PHYSICAL AND CHEMICAL PROPERTIES OF
AMBIENT TEMPERATURE SPUTTERED SILICON CARBIDE FILMS
by
DANIEL THOMAS SHELBERG
Submitted in partial fulfillment of the requirements
For the degree of Master of Science: Engineering
Thesis Adviser: Dr. Chung-Chiun Liu
Department of Chemical Engineering
CASE WESTERN RESERVE UNIVERSITY
May, 2010 CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis/dissertation of
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candidate for the ______degree *.
(signed)______(chair of the committee)
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*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
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List of Tables Table Page Table 1. Diffusion coefficients for Kapton 53 and SiC A.1 Trace elements in sputter target 56 A.2 Nanoindentation data 56
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List of Figures Figure Page Figure 1. Sputtering process illustration 12 Figure 2. Typical XPS of SiC film shows a carbon rich film with 16 oxidation at the surface. Figure 3. XPS of Si 2p peak in SiC film at the surface and after a 1 18 minute sputter. A chemical shift in this peak can be seen between the two layers. Figure 4. XPS of C 1s peak in SiC film at the surface and after a 1 18 minute sputter. A chemical shift in this peak can be seen between the two layers. Figure 5. Sputter yield data for Si and C from nuclear tables. This 19 does not reveal why the films in this study are 3:2 carbon to silicon. Figure 6. Depth profile of SiC / Pt interface shows the thickness of this 21 interface. Figure 7. Uncoated and SiC coated USB flash device used in 24 saltwater corrosion study. Figure 8. Initial saltwater corrosion results show heavy corrosion of 26 one contact point for an uncoated device. Figure 9. Second saltwater test results, main corrosion points on both 28 devices. Figure 10. Image of potassium hydroxide (KOH) etch damage of SiC 30 film on silicon wafers. Figure 11. SEM secondary electron image of 400W etched SiC film 32 on a silicon wafer. Figure 12. SEM backscattered image of 400W etched SiC film on a 33 silicon wafer. Figure 13. XPS of etched 400W sample, carbon peaks show a large 34 chemical shift, and potassium peaks from residual etchant. Figure 14. Loading and indentation curves for nanoindentation of SiC 36 on silicon wafer.
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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
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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.
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List of Abbreviations and Symbols A – constant in power law relation 2 A(hc) – contact area as a function of contact depth (nm ) A1 – integration constant AFM – atomic force microscopy a – constant used in similarity transform b – constant used in similarity transform C – concentration of water (mol/L) – initial concentration of water on dry side(mol/L) – concentration of water on wet side(mol/L) – diffusion coefficient of water through a medium (m2 sec-1) - prefactor for Arrhenius diffusion (m2 sec-1) 2 -1 – diffusion coefficient of water through Kapton (m sec ) 2 -1 - diffusion coefficient of water through SiC (m sec ) - transform variable (seca mb) E – Young’s modulus Er – reduced modulus EPMA – electron probe microanalysis Θ - concentration difference ratio F – force (µN) Fmax – maximum force (µN) h – displacement (nm) H – hardness (GPa) hc – contact depth (nm) hf – constant in power law relation (nm) hmax – maximum displacement k – Boltzmann constant 8.617 10 ⁄ KOH – potassium hydroxide LED – light emitting diode m - constant in power law relation MOSFET – metal oxide semiconductor field effect transistor ν – Poisson’s ratio PECVD – plasma enhanced chemical vapor deposition PVC – polyvinyl chloride r - generation rate of water (mol L-1 sec-1) S – contact stiffness t – time (sec) T – temperature (K) SEM – scanning electron microscopy SiC – silicon carbide USB – universal serial bus V – humidity sensor voltage (V) – initial humidity sensor voltage on dry side(V) – humidity sensor voltage on wet side SiC/Kapton interface (V) – humidity sensor voltage on dry side Kapton/SiC interface (V) – humidity sensor voltage on dry side SiC/Air interface (V) – humidity sensor voltage on wet side(mol/L) XPS – x-ray photoelectron spectroscopy z – distance along the cylindrical z-axis
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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).
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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.
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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
13 2p1/2 and 2p3/2 binding energies would be 99.8 eV and 99.2 eV respectively ,
14 and the peak for SiO2 would be at 103.4 eV . 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
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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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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
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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.
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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.
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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.
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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 18 (standard at 297.3 and 294.6 for 2p1/2 and 2p3/2 respectively .
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
2. 0.75
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2500
2000
1500 (µN)
Force 1000
500
0 024681012141618
Time (s)
2000
1500 (µN)
1000 Force
500
0 0 1020304050
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.
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√ 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
6. C r reduces to
7.
for the setup where C is concentration of water vapor, 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 ∞,C C 8. t 0,C C z 0,C C
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
9. Θ
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 1 12. ∞,Θ 0 0,Θ 1
and then the following relations are then made
Θ Θ Θ 12.
Θ Θ Θ 13.
Θ Θ Θ Θ 14.
Θ Θ Θ 15. 1
Θ Θ Θ 1 16.
Page 49
and substituted into the original equation.
Θ Θ Θ 17. 1
This can then be simplified by choosing a = 1 and b = -1/2.
Θ Θ 18. 2 √
Using equation 11, this can finally be arranged into a solvable differential
equation.
Θ Θ 19. 2
∂Θ 20. 2 ∂Θ
∂Θ ln 21. 4
∂Θ 22.
Θ eA e 23. Θ 1 erf √4 24. C 1 erf C C C 2√ 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.
V V 26. Θ V V
27. V 1 erf V V V 2√
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.
2.6
2.4 (V)
2.2 Voltage 2
1.8 Sensor
1.6
Humidity 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
28. V 1 erf V V V 2
29. V 1 erf V V V 2
V 1 erf V V V 30. 2
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
2.6
2.4 (V)
2.2 Voltage 2
1.8 Sensor
1.6
Humidity 1.4
1.2 0 200 400 600 800 1000
Time (minutes)
2.6
2.4 (V)
2.2 Voltage 2
1.8 Sensor
1.6
Humidity 1.4
1.2 0 200 400 600 800 1000
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 150nm SiC Both Sides 2 10 Test #1 150nm SiC Both Sides 4 10 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 Young’s Hardness Indentation Modulus (GPa) (GPa) 500nm SiC on Si Wafer 37.6 303 500nm SiC on Si Wafer 34.3 288 500nm SiC on Si Wafer 32.8 278 500nm SiC on Si Wafer 32.3 272 500nm SiC on Si Wafer 32.5 269 500nm SiC on Si Wafer 36.1 287 500nm SiC on Si Wafer Edge 25.0 191 500nm SiC on Si Wafer Edge 26.6 196 500nm SiC on Si Wafer Edge 26.9 197 500nm SiC on Si Wafer Edge 27.9 197 150nm SiC on Si Wafer Deposited at 200 Watts 14.4 187 150nm SiC on Si Wafer Deposited at 200 Watts 17.7 222 150nm SiC on Si Wafer Deposited at 200 Watts 21.0 254 150nm SiC on Si Wafer Deposited at 200 Watts 23.6 206 150nm SiC on Si Wafer Deposited at 200 Watts 18.2 213 150nm SiC on Si Wafer Deposited at 300 Watts 18.6 208 150nm SiC on Si Wafer Deposited at 300 Watts 19.0 234 150nm SiC on Si Wafer Deposited at 300 Watts 25.8 193 150nm SiC on Si Wafer Deposited at 300 Watts 13.8 248 150nm SiC on Si Wafer Deposited at 300 Watts 20.4 241 150nm SiC on Si Wafer Deposited at 400 Watts 19.3 277 150nm SiC on Si Wafer Deposited at 400 Watts 16.5 216 150nm SiC on Si Wafer Deposited at 400 Watts 20.3 350 150nm SiC on Si Wafer Deposited at 400 Watts 14.9 264 150nm SiC on Si Wafer Deposited at 400 Watts 24.7 201
Page 57
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