Sputtering Fabrication of Silicon Nitride and
Silicon Oxide Based Dichroic Mirrors
By
ARCHNES
MASSACHUSETTF INSTITI.TE
OF TECHNOLOLGY
Dohyun Bae
S.B. Materials Science and Engineering
JUN 082015
Massachusetts Institute of Technology, 2015
LIBRARIES
Submitted to the Department of Materials Science and Engineering
in Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science
at the Massachusetts Institute of Technology
May2015 [rn
e -015]
C 2015 Dohyun Bae. All rights reserved
The author hereby grants to MIT permission to reproduce and to distribute
publicly paper and electronic copies of this thesis document in whole
or in part in any medium now known or hereafter created.
Signature redacted
.
Signature of Author..........................
.......
.................
Dohyun Bae
Department of Materials Science and Engineering
May 1, 2015
C ertified by .................................................
Signature redacted
Jurgen Michel
Senior Research Scientist & Senior Le turer of Materials Science and Engineering
Thesis Supervisor
A
A
1-1
Signature redacted
A ccepted by ....................................................
.
.. ..
Geoffrey Beach
Professor of Materials Science and Engineering
Chairman, Undergraduate Thesis Committee
1
2
Sputtering Fabrication of Silicon Nitride and
Silicon Oxide Based Dichroic Mirrors
By
Dohyun Bae
S.B. Materials Science and Engineering
Massachusetts Institute of Technology, 2015
Submitted to the Department of Materials Science and Engineering
in Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science
at the
Massachusetts Institute of Technology
Abstract
Thin films in optical materials utilize the properties of multiple materials to obtain
specific and fine-tuned transmission, absorption and reflectance at wavelengths. Dichroic mirrors
exhibit very different reflectance and transmission rates at certain cut-off wavelengths, which
can be adjusted using changes in layer materials and thickness. This is due to constructive optical
interference between alternating layers of two thin films of different refractive indices. This
study explored the sputtering methods of thin-film multilayers to form dichroic mirrors in the
visible spectrum for future solar-cell applications.
Silicon oxide and silicon nitride targets were selected as materials used in the sputtering
process. The sputtered multilayers and films were then characterized and analyzed using
spectrophotometry. The transmission spectrum of the initial multilayer depicted failure in
transmission at wavelengths under 500nm. The components of the multilayer were then sputtered
and analyzed to troubleshoot the problematic nitride films. Transmission spectra were utilized to
select each following process, and both reactive sputtering and cosputtering were explored as
means of creating nitride films with functional properties. Transmission spectra were analyzed
using the Swanepoel method to quantify optical characteristics to assure reactive sputtering of
the targets in a nitrogen environment as a viable direction of mirror construction. Possible further
work include the use of other targets such as titanium oxide, and different chamber gas mixtures
for finer control in the composition of the film layers.
3
4
Acknowledgements
I'd like to take the time here to give thanks to all the people whom without their support,
this would not have been possible.
I'd like to thank my thesis advisor, Jurgen Michel, for the patience and giving me the
proud opportunity of being able to work on this project. I've learned so much and being able to
work with the instruments and team that I did has been something truly special.
I owe many thanks to Vivek Singh and Brian Albert for answering all my questions and
helping me learn and run many of the tests throughout the months. This work wouldn't be
complete without their knowledge and generous assistance throughout these trying times.
5
6
Contents
Introduction ............................................................................................................................................... 11
Background ............................................................................................................................................... 11
1.
Dichroic M irrors ............................................................................................................................. 11
2.
Sp u tterin g ........................................................................................................................................ 1 3
3.
M aterial Selection of Silicon Nitride and Silicon Dioxide ............................................................. 14
4.
UV-Vis-Near IR Spectrophotom etry .............................................................................................. 15
5.
M odel Dichroic M irror Results ....................................................................................................... 15
Experim ental M ethods ............................................................................................................................. 16
1.
Sample Preparation ......................................................................................................................... 16
2.
Instrum ent processes ....................................................................................................................... 17
Results and Discussion ............................................................................................................................... 19
1.
Initial Calibration ............................................................................................................................ 19
2.
M u ltilay er ........................................................................................................................................ 20
3.
Oxide and N itride in Argon ............................................................................................................ 22
4.
N itride in Ar02 ................................................................................................................................ 25
5.
Oxide and N itride in N2Plasm a ...................................................................................................... 27
6.
Sum m ary ......................................................................................................................................... 34
C o n c lu sio n .................................................................................................................................................. 35
R e fe re n ce s .................................................................................................................................................. 37
7
8
List of Figures
Figure 1: How constructive interference occurs in both cases of alternating refractive index (Images
courtesy : Brian A lb ert) ...............................................................................................................................
12
Figure 2: Transmission spectrum for the ideal multilayer dichroic mirror, simulated of an actual design
u sin g T iO 2 and SiO 2 ....................................................................................................................................
16
Figure 3: Sputtering m achine exterior ..................................................................................................
17
Figure 4: Diagram explaining the interior of the load lock and chamber of the sputtering system.....18
Figure 5: Transmission spectra comparison between the initial multilayer deposition of oxide and nitride
in argon onto a quartz substrate compared to the simulated working dichroic mirror............................ 21
Figure 6: Single layer oxide deposition on glass in argon (left) with adhesive and a single layer nitride
deposition on glass in argon (right) with the adhesive removed............................................................
22
Figure 7: SiON deposited substrates cosputtered from Si 3N4 and SiO 2 in an argon plasma................. 25
Figure 8: Si 3N 4 deposited substrates sputtered in an ArO 2 plasma.......................................................
26
Figure 9: Transmission spectra of the Si 3N 4 deposited substrates sputtered in an ArO 2 plasma ........... 27
Figure 10: Si0 2 deposited substrates sputtered in an N 2 plasma............................................................
28
Figure 11: Transmission spectra of Si0 2 deposited substrates sputtered in an N 2 plasma ...................
29
Figure 12: Si 3N 4 deposited substrates sputtered in an N 2 plasma .........................................................
30
Figure 13: Transmission spectra of the Si 3N4 deposited substrates sputtered in an N 2 plasma.............. 31
Figure 14: The transmission spectrum for the Si 3N 4 deposited quartz substrate sputtered in N2 plasma
with fitted env elop es ...................................................................................................................................
33
List of Tables
Table 1: Refractive indices of target and substrate materials................................................................
15
Table 2: Thickness calibration samples QCM and profilometer measurements...................................
20
Table 3: Bond energies of various diatomic species of silicon, nitrogen and oxygen ..........................
23
Table 4: Values of k, TMo, Tmo, and the refractive indices of the substrate and film.............................
34
Table 5: Condensed sum m ary of results ................................................................................................
35
9
10
Introduction
Thin films allow for the use of various properties of multiple materials at once. While
thin films are typically seen in decorative use such as the gilding of stronger metals, thin films
play a very important role in the optical properties of materials. By combining specific thin
films, transmission, absorption and reflectance can be controlled at the finest of levels. For these
reasons, thin film research has been a booming industry for its applications in many fields
including semiconductors, optics and even solar energy.
The major aim of this project was to develop dichroic mirrors that allow for transmission
or reflectance depending on the wavelength of light passing through. These mirrors were to be
made to transmit visible wavelengths of light while reflecting others. By being able to separate
wavelength and frequencies of light passed through a stack of dichroic mirrors, dichroic mirrors
can divide the light onto orthogonal areas, for each respective photovoltaic cell. This increases
the collection efficiency of solar cells [1].
Background
1. Dichroic Mirrors
This project covered the fabrication of dichroic mirrors for future solar cell application.
Dichroic mirrors are mirrors with very different reflectance and transmittance properties at
different wavelengths. Dichroic mirrors are commonly formed as dielectric mirrors, where there
are multiple thin layers of different transparent optical materials. These mirrors are formed by
careful consideration of the refractive index of the materials of each film and the thickness of
each film. Tuning of these two parameters can maximize the reflectance at certain wavelengths.
11
In a dichroic mirror, as light hits the interface of the boundary between films, there is
reflectance that, in an alternating stack of different indices, is also reflected at the next interface
in the same manner. This creates constructive interference throughout the thin-film multilayer
and maximal reflection at these specific wavelengths as seen in Figure 1. This is similar, but the
opposite to the physics behind anti-reflective coating where the interface of increasing refractive
when:
- V/4n2 -+when:
+-
A/4n2
-
indices between the film and substrate allow for destructive interference in the reflections.
No phase
shift (nj > n2
No phase shift
(n2 > n3)
180 phase
shift (n2 < n3)
ni
>
n2
<
n3
nj
<
n2
>
n3
Figure 1: How constructive interference occurs in both cases of alternating refractive index
(Images courtesy: Brian Albert)
Typical use of these multilayer mirrors is at an angle of incidence of 45' where
wavelengths on one side of the cut-off are transmitted in the same direction while the
wavelengths on the other side of the cut-off are reflected at an angle of 90' from its initial
direction.
Thicknesses of the films start with an initial design based on a "quarter-wave" stack
where each thickness of each layer is dependent on the following equation:
12
4n
In Equation [1], d is the thickness of the layer, n is the refractive index of the layer's
material and ko is the center wavelength where reflectivity is aimed to be maximized. This is also
known as a stop band as the signal is not allowed to pass, and is instead reflected around this
range. For a dichroic mirror as compared to a dielectric mirror however, maximizing the
transmission at wavelengths not in the stop band is just as important. For this project,
maximizing transmission above and below the stop band was key.
2. Sputtering
Magnetron sputtering was chosen as the deposition technique to achieve the
aforementioned mirrors for a number of reasons. Compared to chemical vapor deposition (CVD)
methods, sputtering has finer tuning of thickness parameters and it avoids the large hydrogen
content found in CVD techniques. Sputtering is a physical vapor deposition technique that uses
gaseous plasma to deposit a thin film of material from a source onto a substrate. Material is
knocked off a target acting as a cathode by being bombarded with high energy gas ions. Plasma
is first struck by applying a large voltage onto a gas, usually argon, at very low pressures.
Particles from the plasma are positively ionized and accelerated onto the cathodic target in the
chamber. This blasts off the target material into the surrounding area and onto the nearby
substrate. The argon ions soon recombine with free electrons, returning to the original neutral gas
atom before repeating the process. This process allows for a constant uniform deposition rate
onto the nearby wafer with low strain in the deposited film. A Radio Frequency Field (RF Field)
is created by a magnet behind the target in magnetron, confining the plasma and preventing
13
charging of the target when it is not conducting. This confinement of the plasma allows for much
more efficient ionization [2].
Reactive sputtering is a process where the inert gas (argon) is replaced with a reactive
gas, such as nitrogen or oxygen. What happens during this process is that the target material
atoms within the chamber react chemically with the gas and produce a compound that is then
deposited onto the substrate. As the gas is used during this process unlike an inert gas that
becomes ionized and neutralized repeatedly, reactive sputtering has the additional variable of
constantly injecting gas into the chamber. Reactive sputtering can also be done with a
combination of a reactive and inert gas. In these cases, the ratio of each gas has a large influence
on the final thin film produced. Problems of reactive sputtering include the possible deposition of
compounds formed between the target and reactive gas onto the target itself. Known as target
poisoning, this leads into problems of sputtering out the target material as it becomes shielded by
the sputtered composition. This can be solved however by switching targets or letting the target
clean by sputtering with only an inert gas for a while.
3. Material Selection of Silicon Nitride and Silicon Dioxide
Taking into consideration that the final goal product (described further in subsection 5),
the two targets were selected to be silicon nitride (Si 3N 4 ) and silicon dioxide (SiO 2 ). Both of
these are transparent in the visible spectrum as films, they are easy to work with in sputtering and
have differing refractive indices, which allow for the aforementioned constructive interference
within their interfaces. Silicon dioxide and silicon nitride also have been explored in other
research as sputtering targets, so there is an abundance of literature concerning the materials.
Table 1 lists the refractive indices of the two films as well as the various substrates used in this
project.
14
Table
1: Refi-active indices of target and substrate materials
Material
Si3N 4
Refractive Index
2.0167 [3]
SiO 2
1.4585 [4]
Si
3.4434 [5]
Glass
1.46
Quartz
1.55
4. UV-Vis-Near IR Spectrophotometry
Spectrophotometry is a method that is able to measure transmittance or reflectance across
a spectrum of wavelengths. The instrument used in this project was a double beam
spectrophotometer. In this case, one beam is a reference blank sample while one beam contains
the test sample. The transmission through the test sample across the range of wavelengths is
compared to the test samples, giving a quantitative measurement in the form of a fraction of light
that transmits.
5. Model Dichroic Mirror Results
The ultimate goal of this project is to make a dichroic mirror for the visible spectrum.
The thickness would be expected to be anywhere between 1000nm and 2500nm for the needs,
but more importantly, the dichroic mirror would have the following ideal properties:
a) 100% transmission for k above 650nm.
b) 100% reflection for k between 500 nm and 650 nm.
c) 100% transmission for wavelengths below 500 nm.
15
However, obtaining property c) is less critical than achieving the first two. Figure 2 is a
simulated reflection spectrum of a reference design created from TiO 2 and SiO 2. The
transmission spectra obtained from spectrophotometry for the multilayer samples were compared
with this ideal spectrum.
%T
100,
80
60[
- Ideal Multilayer
40[
20
500
1000
1500
2000
2506
I
Figure 2: Transmission spectrumfor the ideal multilayer dichroic mirror.
simulatedof an actualdesign using TiO 2 and SiO 2
Experimental Methods
1. Sample Preparation
Glass, quartz and silicon wafers were prepared as substrates for each sputtering trial.
Unfrosted glass microscope slides were cut in half before being used. Silicon and quartz wafers
were cleaved to roughly 1 inch square pieces. The substrates were cleaned with isopropyl alcohol
and dried before being attached to the substrate holder via double-sided adhesive and then placed
back into the load lock. For each test, two glass slide halves, one piece of quartz and two silicon
16
wafers were used as substrates. For each test, one glass slide was also fitted with a piece of
adhesive on the top in the case of profilometry measurements. A profilometer uses a mechanical
stylus across a length of material to measure changes in the surface. This instrument allows fine
surface characterization.
Figure 3: Sputtering machine exterior
2. Instrument processes
In this project a Kurt J. Lesker sputtering system was utilized. To perform the sputtering
process, first the load lock was pumped by the roughing pump down to roughly 3e-1 Torr. At
that point, the turbo pump was turned on until the load lock reaches roughly le-5 Torr. The load
lock was then ready to be opened to the chamber, which was constantly kept under vacuum and
pumped by the cryo pump. The substrates were loaded into the sample holder within the chamber
using a sample loading arm that travels between the load lock and the chamber. Once the
chamber was isolated again, the sputtering process could begin.
17
Chamber
Load Lock
Load Lock Valve
Shields
Substrate loading arm (knob controlled)
Targets
Figure 4: Diagramexplaining the interior of the load lock and chamber of the sputteringsystem
Gas was then let in through the gun corresponding to one of the three targets within the
chamber and the corresponding power supply was activated. Each target has a shield that can be
opened to expose the target to the plasma, which starts the deposition, as seen in Figure 4. After
the set duration of sputtering, the open shields were closed, and the substrates were removed
once again through the load lock.
The initial sputtering tests were done to calibrate the system and obtain thickness
calibration in relation to time deposited. A power of 170W was deemed usable for the power
sources, giving a workable deposition range without endangering the system. After thickness
calibrations were obtained, a sample multilayer was formed. However this multilayer did not
have the correct properties. To proceed, each layer was then examined individually with a
sputtering of oxide and nitride both in argon. The nitride film was deemed problematic and then
18
various methods such as co-sputtering and reactive sputtering in different plasma were explored
to achieve a functional and transparent nitride film.
Obstacles that were overcome throughout this work included compositional problems in
the deposition itself, such as the lack of nitride being deposited onto substrates. This will be
covered more in the following results section. However other instrumental problems also arose,
such as a malfunctioning software which did not allow for the use of the second mass flow
controller (MFC) which would allow fine control of the input of a gas into the chamber as well
as the guns. This would allow for fine-tuning of reactive sputtering gas mixtures. Due to
software issues this second MFC would not control the flow of a second gas and would
unfortunately simply flood the chamber with the second gas.
Characterization of the transmittance and reflectance of the samples created was done
using a Cary 500i UV-Vis-NIR Dual-Beam Spectrophotometer. Transparent samples of quartz
and glass samples were loaded into the spectrophotometer and measured with respect to a test
sample as mentioned in the previous section.
Results and Discussion
1. Initial Calibration
Time-dependent thickness calibrations were first conducted to obtain a better
understanding of the time-scale of each following deposition. The sputtering system has a quartz
crystal microbalance (QCM) system attached, which provided readings of deposition rate and
total deposition. The QCM works through measuring frequency changes in the resonance of a
19
quartz crystal resonator due to thin film deposition upon it. Due to the piezoelectric effect, an
alternating potential causes oscillation in the quartz, which is affected by mass changes upon its
surface. For each thin film material, the acoustic impedance ratio between the film and quartz,
known as the Z-ratio must be inputted to correct the signal. However, as the sputtering system
had not been used in years, these calibration test were done to verify if the QCM rate readings
were accurate. Silicon nitride and silicon oxide were both deposited separately in argon onto
glass slides. Using Z-ratios found in literature for each material, each deposition took place for
approximately 2 hours with a source power of 170W. After deposition, profilometry was
performed to get an accurate actual result of the deposition thickness. The following table shows
the difference in the QCM reading and the profilometry measurement:
Table 2: Thickness calibrationsamples QCM and profilonefermeasurements
Target
Deposition Time
QCM Thickness
Reading
Profilometry Thickness
Measurement
Si3N 4
SiO 2
2 hours
2 hours
229 nm
187 nm
360 nm
560 nm
The QCM was found to be wildly unreliable, however, thickness measurements
according to time were now found and able to be used to the following tests.
2. Multilayer
The initial multilayer deposition was done with 21 layers on a quartz substrate. It began
with a nitride film of what was expected to be roughly 85nm and then alternated with an 1 l0nm
oxide film before ending with a final nitride layer. However the deposition ended up with less
20
than favorable results. Figure 5 shows the transmission spectra compared to the ideal spectrum in
Figure 2. However, it is clear that the expected spectrum of a dichroic mirror was not achieved.
%T
lOOr
80
60
-
Quartz Sample I
- Quartz Sample 2
Ideal Multilayer
40[
20[
Ail,500
1000
1500
2000
250W
Figure 5: Transmission spectra comparison between the initial multilaver deposition of oxide and nitride in argon
onto a quartz substratecompared to the simulated working dichroic mirror
Comparing this spectrum to that of the ideal dichroic mirror the cut-off that is expected
around 650nm happens at a higher wavelength than expected and much more slowly, over
100nm. The high transmission that is supposed to return at 500nm instead also drops off
gradually, after roughly hitting 475nm at 25% transmission. Because this change is gradual, it is
not expected to be from the reflectance in the interference patterns between the oxide and nitride.
Instead, these results point to some third party involvement of absorption in either of the two
layers. After this initial multilayer, the films were examined separately to figure out the key
underlying problem in this first multilayer.
21
3. Oxide and Nitride in Argon
The tests were now repeated individually for each target in the same atmosphere as
before, argon. While the oxide film was transparent, sputtering of the silicon nitride target in
argon plasma had instantly visible results as seen in Figure 6.
Figure 6: Single layer oxide deposition on glass in argon (left) with adhesive and a single layer nitride deposition
on glass in argon (right) with the adhesive removed.
The substrates were coated with a brownish film that did not change in reflectivity or
color at different angles. This once again pointed towards to absorption due to an abundance of
silicon in its stoichiometry. This can be explained by the bond energies of formation for the
various bonds in these sputtering tests. Silicon dioxide in argon plasma sputters onto the
substrate stoichiometrically. Silicon nitride is however different. Sputtering a silicon nitride
target creates N2 gas as a byproduct.
22
Table 3: Bond energies of various diatomic species (#/silicon, nitrogen and oxygen
Bond
Energy
(kJ/mol)
0-0
498.34
N-N
946
Si-O
798
Si-N
439
Table 3 shows the reason of the differences in sputtering of silicon oxide and silicon
nitride. A Si-O bond is much stronger than an 0-0 bond. Therefore when sputtering, SiO 2 is
favored during sputtering. However, the opposite is true for nitrogen. N-N bond formation is
favored relatively compared to Si-N bonds. This means as the silicon nitride target particles are
sputtered into the chamber, N2 gas molecules tend to form, causing an uneven deposition of
silicon and nitrogen onto the substrates. This abundance of silicon compared to nitrogen on the
substrates in the film explain for the absorptive, non-transparent in the visible spectrum film in
the nitride deposition.
Cosputtering of both the nitride and oxide in an argon was explored next as a possible
replacement of the SixNy film. Cosputtering of silicon nitride and silicon nitride forms SiON and
in previous literature had shown little loss of target atoms by formation of gases [6]. For a
homogenous mixture of oxide and nitride, the stoichiometry would be as follows:
x Si3 N4 + y SiO 2
->
Si3 x+yO2yN4 x
[2]
23
The index of refraction of this cosputtered film would depend on the mole fractions
between the two components to SiON. The equation is as follows:
nSiON =
Si2
+ (1
-
[3]
f)SiN
In Equation 3,f is the mole fraction of SiO 2 in the SiON cosputtered film and nSiON, nSiO2
and nSi3N4 are the refractive indices for the final SiON composition, Si02 and Si3N 4 respectively.
The deposited mole fraction is dependent on the power applied to each of the targets; the more
power applied to the power source for the corresponding gun, the more energy is applied to
nearby plasma particles, increasing the deposition rate. Sandland (2005) states that the deposition
rate of SiO 2 is roughly 0.75 the rate of Si3N 4 of the same power [6]. Using this she developed an
equation that writes the refractive index of the SiON cosputtered film as a function of the target
powers:
nSiON =
0.75 PSi02
0.75 PSio 2 +PSi3 N4
+ -
s
0SiO2
nSi
3 N
4
PSi3N4
0.7 5PSi0 2 +PSi3 N 4 R
Where Psio is the power applied to the silicon dioxide target while
PSi3 N4
[4
[4]
is the power applied to
the silicon nitride target. Using the initial power value for all previous tests of 170W as a
baseline and refractive indices given in Table 1, the power for the silicon dioxide target was
found to be roughly 75 W if the refractive index of the SiON cosputtered film was to be around
1.9.
Cosputtering with the given powers for roughly four hours however gave disappointing
results as seen in Figure 7.
24
Figure 7: SiON depositedsubstrates cosputteredfrom Si3 N4 and SiO 2 in an argon plasma
Similar browning had occurred once again pointing towards absorptive interactions from
an abundance of silicon in the thin film. Because of this absorptivity and lack of high
transmission in the visible spectrum, another method had to be found to get a film with a
refractive index of roughly 1.9 to 2.
4. Nitride in ArO 2
Reactive sputtering of the silicon nitride target in a chamber of both argon and oxygen
was the next step to understand if lowering the concentration of argon would allow for a
transparent and clean Si 3N4 deposition, unlike the previous depositions. This test was done using
a tank of gas with a composition of 90% argon and 10% oxygen. As seen in Figure 8, at first the
deposition looked successful; the film was transparent for once.
25
Figure 8: Si3 N4 deposited substratessputtered in an ArO 2 plasma
However, examination of the transmission spectra shown in Figure 9, provides otherwise.
There is a significant lack of interference bands in both the glass and quartz samples. This
indicates that the refractive index of the film was very close to the refractive indices of the
substrates: roughly 1.4 - 1.5 as seen in Table 1.
26
100
T
80
60
- Glass Sample
- Quartz Sample
40
20
200
400
600
800
1000
Figure 9: Transmission spectra of the Si3N 4 depositedsubstrates sputtered in an ArO 2 plasma
This information along with the fact that oxygen was abundant in the system seem to
point to a SiO 2 dominated film. One again, the values of Table 3 support this argument where the
most favored bonds in the multi-molecular system would be that of nitrogen with itself and
oxygen with silicon. Even with the sputtering of a Si3N 4 target, the oxygen in the environment
had created a product similar to that of the sputtering of a SiO 2 target in argon.
5. Oxide and Nitride in N 2 Plasma
Next, reactive sputtering in a nitrogen plasma was examined. Both oxide and nitride were
tested within this plasma as it would be impractical to change out the gas when making a
multilayer with the existing system. There were problems with target poisoning as well as the
27
striking and holding of the nitrogen plasma at first, but by cleaning the targets and giving extra
time for the chamber to fill the chamber up with N 2 , plasma was able to made and sustained.
The oxide film in the N 2 plasma had a similar outcome to previous oxide depositions.
While nitrogen contamination of the SiO 2 deposition was a very large likelihood, overall, the
transmission spectra seemed to point towards a refractive index of roughly 1.5. The film itself
was clear as it had been in previous oxide depositions. Once again, due do the bond energies, this
was a very expected outcome.
Figure 10: SiO 2 depositedsubstrates sputtered in an N2 plasma.
28
%T
1oor
80
60[
- Glass Sample
- Quartz Sample
40[
20
200
400
600
800
1000
Figure 11: Transmission spectra of SiO2 deposited substratessputtered in an N2 plasma
The nitride deposition in the N2 plasma had a much different outcome than before. The
film as seen below is much clearer than previous nitride depositions. Instead of the silicon rich
brown tint there is more of a darker blue tint. The transparency in the visible regime is much
better.
29
Figure 12: Si3N4 deposited substratessputtered in an N2 plasma
The transmission spectra from the spectrophotometer shows interference bands this time,
meaning the refractive index is much more different than the rough estimates of 1.5 from before.
Using these interference waves, the refractive index of the film can be calculated out using what
is known as the Swanepoel method. Swanepoel proposed a method that uses envelope functions
of the extremes of the interference fringes along with known values such as the refractive index
of the substrate to calculate the refractive index of the thin film at a certain wavelength [7].
30
%T
lOOr
80F
60[
- Quartz sample
- Glass sample
40F
20[
0
200
400
600
800
1000
Figure 13: Transmission spectra of the SiN4 deposited substratessputtered in an N2plasma
Swanepoel considers the following theoretical framework for his method to work. A thin
homogenous film of uniform thickness d and a complex refractive index of n = n - ik is on top of
a thick but finite transparent substrate with a known refractive index of s. The film can also be
characterized by an absorption coefficient a. The system is in air with an index no = 1, and
through the reflections in all interfaces, k2 << n2 . The expression for transmittance T at normal
incidence is then given by Equation [5].
31
T = B-C x
[5]
cosx p+Dx2
Where:
B = (n + 1) 3 (n +s 2 ),
C = 2(n2 - 1) (n2 --s 2 ),
D = (n - 1) 3 (n- s
2
)
A= 16 n 2s,
And:
p=4T n d/X, x = exp(-xd), and k = a/4r.
Setting the condition of cos p in Equation [5] sets the extremes of the interference fringes
of the spectrum. When cos p = 1, the function envelopes the maxima, and when cos 'p = -1,
the function envelopes the minima of the spectrum. Plugging these values into Equation [5], an
Equation [6] for envelope around the maxima, TMo, and an Equation [7] for the envelope the
interference minima, Tmo, are obtained:
TMO =
Ax
B-C x +Dx
2
[6]
TMO
Ax
B+C x +Dx 2
[7]
-
By fitting lines into the envelope to solve for the values above, the real refractive index,
n, can be solved for through the following equations:
n = [N + (N 2
-
s 2 ).5 ].5
[8]
Where:
N=
2
sTMO~TmO +
TMOTmO
2
2
[9
32
For the spectrum shown in Figure 13, the spectrum for the quartz sample was isolated and
examined. Using Equations [6] and [7] as guides, interpolated functions were created as shown
in Figure 14.
%T
100
80
60
40
20
200
400
600
800
1000
Figure 14: The transmissionspec/rum for the Si3N4 deposited quartz substrate sputtered
in N 2 plasma withfitted envelopes
Using the dispersion formula for the refractive index of the quartz substrate as given by
Malitson [4], and Equations [6] and [7] across the values in the above envelops, Table 4 provides
the values for the different wavelengths, the maxima and minima values at the wavelength and
the calculated refractive index for the nitride film.
33
Table 4: Values of X, Two, T,.o, and the refractive indices of the substrate andfilm
Wavelength (nm)
Tmo
TMO
s (SiO 2)
n (thin film)
400
72.79
88.36
1.4701
2.01
450
75.17
90.59
1.4656
1.98
500
77.05
92.18
1.4623
1.96
550
78.55
93.27
1.4599
1.93
600
79.72
93.98
1.4580
1.91
650
80.64
94.36
1.4565
1.89
700
81.33
94.49
1.4553
1.87
750
81.84
94.42
1.4542
1.86
800s
82.21
94.18
1.4533
1.84
The film's refractive index approximately 1.9, much closer to the value of the refractive
index of Si 3N 4 , which is very promising for a functional multilayer stack.
6. Summary
A summary of the depositions so far, bar the calibration samples have been noted in
Table 5. All depositions used a power source of 170W, with substrates of a silicon wafers, glass
slides, and quartz.
34
Table 5: Condensed summary of results
Deposition
Multilayer Oxide and Nitride in Argon,
21 layers, starting with a nitride layer
Silicon oxide in Argon
Silicon nitride in Argon
Oxide and nitride cosputtering in Argon
Silicon nitride in ArO 2
Silicon oxide in N 2 plasma
Silicon nitride in N 2 plasma
Refractive
Index
-
Notes
Failure in transmission <500nm. Nitride layer
found to be the problem
~1.5
Functional
-2.4
High absorption due to high concentration of
silicon in the deposition, brown.
-
-1.5
Brown, not transparent, similar to Si 3N4
deposition in Argon.
Appears to have deposited a film of silicon
oxide due to oxygen in the environment
Functional
-1.9
Sufficiently transparent,
-1.4- 1.5
Conclusion
Various methods of sputtering were explored to find the optimal conditions for the
creation of a dichroic mirror. Moving on from this project, there are many paths to explore. The
first follow-up project would be to try and sputter a multilayer of silicon dioxide and silicon
nitride in the nitrogen plasma. If the software error with the mass flow controller is fixed, mixing
gases would become very simple. A residual gas analyzer (RGA) may also provide great insight,
as they are able to monitor changes in gas composition when reactive sputtering. Other gas
mixtures such as a well-controlled mix of argon and nitrogen may also prove to be fruitful in
obtaining two films of varying refractive index, especially if the films prepared in the full
nitrogen atmosphere prove to be too similar in indices.
Other avenues to explore in further work include sputtering with other targets such as
titanium dioxide. Another commonly used sputtering target, titanium dioxide boasts high
stability, transparency in the visible and NIR range [8] and a refractive index ranging from 2.4 to
35
2.6 [9]. This could be very promising as an alternative to the nitride films. For measuring the
optical properties of films, empirical methods can also be explored through ellipsometry.
In the end, many different methods of sputtering were used to iterate through films to find
a workable pair to form dichroic mirrors. An understanding of the various sputtering methods
were achieved and problems that arose during each deposition were solved by examination and
new tests. Spectrophotometry was used to understand each resultant sample and their flaws,
especially with the contaminated nitride films prevalent in each test. Through this project,
sufficiently transparent silicon nitride films with a practical refractive index were attained. Using
this knowledge and previously successful oxide film depositions, the framework for then
building a functional dichroic mirror of the specified properties has been laid. While the final
goal of a functional dichroic mirror may not have been met, progress through repeated
fabrication and analysis of samples has proven to be fruitful with results to consider moving
forward.
36
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