RELATIONSHIPS BETWEEN OPTICAL PROPERTIES
AND PROCESSING IN A1203-Y203THIN FILM
WAVEGUIDES AND AMPLIFIERS
by
Bethanie J. Hills Stadler
B.S. in Materials Science and Engineering
Case Western Reserve University, May 1990
Submitted to the Department of Materials Science and Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May 1994
(0 1994 Massachusetts Istitute
of Technology. All rights reserved.
Sip-matureof Author
I
Depa.v&en`of Materials Science and Engineering
April 29, 1994
-9--
Certified b
Manuel P. Oliveria, Thesis Advisor
Assistant Professor of Materials Science and Ceramics
Accepted by
Carl V. Thompson II
Professor of Electronic Materials
Chair, Departmental Committee on Graduate Students
MASSArH;JS-`TC1,v
I
AWG18 1994
'7 1,",
. I i ,
i I ;'
L,
FIT
science
To:
My husband,Rob
and
My parents, Dick & Yean tAs
2
RELATIONSHIPS BETWEEN OPTICAL PROPERTIES
AND PROCESSING IN A1203-Y203 THIN FILM
WAVEGUIDES AND AMPLIFIERS
Bethanie J. Hills Stadler
Submitted to the Department ofMaterials Science and Engineering
on April 29, 1994 in partial fulfillment of the requirements of
the Degree of Doctor of Philosophy
ABSTRACT
The application of non-traditional materials and processing to thin film
optical devices was investigated. Current optical devices are limited to single
crystals and traditional glass-formers, and are fabricated via bulk techniques.
This work demonstrated the utility of sputtering in the fabrication of new
amorphous and polycrystalline materials that serve to widen the horizon of
material properties available to the optical communications community.
Specifically, this work systematically examined the effects of processing
parameters on the microstructure and optical properties of films in theY203A1203
system.
Amorphous A1203 (a-AI203) was found to have promising optical
propertiesforplanarwaveguidedevices. The transmissionwindowofa-AI203
extended from 200nm to 9gm. Indices of refraction ranged from 164 to 166,
and optical losses were as low as dB/cm at 632.8 nm. These are some of the
lowest reported losses for as-deposited A1203thin film waveguides. The
refractive index of the films increased with substrate temperature, but
remained constant with%02 in the sputtering gas. The losses, however,
decreased with substrate temperature and increased with%02in
the
sputtering gas.
loss
Processing conditions were harder to identify for the fabrication of lowY203thin film waveguides. Textured bixbyite films had the lowest losses
(5OdB/cmat 632.8nm), and subsequent anneals at 600'C reduced these losses
(2OdB/cm). Films made with high%02 in the sputtering gas had mixed
crystallinity which led to a decrease in the refractive index 1.63 to 133) and
an increase in the optical losses. Although the visible losses were high, the IR
absorption edge of theY203waveguides was located far into the infrared
(>14gm), making them desirable for applications as IR laser amplifier hosts.
For co-depositedY203-A1203 waveguides a compositional region
(roughly the composition of YAG) was identified with the lowest optical losses
(1-5 dB/cm) reported to date. The waveguides had wide transmission windows,
and the edges of the windows shifted with compositiontoward the values of the
endmember edges.
The refractive index was also a function of composition
(1.849-1.679), but was independent Of %02in
the sputtering gas. Several
waveguides of A1203 andY203-A1203were doped with Nd, and a broad
fluorescence peak was observed at 106 gm.
Thesis advisor: Dr. Manuel P. Oliveria
Title: Assistant Professor of Materials Science and Ceramics
3
ACKNOWLEDGMENTS
First and foremost, I would like to acknowledge the support of my
advisor, Dr. Manuel Oliveria. His ideas and insight were invaluable to my
project, and his kindness and supportivenature were invaluable to my
personal growth.
'Manks to my committee, Prof. Harry Tuller, Prof. Lionel Kimerling, and
Dr. Leonard Johnson, for their ears and ideas. Manks also to Dr. Johnson for
his help in polishing and end-coupling analysis of my waveguides.
I would like to thank Sarah E. Kim and Dr. Young Keun Kim for their
support during my studies. I was foliate
to share a lab and ofice with them
for most of my years here at MIT. Manks also to Kathleen Morse for her
hard work and cheerful attitude. I thank Julia Duncan for making my last
year in the office so enjoyable. I hope we stay friends throughout our lives. My
degree would not have been achieved in the same time frame without the help
of Fred Wilson, Pat Kearney, Joe Adario, and Mike Frangillo.
I owe a great debt to Lee Bouthillette for his support and advice last
summer at Hanscom Air Force Base. Also to the whole Hanscom crew for
their help: Dr. Richard Payne, Dr. Joe Lorenzo, Dr. Eric Mai-tin, Dr. Alan
Drehman, Dr. David Weiburn, and especially Steve Spaziani. I look forward to
working with all of you soon! I would like to thank Dr. Robert Folweiler, Dr.
Leonard Andrews, Dr. William Miniscalco and Buce Hall of GTE Laboratories
for a great summer experiencethree years ago and the fuitful repercussions.
My deepest appreciation to Karl and Dian Zeile who have become very
dear to me over the past four years, and to the First Lutheran Church
community.
I thank my parents, Dick and Jean Hills and by sisters Pam and Deb for
their lifelong love and support. I am also grateful to my "Bean Town
Relatives," Walter, Chuck, Barbara, Mike and Missy Romanchuk, Georgia
Dunninz. Dorie. Dana, Flore Ida, Dale and Kathy Nforrow,Barb and Ken
Hiseler, and Cecil and Myrtle Hills for making sure we were taken care of, and
never alone on a holiday! To my new family Chuck, Marlene, Dave, Kathy, and
Sarah Stadler for their love and understanding: we have a lot to look forward to.
With all my love, I thank my husband Rob for making these past four
years the best ever, and for letting me know that life is only going to get better.
,The(ht shines in th, darkness, but
the darknesshas not understoodit.
-yohn 1:5
4
TABLE OF CONTENTS
1.0 INTRODUCTION
..............................................................................................
11
2.0 BACKGROUND..................................................................................................
14
2.1
Materials Selection ................................................................................. 15
2.1.1
Bulk Properties .......................................................................15
2.4.2
Reported Properties of Thin Film s ......................................
25
A1203 Thin Film s ................................................................
25
Y203 Thin Films . ................................................................
27
Y203-AI203 Thin Films .....................................................
28
2.2 Physical Sputtering ...............................................................................
29
2.2.1 Principles of Sputtering .........................................................
30
2.2.2 RF Sputtering .........................................................................
32
2.2.3 Magnetron Sputtering ...........................................................
34
2.3 Processing-Properties Relationships ..................................................
37
2.3.1 Typical Microstructures .......................................................
37
2.3.2 Processing Param eters .........................................................
39
2.4 Optical Considerations .........................................................................
43
2.4.1
Principles of W aveguiding .....................................................43
2.4.2
2.4.3
Index of Refraction .................................................................
47
Attenuation .............................................................................
50
3.0 EXPERIM ENTAL PROCEDURE ................................................................
54
3.1
Fabrication .............................................................................................. 55
3.1.1 A1203 and Cr:A12O3 Thin Films ........................................ 59
3.1.2 Y203 Thin Films .....................................................................
61
3.1.3 Y203- A1203Thin Film s and Nd-Doping .............................
63
3.1.4
3.2
Ridge W aveguides ................................................................... 64
Characterization .................................................................................... 65
3.2. 1.
M icrostructure
and M orphology ..........................................65
3.2.2
Optical Properties ..................................................................
66
The envelope method .........................................................
66
The M-lines technique ........................................................
69
Attenuation . ........................................................................
72
Fluorescence ........................................................................ 73
5
CONTENTS
6
4.0 A1203 AND Cr-A1203WAVEGUIDES ........................................................... 75
4.1
Phase D eterm ination ............................................................................
76
4.2
Film Durability .......................................................................................85
4.3
Optical Properties .................................................................................. 86
4.4.
4.3.1
4.3.2
Index of Refraction .................................................................
86
Attenuation .............................................................................
91
4.3.2
Fluorescence ............................................................................ 96
Conclusion ..............................................................................................
98
5-0 Y203 WAV E GU ID E S .......................................................................................99
5.1 Phase D eterm ination ............................................................................
100
5.2 Film Durability .......................................................................................
114
5.3 Optical Properties ..................................................................................
114
5.4
Conclusion ...............................................................................................
119
6.0 Y203 - A203 WAVEGUIDES AND AMPLIFIERS ...............................
120
6.1 Phase D eterm ination ............................................................................
121
6.2
Film Durability .......................................................................................129
6.3
Optical Properties .................................................................................. 133
6.3.1
6.4
6.5
Index of Refraction .................................................................
133
6.3.2
Attenuation .............................................................................135
N d-Doping ................................................................................................ 138
Conclusion ...............................................................................................143
7.0 SUMMARY AND FUTURE WORK .............................................................
144
REFERENCES ..................................................................................................................
148
Appendix A........................................................................................................................
152
Appendix B........................................................................................................................
153
Appendix
156
C .......................................................................................................................
FIGURES AND TABLES
Chapter 2
Table 2.1-Several Properties of A1203, Y3A15012, Y4AI209., and Y203 ............16
Fig 21 - a) Corundum structure showing only the cation sublattice.
b) Spinel structure . ....................................................................................
18
Fig.2.2 - a) Projection ofthe tetrahedral 02-planes of the bixbyite
structure. b) A base projection of atoms and a packing
direction in the hexagonal uit cell of La2O3. .......................................19
Fig. 23 - Phase diagram of A1203-Y203system . ..................................................2
Fig. 24 - The garnet structure OfY3A15012
. ..........................................................22
Fig. 25 - The perovskite structure of YA103. .........................................................
22
Fig. 26 - a) Important energy levels of Cr3+ in ruby. b) Absorption
bands of ruby . .............................................................................................
23
Fig. 27 - a) Important energy levels of Nd3+. b) Absorption and
fluorescence bands of Nd3+in glass . ......................................................
24
Fig. 28 - Typical sputtering system . .......................................................................
29
Fig. 2.9- Structure of a glow discharge . ...................................................................
30
Fig. 2.10- a) The potential on the target as a function of time
for the first several rf cycles. b) The plasma potential for
an rf powered plasm a discharge . ............................................................
33
Fig. 2 1 I- a) The motion of an electron in a magnetic field perpendicular
and then parallel to the page. b) The motion of an electron
in combined electric and magnetic fields. ..............................................
35
Fig. 2.12- The shape of the magnetic field lines and the resulting drift
path of a radial m agnetron . ......................................................................
35
Fig. 213 - Schematic of an unbalanced magnetron ..............................................
36
Fig.2.14-Thorton diagram showingthin film microstructure as a
function of pressure and temperature . .................................................
38
Fig. 2.15- Schematic demonstration of Snell's Law ..............................................
43
Fig.2.16- An intuitive understanding of guided modes can be gained
through geometrical considerations ........................................................
46
Fig. 2.17- A two-dimensional planar waveguide (asymmetric) and onedimensional ridge waveguide (symmetric, nc = ns)...............................47
Fig. 2.18- Dispersion curves and curve fits of several relevant oxides ..............49
Fig. 2.19- Intrinsic and extrinsic loss mechanisms . ..............................................
50
7
FIGURES AND TABLES
8
Chapter 3
Fig. 3 1- Sputtering chamber and control rack ......................................................
56
Fig. 3.2- Schematic of sputtering chamber .............................................................7
Table 3.1- Deposition Conditions ............................................................................... 57
Fig. 3.3- The envelope method uses the curves Tma,. and Tmin, which
envelope the transmission spectrum .....................................................
67
Fig. 3.4- a) The m-lines technique. b) Schematic of loss measurements ..........70
Fig. 3.5- a)Prism apparatus. b)M-line shown on a screen behind prism
apparatus as light is coupled into a waveguide ....................................
74
Chapter 4
Fig. 4.1- The effects of substrate temperature and
microstructure
02 on the
of A1203 film s.................................................................. 77
Fig. 4.2- The UV absorption edge for A1203films deposited at low % 02 ..........78
Fig. 4.3- Rutherford Backscattering Spectrum and simulations of films
deposited within (5%) and below 3.5%) the processing widow
of transparent aA1203 waveguides . ......................................................
79
Table 4.1- TED Data on Crystallized A1203 ..........................................................
80
Fig. 4.4.a)- X-ray diffraction patterns of as-deposited
and annealed A1203 film s.......................................................................... 8 1
Fig. 4.4.b)- Glancing angle x-ray diffraction patterns of a film that was
deposited with a substrate temperature of 400'C ..............................
82
Fig. 4.5a)- Transmission electron micrograph and diffraction pattern of
an as-deposited
film (a-A1203) .................................................................8 3
Fig. 4.5b)- Transmission electron micrograph and diffraction pattern of a
film annealed by the electron beam (y-A1203).....................................
84
Table 4.2- Optical Properties of A1203 Thin Film Waveguides ...........................
87
Fig. 4.6a)- Dispersion in -3500,&-thick A1203 films as a function of
substrate temperature and %O2. ...........................................................
88
Fig. 4.6b)- Dispersion in ~14m-thick A1203 films as a function of
substrate temperature and %O2. ...........................................................
89
Fig. 4.7- The index of the amorphous A1203 is 94% that of the crystalline
A1203 ph ase ................................................................................................. 9 0
Fig. 4.8a)- LTV/visibletransmission spectra of a 14m amorphous A1203
FIG URES AND TABLES
9
film and a
mm thick sapphire crystal .................................................
92
Fig. 4.8b)- Infrared transmission spectra of a Igm amorphous A203
film and a mm thick sapphire crystal .................................................
93
Fig. 4.9)- RBS data of films deposited at various O2 . ........................................
94
Fig. 4 10- The Cr-O Phase Diagram . ........................................................................
97
Fig. 4 1 I- Fluorescence in Cr:A1203films crystallized to 7AI203 and
a - A 120 3 .......................................................................................................
97
Chapter
Fig. 5.1- Processing windows Of Y203 films a) deposited at room
temperature b) deposited with 30OW......................................................
101
Fig. 5.2- XRD data of transparent Y203 thin films . ..............................................
102
Fig. 5.3.a- Glancing-angle XRD data Of Y203 thin film (bixbyite)
a) on fused-SiO2 and b) on Si .................................................................... 103
Fig. 5.4- GAXRD data of a Y203 film that was deposited at
502
...................
(bixbyite) ........................................................................................................10 4
Fig. 5.5- XRDdata
Of w-02 films (hexagonal
Y203
a) 020 and b) glancing-angle . ..............................................................
106
Fig. 5.6- XRD data of m etallic Y film. .......................................................................
107
Fig. 5.7- XRD patterns of thick (-5500A) Y203 films...........................................108
Fig. 5.8- CRDpatterns OfY203 films deposited at 1% 02 ...................................110
Fig. 5.9- XRD patterns OfY203 films deposited at 3 02 ...................................111
Fig. 5.10- GAXRD pattern OfY203 film deposited at 50OWand 10% 02 . ........112
Fig. 5.11- GAXRDpatterns of annealed Y203 films. ............................................113
Fig. 5.12- Dispersion curves OfY203 films. ............................................................
115
Fig. 5.13-a) UV-VIS transmission OfY203 thin films..........................................117
Fig. 5.13-b) Infrared transmission OfY203 thin films..........................................118
Chapter 6
Fig. 6 1- Summary OfY203 -A1203 co-deposited films with end member
results shown schematically .......................................................................
122
Table 6 1.-Optical Properties OfY203-AI203 Waveguides and Amplifiers .......123
Fig. 6.2- Typical XRD pattern OfY203 -A1203 film . .............................................125
Fig. 6.3- GAXRD pattern OfY203 -A1203films at various compositions . ........126
FIGURES AND TABLES
10
Fig. 6.4- GAXRD pattern Of Y203-A1203
50:50) at varying %02 ................... 127
Fig. 6.5- GAYRD pattern Of Y203 A1203 films deposited onto Si (100)...........128
Table 6.2- TED Data on Crystallized Y203 -A1203 ..............................................
129
Fig. 6.6- Transmission electron micrograph and diffraction patterns of
Y203 -A1203 films with ~5% Y203 ...........................................................
130
Fig. 6.7- Transmission electron micrograph and diffraction patterns of
e-beam.devitrifiedY203 -A1203 films with -5% Y203 . ........................ 131
Fig. 6.8- Transmission electron micrograph and diffraction patterns of
Y203 -A1203 films with -45% Y203 .........................................................132
Fig 6.9- Dispersion curves of Y203 -A1203 ............................................................
134
Fig. 6.10-a) LTV-VIStransmission Of Y203 -A1203 thin films . ..........................136
Fig. 6.10-a) IR transmission Of Y203 -A1203 thin films ......................................137
Fig. 6.1 I- Compositions of Nd-doped Y203 -A1203 films. ....................................
138
Fig. 6.12- GAXRD of Nd-doped Y203 -A1203 film with increasing Y203
con ten t ............................................................................................................. 13 9
Fig. 6.13-a) LW-VIS transmission of Nd-doped Y203 -A1203 thin films . ........140
Fig. 6.13-bl) IR transmission of Nd-doped Y203 -A1203 thin films ....................141
Fig. 6.14- Fluorescence of Nd-doped films. ............................................................
142
Chapter
INTRODUCTION
As optical communications continue to be developed, there is an
increasing need to switch and process optical signals directly, that is, without
converting them into electrical signals. Current optical devices include fiber
and planar devices such as Er fiber amplifiers and LiNbO3 electro-optical
switches. For the most part, fiber devices are limited to traditional amorphous
systems, although some work is done on single crystal fibers despite difficulties
in fabrication. Planar devices are also limited to single crystals and traditional
glasses because diffusion and ion-exchange are used to define waveguide
structures.
Through thin film processing, however, novel amorphous and
polycrystalline waveguide devices can be fabricated. A variety of techniques
are available, such as sol-gel, chemical vapor deposition (CVD), and physical
vapor deposition (PVD).
The present work takes advantage of the
nonequilibrium (fast-quenching) nature of sputtering, a PVD technique, to
fabricate amorphous planar devices composed of A1203 and Y203. The high
energies of sputtering
3 to 20 eV per deposited atom) provide the best
11
CHAPTER 1: INTRODUCTION
12
opportunity to create amorphous films which have high densities and good
adhesion, along with low optical losses.
Fabrication of fine-grained
polycrystalline films is also possible via sputtering, including polymorphs that
are metastable in bulk form, such asy-A1203The purpose of this research was to study the effects of processing
parameters on the microstructure and resulting optical properties OfA1203
and Y203 thin films. In the past, research on oxide thin films has focused on
coatings, where the direction of light propagation is perpendicular to the plane
of the film. (Pawlewicz et. al, 1980, Rainer et.al, 1985) Relatively high losses
have been tolerated because the path length is short, and the relationship
between optical properties and microstructure has not been adequately
explored.
This research helps to fill a substantial gap in the current
understanding of optical films through a systematic study ofthis relationship.
The most important optical properties for waveguides to be used as
devices in integrated optical circuits are optical loss and index of refraction.
Acceptable levels of loss depend on the application and on the path length, but
roughly 0. I- 1.0 dB/cm is desirable for centimeter long planar devices operating
in the IR. For practical reasons, refractive indices between 14 and 19 have
distinct advantages. A index of greater than 14 is desirable for compatibility
with Si technology because SiO2 can be used as a cladding material; an index
less than 19 is best for coupling to fibers. In light of these requirements, bulk
A1203 and Y203 have properties that appear promising. They have very low
intrinsic losses (~O.1 dB/km in the near IR) (Lines, 1986; Mahr, 962; Bendow,
1977) which are comparable to those for SiO2. Ad, at 1.55tm, the refractive
indices of A1203 are 17462 and 17384 (Weber,1982),and the refractive index
OfY203 is 19 (Nigara, 1990).
CHAPTER 1: INTRODUCTION
13
Although the initial application of interest was optical waveguides, the
knowledge gained in this study was used in the development of thin film
amplifiers. A1203 and Y203 have the advantage of transmission windows that
extend well into the infrared. This makes them promising laser amplifier hosts
for near-IR amplifiers because the probability of non-radiative decay is low.
Cr3+ and Nd3+ were chosen as dopants for A203 and Y203, respectively, due
to suitable size and lattice compatibilities. Because Cr3+ is a transition metal,
it also served as a probe for the structure of the A1203 films. Nd3+ was chosen
over other rare earths because of its fluorescence peak at 13 tm. Although
Er3+ doped fibers are commercially available, they only serve applications that
operate at 1.55tm.
system as is Er
Also, Nd3+ is a 4-level system, rather than a 3-level
so a population inversion is more easily attained.
A variety of future applications, such as switches and modulators,
exists for these films due to their processing advantages.
They can be
sputtered onto a variety of substrates without concern for lattice matching.
For example, an amorphous amplifier could be deposited onto an LED (light
emitting diode) for direct laser pumping. Waveguides can also be stacked by
depositing alternating waveguide/SiO2 layers. Interference between layers can
be minimized because sputtering will allow the deposition of a wide range of
materials. Therefore, a higher index difference between the waveguide and the
cladding can be achieved than that which is available through current
techniques
n=0.02-0.05).
Chapter 2
BACKGROUND
This chapter provides background information that is useful for
understanding the thin film waveguidesthat were prepared in this study. First,
known properties of the materials system in question (AI203-Y203) will be
discussed in two parts: bulk properties and thin film properties. Secondly,the
process of rf magnetron sputtering will be described in detail. The third section
discusses the effects of specific processing parameters on the resulting optical
properties. Ad, the final section deals with optical considerations, including
the principles behind waveguiding and a description of optical properties, such
as index of refraction and attenuation.
14
CHAPTER 2 BACKGROUND
15
2.1 Materials Selection
2.1.1
Bulk Properties
In selecting suitable materials for this research, a number of material
requirements had to be met, namely suitable transmission windows and
refractive indices, along with sufficient mechanical and chemical stability.
Applications in telecommunicationsrequire transparency in the near IR (NIR).
Also, materials with multiphonon bands at longer wavelengths have lower
probabilities of nonradiative recombination in laser amplifier hosts.
Along with a high refractive index for waveguiding, an isotropic index is
preferred for polycrystalline waveguides, because scattering at grain
boundaries depends on the relative index between bordering grains. Porosity
negatively affects the index, and it must be eliminated. Water often adsorbs
onto the surface of pores which leads to a variable index. Also, wet and dry
pores will lower the index and scatter light.
The
A1203-Y203
system was chosen because it has suitable
transmission windows and refractive indices. The intrinsic losses are
comparable to SiO2 in the visible and NIR, and the multiphonon bands are at
longer wavelengths. Some properties of bulk crystalline phases are shown in
Table 2.1. SiO2 was chosen as the substrate for all of the films because it is
amorphous, has well characterized optical properties, is available in high
purity, is compatible with Si technology, and has a relatively low refractive
index.
The two well-known phases of A1203 are ot-AI203 andy-AI203.
equilibrium phase is
c-AI203,
The
or corundum. Metastabley-AI203 is produced
in bulk through the decomposition of boehmite,
'y-AIO(OH),at 450'C.
(Wells, 1984) Further heating yields a variety of phases:
CHAPTER 2 BACKGROUND
16
CO
0cq
Q
q40
r--l to
r-4 r-4
C-1m
u
41 m
> co
-4
Lfs
"It co.
I
t-
(M
0
m
c1l
o
tt
r-4
llc
u-z m 6
1R r-4 t7
0
-,t OD
4
ll
co
cli
>-4
Lq
Lo
I
lack
1114
0
N14
Lo (c
C; r-4
l
00
0cq
I-
00 Co
M
-
m
r--l
:
0
(m
Q) >1
11
co
u
u
Q
0
4 5
6 cq
-4
C
Lo
4
-
C6
0
a)
Lo
CO
co
ct c
cq
00
117
Iq
I
:
a)
"It
cqw
m q
I
m m
r--l t-
r--l
(2)
r--4
cq
C)
00
-
I?
C
r--q
Icl
0
m
(M
0cq
:;4
ll
LO
U'D
Lo
LI-
co
C;
r-i
0
cli
CO
lack
Lo t
C6
4
cq
c
II
(t
C.)
la c1l
C m
Lo
c
cli
Lo t- c
(:T 1-4 r-4
>4
Z
14
"t
0
U-
-4-.')
c
ul
't,
4
Lo
If
1:1
CYD
>4
0
CT
L
00
Ci
M
14
0
cq
.-i
10
,C!
0
U)
u
cli
CYD
CO C)
r--q cli
co
4-j
;.4
(1)
E6
W
1.4
cq
;.4
a)
00
c1l 00
P4
0
;4
uI
x
. -4
-c
L)
q
.4 1
(D
c
(1)
t.,
M
0)
4..-a
;L4
;:I
-4 ')
s
4 u
Q)
E C)
z
11
> w
4
;4
m
$:L,
-U
-4-a
m 1 m4
C)
>
Q
"b
I-
,4
>
w
r4
C)
"C
L4
71
=4
cq
11
m
c---E m C'I
co
Q)
u
4
0
IQ
;-4
C)
;-4
(2)
E-4
a
Z
03
(1)
4
10
(Z
(1)
ri)
t- to
-. 4
cli
,,CT
Q
Iq
0
;-4
$a4
(t
>
bio
0
lz
co
cuo
r
71
r-4
0
CY3
C'S
-- 4
11
0
cW
.;:I
4-3
0.
-4-3
;L4
0
0.4
CD
;.4
t!-
=4
L)
1)
V
:
71
(1)
co
:I4
-4
0
.,
a)
$:i
bj0
C)
4-J
z
i
-1
-1
;-4
4-3
m
-4
u
4
.4
o
-1
U)
bm
CHAPTER 2 BACKGROUND
17
4500
boehmite
7500
-------- > y
-------- >
10000
12000
-------- > O+cc -------- >
The y phase is described as a defect spinel in which 21 13 cations are
randomly located in
tetrahedral sites and 16 octahedral sites within a cubic
close packed cell of 32
atoms. In the
phase, the vacancies are ordered in
the octahedral sites and the unit cell is a tetragonal spinel superstructure with
a tripled c-axis. In the
phase, the cations are evenly split between the
tetrahedral and the octahedral sites. The a phase has the corundum structure
in which the
atoms are hexagonally close packed and the cations fill 23 of
the octahedral sites. Except for stacking differencesbetween and cc,these
phases are essentially close packed arrays of anions with structural changes
occurring primarily through cation movement. The corundum and spinel
structures are shown in Fig. 2 1.a) and b), respectively.
Amorphous A1203 can been obtained by anodic oxidation of Al. The
structure of this a-A1203 has been analyzed by electron diffraction and EXAFS
(El-Mashri et.al., 1982; Popova, 1979; El-Mashri, 1983). Most authors agree
that the Al is both octahedrally and tetrahedrally coordinated, although there
is some dispute about the relative amounts of each coordination. El-Mashri
(1982) reports that the bulk phase often become hydrated to form boehmite.
Crystalline Y203 has the bixbyite structure, which is similar to the
fluorite structure in that it has a cubic-close packed cation structure.
However, bixbyite has ordered absences of the anion in the tetrahedral sites
(Fig. 2.2.a). No other room-temperature polymorphs or intermediate oxides are
reported in the Y-0 system, although there is a high-temperature, hexagonal
phase OfY203 (Foex and Traverse, 1966). This presumably has the structure
of La2O3 (Fig. 2.2b).
CHAPTER 2 BACKGROUND
18
a)
M
0
b)
0
Oxypn
1
0
0
Octahedralcation
0
Tetrahedralcation
I
I
i
Fig. 2.1-a) Corundum structure showing only the cation sublattice.
HCP oxygen planes exist between planes of cations.
(o-empty octahedral site, Al3+ ion)
b) Spinel structure. Kingery et.al., 1960)
CHAPTER 2 BACKGROUND
0
0
0
0
19
0
0
0
0
0
0
0
0
0
0
0
0
V8
0
0
0
0
3/8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
b)
.-I.
.0
0 0
0 0
0 0
0 1
7/8
5/8
a)
0
0
0
0
- Oxygen
- Empty Tetrahedral Site
B
F
D
A
E
G
Fig.2.2- a) Projection of the tetrahedral 02-planes of the bixbyite structure
(a defect fluorite structure with a cubic close packed Y lattice).
b) A basal projection of atoms and a packing projection (on the dashed
line) in the hexagonal unit cell of La203- (Wycoff, 1960)
(Large atoms=O, small=La)
CHAPTER 2 BACKGROUND
20
The phase diagram of the A1203-Y203system is shown in Fig. 23 At
room temperature, bulk equilibrium phases show no solid solubility. The two
intermediate phases, Y3AI5012 (YAG)and Y4AI209, possess the garnet
structure (Fig. 24) and an orthorhombic structure, respectively. Another
intermediate phase, YA103, exists at high temperatures
1800-1900'C),and
it has a perovskite structure (Fig. 25).
Cr3+ was chosen as a dopant for the A203 films because it is similar to
A13+in size and behavior. Although the fluorescence of ruby (Cr:AI203) is not
desirable for telecommunication applications, Cr3+ was expected to act as a
probe of the crystal structure of the films. As a transition element, the
fluorescence and absorption of Cr3+is strongly dependent on its environment.
For instance, although the absorption in both ruby and emerald is due to Cr3+
in an oxygen octahedral site, ruby is red and emerald is green. This is due to
differences in the ligand fields of the host materials (corundum and beryl,
respectively) which change the absorption spectrum of the Cr3+ ions. The
optical characteristics of ruby are shown in Fig. 26.
Nd3+ was chosen as a dopant for the Y203 and Y-Al-0 films, again due
to size and coordination considerations.
Also, the fluorescence of Nd3 is
applicable to telecommunications at 13 tm. Currently, Er3+-fiber amplifiers
are commercially available as amplifiers at 1.55 [tm. However, tens of millions
of kilometers of 13 tm-singlemode fiber have already been installed worldwide,
and a suitable amplifier is not commercially available. Besides, Er3+ is a
three-level system whereas a four-level system is preferred in order to more
easily attain a population inversion. Another candidate is Pr+3, but it exhibits
excited-state absorption which makes Nd+3 the preferred choice. As a rareearth element, the fluorescence and absorption peaks of Nd+3 are sharp and
CHAPTER 2: BACKGROUND
21
not strongly dependent on the host lattice. However, the transitions become
rather broad bands in glasses (Fig. 27).
Amorphous structures will be
desirable in this work because they are expected to have the lower losses than
polycrystalline materials.
2400
2200
2000
1800
00
1400
A I
"41,2%j3
M01 %
Y203
Fig, 23 - Phase diagram of A1203-Y203 system (Levin, 1975)
* There is some debate as to whether or not YA103 melts congruently.
CHAPTER 2 BACKGROUND
22
(
.
I
I
I
I
4 2
)
4
(-
4 2I 2
I 0
4
1 )
2
Fig. 24 - The garnet structure OfY3AI5012.(Abrahams and Geller, 1958)
Am
-
to.,
' -
I
Fig. 25 - The perovskite structure of YA103 (Kingery et. al, 1960).
23
CHAPTER 2: BACKGROUND
4
25
F,
a)
20
- 42
I
E
I0
) =2i
15
29 cm-'
El
C
w
2
T
10
5
n.
b)
i
3
0
2
T
52
S6
E
62
0-
v
Z;
I
2
1
2
i,I8
.8
0
5-9
4
0
Wa,,-Ie-qth
(Al
Fig. 2.6
a - Important energy levels of Cr3+ in ruby.
b - Absorption bands of ruby. (Koechner, 1988)
24
CHAPTER 2 BACKGROUND
5
2275
250
J00
'00
500 f"J
'000
I? 00
a
Nd3+
.1
4 -
.
46
2--
K 32
9/2
2
465/2
0
5
a)
I.F
I
7/2
10
32
45
4
3/2
b)
Fig. 48,
Nd"
j
2
i,,
fVj
I
-dw-S
I - 529 ..
,
F &2H
5/2
9/2
I
9 0
ij
-------
FLUOROZIRCONATE
-
FLUOROPHOSPHATE
-,-
SILICATE
;: !V,
Fn
Ujz
z
\ V.,V,.
Ii
I
I
/
". ,V."\ I 1%\-
I
I
1250
I
1300
1400
1350
WAVELENGTH(nm)
F.gglass-
3:
a.ped
W t,,11,11thNd -
3/2-4'13
'
mrs,.,
sectu,
Fig. 2.7.a - Important energy levels of Nd3+.
b - Absorption and fluorescence bands of Nd3+ in a glass.
(Patek, 1970; Miniscalco and Andrews, 1988)
-,
1450
CHAPTER 2 BACKGROUND
2.4.2
25
'ReportedProperties of Thin Films
A1203
Thin Films.
A1203
thin films have been widely studied for
applications such as wear-resistant coating, catalysts, and high temperature
applications. (Hollenbeck et.al., 1990; Vuoristo et.al., 1991; Frederick et.al.,
1991; Chou et.al., 1991) These authors have reported the structural
properties of their films, but seldom have they looked into optical properties.
Most of the optical research on oxide thin films has focused on coatings, where
the direction of light propagation is perpendicular to the plane of the film.
(Pawlewicz et. al, 1980, Rainer et.al, 1985) Since the path length is short,
relatively high losses have been tolerated, and the relationship between optical
properties and processing has not been adequately explored. A few researchers
have investigated oxides for planar optical waveguides. (Kersten,1975; Smit
1986)
Most sputtered-AI203 films in crystalline form have been reported to
consist of the metastable y-AI203phase, rather than the equilibrium phase, CCA1203.
(Pawlewicz et. al., 1980; Hollenbeck et. al., 1990) However, Hollenbeck
et. al. 1990) obtained amorphous films by cooling their substrates to liquid
nitrogen temperatures, 196'C. Vuoristo et.al. 1991) was an exception in
that he deposited amorphous films via rf magnetron sputtering of both A1203
and Al targets and via rf diode sputtering of A1203 targets. However, he also
obtained -AI203 when he deposited films via rf diode sputtering of Al targets.
The as-deposited films of Chou et.al.
1991) consisted of 7-AI203
dispersed in an amorphous matrix. He annealed these films at 800'C for 2
hours and obtained over 90%
7-AI203;
further annealing at higher
temperatures and for longer times produced oc-AI203.Frederick et.al. 1991)
deposited very thin (few nm) A1203 films and annealed them. He found that
CHAPTER2?: BACKGROUND
the y -->
26
transition occurred at 800'C a temperature 200'C below the
reported transition temperature of micron thick films and 400'C below the
transition temperature of the bulk.
Reactive sputtering is often used to vary the stoichiometry of films.
A1203
has been reported to sputter stoichiometrically (Logan et.al., 1990;
Hollenbeck et.al. 1990), but Vuoristo 1991) conducted a study on the
stoichionietry of sputtered A1203 films and found that rf-magnetron-sputtered
films were oxygen deficient unless a partial pressure
present.
Of 02
( - 33%) was
He also found that films deposited by the reactive sputtering of an
aluminum target produced films with an excess of oxygen. Finally, he reported
that regular rf diode sputtering yielded stoichiometric films that contained high
Ar impurities, and introducing a partial pressure
Of 02
would reduce the
impurity evels along with producing an excess of 0.
Pawlewicz et. al.
1980) sputtered
consisted of 7-AI203 with
A1203
films for LV coatings that
35 dB/cm of loss and a refractive index of 17.
Rainer et.al. 1985) and McNally et.al. 1986) deposited films via e-beam
evaporation for coating applications.
They reported losses of >100 and >21
dB/cm and indices of 172 and 168, respectively. Kersten et. al. 1975)
reported the optical properties of a wide variety of evaporated materials for
use as optical waveguides. The A1203 films had refractive indices of 162-1.63
and losses of 10-20 dB/cm. Smit et. al. 1986) have shown that sputtered
A1203
films have promise in the area of Si compatibility by depositing a film
onto an oxidized Si substrate.
sputtering of an
A1203
The films they deposited by low-power rf-
target yielded indices of 156-1.70 and losses of 25
dB/cm, whereas those deposited by electron-beam evaporation yielded indices
of 157-1.63 and losses of 10 dB/cm. Annealing the sputtered films at 800'C
CHAPTER 2 BACKGROUND
27
was found to decrease the losses to dB/cm. Este and Westwood 1984)
reported that A1203 films made by a modifiedmagnetron sputtering technique
had indices of 165 with losses of 500 dB/cm.
This literature on A203 thin films indicated that the films made in this
study would most likely be amorphous-A1203 (a-AI203) ory-A1203- It also
indicated that subsequent anneals in the range of 600-1200'C may lower
optical losses and possibly yield phase changes (a > y and y > cc). It is not
clear from the literature whether the films would be stoichiometric. Finally,
expected values of refractive index ranged from 156-1.7 and expected losses
ranged from 100dB/cm
with only a couple of authors reporting losses below
20 dB/cm. The current study addresses the question of how processing can be
used to tailor the microstructure
of A1203 thin films.
Through a
comprehensive study of the relationships between processing and optical
properties, differences in reported values can be explained and optimal film
properties can be obtained.
Y203 Thin Films.
Again, the literature is found lacking in reports of
the relationship between optical properties and microstructure/processing.
Bezuidenhout and Pretorius 1986)reported textured bixbyite films, fabricated
via thermal evaporation, with the 100> direction normal to the substrate.
Onisawa et.al. 1990), however, found that their films, fabricated by rf
sputtering Of Y203, were oriented along the <332> direction (also bixbyite).
Hollenbeck et.al. 1990)also reported obtaining bixbyite by rf sputtering, but
they did not report any texturing.
CHAPTER2: BACKGROUND
28
Ying et.al. 1991) investigated the effects of oxygen content and
subsequent anneals on e-beam evaporated Y203 films. They reported that
films with oxygen deficiencieshad higher optical absorptions. Their films had
refractive indices ranging 1.80-1.85 632.8 nm). Rainer et.al. 1985) also
fabricated Y203 films via e-beam evaporation. They reported a refractive
index of 21 (at 248nm) with optical absorption of 230 dB/cm. Masetti et.al.
(1990) fabricated films via rf sputtering and attained indices of -1.88 (5801500nm) Bezuidenhout and Pretorius 1986) found that baking their films
(n=1.84) up to 800'C resulted in an increase in normal transmission, and an
increase in 222) x-ray reflections for 1.5gm-thick films.
No reports of
waveguide losses were found.
This literature indicated that bixbyite would most likely be the dominant
phase in the as-deposited films, and that it might be textured. Indices of
refraction from 1.80 to 1.88 were expected in the visible and infrared. The only
reported optical losses were measured normal to the plane of the film, and they
were quite high. These losses should decrease with oxygen content and with
subsequent anneals up to 8000C.
The present work looks at how
microstructure and optical properties can be tailored via processing.
Y203-AI203
Thin Films. Very little work has been done on thin film
combinations OfY203 and A1203. Pawlewicz et.al 1980) deposited Y4A1209
from a pressed powder target that was composed of 2 moles Of Y203 per mole
of A1203. It deposited as a stoichiometric, dense film with an index of refraction
of 1.85, losses of 35 dB/cm. They were not able to determine the phase
conclusively, but indicated that it was most likely orthorhombic. No reports of
co-deposition of Y and Al were found.
CHAPTER 2 BACKGROUND
2.2
29
Physical Sputtering
In this work, thin film waveguides were processed by rf (radio frequency)
magnetron sputtering.
Sputtering was chosen due to its ability to efficiently
fabricate insulating and dielectricmaterials as thin films. Sputtered films tend
to have good adhesion and higher densities than evaporated films since
sputtered atoms arrive at the film substrate with very high energies. Since a
sputtering source has a finite area, unlike evaporation which has a point
source, better cross sectional uniformity can be attained.
However, a
disadvantage of sputtering is that deposition rates are usually less than 2000
A/min. A typical sputtering system is shown in Fig. 28.
Ider
substrate
sputtered al
:)lasma
ititter source
Fig. 2.8- Typical sputtering system.
CHAPTER2: BACKGROUND
2.2.1
30
Principles of Sputtering
Physical sputtering involves bombarding a target with high-energy ions
at sufficiently high voltages that target atoms, along with secondary electrons,
are ejected. Ions are used rather than electrons or photons because the energy
transfer from ions to target atoms is more efficient due to similarities in mass.
These ions can be accelerated with a field to increase their energy before
impact. The potential energy of the ions contributes to electron transitions,
and the kinetic energy of the ions is transferred to the target lattice which
causes the ejection of target atoms from elastic collisions.
Essential to sputtering is a self-sustained glow discharge.
When a dc
voltage is applied between two electrodes in the presence of a low pressure gas,
such as Ar, negligible current flows until the breakdown voltage. However, a
current will flow if an energy source, such as a LTVlight, strikes the cathode
and causes it to emit electrons, but the current will be extinguished if the light
is turned off. This current will increase with the distance between the
electrodes because the electrons will make a fixed number of ionizing collisions
with the gas atoms per unit length. After each ionizing collision, another
electron will add to the flow toward the anode, and the newly created ion will
flow toward the cathode. When ions strike the cathode, or target, they will
eject atoms and secondary electrons. As the applied voltage increases,
breakdown occurs. This will happen at a lower voltage than expected since the
gas does not remain uniform. Rather, as the ionic current increases, ions begin
to accumulate in front of the cathode, creating a space charge. At this point,
the ejected. secondary electrons are sufficient to maintain the discharge so that
it is a 'self-sustained' discharge. This means that the current will continue to
flow even i all external energy sources are turned off.
CHAPTER 2 BACKGROUND
The structure of such a discharge is shown in Fig 29.
31
Due to a large
space charge, the electrons that are ejected from the cathode travel quickly
through Crooke's dark space, also called the sheath. Then at approximately
the mean free path to an ionizing collision, the electrons begin to create ionelectron pairs as they collide with the gas atoms in the negative glow. It should
be noted that this distance is not the total mean free path to any collision,
which is
to 10 times smaller. The electrons create enough ion-electron pairs
that the umber of ions are about equal to the number of electrons; this is
called a "plasma." By the end of the negative glow, electrons have lost enough
energy that they can no longer ionize the atoms with which they collide, and
they begin to accumulate. Since there is no ionization or excitation in this
region, it is also a dark space, called Faraday's dark space. After gaining
energy from the accelerating field, the electrons again begin to have ionizing
collisions and the current is similar to the current that was discussed
previously.
A "normal" glow is defined as one in which only the minimum voltage
(equal to the normal cathode fall) is present, and changes in power simply
increase the cross sectional area of the glow and not the current density. An
11
abnormal" glow occurs when the bounds of the electrodes, or of the smaller
electrode if the system is asymmetric, have been reached and increases of
power result in higher current densities and voltages. Sputtering systems
usually operate in the latter mode since voltages must be sufficiently high for
atoms to be sputtered out of the cathode.
CHAPTER2: BACKGROUND
2.2.2
32
RF Sputtering
DC sputtering techniques are not applicable to insulator and dielectric
materials, such as A1203andY203, because a conductive target is required to
complete the dc circuit. However, if the direct current is replaced by an
alternating current applied to one side of the dielectric target, and a discharge
exists on the other side of the target, a displacement current will flowthrough
the target and into the plasma. Radio frequencies are required for reasons
explained below;
typically
13.56 MHz is used due to government
communications regulations.
Rf sputtering operates on the same principles as dc sputtering, namely
a high voltage is applied between two electrodes which leads to the breakdown
of a gas and the formation of a plasma. The system can be either symmetric,
that is the electrodes are approximately the same size, or asymmetric. The
latter typically involves one small powered electrode, which will be the
sputtering target, and another grounded electrode, which may include the walls
of the chamber. This will be the system of interest in the present work.
When the target is biased positively with respect to the discharge, a
large electron current flows and saturates the surface, causing it to become
negative. As the target potential cycles to negative, an ion current flows to it
that is much smaller than the electron current due to the lower mobility of the
ions. As long as the period is short, as in radio frequencies, the ion current will
fail to fully compensate the electron saturation, and the target surface gains a
net negative bias, Fig. 2.10.a.
CHAPTER,2: BACKGROUND
33
a)
bk
M
;t
0
0)
bio
ct
a)
1=
0
-1-1
,L)
(W
W
0
H
Ions
(a)
Electrode Voltage
(b)
Fig. 2 10- a) The potential on the target as a function of time
for the first several rf cycles. b) The plasma potential for
an rf powered plasma discharge. (after Rossnagel, 1991)
The plasma potential will always be more positive than any part of the
chamber due to the high mobility of the electrons, Fig. 2.10.b. Therefore, the
target will be bombarded with particles accelerated by half of the peak-to-peak
rf voltage, assuming that a low number of collisions occur in the sheath. The
walls and substrate will only be bombarded by particles accelerated by a small
plasma-ground potential. Hence, net sputtering of the target will occur.
An advantage of rf sputtering is an increased ionization efficiency within
the plasma. As in the dc case, the secondary electrons from the target ionize
directly through ionizing collisions and indirectly by adding heat to the plasma.
This heating increases the average electron energy so further ionization can
occur. In rf sputtering, however, the oscillatory nature leads to greater
coupling of the electron energy with the field energy and to a continuous
shifting of the sheath edge. Electrons that are carried on the edge of the
sheath become highly energetic in what is known as the "wave-riding" effect.
CHAPTER.2: BACKGROUND
2.2.3
34
Magnetron Sputtering
Some sputtering systems use magnetrons to contain the plasma above
the target. This creates a more dense plasma, more efficient ionization, and
increased sputtering. With the use of magnetrons, lower Ar pressures and
voltages are required during sputtering. However, magnetrons also cause non-
uniform erosion of the target which leads to poor utilization of the target
material ('20-30%).
A particle of charge q moving in a magnetic field, B, is subject to a force:
Fm = v x B
Eq 21
where v is the velocity of the particle. This "Lorentz" force causes the particle
to move in a helical pattern around the magnetic flux lines, Fig. 2.11.a. The
pattern of motion is further complicated by the simultaneous presence of an
electric field, Fig. 2 Lb, because the particle is accelerated and decelerated
as it
travels with and against the field. The applied fields affect the
electrons since they are very mobile and through electrostatic attraction, the
ions follow the electrons to maintain
a neutral plasma.
The effect of a
magnetic field applied radially across a target is shown in Fig. 212.
CHAPTER2: BACKGROUND
35
X
X
X
X
X
X
X
X
X
X
X
X
(D
0 vl
X
X
X
I to Page
(a)
X
X
X
X
X
X
X
X
X
0
X
X
X
X
X
X
Net Motion
X
Speeded Up
X
X
X
X
X
X
X
X
X
X
X
X
X
(EXB Drift)
Slowed Down
X
X
-1
X
I to Page
N
Fig. 2 1 I- a) The motion of an electron in a magnetic field perpendicular and
then parallel to the page. Motion along the direction of the magnetic
field lines is unaffected by the field. b) The motion of an electron in
combined electric and magnetic fields. In this case, the electric field is
vertical, and the magnetic field is perpendicular to the page. (after Rossnagel, 1991)
B Flux Lines
EXB Drift Path
Cathode
Fig. 2.12- The shape of the magnetic field lines and the resulting
drift path of a radial magnetron. (after Rossnagel,1991)
CHAPTER 2 BACKGROUND
36
Fig. 212 shows the effect of a radial magnetic field that is "balanced",
i.e. the north pole in the center of the target is approximately the same
strength as the south poles on the perimeter of the target. If these poles are
unbalanced", the flux lines will extend beyond the target and may reach the
44
film substrate, Fig. 213. In this configuration, the film would be bombarded by
the energetic particles in the plasma, and deposited atoms may be resputtered.
Field Lines
Mal
Substrate
Fig. 213 - Schematic of an unbalanced magnetron. (after Parsons, 1991)
CHAPTER 2 BACKGROUND
37
2.3 Processing-Properties Relationships
2.3.1
Typical Microstructures
The deposition of atoms into a thin film can be broken down into three
steps. First, incident atoms transfer their kinetic energy to the film lattice and
loosely bond to the film as 'adatoms'.
Second, the adatoms diffuse on the
surface and exchange energy with the film and other surface species until they
either evaporate or become trapped in a lower energy lattice site. Third, these
atoms readjust within the film by bulk diffusion. The parameters affecting the
microstructure of the film for each of these steps will be shadowing,
evaporation, surface diffusion, and bulk diffusion.
The process of sputtering is a nonequilibrium process because high
energy atoms are essentially quenched as they land on the substrate.
Therefore, amorphous films can be fabricated out of many materials at room
temperature. This is especially true of complex materials, such as oxides.
Porosity may occur in these films if shadowingeffects are dominant. Increased
substrate temperatures may provide sufficient energy to allow the adatoms to
move to lower energy sites.
This will lead to densification of amorphous
structures or perhaps to crystallization.
Thorton (Thorton, 1977)studied polycrystalline metallic thin films (Fig.
2.14). This characterization generally applies to polycrystalline ceramic films.
At low pressures and temperatures, Zone 1, surface diffusion is insufficient to
overcome shadowing effects. This leads to open boundaries between deposited
islands of material and a rough film surface results. Preferential nucleation at
substrate defects leads to the formation of crystallites which add to film
roughness.
If the substrate is very smooth, Zone T microstructure results.
CHAPTER 2 BACKGROUND
38
These films will be fibrous with dense boundaries and no crystallites, and they
may possess more bulk-like properties. (Parsons, 1991) For this reason,
several ways of encouraging Zone T structures have been researched.
(Thorton, 1977; Parsons 1991)
At higher temperatures
>0.3 T/T,,,), surface diffusion becomes
dominant and the resulting structure is columnar with platelet- or whisker-like
grains and dense grain boundaries. At even higher temperatures >0.5 T/Tm),
bulk diffusion dominates and an equiaxed, sometimes epitaxial, structure
results. 'These structures are in Zone 2 and Zone 3 in the Thorton diagram,
respectively. A Zone T structure can be promoted at higher temperatures
using higher inert gas pressures.
This is due to lower adatom mobility since
sputtered atoms will experience more collisions before they arrive at the
substrate with less energy. At very high temperatures, these effects vanish.
Fig.2.14- Thorton diagram showing thin film microstructure as a
function of pressure and temperature. (Thorton, 1977)
CHAPTER 2 BACKGROUND
2.3.2
39
Processing Parameters
The processing parameters that have the most significant effect on film
microstructure, and therefore on properties, are substrate temperature and
oxygen artial pressure. Other important parameters include total gas
pressure, rf power, magnetron balance, target-to-substrate distance, and
subsequent anneals. Each of these parameters will be discussed below.
The effects of substrate temperature on polycrystalline films were
discussed in the previous section on microstructure. Increased temperature
will increase the energy of the adatoms so that they can diffuse within the film.
This will result in higher degrees of crystallinity and dense grain boundaries. In
amorphous films, higher substrate temperatures are also likely to lead to
densification. The substrate can be heated with an independent heater, but it
will also be heated by the energy of the depositing atoms, and by resputtering
or any other bombardment.
Increased ratios of the reactive gas will have several effects on the
process. First, if a metal target is being sputtered, the rate of sputtering may
be inhibited by the formation of an oxide layer on the target. If the rate of
oxidation is faster than the rate of sputtering, the oxide will be sputtered rather
than the metal. In general, the sputtering rate of an oxide is much lower than
that of a metal because the bonding energy of an oxide is much larger. Also,
negative ion bombardment of the substrate becomes a concern. As the oxygen
ions collect at the surface of the target, they are likely to be sputtering along
with the target material. Since the target has a net negative bias and the
substrate is part of the anode, these negative ions will be accelerated at the
substrate.
Subsequent bombardment may lead to increased defects, and
therefore higher optical losses.
CHAPTER 2 BACKGROUND
40
In the previous section, it was shown that at intermediate
temperatures a Zone T structure was promoted by using higher total gas
pressures because the adatonis would arrive at the substrate with less energy.
Inert gas pressures will also influence the plasma. At low pressure, the
number of ionizing collisions per unit length will decrease which leads to a
decrease in the number of ions striking the target. The glow will respond by
increasing the length and space charge of Crooke's dark space in order to
produce enough secondary electrons to sustain the glow. If the pressure is
sufficiently reduced, the edge of the dark space will reach the substrate, and
another source of electrons will be required to maintain the discharge. Another
consideration of pressure is that higher pressures tend to yield higher
concentrations of impurities in the films due to gas entrapment.
RF power directly affects the magnitude of the cathode fall, which is the
voltage that accelerates the ions before they strike the target. Therefore, the
energy ofthe sputtered atoms increases with the power of the sputtering
source.
Schuetze et. al. 1963) found that porous films resulted from low
cathode voltages, but when the voltage was increased, the films were dense
and exhibited bulk-like properties. They also reported that these results were
independent of pressure. In optical films, porosity would lead to a decreased
index of refraction and increased losses. Therefore, the rf power should be
sufficiently high for the fabrication of fully-dense films.
The magnetron is usually unbalanced in an attempt to enhance
resputtering, or bombardment ofthe deposited film. Resputtering, along with
low initial substrate roughness, has been found to promote Zone T structure in
polycrystalline films. (Thorton, 1977). Deposited peaks will be sputtered and
the atoms will be redistributed into the film valleys. Thermal energy from
CHAPTER 2 BACKGROUND
41
collisions of the bombarding particles with the film will aid in the redistribution
of matter. Resputtering requires less energy than normal sputtering to fill
voids because atoms just need to be knocked down from a peak, and no
momentum reversal is needed. For similar reasons, resputtering is likely to
help density amorphous films which will result in lower optical losses and higher
refractive indices. Disadvantages of resputtering at low temperatures include
entrapment of the inert gas ( 23 atomic %) which may cause high optical
losses and subsequent blistering.
Substrate bombardment can also be
accomplished through an independent ion beam.
Target-to-substrate distance has several effects. First, the further the
substrate is from the target, the less energy the sputtered atoms will have
after collisions in the gas.
This energy loss is called thermalization.
Thermalization will affect the microstructure of the films because the atoms
will have less energy to diffuse on the film surface after they are deposited.
Thus, shadowing may occur which would lead to porosity. For high-energy
atoms, and for more efficient collection of sputtered material, the substrate
should be as close to the target as possible. The substrate, which is part of the
anode, may be within the positive column, or even within Faraday's dark
space, without affecting the structure of the discharge. If the substrate is
within the plasma, the film will be resputtered and therefore, more dense. A
Zone T structure may result in polycrystalline films. However, if the substrate
extends beyond the plasma to the edge of Crooke's dark space, the source of
ions, and therefore the glow, will be extinguished. The magnetron contains the
plasma directly above the target, so it seems that extinction of the glow is not
a threat in magnetron sputtering. However,if the magnets are unbalanced for
CHAPTER.2: BACKGROUND
42
the purpose of resputtering, care should be taken not to extend the plasma
beyond the substrate.
Subsequent anneals can be used to density the films and anneal defects,
thereby reducing optical loss. Dutta 1990) used a C02 laser to anneal glass
films and found that optical inhomogeneities, and therefore optical losses, were
greatly reduced. However, thermal mismatch should be calculated to ensure
that the film does not experience such extreme temperatures in its processing
and operating lifetime that it will fail. The thermal stress due to mismatch
between the substrate and the film is (Thorton, 1989)
Gth = Ef (off- oc,)AT
Eq 22
where Ef is the elastic modulus of the film, and af and as, are the thermal
expansion coefficients of the film and the substrate, respectively. AT is the
maximum. difference in temperature experienced by the film during processing
and use a positive stress is tensile). If this thermal stress exceeds the yield
strength of the material, the film will either crack or separate from the
substrate during processing.
CHAPTER..2: BACKGROUND
2.4
43
Optical Considerations
2.4.1
Principles of Waveguiding
When a beam of light passes from a medium of high optical density into
a medium of low optical density, the direction and the velocity, v, of the beam
will change according to Snell's law:
nhigh _ sin
njow
sin
_ _ vlow
Ohigh
Eq 23
Vhigh
where nlowand nhigh are the indices of refraction of the respective media, and
the angles are defined in Fig. 215. If 01ow> 90', as in the second beam of Fig.
2.15, the beam of light will be reflected back into the dense medium at an angle
equal to the angle of incidence. This angle of incidence is called the critical
angle, ,
Fig. 2.15- Schematic demonstration of Snell's Law:
1) When light travels from a medium with a high index of refraction, n
to a medium with a low index of refraction, it gains velocity and also changes
direction. 2 When the angle of incidence is large enough, as in Ohigh,2,the
light is reflected back into the medium of high n.
CHAPTER2: BACKGROUND
44
Along with a change direction and velocity, the phase of the light will also
change at the interface. The phase of a ray of light that travels a distance, s,
relative to the starting point (s=O),can be defined as
= - n k s, where k =2n/k.
The light ray is really a simple way of representing plane waves whose
direction of propagation is the same as the direction of the ray. A plane wave
could be thought of as being composed of an infinite number of rays all
traveling parallel to each other, and all in phase. These waves will be either
transverse electric (TE) or transverse magnetic (TM) polarization, meaning
the waves are oriented such that their electric vectors, or magnetic vectors,
are parallel to the interface, respectively. The reflection coefficient of a TE
plane wave at a dielectric interface is given by Marcuse 1989):
(nigh k - P') 1/2- (nfowk2p2)112
r=
kigh k 2
2)1/2+ (n2 w k2p2)1/2
-
Eq 24
lo
where k = 2n/k, is the free-space propagation constant of plane waves and the
dependence on angle lies in the parameter P, (
Ohigh<Oc,
= nhigh k sin0high). When
r is positive and real, so no phase change occurs. However, when
Ohigh>Oc,r is negative and complex. In this case, the wave passing to the next
medium is "decaying." The phase shift experienced by such a reflected TE
wave is:
n2
-2 tan-1
k2
low
n2
high
and that experienced by TM modes is:
k - p2)
Eq 25
CHAPTER 2 BACKGROUND
45
t
-2 an
n2j,
2
-1
(p2
- n2
,
lo
k2)
nlow (n2i,,
k - p2)
h
Eq 26
This property of phase limits the angles of propagation even beyond the
requirement that they be greater than the critical angle 0, . These
limitations lead to the definition of "modes"that travel through the waveguide
at discrete angles. Fig. 216 can be used to gain graphical understanding of
mode guidance. The dashed lines represent the phase fronts of the light waves
(only the phase fronts of waves traveling upward are shown for simplicity).
The phase fronts that go through points A and
belong to the same plane
wave. However, the ray AB travels a shorter distance (d tan0hig - d/tan0high)
than the ray CD (d/cosOhigh).Also, ray CD will go through two additional phase
changes (one at each interface, OCand OD),whereas ray AB will not have any
addition phase changes. Since all of the points on the same phase front must
be in phase, ray CD can only travel distances that are M*2n longer than the
distance traveled by ray AB, where M is an integer. This means that only
waves traveling at angles that satisfy
nhig
d tan Ohigh -
d
tan
Ohigh
d - k0C+0C=2Mn
Eq 27
COS Ohigh
will be able to propagate through the waveguide. This criterion leads to an
eigenvalue equation that is described by Marcuse 1991) with the solutions
(eigenvalues) of
Piwhich are
also called the propagation constants of the
modes, i. This same conclusion can be derived from Maxwell's equations
(Marcuse, 1991), but an intuitive explanation is sufficient for understanding
the scope of this thesis.
describe them as
An even simpler explanation of these modes is to
ial standing waves that are propagating longitudinally.
CHAPTER 2 BACKGROUND
46
phase fronts
A
d
D
ns
Fig.2.16- An intuitive understanding for guided modes can be
gained through geometrical considerations. (after Marcuse, 1991)
The film thickness determines the number of modes which may be
supported. by the waveguide. The minimum thicknesses, Wmin, required to
support TE and TM modes of order m, respectively, are:
MT +tan-1
Wmin
2
nj- n_
Eq 28
k [-f- 4s
and
Mir + tan- I (nf)2
Wmin--
, -n2s n
2
n. A/ nf2---nS
Eq 29
k /`n-j- 4s2
where k= 2n/k, and nf, ns, and nc are the indices of refraction of the film,
substrate, and cladding (or superstrate), respectively.
For instance, the
minimum thickness for an A1203 (n=1.7) waveguide with fused SiO2 (n= 1459)
substrate nd cladding is 745 nm for TEI light waves ( = 13 gm). It is best
to design films to be thicker than the minimum required so that the desired
modes can be realized even in the presence of variations in index and thickness.
CHAPTER.2:BACKGROUND
47
The planar waveguides discussed thus far could consist of a film of high
optical density sandwiched between a substrate and a superstrate of low
optical density. In many applications, it is necessary to confine the light to a
one-dimensional path rather than a two-dimensional plane. This can be done
by using ridge configurations as shown in Fig. 217.
n-
ns
I
___V
planar
waveguide
ridge
waveguide
Fig. 2.17 A two-dimensional planar waveguide (asymmetric)
and one-dimensional ridge waveguide (symmetric: nc = ns).
2.4.2
Index of Refraction
As mentioned in the Introduction, the index of refraction is one of the
most important properties of a waveguide. For the purpose of waveguiding,
the
index must be significantly greater than that of the substrate or cladding.
Also, thin film waveguides must be fabricated such that the index is constant
throughout each sample to avoid scattering losses, and constant between
samples to insure device performance. Scattering at grain boundaries depends
on the relative indices between bordering grains. Therefore, for polycrystalline
waveguides, materials with isotropic indices will be preferred over those with
anisotropic indices.
CHAPTER 2 BACKGROUND
48
Waveguides are typically dispersive dielectric media, so their refractive
indices are dependent on wavelength. This dependence is caused by induced
dipoles that have electronic and lattice contributions. (Klocek, 1986) Typical
dispersion curves and empirically-determined equations are shown in Fig.2.18.
Thin films often have high degrees of porosity, which will lead to an
average decrease in the index of refraction, nf. This difference can be
approximated as the sum of the contributions from the solid, ns, and from the
voids, nv:
nf-- pns + (1-p) nv
Eq 210
where p is the volume fraction of dense solid in the film. More sophisticated
treatments of the solid-void composite index have been discussed by Harris et
al. 1979) and Guenther 1984).
Other causes of change in the refractive index of thin films include
changes in density, microstructure (especially crystalline versus amorphous),
and lattice defects. Post-deposition anneals can be used remove water, pores
and defects, or to crystallize amorphous films. However, crystallization is
usually accompanied by lattice parameter changes that may create new
stresses and defects (and perhaps even cracks) in the film.
CHAPTER 2: BACKGROUND
2.1
I.
49
II I .I . I I I .I II II .I .I II . .I . II . . . . . . . . I
2
1.9
I
r.
0
+4
Q
(z
1.8
u
04
0
X
w
lr
1.7
1.6
1.5
1.4
I
0.2
I
I
I
I
0.4
I
I
0.6
I
I
I
I
I
I
I
0.8
Wavelength
:Al2
I
I
I
I
I
I
1.2
I
I
I
I
I
1.4
1.6
(tm)
n 2= 1.02 X 2/(k 2 -. 004) + 106 ),2/(k 2 .012) + 1
3'
*Si02: n 2= 0.696 2/( 2 005) + 0407
.014) + 1
*YA10 : n2= 2.62),2/(k 2_.012) + 1
3
*YAG: n2= 2.82X2/(k 2_.011
#Y203: n 2 = 13.4x109 /(72,100 2 _
I
2
) + 7.48x105 /(436 2 _
2
Fig. 2.18- Dispersion curves and curve fits of several relevant oxides.
(*-Weber, 1987; #- Palik, 1991)
I
CHAPTER 2 BACKGROUND
2.4.3
50
Attenuation
Loss in optical devices is usually expressed in dB/unit length, where
I dB = 10 log. power output
power input
Eq 211
Each loss of 10 dB is a loss of one order of magnitude of the initial signal.
Acceptable losses vary depending on the application: optical fibers have losses
of 015-0.2 dB/km, good quality optical glasses have losses of about 103
dB/km, and acceptable thin film waveguides would have losses of 104-105
dB/km, or 0.1-1.0 dB/cm. Although the fiber community uses units of dB/km,
the thin film community uses dB/cm because the path length in a film is much
shorter than that in typical fiber applications. The wavelength dependence of
the intrinsic and extrinsic loss mechanisms is shown in Fig. 219.
Other
sources of loss are characteristic of the waveguide application and depend on
the waveguide geometries.
intrinsic bandgap
.ion
A
z
.21
M
::I
,Z)
-.1-1-11,
CZ
r absorption
r.
.-ing
wavelength
-0-
Fig. 2.19- Intrinsic and extrinsic loss mechanisms. (after Mocek, 1986)
CHAPTER 2 BACKGROUND
51
Intrinsic loss mechanisms include electronic absorption at short
wavelengths, multiphonon absorption at long wavelengths, and free carrier,
Rayleigh and Brillouin scattering at all wavelengths. Electronic absorption
can occur either directly by the absorption of a photon, or indirectly by the
absorption of a specific phonon and a photon, either of which occurs during the
generation of electron-hole pairs depending on the nature of the bandgap.
Multiphonon absorption is caused by the coupling of lattice vibrations with the
transmitted light. Free carrier absorption occurs in materials that have
significant conductivities, such as ionic conductivity in AgCl, due to host
Frenkel and Schottky defects. (Butvina et.al., 1990) Rayleigh scattering is
caused by minute density and concentration fluctuations in the waveguide, and
Brillouin scattering results from propagating fluctuations in the dielectric
function that are caused by phonons.
A transmission 'window' exists between the electronic and the lattice
absorption bands. Although Rayleigh and Brillouin scattering occur at all
wavelengths, they are inversely proportional to the wavelength raised to the
fourth power. Therefore, materials that have transmission windows at longer
wavelengths (infrared) will have lower intrinsic losses than materials with
transmission windows at shorter wavelengths. This is only true when free
carrier absorption is low, as in wide-gapmaterials such as A1203 and Y203.
Extrinsic loss mechanisms include absorption by impurities or material
defects, and scattering by structural imperfections. Impurities, water being
the most common, and material defects, such as vacancies, grain boundaries,
and local strains, introduce fundamental vibration modes and localized
electronic states that result in absorption bands within the transmission
CHAPTER 2 BACKGROUND
window
52
the material. The height of each absorption band is proportional to
the concentration of the responsible impurity or defect.
The defects described above are those found in crystalline materials.
Glasses are by definition are defective structures and therefore defects in glass
are described differently. Rather than observing the breaks in long range order,
the defects in glass involve differences in the short range order of the
coordination polyhedra.
Therefore, the defects fall into two categories:
incorrect onds and broken bonds. For instance, in fused silica, each Si should
ideally be connected to four oxygen to form a tetrahedron, and each oxygen
would ideally be connected to two oxygen. The number of shared oxygen
between any two tetrahedra is the degree of freedom which allows a glass to
form. However, Nishikawa et. al. 1989) mention that excess oxygen in high-
purity synthetic silica leads to peroxy linkages (:__-Si-O-O-Si=
which are
incorrect bonds. (The symbol =-Si represents
a Si atom bonded to three
oxygen.) A oxygen deficient silica may have an oxygen vacancy
Si-Si=).
Broken bonds, such as an E' center, contain charges that cause absorption
(Silin, 1985; and Langevin, 1986). In the amorphous structure of A1203,
similar defects may occur with respect to the Al octahedra and tetrahedra.
Structural imperfections, such as bubbles or cracks in the film, or
interfacial inhomogeneities, may scatter the transmitting light. This
scattering is similar to Rayleigh and Brillouin scattering in its relationship to
wavelength, but it is often of a greater magnitude. jKlocek,1986)
Other sources of attenuation are characteristic of the waveguide
application, such as geometry-dependent losses, losses due to laser damage
and losses, due to interfacial effects. Reversible interactions with the laser
include strain, distortion, expansion, temperature rise, non-linear
CHAPTER 2: BACKGROUND
transmittance,
53
and electro-optic effects. For a beam of high intensity,
cracking, pitting, melting, and vaporization may occur. Interfacial scattering
may be caused by large differences in refractive indices and by surface
roughness, and absorption may occur if water has adsorbed onto interfaces or
into voids. Adsorbed water will also change the value of the index of refraction
of the voids and hence the index of the film. This could lead to increased
Rayleigh scattering depending of the size of the voids. Interfacial losses can be
reduced by applying protective coatings and by grading the refraction index
across the interfaces.
Chapter 3
EXPERIMENTAL PROCEDURE
This chapter will discuss the fabrication and characterization of the
A1203,
CrA1203,
Y203,
Y-Al-O thin film waveguides and Nd-doped amplifiers.
The fabrication technique was rf magnetron sputtering. Preliminary results
for fabrication of ridge waveguides will also be discussed. Characterization of
microstructures was accomplished with x-ray diffraction
glancing angle configurations), transmission
-RD) (both 020 and
electron microscopy and
diffraction (TEM and TED), scanning electron microscopy and energy
dispersive spectroscopy (SEM and EDS), and Rutherford Backscattering
(RBS). Optical characterization techniques included the envelope method and
the m-lines technique for measuring index of refraction and dispersion, prism
coupling for measuring attenuation, transmission spectroscopy (visible and
near inftared), Fourier transform infrared spectroscopy(FTIR).
54
CHAPTER 3 EXPERIMENTAL PROCEDURE
3.1
55
Fabrication
A1203
and
Y203
thin films waveguides were made by reactive rf
magnetron sputtering with both metallic and oxide targets.
The sputtering
chamber (Norcal) and a schematic are shown in Fig. 31 and Fig. 32,
respectively. Table 31 contains a summary of the deposition conditions. The
chamber contained three sputtering guns (AJA International) with tilt
mechanisms that allowed for the co-deposition of up to three components. An
Osaka magnetically-levitated turbomolecular pump together with an Alcatel
mechanical pump (equipped with a device to prevent oil back-streaming) were
used to achieve base pressures of 24 x
10-7
Torr in typically 14-16 hours.
The mechanical pump and 'turbo' pump were turned on together for roughing.
Three pressure gauges were used to cover the necessary pressure range: a
convectron from Granville-Phillips (ambient (10-4
1-2
Torr), and a cold cathode
10-8Torr) and capacitance manometer (10- -
-
Instruments.
pumps.
-5
Torr) from MKS
A convectron was also located on the foreline between the
If the chamber was left at ambient for a long time, a UV lamp
(Danielson) was used to speed the pumping of water out of the chamber, and
the turbo pump was heated with a band heater to avoid condensation in the
pump.
Very high purity Ar 99.9996%) was flowed into the chamber at the
desired flow rates, and a gate valve was used to attain the desired pressure.
For reactive sputtering,
02
99.999%)was also fed into the chamber. Flow
rates were controlled electronically with mass flow controllers from MKS. Dry
N2
was used to vent the chamber and to work the shutters on each gun and on
a film thickness monitor (Sycon). Seamless tubes and metal gasket face seal
VCR fittings (Cajon) were used in the gas lines.
CHAPTER 3 EXPERIMENTAL PROCEDURE
56
77----41"
j
-1
4
Fig. 3 - Sputtering chamber and control rack.
a
CHAPTER 3 EXPERIMENTAL PROCEDURE
film thickness
monitor
Q
57
Y- heater
2
-I--,'
substrate
'I
44
02
/I
vacuum
h--, 30'
PUMP
UV lamp
-
U
L
I
.0 Br
target
magnetron
I
Itl
Itl
rf power, Ar &
cooling water
Fig. 3.2- Schematic of sputtering chamber.
Table 3.1- Deposition Conditions
Base Pressure
Deposition Gases
2-4 x 10 -7
Ar
02
0 50 %
40 sccm
4 mTorr
0-50OW
% 02
Total Gas Flow Rate
Deposition Pressure
Total RF Power
Targets
Al 99.99%)
Cr 99.5%)
Y 99.8%)
Y203 99.9%)
Target to Substrate Distances
Typical Rates from Metal Targets
Substrates
02
refers to the ratio of the
16 cm
fused SiO2
(Si or TEM grids)
60 rpm
Substrate Rotation
Substrate Temperature
*
12
100-300kmin
02
RT- 5000C
flow rate to the sum of the Ar/02 flow rates.
CHAPTER3: EXPERIMENTAL PROCEDURE
58
Power supplies and matching networks from RF Plasma Products were
used to supply power to 2-inch targets. The targets were cleaned by sputtering
with Ar alone for
minutes before each deposition. The magnetrons were
sometimes unbalanced by replacing the central magnets with Fe slugs.
Fused SiO2 substrates (from Quartz Plus: 99.999% 12" x 1" x 1mm,
scratch/dig=80/50, see Appendix A) were cleaned ultrasonically for 10 minutes
in acetone, followedby minutes in methanol. They were then either wiped
with methanol, or blown dry with compressed air. One substrate was usually
masked for profflometry (Dektak3, or Tencor Alpha-Step 100) by a -Imm
strip of scotch tape. When the sample was to be analyzed by FTIR a piece of
Si wafer was also attached to the substrate holder. TEM samples are
described in "3.2- Characterization."
To ensure cross-sectional homogeneity,
the substrates were rotated (Bodine motor and controller, RedLion display) at
a sufficient speed to allow the deposition of no more than one atomic
layer/rotation.
Substrate temperature was varied using Omega cartridge heaters
which were located behind the substrates. The power for the heaters and for a
thermocouple had to be passed through a slip ring (Litton PolyScientific)
because the substrates were rotated. Heating and cooling the substrates
required an extra hour on either side of deposition. Annealed films were heated
either in vacuum or in air from 100-1200'C in a Lindberg tube furnace. There
were no noticeable differences between films that
and those annealed in air.
ere annealed in vacuum
CHAPTER 3 EXPERIMENTAL PROCEDURE
3.1.1
59
A203 and Cr:A1203Thin Films
The alumina waveguides were all made with aluminum metal targets
and an Ar/02 sputtering gas. The targets were cleaned by sputtering with Ar
alone for
minutes before deposition began. The bias voltage on a new Al
target was about 670 V, but this was reduced to 300 V after extensive use (on
the order of 100 hours). The rate of deposition varied between 100 and 150
A/min depending on temperature and
02
in the sputtering gas. The
processing parameters of primary interest were substrate temperature,
02,
and subsequent anneals.
It was more difficult to start a plasma with the Al target after it had
been out of the vacuum for a while. This was attributed to the formation of an
oxide layer on the surface. When there was a problem igniting a plasma (due to
shorting between the target and the grounding shield, incorrect matching of the
rf power networks, poor power connection to gun, etc.) there would be zero bias
voltage on the target and most of the power being sent to the target would be
reflected back to the power supply.
When an oxide coating on the target
prevented ignition, however, there would be no reflected power (all forward
power) and no, or low, bias voltage. Wen this was the case, either the Ar
pressure or the forward power could be raised.
As long as there was no
reflected power, the oxide would eventually be destroyed, and a plasma ignite.
A similar problem occurred when oxygen flow rate was increased above
10%. In this high 02 regime, the rate of oxidationof the target was faster than
the rate of sputtering, and the plasma would be extinguished.
If the
capacitance was properly tuned, the plasma would re-ignite with new matching
parameters that corresponded to sputtering the oxide instead of the metal.
The rates of sputtering the oxide were extremely low
15,A/min).
CHAPTER 3: EXPERIMENTAL PROCEDURE
60
Alumina waveguides were doped with Cr by the co-deposition of Al and
Cr metallic targets in a reactive atmosphere A thin film of pure
Cr2O3
was
made initially in order to measure the deposition rate, which was determined to
be abouthalf as fast as that of A1203.
The power to the Al and Cr targets was
kept proportional to the amounts of Cr that were desired in the film allowing for
the slower deposition of Cr. For example, if 10% Cr was desired, 27OW was
supplied to the Al target and 60 W was supplied to the Cr target. It was later
determined that the deposition rate, and therefore thickness, does not vary
linearly with power. Therefore, all of these films had much lower Cr
concentrations than were calculated.
However, this result was somewhat
expected and goals for %Cr were purposefully elevated. Several films had Cr
levels comparable to ruby 0.4-1%), so their fluorescence could be studied.
Desired thicknesses for the films were determined using Eq. 28. For
TEO (single-mode), the minimum thicknesses for
A1203
(n-1.7) planar
waveguides with fused SiO2(n=1.459) substrates and air claddings (n=1.00) are
1019 and 2417A at k= 632.8nm and 1.5Vm, respectively. Single mode films
(-3500A thick) were fabricated initially in order to allow more accurate
comparisons between samples when measuring waveguide losses. However,
due to substrate coupling, it was difficult to measure waveguide losses in these
films, and multi-mode (TE2) films ~Itm thick) were fabricated.
This also
allowed a study of the effects of thickness on the optical properties.
Other processingparameters that were briefly investigated were targetto-substrate distance and sputtering with an unbalanced magnetron.
The
substrates were positioned to be 12 cm from the targets for most of the films,
but a few later films were made with the substrates at 16 cm from the targets.
A magnetron was unbalanced by placing an Fe slug in place of the center
magnet while leaving the magnets on the edge in place.
CHAPTER 3 EXPERIMENTAL PROCEDURE
3.1.2
61
Y203 Thin Films
Fabrication of theY203waveguides was attempted via two routes: with
an oxide target and with a metal target. One disadvantage of sputtering is that
the target material must be available in the correct shape, 2 x 25" discs in
our case. The yttrium metal targets were quite expensive, and yttrium metal
targets were reported to oxidize too quickly to be useful. Therefore, yttrium
oxide targets were fabricated for the initial films
powder was purchased from Johnson-Matthey that had -2gm
Y2(3
particle size.
This powder 65 g) was mixed with a naphthalene binder and
pressed in a die with an inner diameter of 2.25". The binder was burned out at
400'C, and the target was sintered at 1500'C for 2 hours with ramping rates
of 5C/min.
The ceramic targets were ground to size with 600 grit sandpaper,
and then they were bonded to 116" thick Cu discs with Ag-filled epoxy (Pinacle
Technologies, Inc.) to enhance thermal contact to the water-cooled cathode of
the sputtering gun. The epoxy, which adhered best to a surface that was not
ground after sintering, was cured at 200'C for 2 hours.
Many different processing conditions were tried with this ceramic target,
but the deposition rates were not sufficiently high. In many cases, the
substrates were simply resputtered by negative ion bombardment and no film
was deposited at all. The applied power was limited to 20OWto avoid cracking
the target with thermal stresses. The bias voltage, ranging from 450 to 550,
decreased with increasing %O2and with time (use). The%02in the sputtering
gas was varied from
to 50%. Most of the films were deposited for one hour,
but a few films were deposited for 2 hours or 3 hours.
In an attempt to
improve the deposition rates, the target-to-substrate distance was shortened
from 21 to 16 to 12 cm. One film was made on a substrate holder that was
CHAPTER3: EXPERIMENTAL PROCEDURE
62
immediately above the chimney of the gun (-6cm).
Y203
targets were also made with 50 atomic
Y203.
Nd203-Y203
and Er2O3-
However, after the poor
results with the pure Y203 targets, these targets were not used.
Subsequently, films were made using a Y metal target with an Ar/02
sputtering gas. The targets were cleaned by sputtering with Ar alone for
minutes before deposition began. The bias voltage
%02,
which was 730V at low
decreased strongly as a function Of %02 and substrate temperature. At
>5% 02 or at 400'C, the bias voltage was 15OV,and it dropped to 100V over a
20-min deposition. The rate of deposition varied between 150 and 250 A/min
depending on temperature and %02 in the sputtering gas. The Y target did not
have a problem with the formation of a protective oxide coating as did the Al
target, so a plasma was always easily ignited. However, the decrease in bias
voltage and rate (presumably due to oxidation of the target) was very gradual
as a function
Of %02
in the sputtering gas, and it seemed to start at 5% 02.
For the Al target, the oxidation effects occurred as a sharp step function at
just over
10%02.
It was difficult to make thick
Y203
films because the
chimney of the sputtering gun would peel after a short time, and the flakes
would cause arcing. This was especially a problem when the substrates were
heated because the temperature in the whole chamber increased and the
flaking was more pronounced.
Desired thicknesses for the films was determined using Eq. 28. For TEO
(single-mode),the minimum thicknesses for Y203 (n-1.9) planar waveguides
with fused SiO2(n=1.459) substrates and air claddings (n=1.00) are 607 and
1349A at k= 632.8nm and 1.5tni, respectively.
Most of the films were
-3500A tick, which was the maximum thickness for single-mode waveguides.
However, three mulitmode films were made that were ~5500A thick. This was
difficult because the chimney would peel and the flakes would cause arcing.
CHAPTER 3: EXPERIMENTAL PROCEDURE
63
3.1.3 Y203 A203 Thin Films and Nd-Doping
Y203-A1203
waveguides were fabricated by simultaneous reactive
sputtering of Y and Al metal targets.
The power to the Y and Al guns was
varied in the following manner: 100/300, 200/300, 300/300, 300/200, 300/100
(W to Y/W to Al). The targets were set to their respective powers and were
then sputtered in pure Ar for minutes for cleaning. The bias voltages of the Y
and Al at 30OWwere 424-696V and 495-52OV, respectively. The bias voltage
of the Y was again very dependent (decreasing) on the
gas. The
02
was varied between 3 and 6.
02
in the sputtering
A few films were made with
heated substrates (400'C), but most of the films were made at room
temperature. Otherwise, the targets exhibited the same characteristics as
when they were used in single-component depositions. The films were -7000A
thick, which corresponded to similar deposition times as the single-mode
Y203
films. This limit was imposed to avoid peeling within the Y gun.
Several films were doped with Nd by placing a 1" x I" x 0.1 mm foil
(99.9%) from Johnson-Matthey on the Al target.
Nd is easily oxidized so a Nd
target was not guaranteed to sputter efficiently. The foil came packaged in Ar,
and was only slightly tarnished before deposition. The plasma of the Al gun
with the Nd on it was bright blue. All of the other plasmas were a lavender
color. The Nd first wore away in the path of the most dense plasma (a ring
caused by the magnetron). When this happened, the remaining foil was cut
into four pieces which were placed in directly the wear path. The bias voltage
(~490 V) ofthe target was slightly lower than the bias of a plain Al target. One
film was made by co-deposition from all three guns #I- Y, #2- Al, #3- Nd on Al)
at 300/300/100 W, respectively.
The matching networks did not seem well
matched and the plasma pulsed a bit. After this sample, gun 3 seemed to
CHAPTER 3 EXPERIMENTAL PROCEDURE
64
cause the other guns to shut down if they were run simultaneously. This
problem was related to the software parameters of the power supply. Other
films were fabricated with the Y target together with the Al target that had the
Nd foil on it. Films were made with power ratios of 300/100 W and 100/300 W,
respectively and with
and 6
02.
One film was fabricated by co-depositing
from both Al targets (one plain, 30OW;Nd on one, 10OW).
3.1.4
Ridge Waveguides
The goal of this work was not to fabricate and fully characterize ridge
waveguides. Rather, it was the exploration of inexpensive chemical routes to
fabrication for future reference. Such techniques are potentially much less
time consuming and much more economicalthan vacuum etching techniques.
According to the Lide 1991), alumina is insoluble in most common
solvents. However, the amorphous A1203 films were soluble in sulfuric acid,
although sputter-deposited SiO2 was not. This is similar to rocks in which acid
rain (also H2SO4) leaches away the aluminates, but leaves the silicates
behind.
A few sample ridge waveguides were fabricated by masking all but a
strip on the planar
the
A203
A1203,
depositing a layer OfSiO2, and then dissolving all of
except that which was under the SiO2. The dissolution process,
which could take a few days, was much quicker
10 min) when the acid was
heated to .-2000C. Another coating OfSiO2 could be deposited to achieve a
ridge waveguide similar to the one in Fig.2.17. The yttria and
Y203-AI203
were found. to be very soluble in sulfuric acid even at room temperature.
films
In the
future, lithographic techniques could be used to achieve sharper interfaces.
CHAPTER 3 EXPERIMENTAL PROCEDURE
3.2
65
Characterization
3.2.1. Microstructure and Morphology
The film microstructure was characterized by x-ray diffraction (XRD)
using both conventional 020 (Rigaku 300) and glancing angle (Rigaku 200)
geometries. See Appendix
A1203-Y203
for JCPDS diffraction cards of phases in the
system. Glancing angle XRD is used to reduce the signal from the
substrate, and its geometries allow signal collection from more lattice spacings
than conventional XRD. In this work, the glancing angle was set to either 0.5
or 20', and the sample was rotated with the normal to the film as the axis of
rotation. Transmission Electron Diffraction (TED) in an Akashi 002B High
Resolution Transmission Electron Microscope (HRTEM) was also used to
examine the microstructure. TEM samples were made by depositing ~300A of
SiO2 and then -700A of A1203 and/or Y203 directly onto #01800 Cu grids (Ted
Pella, Inc). The grids were 200 mesh Cu that was coated with 35-70nm of
Formvar and a 5-10nm of evaporated carbon.
Elemental
analysis of the Cr:A1203 films was done on a Jeol 840
Scanning Electron Microscope (SEM) with a Tracor Northern Energy
Dispersive Spectroscopy (EDS) package. Since the films were insulators, they
were first coated with ~100A of Pt. A low accelerating voltage (10keV) was
used to minimize the substrate signals. However, 0.9% Cr was the limit of
resolution by EDS, so optical absorption was used to estimate the Cr content
in films with < 09% Cr.
Rutherford Backscattering Spectroscopy (RBS) with 2 MeV 4He+ ions
was also used to look at variations in stoichiometry in the
films were sputter-coated with Au to prevent charging.
A1203
films. The
CHAPTER 1'1:
EXPERIMENTAL PROCEDURE
66
Co-deposited Y-Al-O films and Nd-Y-Al-O films were analyzed on a
Camscan SEM with a KEVEX EDS package. An accelerating voltage of 0
keV was used to focus the beam. Once in focus, the beam raster was turned
off, and signal was accumulated for 60 seconds. The escape peaks and
background were subtracted using the automatic parameters of the software.
Due to the low accelerating voltage, the Y L-line and Al K-line were used for
calculations. If the voltage was raised to 20 keV, the Y K-line could be used,
but the substrate peaks were detected, and they overlapped with the Al peak.
Three different conductive coatings (-100A) were used on the films: Au, Pd,
and C. The peak for Y overlapped with the peak for Au, so it was not the best
coating. There was no difference in the results for the Pd-coated and the Ccoated samples.
3.2.2
Optical Properties
The envelope method. Index of refraction was measured by two
techniques: the envelope method and the m-lines technique. In the envelope
method of Manifacier et.al. 1976), light is passed normal to a film, and the
transmission through the film is detected as a function of wavelength by a
spectrometer (here, a Perkin-Elmer Lambda 9
When the optical thickness
(n*thickness) of the films is of the same order of magnitude as the wavelength
of the light, several maxima and minima (interference fringes) occur due to the
interference of the reflected light from both surfaces of the film. The index of
the film can then be calculated from the curves of transmission, T, of the
CHAPTER,3:EXPERIMENTAL PROCEDURE
67
II
0.9
Z
0.8
.2M
.4
M
0
(t
EZ
0.7
0.6
n _r;
300
400
500
600
700
800
Wavelength (nm)
Fig. 3.3- The envelope method uses the curves Tmax and Tmin,
which envelope the transmission spectrum.
900
CHAPTER 3 EXPERIMENTAL PROCEDURE
68
extrema which envelope the spectrum (Fig. 33). The following equations were
used:
n = IN + (N2 -
n2n2)1/2]1/2
C S
Eq 31
where
N
+ 2ncn, Tmax - Tmin
2
Eq 32
Tmajmin
The thickness can also be calculated by
t=
MXjk2
Eq 33
2 (n(k I) X - n(k2) XI)
where M is the number of oscillations between the two extrema (M=1 between
two consecutive maxima and minima). The envelope method assumes that
the films have negligible losses and that the substrate is infinite. This method
also gives a measure of dispersion since the index is calculated at several
wavelengths.
The maxima and minima curves were determined graphically. Since the
maxima and minima do not occur at the same wavelengths, the value of the
curves had. to be determined for the alternate points. For example, the value of
Tmax had to be found at each of the wavelengths at which a minima occurred.
An alternative method would involve curve fitting the data and then using the
equation for the curve fit as a continuum of points with which to calculate the
index of refraction. Due to the UV absorption which begins to play a role at the
shorter wavelengths, the curve:
T0 = T- - exp(-(Ak)2+B)
fits the data fairly well.
Eq 34
CHAPTER3: EXPERIMENTAL PROCEDURE
69
The M-lines technique. The mlines technique can also be used to
determine the index of a thin, transparent, dielectric film. (Ulrich and Torge
1973) In this method, the film is pressed up against the base of a prism such
that only a thin ~1/2 k) gap with a low refractive index separates them
(Fig. 3.4.a). A polarized laser beam then enters the prism which must have a
sufficiently high index so that the beam would ordinarily be totally reflected
upon reaching the prism-gap interface. Under the right conditions, however,
the beam can be coupled into the film via optical tunneling.
These coupling
conditions are that the beam must have an angle of incidence such that the
evanescent fields in the gap travel with the same phase velocity as the mode
to be excited in the film. Then, upon entry into the film, the beam will have an
allowed propagation constant, P.
With a symmetric prism, a small portion of the light is also coupled back
out of the prism, and a series of lines, called m-lines, can be seen (Fig. 34.a).
Each line is caused by a different mode in the film. At least two modes are
needed to determine the index and thickness of the film. However,if thickness
is already known, index can be calculated from a single m-line, for instance the
single line of a single mode waveguide. In this study, a TE polarized HeNe laser
beam was used with an LaSF-N9 flint glass prism (n=1.96)for the
(YA1)203 films and a rutile prism (n=2.8) for the
Y203
A1203
and
films. The accuracy of
measuring the coupling angle was 0.1', which led to an accuracy of -0.00
in
refractive index.
If the film has at least two modes, TEOand TE1 for example, the angles
at which oupling occurs can then be used to determine the index and the
thickness of the film. In calibrating the apparatus, the prism face should be
normal to the laser beam at 45'. First, the propagation constants,
(respectively), are found for the modes:
Po and 1
CHAPTER,3:EXPERIMENTAL PROCEDURE
70
coupled mode
a)
I
I
I
I
I
I
I
I
I
J
I
!reen
pole
substrate
pressure point
b)
20x
Si
ode
Fig. 3.4- a) The m-lines technique.
b) Schematic of loss measurements.
CHAPTER 3.-EXPERIMENTAL PROCEDURE
71
= sin0o cosOP + VnP - sin 200 sinOP
Eq 35
P = sin0i cosOP + VnP - sin 20, sinOP
Eq 36
Po
and
where O is the prism angle shown in Fig. 3.4.a. and 00 and 01 are the coupling
angle of the modes, respectively. The index is then calculated iteratively using
the following equations:
nf
2 XV2
PO
I _Pi2 p0
W1
Eq 37
Wo
where
1#0 tan-
n2
2
N - PO
+ tan-
-
2
Eq 38
2
NJ - Po
and
7t+ tan-
2
NJ - Po
+ tan-
ns2
2
NJ - 00
Eq 39
nc, ns and nf are the indices of the cladding, substrate, and film, respectively.
Nf is initially approximated as LOPP0, but is replaced by the calculated nf in
following iterations until the calculations converge on a value. It is important
to remember to convert angles to radian units for these calculations.
To facilitate initial coupling, an approximate index was entered into Eq.
3.7, and the approximate coupling angle can be determined. This is most easily
accomplished if Eq 35 - 39 are in a spreadsheet, so a range of possible angles
can be quickly determined by using a range of expected indices. Once coupling
angles were found, a QuickBasic program was used to calculate indices.
CHAPTER 3 EXPERIMENTAL PROCEDURE
72
For the single-mode films, the index of the films can be calculated using
one coupling angle and the thickness, t, as measured by the profilometer. The
index can be calculated by
nf
k2
t
Po
Eq 3 10
where k=2n/k. All other parameters are defined as before, and Nf is again
iteratively replaced by nf until the value converges.
Attenuation.
A Perkin-Elmer Lambda 9 LV/VIS/NIR spectrometer
and a Perkin-Elmer Fourier Transform Infrared (FTIR) spectrometer were
used to determine the overall transmission window of the thin films. Doublepolished Si was used for the substrates of the FTIR films.
Several configurations for measuring actual losses within the film were
also tested, including detection of the streak of scattered light at the surface of
the film, end-coupling into and out of the film, and prism-coupling into and out
of the film (both with right-angle and symmetric prisms). The most suitable
geometry proved to be prism-coupling at the input and end-coupling at the
output of the signal (Fig. 3.4.b). The set-up was similar to that for m-lines, only
the end of the sample was cleaved and a OXobjective lens was used to gather
the output and focus it onto a Si photodiode. In this technique, the mlines
were useful for aligning the optical components efficiently. The prism was
translated away from the edge so that the light would be guided through
various lengths of the film. This allowed the cancellation of coupling losses and
hence a more accurate measurement of actual waveguide losses. Losses due
to the nature of waveguiding were not studied here since the films all had
similar geometries and coupling losses were canceled by the measurement
technique.
CHAPTER 3 EXPERIMENTAL PROCEDURE
73
Fig. 3.5.a shows the actual prism apparatus that was used in this study.
Fig. 3.5.b shows an m-line as it appears on a screen located behind the prism
apparatus as 106 tm light is coupled into a Y203-AI203 waveguide.
Fluorescence. Fluorescencein the Cr:A1203films was at first detected
by illuminating a film with the 514.5 nm line of an Ar laser and viewing it
through Ar safety glasses to see if any red could be observed. The films were
later setup such that the 514.5 nm light was passed through a fiber that was
butted up against the film. Any fluorescence was gathered back through the
fiber and focused onto the slit of a monochrometer with a liquid N2-cooled Ge
detector at the output. The latter apparatus was also used to measure
fluorescence in a few of the Nd-doped films.
CHAPTER 3 EXPERIMENTAL PROCEDURE
74
m
0
CZ
;-4
cd
I:Ll
04
CZ
.r.., Id
"a) h
m
", (p
It0 .
0 .0
-W
4
lt.,
w 9b
a)
;-4 W
Q ;>
CD
cd
q :
0
a)
d
o
lm -a
"I
It
,a S
0.
,6 :j
0 0u
1
;-4
ul
1"
cd -42L" gb4
CZ
m
rn CZ
-4
$-4
PL,
cd
16
C6
Chapter 4
A1203AND Cr-.AI203WAVEGUIDES
This chapter evaluates amorphous
A1203,
fabricated by reactive rf
magnetron sputtering, for use as a planar waveguide. The microstructure and
optical properties of A1203planar waveguides are examined as a function of
deposition (substrate temperature, 02flow rate) and annealing conditions.
Microstructural analysis is discussed first, followed by a discussion of the
general durability of the films. Finally, optical properties, such as transmission
windows, index of refraction, dispersion, attenuation and fluorescence are
reported.
75
CHAPTER, 4 A1203 AND CrA1203 WAVEGUIDES
4.1
76
Phase Determination
The nonequilibrium nature of reactive rf magnetron sputtering was
found to allow deposition of amorphous
A203
films within a wide processing
window (Fig. 41). Substrate temperatures extended from room temperature
to 500'C and oxygen contents in the sputtering gas varied from 4 to 10%
Above 10%
prohibitively slow
02
02-
the Al target oxidized and the rate of deposition was
15,A/min). Below 4
02,
films became defective and the
TVedge of the optical window shifted to longer wavelengths as shown in Fig.
4.2. Similar stoichiometric-dependent shifts in the UV absorption edge have
been observed in
SnO2-x
films
prepared in our laboratory. Rutherford
Backscattering Spectroscopy (RBS) revealed that these films had higher
ratios of Al/O (Fig.4.3) than the films made within the processing window (410% 02
) of transparent aA1203 (amorphous-AI203)waveguides. The peak in
the RBS data is due to an overlap in the Si (from the substrate) and the Al
spectra. It is thickness dependent. The simulation indicates that the
films were off stoichiometry Al/0=0.667). These films were annealed at 200
and 300'C in air in an attempt to make them transparent while retaining their
amorphous structure. After 30-hour anneals, they were lighter in color, but
still very lossy. At less than 2
02,
the films were metallic Al.
77
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
600
defectiv e
Al 20 3
Al
amorphous
o)ddized
A1203
target
500
f-I
0
400
0
U
0)
4
M2
0
-3500
0
-3500 nim
x
-3500 nm
0
- I PM
nim
300
.4
w
:z
w
0
E-
1:1
200
1(O
I
x
D
0
I
0
0
2
4
6
I
8
10
%0 2
Fig. 4.1- The effects of substrate temperature and
on the microstructure of A1203 films.
02
12
CHAPTER 4 A1203 AND CrAI203 WAVEGUIDES
78
1
0.8
0.6
00
M
.41
E
M
zCZ
0.4
E--f
Z:-
0.2
0
A
-U.4
200
300
400
500
Wavelength
rum)
600
700
Fig. 4.2- The LTVabsorption edge for A203 films deposited at low % 02.
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
79
25
20
10
W
15
lc
W
M
0
7
10
5
0
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Energy (MeV)
Fig. 4.3- Rutherford Backscattering Spectrum and simulations of films
deposited within (5%) and below 3.5%) the processing widow
of transparent a-AI203 waveguides.
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
The amorphous
A203
80
films were shown to be quite stable and could be
annealed up to 800'C without crystallizing. However, films annealed at
1000'C rystallized intoy-AI203, and a 1200'C anneal yielded the equilibrium
phase, oc-AI203or corundum. The x-ray diffraction patterns of these films are
shown in Fig. 4.4a. The film annealed at 1000'C had broad peaks which
indicates smaller crystallite sizes, whereas the sharp corundum peaks indicate
that grain growth had occurred at 1200'C. Films were annealed in both
vacuum and air for two hours.
To further confirm that the films were amorphous, they were analyzed
by GAXRD, high resolution TEM (HRTEM) and transmission electron
diffraction (TED). GAXRD,used to reduce substrate signal, verified that the
films were amorphous on both SiO2 and Si substrates (Fig. 4.4.b). Glancing
angle x-ray diffraction (GAXRD)was
HRTEM also revealed that the as-
deposited films were amorphous, but they were crystallized (7-AI203)by the
electron beam of the TEM within a few minutes. Micrographs and TED
patterns are shown for as-deposited films and electron-beam-annealed films in
Fig. 45. Table 41 shows the calculated d-spacings.
Table 4.1- TED Data on Crystallized A203
JQPDS 10-425 (y-AI 031
TEM Samp1p,
4.560
2.800
2.390
2.280
4.655
2.8848
2.4383
2.3013
1.977
1.520
1.395
1.140
1.027
0.989
2.0080
1.5400
1.4126
1.2265
1.1605
1.0040
0.884
0.9023
0.806
0.8360
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
81
7000
6000
5000
Cn
:t
0
D
6
M
911)
4Q,
4000
3000
0
2000
1000
0
20
30
40
50
60
70
80
20
Fig. 4.4.a)- X-ray diffraction patterns of as-deposited
and annealed A203 films.
90
100
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
82
350
300
250
C
r.
D
-6
M
za)
200
150
r.
4_
100
50
0
20
30
40
50
60
70
80
90
20
Fig. 4.4.b)- Glancing angle x-ray diffraction patterns of a film that
was deposited with a substrate temperature of 400'C.
100
CHAPTER 4 A1203 AND Cr.-AI203 WAVEGUIDES
Fig. 4.5 a) Transmission electron micrograph and diffraction pattern of
an as-deposited film (a-AI203).
83
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
Fig. 45 b) Transmission electron micrograph and diffraction pattern of
a film annealed by the electron beam (y-AI203).
84
CHAPTER 4 A1203 AND CrAI203 WAVEGUIDES
4.2
85
Film Durability
As long as the substrates were sufficiently clean, it was not difficult to
achieve adherent, chemically-stable, hard, and wear-resistant amorphous
A1203 films. The films could be cleaned in an ultrasonic cleaner, or by wiping
with a solvent-wetted cloth, and they survived repeated tape tests without
damage. The films also proved to be durable when the edges had to be polished
in order to test end-coupling configurations for loss measurements.
This end-
polishing was accomplished by gluing two films face-to-face with a
cyanoacrylate adhesive such that they were sandwiched between their
substrates. This assemblage would then be mounted in epoxy and polished.
The films endured, without damage, both the adhesives and the various
solvents (acetone, methanol, toluene, tetrahydrofurane, MasterBond MB6)
that were required to release the polished films. Another indication of the
durability of the films was seen in prism-coupling. Glass and garnet prisms
were pressed sharply against the films, and they were not scratched or worn.
Thermal expansion mismatch problems were anticipated for the films
made at levated substrate temperatures.
Using Eq. 22, a corundum film (Ef
= 372 GN/M2 and af = 8.8xIO-6 C-1) (Kingery et. al., 1960) can sustain a
temperature difference of 170'C before its yield strength 0265 GN/M2)
(Kingery et. al., 1960) is overcome if it is deposited onto a fused silica substrate
(as =0.55xlo-6 oC-1). (Kingery et. al., 1960). The film may see 80'C from
heating due to sputtering alone, and it will be cooled to room temperature after
deposition. However, none of the as-deposited films in this study, including
those that were deposited at 400-500'C, experienced any cracking or crazing.
This suggests that the thermal expansion coefficient of the amorphous
A1203 films was closer to that of a-SiO2 than oc-AI203. However, annealed
films had flaws at ~400'C, and crystallized films were frosted.
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
4.3
Optical Properties
4.3.1
Index of Refraction
86
Within the processing window of the amorphous-A1203 (a-AI203) films,
the optical properties were found to be dependent on the processing conditions.
A summary of the optical properties is given in Table 42.
The index of the films averaged 165 (at 632.8 nm) and increased slightly
(1.64 to 166) with substrate temperature, but remained unaffected by
the sputtering gas, target-to-substrate distance and thickness.
maximum variation in index across any particular film was
02in
The
00017,
corresponding to ± .1' difference in coupling angle. Since this is roughly the
error in the apparatus, the variation in index must be negligible. The dispersion
curves of the single-mode films demonstrate that the refractive index increases
with substrate temperature, but is constant withO2flow rate throughout the
UV and visible (Fig. 4.6a). The same trends hold true for the multimode films
(Fig. 4.6b), and the absolute values for the refractive index correspond with
those in the single-mode films. The increase in data points for the thicker
multimode films results from the increased number of extrema in their
transmission curves. The films deposited at 65 and 8.5%02in the sputtering
gas were made at a different target-to-substrate distance 12 cm instead of
16cm) than the others in Fig. 4.6b, yet the index does not seem to be affected.
The average index of the aA1203films is roughly 94% of the index of
crystalline
A1203,
as shown in Fig. 47. Although a reduced index sometimes
indicates porosity, this index reduction is the same as that between fused and
crystallineSiO2, and is therefore attributed to the amorphous nature of the
films. Anealed
films had the same indices as the as-deposited films until the
films were crystallized. Upon crystallization the indices became similar to
those of bulk A1203 1.768 and 1760 for oc-AI203).(Lide, 199 1)
CHAPTER 4 A1203 AND Cr.-A1203 WAVEGUIDES
87
Table 4.2- Optical Properties of A1203 Thin Film Waveguides
No.
%02
Flow
Rate
Substrate
Temperature
(0c)
SinLyle Mode Films
SM1
4
SM2
5
SM3
5
SM4
5
SM5
10
T-S Dist# Thickness Refractive
Losses
(cm)
(A)
Index
(dB/cm)
at 632.8 nm at 632.8nm
20
20
250
400
20
12
12
12
12
12
4663
4604
4057
3945
4153
1.637
1.634
1.649
1.66
1.637
1.4
-
SM6
10
250
12
3993
1.648
SM7
I
500
12
3870
1.66
-
20
400
20
400
400
20
20
20
20
16
16
16
16
16
12
12
12
12
12
12
12
12
9017
7750
7125
5650
7550
8868
10904
11363
10715
13229
13229
10185
10185
1.644
1.658
1.645
1.664
1.666
1.644
1.645
1.637
1.637
1.77
1.75
1.647
1.650
4.8
1.5
2.5
2.5
2.3
12
12
12
12
12
12
12
12
8868
14755
14113
13050
10715
13620
13975
13325
1.644
1.644
1.645
1.650
1.645
1.642
1.646
1.652
Multimode Films
mmi
MM2
MM3
MM4
5
5
10
10
MM5*
MM6
MM7
MM8
5
5
6.5
8.5
MM9
10
MM10A
10
MM10E
10
MM11A
MM11B
5
5
Cr Doj2ed (RT)*
CD1
CD2
CD3
CD4 CD5
CD6
CD7
CD8
5
5
5
5
20(1200'C anneal)
20(1000'C anneal)
20 (650'C anneal)
20 (400'C anneal)
2Lr0
< 0.1
0.3
1.0
6
13
51
3
-
2-
15 - 20
30 - 45
9 - 13
10
0
< 0.1
10
1 - 16
30 - 45
10
0.3
10
1.0
60 - 96
# Target to Substrate Distance. Made with unbalanced magnetron. -- Too high to measure.
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
-1 r7
-L.
88
-1
1.7
1.69
90
1.68
-4--)
U
z
a)
P4
1.67
4-4
0
x
a)
0
1.66
1.65
1.64
1X. 0VO0
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 4.6a)- Dispersion in -350OA-thick A1203 films as a function of
substrate temperature and %02. ( - ,
- 10%02)
650
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
1.71
T
I
I
I
f
I
I
89
I I
I
1.7
1..69
0
1.68
a)
1.67
0
.-j1-4
U
M
;q
I+--4
P4
IL
4--4
0
x
a)
llc
0
C3
0
N
%%
400
1.66
C
1.65
FQ-.
1.64
1 -I. lo O
RT
- --- W-. .
I
3 00
I
I I
I
I
I I
I
I I
400
I I
I
I I
I
I
I I
I
I I
I
I I
500
I
I I
I I
I
I
600
I I
I
I I
700
Wavelength (nm)
Fig. 4.6b)- Dispersion in -14m-thick A1203 films as a function of substrate
temperature and %02. (o -5, Li- 65,
- 8.5, - 10 02)
CHAPTER, 4 A1203 AND CrAI203 WAVEGUIDES
90
crystalline
Al2 0 3
1.79
e
1.75
0
.- 0.-_I
w
M
I*-4
w
1.71
W
A
4-1
0
X
a)
amorphous
films
1.67
-EB
A
0
-EB
1.63
I
300
I
I
I I I
350
I I
I I I
400
1 1
450
I
I
I
I
I
500
I
I
I
in I
550
Wavelength (nm)
Fig. 4.7- The index of the amorphous A203 is 94% that of the
crystalline A203 phase. (*Lide, 1991)
I I
I I
600
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
4.3.2
91
Attenuation
The A203
films had wide transmission windows (200nm to 9gm, Fig 48).
The waveguide losses decreased with increasing substrate temperature,
decreasing target-to-substrate distance, and decreasing 02 flow rate. Due to
substrate coupling complications, the only single mode film that had
measurable losses was the one made under the best conditions (high substrate
temperature,
lw
02
flow rate), and therefore, probably the lowest losses.
These losses were similar to those of the equivalent multi-mode film (high T,
low
02). With the other single-mode films, it was too difficult to distinguish
the output signal from the substrate signal because of the end-viewing
geometry, Fig. 3.4b.
Increased substrate temperatures led to decreased losses due to
densification and defect annealing. The higher density of these films also
explained their increased refractive index. Similarly, the effect of target-tosubstrate distance was probably due to increased densities because the
deposited atoms have higher energies with decreasing distance.
An increase of the losses with increased 02 flowrate indicated that there
could be some stoichiometric effects. Although Logan 1990) and Hollenbeck
(1990) report stoichiometric A1203 films, Vuoristo 1991) reported that the
stoichiometry of A1203 films deposited by various techniques could differ quite
substantially.
Within the limits Al/O= 0667 ± 0022) of Rutherford
backscattering spectroscopy (RBS), it was determined that the stoichiometry
of our films did not vary with the oxygen content of the sputtering gas (Fig.4.9).
The peak in the data, due to an overlap in the Si and Al spectra, was thickness
dependent. It can be used to distinguish the samples, since their thicknesses
are slightly different, but their spectra are otherwise indistinguishable.
92
CHAPTER 4 A1203 AND Cr.AI203 WAVEGUIDES
I
-1
0.9
9
0
0.8
co
(n
co
zCZ
;-4
E--i
Z
0.7
0.6
n r
%J.
"
200
600
1000
1400
1800
Wavelength (nm)
Fig. 4.8a)- LTV/visibletransmission spectra of a Itm amorphous A1203
film and a mm thick sapphire crystal.
(Note: the oscillations are interference fringes.)
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
93
100
80
I-co
-j
0
D
,6
60
r.
0
co
M
40
0
0
E---,
20
rl
U
2
4
6
8
10
12
Wavelength (gm)
Fig. 4.8b)- Infrared transmission spectra of a 1tm amorphous A203
film and a I mm thick sapphire crystal.
(Note: the oscillations are interference fringes.)
14
94
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
25
20
lc
a)
15
4
lc
a)
N
I
z0
10
5
U
0.2
0.4
0.6
0.8
Energy (MeV)
Fig. 4.9)- RBS data of films deposited at various %02.
1
1.2
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
95
Increased oxygen content in the sputtering gas may lead to increased
negative ion bombardment of the film during deposition, thereby causing higher
levels of defects in the films. Since substrate bombardment was suspected of
causing higher losses, a film was made with an unbalanced magnetron. Such a
film was more likely to have been bombarded during deposition since the flux
lines would extend further away from the target than in the case of a balanced
magnetron. This film did indeed have slightly higher losses (Table 4 1).
Subsequent anneals above 400'C produced visible flaws in the films, but
little change to the optical properties (Table 41). The index represents an
average index, which may include a densified film together with low-index flaws.
The losses of the 400'C-annealed film did not change from the as-deposited
film, perhaps again due to an averaging effect. However, the 600'C anneal
produced sufficient flaws that the losses were too high to measure.
The refractive index and loss results found here can be used to explain
inconsistencies in reported optical properties. All of the films made by
evaporation techniques had low refractive indices 1.56-1.63)and high optical
losses 10-20 dB/cm). (Rainer, 1985; McNally, 1986) These films probably had
varying amounts of porosity throughout an amorphous A203 matrix. Films
deposited by sputtering had indices of 1.57 to 170 with losses from
to >20
dB/cm. Of these films, the lower index films also had higher losses and were
again likely to contain porosity. Films with indices of 1.70 were probably
partially crystalline, although no electron microscopy or diffraction data were
reported for these films. Smit 1986)determined that the optimum annealing
temperature for reduction of losses is 800'C. Higher-temperature anneals
produced very lossy films. This can be explained by our findings that the films
are amorphous below 8000C, above which they crystallize. Anneals below
CHAPTER 4 A1203 AND CrA1203 WAVEGUIDES
96
8000C must have partially reduced film defects (there was no reported effect
on the refractive index), and yet not produced the visible flaws that were seen
after our annealed films. Along with using different substrates (oxidized Si
wafers), the anneals of Smit (30-min) were shorter than ours 2 hours).
4.3.2
Fluorescence
Films with
Al targets.
1%Cr were fabricated by reactive co-sputtering of Cr and
Cr content was analyzed by EDS, but the lower limit for this
technique was 09%. Optical absorption was used to determine the Cr content
of the other films, using the equation:
I/Io = exp(-F-cx)
where, I is the beam intensity,
Eq 42
is extinction coefficient, and c is the
concentration of Cr.
None of the as-deposited films tested for fluorescence showed any
response, but
Cr3+
was not necessarily the only valence state present. In
fact, the films exhibited some LTVabsorption which is attributed to Cr6+ in the
literature
Therefore, the films were annealed at various temperatures to
control the valence state of the Cr. The Cr-O phase diagram (Fig. 4 10)
indicates that
Cr3+
is stable above 600'C. However, fluorescence was
observed only when the annealing temperatures were sufficient to crystallize
the films.. This happened at 750'C for Cr-doped films. Films with the defect
spinel structure (y-AI203) were observed to fluoresce red, but the fluorescence
was such that it was not substantially above the noise level of the Ge detector
(Fig.4.11). Films with the corundum structure (x-Al2O3)exhibited the sharp
fluorescenceof ruby with a peak at 694 nni and a shoulder at 693 nm.
CHAPTER 4 A1203 AND CrAI203 WAVEGUIDES
97
1000
Boo
1q,
5
- 00
I
i
0(
200
0
200
T-P.
400
M.)
600
Fig. 4.10- The Cr-O Phase Diagram. (Levin, 1975)
1
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
2
3
I
0.8
0.6
w
:t
0
D
6
0.4
0.2
0
-0.2
6200
-\,-/-"
1
1
1
1
6400
1
1
1
1
1
1
6600
Q
1
_Y-Al
1
6800
1
1
7000
Wavelength (m)
Fig. 4 1 1- Fluorescence in CrAI203 films crystallized to
y-AI203 and x- A1203.
1
7200
7400
CHAPTER4: A1203 AND CrAI203 WAVEGUIDES
4.4.
98
Conclusion
Amorphous
was found to have promising optical and mechanical
A1203
properties that will be suitable for planar waveguide devices. Sputtered
A1203
films were easily fabricated in amorphous form. The amorphous structure was
verified by XRD, transmission electron diffraction, HRTEM, and fluorescence
measurements. These
A1203
films have transmission windows that extend
from 200nm to 9gm, with an average refractive index of 165 and optical losses
as low as -1 dB/cm at 632.8 nni. They also were hard, chemically-stable,
wear-resistant, and adherent. The optical properties of amorphous A1203 can
be optimized by varying the processing conditions. The refractive index of the
films was 94% of the crystalline value (the same difference occurs between
fused silica and quartz) due to the amorphous nature of the films. With
increasing substrate temperature, the index was found to increase and the
losses to decrease; this is attributed to densification and defect annealing.
With increasing 02 in the sputtering gas, the index remained constant, but the
losses increased probably due to defects from negative-ion bombardment of the
films. The results of this study demonstrate the feasibility of producing thin
film waveguides with good optical properties from oxides which are not
traditional glass formers.
Chapter
Y203WAVEGUIDES
This chapter discusses Y203 thin films that were fabricated by rf
magnetron sputtering. Various processing conditions (%02 in the sputtering
gas, substrate temperature, and rf power) were tested in an attempt to
achieve films with low optical losses. The microstructure and durability of the
films are discussed first. Then, the optical properties are reported. This
chapter aso serves to provide information on the end-member of the Y203A1203 co-sputtered films. The knowledge gained here was useful in determining
the process variables for the films in the next chapter.
99
CHAPTER 5. Y203 WAVEGUIDES
5.1
100
Phase Determination
The room-temperature processing window of transparent
(-3500A) ranged from
-6
02
Y203 films
(Fig. 5.1). These transparent films were
crystalline, but they had mixed crystallinity at
502
bixbyite is the only reported room temperature phase
(Fig. 52).
Of Y203,
Although
it was possible
that the films were composed of the high temperature hexagonal phase
because sputtering is a non-equilibrium process.
From conventional 020 x-ray diffraction (XRD),it was unclear whether
the films were comprised of bixbyite (cubic) or the high-temperature hexagonal
phase (Fig. 52). It was clear that the films were textured, however, with the
close-packed oxygen planes parallel to the substrate: (111) for the cubic phase
and (001) for the hexagonal phase. Glancing-angle XRD (GAXRD)was used to
observe the lattice planes that were not parallel to the surface (Fig. 5.3.a).
From these data it is clear that the films are made up of bixbyite. GAYRD was
also used to show that
Y203
deposited onto crystalline Si (100) also has the
bixbyite structure (Fig. 5.3.b). GAXRD allows the film to be probed without
sampling the substrate.
The XRD peaks were different for films that were deposited with
5
02
in the sputtering gas. One broad Y.RDpeak was observed at 30' (Fig. 52),
and GAXRD revealed two broad peaks (Fig. 54). The optical losses were high
in these films, which suggests density fluctuations. One likely microstructure
would be interspersed nanocrystallites in an amorphous matrix.
microstructure was verified in the
Y203-AI203
This
films in Chapter 6 by
HRTREM. The deposition rates slowly decreased with increasing
O2
until
the rates were prohibitively slow at 75%. This gradual oxidation of the target,
which was not observed with the Al target, could be a result of the type of oxide
CHAPTER 5: Y203 WAVEGUIDES
= n
Jav
A
101
%r
transparent Y202
500
A
defective
mixed crystallinity
cryst
450
M
CZ
400
a)
350
0
oxidized
target
0
PL,
0
300
X4
0
P4
250
0
200
150
0
2
4
6
8
10
12
%O 2
500
0
400
300
U
0
a)
M
II
200
E-cn
100
,n
Q
0
0
0
-100
0
0
0
0
I I I I I I I I I
I I I I I I I
1
3
2
4
i
A
I I I I I
5
6
7
%O2
Fig. 5.1- Processing windows OfY203 films a) deposited at room temperature
b) deposited with 30OW
8
CHAPTER,5: Y203 WAVEGUIDES
102
8000
6000
C
0
,6
r.
a)
M
4000
4-
2000
0
20
30
40
50
60
70
so
20
Fig. 5.2- XRD data of transparent Y203 thin films.
90
100
CHAPTER 5: Y203 WAVEGUIDES
103
14000
12000
10000
0
8000
,6
>-I
-4-
M
0
0
Q)
-4--)
6000
4000
2000
A
v
20
30
40
50
60
70
80
90
20
Fig. 5.3, Glancing-angle XRD data OfY203 thin film (bixbyite):
a) on fused-SiO2 and
b) on Si.
100
CHAPTER 5: Y203 WAVEGUIDES
104
3000
2500
2000
1500
1000
500
0
20
30
40
50
60
70
80
20
Fig. 5.4- GAYRD data of a Y203 film deposited at 5%02.
90
100
CHAPTER 5: Y203 WAVEGUIDES
layer that was formed
(A1203
105
forms a protective layer on Al). The oxidation of
the target, was faster at higher temperatures. At 400'C and 5% 02, the rates
were prohibitively slow.
Below 1.0 %
02
at room temperature and below 3 at 400'C, the films
became defective and dark. Oxygen deficiency increased with increased
substrate temperatures, presumably due to the increased rate of evaporation
of 0, which has the lower sticking coefficient of the two species. These films
had high absorption in the UV similar to the
IOW-02
were composed of the high temperature hexagonal
films of A1203, and they
Y203
phase (Fig. 5.5).
Many oxides experience a reduction in valence upon heating, so it corresponds
that the high-temperature phase would be encouraged by an oxygen deficiency.
Below 075%
02,
the films were textured (001) Y metal. (Fig. 56).
It was difficult to measure the optical losses in the single-mode
(-3500A) films that have been discussed thus far. Thicker (~5500A) films
could only be fabricated with lw
O2
in the sputtering gas. At higher
02,
the chimney of the gun would begin to flake, probably due to thermal mismatch
between the metal and the oxide that was deposited there. Flakes that fell into
the gap of the sputtering gun would cause arcing before a thick film could be
deposited. This flaking phenomenon became more dramatic at elevated
substrate temperatures because the temperature of the entire chamber was
elevated. Increased thicknesses led to bixbyite films that were not as textured
as the thin films. (Fig. 57) Peaks from lattice spacings other than the (111)
family were resolved by conventional XRD.
CHAPTER 5: Y203 WAVEGUIDES
106
12000
M
10000
:4
r.
D
-6
-4.
8000
6000
M
r.
a)
4000
2000
u
20
30
40
50
60
70
80
90
100
20
6000
5000
W
:t
z
4000
D
-6
3000
M
r.
a)
-4-1
2000
1
1000
0
20
30
40
50
60
70
80
20
Fig. 5.5- XRD data Of w-02 films(Hexagonal Y203):
a) 020 and b) glancing-angle.
90
100
CHAPTER 5: Y203 WAVEGUIDES
107
16000
14000
12000
C
:t
r.
10000
D
-6
8000
'95
00)
+j
r.
6000
4000
2000
0
20
30
40
50
60
70
20
Fig. 5.6- XRD data of metallic Y film.
80
90
100
CHAPTER 5: Y203 WAVEGUIDES
108
5000
4000
r.
3000
6
W
11)
4.
9
2000
1000
0
20
30
40
50
60
70
80
20
Fig. 5.7- XRD patterns of thick (-5500A') Y203 films.
90
100
CHAPTER 5.-Y203 WAVEGUIDES
109
In an attempt to make amorphous films, the power was varied from
20OW to 40OW (Fig. 5.8). At 20OW,the microstructure was the same as that
for films made at 300W. But, at
1% 02
a film deposited at 40OWwas less
crystalline than the film deposited at 30OW, probably due to the faster
deposition rates. However, the deposition rate of the metal appears to have
been too rapid to allow proper oxidation of the film, and the film was dark
(similar to the defective,
IOW-02 films).
When the oxygen content of the
sputtering gas was raised again, the structures of the 300- and 400-W films
were similar (Fig. 59). One film was made at a very high power (50OW) and
oxygen content (10%) to encourage an amorphous structure that would not be
oxygendeficient. However,this resulted in a structure with mixed crystallinity,
similar to the high-02 films that were made at 30OW(Fig. 5.10).
From this study it appears that
Y203
has a strong tendency to order, as
opposed to A1203 which was always amorphous as deposited. This tendency
could be due to the stacking preferences of Y and
1.02A and 138A, respectively.
which have similar sizes:
The size of Al is 039 A in tetrahedral
coordination and 054 A when in octahedral coordination. (Shannon and
Prewitt, 1969). Also, the bixbyite structure has an intrinsic disorder that may
make crystallization easier.
AnnealingY203 films produced very little change in the GAXRD data up
to 800'C (Fig. 5. 1). The peaks became a little sharper and shifted slightly,
indicating that some grain growth and lattice adjustments may have occurred.
Above 1000'C, grain growth began to occur along new crystallographic
directions. The 211) peaks became especially prominent.
CHAPTER 5.,Y203 WAVEGUIDES
110
---fovvv
5000
4000
I..""I
0
6
3000
4-,
co
0
Q)
+j
z
2000
1000
n
20
30
40
50
60
70
80
20
Fig. 5.8- XRD patterns OfY203 films deposited at 1% 02.
90
100
ill
CHAPTER 5: Y203 WAVEGUIDES
10000
8000
M
:t
Z
6000
D
"6
1-1
cn
a)
.4
r,
4000
2000
0
20
30
40
50
60
70
80
90
20
Fig. 5.9- XRD patterns OfY203 films deposited at 3
02.
100
CHAPTER 5: Y203 WAVEGUIDES
112
3000
2500
2000
c
:t
0
D
.6
1500
4-1
M
0
a)
-4-."
0
1000
500
0
20
30
40
50
60
70
80
90
20
Fig. 5.10- GAXRD pattern OfY203 film deposited at 50OW and 10% 02.
100
CHAPTER 5: Y203 WAVEGUIDES
'30000
,
25000
,
,
,
i
,
113
i
i
,
i
i
r-
1
,
r
-i
i
i
i
i
,
I
i
i
,
i
I
,
T
i
i
I - i
,
-T --T
-
,--I
,--I
cq
N
N
20000
z
D
..6
15000
m
z
W
4-j
0
O
z
't
I
10000
co
co
cq
't
co
M,-
cq
cq
1--f
CT
.c
t-
cq
M
t-
12000C
100011c
000c -
5000
Rnnor
-,."NI. i
1.1w
I I I I I
0 20
. . . -
30
40
50
60
70
-
80
400-C
,
I I .- .-
90
100
20
30000
r.
D
20000
.6
10000
w
0w
z
0
20
22
24
26
28
20
Fig. 5.11- GAXRD patterns of annealed Y203 films.
30
32
CHAPTER 5 Y203 WAVEGUIDES
5.2
114
Film Durability
The
Y203
films appeared to be as mechanically durable as the
A203
films. As long as substrates were sufficiently clean, it was not difficult to
achieve adherent, hard, and wear-resistant
films. Again, the films could be
cleaned in an ultrasonic cleaner, or by wiping with a solvent-wetted cloth. And,
they survived repeated tape tests and prism coupling without damage.
However, the
Y203
films were not as chemically resistant as the A203
films.
Whereas the A1203 films had to be etched with sulfuric acid at an elevated
temperature (~200'C) for several minutes, the Y203 films could be etched in
room temperature sulfuric acid in several seconds. Also, most other common
acids (HN03, HCI) could be used to etch the Y203 films.
Thermal expansion mismatch was anticipated.
None of the as-
deposited. films in this study experienced visible flaws. Films that were
annealed did not show any visible flaws until 1200'C, at which point they
became frosted in appearance. Anneals up to 800'C actually lowered the
optical losses, which indicates that defects were annealed. However, the
optical losses increased greatly with an 1000'C anneal.
5.3
Optical Properties
The index of refraction of these Y203 films was seen to decrease with
increasing 02 (Fig. 512). This correlates with the XRD data that shows a
decrease in crystallinity with increasing %02. The index of all of the films was
lower than the index of bulk Y203, indicating the presence of porosity. Using
equation 29, the volume fraction of solid (or the packing density) of the films
CHAPTER 5: Y203
AVEGUIDES
115
2.01
1% 02
L
1.96
=
3% 0
.2
C.)
CZ
C)
4
11,
1. N .
0
xa)
IC
I-4
r.
Nl-D
1.91
1.86
0.3
I
I I
I
0.35
I
I I I
0.4
I I
I I
I
0.45
Wavelength
I
I I I I
0.5
(tm)
Fig. 512 - Dispersion curves OfY203 thin films.
I I
0.55
I I I
0.6
CHAPTER 5: Y203 WAVEGUIDES
116
can be calculated. Assuming that the index of bixbyite is 191 (Fig. 218) and
the index of the pores is 1.0 (air), the packing density of the higher index films is
95
.
The transmission window of the
Y203 film
i
shown in Fig. 513
absorption from electronic transitions becomes significant at
UV
250 nm. The
IR absorption does not have as sharp a cutoff as was seen in the A203 films.
Instead, the transmission gradually begins to decrease, starting at
5gm.
The oscillations at shorter IR wavelengths and in the visible are interference
ftinges.
The relative losses were evaluated qualitatively during prism coupling
by observing the streak of scattered light out of the surface of the film. The
losses increased with
02, which could indicate that density fluctuations are
causing increased Rayleigh scattering. The microstructure is most likely
composed of nanocrystallites in an amorphous matrix; a similar
microstructure was observed in the Y203-AI203 films by HRTEM. The only
films that had measureable losses were multi-mode (TE1) films made at 1 02
(the lowest
02). Their losses, measured to be ~50 dB/cm, were close to the
limit of this technique. Subsequent anneals at 600'C lowered the losses to
2OdB/cm. The 3-02
films did not appear to have much higher attenuations,
but the films made with
5% were very lossy. Substrate coupling made loss
measurements on the single-mode films very difficult via end coupling because
it was difficult to isolate the output signal of the waveguide.
CHAPTER 5.-Y203 WAVEGUIDES
117
1
0.8
00
0.6
W
-2
E
co
0
z
E
0.4
0.2
0
200
650
1100
Wavelength
1550
m)
Fig. 5.13a) LTV-VIStransmission OfY203 thin films.
(Note: oscillations are due to interference fringes)
2000
CHAPTER 5: Y203 WAVEGUIDES
118
II
0.9
0.8
z
.2M
n
E
M
0.7
I"I
E..4
-12§1
0.6
0.5
V.
A2
2
4
6
8
10
Wavelength
12
14
16
(Im)
Fig. 5.13b) Infrared transmission OfY203 thin films.
(Note: Short wavelength oscillations due to interference fringes)
18
20
CHAPTER 5: Y203 WAVEGUIDES
5.4
119
Conclusion
Despite extensive attempts, processing conditions were not found for the
fabrication Of w-10ss Y203 thin films. The single-mode films that had the
lowest losses were textured bixbyite films with the close-packed oxygen plane
parallel to the substrate. Multi-mode films were less textured. For both single-
and multi-mode films, the losses increased with
02 of the sputtering gas.
This was due to a decrease in crystallinity, resulting in density fluctuations
throughout the material. Correspondingly the index of refraction decreased
and optical losses increased with %02. Subsequent anneals up to 800'C
lowered the losses of the films. Anneals at temperatures
1000'C caused
grain growth which led to increased losses from grain boundaries and
microcracks. Although the losses at 632.8nm were high, the Y203 films had IR
absorption bands very far into the infrared
15gm), which renders them
desirable -forapplications as laser amplifier hosts. This would especially be
true for NIR and IR applications because the losses from Rayleigh scattering
will be lower at longer wavelengths.
Chapter 6
Y203-AI203 WAVEGUIDES AND AMPLIFIERS
From Chapter 4 and Chapter 5, the processing window of co-deposited
Y203-A1203
waveguides was estimated, and in this chapter it is verified.
Glancing angle x-ray diffraction (GAXRD)was again used to examine the
microstructures of these waveguides, along with high resolution TEM
(HRTEM) and transmission electron diffraction (TED). The optical properties
of co-deposited Y203-AI203
waveguides were determined, including index of
refraction, dispersion, overall transmission windows, and optical losses. The
microstructures and optical properties of Nd doped films are reported in the
final section of this chapter.
120
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
6.1
121
Phase Determination
A summary of the films made by co-deposition of Y and Al are shown in
Fig. 6.1. The compositions were determined by energy dispersive spectroscopy
(EDS) in a Camscan SEM (Appendix C). Films of various compositions were
fabricated at 4
and 6
02
by varying the power to the Y and Al guns as
follows: 300:100, 300:200, 300:300, 200:300, 100:300 (Watts to Y: Watts to
Al). The compositions of the films was relatively independent of % 02, except
in the case of the films made with 100W Y: 30OWAl. In the latter, deposition
at and 6
02
yielded significantlylower amounts Of Y203 than deposition at
4% 02. Recall in the study of the pure Y203 films that the rate of deposition
decreased with increasing
02
in the sputtering gas. In these co-deposition
cases, the power to the Y gun was sufficient to sputter at a consistent rate for
all oxygen contents, except in the case of the lowest power (100W).
In the 5-02
film made with high Y content, a large Si (substrate) peak
appears in the EDS spectra. This film was thinner
2000 A) than the others
(-6500,k) because the chimney of the Y gun began to flake. When a flake falls
onto the target, it causes shorting, and the deposition must be halted. As
mentioned in the last chapter, this was also a barrier to making multiniode
films of Y, especially at high oxygen contents or elevated temperatures.
made with the same Y content at 6
02
A film
was too thin to support a TEOmode.
CHAPTER 6: Y203-AI203 WAVEGUIDES AND AMPLIFIERS
10
.
.
.
.
.
122
I
I .
I
amorphous
mixed crystallinity
8
I
amorphous
mixed
xtal
0
6
0
0
A
0 0
A
Cq
0
0
e_
4
0
0
U
0
0
A
defective
defective
A1203
0
bixbyite
2
7-1
Y
metal
L'.
I
0
Al
defective
metal
n
2-3
I
I
I
20
I
I
.
.
.
40
.
.
.
60
Y4AI209
.
.
.
.
80
Y3AI5012
atomic
Fig. 6.1- Summary OfY203-AI203 co-deposited films with end member
results shown schematically.
100
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
123
Table 6.1- Optical Properties OfY203-AI203 Waveguides and Amplifiers
Sample No.
Y203
%A1203
% 02
Flow
Thickness
(A)
Rate
Refractive
Index
Losses
(dB/cm)
at 632.8nm
at 632.8 nm
Undoped Films
YA31
~50
-50
3
-9000
YA41
YA42
YA43
84.4
65.8
53.2
~50
43.3
23.4
15.6
34.2
46.9
4
4
4
4 (4000C)
4
4
7222
7827
13.8
35.4
52.0
5
5
-50
5 (4000C)
YA55*
86.2
64.6
48.0
-50
41.8
5.0
58.2
95.0
5
5
2942
7255
9183
7868
6970
6300
YA61
YA62
YA63
YA64
YA65*
97.0
64.6
55.0
41.3
4.5
3.0
35.4
45.0
84.7
95.5
6
6
6
6
3.1
12.4
14.2
45.7
68.0
84.2
86.2
6
YA43a
YA44
YA45
YA51
YA52
YA53
YA53a
YA54
-50
56.7
76.6
5
6
6600
7543
6412
6142
2072
7684
7353
7276
5706
Nd-Doped Films*
(Y/Nd)
2.6/ 94.3
YAN1
9.1/ 78.6
YAN2
14.6/ 71.2
YAN3
8.8/ 45.5
YAN4
13.6/ 18.39
YAN5
11.0 48
YAN6
YAN7
13.8/ 0
* Index by envelope method.
5
5
5
5
6
5
3930
3240
2631
2917
5752
5632
8137
dark
1.849
1.804
1.772
18
20.3
53
dark
1.745
1.679
18.2
15.3
1.844
1.791
1.788
9.3
9.5
50
dark
1.732
1.665
5
15
thin
1.801
1.781
1.727
1.66
15.8
13
1.0
15
CHAPTER 6: Y203-AI203 WAVEGUIDES AND AMPLIFIERS
124
The conventional 0-20) X-ray diffraction patterns indicated that all of
the films were amorphous (Fig. 62). However, when the samples were
analyzed by glancing angle x-ray diffraction (GAXRD) a very broad peak was
resolved at 30' for films with
40
Y203
(Fig. 63). This is roughly the
composition of Y3AI5012 (YAG). The films in Fig. 63 were deposited at 5% 02,
except the 23.4%
Y203
film which was deposited at 4
02.
This film was
scanned at a slightly larger glancing angle, so it does not fit conveniently on the
scale of the other films.
The microstructure of the films did not appear to vary significantly with
%
2
(Fig. 64).
In both amorphous and mixed crystallinity cases, the
structures of the films deposited onto crystalline Si (Fig. 65) were similar to
those of te films deposited onto the fused-SiO2 (Fig. 63).
Oxygen contents of 6
At less than 4
and
Y203
02,
02
were used to avoid oxidation of the Y target.
the films were defective (dark), similar to the
IOW-02 A1203
films. Two films were made with elevated substrate temperatures
(400'C): one at 4
02
and one at 5% 02. These films were also defective. This
increase in oxygen defects with substrate temperature is again attributed to
an increased rate of evaporation of the
adatoms which would result in lower
0 contents in the films. Again, there was a problem with the chimney of the Y
gun flaking which made it difficult to make thick films at elevated temperature.
And since these initial high-temperature films were very lossy, no other films
were fabricated at elevated temperatures.
Subsequent anneals at
temperatures:! 8000C did not alter the diffraction patterns of the films.
Transmission electron microscopy (TEM) and diffraction (TED) were
used to verify the GAXRD microstructures. The film composed of -5%
Y203
was verified to be amorphous (Fig. 66.). A concentrated electron beam was
used to in an attempt to devitrify the film (Fig. 67). The film crystallized into
125
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
2000
1750
1500
C
-4-j
1250
Z
6
1000
4-4
W
z
4V
0
-4
750
500
250
0
20
30
40
50
60
70
80
20
Fig. 6.2- Typical XRD pattern OfY203-AI203 film.
90
100
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
126
2500
2000
-(;i
1500
0
D
-6
1000
W
Q)
r.
-4-;'
0
500
0
20
30
40
50
60
70
80
90
100
20
3000
Z;
2500
0
D
.6
2000
1500
:t4
zQ)
4
9
1000
500
0
20
30
40
50
60
70
80
90
20
Fig. 6.3- GAYRD pattern OfY203-AI203 films at various compositions.
All films were deposited at 5% 02, except the one marked 4%02.
100
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
127
---
louu
1000
r.
D
,6
U)
%.1
a)
0
500
0
20
30
40
50
60
70
80
90
20
Fig. 6.4- GAY.RDpattern OfY203-AI203
50-50) films at varying % 02.
100
CHAPTER 6: Y203-A1203 WAVEGUIDES AND AMPLIFIERS
128
1000
800
M
:t
z
600
D
-6
4i
W
00)
-4-1
z
400
200
0
20
30
40
50
60
70
80
90
20
Fig. 6.5- GAXRD pattern OfY203-AI203 films deposited onto Si (100).
100
CHAPTER 6: Y203-AI203 WAVEGUIDES AND AMPLIFIERS
129
7-AI203 (Table 62) a phenomenon similar to the accidental annealing
experienced by the
Y203
A203
films in the TEM. A film that was composed of 45%
was verified to have mixed crystallinity (Fig. 68). Although no large
crystallites were found, there appeared to be an amorphous matrix with
branch-like structures
5-9 A wide) that were made up of evenly spaced (-3.4
A) lattice planes. The diffraction pattern contained rings that were only
slightly sharper than the amorphous pattern. This film did not crystallize
further, even upon subjection to a concentrated beam for 20 min.
Table 6.2- TED Data on Crystallized Y203-A1203
JQPDS 10-425 (y-Al 031
2.390
2.280
1.977
1.520
1.395
1.140
1.027
0.989
0.884
0.806
6.2
TEM Sample
2.410
2.251
1.989
1.463
1.393
1.151
1.125
0.985
0.879
0.794
Film Durability
The
Y203-AI203
films appeared to be as durable as the
Y203
films.
Again, it was not difficult to achieve adherent, hard, and wear-resistant films.
And, the films could be cleaned in an ultrasonic cleaner, or by wiping with a
solvent-wetted cloth. They survived repeated tape tests and prism coupling
withoutdamage.TheY203-AI203 films could also be etched in several seconds
by sulfuric acid, and most other commonacids, at room temperature. Thermal
expansion mismatch problems were again anticipated, but none of the asdeposited films in this study experienced any visible flaws. An 800'C anneal
did not produce any visible flaws, but the films became very lossy, possibly due
to thermally-induced microcracks.
CHAPTER 6 Y2(3-A1203 WAVEGUIDES AND AMPLIFIERS
Fig. 6.6- Transmission electron micrograph and diffraction pattern of
Y203-AI203 films with ~5% Y203.
130
CHAPTER 6 Y2(3-A1203 WAVEGUIDES AND AMPLIFIERS
Fig. 6.7- Transmission electron micrograph and diffraction pattern of
e-beam devitrified Y203-AI203 films with -5% Y203.
131
CHAPTER 6 Y2(3-AI203 WAVEGUIDES AND AMPLIFIERS
Fig. 6.8- Transmission electron micrograph and diffraction pattern of
Y203-AI203 films with 45% Y203.
132
CHAPTER 6: Y203-AI203 WAVEGUIDES AND AMPLIFIERS
6.3
Optical Properties
6.3.1
Index of Refraction
133
The measured indices of refraction of theY203-A1203films are shown in
Table 6 1. The index decreased dramatically with increasing amounts of A1203.
This was expected since the index of pure A1203 is much lower than that of
pureY203- When the index decreased sufficiently the films became singlemode. The indices of the films with the highest amounts of A1203could not be
measured with the rutile prism because their indices were too low, but a highindex glass prism was sufficient. The index was not dependent
on%02.
This is
different from the case of the pureY203films, but the microstructures of these
co-deposited films did not vary with%02.
The indices of the films with only 5%Y203were higher than the indices
of the pure A1203films. However, the indices of films with up to 15% A1203
were still comparable to that of the pureY203films.
The combined sputtering
power to the Y and Al targets was always more than the standard 30OWused
in fabricating the end-member films. Therefore, it was expected that the codeposited films would have higher indices than a simple average of the indices
of the pure A1203 and pure Y203 films. It has already been shown that an
increase in power results in a more dense film. Only the films with the highest
Y contents had indices that were higher than the bulk index 1.840) of YAG
(yttrium aluminum garnet).
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
134
1.95
1.9
1.85
$--i
.2
Q
CZ
14i
w
P4
t-
1.8
0
x
Q)
,I=
z
*--4
1.75
1.7
1.65
0.3
0.35
0.4
0.45
0.5
0.55
0.6
Wavelength (tm)
Fig. 6.9- Dispersion curves OfY203-AI203 films.
0.65
0.7
CHAPTER 6 Y203-A1203 WAVEGUIDES AND AMPLIFIERS
6.3.2
135
Attenuation
The transmission windows of theY203-A1203films are shown in Fig.
6.10. The UV absorption edge shifted from the UV absorption of pure
A203
(-200nm) to the absorption of pureY203 (-250nm) with increasing amounts of
Y203.
Similar shifts were seen in the IR absorption bands.
In general, these films had waveguide losses of 10-20 dB/cm (Table 61).
However, the films with compositions similar to YAG that were fabricated with
> 5%02had
much lower losses (1-5 dB/cm). The breadth of the GAXRD peak
indicated that the crystallites in these films were smaller than in the other
films. This was also the region in which the TEM sample of mixed crystallinity
was fabricated, and the micrograph (Fig. 68) shows that the crystallites are
very small. Therefore, Rayleigh scattering in these samples may have been
much lower than in samples with larger crystallites. Two samples at 4 and %
02
(~5Oy2O3)were
anomalously lossy. These films were the firstY203-
A1203films to be made, and the targets may have had contaminated surface
layers from being exposed to air.
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
136
I
0.8
0.6
.2In
.4
0
ct
EZ:
0.4
0.2
n
V
200
600
1000
1400
Wavelength (nm)
Fig. 6. 10a) UV-VIS transmission OfY203-AI203 thin films.
(Note: oscillations are due to interference fringes)
1800
CHAPTER 6 Y203-A1203 WAVEGUIDES AND AMPLIFIERS
137
I
0.9
0.8
0.7
z0
W
.4
F.
U)
0
0.6
M
EI
e
0.5
0.4
0.3
n 9
2
4
6
8
10
12
Wavelength (tm)
Fig. 6.10b) IR transmission OfY203-AI203 thin films.
(Note: short wavelength oscillation is due to interference fringes)
14
CHAPTER 6 Y203-A1203 WAVEGUIDES AND AMPLIFIERS
6.4
138
Nd-Doping
A summary of the films doped with Nd is shown in Fig. 6 11 and in Table
6.1. The compositions were again determined by EDS and microstructures
were analyzed via GAXRD. The trends in the microstructure with respect to
Y203
content was the same as those for the undoped films. The film with the
highest
Y203
content 93.3%) was textured bixbyite (Fig. 612). Films with
intermediate Y203 contents had mixed crystallinity, and the remaining films <
5% Y203)
were amorphous. The edges of the transmission windows showed
similar compositional shifts as the undoped films.
The fluorescence of the Nd in these glassy and mixed crystalline hosts
was quite broad (Fig. 618). The sharp peaks and dips seen in the spectra are
characteristic of the measurement apparatus.
This fluorescence may be
useful for a tunable source, but it is too broad for many applications. However,
the host material has infrared transmission far into the infrared which makes
this system an attractive subject for future research.
Nd 20 3
Y'O'
Z
20
40
60
80
Fig. 6.11- CmPOsitions of Nd-doped Y203-AI203 films.
Al - -
-- 2
3
CHAPTER 6 Y203-A1203 WAVEGUIDES AND AMPLIFIERS
139
15000
10000
Z
D
6
M
a)
r.
4-z
93.3%
5000
y203
79%
71%
45.5%
18%
5%
0
u /O
20
30
40
50
60
70
80
20
Fig. 6.12- GAXRD of Nd-doped Y203-AI203 film
with increasing Y203 content.
90
100
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
140
1
0.8
0
.2
M
0.6
M
Cn
0
z
E-1
Z:-
0.4
0.2
0
200
600
1000
1400
1800
Wavelength (nm)
Fig. 6.13a)
TV-VIStransmission of Nd-doped Y203-AI203 thin films.
(Note: oscillations are due to interference fi-inges)
CHAPTER 6.-Y203-AI203 WAVEGUIDES AND AMPLIFIERS
141
1
0.9
0.8
0.7
r.
0
W
-2
E
W
r.
M
0.6
E
e0.5
0.4
0.3
A
V.
61
2
4
6
8
Wavelength
10
12
(tm)
Fig. 6.13a) IR transmission of Nd-doped Y203-A1203 thin films.
(Note: short wavelength oscillation is due to interference fringes)
14
142
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
0.80
nV.t 0 -
I
0.60-
n
t
e:.
0.50-
I
I0
i
V
Ii
i
11!
I
. rv
S
I
f
\
,
,
"
'A 1;
A!,
1
i
i
i
t:
N
;I'I
0.40-1
1
,
i
II
Y'l
I
,
i
I V
0.30-:
I
i
i
i
0.20-I
I
0.10-
i
I
I
0.000.80
i A '[
'tvv
!I
I
V.J:
I
;
/ ii
i,
v
N.1
- IL
0.90
1.00
1.20
1.10
. 1.
1.30
W-valelowth
UM)
Fig. 6.14- Fluorescence of Nd-doped films.
1.40
I
I;
1.50
1.60
i
,
CHAPTER 6 Y203-AI203 WAVEGUIDES AND AMPLIFIERS
6.5
143
Conclusion
planar waveguides were fabricated by reactive co-
Y2(3-AI203
deposition of Y and Al targets.
amorphous with
Y203
These waveguides were verified to be
contents as high as 23.4% by XRD, GAY-RD, HRTEM,
and TED. Above 41.8% Y203, the waveguides had mixed crystallinity and
contained
branch-like structures (5-9A wide) dispersed throughout an
amorphous matrix. These microstructures appeared to be unaffected by the
%02
in the sputtering gas. The index of refraction was found to increase
strongly as a function of
Y203
content, but remain constant with
%02.
Although the waveguides had wide transmission windows, the edges were
observed to shift with compositiontoward the values of the endmember edges.
In general., the waveguides had optical losses of 10-20 dB/cm. However, films
with compositions roughly equivalent to that of YAG had lower losses (1-5
dB/cm).
Several waveguides were doped with Nd. The microstructural
dependence on Y203
content was the same as that of the undoped waveguides.
The transmission windows were also comparable. The fluorescence of Nd was
measured to be quite broad.
Chapter 7
SUMMARYAND SUGGESTIONS FOR
FUTURE RESEARCH
The relationships between optical properties and processing OfA1203,
Y203 and. Y203-AI203
thin film waveguides and amplifiers, fabricated by
reactive rf magnetron sputtering, have been examined. The purpose of the
research was to study the feasibility of producing waveguides and laser
amplifier hosts with good optical properties from oxide thin films which were
neither single crystals nor traditional glass formers.
The effects of substrate temperature and oxygen content of the
sputtering gas on the index of refraction and attenuation of amorphous A203
films were examined.
The index 164-1.66) increased as a function of
substrate temperature due to densification and annealing of defects, but was
relatively insensitive to the oxygen content of the sputtering gas. The losses,
however, ecreased with substrate temperature and increased with 02 in the
sputtering gas.
Losses of 13 dB/cm were obtained in films made with
144
CHAPTER 5: Y203 WAVEGUIDES
145
substrate temperatures of
400'C and with 5% 02. Losses as low as a few
dB/cm were attained at room temperature with the same oxygen content.
These are some of the lowest losses that are reported for as-deposited A203
thin films.
Y203
thin films were also fabricated with varying oxygencontents in the
sputtering gas. The films had mixed crystallinity when made with 5% 02, and
had a textured bixbyite structure with the close-packedplanes parallel to the
substrate at
402.
However, these films exhibited relatively high losses.
The films with the lowest losses (50 dB/cm) were fabricated with
1% 02,
and
subsequent anneals up to 600'C reduced these losses to 20 dB/cm. The losses
increased as a function of oxygen content due to increased fluctuations in
density. Despite variations in rf power, substrate temperature and thickness,
attempts to fabricate amorphous films, which would eliminate attenuation due
to grain boundaries, did not succeed. However, the transmission windows of
the Y203 films extended far into the infrared which makes them promising IR
laser amplifier hosts.
The optical properties of co-deposited
Y203-AI203
films were a strong
function of composition, but were independent of oxygen content in the
sputtering gas. The films had losses of 10-20 dB/cm, except the films that had
compositions similar to yttrium aluminum garnet (YAG,Y3AI5012).
films had losses as low as
dB/cm, which are the best value obtained so far for
Y203-AI2(3 films. Compositions of
and
These
23.4% Y203 yielded amorphous films,
4110/c,
Y203 films had mixed crystallinity. The edges of the transmission
windows of these films were dependent on composition, but in all cases they
were relatively transparent up to 10tm. Several A1203 and Y203-A1203 films
were doped with Nd. The fluorescence was very broad with a maximum at
CHAPTER
-1.06gm.
: Y203 WAVEGUIDES
146
These results demonstrate that it is feasible to produce thin film
waveguides with good optical properties from oxides which are not single
crystals or traditional glass formers.
This work was unique in its systematic approach to studying the
relationships between sputtering processing parameters, microstructure and
properties. We also note that this work was unique in studying a binary
system via co-deposition.
Several new questions arise out of this work. One question is: what are
the losses of these A1203,
Y203
and Y203-AI203
waveguides in the infrared?
This information would be useful for applications in the infrared, but it would
also provide information on the loss mechanisms within the films, especially if
these losses were measured at several wavelengths. Although we feel that
Rayleigh scattering is dominant in our films, there could be losses due to
interfacial scattering and absorption by contaminants or defects. Also, only
planar devices with no cladding were investigated in this work. A study of
waveguides with various claddings, beginning with SiO2, and confinement
geometries would prove useful in actual device application. Confinement can
be achieved with photolithographic techniques, where H2SO4
an etchant for these
Y203
i
suggested as
and Y203-AI203 waveguides
There are several other extensions to this work. Further study of films
with compositions similar to YAG may prove fruitful, since this work
demonstrated low optical losses in this region. Also, this system seems
particularly suitable for application to infrared coatings for which there is a
great demand. For applications as laser amplifiers, optimization of Nd-doping
is required.
The doped films in this study covered a variety of host
compositions, but all of the Nd concentrations were similar. A study of Nd
CHAPTER 5: Y203 WAVEGUIDES
concentration
147
of 001-20% Nd would be beneficial, followed by a study of
subsequent anneals. Other dopants, such as Er and Pr could also be tested
and optimized for use as laser amplifiers.
This work also suggests that other oxides, including other binary
systems, should be studied for optical device applications. Many materials
exhibiting properties of interest in bulk optics can be integrated with the
microelectronics community through similar studies of thin film processing.
Also, we found that some oxides that are not traditional glass formers can be
fabricated in amorphous form. A study of other oxide systems is required to be
able to establish trends that would allow future predictions of which oxides are
likely to be amorphous as-sputtered.
REFERENCES
Andrews, S.C., and S. Geller, Acta Crystallographa 11 437-441 1958).
Bendow, B., Ann. Rev.Mater. Sci. 7 23-53 1977).
Bezuidenhout, D.F., and R. Pretorius, Thin Solid Films 139 121-132 1986).
Butvina, L.N., E.M. Dianov, Y.G Kolesnikov, V.A. Prokashev, SPIE vol. 1228,
Infrared Fiber Optics 1, 155 1990).
Chou, T.C., T.G. Nieh, S.D. McAdams, G.M. Pharr, Scripta Metallurgica et
Materialia, 25 no. 10, 2203-2208 1991).
Dutta, S., H.E. Jackson, J.T. Boyd, Appl. Phys. Let., 37 no.6, 512-514 1990).
El-Mashri SM, A.J. Forty in Electron Microscopy and Analysis 1981, The
Institute of Physics, Bristol 1981).
El-Mashri, S.M., R.G. Jones, A.J. Forty, Philisophical Magazine A, 48 [51, 665683 1983).
Este, G. ad W.D. Westwood, Journal of Vacuum Science Technology A2 3)
1238-1247 (July-Sept 1984).
Foex and Traverse, Revue do Hautes Temp. Refract. 3 429-453 1966).
Frederick,, B.G., G. Apai, and T.N. Rhodin, Surface Science, 244 no. 12, 67-80
(Mar. 1, 1991).
Guenther, Karl H., Applied Optics, 23, 3612-3632 1984).
148
REFERENCES
149
Harris, M., H. A. Macleod, S. 0gura, E. Pelletier and B. Vidal, Thin Solid Films,
57, 173-178 1979).
Hollenbeck, J.L., and R.C. Buchanan, J. Mat. Res.,
no.5, 1058-1072 (1990).
JCPDS Centre for Diffraction Data, Powder Diffraction File, Swarthmore
(1988).
Kersten, R.T., H.R. Mahlein and W. Rauscher, Thin Solid Films, 28, 369-374
(1975).
Kingery, W.D., H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd
Edition. John Wiley Sons, New York 1960).
Klocek, Paul, MRS Bulletin, 41 (May/June 1986).
Koechner,W., Solid State Laser Engineering, 1988
Langevin, Y., E. Dooryhee, J. Borg, J-P. Duraud, C. Lecomte, E. Balanzat,
Applied Physics Letters 49 251 1699-1701 1986).
Levin, E.M. and H.F. McMurdie, Phase Diagrams for Ceramists, 1975
Supplement, Columbus: The American Ceramic Society, 132 1975).
Lide, D.R., CRC Handbook of Chemistry and Physics 72nd Ed, Boston: CRC
Press 1991).
Lines, M.E., Ann. Rev.Mater. Sci., 16 113-35 1986).
Logan, Joseph S., Thin Solid Films, 188, 307-321 1990).
Mahr, H., Physical Review, 125 [51 1510-1516 1962).
Manifacier,J.C. , J. Gasiot and J.P.Fillard, Journal of PhysicsE: Scientific
Instruments, 9 1002-1004 1976).
Marcuse, D., Light Transmission Optics, 2nd Ed. Van Nostrand Reinhold,
Princeton, NJ 1982), and Robert E. Krieger, MaLabar, Florida, 1989).
Marcuse, D., Theol:y of Dielectric Optical WavegUides, 2nd Ed. New York:
Academic Press, Inc 1991).
Masetti, E., A. Peigari, and A. Tirabassi, SPIE vol 1270, Optical Thin Films
and Applications 125-132 1990).
McNallyJames J., F. L. Williams and J. R. McNeil,SPIE vol 678, Optical Thin
Films 11:New Developments, 151-159 1986).
149
REFERENCES
150
Miniscalso, W.J., and L.J. Andrews, Materials Science Forum 32-33 501-510
(1988).
Onisawa, K, M. Fuyama, K. Tamura, K. Taguchi, T. Nakayama, Y.A. Ono,
Journal ofApplied Physics 68 719-723 1990).
Nigara, Y., Japanese Journal ofApplied Physics 7 41 404-408 1968).
Nishikawa, H., R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, Journal Applied
Physics 65 121 4672-4678 1989).
Patek, K., Glass Lasers, London: Butterworth
Co. 1970).
Palik, E.D. (ed.) Handbook of Optical Constants of Solids II, 761-774 and 10791096, New York: Academic Press 1991)
Parsons, Robert, in Thin Film Processes II edited by John L. Vossen and
Werner Kern, 177-208, New York: Academic Press 1991).
PawlewiczW.T.,K.K.Haysand P.M.MartinThin Solid Films, 73,169-75
(1980).
Popova LA, Inorganic Materials, 14, 1503-1505 1978).
Rainer, F, W.H. Lowdermilk, D. Milam, C.K. Carniglia, T.T. Hart, and T.L.
Lichtenstein, Applied Optics, 24 no. 4 496-500 1985).
Rossnagel, S.M., in Thin Film Processes II edited by John L. Vossen and
Werner Kern, 11-77, New York: Academic Press 1991).
Schuetze, H. J., H.W. Ehlbeck and G.G. Doerbeck, Trans. 10th Natl. Vacuum
Symp. 434 1963).
Shanon, R.D., and C.T. Prewitt, Acta Crystallographa B25 925 1969).
Silin, A.R., and L.N. Skuja, Journal of Non-CrystallineSolids 71 443-445
(1985).
Smit M.K., G.A. Acket, and C.J. Van Der Laan, Thin Solid Films 138 171181
1986).
Thornton, John A., Annual Review of Materials Science 7 239-260 1977).
Thornton, John A. and D. W. Hoffman, Thin Solid Films 171 531 1989).
Ulrich, R. and R. Torge, Applied Optics 12 1212901-2908 1973).
150
REFERENCES
151
Vuoristo, P., T. Mantyla, and P. Kettunen, Thin Solid Films 204 297-311
(1991).
Weber, M.J. (Ed.), CRC Handbook of Laser Science and Technology 4 21 24-28
(1982).
Wells, A.F., Structural Inorganic Chemista, 5th Ed., Oxford: Clarendon Press
(1984).
Wyckoff, Ralph W.G., CLrystalStructures Vol.2, 2nd Ed., New York: John
Wiley Sons, 1-8 1960).
Ying, X., A. Feldman, and E.N. Farabaugh, SPIE Vol. 1519, Int. Conf Thin
Film Phys. and Appl., 321-325 1991).
151
Appendix A
PROPERTIES OF FUSED SiO2 SUBSTRATES
Physical Properties
Density
2.2 gm3
Mohs Hardness
5.5-6.5
4.8x107 Pa
> I. x109 Pa
3.7x101 Pa
3.1x101 Pa
7.2x101 Pa
0.17
Tensile Strength
Compressive Strength
Bulk Modulus
Rigidity Modulus
Young's Modulus
Poisson's Ratio
Thermal Expansion Coeff (RT)
Thermal Conductivity (RT)
Specific Heat (RT)
Softening Point
Electrical Resistivity
Dielectric Constant
Index of Refraction
0.55x10-6 oC-1
3.3 mg cal/cm s 'C
0. 16 g cal/g
16830C
7x109 Qcm (350'C)
3.75
1.4585
Surface Conditions
Edges
±0.01011
Parallelism
Scratch and Dig
Thickness
Flatness (per 1"dia. aperature)
nominally 0.005"
nominally 80-50
Maximum Permissagle
Bevel or Edge Chip
±0.01011
nom. 10 wavelenghs
0.020"
152
Appendix B: RELEVENT JCPDS CARDS
120
100
4-
M
0W
80
a)
60
-4-J
0
-4M
a)
C4
40
20
v 20
30
40
50
60
70
80
90
100
20
JCPDS Card 4-787 Al
120
100
M
r.
11)
4-a
80
9
a)
60
-41
.5
Q)
40
P4
20
0
36
52
68
84
100
20
JCPDS Card 10-425 y1203
120
>1
100
;t
z1
)
4-4
0
80
(3)
60
.1-1
M
4C
(1)
P4
2C
O
30
40
50
60
70
80
20
JCPDS Card 10-173: oc-A1203
/ corundum
153
90
100
APPENDIX B: RELEVANT JCPDS CARDS
120
I -
154
I I I -I I I I I I I
ii
ii
ii
i
i
100
:t
W
0(1)
80
a)
60
r.
4-)
-41
0)
40
P4
20
I
0
I
M
30
40
50
-,
,1
60
r
70
r
F
I
P:
80
90
10(
80
90
101
90
100
20
JCPDS Card 33-1458: Y
120
4--1
100
M
0
0)
80
4-
r,
(1)
-4j
0
a)
60
40
P4
20
0
20
30
40
50
60
rr
70
20
JCPDS Card 25-1200: Y203 bixbyite
120
100
80
60
40
20
0
j,
20
30
40
50
60
70
80
20
JCPDS Card 20-1214: hexagonal Y203 (at 2300'C)
APPENDIX B: RELEVANT JCPDS CARDS
I
I
120
I
I
I
I
I
I
I
I
155
I I I
I
I
I
I
,
.
I
.
.
I
I
I
I
I
I
100
W
z(V
80
a)
60
-4--l
z
.1-1
C's
a)
40
04
20
0
9O
II
-
I...
30
I
f
. .
40
-I."[
50
.t
60
1.
70
.
I
Lf
80
f
. .
90
0
20
JCPDS Card 33-40 YW5012
Peaks with intensities
10
garnet (YAG)
not shown
120
100
4-,
W
el)
r.
80
W
60
CZ
(W
40
4
20
0
20
40
30
50
60
70
80
90
100
20
JCPDS Card# 33-41: YA103
Peaks with intensities <5 not shown
120
;P.,
100
-4-
r.
a)
r.
m
80
4--l
a)
60
41
CZ
(W
W
40
20
0
.
20
Jk.
30
',
40
I ..
-
l
Al.
50
-..
60
1-....
-
I
-. 11-1.1,
....
I....
I I
I -
I 70
20
JCPDS Card
33-40: Y4AI209
Peaks with intensities <5 not shown
80
90
100
O
O
U)
6O
P-4
5
m
C)
C)
N
r-i
I
I
I
--7- ->
...
4
W4
E*
w
in
00
4
m
U)
P4
O
PA
O
CeD
0.4
6O
U)
co
0*
9z
19
i
i
Ii
I
i
00
Q
m
iI
i
-7E
-->
1
* P04
Ft
0
(1)
04
04
-41
C)
L6
Lo
w
C)
C)
CYID
1-1
,7
C)
tz
m
r-4
:; I
-,i
C9
5
F-4
6C)
eq
N C)
=i
r.
I
e
km
-I
-:w,
-4-D
q.
eq (Z
Po
a)
I-i
CZ
4
ai
-+.a
i
i
I
-',#
(1)4
g: , %
9)
CI
-
-->
m
7
9z
w
I
C)
OD
10
"Itil
U)
4..')
4-01
tt
C
C)
C)
C)
cq
7!
4
M
-11,
F.4
Q
-LI
aI
-7-
-4.0
CZ
---7
6C)
C)
C)
co
CYD
IJ
ct
L ..
i
I
I
I
f
5
E
j..
4
-7--
1-
i
I
iII
1
1
5
i
r--
5:
I
N
16
00
CD
o6
C)
O
m
m
00
r--i
cq
4
m
aq
cq C)
I e
m,"
0
e
f-1-4
.41
CZ
Po
0
-+.j
CZ
CZ
;o
PO
4.,
Q
Q) 'm
0-
9)
l%
U)
w
co
cq
cl
cl
N
a)
-4--)
-4-1'
(z
O
cn
t
lz
1,-;4
w
4--')
6a)
Cy:)
0
C)
m
co
6
co
m
5V4
cm
I
i- `
I -
I
I
i
.7- -),
- -- ---2-
I
1".-
1ZL
--
cq
cy
Lo
V
A.