cross section of the
sample.
treatment on. The couch was raised right up to the collimator until it couldn’t come
any closer. The gap distance was about 5 cm. This was done so that the electron
beam would not scatter in the air. Also, no attenuation cones were needed to direct
the beam because it was so close to the sample. Attenuation cones are used in the
treating of patients. They help direct the flow of electrons over long distances through
the air. Since the sample was raised right up to the collimator, no cones were needed.
The machine was then set to deliver 6 MeV to the sample over 15 minutes. After
this had occurred the machine was given 5 minutes to cool before another 15 minutes
of irradiation was completed. This was done until 10 hours of irradiation had been
reached. The irradiation took over several nights to complete because it takes a long
time and patients use the machine during the day.
After this sample was irradiated for 10 hours, it was imaged by the SEM. The sample
was then sent back to the medical accelerator to receive another 10 hours of irradi-
2.3 Ion Accelerator
9
ation. This was the only sample to go through two rounds of having its whiskers
counted and then being irradiated. The other three samples being irradiated only
went through the process once.
2.3
Ion Accelerator
This sample had its whiskers counted at the SEM. After the sample had its whiskers
counted it was discovered to be too big to fit into the accelerator, so it was cut
to about a quarter of its original size using metal shears. It did not need to have
its whiskers recounted since the whisker density on the sample was not affected by
decreasing the size of the sample. A piece of glass was attached to the underside of
the sample, so it would be insulated and keep a charge while it was bombarded with
ions. The type of particle it was bombarded with was tin-120. The reason this was
done is because tin-120 was what was currently in the accelerator. The tin was at
130 KeV and the beam current was 110 nA. The sample was irradiated for a total of
1 hour in two
2.4
1
2
hour sessions.
Control
The control sample was not irradiated at all. It had its whiskers counted at the
beginning of the experiment with the rest of the samples. After images were acquired
of the other samples, the surface of the control sample was imaged with the SEM.
10
Chapter 2 Procedure
Chapter 3
Results
What follows are the measured results of the experiment. The table contains whisker
densities before and after irradiation. It also contains the change in the densities.
Figure 3.1 Collected Experimental Data
11
12
3.1
Chapter 3 Results
Control Sample
The control sample saw a 23%
growth without any irradiation.
This means it is possible to grow
whiskers without being irradiated by an accelerator. Dipoledipole interactions in the sample
may be the cause of this. Since
the peaks are so close together, a
two mean independent samples
Figure 3.2 Control Sample Densities
test was done in order to determine that the distributions were
actually different. This was done by using a null hypothesis of µ1 = µ2 and an alternative hypothesis of µ1 6= µ2 . Then an independent samples t test was run and a
t value was derived. Comparing this t value to a t distribution and the significance
level of 0.05, it was determined that the distributions had different means, and are
therefore different distributions.
3.2 SEM Grounded Sample
3.2
13
SEM Grounded Sample
The Grounded SEM sample saw
a 30% growth in whiskers. The
whisker density changed by 7
whiskers/mm2 .
This is very
close to the amount that the
control changed, which was 5
whiskers/mm2 .
Because of
this, it can be inferred that irradiating a sample is not enough
Figure 3.3 SEM Grounded Sample Densities
to stimulate whisker growth. In
order to stimulate growth, the
sample must not be grounded. Charge must be kept on the sample to create a
continuous electric field. Once again, the peaks are close together, so a two mean
independent samples test was done. Using a significance level of 0.05, it was determined that these distributions are different.
14
3.3
Chapter 3 Results
SEM Ungrounded Sample
The ungrounded SEM sample
saw a 53% increase in the number of whiskers on its surface.
This is more growth
than the control or ungrounded
sample saw.
This supports
the idea that charging the
sample will promote whisker
growth.
Figure 3.4 SEM Ungrounded Sample Densities
3.4
Tin Beam Sample
The tin beam sample was the
only one to have its whisker
density decrease. The density
decreased by 63%.
This may
have occurred because the ion
beam used tin rather than electrons. The tin may have interacted with the zinc oxide plating causing it to corrode. This
Figure 3.5 Tin Sample Densities
would have caused the number
of whiskers to decrease on this
sample.
3.5 Medical Sample
3.5
15
Medical Sample
After 10 hours of irradiation, the
medical sample’s whisker density increased by 38%. After an
additional 10 hours, the density
had increased by 92%. Comparing this with the control sample
shows that irradiating the sample caused charge to build up,
and whiskers to grow.
Figure 3.6 Medical Sample Densities
16
Chapter 3 Results
Chapter 4
MCNP
MCNP (Monte Carlo N-Particle code) is a software package for Monte Carlo modeling
of radiation transport. MCNP can handle large simulations and is used extensively
in fields that involve simulating particle collisions with specific materials. A code
was written in MCNP to simulate the irradiation of the medical sample and the
ungrounded SEM sample. The purpose in creating the code was to calculate the
amount of energy deposited in the system and determine if energy deposition and
whisker growth are related.
First, the simulated samples needed to be created. Both samples were a 4 cm x
4 cm piece of .9 mm steel with a 0.1 mm zinc oxide layer on top. Beneath the zinc
and steel layers there was a 1 cm thick piece of glass. The environment surrounding
the sample differed in that the SEM sample was under vacuum, and the medical
sample was exposed to the atmosphere.
17
18
Chapter 4 MCNP
Two things were measured for each simulation. First, the electron flux through each
surface and the energy that these particles had was measured. Second, the amount
of energy deposited in each layer was measured.
Figure 4.1 This is a screenshot of a Visual Editor that shows what the
sample looks like to MCNP.
19
Figure 4.2 The density of particles that made it through each layer of the
sample versus the Energy that each particle had.
The medical sample (Fig. 4.2) had particles passing through each surface. The number
of particles decreases deeper into the sample because they were being absorbed. The
stair-like structure on the zinc oxide curve is most likely due from backscatter. The
code does not take into account the starting angle or the trajectory of a particle when
it passes through a layer.
20
Chapter 4 MCNP
Figure 4.3 The density of particles that made it through each layer of the
sample versus the Energy that each particle had.
On the ungrounded SEM sample (Fig. 4.3), no particles made it past the zinc oxide
layer. This is due to the energy that the electrons have when they come into contact
with the sample. The stopping power of zinc oxide to 10 KeV electrons is 3.717 ∗
10−4 g/cm2 . [2] Taking the stopping power and dividing it by the density of zinc oxide
gives the penetrating distance of the electrons. This means that the electrons should
only be able to penetrate up to .663µm. The simulated zinc oxide layer is 100µm
thick so these simulated results are reasonable.
21
Figure 4.4 The amount of energy deposited in each layer of the sample.
In order to apply these numbers to the experiment, the electron fluence of each
machine must be used. The energy that would be most responsible for whisker growth
would be the energy deposited in the zinc oxide layer. The zinc oxide layer is where
whisker growth occurs.
Figure 4.5 The amount of energy deposited in the Zinc Oxide layer over a
specific amount of time.
The ungrounded SEM sample had two orders of magnitude more energy deposited in
its zinc oxide layer. If energy and whisker growth were related, then the ungrounded
22
Chapter 4 MCNP
SEM sample should have many more whiskers on it than the medical sample. In
the experiment, after 10 hours of irradiation for both samples, it was found that the
medical sample had a 38% increase in its whisker density, while the ungrounded SEM
sample had a 53% increase in its whisker density. The closeness of numbers imply
that energy deposition and whisker growth are not related. The ungrounded SEM
sample would have more whiskers and this isn’t the case.
Chapter 5
Conclusion
Metal whiskers have been causing problems for a long time. The purpose of this
experiment was to stimulate the growth of metal whiskers and determine the cause.
After running the MCNP code, and comparing it to the data collected, it can be
seen that whisker growth is related to the charging of the sample, but not the energy
deposited. The control sample shows that even when whiskers are left alone, they
will grow. Comparing the ungrounded SEM sample to the grounded SEM sample
shows that whisker growth is not stimulated merely by bombarding the sample with
particles. Charge must be allowed to build up on the surface. This charge creates an
electric field in the sample which causes whiskers to grow.
Some future work would include running the experiment again, but with a Van de
Graaf Generator. A Van de Graaf generator would put charge on a sample without
bombarding it with electrons. If the particle beams were causing any whiskers to
deteriorate, a Van der Graaf sample would show this.
23
24
Chapter 5 Conclusion
Knowing how metal whiskers grow is important. The main goal of studying metal
whiskers is to find a way to stop them. Once metal whiskers are understood, then
steps can be taken to stop their growth.
Bibliography
[1] Panashchenko, L., 2012. The art of metal whisker appreciation: A practical guide
for electronics professionals.
[2] National Institute of Standards, and Technology, 2015. Stopping-power and range
tables for electrons, protons, and helium ions.
[3] Brusse, J. A., Ewell, G. J., and Siplon, J. P., 2002. Tin Whiskers: Attributes and
Mitigation.
[4] Panashchenko, L., 2009. Evaluation of environmental tests for tin whisker assessment.
[5] Umalas, M., Vlassov, S., Polyakov, B., Dorogin, L. M., Saar, R., Kink, I., Lohums, R., and Romanov, A. E., 2014. “Electron beam induced growth of silver
nanowhiskers.” Journal of Crystal Growth.
[6] Vasko, A. C., Warrel, G. R., Parsai, E., and Karpov, V. G.and Shvydka, D., 2015.
Evidence of rapid tin whisker growth under electron irradiation.
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BIBLIOGRAPHY