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Silica sol-gel thin film coatings for integrated photonic devices

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Silica sol-gel thin film coatings for integrated photonic devices

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SILICA SOL-GEL THIN FILM COATINGS FOR INTEGRATED PHOTONIC DEVICES

by

Brian Andrew Rose

A Thesis Presented to the
FACULTY OF THE USC VITERBI SCHOOL OF ENGINEERING
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CHEMICAL ENGINEERING)

August 2012

Copyright 2012 Brian Andrew Rose

ii
Acknowledgements

I would like to thank Dr. Andrea Armani for her continued support and allowing me the
opportunity to work in the Armani Lab. I would like to thank Dr. Steven Nutt and Dr. Alan Willner
for their support and serving as committee members. I would like to acknowledge and thank
Ashley Maker for her contributions and help throughout my time in the Armani Lab. I would like
to thank Hong-Seok Choi and Simin Mehrabani for helpful discussions and thoughtful
suggestions.

iii
Table of Contents

Acknowledgements ....................................................................................................................... ii
List of Figures ................................................................................................................................ v
Abstract ........................................................................................................................................ vi
Chapter 1. Introduction ............................................................................................................... 1
1.1. Motivation ........................................................................................................................ 1
1.2. Chapter overview ............................................................................................................. 2

Chapter 2. Silica Sol-gels Materials ............................................................................................ 3
2.1. Introduction (include applications in optics) ..................................................................... 3
2.2. Basic mechanism of Sol-gel Formation ........................................................................... 5
2.2.1. Chemical Reaction .................................................................................................... 5
2.2.2. Annealing Methods ................................................................................................... 7
2.3. Characterization Methods ................................................................................................ 9
Chapter 2 References ............................................................................................................. 11

Chapter 3. Whispering Gallery Mode Optical Resonant Cavities ............................................. 13
3.1. Introduction .................................................................................................................... 13
3.2. Fundamental Properties ................................................................................................ 14
3.2.1. Quality Factor and material loss .............................................................................. 14
3.2.2. Thermo-optic coefficient .......................................................................................... 16
3.2.3. Fabrication of toroidal cavities ................................................................................. 17
3.2.4. Fabrication of hybrid sol-gel coated microtoroids .................................................... 19
3.3. Finite Element Method modeling ................................................................................... 21
Chapter 3 References ............................................................................................................. 23

Chapter 4. High Refractive Index Silica Sol-gel ........................................................................ 24
4.1. Background ................................................................................................................... 24
4.2. Synthesis ....................................................................................................................... 25
4.3. Deposition ...................................................................................................................... 26
4.4. Characterization (FTIR, Ellipsometry) ........................................................................... 28
4.5. Material Loss ................................................................................................................. 31
4.6. Thermo-optic coefficient ................................................................................................ 36
Chapter 4 References ............................................................................................................. 39

iv
Chapter 5. UV-Curable Silica Sol-gels ..................................................................................... 41
5.1. Background ................................................................................................................... 41
5.2. Synthesis ....................................................................................................................... 42
5.3. Deposition ...................................................................................................................... 44
5.4. Characterization ............................................................................................................ 45
5.5. Device Fabrication ......................................................................................................... 48
Chapter 5 References ............................................................................................................. 52

Chapter 6. Porous Sol-gels ...................................................................................................... 53
6.1. Background ................................................................................................................... 53
6.2. Synthesis ....................................................................................................................... 54
6.3. Deposition ...................................................................................................................... 55
6.4. Characterization ............................................................................................................ 57
Chapter 6 References ............................................................................................................. 59

Chapter 7. Future Work ............................................................................................................ 60
Comprehensive Bibliography ...................................................................................................... 61

v
List of Figures

Figure 1: TEOS sol-gel reaction mechanism ................................................................................ 5

Figure 2: Reaction mechanism of the TEOS silica polymer network ............................................ 6

Figure 3: Microtoroid fabrication procedure ................................................................................ 19

Figure 4: SEM image of silica microtoroid before (a) and after (b) spin coating ........................ 25

Figure 5: FTIR spectroscopy comparison of thin films ............................................................... 29

Figure 6: Spectroscopic ellipsometry measurements for MTES R=0.3 thin films ....................... 30

Figure 7: Refractive index of composite thin films ...................................................................... 31

Figure 8: FEM simulations of optical field distribution in sol-gel microtoroids ............................ 33

Figure 9: Characterization Set-up ............................................................................................... 35

Figure 10: Representative transmission spectra of coated toroids ............................................ 35

Figure 11: Representative change in resonant frequency wavelength vs. temperature ............ 37

Figure 12: Molar ratio formula and procedure for TEOS silica sol-gel synthesis ....................... 43

Figure 13: FTIR comparison of UV curing effect on TEOS silica films at 150 C ........................ 46

Figure 14: FTIR comparison of UV curing effect on TEOS silica films at 400 C ........................ 46

Figure 15: Waferskan ellipsometry of TEOS sol-gel thin film ..................................................... 48

Figure 16: UV cured sol-gel cracking. ........................................................................................ 49

Figure 17: Attempts to pattern UV cured sol-gel films ................................................................ 50

Figure 18: Surfactant sol-gel mass ratios and procedure for synthesis ..................................... 55

Figure 19: Porous sol-gel thin films after spin coating and thermal treatment ........................... 56

Figure 20: SEM images of silica microtoroids before surfactant sol-gel coating ........................ 57

Figure 21: SEM images of microtoroids coated with surfactant sol-gel ...................................... 58

vi
Abstract

The following thesis will discuss new applications of sol-gel derived materials designed
to tune specific optical properties of optical resonant cavities. Sol-gel thin films are modified
through the use of titanium precursors, surfactant templating models, and annealing method.
Optical properties of the sol-gel thin films can be studied using high Quality factor microcavities.
Characterization is also preformed using Fourier Transform Infrared (FTIR) Specroscopy,
Spectroscopic Ellipsometry, and Scanning Electron Microscopy (SEM).
Using a combination of Finite Element modeling and resonant wavelength tracking, we
can determine the thermo-optic coefficient and materials loss of a high refractive index
composite sol-gel thin film. Determining a material’s thermo-optic coefficient is important to
determine its suitability in optical switches. For doping purposes, a low temperature curing
method is studied using ultraviolet radiation as a substitute for thermal energy will be discussed
in Chapter 5. Finally, Chapter 6 will study the deposition of an ordered porous thin film can be
templated using organic surfactant molecules.
These three methods of modifying sol-gel thin films present important applications to that
can modify the optical properties of resonant cavities. Further examination of the thin films
properties is important for their applications in optical sensing and I will discuss the ramifications
of their applications in integrated optical systems.

1
Chapter 1. Introduction

1.1. Motivation

Over the past decade, researchers have focused on developing new methods to
fabricate optical devices for applications in biodetection and in telecommunications.
Researchers have demonstrated both higher performance and smaller devices, ultimately
reaching the theoretical limits of both values. Therefore, despite the numerous advances in
fabrication technology, the device size is currently limited by the fundamental material
properties. By developing new materials, it will be possible to continue the pursuit of high
performance optical devices which can be densely integrated on a silicon wafer.
There are many materials which can be studied for this purpose, including silicon and
silicon derivatives and polymeric materials. However, these materials have high loss in the
visible and some have electro-optic coefficients. Additionally, polymeric materials are unstable
at high temperatures and are not compatible with many standard CMOS fabrication procedures.
By using a silica material, it will be possible to increase the refractive index of the device while
simultaneously maintaining the device performance.
Therefore, in the present work, I investigated the feasibility of using silica sol-gel glasses
to modify the refractive index of optical devices, thereby improving the device performance.
Silica sol-gel glass, also known as spin-on glass, is formed through a hydrolysis condensation
reaction of an alkoxysilane precursor. While it is a liquid, additional materials or dopants can be
easily mixed in, allowing the refractive index to be controllably and uniformly modified. Several
different studies were performed during the course of this work.
First, a series of increasing refractive index materials were synthesized and
characterized and integrated into an optical device. The refractive index was increased through
the addition of titanium butoxide to the initial sol-gel matrix. The optical performance of the
device was also determined. The high refractive index of the film enabled control over the

2
location and size/shape of the optical mode volume in the device. As part of this work,
complementary finite element method simulations were performed.
Additionally, micro-porous silica sol-gel films were synthesized by incorporating a
surfactant (CTAB) in the sol-gel film. The porosity of the film was characterized, and the
feasibility of creating a porous film on an optical device was explored.

1.2. Chapter overview

The following chapters will discuss the importance and significance of sol-gel synthesis
on integrated optical resonators. I will begin by providing a background on sol-gel synthesis and
characterization methods. In Chapter 3, I will then discuss whispering gallery mode optical
resonators, and their role as highly sensitive optical sensors in characterizing the thermo-optic
coefficient and materials loss of thin films. Chapter 4 will discuss in detail the characterization of
the thermo-optic coefficient and materials loss of a high refractive index, composite silica-titania
thin film. A proposed low-temperature curing method utilizing ultraviolet radiation will be
discussed in Chapter 5. The integration of a surfactant templated porous silica film on a
microtoroid resonator is studied in Chapter 6. Finally, Chapter 7 will discuss the future
applications of the presented work.

3
Chapter 2. Silica Sol-gels Materials

2.1. Introduction (include applications in optics)

Silica and composite sol-gel thin films retain the ability to conduct an electromagnetic
field much like thermally grown silicon dioxide. Silicon dioxide is commonly used in waveguide
applications because its low propagation losses in the visible and near-infrared wavelengths.
Silica is naturally abundant and inexpensive which lends to its status as the industry standard in
telecommunications and optical applications. Silica-on-silicon wafers facilitate easy device
fabrication through the use of conventional photolithography. Silica offers a viable medium for
fabrication of waveguides because of its low materials loss and high refractive index. As
conventional fabrication techniques are responsible for the development of highly sensitive
optical sensors, the utilization of sol-gel thin films allows the further applications of these
devices. Sol-gel synthesis remains a unique processing method allowing thin oxide films to be
deposited in unique and creative ways.
Sol-gel synthesis is a unique method of depositing thin films through the use of liquid
alkoxysilane precursors catalyzed to form an amorphous polymer matrix. Thin films can be
produced using an assortment of different precursors, or in combination with various optically
active materials to enhance the optical properties of the oxide film
[1-5]
. Sol-gel thin films are
produced through an initial hydrolysis and polycondensation reaction of alkoxysilane precursors
to produce an oxide polymer matrix. This polymer matrix can be reacted and manipulated at
room temperature in the liquid state then annealed at high temperatures to form thin oxide film.
Because of the liquid-gel nature of the oxide polymer, the thin films can be deposited through a
variety of different methods including spin coating, dip coating, ink-jet screen-printing, spray
coating, etc
[6-8]
. The solution-gelation processing of sol-gels allows the ability to template the
deposited oxide according to specifications for each application.

4
Various applications of sol-gel thin films include surfactant templating of the silica thin
film. Not only are sol-gels uniquely able to incorporate the use of different chemical precursors,
but also varying the annealing method can tune the optical properties of the finished thin film.
Templating thin films using surfactant micelles can form stable porous structures well suited for
catalyst structures. Additionally, sol-gel processing at room temperature and pressure allows the
ability to dope thin films with organic molecules that would normally degrade at high
temperatures. Dopants such as rare-earth elements, proteins, even DNA can be encapsulated
in the sol-gel polymer matrix, creating a biologically or optically active thin film.
Doping and encapsulation is one of the advantages sol-gel synthesis has compared to
thermally grown silica. Biologically active silicate glasses have recently become of great
interest, as studies have shown that biomolecules immobilized in the host matrix of a sol-gel
solution still retain a majority of their functional characteristics
[9,10,1]
. Doped sol-gels retain the
specificity and reactivity of biological molecules in a solid state, optically transparent glass
because the amorphous nature of the silicate polymer does not impart geometric order on the
entrapped molecule. In fact, through the introduction of surfactant micelles, the thin film can be
templated by the surfactant molecules. Although the dopant is trapped in a solid material, the
captured dopant retains many of the characteristics of the liquid state of the functional
molecule
[1]
.
Rare-earth ion doping is significant because of applications in a wide variety of
applications including integrated optical amplifiers and laser systems
[11]
. Sol-gel waveguides
offer the potential to behave as an effective optical medium for light propagation and
luminescence enhancement when doped with rare-earth ions. Because of their high Q-factors
and small optical mode volumes, whispering gallery mode optical resonators offer an ideal
geometry to decrease the lasing threshold of rare-earth doped glasses. Sol-gels can easily be
applied to cover microtoroid resonators and provide an excellent medium for entrapment of the
rare-earth ions

5
Due to the reaction mechanism of sol-gel synthesis, more than one precursor can be
used to create a composite oxide thin film. For example, the use of a titanium precursor can be
used in the reaction to increase the refractive index of the deposited thin film. Increasing the
refractive index can encourage total internal reflection that may, in turn, decrease the
propagation losses within the thin film. The incorporation of a higher refractive index thin film on
the surface of a biological sensor may be able to increase evanescent field intensity around the
surface of the optical sensor. This hypothesis is of particular interest and will be studied in
further chapters.

2.2. Basic mechanism of Sol-gel Formation

2.2.1. Chemical Reaction

Sol-gel synthesis is generally used to describe the process in which solutions of
monomeric, oligomeric, polymeric or colloidal precursors are reacted through catalysis,
hydrolysis, condensation, aging, and densification to form a solid material. The process is begun
when colloidal particles suspended in a solvent are catalyzed using either an acid or base.
Hydrolysis refers to the reaction of the metal alkoxide precursor with water to form metal
hydroxide. Condensation then occurs when two metal hydroxides combine to give a metal oxide
species. Following the catalysis, hydrolysis, and condensation, the solution is aged at room
temperature and pressure to allow the propagation of reaction. The chemical reaction of
tetraethyl orthosilicate (TEOS) is presented below in Figure 1.

Figure 1: TEOS sol-gel reaction mechanism

6

The polymerization reaction propagates to form a gel consisting mainly a three-
dimensional solid polymer network whose pores are filled by the solvent solution. With aging,
the interconnections between particles increase the viscosity of the solution and eventually
leads to the formation of a solid gel. The resultant gels are commonly referred to as aquagels,
hydrogels, or alcogels depending on the solvent. If the solvent is removed and replaced by air
without deformation of the network structure or volume, the resultant gel is called an aerogel.
One possible substitute for the thermally grown oxide used in waveguide applications is
the use of a high purity alkoxysilane precursor in an alcohol solvent. The reactions in sol-gel
processing depend on many parameters including the composition and concentration of
alkoxides and solvent, the amount of water present in the solution, the type and concentration of
catalyst used, and additives such as chelating agents or desiccating controlling chemical
additives (DCCA). Additionally, the sequence in which the components are added, the time
allowed for mixing and hydrolysis, and the temperature at which the reaction takes place can all
affect the final product of the thin film. Optimizing the selected parameters for the reaction offers
the potential advantages such as low temperature processing, small thickness, high optical
quality, and a very high purity level.

Figure 2: Reaction mechanism of the TEOS silica polymer network

7
After the point of gelation, the structure of the gel continues to condense as long as the
solvent remains in its pores. As the polymer matrix continues to cross-link during aging, the gel
undergoes shrinkage as the liquid solvent is forced from the pores of the structure. Once aged,
the viscous polymer gel can be applied in a variety of techniques such as spin coating, dip
coating, screen printing, or manipulated in practically any way needed. Spin coating is of
particular importance because of its applications in the microelectronics industry. Spin coating
can apply films of uniform thickness with low contamination levels. With an excess amount of
liquid dispensed on top of a wafer at rest, the spin coating process begins with an accelerating
spin-up step usually on the order of hundreds of revolutions per minute. Then the substrate is
accelerated up to speeds on the order of thousands of revolutions per minute. This step is
known as the spin-off stage, in which the liquid flows radially off the substrate due to centrifugal
force. Over time, film thinning by centrifugally driven flow decreases until it becomes
comparable to the rate of film thinning by solvent evaporation. At this step, the viscosity is
drastically increased and the centrifugally driven convective flow is ceased, and film thinning
occurs only due to solvent evaporation. Thus spin coating functions as a way to give very
uniform substrate coverage as well as expel the solvent contained in the porous polymer
structure.
2.2.2. Annealing Methods

Once the gel has been applied to a substrate through spin coating, the film must be
annealed at higher temperatures to remove the remaining organic components or any remaining
unbonded alkoxysilane groups in the film. Annealing must be performed at high enough
temperatures for densification and thermal sintering of the oxide polymer to occur. Not only do
annealing methods greatly affect the optical properties of the thin film but also it may affect the
rigidity and porosity of the finished product. Extensive research has been devoted to the
examination of the microstructural evolution of the silica film as a function of temperature
[12]
.

8
There are several regions in which silica sol-gels undergo microstructural evolution before
reaching a densified thin film. The first region of thermal treatment occurs between 25 °C and
300 °C. Thermogravimetric analysis and Fourier Transform Infrared Spectroscopy have shown
that in this region structural density remains almost constant with no densification of the
structure even though there is a considerable amount of weight loss due to evaporation of
solvent and water.
The second region occurs between 300 and 850 °C in which there is a slow decrease in
the pore volume and consequently an increase in film density. The density variation can be
related to the weight loss in this region. In this region, the weight loss can be correlated to the
evaporation of the water formed during the polycondensation reaction and the removal of
organic groups through film oxidation, although it is suggested that polycondensation reactions
and classical sintering mechanisms are mainly responsible for the rearrangement of the
microstructure of the film. The density variation in this region is approximately 75% of the total
density change in the film.
[12]

A third region in the microstructural evolution occurs between 850 and 1100 °C that
indicates an abrupt change in the pore volume. Thermogravimetric analysis (TGA) typically
shows only a small weight loss in this region, with most of the weight loss occurring between
300 and 850 °C
[12]
. A rapid change in the pore volume without significant weight loss suggests
the possibility that the film undergoes viscous flow that allows the pores in the gel to collapse.
This indicates that densification occurs in this temperature region. Densification represents the
final step in the transition from viscous polymer gel to rigid thin film. Rigid thin films deposited
and annealed according to the sol-gel method can be patterned into waveguides or optical
resonators through conventional photolithography, wet etching, and a dry silicon etching
process.

9
2.3. Characterization Methods

Because we are primarily interesting in sol-gel thin films for their optical properties,
characterization methods such as Fourier Transform Infrared Spectroscopy, Ellipsometry,
Optical and Scanning Electron Microscopy are of primary interest. Fourier Transform Infrared
Spectroscopy, or FTIR, analysis is a method by which we can identify the chemical composition
of the synthesized thin films. FTIR analysis can confirm the degree to which the thin films have
been annealed and whether or not the thin film contains trace amounts of organic components
that degrade the optical quality of the thin film. FTIR analysis is an important method of
confirming the complete removal of solvent or surfactant.
The infrared spectrum analysis is key to understanding the material present in the thin
film because the spectrum reveals characteristic vibrations of atoms in molecules. Atomic
vibrations in thin films and their effect on the infrared spectrum have been extensively studied to
reveal correlations between infrared bands and the molecular structure in the thin film
[13-15]
.
These correlations have been well documented and correlate to the thin films we will be
examining allowing for a better understanding of the materials in the deposited thin film.
By examining the infrared spectra of a sample we can determine the amount of organic
material left in the thin film. An excess amount of organic material causes larger propagation
losses in the material. Thermal annealing is specifically designed to remove the excess organic
molecules through degradation at high temperatures. It is possible to monitor the removal of the
organic material by comparing the peaks that correspond to organic molecules before and after
the annealing process. Additionally, the FTIR spectrum allows us to examine the extent of the
silica precursor’s bonding and polycondensation. FTIR characterization will be discussed further
in the Characterization sections of Chapters 4, 5, and 6.
Because we are concerned with the optical properties of our deposited thin films, we will
characterize our thin films by ellipsometry and spectroscopic ellipsometry measurements.

10
Ellipsometry remains an excellent technique to characterize thin films surfaces, and material
microstructures. Ellipsometry measurements can achieve very sensitive measurements from the
determination of the relative phase change in the beam of reflected polarized light. Ellipsometry
is considerably more sensitive than intensity reflectance measurements and is more accurate
because the absolute intensity of the reflected light does not have to be measured for
reference
[16]
. Important information such as the film thickness and refractive index are solved for
using the measurements of he polarization state of a light beam. In ellipsometry of thin films, the
sample film acts as an “optical system” that modifies the polarization state of a beam of light.
The ellipsometer measures parameters psi (Ψ) and delta (Δ) with are related to the ratio of
Fresnel reflection coefficients for p- and s- polarized light.
The optical parameters are never directly measured but they are derived from a quantity
that is a function of the parameters of interest, in this case, the thickness and the refractive
index. Fitting the measured data to a model then solves for the parameters of interest that best
estimates the values of the sample’s predicted model. It is necessary to construct a model that
accurately predicts the unknown physical parameters using some of the known parameters
such as the wavelength of the incident light, the polarization state, and the angle of incidence.
The best-fit model is then determined using the Mean Square-Error method with gives the best
fitting values of the physical parameters.
Optical and Scanning Electron Microscopy are also vital methods for the characterization
of sol-gel thin films. Optical microscopy is generally used to determine the homogeneity of the
thin films and SEM imaging is used to examine the microstructure of toroids. Using a
combination of these characterization methods, we are able to determine vital information
regarding the optical and mechanical properties of the deposited thin films.

11
Chapter 2 References

1. Dave, B. C. "Sol-Gel Encapsulation Methods for Biosensors." Analytical Chemistry 66.22
(1994): 1120-1127.

2. Livage, J. "Sol-gel Chemistry." Journal of Non-Crystalline Solids 145 (1992): 11-19.
Oxborrow, M. "How to Simulate the Whispering-Gallery-Modes of Dielectric
Microresonators in FEMLAB/COMSOL." n.d.

3. Yang, L. "Fabrication and Characterization of Low-loss, Sol-gel Planar Waveguides."
Analytical Chemistry 66 (1994): 1254-1263.

4. Jung, J. I. "Preparation and Characterization of Structurally Stable Hexagonal and Cubic
Mesoporous Silica Thin Films." Journal of Sol-Gel Science and Technology 31 (2004):
179-183.

5. Beck, J. S. "Molecular or Supramolecular Templating: The Defining Role of Surfactant
Chemistry in the Formation of Microporous and Mesoporous Molecular Sieves." Chem.
Mater. 6 (1994): 1816-1821.

6. Wang, J. "Screen-Printable Sol-Gel Enzyme-Containing Carbon Inks." Anal. Chem. 68
(1996): 2705-2708.

7. Altintas, Z. "Preparation of photocurable silica-titania hybrid coatings by an anhydrous sol-gel
process." J. Sol-Gel Sci Technol 58 (2011): 612-618.

8. Han, Y. H. "UV curing of organic-inorganic hybrid coating materials." J. Sol-Gel Sci Technol
43 (2007): 111-123.

9. S. Braun, S. Rappoport, R. Zusman, D. Avnir, M. Ottolenghi. Mater. Lett. 10 (1990): 1.
Shadbolt, P. J. "Generating, manipulating and measuring entanglement and mixture with
a reconfigurable photonic circuit." Nat. Photonics 6.1 (2011): 45-49.

10. L. M. Ellerby, C. R. Nishida, F. Nishida, S. A. Yamanaka. Science 255 (1992): 113.
Launer, P. J. "Infrared analysis of organosilicon compounds: spectra-structure
correlations." Silicone Compounds Register and Review (1987): 100-103.

11. Hsu, H. S. "Low threshold Erbium/Ytterbium co-doped microcavity laser." Integrated Optics:
Devices, Materials, and Technologies XIV 7604 (2010).

12. Folgar, C. "Microstructural evolution in silica aerogel." Journal of Non-Crystalline Solids 353
(2007): 1483-1490.

13. Anderson, D. R. "Analysis of Silicones." Ed. A. Lee Smith. New York: Wilery-Interscience,
1974. Chapter 10.

14. Bellamy, L. J. "The Infra-red Spectra of Complx Molecules." 3rd Edition. London: Chapman
and Hall, 1975. Chapter 20.

12
15. Launer, P. J. "Infrared analysis of organosilicon compounds: spectra-structure correlations."
Silicone Compounds Register and Review (1987): 100-103.

16. J.A. Woollam Co. Guide to Using WVASE Spectroscopic Ellipsometry Data Acquisition and
Analysis Software. Lincoln: J.A. Woollam Co., 1994.

13
Chapter 3. Whispering Gallery Mode Optical Resonant Cavities

3.1. Introduction

An optical resonator refers to a device that has a specific geometry designed to confine
light in small volumes through resonant recirculation. Optical resonators can circulate light
through a selective arrangement of mirrors or through a whispering gallery mode. Whispering
gallery mode resonators are typically circular waveguiding structures that conform optical fields
in a closed path around the device made of dielectric media through total internal reflection.
Current research topics in this area are devoted to the design and improvement of the optical
resonant cavities with very low energetic losses.
Currently there are a number of geometries and devices that can function as optical
resonant cavities, of which whispering gallery modes have received significant attention
because of their ultrahigh Quality factors. The Quality factor, or Q-factor, is one representation
of a microcavity’s ability to store energy. The Q-factor is the energetic losses of a device due to
a number of factors. An optical resonator’s Q-factor is inversely related to the amount of energy
lost through surface scattering, materials loss, curvature losses, and surface contamination
losses. The higher the Q-factor the better the microcavity can circulate light; an ideal optical
resonator can circulate light for an indefinite period of time. Further discussion on the quality
factor of microcavities will be saved for Section 3.2.1. There are a number of different
geometries of microcavities that function as optical resonators of which whispering gallery mode
resonators such as microspheres and microtoroids have ultrahigh Q-factors as compared to
microdisks and micropost resonators
[1]
.
In order for optical cavities such as microtoroids and microspheres to confine light, light
must be coupled into the device through a waveguide. Coupling into the microcavity is
commonly done through the evanescent field of a tapered optical fiber. Using a piezoelectric
stage, the tapered fiber can be brought into the immediate proximity of the device, so that the

14
evanescent field excites a whispering gallery mode. One end of the tapered fiber is attached to
a tunable laser that can scan through possible resonant wavelengths. The other end of the fiber
is attached to a photodetector that is able to record the intensity of transmitted light. As the laser
scans through wavelengths, resonance is achieved by tuning the laser to a wavelength that is
equal to the microsphere circumference an integer multiple of orbital wavelengths.
A device’s resonant wavelength has a very fine sensitivity to the surface conditions of
the microresonator. Changes in the microresonator’s radius or effective refractive index, will
cause a shift in the resonant frequency of the device. Resonators with high enough Q-factors
have the potential to track the adsorption of a chemical species to the surface by recording the
changes in the resonant frequency of the device. Higher Q-factor devices give better resolution
allowing researchers to better track slight changes in the resonant frequency of the device. The
resolution of the device is especially critical for the detection of analytes at extremely low
concentrations or analytes with low molecular weight.
Resonators with high Q-factors allow for higher resolution detecting with lower detection
limits when used as optical sensors. Spheres and toroid resonators can be made of dielectric
material, most commonly thermally grown silica, but can also be made of a variety of
transparent electromagnetic conducting materials. The materials loss of silica limits the ultimate
Q-factor of microtoroids and microspheres. For this reason, sol-gels can be integrated into the
device design to utilize their many unique optical properties in an attempt to gain further
understanding of their applications.

3.2. Fundamental Properties

3.2.1. Quality Factor and material loss

The Quality Factor of a resonator, also known as the Q-factor, is a measure of the
resonators ability to store energy. The Q-factor is representative of the electromagnetic losses

15
due to contributing factors of the resonator. The Q-factor of a device may be measures in two
different ways, the first of which the linewidth of cavity is measured by scanning a tunable laser
across a series of wavelengths until a resonant wavelength is identified. The quality factor is
then equal to the resonant wavelength divided by the linewidth of the device. The second
method of measuring the quality factor is measuring the photon lifetime in the resonant cavity.
This is performed through the cavity ringdown method. Both forms are equivalent.
Factors that contribute to the losses in the resonator and lead to lower Q-factors include
curvature losses, scattering losses on at the surface of the device, surface contaminant losses,
materials losses, and coupling losses. Curvature losses occur due to the curve of the device
geometry and vanish exponentially with increasing device size above D/λ > 15, where D is the
device diameter and λ is the wavelength
[2]
. Surface scattering losses due to inhomogeneities is
usually the dominant loss factor in intermediate-sized resonators. By controlling the coupling
losses and in the absence of surface contaminants on a sufficiently large microsphere or
microtoroid resonator, the Q-factor may then be limited by the materials loss of the device.
Although the use of optical resonators is a relatively new subject in optics, the loss
mechanism of microtoroids and microspheres have been significantly studied and reported in
literature. As previously mentioned, the intrinsic curvature losses of a device may be ignored in
devices of sufficient size. Surface scattering losses on atomically smooth devices are less
significant than other means of energy loss. In spheres with diameters much larger than 100
micrometers the surface scattering losses become sufficiently small such that a sphere without
surface contaminants can reach the Q-factor limit defined by material losses. For high-purity
fused silica, microsphere may have an ultimate Q-factor of 9 x 10
9
. Microtoroids have suitably
high Q-factors on the order of magnitude of 10
8 [1]
. For comparison, microdisk resonators can
given Q-factors as high as 12,000, micropost resonators can have Q-factors as high as 2,000,
and semiconductor polymer resonators can achieve Q-factors as high as 7,000
[1]
. Thus,

16
microtoroid and microspheres offer device improvement orders of magnitude higher than other
devices currently being studied.
Given that the materials loss is the dominant mechanism of energy loss in microspheres
and microtoroids of a given size, the overall quality factor measured is easily relatable to the
device’s optical materials loss, α. Optical microcavities provide significant tools for the
determination of the optical properties of the sol-gel thin films we have developed. By coating a
microtoroid in a transparent electromagnetic conducting sol-gel thin film, we can measure the
optical loss of the hybrid device, and consequently the optical loss of the thin film. The optical
loss of the sol-gel thin film is an important consideration that will be useful in determining future
applications of sol-gel materials.

3.2.2. Thermo-optic coefficient

The thermo-optic coefficient of a material is very significant in optical applications. In
communication devices, optical waveguide switches can be tuned by controlling the
temperature of the device
[3-5]
. The incorporation of organic/inorganic hybrid materials in optical
switches has recently drawn attention to the importance of materials’ thermo-optic coefficient
[6]
.
Electrical device tuning of ultrahigh-Q microtoroids has previously been demonstrated using
aluminum contacts on the device
[7]
. Thermally controlled microcavities have potential
applications ranging from tunable optical filters to a tunable laser source. Further study of the
control of the thermo-optic coefficient provides vital information of the applicability of these sol-
gel thin films.
A change in the refractive index of the device will cause a change in the resonant
wavelength. Changes in the refractive index can be determined with a resolution as small as
10
-7 [8]
. With the use of a controlled heating stage, microtoroids can allow for ultrafine resolution
of the thermo-optic coefficient of the hybrid device, and thus the deposited thin film.

17
The thermo-optic coefficient (dn/dT) of a material is the change in the refractive index
per unit change in temperature. It is a result of the competition between a material’s
polarizability and thermal expansion. In dielectric materials, the polarizability term is typically
dominant, resulting in positive values of dn/dT. Thermo-optic coefficients have drawn significant
interest because of their potential use in optical switches
[6]
. To determine the thermo-optic
coefficient, it is necessary to measure the value of the change in resonant frequency as a
function of the change in temperature. This can be done through the tracking of the resonant
wavelength of a device as a function of the temperature of the device environment. High quality
resonant cavities allow high resolution tracking of the resonant wavelength.

3.2.3. Fabrication of toroidal cavities

Because of the functionality of microtoroids, the majority of my research is focused on
the integration of sol-gel thin films with microtoroid cavities. The fabrication of microtoroids
occurs through photolithography patterning, a wet buffered-oxide etch, a xenon difluoride
isotropic etch, and a CO
2
laser reflow. Microtoroid fabrication requires the design of a
photomask that will be used in conjunction with UV photoresist to pattern the silica pads used
for microcavity production.
A dielectric material used to for microcavity confinement of light is silicon dioxide.
Standard production of microtoroids uses 2-µm thick thermally grown silica on silicon wafers.
Microtoroid production is particularly advantageous because the microcavities remain attached
to the silicon substrate, potentially providing the ability to integrate these devices into a lab-on-a-
chip sensing device. Microtoroid fabrication is always preformed in the Class 100 cleanroom on
campus to mitigate any defects that can be caused by particulate matter or contaminations. In
the cleanroom, the silica on silicon wafers are thoroughly cleaned by washing with acetone,
methanol, isopropanol, and deionized water to remove any surface contaminants. Samples are
then blown dry using nitrogen or compressed air and placed on a 120°C hot plate for 2 minutes

18
to completely dry the sample. Wafers are then cooled and exposed to hexamethyldisilazane
(HMDS) for 2 minutes using chemical vapor deposition. HMDS is used to increase the adhesion
of photoresist to the surface of the substrate.
Photoresist is applied to the sample by spin coating for 45 seconds at 3000rpm. The
photoresist is then soft baked at 95°C for 2 minutes. Then using a UV mask aligner with the
desired photomask, the photoresist is then exposed to 80 mJ/cm
2
of UV radiation. The exposed
samples are then immersed in a developer that removes the photoresist that was exposed to
UV light. Once the unwanted photoresist has been dissolved in the developer, the samples are
rinsed thoroughly with water and dried with compressed air or nitrogen. The samples are then
hard baked above the glass transition temperature at 110°C for 2 minutes to reflow the
photoresist and repair surface roughness. The UV-exposed photoresist acts to mask the
patterned circular silica pads on the silicon wafer when the sample is exposed to a buffered
oxide etch, containing HF acid.
Fifteen to twenty minutes of exposure to BOE removes the unwanted silica not covered
by photoresist, exposing the silicon substrate underneath the thermal oxide layer. The
photoresist can then be removed using acetone, methanol, isopropanol, and deionized water.
After the samples have then been dried, the silicon beneath the circular silica pads is undercut
using a XeF
2
etcher. XeF
2
isotropically etches the silicon wafer, but not the silica pad causing an
undercut beneath the silica to suspend the circular silica pads from a pillar of silicon attached to
the substrate. The undercutting forms a microdisk, which, when reflowed with a focused CO
2

laser, forms the final microtoroid device. The microdisks are exposed to a CO
2
laser of 12 W
intensity for about 3 seconds to reflow the silica disk into the toroid. Microtoroid fabrication is
complete after the final CO
2
reflow step, leaving an atomically smooth microcavity capable of
numerous important applications because of their long photon lifetimes.

19

a) b) c)

d) e) f)
Figure 3: Microtoroid fabrication procedure
(a) rendering of silica pads on silicon substrate, (b) rendering silica microdisks after xenon difluoride etching,
(c) rendering silica mircotoroid following CO2 laser reflow (d) SEM image of silica disk on silicon substrate
(e) SEM image of silica microdisk follow xenon difluoride undercutting of silicon substrate (f) SEM image of
silica microtoroid following CO 2 laser reflow.

3.2.4. Fabrication of hybrid sol-gel coated microtoroids

As previously discussed in Chapter 2, the optical properties of sol-gels are of vital
importance to their applications in waveguides and optical cavities. A promising new method
developed to characterize a material’s optical transmission loss and thermo-optic coefficient that
utilizes the microtoroid as an optical sensor capable of studying a material’s optical behavior. By
applying a sol-gel thin film to the surface of a microtoroid, we can probe the material’s optical
loss and thermo-optic coefficient through the behavior of the hybrid microtoroid resonator.
Because of the film’s interaction with the resonator’s Quality factor and the mode volume, we
can obtain select optical properties of the deposited thin film, such as materials loss, α, and the
thermo-optic coefficient, dn/dT.
In order to probe the optical properties of the sol-gel thin film, a hybrid microcavity must
first be prepared by applying a thin film of sol-gel on top of the microtoroid that has been
previously fabricated. Sol-gel synthesis allows for simple fabrication, providing uniform coverage

20
through spin coating, despite the irregular geometry of the device. Sol-gels prepared with
unique precursors or structured with surfactant can be applied by spin coating due to their
unique deposition mechanism, allowing for the integration of the unique optical properties of
these films. For two of my projects specifically, I will be applying porous sol-gel thin films
templated through the use of surfactant and applying high-refractive index composite thin films
using both silica and titania precursors. Further details on the synthesis, deposition, and
characterization of silica-titania composite thin films will be discussed in Chapter 4 and
surfactant templated thin films will be discussed in Chapter 6.
Sol-gel thin films can be deposited on the previously fabricated microtoroids by applying
properly aged sol-gel solution directly on the substrate in a spin coater. Individual preparation of
each type of sol-gel will be described in details to follow, but each sol-gel must undergo aging to
propagate the polymer matrix that forms the silica structure. Sol-gels are ready to be applied
once the aging is complete. Dropping 0.1 mL of sol-gel on the surface of the wafer, the sol-gels
are then spun at 7000rpm for 30 seconds to apply a uniform thin film around 350nm thick. The
sample is then hard baked for 5 minutes on a hot plate pre-heated to 75°C. Samples are then
annealed in a tube furnace with a program developed distinctly for each type of sol-gel. The
silica-titania sol-gel thin films were heated from 25°C at a rate of 5°C per minute up to 1000°C,
held at 1000°C for 1 hour, then cooled down at a rate of -5°C per minute back to 25°C.
Annealing at 1000°C ensures the removal of organic components and solvent that would cause
significant optical losses.
Surfactant templated thin films can be deposited through the same method of spin
coating, but require a much more delicate annealing method. Surfactant templated thin films
require a three-level program designed to remove the sol-gel solvent, decompose and remove
the surfactant molecules, and strengthen the silica polymer skeleton. To remove the excess
solvent in the film, the samples are heated to 140°C for 1 hour. Then, the samples are gradual
heated at a rate of 1°C per minute up to 210°C for 1 hour to decompose and remove the

21
surfactant molecules. Samples are then heated to at the same ramp rate to 450°C in order to
strengthen the silica film and remove the remaining hydroxyl groups within the silica polymer
network. Thermal treatment of surfactant templated sol-gel films usually do not exceed 600°C
otherwise the porous structure collapses.

3.3. Finite Element Method modeling

COMSOL Multiphysics Modeling can be used in conjunction with Maxwell’s equations to
provide calculations of the frequencies, electromagnetic field patterns, mode volumes, filling
factors, etc. of the whispering gallery modes of axisymmetric microresonators. Axisymmetric
resonators can be simply modeled by their 2-dimensional cross-section due to their continuous
rotational symmetry. Using the Oxborrow method of modeling, the magnetic or electric field is
assumed to lie everywhere parallel to the resonator’s axis of rotational symmetry allowing for
the 2-D simplification of the 3-D microcavity
[9]
. Defining the geometry and material properties of
the coated microcavity allows for the determination of the optic field distribution in the air, sol-gel
coating, and the silica microcavity. The optic field distribution can then be used to determine the
effect each layer of the hybrid microcavity has on the overall propagation loss and the change in
refractive index of cavity as a function of temperature. The properties of the thin film can then be
determined from these calculations.
The optical mode volume, or the distribution of the optical field in the microcavity, is a
useful property in some non-linear optics research. The sol-gel coatings with modified refractive
indices can potentially be used to optimize the mode volume of the microcavity. COMSOL
modeling provides a useful tool to determine the mode volume of new hybrid microcavities.
Mode volume shifting occurs due to the high refractive index region of the sol-gel thin film.
Forming a microcavity of alternating layers of high and low refractive index materials thinner
than the mode volume causes a compression of the electric field in the high refractive index

22
region. The degree of the compression of the electric field is an important parameter for
studying nonlinear phenomena and could be very useful in integrated laser applications.
COMSOL modeling allows us to study the magnitude of the shift in mode volume and the
degree of mode volume compression.

23
Chapter 3 References

1. Vahala, K. J. "Optical Microcavities." Insight Review Articles (2003): 839-845.

2. Gorodetsky, M. L. "Ultimate Q of optical microsphere resonators." Optics Letters 21.7 (1996):
453-455.

3. Kartalopoulos, S.V. "Introduction to DWDM Technology." (2000): 141.

4. X. Wang, L. Xu, D. Li, L. Liu, W. Wang. Journal of Applied Physics 94 (2003): 4228.

5. Kribich, K. R. "Thermo-optic switches using sol-gel processed hybrid materials." Integrated
Optics and Photonic Integrated Circuits (2004): 518-527.

6. Zhang, Y. "Compact asymmetric 1 x 2 multimode interference optical switch." J. Opt. A: Pure
Appl. Opt. 11 (2009): 105401.

7. Armani, D. "Electrical thermo-optic tuning of ultrahigh-Q microtoroid resonators." Applied
Physics Letters 85.22 (2004): 5439-5441.

8. Choi, Hong-Seok. "Thermal non-linear effects in hybrid optical microresonators." Appl. Phys.
Lett. 97.22 (2010): 223306.

9. Oxborrow, M. "How to Simulate the Whispering-Gallery-Modes of Dielectric Microresonators
in FEMLAB/COMSOL." n.d.

24
Chapter 4. High Refractive Index Silica Sol-gel

4.1. Background
Unlike conventional oxides which are deposited or grown with limited flexibility in the
processing, silica sol-gel materials are synthesized using an acid or base catalyzed hydrolysis
and condensation reaction
[1,2]
. The optical and mechanical properties of the final sol-gel layer
are governed by the specific chemicals used as well as the temperature, pH and annealing
conditions
[3]
. Additionally, as a result of the liquid synthesis method, it is possible to directly
incorporate dopants into the sol-gel. Therefore, silica sol-gel materials have found numerous
applications in integrated photonics. For example, by judicious selection of reactants, it is
possible to tune the refractive index by over 40%, enabling the fabrication of silica-on-silicon
waveguides for quantum computing applications
[4]
. By adding rare earth elements, such as
Erbium and Ytterbium, low threshold microlasers have been demonstrated
[5]
. However, while it
is straightforward to characterize the refractive index of a material using methods like
ellipsometry, it is more challenging to characterize the more subtle thermal non-linear properties
of these sol-gel materials. One approach is to fabricate a device from the material, and use the
device as a sensor to probe its material properties
[6]
. Clearly, it is critical that the device is
sufficiently sensitive. Because silica sol-gel materials have very low optical loss and negligible
thermo-optic coefficients, this requirement is very challenging to satisfy.
Recently, an approach based on hybrid high quality (Q) factor whispering gallery mode
microcavities was used to characterize the material loss and thermo-optic coefficient of polymer
thin films
[7]
. High Q optical microcavities confine light of a specific wavelength, also known as
the resonant wavelength of the cavity, which is governed by the cavity geometry and material
[8]
.
Because the circulating optical field interacts with the entire material system, the resonant
wavelength can be used to characterize the material properties and optical response.

25

Figure 4: SEM image of silica microtoroid before (a) and after (b) spin coating

In the present work, ultra-high-Q microcavities are conformally coated with high
refractive index silica sol-gel materials synthesized using either tetraethyl orthosilicate (TEOS)
or methyltriethoxysilane (MTES) (Figure 4). Sol-gel synthesis represents a simple yet versatile
method for tuning the optical properties of a deposited thin film suitable for use in waveguide
applications
[9]
. In addition to characterizing the basic material properties of the films using
Fourier Transform Infrared Spectroscopy (FTIR) and spectroscopic ellipsometry, the
transmission loss of the sol-gel layer and the thermo-optic behavior of the material are
determined using the optical resonant cavity and complementary Finite Element Method (FEM)
simulations.

4.2. Synthesis

Two distinctly different types of sol-gel composite thin films were synthesized. A titanium
precursor, titanium butoxide (Ti(OBu)
4
) (Aldrich, 97%), was added in order to increase the
refractive index of the material. In the high refractive index composite sol-gel,
methyltriethoxysilane (MTES) (Alfa Aesar, 98%) and titanium butoxide Ti(OBu)
4
were used as
precursors. In the second sol-gel, tetraethyl orthosilicate (TEOS) (Alfa Aesar, 99.999+%) was
used as the silica sol-gel precursor. Both sol-gel silica materials were prepared in an ethanol
solvent and catalyzed by hydrochloric acid (HCl) (EMD, 36.5-38.0%). The sol-gels were
prepared holding molar ratios between solvent, precursor, catalyst and water constant.

26
MTES R=0.3 sol-gels held a constant molar ratio of 0.3:1 Ti(OBu)
4
to MTES precursor,
in 16 moles anhydrous ethanol with 0.1 grams HCl. Similarly, MTES R=0.1 sol-gels were
prepared using a 0.1:1 molar ratio of Ti(OBu)
4
to MTES precursor in 16 moles ethanol and 0.10
grams HCl. Both MTES sol-gels were prepared without any added water to avoid forming a
precipitate. The composite solutions were prepared by adding the MTES precursor to the
solvent, then adding HCl to hydrolyze the MTES, stirring for 20 minutes, and finally adding the
Ti(OBu)
4
to the pre-hydrolyzed silica precursor. The titanium precursor hydrolyzes much more
rapidly, so in order to avoid an irreversible precipitate the silica precursor must be allowed to
hydrolyze first. Adding Ti(OBu)
4
before adding the catalyst formed solid white precipitate. After
mixing for 2 hours, the sol-gels were aged for 24 hours to allow propagation of polymer matrix.
TEOS sol-gels were prepared using 1:4:0.1:2 molar ratios between TEOS silica precursor,
ethanol solvent, HCl catalyst, and deionized water. Similar to the MTES sol gel materials, the
sol gel was prepared by adding the TEOS precursor to the solvent, followed by H
2
O, then HCl to
hydrolyze the TEOS, and stirring for 5 minutes between each addition. After mixing for 2 hours,
the sol-gels were aged for 24 hours to allow propagation of polymer matrix. Synthesis
procedures for both the TEOS and MTES-Ti(OBu)
4
sol-gels were performed at room
temperature and atmospheric pressure in a chemical fumehood.

4.3. Deposition

The method used for depositing and annealing the sol-gel thin films was the same for
TEOS sol-gels, MTES R=0.1, and MTES R=0.3 sol-gels. After sol-gels were properly aged, the
solutions were applied to the control wafers and the toroid devices by spin coating. The films
were spun at 7000 rpm for 30 seconds to apply a homogeneous thin film. The remaining solvent
was removed by heating on a hot plate at 75 degrees Celsius for 5 minutes. Then the deposited
TEOS films, MTES R=0.1 films, and MTES R=0.3 films were annealed using a Lindberg/Blue M
Tube Furnace from Thermo Scientific. All samples were heated from 25 °C at a constant ramp

27
rate of 5 °C per minute up to 1000 °C and held at this temperature for 1 hour to remove any
remaining organic components and complete densification through thermal sintering. After 1
hour of annealing at 1000 °C, samples were then cooled down to 25 °C at a ramp rate of -5 °C
per minute. No further annealing or reflowing steps are used.
By spin coating the TEOS and MTES sol-gels onto silica toroids, we can further
characterize the material loss and thermo-optic coefficient of these unique materials. The silica
toroids are fabricated as follows. First, 160µm diameter circular silica pads are defined on
silicon wafers using standard photolithography and BOE etching procedures. Next, XeF
2
is
used to isotropically etch silicon underneath the silica pads, forming elevated silica microdisks.
Finally, the silica microdisks are reflowed using a CO
2
laser to form ultra-high Q toroids
[10]
. A
uniform coating of sol-gel is subsequently applied to the finished toroids by spin coating at
7000rpm and annealed, as described in the previous section.
Several different methods were used to characterize the material properties. Scanning
electron microscopy (SEM) was used to image the material surface for porosity. Colorimetric
analysis was performed to determine the thermal expansion coefficient
[11]
. FTIR spectroscopy
confirmed the removal of solvent and organic components, and spectroscopic ellipsometry was
used to measure film thickness and refractive index of the material. The SEM images showed
no evidence of porosity, although sub-nm pores are beyond the resolution of the SEM. The
colorimetric analysis was performed over a temperature range from room temperature to 100C,
and no color change was observed. Therefore, the assumption that the polarizability term in the
dn/dT equation is dominant is valid for these materials. The FTIR spectroscopy and
spectroscopic ellipsometry results are discussed in more detail in subsequent sections.

28
4.4. Characterization (FTIR, Ellipsometry)

Thin film characterization was performed using a Bruker Optik ALPHA-P FTIR
spectrometer in combination in order to confirm the removal of solvent and additional organic
groups (Figure 4). The Bruker Opik ALPHA-P measurement module uses attenuated total
reflection (ATR) to measure the absorption intensity spectra. The ALPHA-P module uses an
anvil to place sufficient pressure on the sample in order to make contact with the ATR diamond
used for reflectance measurements. At each reflectance point of the ATR diamond, the sample,
in contact with the diamond, absorbs IR light according to its signature chemical bond
vibrations. The IR light absorbed by the sample is then missing in the reflected beam. The
reflected beam changes in intensity depending on the wavenumber, allowing us to record an
absorption spectrum for the sample. The absorption intensity spectra are then compared to
literature values to determine the identity of the chemical bonds in the thin film.
All thermal silicon dioxide films show strong characteristic peaks between 1110-1080
cm
-1
. Additionally, we were able to identify peaks characteristic of the Si-O-Ti vibrational mode.
The Si-O-Ti characteristic peak occurs around 905 cm
-1
and is seen in the composite films
prepared with the Ti(OBu)
4
precursor
[12]
. After annealing at higher temperatures, it was seen in
our composite films (Figure 5, arrow). This peak is indicative that titania is coordinating with four
oxygen atoms, forming TiO
4
[9]
.
Additionally, the sol-gel films identified broad peaks near 1130 cm
-1
corresponding to Si-
O-Si siloxanes bonds
[13]
. As siloxane bonds increase throughout the polymer network, it is
common to see these peaks broaden and become more complex. This may be due to two or
more overlapping bands in this range. A broader, weaker band around 810-800 cm
-1

corresponds to absorption by amorphous silica, rather than the α-Quartz crystalline form of
SiO
2
,

which would have been indicated by a sharp doublet at 800 cm
-1
and 780 cm
-1
. As
expected, the TEOS thin films were absent of peaks around 905 cm
-1
due to the lack of the

29
titania precursor. Peaks in the range of 1030 cm
-1
and 1160 cm
-1
were indicative of silica bond
vibrations. Similarly, thermally grown oxide contained peaks very similar to that of the TEOS
silica thin films except for a specific peak occurring around 460 cm
-1
. This peak is attributed to
the rocking motion of the bridging oxygen atom perpendicular to the Si-O-Si plane commonly
seen as a result of silica gel preparation
[14]
. As sol-gel films are heat treated at temperatures as
high as 1000 °C the peak at 460 cm
-1
gradually shifts up in frequency, suggesting a
strengthening of the silicate network due to densification of the porous thin film
[13]
.

Figure 5: FTIR spectroscopy comparison of thin films
Comparison between thermally grown silicon oxide, TEOS sol-gel thin films, MTES R=0.1 and MTES R=0.3.
The arrow on the graph highlights the peak at 905cm
-1
that comfirms the presence of Si-O-Ti bond vibrations
Spectroscopic ellipsometry was performed to measure the thickness of the deposited thin film,
in addition to determining the refractive index of the material. Using a V-VASE (J.A. Woollam
Co.) variable wavelength and angle of incidence ellipsometer, psi and delta measurements were
gathered for three angles of incidence (64°, 69°, and 74°) scanning from 550 nm to 1350 nm.
The measured values, psi (ψ) and delta (Δ), are related to the ratio of the Fresnel reflection
coefficients R
p
and R
s
for p- and s- polarized light respectively. The Fresnel reflection
relationship is related to psi and delta by:

R
p
R
s
= tan(ψ)e
iΔ

30
The psi and delta parameters were fit using regression analysis by WVASE32
Ellipsometry Analysis Software to determine layer thickness and refractive index. The measured
experimental data acquired from the V-VASE Ellipsometer is first compared with a generated
model. The model can be adjusted to minimize the difference between the generated data and
measured data through series of iterations. The WVASE32 software fits the thickness and
refractive index by defining a quantity called the maximum likelihood estimator, which
represents the quality of the match between the data calculated from the model and the
experimental data. WVASE32 determines the maximum likelihood estimator by the method of
Mean-Squared Error (MSE) and is defined as the following:

The MSE reduces the fitting problem to finding a set of values for the variable model
parameters, which gives a single unique minimum of the MSE corresponding to a single set of
variable parameters. The MSE was minimized using the Levenberg-Marquardt algorithm to
evaluate the desired parameters, thickness and refractive index.

Figure 6: Spectroscopic ellipsometry measurements for MTES R=0.3 thin films
(a) Psi with WVASE32 parameter fitting and (b) Delta with WVASE32 parameter fitting
A representative measurement of the psi and delta parameters is shown in Figure 6, and
the determined values of thickness and refractive index are in Table 1. Because the same
MSE=
1
2N −M
Ψ
i
mod
−Ψ
i
exp
σ
Ψ
i
exp
#
$
%
%
&
'
(
(
2
+
Δ
i
mod
−Δ
i
exp
σ
Δ
i
exp
#
$
%
%
&
'
(
(
2
*
+
,
,
-
.
/
/
i=1
N
∑

31
synthesis, spin-coating and annealing conditions were used, the film thickness is nearly
constant across the different sol-gel materials. However, in similar systems, it has been shown
that the density of the film will increase with increasing titanium butoxide content
[12]
. This change
is partially responsible for the increase in refractive index in the present system.

a) b)
Figure 7: Refractive index of composite thin films
Spectroscopic ellipsometry fitting of refractive index of silica-titania composite sol-gel thin films between
550nm and 1350nm. Sol-gels films prepared with less titanium precursor, MTES R=0.1, shown in (a) had
lower R.I. than MTES R=0.3 (b).

As mentioned, the material loss α
eff
and thermo-optic coefficient dn/dT of the TEOS and
MTES coatings can be determined by experimentally measuring the intrinsic quality factor and
the Δλ/ΔT values of the coated toroids. All measurements were performed on multiple toroidal
devices. Additionally, it is important to note that all measurements were taken in an ambient
environment.

4.5. Material Loss

The quality factor (Q) of a resonant cavity is determined by both the intrinsic losses and
the extrinsic losses of the cavity (Gorodetsky). In silica hybrid cavities, the intrinsic loss is
dominated by the material loss (Q
mat
) while the extrinsic loss is dominated by the coupling loss
(Q
coupl
)
[15]
. The analytical form for Q
mat
is Q
mat
=2πn
eff
/λα
eff
, where n
eff
is the effective refractive
index, λ is the wavelength, and α
eff
is the effective material absorption. While Q
mat
or the

32
intrinsic Q of a given resonant cavity is constant, Q
coupl
is dependent on the amount of power
coupled into the cavity. Therefore, by judicious experimental design, it is possible to isolate this
loss mechanism.
To determine the material loss of a cavity, this equation can be re-arranged:

𝛼
!""
=
!!!
!""
!"
(1)
and
(2)
(3)

where α
eff
is the loss in the resonant cavity-film-air system, n
eff
is the refractive index of the
cavity-film-air system, and λ is the resonant wavelength of the cavity. Therefore, to determine
α
eff
, it is necessary to know the refractive index values for both materials and the material loss of
silica. The refractive indices were determined using spectroscopic ellipsometry, and the
material loss of the thermal oxide was determined from previous measurements of this
material
[16]
. The coefficients (χ, γ, δ) are the percent of the field in the cavity, film and air,
respectively. These values are determined by Finite Element Modeling (FEM) simulations.
Therefore, given the previous values, it is possible to determine the material loss of the film with
a high degree of accuracy by simply measuring the Q of the cavity.
In order to determine the material loss of the sol-gel thin film, the optical field distribution
was modeled using COMSOL Multiphysics FEM. The field distribution was determined using the
measured major (98-115µm) and minor (9-11µm) diameters of the toroidal cavities. The sol-gel
film thickness and refractive index values were determined using spectroscopic ellipsometry and
are in Table 1. With the defined geometry and optical properties of the thin films, we were able
to model the distribution of the optical field in our devices by controlling the azimuthal mode

33
order (M) in the cavity. The mesh size used in the simulations was 0.021 µm
2
. All constants,
such as refractive index and film thickness, used in the simulations are either included in Table
1 or were taken from the COMSOL Library.
Representative simulations are shown in Figure 8 for 633 nm wavelength. The optical
field distribution was calculated by measuring the magnitude of the electric field squared in the
toroid, the film, and the air; therefore the units of the optical intensity are V
2
/µm
2
. The optical
field distribution in the toroid, sol-gel film, and air was then found by dividing the optical field
intensity of each section by the total optical field intensity in all three sections. From these
simulations, the precise values for χ, γ, δ are determined.

Figure 8: FEM simulations of optical field distribution in sol-gel microtoroids
Finite element modeling of optical field distribution of 633 nm wavelength in microtoroid coated with
(a) TEOS sol-gel (n = 1.454 at 633 nm), (b) MTES R=0.1 sol-gel (n = 1.518 at 633 nm), and (c)
MTES R=0.3 sol-gel (n = 1.618 at 633 nm). The optical field distribution was determined by finding
the magnitude of the electric field squared, thus the units of the scale bar are in V
2
/µm
2
. As the
refractive index of the sol-gel coating increased, the optical mode shifted to the coating, resulting in
a higher percentage of the optical field being contained in the sol-gel film.

Several important conclusions can be drawn from these simulations. The most immediately
apparent is the shift in the location and shape of the optical field. As the refractive index of the
film increases, the optical mode transitions from only slightly interacting with the film to being
completely confined within the film. This provides one mechanism for increasing the mode
confinement and interaction with the film.
To determine the material loss of the sol-gel coated toroids, it is necessary to measure
the quality factor of the devices. In the present series of experiments, the linewidth

34
measurement method is chosen. In this approach, the linewidth (Δλ) of the cavity is measured
by scanning a tunable laser across a series of wavelengths until a resonant wavelength is
identified. The quality factor can then be calculated from the simple expression Q=λ/Δλ.
However, to determine the intrinsic quality factor, it is necessary to isolate and remove the
coupling losses from the measurement. When using tapered fiber waveguides, this process is
straightforward, and requires performing the linewidth measurement under a range of coupling
conditions. This is accomplished by tuning the gap between the toroid and the tapered fiber. If
there are only two loss mechanisms (Q
mat
and Q
coupl
), the quality factor will vary linearly with the
percentage of power coupled into the device, allowing a linear fit to be applied to the Q versus
coupling data. The quality factor at zero percent coupling is the intrinsic Q and is used to
calculate the material loss.
To measure the linewidth of the sol gel coated toroid, a nanopositioning stage and
top/side view microscope cameras were used to align the sol-gel coated toroid with a tapered
optical fiber (Figure 9). The tapered optical fiber evanescently couples light from a tunable
633nm or 1300nm laser (New Focus) into the sol-gel coated toroid samples with high efficiency
and low loss
[17]
. Transmission data from the tapered optical fiber is recorded in real-time on a
computer for analysis. The linewidth is then determined from a Lorentzian fit to the spectra.
The error in a given material loss measurement is proportional to the error in the related quality
factor measurement and is approximately ±0.01m
-1
. However, the overall error of the
measurement method is larger, as it includes the error due to variations between the Q factors
of the different devices.

35

Figure 9: Characterization Set-up
The characterization set-up used for measuring the thermo-optic coefficient and materials loss of
sol-gel thin films. a) A schematic of the optical device characterization set-up. A tunable laser
(Laser input) is used to couple light into the cavity using a tapered fiber, and the output light is
detected on a photodetector (PD). The initial alignment is imaged with a machine vision system.
The signal is recorded using a high speed oscilloscope/digitizer (PCI card). The laser is controlled
using a function generator (FG) and a GPIB PCI card. b) An optical image of the toroid coupled to
a tapered optical fiber.

The quality factor of all three materials was determined at 633nm and at 1300nm to
study the dependence of the material loss on wavelength. Example spectra are shown in
Figure 10 and the material loss values are summarized in Table 1. As can be observed in
Figure 6, there is no lineshape distortion because low input powers were used. However,
occasionally, linewidth splitting occurred (Figure 10a). This behavior is commonly observed in
ultra-high-Q cavities and is the result of coupling into the clockwise and counterclockwise
modes of the cavity simultaneously.

Figure 10: Representative transmission spectra of coated toroids
Representative transmission spectra of toroids coated with a) TEOS, b) MTES R=0.1. and c) MTES
R=0.3 films. By fitting a Lorentzian curve to the spectra, the quality factor can be measured. The
presence of titanium in the coating increases the film’s material loss, therefore reducing the quality
factor of the coated toroids from over 10
7
(TEOS film) to 10
5
(0.3 MTES film).

The TEOS films have significantly lower material loss than the titanium butoxide MTES
films. Additionally, the loss of the TEOS films was nearly identical at both wavelengths studied,

36
whereas the loss of the MTES films was strongly dependent on the wavelength and the
concentration of titanium butoxide. This dependence is the result of the strong absorption of
titanium butoxide
[12]
.

4.6. Thermo-optic coefficient

The thermo-optic coefficient (dn/dT) of a material is the change in the refractive index
per unit change in temperature. It is a result of the competition between a material’s
polarizability and thermal expansion. In dielectric materials, the polarizability term is typically
dominant, resulting in positive values of dn/dT
[18]
. In the context of an optical cavity, the dn/dT of
the cavity material results in a change in the resonant frequency (Δλ) when the cavity is
exposed to a change in temperature according to the following relation: Δλ/ΔT=(dn
eff
/dT)(λ/n
eff
),
where Δλ/ΔT is an experimentally measured value and dn
eff
/dT is the effective thermo-optic
coefficient. To calculate the dn/dT of the material (dn
film
/dT), dn
eff
/dT is expanded
[19]
:

(4)

By combining this expression with the values for χ, γ, δ determined from the FEM simulations, it
is possible to determine the dn/dT of the sol-gel film.
To determine the thermo-optic coefficient, it is necessary to measure the value of Δλ/ΔT
experimentally. This is done by recording the change in resonant wavelength as a function of
temperature. Using a custom-built temperature control stage which included a heater and
thermo-couple (Omega), the resonant wavelength λ
0
was recorded at constant coupling as the
coated samples were incrementally heated from approximately 20 to 60°C in ~1
o
C steps
[7]
. All of
these measurements were performed in the under-coupled regime at low input powers to

37
eliminate additional contributions from thermal non-linear effects inherent to high-Q cavities,
such as thermal bistability.
Representative results from the series of thermo-optic characterization measurements
are shown in Figure 10. As detailed previously, the change in refractive index (Δn) is directly
related to Δλ. Over the entire temperature range, the change in refractive index (dλ/dT or
dn
eff
/dT) is extremely linear, indicating that there are no additional material or optical effects
present, such as solvent evaporation or electro-optic effects. Using these results, the calculated
thermo-optic coefficients of the three different material systems are summarized in Table 1.

Figure 11: Representative change in resonant frequency wavelength vs. temperature
Representative Δλ versus ΔT data for toroids coated with a) TEOS, b) MTES R=0.1, and c) MTES R=0.3 films.

Table 1: Thermo-optic coefficient and material loss measurements.

Material λ
nm
Thickness
nm
n
sol-gel
α
eff

m
-1

α
film
m
-1

Δλ/ΔT
nm/
o
C
dn
eff
/dT
o
C
-1
x10
-5

dn
film
/dT
o
C
-1
x10
-
5

TEOS 633 350.17 ±
0.284
1.4547 0.181 0.978 0.0069 1.58 4.43
1300 350.17 ±
0.284
1.4479 0.085 0.969 0.0144 1.59 7.13
MTES
0.1
633 373.31 ±
0.303
1.5181 15.81 28.41 0.0069 1.62 2.01
1300 373.31 ±
0.303
1.5066 8.27 48.06 0.0142 1.60 3.96
MTES
0.3
633 360.96 ±
0.365
1.6183 63.51 79.12 0.0069 1.71 1.89
1300 360.95 ±
0.365
1.5895 34.41 105.61 0.0146 1.66 2.83

38

As is clearly observable in Table 1, the three sol-gel materials have distinctly different
thermo-optic coefficients. This result is somewhat surprising given the extremely similar
responses observed in Figure 11. Namely, the dn/dT of TEOS is twice that of both MTES
R=0.1 and MTES R=0.3 films. However, as expressed by equation 4, the total shift (dλ/dT) is
related to the dn/dT of the film (dn
film
/dT) times the amount of the optical field located in the film.
As observed in the FEM simulations in Figure 8, the optical field confinement in the film
increases with increasing refractive index. Therefore, a decrease in the dn
film
/dT can be
compensated by an increasing optical field overlap.

39
Chapter 4 References

1. Dave, B. C. "Sol-Gel Encapsulation Methods for Biosensors." Analytical Chemistry 66.22
(1994): 1120-1127.

2. Livage, J. "Sol-gel Chemistry." Journal of Non-Crystalline Solids 145 (1992): 11-19.

3. Dumas, R. L. "Dependence of SiO2 gel structure on gelation conditions and sol reaction
temperature as followed by FTIR and Nitrogen adsoprtion measurements." J. Porous
Mater. 5.2 (1998): 95-101.

4. Shadbolt, P. J. "Generating, manipulating and measuring entanglement and mixture with a
reconfigurable photonic circuit." Nat. Photonics 6.1 (2011): 45-49.

5. Hsu, H. S. "Low threshold Erbium/Ytterbium co-doped microcavity laser." Integrated Optics:
Devices, Materials, and Technologies XIV 7604 (2010).

6. Choi, H. S., X. Zhang, A. M. Armani. "Hybrid silica-polymer ultra-high-Q microresonators."
Optics Letters 35.4 (2010): 459-461

7. Choi, H. S., D. Neiroukh. "Thermo-optic coefficient of polyisobutylene ultrathin films measured
with integrated photonic devices." Langmuir 28.1 (2012): 849-854.

8. Gorodetsky, M. L. "Ultimate Q of optical microsphere resonators." Optics Letters 21.7 (1996):
453-455.

9. Yang, L. "Fabrication and Characterization of Low-loss, Sol-gel Planar Waveguides."
Analytical Chemistry 66 (1994): 1254-1263.

10. Armani, D. K., Kippenberg, T. J., Spillane, S. M., and Vahala, K. J., “Ultra-high-Q toroid
microcavity on a chip,” Nature 421(6926), 925-928 (2003).

11. Larson, D. T., Lott, L. A., and Cash, D. L., “Surface film thickness determination by
reflectance measurements,” Appl. Opt. 12(6) 1271-1275 (1973).

12. Abdel-Baki, M. "Optical Characterization of xTiO2-(60-x)SiO2-40Na2O glasses I. Linear and
nonlinear dispersion properties." Mater. Chem. Phys. 96.2-3 (2006): 201-210.

13. Launer, P. J. "Infrared analysis of organosilicon compounds: spectra-structure correlations."
Silicone Compounds Register and Review (1987): 100-103.

14. Innocenzi, P. "Infrared spectroscopy of sol-gel derived silica-based fillms: a spectra-
microstructure overview." J. Non-Cryst. Solids 316 (2003): 309-319.

15. Choi, H. S. "Studying polymer thin films with hybrid optical microcavities." Optics Letters
36.11 (2011): 2152-2154.

16. Zhang, X. "Ultimate quality factor of silica microtoroid resonant cavities." Applied Physics
Letters 96.15 (2010): 153304.

40
17. Birks, T. A. "The shape of fiber tapers." J. Lightwave Technol. 10.4 (1992): 432-438.

18. Pokrass, M. "Thermo-optic coefficient in some hybrid organic/inorganic fast sol-gel glasses."
Opt. Mater. 32.9 (2010): 975-981.

19. Choi, Hong-Seok. "Thermal non-linear effects in hybrid optical microresonators." Appl. Phys.
Lett. 97.22 (2010): 223306.

41
Chapter 5. UV-Curable Silica Sol-gels

5.1. Background

In Chapter 3 it was discussed that sol-gel thin films are extremely useful because of their
ability to encapsulate active chemical or biological molecules. Doped sol-gel thin films facilitate
easy processing of chemically and biologically active glasses due to the sol-gel synthesis
mechanism. While sol-gels are easily able to encapsulate dopants when prepared at room
temperature, the method of annealing the deposited thin films can affect the stability of the
dopant in the thin film. In previous sol-gel processing, high temperature annealing was required
to remove organic components and strengthen the silica polymer skeleton structure. Thermal
treatment up to 600 °C is required to fully remove the solvent from the pores and strengthen the
silica structure
[1]
. Annealing at temperatures 600 °C and above will eventually lead to the
closing of pores and densification through thermal sintering. Biological dopants treated at such
high temperatures either lose their biological activity or simply degrade all together. Therefore
some sol-gel applications with temperature sensitive substrates or dopants are limited by the
chosen annealing method due the high temperatures required for thermal annealing. For this
reason, there has been considerable investigation into alternative methods of curing sol-gel thin
films.
It has been recently proposed that ultraviolet radiation may serve as substitute for some
thermal energy used to anneal sol-gel thin films. Ultraviolet curing, it was suggested, could
decrease the temperature needed to treat and anneal thin films while removing solvent and
strengthening the silica network. UV radiation curing is used to promote more cross-linking of
siloxane groups through reactions between Si-OH bonds. It has been documented that the
silanol condensation reaction is one of the main chemical reactions that occurs during UV
curing
[2,3]
. Potentially this method of annealing could replace some of the thermal energy
required to promote siloxane bonds through the condensation of hydroxyl bonds.

42
UV curing has a considerable number of advantage over thermal-induced curing
process, leading to shorter processing times at room temperature, lower energy costs for
treatment, and feasible application of a temperature sensitive substrate. UV-cured sol-gels are a
type of fast sol-gel processing technique with can be used in applications for solid-state dye
lasers and non-linear optical switching. Low temperature synthesis remains very important to
dye-doping of sol-gel glasses. Optical sensing, based on the measured transmission or
emission, has been demonstrated in sol-gel glasses doped with organic dyes
[4]
.
Annealing at lower temperatures can allow for the entrapment of organic and biological
molecules that are unstable at high temperatures. It has been demonstrated that doping
enzymes can be retained in the aqueous microenvironment inside the pores of the silicate
glass. Enzymes, proteins, and other biological molecules are able to retain their stability and
reactivity due to the lack of covalent interactions between the biological molecule and the
surrounding silicate medium. UV curing, then, is a proposed method of strengthening the silicate
structure surrounding biological molecules without damaging the entrapped molecule or
collapsing the pore by high temperature annealing.

5.2. Synthesis

As previously mentioned in Section 3.2.3, microtoroid cavities are produced using
thermally grown silica because of its low optical loss in the visible and near-infrared spectrum.
Although thermal oxide is readily available, it is very difficult to tune its optical properties such as
refractive index or dielectric constant. Sol-gels offer the ability to tailor the optical properties of
their deposited thin film by incorporating a variety of catalysts and precursors. Therefore, our
main focus will be finding a substitute method of fabricating the thin films from which optical
waveguides are produced. Sol-gel synthesis can offer an alternative method for depositing new
materials for the fabrication of microtoroids. UV curing at low temperatures allows for the
incorporation of temperature sensitive dopants. The goal of this project is to determine the

43
viability of UV radiation as a means of replacing some of the thermal energy used in annealing.
The motivation for this is that lower temperature annealing can open the door to new types of
dopants, such as organic- active molecules, in the sol-gel that would degrade at high
temperatures.
This experiment will be focused on investigating the effects of UV curing on a silica
based sol-gel thin film. Silica based sol-gel are synthesized using tetraethyl orthosilicate in an
ethanol solvent, hydrolyzed by deionized water, and catalyzed with hydrochloric acid. The
following table includes the molar ratios between precursor, solvent, water, and acid.

Figure 12: Molar ratio formula and procedure for TEOS silica sol-gel synthesis

The silica precursor, tetraethyl orthosilicate (TEOS) was added to anhydrous ethanol
and mixed for 5 minutes on a magnetic stir plate stirring at 300rpm. Deionized water was then
added and stirred thoroughly for another 5 minutes at 300rpm. Finally, hydrochloric acid is
added to begin the catalysis of the silica precursor. The solution is then mixed for 2 hours at
300rpm to ensure complete mixing while the precursor is catalyzed and hydrolyzed. After

44
mixing, the solution is then filtered using a 0.22 µm filter and placed to rest for 24 hours. Aging
for 24 hours allows the propagation of the silica polymer matrix. Polymer propagation increases
the viscosity of the solution and after 24 hours the solutions are ready deposition by spin
coating.

5.3. Deposition

Sol-gels are applied after aging through the use of a spin coater. Thin film samples are
applied to bare silicon substrates, which will then be treated with UV radiation in addition to
thermal treatment on a hot plate. Samples were UV cured using a Sylvania 100 Watt Mercury
Lamp. For comparison, control samples will be cured without the use of UV radiation. FTIR
comparison will be preformed to examine the effect the UV radiation has on the components of
the film.

Thin films were applied following the described process:

Goal: to form an ~ 1 micrometers thick coating of sol-gel on the surface of bare (100) Si
wafers and treat with UV radiation
Chemicals: Sol-gel
(100) Si (bare) wafer
Source: Montco Silicon
Amount Needed: drop 4 per 1 cm x 1 cm piece
Procedure:
1. Set a hot-plate to 75 °C.
2. Blow off the 1 cm
2
wafer samples using dry argon or dry nitrogen.
3. Place the wafers in dip boats, polished side facing the wall of the boats. If you are not
using dip boats, place the wafers carefully in a beaker or glass vial, device side up,
being careful to not damage the surface.
4. Submerge the dip boats or wafers in trichloroethylene, then acetone, then methanol,
then deionized H
2
O for 5 minutes each under mild agitation.
a. Preferred: use a rocker or tilt tray to gently mix
b. Reason: degreases substrate
c. In between rinses, decant 1⁄2 of beaker, refill with the next solvent, never
exposing the surface of the wafer
5. Setup the spin-coater system.
a. Turn on the air compressor to ~ 60 psi. Check the gauge. (< 100 psi for best
operation)
i. < 100 psi for best operation
ii. Turn “push to lock” knob to obtain the correct pressure if needed.

45
iii. Check the spin-coater – if “CDA” is flashing, then you will need to turn up
the air pressure.
b. Turn on the vacuum pump.
c. Place an appropriately-sized chuck on the spin-coater. Place your sample on the
chuck. Press vacuum.
i. Make sure the coater gives a vacuum reading and stops flashing.
d. Select the program you want by pressing the “program select button” to scroll through
the program numbers.
i. While you are in this mode, “PGM” is on the spin-coater screen.
ii. Usually: Program A, 1 step, 7000 rpm.
e. Close the lid
6. Drop enough sol-gel onto the wafer to completely cover the surface.
7. Press “run” on the spin-coater.
8. Remove the sample by opening the lid, pressing “vacuum” to turn off the vacuum. Turn
off the vacuum pump and the compressor when you are done.
9. Place the sample on a 75 °C hot plate for 5 minutes to drive off the water and ethanol in
the sol-gel film. a. Time: b. Temperature:
10. Place on hot plate increasing temperature in equal time increments to 50 °C to 100 °C to
200 °C to 300 °C to 400 °C while exposed to UV lamp.
11. Expose for 60 minutes to UV radiation then turn off lamp.
12. Slowly reduce temperature from 300 °C back to 25 °C, manually reducing temperature
by 5°C per 5 minutes.

Thin films applied in this manor deposited films about 350 nm thick once fully annealed at
1000°C. In order to have enough material for microtoroid fabrication, the sol-gel thin film must
be at least a micrometer thick. Therefore it is necessary to deposit and cure three sol-gel layers
before attempting to fabricate using lithography.

5.4. Characterization

In order for UV curing to be a viable substitute for thermal annealing, it must be
confirmed that the curing promotes siloxane condensation and promotes the removal of ethyl
leaving groups. Remaining hydroxyl groups and organic components can ultimately degrade the
optical quality of the film. FTIR analysis can provide a qualitative analysis on the presence of
hydroxyl or organic components remaining in the cured film. Comparisons between samples
cured at 400 °C and samples cured at 150 °C have been preformed. The highest temperature of
curing was 400 °C so as not to collapse the porous silica network.

46

Figure 13: FTIR comparison of UV curing effect on TEOS silica films at 150 C

Figure 14: FTIR comparison of UV curing effect on TEOS silica films at 400 C

As expected, the samples cured at 400 °C shows a significant reduction in organic and
hydroxyl peaks as compared to the samples cured at 150 °C. The most notable differences
between the samples prepared at 150 °C and 400 °C showed was a reduction in peaks located
around 800 cm
-1
, 960 cm
-1
and 1100 cm
-1
when cured at higher temperatures. The peak located
at 800 cm
-1
is associated with the Si-OH groups still present in thin film, meaning the film has
not yet undergone polycondensation at 150 °C. A comparison of samples prepared at both
temperatures with and without UV exposure showed no discernible difference between the two
curing methods.

47
The peak located at 960 cm
-1
is associated with Si-OCH
2
CH
3
groups. While this peaks is
present in the 150°C cured samples, the 400°C samples showed a noteworthy decrease,
signifying that the leaving groups bonded to the central silicon atom have not been fully
removed at 150°C. For comparison, samples annealed at 1000°C were included to show the
FTIR spectra of a fully annealed film. Samples annealed at 400°C showed comparable spectra
to the 1000°C annealed samples.
Thicknesses of the sol-gel thin films were measured following each heat treatment to
observe the densification of the thin film. Using the TEOS sol-gel, bare silicon substrates were
covered with a single layer of sol-gel thin film. Spin coating was preformed at 7000rpm for 30
seconds in each sample. Select samples were then placed on a hot plate preheated to 75 °C for
5 minutes. This step was consistently used in all spin coating applications and was believed to
remove the excess solvent in the thin film. Another batch of samples were placed on a hot plate
preheated to 150 °C for 1 hour, while another batch were placed on a hot plate preheated to
400 °C for 1 hour.
Ellipsometry was preformed on the samples following each thermal treatment step using
a Gaertner Stokes Waferskan ellipsometer. Following the spin coating and heating to 75 °C the
samples had an average thickness of 586.2 nm. The thickness did not significantly decrease
when heated to 150 °C for 1 hour. These samples had an average thickness of 582.6 nm.
However, it was seen that the samples heated to 400 °C showed a significant decrease in
thickness, with an average thickness of 456.4 nm. Samples annealed at 1000 °C showed final
thicknesses around 350 nm. The decrease in thickness was due to the loss of organic material,
which was confirmed by FTIR. Porosity was removed when annealed at 1000°C. A
representative measurement of a sample cured at 400°C is provided below. A sample
measurement over a 10mm diameter section on the wafer is presented in Figure 15,
thicknesses are measured in Angstroms.

48

Figure 15: Waferskan ellipsometry of TEOS sol-gel thin film

5.5. Device Fabrication

Once the sol-gel has been hard-baked on the hot plate at 75 °C the wafers are
transferred to a hot plate under the UV lamp. The samples are slowly brought up to temperature
and the UV lamp is turned on. Initial work was done at the highest temperature allowed by the
hot plate, 400 °C. This resulted in massive cracking on the first layer. The max temperature was
then reduced to 250 °C, 200 °C and 150 °C. The first layer could be applied without cracking,
but the second layers would crack and peel such that applying a third layer is not possible. The
first layer could be applied without cracking most likely due to the fact that the thermal
expansion coefficients were of similar magnitude or the presence of solvent still in the previous
layer.

49

Figure 16: UV cured sol-gel cracking.
UV curing samples showing poor adhesion between first and second applied layer of TEOS sol-gel, further fabrication
was not possible

The cracking was thought to occur because of a mismatch between the thermal
expansion coefficients of the applied layers of sol-gel. Since the cracking occurred during the
annealing of the second layer, it was also thought that the first layer might not totally been
annealed. For this reason, it is necessary to anneal the first layer in the tube furnace instead of
on the hot plate at a higher temperature. Initially, this annealing was done at 500 °C in the tube
furnace for 1 hour. Annealing the first layer in the tube furnace at 500 °C allowed the application
of a second and third layer, both of which were baked on a hot plate at 150 °C with UV
exposure.
It was found that the second and third layers could be hard-baked, with UV exposure
without cracking after the first layer was annealed in the tube furnace at 500 °C and higher.
Once the third layer was applied, the film was thick enough for the fabrication of toroids, and we
tried to pattern the samples using photolithography and BOE etching. Xiaomin Zhang preformed
the lithography and etching of the UV annealed samples. Fabrication of microtoroids from the
UV cured samples was preformed following the same protocols stated in Section 3.2.3.
Photoresist was applied after samples were cleaned. Then samples were patterned with a UV
photomask, developed and then etched using HF buffered oxide etch. After short exposure to
BOE etching, the samples were inspected using an optical microscope. But when annealed at

50
only 500 °C the oxide pads did not adhere to the silicon substrate. In fact, during the buffered
oxide etch, the patterned pads floated around the substrate (Figure 17a), making XeF2 etching
impractical.

a) b)
Figure 17: Attempts to pattern UV cured sol-gel films
(a) first layer annealed at 500 C showed poor adhesion (b) first layer annealed at 1000 C were damaged by BOE etching

In order to increase the adhesion of the oxide pads to the silicon substrate, a higher
temperature was needed for the first layer of the film. In this case, I began annealing the first
layer of the thin film at 1000 °C. In this instance, the first layer stuck to the substrate and was
able to be patterned using lithography, but the upper two layers would peel during the buffered
oxide etch (Figure 17b), again making the XeF2 etching unusable.
The problems that occurred during the fabrication process are most likely due to the
reduced temperatures used during annealing of the thin films. Conventional production of TEOS
sol-gel thin films use very high temperatures to ensure densification of the oxide matrix as well
as thermal sintering to condense the porous structure. The problems seen while patterning the
UV annealed samples were likely due to the films not being completely annealed. In this case,
higher temperatures are needed to create rigid enough films to withstand lithographic
patterning. Film rigidity is the key issue when trying to fabricate the microtoroids from the sol-gel
thin films. Unfortunately UV curing could not deposit a thin film rigid enough to withstand the
fabrication methods. Instead, a new approach to depositing a biologically active sol-gel glass
could be applied in a similar fashion to the high refractive index sol-gel thin films.

51
A biologically doped sol-gel could be used to measure the binding kinetics of an analyte
to the entrapped molecules in the sol-gel silica using high Q microresonators. Tracking the
response of a high Q microtoroid upon the introduction of a responsive analyte could be used to
test the functionality of the entrapped molecules in the sol-gel thin film. Instead of using high
temperature annealing, a lower temperature method, such as heat curing on a hot plate around
150 °C could be used to entrap the molecule in a hard baked sol-gel thin film without losing its
porosity due to densification. FTIR analysis shows the presence of organic molecules in the sol-
gels cured at 150 °C demonstrating this is a viable method for curing thin films that may retain
organic components.

52
Chapter 5 References

1. Gvishi, Raz. "Fast sol-gel technology: from fabrication to applications." J. Sol-Gel Sci.
Technol. 50 (2009): 241-253.

2. Attia, S. M. "Review on Sol-Gel Derived Coatings: Process, Techniques and Optical
Applications." J. Mater. Sci. Technol. 18.3 (2002): 211-217.

3. Drobny, Jiri. Radiation Technology for Polymers. Boca Raton: CRC Press, 2003.

4. Wirnsberger, G. "Mesostructured materials for optical applications: from low-k dielectrics to
sensors and lasers." Spectrochimica Acta Part A 57 (2001): 2049-2060.

53
Chapter 6. Porous Sol-gels

6.1. Background

One advantage of the sol-gel technique is the ability to template the oxide through the
use of different surfactants. Because the sol-gel thin film begins as a solution, the surfactants
are not restricted in motion and can arrange themselves into micellular structures. The scientists
at the Mobil Corporation first developed Mesoporous thin films two decades ago
[1,2]
.
Mesoporous thin films offer the advantage of high surface area and through the organization of
ordered molecular sieves. Using a variety of surfactant templating agents can control pore size
and distribution
[5]
. Mesoporous silica can be deposited on substrates for a number of practical
applications including catalysis, adsorption, separations, sensing, and nanotechnology
applications
[3-5]
. Mesoporous silicates can also be used as hosts or reactors in some
polymerization reactions to decrease the reactor volume.
An ordered mesoporous arrangement can be achieved through the use of amphiphilic
surfactant molecules that arrange themselves in solution to reduce their interface energy.
Surfactant molecules arrange themselves into micellular structures at their thermodynamic
minimum by reducing interface energy between the solution and the hydrophobic end of the
surfactant. This technique has been documented in sol-gel synthesis and is known as liquid
crystal templating
[3]
. Once the solution has been spin coated, the template can be removed
through a heat treatment to give an ordered porous structure to the oxide.
Liquid crystal templating plays an important role in thin film patterning, as it would
increase the surface area of the toroid that is within the evanescent field used for sensing
applications. Because optical resonators are label-free evanescent field sensors, a larger
percentage of the optical field that circulates in the porous material would allow for better
sensing. These sensors could also potentially be used to monitor the adsorption and desorption
of a species through the porous thin films. Also, a more porous oxide would have a lower

54
dielectric constant perhaps enabling lower detection limits on our devices. With such a variety of
applications, porous sol-gels represent a significant step in the advancement of chemical
sensing using optical resonators.

6.2. Synthesis

Surfactant templated sol-gel thin films are synthesized in a manor very similar to
standard silica sol-gel thin films. The synthesis follows the same procedure as the tetraethyl
orthosilicate sol-gels with the addition of a templating surfactant component added to the
mixture. Because of the hydrophilic/hydrophobic nature of the surfactant molecule, micelles
formation spontaneously occurs in solution. Hexadecyltrimethylammonium chloride (CTACl)
was the surfactant used to template the silica thin films in this experiment. CTACl is a long chain
organic species with hydrophilic/hydrophobic properties. When mixed in solution, the molecules
arrange themselves to lower the interface energy.
The most important factor in forming stable, relatively uniform pores is finding an
optimum surfactant concentration. The surfactant mass ratio that resulted in formation of pores
occurred in the range between 1.25 - 2.00 grams surfactant solution in a basic TEOS sol-gel
with the same molar ratios between solvent, precursor, catalyst, and water. Following the
previously described procedures for TEOS sol-gel synthesis, the surfactant is added to the
solution, which is then stirred for 2 hours to ensure complete mixing. The sol-gels are then aged
24 hours at rest, and are then moved to a refrigerator. Initially, newly aged surfactant sol-gels do
not form pores in the deposited thin film. After a week in the refrigerator, the surfactant sol-gels
form porous thin films due to micelle formation in the sol-gel solution with the additional aging
time at a lower temperature. The higher concentrations showed better formation of pores in the
oxide, but concentrations higher than 2.00 grams caused non- uniformities in the sol-gel film
because the surfactant solution contains too much water.

55

Figure 18: Surfactant sol-gel mass ratios and procedure for synthesis

6.3. Deposition

Initially, the surfactant sol-gels were applied to control wafers to confirm the presence of
pores in the thin film. Surfactant sol-gels were applied through spin coating on silica-on-silicon
substrate. The silica-on-silicon substrate was used to determine if the thin films would be
applicable to the silica surface of the microtoroids. When sol-gels were aged properly, the
following method was used for spin coating.

Goal: to form a porous coating of sol--‐gel on the surface of microtoroid
Procedure:
6. Set a hot--‐plate to 140 C.
7. Setup the spin--‐coater system.
a. Turn on the air compressor to ~ 60 psi. Check the gauge. (< 100 psi for best
operation)
i. < 100 psi for best operation
ii. Turn “push to lock” knob to obtain the correct pressure if needed.
iii. Check the spin--‐coater – if “CDA” is flashing, then you will need to turn
up the air pressure.
b. Turn on the vacuum pump.
c. Place an appropriately--‐sized chuck on the spin--‐coater. Place your sample on
the chuck. Press vacuum.
i. Make sure the coater gives a vacuum reading and stops flashing.

56
f. Select the program you want by pressing the “program select button” to scroll through
the program numbers.
i. While you are in this mode, “PGM” is on the spin--‐coater screen.
ii. Usually: Program A, 1 step, 4000 rpm.
g. Close the lid
8. Drop enough sol--‐gel onto the wafer to completely cover the surface.
9. Press “run” on the spin--‐coater.
10. Remove the sample by opening the lid, pressing “vacuum” to turn off the vacuum. Turn
off the vacuum pump and the compressor when you are done.
11. Place the sample on a 140 C hot plate for 1 and half hours to drive off the water and
ethanol in the sol--‐gel film.
12. Increase heat of hot plate to 210 C in order to decompose and remove the surfactant
template
13. Perform thermal annealing, standard procedure is to anneal at 450 C for 12 hours,
increasing and decreasing ramp rate of 1 C/min.

The deposited porous thin films must undergo a modified annealing method designed to
remove the organic surfactant template. The hot plate treatment, along with the annealing
program is designed to remove the solvent first at 140 °C and then decompose and remove the
organic surfactant at 210 °C. Using a programmable tube furnace the films are then heated to
450 °C at a constant ramp rate of 1°C per minute. The samples are then held at 450 °C for 12
hours to completely remove residual surfactant. After the 12 hours the films are cooled back to
room temperature at a rate of -1°C per minute to provide a porous silica film free of solvent and
surfactant. Pore diameter ranged from 2-4 µm.

Figure 19: Porous sol-gel thin films after spin coating and thermal treatment

57
Once it was possible to deposit porous thin films on control wafers, the next step was to
deposit the porous films on the surface of the microtoroid. The surfactant templated sol-gels
could be applied to the surface of the microtoroid through the same spin coating method used to
apply composite silica-titania thin films discussed in Chapter 4. Sol-gel spin coating was
performed at 7000 rpm for 30 seconds, and then followed by the same thermal treatment used
on the silica-on-silicon control samples. Characterization of the porous film structure on the
surface of the microtoroid was performed using Scanning Electron Microscopy. FTIR analysis
was also used to examine the extent of the removal of the organic surfactant.

6.4. Characterization

The deposited thin films were examined by SEM imaging to confirm or repudiate the
presence of pores on the surface of the microtoroid. Microtoroids were imaged before the
application of surfactant sol-gel and after the thermal treatment of the thin film.

Figure 20: SEM images of silica microtoroids before surfactant sol-gel coating

58

Figure 21: SEM images of microtoroids coated with surfactant sol-gel
While we have demonstrated the ability to pattern our sol-gel films with surfactant, the
complexity of spin coating on toroids caused unexpected results. The toroids’ complex geometry
results in the formation of pores everywhere on the sample expect for the toroid surface. Further
work needs to be spent finding a solution to this problem, but we now know that surfactant
templating will work with our conventional sol-gels. Spin coating at slower speeds would allow a
thicker film on the surface; potentially allowing the micelles to form pores on the equatorial rim
of our devices. Additionally, another surfactant with a shorter chain could produce smaller pores
that might be able to arrange themselves on the surface. The complexity of the microtoroid’s
geometry has caused the uneven formation between the wafer surface and the microtoroid
surface.

59
Chapter 6 References

1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck. Nature 359 (1992): 710.

2. Beck, J. S. "Molecular or Supramolecular Templating: The Defining Role of Surfactant
Chemistry in the Formation of Microporous and Mesoporous Molecular Sieves." Chem.
Mater. 6 (1994): 1816-1821.

3. Jung, J. I. "Preparation and Characterization of Structurally Stable Hexagonal and Cubic
Mesoporous Silica Thin Films." Journal of Sol-Gel Science and Technology 31 (2004):
179-183.

4. Stein, Andreas. "Hybrid Inorganic-Organic Mesoporous Silicates - Nanoscopic Reactors
Coming of Age." Advanced Materials 12.19 (2000): 1403-1417.

5. Raman, Narayan. "Template-Based Approaches to Preparation of Amorphous, Nanoporous
Silicas." Chem. Mater. 8 (1996): 1682-1701.

60
Chapter 7. Future Work

The integration of sol-gel thin films with high Quality optical cavities has opened the door
to new applications in biosensing and telecommunications. High refractive index coatings
applied to the surface of the microcavities has demonstrated a new method to tune the Purcell
factor of the microcavity, an important consideration in nonlinear optics and lasing applications.
Rare-earth lasing can be performed at low power thresholds due to the high intensity of the
optical field in microresonators. Using a smaller mode volume and higher optical intensities can
reduce rare-earth element lasing thresholds. Using high refractive index sol-gel thin films can
increase the optical intensity and reduce the mode volume in the microresonator. Future work
should investigate the doping of the high refractive index sol-gels with rare-earth elements to
potentially reduce the lasing threshold even further.
UV curing has been studied to examine the potential for the biological doping. While UV
cured films are not able to withstand the fabrication techniques for microtoroids, these thin films
could be applied on top of microtoroids while still retaining their biological functionality. Future
work should be done to incorporate an organic dye or biologically active material. These films
could shed light on the retained functionality of biological molecules in entrapped in the sol-gel
thin film. Doped sol-gel thin films lend to optical sensing methods discussed in previous
chapters.
Silicate surfactant sol-gel films could give valuable information on the diffusion of specific
molecules through porous media. Because of their ability to conduct light, and provide a porous
medium, porous silica films can be easily integrated into an optical sensing device. While future
work will need to be done to apply the surfactant sol-gel to the surface of the microtoroids, there
remain abundant applications of a hybrid porous optical sensor. The porous silica’s effect on the
quality factor will need to be studied further. Low-dielectric constant thin films could potentially
increase the Q-factor of microresonators.

61
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Abstract (if available)

Abstract The following thesis will discuss new applications of sol-gel derived materials designed to tune specific optical properties of optical resonant cavities. Sol-gel thin films are modified through the use of titanium precursors, surfactant templating models, and annealing method. Optical properties of the sol-gel thin films can be studied using high Quality factor microcavities. Characterization is also preformed using Fourier Transform Infrared (FTIR) Specroscopy, Spectroscopic Ellipsometry, and Scanning Electron Microscopy (SEM). ❧ Using a combination of Finite Element modeling and resonant wavelength tracking, we can determine the thermo-optic coefficient and materials loss of a high refractive index composite sol-gel thin film. Determining a material’s thermo-optic coefficient is important to determine its suitability in optical switches. For doping purposes, a low temperature curing method is studied using ultraviolet radiation as a substitute for thermal energy will be discussed in Chapter 5. Finally, Chapter 6 will study the deposition of an ordered porous thin film can be templated using organic surfactant molecules. ❧ These three methods of modifying sol-gel thin films present important applications to that can modify the optical properties of resonant cavities. Further examination of the thin films properties is important for their applications in optical sensing and I will discuss the ramifications of their applications in integrated optical systems.

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Creator Rose, Brian Andrew (author)

Core Title Silica sol-gel thin film coatings for integrated photonic devices

School Viterbi School of Engineering

Degree Master of Science

Degree Program Chemical Engineering

Publication Date 06/11/2012

Defense Date 06/01/2012

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,optical microcavity,sol-gel,thin films

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Advisor Armani, Andrea M. (committee chair), Nutt, Steven R. (committee member), Willner, Alan E. (committee member)

Creator Email brian.a.rose11@gmail.com,brianaro@usc.edu

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