Download PHYSICAL AND CHEMICAL PROPERTIES OF AMBIENT

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
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CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis/dissertation of
Daniel Thomas Shelberg
______________________________________________________
Master of Science: Engineering degree *.
candidate for the ________________________________
Chung-Chiun Liu
(signed)_______________________________________________
(chair of the committee)
R. Mohan Sankaran
________________________________________________
Heidi B. Martin
________________________________________________
________________________________________________
________________________________________________
________________________________________________
March 24, 2010
(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
Table 1. Diffusion coefficients for Kapton
and SiC
A.1 Trace elements in sputter target
A.2 Nanoindentation data
Page
53
56
56
Page 2 List of Figures
Figure
Figure 1. Sputtering process illustration
Figure 2. Typical XPS of SiC film shows a carbon rich film with
Page
12
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
A(hc) – contact area as a function of contact depth (nm2)
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)
– diffusion coefficient of water through Kapton (m2 sec-1)
- diffusion coefficient of water through SiC (m2 sec-1)
- 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 barrier properties, and
chemical resistance. Thin film SiC has been used and applied in packaging
sensors, power sources, and microelectronics. One major problem with the
current methods of depositing SiC thin films is that high temperatures are
required in the deposition process. Sputtering is a physical deposition method in
which a target material is bombarded with ions from a plasma source, which
knocks off atoms of a target material. The atoms which are released and will
deposit onto the substrate because of the principle of mean free path in a high
vacuum environment. If a well defined deposition condition can be defined, this
sputtering process can be carried out at relatively low temperature. This ambient
temperature sputtering process allows the SiC thin film to be deposited on
materials such as polymers and or micro-electronic devices which cannot survive
at high temperatures.
Silicon carbide is a material known for its hardness, chemical resistance,
and thermal properties. Due to its many favorable properties, silicon carbide is an
ideal
candidate
for
packaging
sensors,
power
sources,
and
other
microelectronics. It has been used in blue light emitting diodes and as a
substrate for nitride based LEDs1. Silicon carbide is also used in high power
applications such as Schottky diodes and MOSFETs2, and highly corrosive
environments such as phosphoric acid fuel cells3.
This study examines silicon carbide for use as a protective coating for use
in electronics packaging. Its favorable physical, chemical, and electrical
Page 9 properties make it an attractive protective coating for devices that need shielding
from moisture, chemicals, or physical wear. Electronics that must be protected
but cannot be sealed off from the environment could be coated with silicon
carbide. Devices printed on plastics which must remain flexible, but also endure
solvents or other corrosive chemicals could be fabricated. The possibilities for
applications are numerous, but each one hinges on the integrity of the film and
the requirement that nanometer scale films provide an adequate barrier.
Therefore this study examined the moisture, corrosion and etchant
resistance, indentation hardness, film flexibility, and composition to determine the
efficacy of silicon carbide films sputtered at ambient temperatures. Composition
and the repeatability of producing films were examined using x-ray photoelectron
spectroscopy (XPS). Resistance to salt water corrosion, potassium hydroxide
etching, and moisture diffusion were tested followed by XPS and scanning
electron microscopy (SEM). Film adhesion was tested by bending films around a
radius of curvature and examining the cracking with SEM. Hardness was tested
using an atomic force microscope to make nanoindentations in the silicon carbide
films. Each of these tests showed favorable results and support the validity of
silicon carbide films used for protective coatings.
Page 10 Chapter 2: Theoretical Background
Sputtering is a physical process by which high energy ions remove
material from a target. The sputtering process is often used for the cleaning or
the etching of a material, but the atoms ejected from the target material may also
deposit on a substrate. Historically, two sputtering mechanisms were proposed:
thermal
vaporization
theory
and
momentum
transfer
theory4.
Thermal
vaporization as the dominant mechanism was supported by Sommermeyer5, and
Townes6 in the first half of the 20th century. In contrast, studies done by Wehner
in 1956 suggested momentum transfer as the dominant mechanism. Modern
theory developed by Sigmund found a linear cascade of momentum transfer
accurately models sputtering behavior for amorphous and polycrystalline
targets7.
Sputter yield depends on the energy and the angle of the incident
particles, the target material, and the crystal structure of the target. There exists
a threshold energy for sputtering to occur, and this threshold energy has been
measured by Stuart and Wehner to be in the range of 15 to 30 eV8. Near the
threshold region of incident energy, sputter yields are found to be very low at 10-4
to 10-5 ejected atoms per incident ion. At energies of a few hundred eV sputter
yield is proportional to the incident ion energy, and is on the order of 0.1 to 1
atoms ejected per incident ion9. Under high incident ion energy of 10 keV,
clusters of atoms are knocked off and yield can be greater than unity.
Atoms sputtered by a few hundred eV, which is typical for conventional
sputtering systems, have an energy of 10 to 30 eV10. This is orders of magnitude
Page 11 greater than the thermal evaporation energy and corresponds to a sputtered
atom velocity of 3 × 105 to 7 × 105 cm/sec. The energy does depend on the
sputtering ions and ion energy, and was studied extensively by Stuart and
Wehner11. Additionally, the incident and escape angles of the ions and atoms can
also have an impact on the energy of sputtered atoms12.
Figure 1 demonstrates the process of sputtering and the damage caused
to material layers. The bombarding ion penetrates the material and transfers
energy through elastic collisions. The linear transfer of momentum can eventually
be scattered back towards the surface of the material, ejecting atoms. Due to the
random motion caused by the penetrating ion, the layers within the target
become inter-mixed, with a thickness on the order of a few nanometers.
Atomic layer inter-mixing is an important consideration when determining
thin atomic layers or boundary layer thickness in techniques which use sputtering
to mill the surface of a sample. In this study, x-ray photoelectron spectroscopy
(XPS) was used frequently to determine composition. Inter-mixing was not an
important consideration due to the uniformity of film composition as a function of
depth.
Page 12 Figure 1. Process of sputtering at increasing points in time. The bombardment of
ions causes an intermixing of atom layers. At t0 the bombarding ion strikes the
target surface. At t1 the bombarding ion transfers momentum in a linear cascade
of collisions. The momentum can eventually translate to freeing atoms from the
target surface, as in t2. Intermixing of atomic layers happens as a result of the
random momentum transfer, shown at time t3.
Page 13 Due to the use of charged particles to bombard the target, insulators such
as silicon carbide cannot be sputtered by conventional means. Charge buildup
does not allow for subsequent ions to reach the target, and therefore the sputter
glow discharge cannot be sustained. This problem is avoided by the use of an
RF power supply in the sputtering system, where the varying anode/cathode
polarity neutralizes the surface charge of the target. The use of a magnetron is
another enhancement to the process, which forces electrons in helical paths and
increases their chance to ionize the gas. In this study, all samples were
deposited using RF magnetron sputtering from silicon carbide compound targets.
Page 14 Chapter 3: Deposition Conditions
Deposition conditions such as base pressure, sputtering power, target and
substrate angles can have a large impact on film quality and composition.
Therefore it was imperative to closely document deposition parameters to
produce repeatable results. The silicon carbide films used in this study were
deposited with a Denton Vacuum Explorer 14 RF magnetron sputtering system
(1259 North Church Street, Morestoon, NJ08057). In this study the base
pressure achieved for each deposition was approximately 7.0
10
torr, and a
working pressure of 3 millitorr Argon. In order to achieve the lowest base
pressure possible, the chamber was allowed to pump down overnight, namely 16
hours. The target was placed 130 mm from the center of the substrate at an
angle of elevation of 40 degrees. Deposition power ranged from 200W to 400W;
higher power may cause the target to crack due to thermal stress. Deposited
films thickness ranged between 150nm and 500nm, and chamber temperature
during deposition was approximately 27°C. The deposition rate was 0.9Å/sec
leading to deposition times on the order of 30 minutes to 1.5 hours depending on
the thickness of the deposited film. The substrate station was rotated during
deposition in order to increase the film uniformity. The 3 inch (7.62 cm) SiC target
was acquired from Kurt J. Lesker Company (1925 Route 51, Clairton PA, 15025)
and contained 99.5% SiC with a density of 2.95 g/cm3 (Appendix A.1).
Page 15 Chapter 4: Film Composition and Bonding
Slight variations in silicon carbide film composition and bonding could
have a large impact on film properties. The deposited SiC film composition was
consistently checked with x-ray photoelectron spectroscopy (XPS), and revealed
films with a carbon to silicon ratio of approximately 3:2. The sputtering power did
not significantly affect this ratio; for powers of 200W, 300W, and 400W
composition was within 1%. Figure 2 shows a typical XPS spectrum for the
surface of a SiC film deposited at 400W. X-rays were monochromated aluminum
Kα photons (1.487 keV) with a 300 micron spot size. There was a large amount
of oxygen present; a thin oxide layer formed when the film was exposed to air.
The oxide layer formed would be on the order of a few nanometers, but would
grow slowly with time. Keeping the samples under nitrogen until the XPS analysis
would prevent an oxide layer from forming, though oxygen contamination was
still present. Trace elements found in the target (see Appendix A.1) were not
quantitatively significant to be detected by XPS.
Page 16 Figure 2. Typical XPS spectrum for the surface of a SiC film. Degree of oxidation
can vary greatly with film age.
Figure 3 shows the chemical shift observed for the silicon and carbon
binding energy peaks for a 2 month old sample deposited at 300W. The
presence of Si-O bonds increased the binding energy of silicon 2p electron, and
a higher energy shoulder appeared. Both show peaks at 100.4 eV, but the
surface sample also has peak approximately 2.5 eV higher. For pure silicon the
2p1/2 and 2p3/2 binding energies would be 99.8 eV and 99.2 eV respectively13,
and the peak for SiO2 would be at 103.4 eV14. Our results indicated that the
silicon was fully oxidized to SiO2. A slight shift of the carbon peak was also
visible indicating C-O bonds (Figure 4), but this could also be attributed to the
residue of alcohol used in the cleaning process before testing a sample. The
Page 17 standard carbon 1s peak would be at 284.2 eV15, and a slight decrease was
visible due to the bonding with silicon. The oxidation could be removed with only
one minute of sputtering, which is approximately 5nm surface materials removed.
A slight variety in the composition of the deposited SiC film could be
observed between the center and the edge of the wafer. This would be the result
of carbon and silicon preferentially sputtering at slightly different angles from the
SiC target. The thickness of the deposited film also decreased near the edge,
and it was evident in a decrease in nanoindentation hardness. Due to decreased
film thickness near the edge of the wafer, there was a larger contribution from the
underlying softer silicon wafer. XPS depth profiles showed a constant
composition, which would be a result of the deposition and depth profile
preferential sputtering effects counteracting each other.
The 3:2 ratio of carbon to silicon ratio in the deposited SiC films was not
totally clear, as the sputter yield ratio for carbon to silicon would not reach that
ratio within conventional incident ion energies (Figure 5). The incident ion angle
does have a significant impact for crystalline targets and may sufficiently alter the
yields of carbon and silicon. However, crystalline films with a higher content of
carbon could yield superhard films of greater than 40 GPa16, and therefore this
increase in carbon in the composition of the SiC film may not be a disadvantage.
Page 18 Figure 3. Comparison between silicon 2p peak for surface and 5 nm sputter. The
surface silicon 2p peak has a shoulder at a higher binding energy that indicated
Si-O bonding in the film.
Figure 4. Comparison between carbon 1s peak for surface and 5nm sputter. The
surface carbon peak has a slight shoulder which may indicate bonding to oxygen.
This could be due to residual cleaning alcohol.
Page 19 Figure 5. Sputter yield data for carbon and silicon. Yield ratio does not reach 3:2
carbon to silicon within conventional sputter deposition energies.
A difficulty in analyzing the results documented RF sputtering is the lack
of reported incident ion energy. It is often that the sputtering power is reported,
however the sputter yield of the deposited compounds is not given. Fujiyama et
al. discuss the effect of silicon carbide target composition on the film composition
using mixture and compound targets17. A nonlinear trend of increasing film
carbon content with target carbon content is reported, and the film carbon
content is always lower than the target carbon content. Therefore the sputter
yield of carbon may always be lower than that of silicon. In this study,
compositions were obtained with electron probe microanalysis (EPMA), which
Page 20 was not ideal for finding the composition of SiC. The x-ray yield for light elements
such as silicon and carbon are particularly low and quantification may be difficult
and inaccurate. Furthermore, the carbon and silicon mole percentages could not
be added up to 100%, and the source of this discrepancy was unreported in the
paper. The missing composition was most likely oxygen contamination and the
oxidation in the SiC film. Therefore it was not possible to conclude whether the
lower carbon content was a result of the low incident ion energy and therefore a
less than unity carbon than silicon sputter yield ratio, or due to any error in the
methods used. However, the study does reveal that the sputtering process did
not reach a state in which the target surface composition equaled the inverse of
the sputter yield ratio, thereby causing the film composition to equal the bulk
target composition.
The surface composition of all films sputtered in this study was not found
to be a function of film thickness, and therefore the surface composition was not
a function of sputter time. This indicated that the surface composition of the
target was not significantly changing over time. Therefore the time constant for
the change in surface composition due to preferential sputtering was relatively
large, or target heating was sufficient to cause significant diffusion within itself.
The long term change in the target composition as a function of time was not
monitored due to the inability of the target to fit in the XPS chamber.
Devices were also fabricated on an alumina (Al2O3) substrate and
consisted of 300nm silicon carbide sandwiched between two platinum electrodes.
The initial purpose of this was to produce capacitors with a silicon carbide
Page 21 dielectric material to determine the dielectric constant. This failed however, due
to the relatively low resistivity of the silicon carbide films in comparison to the film
thickness.
Therefore the devices were only used to examine the interface
between sputtered platinum and silicon carbide (figure 6). Due to an overlap of
the silicon 2p peak with platinum 5s peak (101.7 eV)18, silicon appeared to be in
the platinum layer. Therefore, the presence of carbon was a better indication of
the interface bounds.
Figure 6. Depth profile of platinum / silicon carbide interface. Sputter interval was
30 seconds. The width of the interface is approximately 25 nm without
accounting for layer mixing due to sputtering. An overlap of silicon 2p with
platinum results in a misrepresentation of silicon and platinum concentrations.
Page 22 Platinum sputtered at approximately twice the rate of silicon carbide,
making the width of the interface difficult to determine. This was not improved by
the atomic layer mixing effect caused by sputtering, and resulted in a larger
interface. Therefore the interface width was approximated at 25 nm. This also
indicated the surface roughness was less than 25nm, and was confirmed with
AFM and SEM.
In summary, the silicon carbide films deposited were carbon rich and
displayed oxidation at the surface. The ratio of carbon to silicon cannot be
explained due to a lack of knowledge of the average incident ion energy and
target crystal orientation. Depth profiles of the film, as well as surface
composition of varying film thickness give no indication of the film composition
approaching the target composition. However, the silicon carbide film
composition is not a function of depth and allows for repeatable and consistent
films.
Page 23 4.1 Evaluation of the Chemical Resistance of SiC Films
Silicon carbide is an extremely inert material and as a surface coating thin
films may be used as protective coating for electronic devices. Resistance to salt
water corrosion and potassium hydroxide etching were tested and assessed to
determine the suitability of silicon carbide films to protect against these
chemicals. Salt water corrosion resistance has applications in protecting
electronic devices in ships or coastal buildings, as well as portable electronic
devices. Potassium hydroxide is a common electrolyte as well as an etchant in
electronics fabrication. Therefore the suitability of silicon carbide films for these
chemically sensitive environments for many practical applications will rely on the
stability of the SiC film in these potential applications.
If a silicon carbide film can be a suitable protective coating, the films must
demonstrate that the coating will not damage the electronic device, first and
foremost for a USB or similar storage device. In this study, seven USB flash
drives were coated with silicon carbide and then the performance of the devices
and coatings was assessed. Deposition was done on both sides of the USB flash
drives at 400W and the drives were arranged randomly on the stage, and Kapton
tape was used to protect the contacts. Figure 7 shows a device before and after
deposition.
Page 24 Figure 7. USB flash device before (left) and after (right) coating with SiC. Color
gradient areas near features indicate uneven film.
Areas in the shadow of large device features show a color gradient in
Figure 7, indicating an uneven film thickness. Uneven coating of millimeter scale
features may reduce coating performance and efficiency. The deposition did not
damage any of the devices and each one remained in working order. Therefore
the performance of the electronic device was unaltered and demonstrated silicon
carbide films may safely be applied to selected electronic devices.
Page 25 Investigation of the Potential Salt Water Corrosion of
SiC Coated and Uncoated USB Devices
Many electronic devices with printed circuit boards and surface mounted
chips are used in saltwater environments. A ship’s computer and control systems
may be exposed to salt water over the course of its lifetime. In this study, USB
storage devices are used as an analog to electronic components on a ship. In
order to test the resistance of the SiC film to salt water corrosion, a saturated
sodium chloride solution was prepared (6.14 mol/L). This would be a higher salt
concentration than electronic devices on a ship would experience in seawater (on
average 0.6 mol/L). In this study eight uncoated and seven coated USB flash
drives were placed in the prepared NaCl solution for 24 hours. The devices were
then removed but not cleaned or dried, and placed in a sealed environment for
one week. This was followed by a 24 hour soak in deionized water in order to
remove any salt buildup from the devices and then allowed to dry. Figure 8
shows damage to a coated and uncoated device that were exposed to the salt
solution.
Page 26 Figure 8. Uncoated and coated USB device after a one day soak in salt water
with seven more days kept wet. Note for the uncoated device, one of the
connections between the contacts in the case and the circuit board has corroded.
The coated device has the contacts appropriately covered.
As a result of the corrosion, four uncoated USB devices failed to function
and only one coated device failed. There were two failure modes for the devices,
indicating corrosion in different places. If the device failed to be detected by the
computer, then one of the four connection points to the board was corroded (they
supply power and carry data). This behavior was shown only by the uncoated
devices, and a clear corrosion at these points can be seen (Figure 9). In the
second failure mode a USB device was detected by the computer but not
accessible, indicating a failure of the contacts between the board and surface
mounted chip. This was common in uncoated devices but was also present in
coated devices due to an incomplete coverage of silicon carbide.
Page 27 Immediately following the first test, a second test was performed on the
devices which had survived. The devices were soaked for three days in the
prepared salt water solution. They were then removed but kept wet for 6 days in
a sealed container, and then rinsed and dried before testing. Dried salt was
visible on all devices due to the short rinse time. Two additional uncoated
devices failed and one coated device, and a higher degree of corrosion is visible
(Figure 8). There was a significant amount of silicon carbide that flaked off the
iron contact casing and the surfaces of the mounted devices. This indicated poor
adhesion to iron or matte surfaces.
Due to the small sample size used in this study a conclusion could not be
easily drawn. A total failure rate of 75% for the uncoated devices and 25% for
coated devices in a 6.14 molar salt solution indicated that optimizations must be
made in the coating process. In order to better determine the effectiveness of
silicon carbide coatings for protection against salt water corrosion. A larger scale
test with more control would need to be performed. Different devices could be
also chosen as testing samples in this aspect study, or the coating process could
be modified to provide a more adequate coating for the application in the
presence of salt solutions.
Page 28 Figure 9. Uncoated and coated device after a second test. Uncoated devices
had severe corrosion of board connections and surface mount connections. The
coated device suffered mainly from surface mount connection corrosion (see
circled areas).
Page 29 Potassium Hydroxide Etch Resistance of Silicon Carbide Films
Potassium hydroxide (KOH) is a common electrolyte and etchant in
electronics fabrication processes. Therefore silicon carbide’s resistance to
industrial strength etch solutions may be a crucial property. In order to test
resistance to potassium hydroxide etch, silicon wafers were coated with silicon
carbide and immersed in 33 wt% KOH solutions. Samples of SiC deposited at
200W, 300W, and 400W with a film thickness of 150nm were used, as well as
one 500nm sample deposited at 400W. At room temperature there was no
damage to the silicon carbide on any sample after a 7 day period of sitting in the
prepared KOH solution. This was not surprising, for silicon carbide was expected
to be highly inert to alkaline chemicals. The potassium hydroxide solution was
then placed on a temperature controlled hotplate in order to increase the etch
rate. After raising the temperature to 70°C for 1.5 hours, there was still no visible
etching of SiC. Lithography may potentially be performed on higher film layers
without damaging the underlying SiC layer.
The potassium hydroxide bath was then raised to 80°C and was set for 20
hours. At this temperature the etch rate of 100 silicon would 75 µm/hr19 and
therefore nearly reached the wafer thickness. After this extended etch period,
differentiation was finally visible between samples. Figure 10 shows the 400W,
300W, and 200W samples after etching, with the 400W and 200W samples
showed severe damage. The test was repeatable with pieces of the same wafer,
and the 300W samples performed relatively better than the other samples. The
500nm samples deposited at 400W also performed well, but this may be due to
Page 30 the slow rate of SiC etching. If the damaged samples were considered in this
evaluation, then the etch rate was at least 7 nm/hr at 80°C. However, considering
the undamaged samples, the etch rate could be far less than that. This large
difference in the chemical resistance could not be attributed to the different
deposition powers due to the lack of control and monitoring of other process
variables.
Figure 10. 400W (left), 300W (middle), and 200W (right) samples of 150nm SiC
on silicon after 30 wt% KOH etching for 20 hours at 80°C. Results from a second
test can be seen above each sample in the image, reveal the same etch pattern.
In an attempt to evaluate any difference between the samples of the SiC
film on silicon wafers, SEM, EPMA, and XPS were used to inspect the physical
or chemical differences of these samples. The overall compositions found by
XPS were within 1 mol percent and did not show a correlation with the etch
Page 31 severity. There was also no visible chemical shift on the surface of the film for
any sample, indicating similar bonding. With no evidence for different
composition or structure causing the disparity in etch rates; possible local
property variations were examined.
Electron microscopy images of non etched surfaces were not possible to
obtain; the films did not have any features that could be focused upon. The
microscope used a field emission source with a resolution approaching 1 nm.
Therefore any defects in the film that may increase etch rate are on the atomic
scale or nonexistent. Electron probe microanalysis was attempted to be used in
this study, but due the low x-ray emission of the light elements a sufficient signal
was not produced. Images of the etched samples revealed that there may be a
local property variation that increases etch rate (Figure 11). An image of a 400W
sample revealed large circular holes almost exclusively in the lighter shaded
region of the image. A local variation of composition or structure caused uneven
etch rates.
Page 32 Figure 11. Image of etched 400W sample, depicting large holes predominately
in the lighter areas. This indicates there may be local variation in composition or
structure that affects etch rate.
When the sample was examined closer with backscattering images (Figure 12),
many of the holes were found to be near perfect circles. This indicated the
starting point for the etch crater was a point source that etched evenly in the
radial direction. Even etching of the underlying silicon substrate was visible in
many of the holes. The darker areas indicate higher nuclear density atoms, and
could be attributed to residual potassium salts.
Page 33 Figure 12. Backscattering image of 400W etched sample. Note the near perfect
circular pits indicating point sources for the start of corrosion.
Photoelectron spectroscopy of the surface indicated a majority of carbon
and residual potassium and traces of chlorine, sodium, and sulfur contamination.
Silicon showed a complete shift in binding energy from 100.4 eV to 102 eV
revealing any surface silicon was oxidized. The carbon 1s split into two peaks
(Figure 13), one at 284 eV and one at 287 eV indicating some additional bonding
to more electronegative elements such as oxygen, chlorine, or sulfur. A decrease
in the presence of silicon indicated a preferential etching of the silicon from the
film. Therefore the relatively poor resistance of some samples to other samples
may be the result of local pockets of silicon rich film.
Page 34 Figure 13. Carbon peak at slightly below 284.2 eV indicating bonding to silicon.
A much higher peak also appears at 287 eV, most likely belonging to carbon
bonding to other species. Potassium peaks are shifted down in binding energy
(standard at 297.3 and 294.6 for 2p1/2 and 2p3/2 respectively18.
The results of these etch tests showed that silicon carbide films could
provide excellent resistance to concentrated alkaline solutions at room
temperature for extended durations or at higher temperatures for shorter periods.
It was also revealed that under extreme etching conditions, differences in the
silicon carbide films can be seen. Therefore more control must be exerted over
the deposition process to optimize the films. The source of the difference in
chemical resistance remained to be unknown; there were no visible chemical
shifts between samples or nano sized features between films.
Page 35 Hardness and Young’s Modulus
The ability to deposit silicon carbide films on polymers could be used to
increase the durability of polymer substrates. Hardness acts is a good indicator
of wear resistance, and therefore hard films will result in more durability. Tests
were performed using a Hysitron nanoindenter with a diamond Berkovich probe.
In situ imaging was available to find smooth sites for indentation, as well as to
confirm a well defined three sided pyramidal indentation. Surface roughness was
not greater than 2 nm for any sample. Each test was performed using a
maximum force of 1050 µN or 2050 µN for film thicknesses of 150nm and 500nm
respectively. A five second linear load, 5 second hold at the maximum force, and
a 5 second linear unload was used for both film thicknesses. Figure 14 shows
force versus time and force versus displacement data for a typical indentation.
Hardness and the reduced modulus are calculated from the unloading
portion of the force versus displacement curve. First a selected portion of the
unloading curve is fit to the power law relation
1.
where F is the force applied, h is the displacement, and A, hf, and m are
constants. The derivative of the resultant equation with respect to the
displacement at the maximum load is the contact stiffness S. The contact depth
is then calculated with the following equation
0.75
2.
Page 36 2500
Force (µN)
2000
1500
1000
500
0
0
2
4
6
8
10
12
14
16
18
Time (s)
2000
Force (µN)
1500
1000
500
0
0
10
20
30
40
50
Displacement (nm)
Figure 14. (Top) Typical loading curve for a nanoindentation test. (Bottom)
Typical indentation curve for silicon carbide on a silicon wafer. The bottom
portion of the curve is used for calculating the hardness and reduced modulus.
where hc is the contact depth, hmax is the maximum displacement, and Fmax is the
maximum load. The hardness and reduced modulus are then calculated with the
following equations
3.
Page 37 4.
√
2
where H is the hardness, Er is the reduced modulus, and A(hc) is the contact
area, which is a function of the contact depth. Young’s modulus can be extracted
from the reduced modulus according to the relation
1
1
1
5.
where E is Young’s modulus and v is Poisson’s ratio. The diamond tip of the
indenter had a modulus of 1140 GPa and a Poisson’s ratio of 0.07, and the
sample’s Poisson’s ratio was assumed to be the bulk value of 0.14.
Initial tests were performed on three silicon wafers with 150 nm silicon
carbide deposited with varying powers of 200W, 300W, and 400W. Three
random indentation points were chosen for each wafer. The maximum force for
the indentation was set to 1050 µN, which resulted in an average maximum
displacement of 43.5 nm. ASTM standards require the indentation depth be ten
times greater than the film thickness, in order to minimize substrate effects20.
Therefore, the resulting hardness of 19.2 ± 3.6 GPa and modulus of 197 ± 28
GPa had high substrate effects.
A second test was performed on a silicon wafer with 500nm SiC,
deposited at 400W. The maximum force used was 2050 µN, which resulted in an
average maximum displacement of 55.2 nm. Six indentations were made at
random places on the wafer, and four more were made within 1 cm from the
wafer edge. Visual inspection of the wafer showed a color change gradient at
approximately 1 cm from the edge, indicating decreasing film thickness. The
Page 38 hardness for the bulk of the wafer was found to be 34.3 ± 2.2 GPa and the
modulus to be 283 ± 13 GPa. Near the edge of the wafer, the hardness was
found to be 26.6 ± 1.2 and the modulus to be 195 ± 3 GPa. The decrease in
hardness near the wafer edge was due to the decrease in film thickness and an
increase in substrate effects. All Results can be found in table A.2 in the
appendix.
Literature reported results for silicon carbide films vary greatly, depending
on deposition method and carbon content. Films deposited by activated reactive
evaporation reported in Thin Solid Films (1994) had an average hardness of 19.9
± 6.9 GPa, with a maximum value of 33 GPa reported21. Liao and Girshick
reported hardness values between 30 and 50 GPa, depending on substrate
temperature during PECVD deposition22. Costa and Camargo achieved a
hardness of 30.6 ± 1.9 on WC-Co cutting tools sputtered with SiC23. Fujiyama,
Nakamura, and Sumomogi reported hardness values between 25 and 50 GPa
depending on the carbon content of the target17.
Abrasive wear resistance has an inverse proportionality to hardness, and
therefore is an important property for SiC films used as protective coatings24.
Amorphous silicon carbide films deposited by RF sputtering have high hardness
and therefore are a promising coating material for improving wear resistance. A
variety of techniques can be used to deposit SiC films but sputtering allows for a
low temperature deposition. Therefore wear resistance for low melting point
polymer substrates could be improved significantly with SiC films.
Page 39 Flexibility and Film Adhesion
The ability to deposit silicon carbide films at ambient temperature allows
for many flexible and low melting point polymers to be used as the substrate
materials. Therefore, film flexibility and adhesion to the substrate are important
film properties. Internal stress increases with film harness, and negatively affects
adhesion. Kiyotasa Wasa et al. reported the use of a borosilicate glass to
improve film adhesion for silicon carbide sputtered films, as well as a silicon
carbide layer to improve adhesion for boron carbide films4. J.L. He et al looked at
adhesion of amorphous SiC deposited on glass by PECVD and found better
adhesion at higher deposition powers. At low RF powers softer films were
formed, and therefore negatively affected film adhesion25. This study focuses on
silicon carbide films for flexible substrates and demonstrates the adhesion under
bending scenarios.
Films of sputtered SiC were under high compressive stress; when on a
thin and flexible substrate, it curled up on itself (Figure 15). The film did not flake
off or crack unless bent around a small radius of curvature. In order to examine
the physical strength under compressive and tensile stress, strips of Kapton
coated with SiC were pulled around a known diameter (Figure 16), and then
examined under a scanning electron microscope for fractures. The forces used to
were large enough to tightly press the film to the wire, but not stretch the film or
substrate. Silicon carbide films of 150nm and 300nm were pulled around wires of
diameter 0.32mm and 0.81mm. Films were also bent directly in half to approach
the limit of diameter = 0 mm.
Diameters greater than approximately 1mm
showed little to no damage to the films.
Page 40 Figure 15. 500nm of silicon carbide on 0.02 mm thick Kapton curls up due to
high compressive stress.
Figure 16. Silicon carbide film on Kapton pulled around a wire under tensile
stress. Compressive stress is achieved by switching the SiC layer to inner
diameter.
Page 41 Figure 17 shows films of 150nm and 300nm bent around 20 AWG wires
(0.81mm) compressively. Films bent under tension showed no damage and are
therefore not shown. The 150nm film showed cracking spaced at approximately
50µm and the 300nm film cracked on the order of 100µm intervals. Figure 18
shows films of 150 and 300nm bent around 28 AWG wires (0.32mm)
compressively. Again, films put under tensile stress at this diameter showed no
damage. Significant cracking could be seen at 20-50µm intervals for both 150nm
and 300nm samples.
Figure 17. 150nm (left) and 300nm (right) bent around 20 AWG wires (0.81mm)
compressively.
Figure 18. 150nm (left) and 300nm (right) bent around 28 AWG wires (0.32mm)
compressively.
Page 42 It was not until samples were bent completely in half that cracking was
observed for samples under tensile stress. Figure 19 shows both 150nm and
300nm under tensile and compressive stress when bent in half. Significant
damage could be seen for both compressive stress samples and the 150nm
tensile stress sample, where crack spacing was under 10µm. Only one crack was
visible for the 300nm tensile stress sample, which may not have been the result
of bending under tension.
Figure 19. 150 nm (left) and 300 nm (right) bent under tensile (top) and
compressive (bottom) stress.
Page 43 Silicon carbide films deposited by RF magnetron sputtering provided
excellent adhesion to substrates and flexibility. They may provide protective
layers for chemical and wear resistance in flexible devices which can be bent to
short of creasing the device. Thicker films showed more resistance to cracking.
Therefore nanometer scale silicon carbide films could be used to provide a
protective layer for electronic devices.
Page 44 Examining Moisture Diffusion Through SiC Films
Many electronic devices are sensitive to moisture, and if a polymers often
swell due to high water retention. Therefore the ability for silicon carbide films to
prevent moisture diffusion makes them attractive coatings for many devices. In
order to control relative humidity, this experiment used saturated salt solutions.
Air above a saturated solution is maintained at a constant relative humidity at a
given temperature due to the vapor-liquid equilibrium defined by the saturated
solution26. A difference in relative humidity was used as the driving force for
diffusion in this setup, where the changing boundary condition is recorded
(Figure 20). There is currently no complete model for diffusion of vapor through a
thick semi permeable membrane. It is know that there are two major factors that
affect diffusion: the rate at which water travels through a membrane and the
absorption/desorption rate at the membrane surface27.
Figure 20. Setup to test moisture diffusion. One side of a chamber is kept at a
constant humidity with a saturated salt solution. The diffusion through a
membrane is modeled, and the boundary condition changes with time.
Page 45 To test moisture resistance, this study used four custom built PVC
containers (Figure 21). Each container had a separable top and bottom, which
were bolted together and had a double o-ring seal. The bottom chambers were
fitted with vacuum wire feedthroughs to allow for humidity and temperature
sensors. The top chambers were fitted with vacuum wire feedthroughs for
sensors and valves that allowed for a nitrogen purge. Each bottom chamber
contained a saturated sodium chloride solution to keep the relative humidity at
75%. Humidity in both the top and bottom chambers was monitored using an
Ohmic Instruments UPS-500 relative humidity sensor with an SC-500 signal
conditioner. Data was recorded by a computer with an analog data acquisition
card.
Figure 21. Test chambers for humidity experiments. Each chamber is equipped
with to humidity sensors, temperature sensor, and purge valves.
Page 46 The substrate used in this setup was DuPont Kapton, a polyimide film
commonly used in flexible electronics packaging. Each piece of Kapton was laser
cut to fit the chambers and then cleaned with alcohol and acetone. Following
cleaning, Kapton pieces were sputtered with 100, 300, and 500 nm of SiC, and
an additional piece was sputtered on both sides with 150nm of SiC. Each piece
of single sided SiC sputtered Kapton showed high internal compressive stress.
The inherent stress from both films of the double coated Kapton counteracted
each other and did not tend to roll up on itself.
Initial tests were run for a period of one to three days, with data logged
every minute. The silicon carbide layer was put on both the wet and dry side of
the chamber, and showed no difference. In some tests, the Kapton and SiC
pieces were prebaked at 110°C to dry them. This only caused an initial period in
which the Kapton absorbed water from the top chamber, but did not affect the
water vapor diffusion rate. After analysis of the data, it was determined that the
voltage outputs from the humidity sensors were extremely noisy and did not
output the correct voltage range. RC filters were added to each voltage output to
improve the signal quality. Additionally, the top chambers were pumped with
nitrogen rather than using room humidity as the initial condition.
Although initial tests contained signal noise and shifted signal range, the
results were reproducible and consistent. Therefore although the voltages
recorded could not be translated directly to relative humidity; they still
represented rates of water vapor diffusion. The tests showed that thicker layers
of silicon carbide provided significant performance increases. However, when the
Page 47 Kapton sample that was coated on both sides with 150nm SiC was tested, it
outperformed a singled sided 300nm SiC. This was because the rate of diffusion
was affected by the rate of absorption/desorption at the surface. With both sides
of the Kapton covered, the absorption from the higher humidity environment and
desorption into the lower humidity environment was made insignificant by the low
diffusion coefficient of SiC. The rate of water vapor diffusion was directly affected
by the difference in humidity of the two environments.
To find the diffusion coefficients for water through Kapton and SiC, a
standard mass balance was used. Examining the mass transfer equation
C
r 6.
reduces to
for the setup where C is concentration of water vapor,
7.
is the diffusion
coefficient for water vapor through a medium, z is the axial direction in cylindrical
coordinates, t is time, and r is the generation rate of water vapor. The
experimental setup assumes the boundary conditions
t
t
z
∞, C
0, C
0, C
C C C
8.
Page 48 where C is the concentration on the humid side, and C
is the initial
concentration on the dry side. The differential equation can be scaled to include
these conditions with
Θ
9.
Θ
Θ
10.
where Θ is the scaled concentration. Solving this partial differential equation
requires a similarity transform in the form of
11.
where η is the transform variable. This changes the boundary conditions to
0, Θ
∞, Θ
0, Θ
1 0
1
12.
and then the following relations are then made
Θ
Θ
Θ
Θ
Θ
Θ
12.
13.
Θ
Θ
Θ
Θ
14.
Θ
Θ
1
Θ
15.
Θ
1
Θ
Θ
16.
Page 49 and substituted into the original equation.
Θ
Θ
1
Θ
17.
This can then be simplified by choosing a = 1 and b = -1/2.
Θ
Θ
2 √
18.
Using equation 11, this can finally be arranged into a solvable differential
equation.
Θ
Θ
2
19.
∂Θ
2
20.
∂Θ
ln
4
∂Θ
21.
∂Θ
22.
eA
Θ
e
23.
Θ
1
erf
√4
24.
C
1
erf
2√
C
C
C 25.
Page 50 As a result of the linear relationship between concentration and relative
humidity as well as the linear response of the humidity sensors used, the sensor
voltages V, V , and V . The only unknown in equation 25 is the diffusion
coefficient
, and therefore can be fit to the data.
Θ
V
1
erf
2√
V
V
V
V
V
V
26.
27.
V
Results of a simple least squares fit to the data yielded diffusion
coefficients on the order of 10
for Kapton. In comparing the data and model
(Figure 22), it could be seen that the model increases more quickly than the data.
The rates of adsorption onto the Kapton and desorption from the Kapton were
not negligible and resulted in a slower than expected rate of diffusion.
Humidity Sensor Voltage (V)
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
0
200
400
600
800
1000
Time (minutes)
Figure 22. Diffusion model for only Kapton does not appropriately match curve.
The blue line represents the data and the red line represents the model.
Page 51 The silicon carbide coated Kapton did not have this problem, as the SiC
was the most rate limiting layer. In the case of 300nm SiC coating and 150nm
double sided SiC coating, two and three equations were fit for the diffusion
coefficient. In the case of the doubled sided coating, both silicon carbide diffusion
coefficients were set to be equivalent. The following set of equations were used
V
1
1
V
V
erf
erf
1
2
V
V
2
erf
V
2
V
V
V
28.
V
V
V
29.
30.
where V1, V2, and V3 are the modeled humidity sensor voltages at the first
SiC/Kapton interface, Kapton/SiC interface, and SiC/air interface respectively.
The diffusion coefficient
was set to the coefficient found in the only
Kapton test. Results are shown in figure 23, where the data and diffusion model
matched more closely. This resulted in SiC diffusion coefficients on the order of
10
, which is five orders of magnitude lower than that of Kapton. Diffusion
coefficients are summarized in table 1. Therefore nanometer scale coatings of
silicon carbide are able to provide a significant moisture barrier improvement.
Page 52 Humidity Sensor Voltage (V)
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
0
200
400
600
800
1000
800
1000
Time (minutes)
Humidity Sensor Voltage (V)
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
0
200
400
600
Time (minutes)
Figure 23. Top: Kapton with 300nm SiC. Bottom: Kapton with 150nm SiC on
both sides. The blue lines represent the data and the red lines represent the
model.
Page 53 Table 1. List of Measured Diffusion Coefficients
Kapton Test #1
6
10
Kapton Test #2
4
10
300nm SiC Test #1
6
10
300nm SiC Test #2
2
10
2
10
4
10
150nm SiC Both Sides
Test #1
150nm SiC Both Sides
Test #2
The diffusion of moisture through amorphous SiC has been modeled by
Haying He et al, and it was found the rate depended mainly on the SiC ring
morphology28. If the diffusion coefficient can be modeled by the Arrhenius
relation
31.
exp
where
is a constant, Ea is the activation energy for diffusion, k is the
Boltzmann constant, and T is the temperature. If 3.5
10
is used for the
diffusion coefficient, the calculated activation energy is 0.7 eV. By comparing this
to values calculated by Haying He et al, this indicated the amorphous SiC was
made up of mainly five member rings. The diffusion coefficient of the silicon
carbide films could be substantially lowered if the average ring size was reduced,
thereby reducing atomic spacing.
Silicon Carbide deposited via sputtering provided a moisture barrier when
deposited on the polyimide film Kapton. Coating both side of the polyimide film
Page 54 with SiC not only reduced the film stress, but provided a greater moisture barrier
as well. Further tests should be done using different substrates, as well as
different temperatures. This research showed consistent repeatable results for
the membranes tested, and a mathematical model that fits the data. SiC shows
potential as a way to improve electronics packaging that require low temperature
solutions. Further tests should be performed to determine the amorphous silicon
carbide ring morphology and attempt to engineering ring size.
Page 55 Conclusions and Recommendations
This study has shown silicon carbide sputtered at ambient temperatures
exhibits many properties that make it favorable for coating applications. The films
were 3:2 carbon to silicon and the silicon near the surface fully oxidized. Films
can be deposited pinhole free with good adhesion on many substrates. The
combination of the flexibility of silicon carbide films and the low deposition
temperatures may prove useful in protecting polymer surfaces. A small diffusion
coefficient for moisture can also prevent polymers from absorbing excess
moisture and swelling. The high hardness of silicon carbide films indicates good
wear resistance. For standard electronics coating applications, silicon carbide’s
chemical resistance is more than needed for any circumstance. Extreme tests
reveal the possibility to engineer the films to have even greater chemical
resistance.
Future work needs to study the effects of deposition power, pressure, and
incident ion energy on the nature of the films. These variables could be
manipulated to engineer properties for the films. The electrical properties of the
films should also be examined, as they may be different than bulk silicon carbide.
Page 56 Appendix
Table A.1 Silicon carbide sputter target composition
Component
Weight %
SiC
99.5
Al
0.016
B
0.079
Fe
0.098
Mn
0.062
Ni
Trace
O
0.041
Ti
Trace
Table A.2 Nanoindentation results for all tests
Indentation
Hardness
(GPa)
500nm SiC on Si Wafer
500nm SiC on Si Wafer
500nm SiC on Si Wafer
500nm SiC on Si Wafer
500nm SiC on Si Wafer
500nm SiC on Si Wafer
500nm SiC on Si Wafer Edge
500nm SiC on Si Wafer Edge
500nm SiC on Si Wafer Edge
500nm SiC on Si Wafer Edge
150nm SiC on Si Wafer Deposited at 200 Watts
150nm SiC on Si Wafer Deposited at 200 Watts
150nm SiC on Si Wafer Deposited at 200 Watts
150nm SiC on Si Wafer Deposited at 200 Watts
150nm SiC on Si Wafer Deposited at 200 Watts
150nm SiC on Si Wafer Deposited at 300 Watts
150nm SiC on Si Wafer Deposited at 300 Watts
150nm SiC on Si Wafer Deposited at 300 Watts
150nm SiC on Si Wafer Deposited at 300 Watts
150nm SiC on Si Wafer Deposited at 300 Watts
150nm SiC on Si Wafer Deposited at 400 Watts
150nm SiC on Si Wafer Deposited at 400 Watts
150nm SiC on Si Wafer Deposited at 400 Watts
150nm SiC on Si Wafer Deposited at 400 Watts
150nm SiC on Si Wafer Deposited at 400 Watts
37.6
34.3
32.8
32.3
32.5
36.1
25.0
26.6
26.9
27.9
14.4
17.7
21.0
23.6
18.2
18.6
19.0
25.8
13.8
20.4
19.3
16.5
20.3
14.9
24.7
Young’s
Modulus
(GPa)
303
288
278
272
269
287
191
196
197
197
187
222
254
206
213
208
234
193
248
241
277
216
350
264
201
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