design, fabrication, and testing of an integrated … design, fabrication, and testing of an...
TRANSCRIPT
Design, Fabrication, and Testing of an Integrated Optical Hydrogen and Temperature Sensor
by
Nicholas Osawa Carriere
A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science
Graduate Department of the Edward S. Rogers Sr. Department of Electrical and Computer Engineering University of Toronto
© Copyright by Nicholas Osawa Carriere 2013
ii
Design, Fabrication, and Testing of an Integrated Optical
Hydrogen Sensor
Nicholas Osawa Carriere
Masters of Applied Science
Graduate Department of the Edward S. Rogers Sr. Department of Electrical and Computer Engineering
University of Toronto
2013
Abstract
In this thesis, the details of the design, fabrication, and characterization of an optical, integrated
hydrogen gas and temperature sensor are explored. The hydrogen sensor is implemented by
coating a ridge waveguide with a thin layer of palladium and shows very good response time and
detection response for hydrogen concentrations ranging from 0.5-4%, both of which compare
very favourably to similar existing technologies. Multiple film thicknesses were tested and it
was found that thinner films give a faster response time at the expense of a reduced detection
response. The temperature sensor is implemented with a multi-mode interferometer coupled ring
resonator and has a sensing range of 100 K with good sensitivity. Both sensors are fabricated on
a silicon-on-insulator platform and could easily be integrated together onto a single chip as part
of an optical nose technology that would have the ability to sense multiple environmental factors
simultaneously.
iii
Acknowledgments
First and foremost, I would like to thank my supervisor, Professor Stewart Aitchison. His
helpful guidance and support throughout my studies have been invaluable. Someone I would
also like to thank, who had a role which was almost equally as important, is Muhammad Alam.
This work would not have been possible without him and I owe him a great deal of gratitude for
his patience and help.
There are many other colleagues and professors who I have to thank for their help throughout my
studies: Farshid Bahrami for his help in the cleanroom and for his support in the early part of the
project, Sean Wagner for his instruction and patience in the lab, Arash Joushaghani for his
insights and help with testing and fabrication, Pisek Kultavewuti for his support throughout the
project, Xiao Sun for her help with fabrication, Niklas Caspars and Ksenia Dolgaleva for their
support in the characterization lab, Mariya Yagnyukova and Sutha Sathananthan for the helpful
discussions concerning fabrication, Professors Mo Mojahedi and Glenn Hibbard for the
discussions and support, and last but not least, Alex Tsukernik, Henry Lee, and Yimin Zhou for
ensuring that my cleanroom visits were efficient and productive.
Finally, I would like to thank my family and my girlfriend, Emily. Their continuing love and
support is very much appreciated.
Ackn
Table
List o
List o
Chap
In1
1.
1.2
Chap
Li2
2.
2.2
2.3
2.4
Chap
Li3
3.
3.2
Chap
nowledgmen
e of Content
of Tables ....
of Figures ...
pter 1 Introdu
troduction ..
1 Motivatio
2 Summary
pter 2 Literat
iterature Rev
1 Existing H
2 The Palla
3 Palladium
2.3.1 In
2.3.2 Gr
2.3.3 In
4 Table of C
pter 3 Literat
iterature Rev
1 Integrated
2 Fibre Sen
pter 4 Hydrog
nts ................
s .................
...................
...................
uction .........
...................
on for Optica
y of Thesis ..
ture Review:
view: Hydrog
Hydrogen Se
dium Hydro
m-Based Hyd
nterferometri
rating-Based
ntensity-Base
Comparison
ture Review:
view: Tempe
d Sensors ....
nsors ............
gen Sensor D
Tab
....................
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....................
....................
....................
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al Hydrogen
....................
: Hydrogen S
gen Sensors
ensor Techno
ogen System
drogen Senso
c-Based Sen
d Sensors .....
ed Sensors ...
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: Temperatur
erature Senso
....................
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Design .........
iv
ble of Co
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Sensors ......
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Sensors .......
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ologies ........
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ors ...............
nsors ............
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re Sensors ...
ors ...............
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Hy4
4.
4.2
Chap
Hy5
5.
5.2
5.3
5.4
Chap
Hy6
6.
6.2
6.3
6.4
6.5
Chap
Th7
ydrogen Sen
1 Waveguid
2 Palladium
pter 5 Hydrog
ydrogen Sen
1 Waveguid
5.1.1 Pa
5.1.2 Pa
2 Lift-Off P
3 Metal Dep
4 Hydrogen
pter 6 Sensor
ydrogen Sen
1 Experime
2 Losses ....
3 Temporal
6.3.1 Re
6.3.2 Re
4 Hysteresi
6.4.1 Re
6.4.2 Re
5 Response
6.5.1 Re
6.5.2 Re
pter 7 The Op
he Optical N
nsor Design .
de Width and
m Layer Dim
gen Sensor F
nsor Fabricat
de Fabricatio
attern Expos
attern Etchin
Preparation .
position and
n Sensor Fab
r Characteriz
nsor Characte
ental Setup ..
...................
l Response ..
esults for Pa
esults for Pa
s .................
esults for 2-5
esults for 1 n
Time .........
esults for 2-5
esults for 1 n
ptical Nose:
Nose: Design
....................
d Pedestal H
mensions .......
Fabrication ..
tion ..............
on ................
ure and Dev
ng .................
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d Lift-Off .....
brication Res
zation ...........
erization .....
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alladium Dep
alladium Dep
....................
5 nm Palladi
nm Palladium
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5 nm Palladi
nm Palladium
Design and
n and Implem
v
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Height ..........
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velopment ....
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sults .............
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positions Ran
position of 1
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ium Deposit
m Deposition
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ium Deposit
m Deposition
Implementa
mentation of
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nging from 2
nm .............
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tion ..............
n .................
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tion ..............
n .................
ation of a Te
a Temperatu
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emperature S
ure Sensor ...
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7.
7.2
7.3
7.4
Chap
Co8
8.
8.2
Refer
1 The Optic
2 Temperat
3 Temperat
4 Temperat
7.4.1 Ex
7.4.2 Re
pter 8 Conclu
onclusions a
1 Summary
2 Future W
8.2.1 Pa
8.2.2 Ex
rences .........
cal Nose .....
ture Sensor D
ture Sensor F
ture Sensor C
xperimental
esults ..........
usions and F
and Future W
y of Contribu
ork .............
alladium All
xtensive Cha
...................
....................
Design .........
Fabrication ..
Characteriza
Setup ..........
....................
uture Work .
Work .............
utions ...........
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oys to Redu
aracterization
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vi
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ation .............
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uce Hysteresi
n of Thin Pa
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is Effects .....
alladium Film
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vii
List of Tables
2.1 Summary of existing hydrogen sensor technologies.
5.1 Parameters used for the RIE etching recipe.
5.2 Details of the lift-off recipe used for hydrogen sensor fabrication.
viii
List of Figures
2.1 Phase diagram of the palladium-hydrogen system showing α and α’ (β) phases.
2.2 The absorption process of hydrogen into palladium. a) Palladium atoms before hydrogen
absorption. b) Palladium lattice is in the α-phase as hydrogen atoms are absorbed. c)
Lattice expands as more hydrogen is absorbed and lattice enters the β-phase. d) After the
palladium is flushed with nitrogen, the lattice mostly recovers, but retains some
deformation.
2.3 MZI-based hydrogen sensor by Butler.
2.4 Schematic of the LPG-based hydrogen sensor by Kim.
2.5 Schematic of the fs-laser processed micro-cavity hydrogen sensor by Wang.
2.6 Schematic (a) and cross-section (b) of the integrated MZI sensor by Bearzotti.
2.7 Schematic of the intrinsic cavity sensor by Maciak.
2.8 Schematic of the extrinsic FP cavity sensor by Yang.
2.9 Schematic of a typical fibre Bragg grating and refractive index profile.
2.10 Reflection spectrum of a typical Bragg grating.
2.11 Schematic of the Bragg grating hydrogen sensor by Sutapun.
2.12 Schematic of the Bragg grating sensor by Schroeder.
2.13 a) Sensor response as a function of hydrogen concentration for T = 30oC. b) Sensor
response as a function of hydrogen concentration for T = 100oC, T = 150oC, T = 200oC.
2.14 Structure of a typical micromirror sensor.
2.15 Detection response (a) and response time (b) of Pd micromirror sensor as a function of
gas temperature at when exposed to a constant 4% atmospheric hydrogen concentration.
ix
2.16 Detection response of the micromirror sensor as a function of optical power for a variety
of hydrogen gas temperatures.
2.17 Detection response of stripped MMF sensor to 0.6% hydrogen.
2.18 Structure of tapered SMF hydrogen gas sensor.
2.19 Detection response of tapered SMF sensor to hydrogen concentrations ranging from 1.8-
10%.
2.20 Schematic of SMF embedded into quartz fibre-holding circular groove.
2.21 Detection response of sensor coated in (a) 20 nm Pd and (b) 100 nm Pd, also showing
response and recovery times. c) Simulated transmission losses of the sensor when coated
in Pd layers ranging from 5-40 nm.
3.1 Schematic of the Fabry-Perot sensor.
3.2 Transmitted intensity of the device for cavity lengths of 1700 µm and 100 µm.
3.3 Schematic and SEM (inlaid) of the silicon ring resonator device making use of grating
couplers for input and output coupling.
3.4 Measured transmission spectrum of the ring resonator temperature sensor.
3.5 a) Transmission spectrum, showing a single resonance peak, for a range of temperatures
from 16-45oC
b) Sensitivity of the device for a range of waveguide widths.
3.6 a) Transmitted spectrum of the device, showing a single resonance peak, for temperatures
ranging from 21-22oC b) Location of the measured resonance peak over a range of
temperatures, showing a sensitivity of 77 pm/K.
3.7 Schematic of the bi-modal Y branch temperature sensor.
3.8 Spatial interference profile and output intensities for temperatures of 25oC (a) and for
300oC (b).
x
3.9 Output power at outputs A and B for temperatures ranging from 25-300oC.
3.10 Schematic of the multimode interferometer sensor.
3.11 Results of the multimode interferometer sensor compared to the bi-modal Y branch
sensor.
3.12 Asymmetric transmitted intensity as a function of temperature.
3.13 Simulated results showing how the sensitivity of the device can be tuned by the length of
the fibre ring.
3.14 Diagram of the fluorescent epoxy micromirror sensor.
3.15 Detected spectrum showing both the fluorescence signal as well as some of the input light
that has been reflected off the micromirror and coupled into the detection fibre.
3.16 a) Fluorescence intensity of the sensor with respect to temperature (ranging from 20-
100oC)
b) Time response of the fluorescent sensor compared to the time response of a
commercially available thermocouple.
3.17 Schematic of the Fabry-Perot cavity sensor.
3.18 a) Simulated reflectivity of the Fabry-Perot cavity as a function of refractive index of the
cavity material b) Reflected signal intensity detected by the OTDR for temperatures
ranging from -30-80oC.
3.19 OTDR trace for three temperature sensors on the same fibre, showing the detected
response for -30oC (indicated by line a, in black) and for 30oC (indicated by line b, in
red).
3.20 Diagram of the tapered fibre MMI sensor.
3.21 a) Spectrum of the device output for temperatures ranging from -21.6-78.8oC
b) Output power of the device for temperatures ranging from -20-80oC.
xi
4.1 Schematic of the Pd-coated hydrogen sensor waveguide.
4.2 Structure of hydrogen sensor waveguide used in Lumerical MODE Solutions (Pd
thickness exaggerated for easy viewing).
4.3 Simulation results showing the simulated metal losses for different waveguide
configurations.
4.4 Simulated TE mode profiles for waveguides with a) no pedestal and b) 90 nm pedestal.
4.5 Simulation results showing the simulated metal losses for different waveguide
configurations assuming full sidewall deposition of the metal.
4.6 Simulated TE mode profile with no pedestal and full sidewall metal deposition.
5.1 CAD layout showing a section of the hydrogen sensor. Each cell (separated by crosses)
contains waveguides of different widths. Crosses are used for alignment during lift-off
stage.
5.2 CAD layout of the designed taper. Red indicates the non-exposed area where the
waveguide is defined. Blue indicates an area of low resolution exposure and green
indicates an area of high resolution exposure.
5.3 CAD layout of a section of waveguide.
5.4 Diagram showing the process of resist application and pattern etching. a) Un-etched SOI
wafer. b) Un-etched SOI wafer with positive resist that has been applied, exposed, and
developed into the desired pattern. c) SOI wafer after etching. d) SOI wafer after etching
and resist removal showing the waveguide ridge.
5.5 CAD layout of the lift-off mask. Red areas are chrome (opaque).
5.6 CAD layout of the waveguide and lift-off masks to illustrate how the lift-off procedure
works. a) CAD layout of a section of the waveguide mask. b) CAD layout of a section
of the lift-off mask. c) The lift-off mask superimposed on top of the waveguide mask.
The thin strips align perfectly with the waveguides, creating a window for metal
deposition. The square block is used as a marker and for thickness testing purposes.
xii
5.7 Diagram showing the metal deposition process. a) Etched SOI wafer. b) SOI wafer after
resist is applied, exposed, and developed. Resist creates a window for metal deposition.
c) Wafer after metal deposition. Metal is deposited only within the windows created by
the resist. d) Wafer after metal deposition and final resist removal. The end result is a
ridge waveguide with a strip of metal coating a section of it.
5.8 SEM image of the two adjacent troughs that were etched in order to create a single
waveguide.
5.9 Zoomed-in SEM image of a single etched waveguide.
5.10 SEM image of a 2 nm thick deposition of palladium (of lengths 8 µm and 12 µm) onto
two separate waveguides.
5.11 Zoomed-in SEM image of a 2 nm thick palladium deposition onto a single waveguide.
6.1 Schematic of the experimental setup for hydrogen sensor characterization.
6.2 Metal losses of the various Pd-coated waveguides.
6.3 Temporal response of a Pd-coated waveguide when exposed to hydrogen gas.
6.4 a) Detection response values for different lengths of palladium films (2 nm thickness) and
waveguides with different pedestal heights b) Average detection response of waveguides
for varying pedestal heights (2 nm palladium thickness).
6.5 Detection response for varying hydrogen concentrations for a waveguide with pedestal
height of 90 nm, palladium thickness of 3 nm and length of 20 µm.
6.6 Temporal response of a waveguide coated in 1 nm of Pd.
6.7 Plot showing the hysteresis effect present in a typical Pd-coated waveguide when
exposed to hydrogen.
6.8 Hysteresis loops for palladium thicknesses of 2,3, and 5 nm Pd depositions.
6.9 Hysteresis loop for palladium thickness of 1 nm.
xiii
6.10 Response time data for palladium thicknesses of 2,3, and 5 nm.
6.11 Theorized phase diagrams for thick and thin palladium films. a) Phase diagram for thick
palladium film (approaching bulk) b) Theorized phase diagram for very thin palladium
film.
6.12 Response time data for palladium thickness of 1 nm.
7.1 Diagram showing the relevant dimensions when designing an MMI coupler. i) Height of
the coupler (equal to the silicon top layer height minus the etch depth) ii) Center-to-
center input/output waveguide separation iii) MMI width iv) MMI length.
7.2 SEM image of a 4.5 µm long MMI coupler.
7.3 Zoomed-in SEM image of the input waveguide gap on an MMI coupler.
7.4 SEM image of MMI coupled ring resonator.
7.5 Schematic showing the experimental setup for temperature sensor characterization.
7.6 Transmission spectrum for one of the ring resonator temperature sensors (50/50 coupler,
4.5 µm wide coupling section, 5 µm ring radius).
7.7 A single resonance peak of the sensor for temperatures varying from 20-50oC.
7.8 Free spectral range (a), sensitivity (b), and Effective sensing range (c) of the device for
varying ring radii.
7.9 Intensity-based sensing scheme for the ring resonator temperature sensor. a)
Experimental data for a single resonance peak showing the transmitted intensity at a
specified wavelength for each temperature
b) Normalized transmission of the sensor as a function of temperature.
1
1.1
The h
sourc
Acco
cell m
colou
have
conce
Curre
devel
and
disad
to cre
Optic
comp
insula
fabric
integr
platfo
an op
packa
poten
1.2
This
chapt
Introd
Motiva
high price a
ces of energ
ording to the
market is po
urless, tastele
an inexpe
entrations in
ently, many
lopment incl
mass spect
dvantages, su
eate electrica
cal hydrogen
pact size, in
ator (SOI) is
cate using C
rate electron
orm, the num
ptical nose.
age, there w
ntially explo
Summ
thesis will b
ters 2 and 3
uction
ation for O
and volatile s
gy has creat
e Canadian H
oised to be
ess, and can
ensive devi
n case there i
different te
luding: semi
troscopy [3
uch as large
al sparks tha
n sensing off
ncreased saf
s an excellen
CMOS fabri
nics and pho
mber of sens
As there w
would be no r
sive environ
ary of Th
begin with a
respectively
I
Optical H
supply sourc
ted an inter
Hydrogen an
worth $8.5
be explosiv
ice that ca
is a leak.
echnologies
iconductor s
]. Unfortu
size, high c
at would be d
fers a numbe
fety and imm
nt platform fo
ication techn
otonics on th
sing element
would be no
risk of electr
nments.
hesis
a literature re
y. The detai
Chapterntroduct
Hydrogen
ce of oil as
rest in the u
nd Fuel Cel
billion by
ve in volume
an quickly,
for hydrog
sensors, ther
unately, the
ost, depende
dangerous in
er of advant
munity from
for implemen
nology, it is
he same platf
ts on a singl
need to hav
rical spark,
eview of rel
ils of the pal
r 1 tion
n Senso
well as the
use of altern
ll Associatio
2016 [1].
etric quantiti
, reliably
gen detectio
rmoelectric s
ese sensing
ence on the
n explosive e
tages over co
m electroma
ntation of op
s very comp
form. More
le chip coul
ve any elect
which make
levant hydro
lladium-hyd
rs
desire to m
native fuels
on, the globa
As hydrog
ies as low as
and safely
on are being
sensors, elec
g methods
presence of
environment
ompeting te
agnetic inter
ptical sensors
pact, and it
over, by usi
d be increas
trical compo
es the sensor
ogen and tem
drogen system
move towards
s, such as h
al hydrogen
gen gas is o
s 4% [2], it i
monitor h
g used or a
ctrochemical
have a nu
f oxygen, or
ts.
chnologies i
rference. Si
s as it is easy
has the pot
ng an integr
sed in order
onents on th
r very safe f
mperature se
m as well as
1
s greener
hydrogen.
and fuel
odourless,
is vital to
hydrogen
are under
l sensors,
umber of
potential
including
licon-on-
y to mass
tential to
rated SOI
to create
he sensor
for use in
ensors, in
s existing
2
palladium-based hydrogen sensors will be the subject of chapter 2, while chapter 3 will focus on
a review of integrated and fibre-based temperature sensors.
Chapters 4, 5, and 6 will focus on the design, fabrication, and characterization of the hydrogen
sensor. Chapter 4 will include the design details and decisions that are relevant to the hydrogen
sensor. Chapter 5 will include all of the fabrication steps and processes that were used in order
to create working samples of the hydrogen sensor. Chapter 6 will discuss the experimental
results that were obtained from testing the hydrogen sensor samples.
The topic of chapter 7 will be the idea of an optical nose as well as a theoretical and
experimental demonstration of a temperature sensor that could be integrated onto the same
platform as the hydrogen sensor. The design, fabrication and characterization of the temperature
sensor will all be thoroughly detailed.
Finally, chapter 8 culminates with a summary of the contributions of this thesis as well as some
possible area that future work could be done in order to improve and better understand the work
that this thesis is focused on.
2
2.1
Befor
woul
streng
exist
hydro
goes
Befor
and d
sensin
table
Tech
Catal
Therm
Cond
Elect
Resis
Literat
Existin
re we go int
d be good to
gths and we
both in acad
ogen sensor
into far mor
re discussing
disadvantage
ng can fit in
2.1.
hnology
lytic
mal
ductivity
trochemical
stance
Litera
ture Re
ng Hydro
to more det
o first get an
aknesses. T
demia and co
technologie
re details tha
g optical hyd
es of other ex
the current
Advantag
Ro
Wi
Sim
Re
Lo
Fa
Lo
Lo
ature Rev
eview: H
gen Sen
tails about th
n idea of the
There are a n
ommercially
es, an excell
an presented
drogen sensi
xisting techn
landscape. T
ges
obust
ide operating
mple configu
esistant to po
ow cost
st response t
ow power co
ow cost
Chapterview: Hy
Hydroge
nsor Tech
he theory an
other techno
number of di
y. For a mor
lent paper b
here.
ing in more d
nologies in o
These advan
g temperatur
uration
oisoning
time
nsumption
r 2 ydrogen S
en Sens
hnologie
nd operation
ologies that a
ifferent tech
re detailed d
by Hubert [3
detail, I will
order to bette
ntages and d
Disadv
re
Sensors
ors
s
n of optical
are available
hnologies an
description an
3] was recen
first summa
er understand
disadvantage
vantages
Not compl
hydrogen g
Requires o
the atmosp
Doesn’t ha
detection li
Cross sens
High cost
Cross sens
Limited lif
Humidity a
hydrogen se
e and their re
nd configurat
nd review of
ntly publishe
arize the adv
d where opti
s are summa
etely selectiv
gas
oxygen (5-10
phere for ope
ave a very lo
imit
itive to heliu
itive to CO
fetime
and temperat
3
ensors, it
espective
tions that
f existing
ed which
vantages
ical
arized in
ve to
0%) in
eration
w
um gas
ture
4
Fast response time interference
Poor selectivity
Requires oxygen for operation
Work Function Low cost
Small size
Possible to mass produce
with existing infrastructure
Sensor response drift
Hysteresis
Saturates at low concentrations
Mechanical Small size
No source of ignition (usable
in potentially explosive
environments without risk)
Slow response time
Susceptible to poisoning from
other gases
Table 2.1.: Table comparing the advantages and disadvantages of the different types of hydrogen sensors
2.2
The
pallad
(Tc =
figure
Fig
At ro
exists
under
phase
to th
hydro
phase
pallad
hydro
The Pa
palladium h
dium is exp
= 293oC) [4],
e 2.1.
g. 2.1: Phase dia
oom tempera
s solely in t
rgoes a phas
e. As a resu
he β-phase,
ogen that ha
es is observe
dium lattice
ogen atoms
alladium
hydrogen sy
osed to hyd
, PdH is a tw
agram of the p
atures (~300
he α-phase.
se transforma
ult of this, as
there is a
as been abso
ed. The α-ph
without for
bond with th
Hydroge
ystem is on
rogen gas, i
wo-phase sy
alladium-hydro
(1994) wit
K), at H/Pd
At H/Pd r
ation into th
s the materia
volume exp
orbed [6]. F
hase is a soli
rming chemi
he palladium
en Syste
ne of the m
it will absorb
ystem. The p
ogen system sh
th permission f
ratios lowe
ratios highe
he β-phase, w
al undergoes
pansion of a
For intermed
id solution p
ical bonds.
m lattice to f
em
most studied
b hydrogen
phase diagra
howing α and α
from Springer.
r than appro
er than appro
which has a l
s full phase
approximate
diate H/Pd r
phase where
The β-phas
form Pd-H b
d metal hyd
atoms. Bel
am of this sy
α’ (β) phases.
.
oximately 0.
oximately 0
larger lattice
transformati
ely 10% du
ratios, a mix
the hydroge
se is a hydri
bonds. The
drogen syste
low the criti
ystem can b
Reprinted from
015 [5], the
.6, the medi
e constant th
ion from the
ue to the am
xture of the
en atoms mix
ide phase, w
phase diagr
5
ems. As
ical point
e seen in
m [4], ©
e medium
ium fully
han the α-
e α-phase
mount of
α and β-
x into the
where the
ram is an
intere
comp
temp
and t
pallad
Fig.
P
absor
but r
Anoth
[6]. A
of fre
index
2.3
Palla
respo
senso
sensin
Each
explo
2.3.
Fund
hydro
esting one as
ponents of t
eratures abo
the solution
dium is illus
. 2.2: The abso
alladium lattic
rbed and lattice
etains some de
her notable p
As palladium
ee electrons
x of the mate
Palladi
dium is an
onse times,
ors can be d
ng. The th
of these c
ored below.
1 Inter
damentally, i
ogen, it sees
s it displays
the system
ove the uppe
n will behav
strated in figu
rption process
e is in the α-ph
e enters the β-p
eformation. Rep
property of p
m absorbs hy
. This chan
erial, which i
ium-Bas
excellent tr
and minim
divided into
hree categor
onfiguration
rferometric
nterferometr
a volume ex
an upper cri
(Pd and H)
er critical so
ve like the
ure 2.2.
of hydrogen in
hase as hydrog
phase. d) After
printed from [7
palladium th
ydrogen, the
nge in electr
it a very use
ed Hydro
ransducer fo
mal cross-sen
three diffe
ries are: int
ns offers the
c-Based S
ric-based sen
xpansion as
itical solutio
) are miscib
olution tempe
α-phase.
nto palladium.
en atoms are a
r the palladium
7], © (2010) w
Hydrogen Ene
hat changes a
e reduced de
ron density l
ful transduc
ogen Se
or hydrogen
nsitivity. T
rent categor
erferometric
eir own adv
Sensors
nsors rely on
well as a ch
on temperatu
ble in all pr
erature, ther
The absorp
a) Palladium a
absorbed. c) La
m is flushed with
with permission
ergy.
as it absorbs
ensity of the
leads to a c
er for hydro
ensors
n sensors as
The majority
ries, based
c-based, gra
vantages and
n the fact th
hange in its c
ure (labeled
roportions.
re is only a
ption proces
atoms before h
attice expands
h nitrogen, the
n from the Inter
s hydrogen is
metal leads
change in the
ogen gas dete
s it offers g
y of palladi
on the optic
ating-based,
d disadvant
hat when pal
complex ref
Tc), above w
This mean
solid solutio
ss of hydro
hydrogen absor
as more hydro
e lattice mostly
rnational Asso
s the electro
to a reduce
e complex r
ection.
good sensitiv
ium-based h
cal scheme
and intensi
tages, which
lladium is ex
fractive index
6
which the
ns that at
on phase,
ogen into
rption. b)
ogen is
y recovers,
ciation of
n density
d density
refractive
vity, fast
hydrogen
used for
ty-based.
h will be
xposed to
x. These
7
two changes can change the properties of an interferometric device by changing the optical path
length. This change in optical path length can be detected at the output of the interferometer and
correlated to the hydrogen concentration.
Interferometric sensors were the first palladium-based hydrogen sensors to be demonstrated in
the literature by Butler, in 1984, who was able to demonstrate a fibre-based Mach-Zehnder
interferometer, with one of the fibre arms having a 3 cm long section coated in a 10 µm thick Pd
layer [8]. Along with the Fabry-Perot cavity, the Mach-Zehnder is one of the two most
commonly used devices for interferometric-based hydrogen sensing. These two sensing
modalities will be explored shortly.
Mach-Zehnder Interferometer
A typical Mach-Zehnder interferometer sensor works by splitting a single input light source into
two separate arms: a reference arm and a sensing arm. The reference arm is isolated from the
external variation that is to be detected and the sensing arm is exposed to this variation. In order
to make the sensing arm respond to hydrogen, the sensing arm requires some sort of transducer
(i.e. Palladium) that is sensitive to hydrogen and is able to induce a change in optical path length
in the sensing arm. In this case, the change in optical path length is induced by the volume
expansion and change in refractive index of the palladium transducer upon exposure to
hydrogen. By not having a similar transducer on the reference arm, we are effectively
preventing it from having any significant change in optical path length due to hydrogen
exposure.
In this scheme, we are able to detect a change in the output intensity of the device:
1 cos ∆ . (2.1)
The Δφ term is the relative phase shift of the light in the reference and sensing arms, which is
induced by the change in optical path length in the sensing arm during hydrogen exposure.
As previously mentioned, the first demonstrated MZI hydrogen detector was by Butler in 1984
[8]. The device consisted of two single mode fibres which served as the reference and sensing
arms. A 3 cm section of the sensing arm was coated in a 10 µm thick layer of Pd which was
sputte
figure
Fig
To d
chang
senso
Subse
28 cm
senso
than 3
Whil
stead
shift
LPGs
[10].
in the
must
and t
outer
ered over a
e 2.3.
g. 2.3: MZI-ba
detect chang
ge in optical
or was able
equent impro
m and reduc
or was able t
30 seconds.
e traditional
dily abandon
to the inline
s are a type
The functio
e core of th
be set such
the phase ma
r cladding m
10 nm thick
ased hydrogen
ges in hydro
l path length
to detect 0
ovements to
cing the Pd l
to detect hy
l MZIs use t
ned in favour
e MZI was
of fibre Bra
on of an LP
e fibre to th
that the dif
atching vect
odes. The p
k titanium la
sensor by Butl
ogen concen
h of the sen
.6% (v/v) h
o this configu
layer thickn
drogen conc
two separate
r of inline M
precipitated
agg grating
G is to trans
he cladding
fference betw
tor of the gr
principle of o
ayer for impr
ler. Reprinted
Physics Lette
ntration, the
sing arm wa
hydrogen wi
uration invol
ness to 1 µm
centrations a
e arms for s
MZIs, which
d by the adv
(FBG) with
sfer a portion
modes. In
ween the pro
rating is equ
operation of
roved adhes
from [8], © (1
ers.
number of
as counted a
ith a respon
lved extendi
m [9]. With
as low as 2 p
sensing and
h use only a
vent of the l
h periods typ
n of the pow
order to acc
opagation co
ual to the pro
LPGs can b
sion. The de
984) with perm
f shifted fri
at the output
nse time of
ing the sensi
the thinner
ppb with a r
reference, th
single arm
long period
pically betwe
wer from the
complish thi
onstant of th
opagation co
e seen in mo
evice is illus
mission from A
nges created
t of the dev
less than 3
ing arm to a
palladium l
response tim
his scheme
instead of tw
fibre grating
een 100 µm
e fundament
is, the gratin
he fundamen
onstant of on
ore detail in
8
strated in
Applied
d by the
ice. The
minutes.
length of
layer, the
me of less
has been
wo. The
g (LPG).
m – 1 mm
tal modes
ng period
ntal mode
ne of the
[11].
A pop
core
and t
separ
requi
In ter
(with
The m
sectio
a clad
effect
absor
blue
time
with
Fig.
Pd-ba
demo
bulk
inline
portio
pular techni
to the fibre
the core serv
rate arms as
ired.
rms of hydro
h 500 µm gr
method of o
on containin
dding mode.
tive index is
rbs hydrogen
shift in the
for this devi
the sensitivi
. 2.4: Schemati
ased MZI hy
onstrated a M
silica substr
e MZI. The
on of the fib
que in sensi
cladding. I
ves as the re
it is easy to
ogen sensors
rating period
operation fo
ng the grating
. The claddi
s affected by
n, the effecti
output spec
ice was rathe
ity saturating
ic of the LPG-b
ydrogen sen
MZI hydroge
rate, as seen
path through
re core acts
ng involves
n this case,
eference arm
fabricate an
s, Kim demo
d) coated in
or this senso
g, a portion o
ing mode is
y the refracti
ive index of
ctrum of the
er slow as it
g at approxim
based hydrogen
nsors have al
en sensor us
n in figure 2
h the cavity
as the refere
using a LPG
the fibre cla
m. This met
nd is much e
onstrated an
a 50 nm lay
or relies on
of the power
close enoug
ive index of
f the claddin
e device whe
t exhibited o
mately 8 min
n sensor by Ki
from SPIE
lso been dem
sing a micro
2.5 [13]. In
acts as the s
ence arm.
G to couple
adding effec
thod is muc
easier to cha
LPG sensor
yer of pallad
the properti
r in the fund
gh to the oute
the palladiu
ng mode is in
en exposed
only -0.29 nm
nutes.
im. Reprinted
E.
monstrated u
o-cavity MZI
n this case, t
sensing arm
a portion the
ctively serve
h more prac
aracterize as
r that used a
dium, as see
ies of the L
damental mo
er surface of
um layer. A
ncreased, wh
to 4% hydr
m/min reson
from [12], © (
using fs-lase
I that was fs
the micro-ca
while the pa
e light from
es as the sen
ctical than u
only a singl
a 50 mm lon
en in figure
LPG. In the
de in pushed
f the claddin
s the palladi
hich causes
rogen. The
ant wavelen
(2008) with per
er processing
fs-laser ablat
avity functio
ath through t
9
the fibre
nsing arm
using two
le fibre is
ng grating
2.4 [12].
e 50 mm
d out into
ng that its
ium layer
a 2.3 nm
response
ngth shift,
rmission
g. Wang
ted into a
ons as an
the lower
Fig. 2.
A Sp
wave
powe
Micro
appro
and
hydro
for th
Even
been
hydro
on a
layer
30 se
Fig
.5: Schematic o
pectral Phys
elength of 80
er of 12 mW
o-cavity len
oximately 11
125 µm, re
ogen concen
he 40 µm cav
n though the
some work
ogen sensor
LiNbO3 pla
, as seen in
econds. The
. 2.6: Schemati
of the fs-laser p
sics fs-laser
00 nm, puls
W) and a m
ngths of 40
10 nm thick
spectively.
ntration, the
vity and 0.04
bulk of MZ
k done in i
on a LiNbO
atform, with
figure 2.6.
fall time wa
ic (a) and cross
processed micr
with perm
r was used
e width of 1
magnetron sp
µm and 1
. The core
When plot
sensor was a
42 nm/% for
ZI-based sen
integrated p
O3 platform [
a portion o
The sensor w
as also very f
s-section (b) of
(1992) wi
ro-cavity hydro
mission from Op
to create t
120 fs, repet
puttering sys
00 µm are
and cladding
tting the re
able to exhib
r the 100 µm
sors have be
platforms. B
14]. The de
of the sensin
was able to
fast, at only
f the integrated
ith permission
ogen sensor by
ptics InfoBase.
the micro-ca
tition rate o
stem is used
tested and
g diameters
sonance shi
bit a resonan
m cavity.
een demonst
Bearzotti de
evice consist
ng arm coat
detect 1% h
10 seconds.
d MZI sensor b
from Elsevier.
y Wang. Repri
.
avity (laser
of 1 kHz, av
d to deposit
the Pd film
of the singl
ift versus ch
nce shift as
trated in opt
emonstrated
ts of an integ
ed in a 120
hydrogen wit
by Bearzotti. R
inted from [13]
parameters
verage on-tar
t the palladi
m thickness
le-mode fibr
hance in vo
great as 0.1
tical fibres,
d an integra
grated, two-
nm thick p
th a respons
Reprinted from
10
], © (2012)
s: central
rget laser
ium film.
s used is
re are 8.2
olumetric
55 nm/%
there has
ated MZI
arm MZI
palladium
e time of
m [14], ©
Fabr
A Fa
distan
either
cavity
pallad
The t
F is
length
pallad
the c
which
The
optic
stand
layer
as see
Fig. 2
ry-Perot Cav
abry-Perot c
nce, d (cavit
r constructiv
y. A hydrog
dium as one
transmission
the finesse
h, and n is
dium metal
avity will ch
h can be dete
most comm
al fibre (kno
dard multimo
which is de
en in figure
2.7: Schematic
vity
cavity is for
ty length). B
vely or destr
gen gas sens
of the reflec
n through the
of the cavit
the refractiv
absorbs hyd
hange. Both
ected at the
monly used c
own as an in
ode fibre wit
eposited on t
2.7.
c of the intrinsi
rmed by cre
By changing
ructively at
sor can be c
ctive surface
e cavity can b
ty and is re
ve index of
drogen gas, t
h of these ch
output and c
configuratio
ntrinsic cavit
th 62.5 µm c
op of a 155
ic cavity sensor
eating two p
g the cavity l
certain wav
reated by co
es.
be expressed
1
1
lated directl
the cavity.
the cavity len
hanges will
correlated to
on is to use
ty). Maciak
core diamete
nm NiOx lay
r by Maciak. R
Elsevier.
partially ref
length, the b
velengths, du
oating the ca
d as:
2 .
ly to the mi
If the cavit
ngth will inc
shift the res
ambient hyd
a Fabry-Pe
k was able to
er [15]. The
yer which is
Reprinted from
flecting surf
beam can be
ue to the re
avity with p
irror reflecti
ty is coated
crease and t
sonant wave
drogen gas c
erot cavity
o demonstra
device is ma
s deposited o
m [15], © (2007
faces separa
made to inte
esonant natu
alladium or
ivity, d is th
in palladium
the effective
elength of th
concentratio
contained w
ate such a de
ade up of a 1
on the tip of
7) with permis
11
ated by a
erference
ure of the
by using
(2.2)
he cavity
m, as the
index of
he cavity,
n.
within an
evice in a
10 nm Pd
the fibre,
sion from
The N
interf
interf
the c
senso
hydro
time
Anoth
know
use h
less e
align
cavity
made
then
senso
Fig. 2
The m
the P
the ou
into t
was t
length
(appr
NiOx layer s
face of the s
face of the N
changing sec
or was tested
ogen, the se
of 30 second
her way to c
wn as an extr
higher reflect
expensive to
ed and prese
y sensor has
e up of a hol
inserted into
or can be see
2.8: Schematic
method of o
Pd-Ag layer
uter tube dir
the tube. Th
tested in hyd
h of up to
roximately 2
serves as the
silica fibre an
NiOx layer an
cond mirror
d in hydroge
nsor showed
ds.
create a Fab
rinsic cavity.
tivity mirror
o fabricate.
ent notable p
s been demo
llow silica tu
o the tube fr
en in figure 2
of the extrinsic
operation for
on the silica
rectly increa
his increase i
drogen conc
approximat
20-30 minute
e cavity of t
nd the NiOx
nd the Pd lay
reflectivity
en concentra
d a change
bry-Perot cav
. Extrinsic s
rs (higher in
Some notab
packaging ch
nstrated by
ube, which h
rom both en
2.8.
c FP cavity sen
r the sensor
a tube absorb
ases the leng
in gap length
entrations v
tely 9 nm w
es).
the device.
x layer and th
yer. In this c
as the pall
ations rangin
in transmiss
vity is by us
sensors are a
ndex differen
le disadvant
hallenges [16
Yang, in 20
has 100 nm o
nds, leaving
nsor by Yang.
Optics InfoBa
is as follow
bs hydrogen
th of the air
h changes th
arying from
was observe
The first re
he second re
case, there is
ladium layer
ng from 0.5-3
sion of appr
sing an air g
advantageou
nce between
tages are the
6]. One exa
010 [17]. Th
of Pd-Ag de
an air gap i
Reprinted from
ase.
ws: when the
n and expand
r gap betwee
he output spe
m 0.2-4%. A
ed, but the
eflective surf
eflective sur
s a change in
r is exposed
3% in air. W
roximately 4
gap between
us in the way
n air and silic
e fact that th
ample of an e
he outer clad
eposited on i
in the middl
m [17], © (201
e sensor is e
ds in volum
en the two fi
ectrum of th
At 4% hydrog
response tim
face is locat
rface is locat
n transmissio
d to hydrog
When expos
40% with a
n two fibres
y that they ar
ca) and are g
hey must be
extrinsic Fab
dding of the
it. A 130 µm
e. A diagra
10) with permis
exposed to h
e. The expa
bres that are
he cavity. Th
gen, a chang
me was rat
12
ted at the
ted at the
on due to
gen. The
ed to 3%
response
. This is
re able to
generally
precisely
bry-Perot
sensor is
m fibre is
am of the
ssion from
hydrogen,
ansion of
e inserted
he sensor
ge in gap
her slow
2.3.2
The
refrac
chang
When
senso
but th
mech
Brag
A Br
struct
typic
By al
By c
interf
figure
2 Grat
majority of
ctive index
ges the spect
n it comes
ors in use: B
he key simil
hanism.
gg Gratings
ragg grating
ture such as
al device.
Fi
lternating re
ontrolling th
fere construc
e 2.10 for a t
ing-Based
f grating-bas
when palla
tral response
to palladium
ragg grating
arity is that
g is created
an optical fi
ig. 2.9: Schema
fractive indi
he grating p
ctively at a d
typical refle
d Sensors
sed hydroge
dium is exp
e of the grati
m-based hyd
gs and long-p
both device
by alternati
fibre or an in
atic of a typica
ices, the dev
period and t
desired wave
ction spectru
s
en sensors r
posed to hy
ing and is th
drogen sens
period gratin
s make use o
ing the refra
ntegrated wa
al fibre Bragg g
vice creates a
he refractiv
elength, whi
um of a Brag
rely on the
ydrogen. Th
e sensing me
ors, there a
ngs. The me
of a shifting
active index
aveguide. Se
grating and refr
a series of su
e indices, th
ich effective
gg grating.
fact that th
his changed
echanism of
are chiefly t
ethods of op
g spectral res
x in a period
ee figure 2.9
fractive index p
uccessive ba
hese reflecti
ly creates a
here is a ch
d in refracti
f the device.
two types o
peration are d
sponse as the
dic fashion
9 for a schem
profile.
ackwards ref
ions can be
stop band fi
13
hange in
ve index
f grating
different,
e sensing
within a
matic of a
flections.
made to
ilter. See
The c
as:
Wher
theor
The f
Sutap
wave
the in
sectio
µm d
of the
center wavel
re neff is the
ry and operat
first palladiu
pun [19]. T
elength, etch
nteraction be
on was etche
diameter. Fr
e sensor can
length of the
effective in
tion of Brag
Fig. 2
um-based hy
The device c
hed into a 12
etween the c
ed with chlo
rom here, a 5
be seen in f
e stop band i
ndex of the g
g gratings ca
2.10: Reflectio
ydrogen sens
consisted of
25 µm single
core fibre m
oroform to d
560 nm Pd l
figure 2.11.
is referred to
2
grating and Ʌ
an be seen in
on spectrum of
sor using a B
f a 2-3 cm l
e mode fibre
mode and the
decrease the
layer was de
o as the Bra
Λ .
Ʌ is the grat
n [18].
a typical Brag
Bragg gratin
long Bragg
e with 5-10
e palladium c
fibre diame
eposited onto
gg waveleng
ting period.
gg grating.
ng was demo
grating, wit
µm diamete
coating, the
eter from 12
o the grating
gth, and is c
More detai
onstrated in
th 829.73 n
er core. To
2-3 cm lon
5 µm diame
g section. A
14
calculated
(2.3)
ils on the
1999 by
nm Bragg
o increase
g grating
eter to 35
A diagram
Fi
When
Due t
expos
could
layer
Schro
This
create
The b
is the
figure
Fig
When
respo
ig. 2.11: Schem
n exposed to
to the fact th
sed to hydro
d be reduced
are chromiu
oeder was ab
sensor is m
e a large ev
bent section
en polished
e 2.12 for a
. 2.12: Schema
n exposed t
onse time o
matic of the Br
o 1.8% hydro
hat a relative
ogen concent
d or an adhe
um or titaniu
ble to demo
made up of a
anescent fie
of the fibre
and a 50 nm
schematic of
atic of the Brag
to 4% hydro
f 30 s. A
agg grating hy
perm
ogen gas, the
ely thick pal
trations high
esion layer c
um [20].
onstrate a sim
a 1.5 mm lo
eld compone
is then cem
m palladium
f the sensor.
gg grating sens
ogen, the se
At lower con
ydrogen sensor
mission from E
e sensor exh
lladium laye
her than 1.8%
could be em
milar sensor
ong Bragg g
ent, the fibre
mented into a
m layer is de
sor by Schroede
from Elsevie
ensor exhibi
ncentrations
by Sutapun. R
Elsevier.
hibited a 5 pm
er was used,
%. To comb
mployed. Ty
r with a diff
grating etche
e is slightly
a glass block
eposited ont
er. Reprinted
er.
ited a 4 pm
(1%), the
Reprinted from
m red-shift i
the film ten
bat this issue
ypical mater
fferent type
ed into a sin
bent with a
k. The surfa
o the surfac
from [21], © (
m red-shift w
sensor resp
m [19], © (1999
in Bragg wav
nded to peel
e, the layer t
rials for an
of construct
ngle-mode fi
bend radius
ace of the gla
ce of the blo
2009) with per
with a relati
ponse red-sh
15
9) with
velength.
off when
thickness
adhesion
tion [21].
fibre. To
s of 2 m.
ass block
ock. See
rmission
ively fast
hifted by
appro
pallad
hydro
Long
Long
sectio
gratin
fibre
top o
When
with
at hig
reduc
expec
temp
figure
Fi
fun
oximately 1
dium were a
ogen).
g-Period Gr
g-period grat
on 2.2.1) an
ng-based LP
with a CO2
f the grating
n exposed to
a relatively
gher tempera
ced. The sen
cted, as the
eratures [5].
e 2.13.
ig. 2.13.: a) Sen
nction of hydro
pm and ha
also tested,
ratings
tings are ver
d grating-ba
PG sensor in
laser. Foll
g section by D
o 4% hydrog
fast respons
atures. At 1
nsor only sh
e hydrogen
. The senso
nsor response a
ogen concentrat
ad a respons
but they sh
rsatile as th
ased configu
2008 [22].
owing LPG
DC magnetr
gen gas, the
se time of 70
100oC, the re
howed a 0.4
absorption
or response f
as a function o
tion for T = 10
perm
se time of a
owed a muc
hey can be u
urations. We
The 500 µm
fabrication,
ron sputterin
e sensor exh
0 s at a temp
esponse time
1 nm blue-s
capacity o
for temperat
of hydrogen con
00oC, T = 150oC
mission from E
approximatel
ch lower sen
used in both
ei was able
m period LPG
, a 70 nm pa
ng.
hibited a blu
perature of 3
e was simila
hift at the h
of palladium
tures varying
ncentration for
C, T = 200oC.
Elsevier.
ly 2 minute
nsitivity (2
h interferom
to demonstr
G was etche
alladium lay
ue shift of ap
30oC. The se
ar, but the se
higher tempe
m is greatly
g from 30-2
r T = 30oC. b)
Reprinted from
es. Thinner
pm red-shif
etric-based
rate an exam
ed into a sing
yer was depo
pproximately
ensor was al
ensitivity wa
erature. This
y reduced a
200oC can b
Sensor respon
m [22], © (200
16
layers of
ft for 4%
[12] (see
mple of a
gle mode
osited on
y 4.3 nm
lso tested
as greatly
s is to be
at higher
e seen in
nse as a
08) with
2.3.3
Unlik
senso
expan
light
three
The t
which
Micr
Micro
upon
struct
diam
Pd fi
illustr
Fig.
When
22%,
Intere
work
diam
Bene
funct
3 Inten
ke interferom
ors generally
nsion. Fund
signal upon
major types
three types a
h will all be
romirror Se
omirror sens
exposure to
ture of the d
eter of 50 µm
ilm. The de
rated in figu
2.14.: Structur
n exposed to
, but no relia
ested by the
k by Butler b
eter MMF c
vot’s paper
tion of gas te
nsity-Base
metric and
y relies only
damentally, i
n exposure t
s of intensity
are micromir
discussed in
nsors
sors rely on
o hydrogen g
device was q
m and cladd
evice, which
ure 2.14.
re of a typical m
o hydrogen g
able response
e inherent si
by creating
coated in a h
is a thoroug
emperature.
ed Sensor
grating-base
on the chang
intensity-bas
to hydrogen,
y-based sens
rror sensors,
n more detail
the changin
gas. The firs
quite simple
ding diameter
h is very sim
micromirror se
gas, the refle
e time inform
mplicity of
a sensor co
hard polyme
gh analysis o
As discusse
rs
ed sensors,
ging refracti
sed sensors r
, whether it
sors, each ha
, evanescent
l.
ng reflectivit
st such devic
and was co
r of 125 µm
milar in stru
ensor. Reprinte
ectivity of th
mation was p
the sensor,
onsisting of
er layer [25]
of the senso
ed in section
the hydrog
ive index of
rely on a dir
is in reflec
aving a sligh
t field senso
ty of a meta
ce was presen
omposed of
m) with its cle
ucture to ne
ed from [24], ©
he micromir
presented.
in 2000 Be
a 13 nm th
]. The uniq
or response t
n 2.1, as tem
gen respons
f the metal, ra
rect change i
ction or tran
htly different
ors, and surfa
al as its refra
nted by Butl
a cleaved m
eaved end co
early all mic
© (2003) with
rror was red
nevot follow
hick Pd laye
que and mos
time and det
mperature dec
e of intens
ather than it
in the intens
smission. T
t sensing me
ace plasmon
active index
ler in 1991 [
multimode fib
oated in a th
cromirror se
permission fro
duced by a n
wed up on t
er on a 400
t informativ
tection respo
creases, the
17
ity-based
ts volume
ity of the
There are
echanism.
n sensors,
x changes
23]. The
bre (core
hin 10 nm
ensors, is
om IEEE.
noticeable
the initial
µm core
ve part of
onse as a
ability of
pallad
senso
70 oCoC, th
hydro
hydrooC w
incre
to de
film r
Fig. 2
w
A sim
temp
the am
The a
in the
the m
dium to abso
or. This effe
C, while keep
he sensor re
ogen concen
ogen is incre
where the Pd
ase in the de
crease, the r
reaching its
.15.: Detection
when exposed t
milar effect
eratures (in
mount of hy
authors also
e metal. By
micromirror,
orb hydroge
ect is illustra
ping the hyd
esponse is re
ntration. As
eased, and th
film is able
etection resp
response con
limit as to th
n response (a) a
to a constant 4
is seen for
the α-phase)
drogen able
had one mo
y increasing
, which sup
n gas increa
ated in figure
drogen conce
elatively low
s the temper
he detection
e to enter the
ponse is seen
ntinues to inc
he amount of
and response ti
4% atmospheric
perm
the respons
), the respon
to be absorb
ore degree of
the laser sou
ppresses th
ases. This ca
e 2.15a, whe
entration con
w, as the Pd
rature is dec
response inc
e β-phase at
n around this
crease until
f hydrogen i
ime (b) of Pd m
c hydrogen con
mission from E
se time of th
nse time is qu
bed is increa
f freedom in
urce power,
e phase tra
auses a direc
ere the gas te
nstant at 4%
d layer is un
creased, the
creases. Ev
t 4% hydrog
s temperatur
beginning to
it can absorb
micromirror sen
ncentration. R
Elsevier.
he sensor, s
uite fast, but
ased the resp
n controlling
they were a
ansition in
ct increase in
emperature w
%. Starting a
nable to ente
ability of th
ventually, a p
gen concentr
re. As the te
o hit a platea
b) in detectio
nsor as a funct
Reprinted from
shown in fig
t as the temp
ponse time in
g the point of
able to creat
an otherwi
n the respon
was varied f
t a temperat
er the β-pha
he Pd film t
point is reach
ration and a
emperature c
au (signifyin
on response.
tion of gas tem
[25], © (2000)
gure 2.15b.
perature is de
ncreases.
f the phase t
te a heating
ise low tem
18
nse of the
from -50-
ture of 70
ase at 4%
to absorb
hed at 36
dramatic
continues
ng the Pd
mperature at
) with
At high
ecreased,
transition
effect on
mperature
envir
detec
Fig. 2
Evan
Hydr
of the
on th
hydro
transm
Senso
first
diam
manu
of pa
regio
in nit
ronment (see
ction respons
2.16.: Detection
t
nescent Field
rogen sensor
e waveguide
he change i
ogen gas, du
mitted powe
ors of this ty
such device
eter and 125
ually stripped
alladium (va
n. The sens
trogen) and
e figure 2.1
se of the dev
n response of th
temperatures.
d Sensors
rs based on e
e mode with
in absorptio
ue to the decr
er.
ype have be
was presen
5 µm claddi
d with hydro
arying in thic
sor was only
showed a m
6). This is
vice in a low
he micromirror
Reprinted from
evanescent fi
the palladiu
n of pallad
reased free e
en demonstr
nted by Tabi
ing diameter
ofluoric acid
ckness from
y tested in c
modest incr
s a very use
temperature
r sensor as a fu
m [25], © (200
field sensing
um transduce
dium upon e
electron dens
rated almost
ib-Azar in 1
r) with a 1.5
d and the cor
m 10-100 nm
concentration
rease in tran
eful way to
e environme
unction of optic
0) with permis
rely on the
er. This type
exposure to
sity of pallad
t exclusively
1999 and co
5 cm length
re then clean
m) were then
ns varying f
nsmission of
o control the
nt.
cal power for a
ssion from Else
interaction o
e of sensor g
hydrogen.
dium, there w
y on a fibre
onsisted of a
h that has ha
ned with acet
n deposited
from 0.2-0.6
f approxima
e response t
a variety of hyd
evier.
of the evane
generally reli
Upon exp
will be an in
optic platfo
a MMF (50
ad the fibre
tone [26]. T
on the expo
6% hydrogen
ately 1%. R
19
time and
drogen gas
escent tail
ies solely
posure to
ncrease in
orm. The
µm core
cladding
Thin films
osed core
n (diluted
Response
times
transm
Fig
The m
diam
this i
stripp
using
The d
begin
coatin
Fig.
s for 0.2%
mission resp
g. 2.17.: Detect
major downs
eter fibre co
ssue by usin
ped [27]. A
g the travellin
device relies
n to actually
ng. The dev
. 2.18.: Structu
and 0.6%
ponse of the
tion response o
side of the ap
re, which is
ng a more res
single mode
ng-burning t
on the fact t
expand beyo
vice structure
ure of tapered S
hydrogen c
sensor to 0.6
of stripped MM
perm
pproach by T
very brittle
silient fibre s
e fibre was a
technique [2
that the mod
ond the clad
e can be seen
SMF hydrogen
concentration
6% hydrogen
MF sensor to 0.6
mission from E
Tabib-Azar i
and prone to
structure wh
adiabatically
8] and then
de size will g
dding air inte
n in figure 2
gas sensor. R
IEEE.
ns were 30
n can be see
6% hydrogen.
Elsevier.
is the extrem
o cracking. V
hich did not r
tapered dow
coated in a 1
greatly incre
erface in ord
2.18.
Reprinted from
0s and 20s,
en in figure 2
Reprinted from
me delicacy o
Villatoro att
require the c
wn to a waist
12 nm thick
ease in the ta
der to interac
[27], © (2001)
, respectivel
2.17.
m [26], © (199
of a stripped
tempted to c
cladding to b
t diameter of
layer of pall
apered region
t with the pa
) with permissi
20
ly. The
99) with
d, 50 µm
ounter
be
f 25 µm
ladium.
n and
alladium
ion from
In ord
was u
obtain
mode
conce
high
The d
incre
respo
but ap
Fig. 2
In an
embe
show
thickn
der to make
used, where
n a quasi-cir
es will see th
entrations as
for Pd-based
detection res
ase in respon
onse expecte
ppears to be
2.19.: Detection
attempt to e
edded a side-
wn in figure 2
nesses varyi
the device a
the fibre wa
rcular deposi
he same thick
s high as 10%
d evanescent
sponse result
nse around t
d around the
relatively sl
n response of ta
f
even further
-polished, sin
2.20 [29]. Pa
ing from 20-
as polarizatio
as rotated 120
ition of palla
kness of pall
%, and show
t field sensor
ts can be see
the 2% hydro
e α to β-phas
low (~100s f
apered SMF se
from [27], © (2
increase the
ngle mode f
alladium wa
-100 nm.
on insensitiv
0o between c
adium onto t
ladium. The
wed a transmi
rs.
en in figure 2
ogen point, w
se transition
for 1.8% hyd
ensor to hydrog
2001) with perm
e durability o
fibre into a q
as then depos
ve as possible
consecutive
the fibre and
sensor was
ission increa
2.19, and not
which is con
point. Resp
drogen).
gen concentrat
mission from I
of a fibre-bas
quartz fibre-h
sited onto th
e, a two-stag
depositions.
d ensure that
exposed to h
ased of up to
tice that ther
nsistent with
ponse time d
tions ranging fr
IEEE.
sed evanesce
holding circu
he surface of
ge deposition
This was d
t the TE and
hydrogen
o 60%, which
re is a signif
h the large in
data was not
rom 1.8-10%.
ent field sen
ular groove,
f the quartz h
21
n process
done to
TM
h is quite
ficant
ncrease in
reported,
Reprinted
nsor, Kim
as
holder in
Fig.
In thi
to be
hydro
incre
100 n
layer
figure
2.20.: Schemat
is configurat
able to dete
ogen gas.
ase in transm
nm Pd layer.
, the higher
e 2.21.
tic of SMF em
tion, the TM
ect an increa
The sensor
mission) for
. Response
the insertion
mbedded into qu
with
M mode is ab
ase in the tr
r detection r
the 20 nm P
times ranged
n loss and re
uartz fibre-hold
permission fro
le to penetra
ransmission
response va
Pd layer to 1
d from 90-2
esponse time
ding circular gr
om IEEE.
ate into the m
at the outpu
aried from
.4 dB (38%
200s. As exp
e of the devi
roove. Reprint
metal suffici
ut when the
approximate
increase in
pected thoug
ice. These r
ted from [29],
iently deeply
sensor is ex
ely 0.55 dB
transmission
gh, the thick
results can b
22
© (2007)
y in order
xposed to
B (13.5%
n) for the
ker the Pd
be seen in
Fig
reco
g. 2.21.: Detect
overy times. c
tion response o
) Simulated tra
Reprin
of sensor coate
ansmission loss
nted from [29]
d in (a) 20 nm
ses of the senso
, © (2007) with
Pd and (b) 100
or when coated
h permission fr
0 nm Pd, also s
d in Pd layers r
from IEEE.
showing respo
ranging from 5
23
nse and
5-40 nm.
24
2.4 Table of Comparison
This section offers a summary of the technologies that were covered, shown in table 2.1.
Sensor Detection
Response
Response
Time
Comments
Fibre MZI, 10 µm thick Pd
[8]
- 0.6% detection
limit
- < 3 mins - fringe shift at the output is the method of
detection
50 mm long LPG (500 µm
grating period), 50 nm
thick Pd [12]
- 2.3 nm blue shift
(4% hydrogen)
- 8 minutes
fs-laser etched micro-
cavity MZI, 110 nm thick
Pd [13]
- 0.155 nm/%
hydrogen
(resonance shift)
- Not reported - 40 µm and 80 µm cavity lengths were used
(40 µm cavity exhibited greater sensitivity)
LiNbO3 integrated MZI,
120 nm thick Pd [14]
- Detected 1%
hydrogen
- 30 seconds
Fibre FP cavity, 10 nm Pd
micromirror [15]
- 40% change in
transmission for
3% hydrogen
- 30 seconds - tested in concentrations ranging from 0.5-3%
Extrinsic fibre FP cavity,
100 nm thick Pd-Ag film
[17]
- 9 nm air gap
length change (4%
hydrogen)
- 20-30
minutes
- tested in concentrations ranging from 0.2-4%
2-3 cm long Bragg grating,
560 nm thick Pd layer [19]
- 5 pm red-shift
(1.8% hydrogen)
- Not reported - 560 nm thick Pd film tended to peel off after
repeated exposures
1.5mm long Bragg grating
embedded in glass block,
50 nm thick Pd layer [21]
- 4 pm red-shift
(4% hydrogen)
- 1 pm red-shift
(1% hydrogen)
- 30 seconds
(4% hydrogen)
- 2 minutes
(1% hydrogen)
500 µm period fibre LPG,
70 nm thick Pd layer [22]
- 4.3 nm blue-shift
(4% hydrogen)
- 70 seconds - sensor was tested in temperatures ranging
from 30-200oC
Fibre micromirror, 10 nm - 22% change in - Not reported
25
thick Pd tip [23] transmission
MMF Fibre micromirror,
13 nm thick Pd tip [25]
- ~13% change in
transmission (4%
hydrogen at room
temperature)
- ~100 seconds
(4% hydrogen
at room
temperature)
- sensor was tested using gas temperatures
ranging from -50-70oC
Stripped MMF fibre, 10-
100 nm thick Pd layer [26]
- 1% increase in
transmission (0.2-
0.6% hydrogen)
- 30 s (0.2%
hydrogen)
- 20 s (0.6%
hydrogen)
- tapered and stripped fibre section was very
delicate and prone to cracking
Tapered fibre (non-
stripped), 12 nm thick Pd
layer [27]
- 60% increase in
transmission (10%
hydrogen)
- ~100 seconds
for 1.8%
hydrogen)
- accurate and thorough response time data not
reported
Fibre embedded into
quartz block, 20-100 nm
thick Pd layer [29]
- 13.5% increase in
transmission for 20
nm Pd, 38%
increase in
transmission for
100 nm Pd (4%
hydrogen)
- 90 seconds
for 20 nm Pd
- 200 seconds
for 100 nm Pd
Table 2.1.: Summary of existing hydrogen sensor technologies.
3
3.1
The m
index
therm
which
wave
order
can b
of the
Fabr
The t
wher
the in
devic
The m
The p
Literat
Integra
majority of
x and size o
mo-optic coe
h is relativel
elength of 15
r of magnitu
be ignored in
ese propertie
ry-Perot
transmitted l
e Io is the in
ntracavity lo
ce (dependen
mirror reflec
phase factor,
Literatu
ture Re
ated Sen
silicon-base
f silicon are
efficient (dn/
ly high, and
500 nm) [31
ude smaller (
n most simu
es of silicon
light intensit
nput intensit
sses of the d
nt on mirror
ctivity, R, is
, φ, is given
ure Revie
eview: Te
nsors
d integrated
e both tempe
/dT = 1.8 x
the thermal
]. The ther
(dn/dT = 1.0
ulations. Pho
and differen
ty through a
11
ty, L is the c
device, R is t
reflectivity)
given by:
by:
Chapterew: Tem
Tempera
d temperatur
erature depe
10-4 K-1 at
expansion c
rmo-optic co
0 x 10-5 K-1
otonic devic
nt device con
silicon wave
cavity length
the mirror (e
, and φ is a p
2
r 3 mperature
ature Se
re sensors re
endent. The
300 K for a
coefficient (α
oefficient of
at 300 K for
ces mainly re
nfigurations
eguide Fabry
4
h, α is the ab
end-facet) re
phase factor
e Sensor
ensors
ely on the fa
ese changes
a wavelengt
α = ~2.5 x 1
the insulatin
r a wavelen
ely on the ch
will be discu
y-Perot cavit
bsorption coe
eflectivity, FR
.
rs
act that the r
are governe
th of 1550 n
0-6 K-1 at 30
ng layer (Si
gth of 1550
hange in on
ussed furthe
ty is given b
efficient repr
R is the fines
26
refractive
ed by the
nm) [30],
0 K for a
O2) is an
nm) and
e or both
er.
by:
(3.1)
resenting
sse of the
(3.2)
(3.3)
The r
length
reflec
Coco
earlie
subst
wave
tested
in fig
Fig
Infrar
collec
24-36
These
reason that
h changes
ctivity and p
orullo demon
est demonst
trate is silico
eguiding n- d
d range in le
gure 3.1.
g. 3.1.: Schema
red light at a
cted at the o
6oC and sho
e results can
a Fabry-Per
length in d
hase factor a
nstrated a Fa
trations of a
on-on-silicon
doped struct
ngth from 2
atic of the Fabr
a wavelength
output with a
wed a notic
n be seen in f
rot cavity is
different tem
are both sens
abry-Perot ba
a silicon-ba
n, with an n
ture has dim
5 µm to sev
ry-Perot sensor
h of 1.55 µm
a photodetect
eable, perio
figure 3.2.
sensitive to
mperatures d
sitive to chan
ased temper
ased integrat
n- doped gui
mensions of 1
eral millime
r. Reprinted fr
m was coupl
tor. The sen
dic change i
o temperatur
due to ther
anges in the r
rature sensor
ted optical
iding layer a
15 µm wide
etres. The st
rom [32], © (19
led into the c
nsor was test
in output po
re changes i
rmal expans
refractive ind
r in 1997, in
temperature
and an n+ d
e and 4 µm h
tructure of th
997) with perm
cavities with
ted in tempe
ower with ch
is because th
sion and th
dex of silico
what was o
e sensor [32
doped substr
high and the
he device can
mission from E
h an optical
eratures rang
hanging tem
27
he cavity
he mirror
on.
one of the
2]. The
rate. The
e cavities
n be seen
Elsevier.
fibre and
ging from
mperature.
Fig.
One
appli
becau
becau
Ring
A rin
wave
accom
coupl
integ
the re
wher
integ
Comb
clear
The c
chang
3.2.: Transmit
advantage o
cation by ch
use of the st
use of the low
g Resonators
ng resonator
eguide into a
mplished eit
led into the
er multiple
esonator are
e neff is the
er. It is evid
bine this wi
why the ring
change in re
ge in resonan
tted intensity o
of the devic
hanging the
eeper slope
wer free spe
s
r is a reson
an adjacent
ther by dire
ring sectio
of the wave
given by:
effective in
dent that this
ith the fact t
g resonator i
esonant wav
nt waveleng
of the device fo
(2007) wi
ce is that th
cavity lengt
of the reson
ectral range w
nant device
ring or race
ectional cou
n of the wa
length, then
ndex of the
s relationship
that silicon
is a viable so
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2 µm with a
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32
sensor in
substrate,
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temperature
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as seen in fi
s for temperatu
th permission f
e range of 2
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figure 3.9.
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from SPIE.
25-300oC.
region chang
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As the tem
ge, and this
33
C (b).
mperature
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-300oC. Reprin
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nted from [38]
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l region. Th
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nsors.
in 2003 [39]
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chematic of
03) with permis
34
, © (2001)
the
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]. The
layer and
cladding
the
ssion from
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Fibre S
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Sensors
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using in-hou
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[39], © (2003)
on coefficie
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s InfoBase.
quite low an
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35
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as ΔI/ΔT,
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Reprinted
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es offer a
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of metres)
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d range of th
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36
nators, as
where the
order to
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This was
shifter on
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ty Enhancin
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enhance the
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amount of en
m for a cert
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as a fluoresce
sor in 2006
fluoresce in
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ut light has w
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, © (2004) wit
ng Membra
mo-optic coe
sensitivity o
mal expansion
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6 [43]. The
the near-UV
ce consists o
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reflectivity a
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th permission f
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of a fibre te
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orescing glu
be tuned by the
from SPIE.
fused silica
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length of the f
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change.
bre-based flu
umber of p
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two fibres a
old-coated a
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37
fibre ring.
ow, other
ice. One
by using
uorophore
diation or
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ht. These
ature and
uorescent
olycyclic
cited with
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Fig.
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refle
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asing and dec
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in figure 3.1
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creasing tem
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from Elsevie
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re is a need
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fluorescence s
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anging from
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bre. Reprinted
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m 20-100oC a
order to te
ceable decre
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cles. The re
d from [43], ©
ptical fibres
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as some of the
d from [43], ©
and the resul
st accuracy.
ease with inc
sensor as it
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where its in
being reflec
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(2006) with pe
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. As can be
creasing tem
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e compared f
38
ermission
ntensity is
cted back
meter for
um of the
t has been
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mperature.
hrough a
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at of the ther
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Fig. 3.16.: a)
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th
ry-Perot Cav
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to fabricate
ting a temp
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3.17.: Schemat
rmocouple, w
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vity
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which was l
intensity of the
orescent senso
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ood choice fo
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listed at less
e sensor with re
r compared to
m [43], © (200
or a tempera
vity to chang
erting two s
rial in betw
ed was meas
be seen in fig
sensor. Reprin
s than 10 sec
espect to tempe
the time respo
06) with permis
ature sensor
ges in refract
single mode
ween them to
sured to hav
gure 3.17.
nted from [44],
conds on the
erature (rangin
onse of a comm
ssion from Els
in either an
tive index o
e fibres into
o serve as
ve a refractiv
© (2008) with
e manufactur
ng from 20-100
mercially availa
evier.
n integrated s
f the cavity.
o a hollow
a cavity [4
ve index of 1
h permission fr
39
rer’s data
0oC)
able
silicon or
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tube and
44]. The
1.407. A
rom IEEE.
40
One major advantage of this sensor is that it is intensity based and works by detecting the
reflected light, rather than the transmitted spectrum. Since the refractive index of the cavity is
very close to the index of the fibre, the reflectivity of the interfaces are very low, and the overall
reflectivity can be approximated by the two-beam interference equation given by [45]:
1 2 1 cos (3.6)
where r is the cavity reflectivity at each interface and is given by:
(3.7)
and φ is the round-trip phase shift given by:
4
(3.8)
The cavity length, L, was assumed to be constant. The reflectivity has a periodic nature to it and
was simulated (see figure 3.18a). The cavity length was chosen to be approximately 15 µm
order to ensure the sensor operates in the near-linear regime over the desired temperature range
of -30-100oC and with a cavity refractive index of 1.407. A commercially available optical time
domain reflectometer was used as the source and detection device. This device works by
sending out a 5 ns optical pulses at 1550 nm and measuring the reflected power as well as the
corresponding delay. As the temperature around the cavity increases, the refractive index of the
cavity also increases, which causes an increase in the reflected power, which can be measured by
the OTDR. The temperature response of the device can be seen in figure 3.18b.
Fig. 3
b) Re
The m
sourc
on th
of sen
spect
trace
fibre
Fig.
(in
3.18.: a) Simula
eflected signal
main advan
ce and measu
he same fibre
nsors on the
trum of each
for a netwo
optic cable c
3.19.: OTDR
ndicated by lin
ated reflectivit
intensity detec
ntages of thi
urement dev
e to create a
e same cable
h of the dev
rk consisting
can be seen
trace for three
ne a, in black) a
ty of the Fabry-
cted by the OT
© (2008)
is device are
vice (the OTD
series of di
e, separated
vices, since i
g of three se
in figure 3.1
temperature se
and for 30oC (i
pe
-Perot cavity a
DR for temper
with permissio
e in the com
DR) and in t
stributed sen
by delay lin
it also meas
eparate cavit
19.
ensors on the s
indicated by lin
ermission from
as a function of
ratures ranging
on from IEEE.
mmercially
the ability to
nsors on a si
nes, the OTD
sures the ref
ty sensors se
same fibre, sho
ne b, in red). R
m IEEE.
f refractive ind
g from -30-80oC
available an
o attach mult
ingle cable.
DR is able to
flected pulse
eparated by o
owing the detec
Reprinted from
dex of the cavit
C. Reprinted f
nd industry
tiple cavities
If you have
o detect the
e delay. Th
over 50m on
cted response f
m [44], © (2008
41
ty material
from [44],
standard
s together
e a series
reflected
he OTDR
n a single
for -30oC
8) with
Mult
MMI
struct
the f
temp
mater
canno
new
incre
thoro
Fig.
The s
same
refrac
modi
devic
3.21.
timode Inter
Is fabricated
ture on an S
fact that it
erature sens
rial [46]. B
ot contain th
core and the
ase in effect
ough analysis
3.20.: Diagram
sensor was t
as that of
ctive index o
fy the outpu
ce can be tun
rferometer
d within a
SOI platform
has to be
sor in an opt
By tapering t
he mode, and
e temperatur
tive core diam
s of the math
m of the tapere
tested in tem
f the integra
of the tempe
ut power of
ned by the w
fibre functi
m, but the str
contained w
tical fibre b
the fibre dow
d the mode e
re sensitive c
meter, the ta
hematics of t
d fibre MMI se
mperatures ra
ated SOI M
erature sensi
the MMI. T
waist diamet
on upon th
ructure of th
within an o
by using a ta
wn to a ver
expands to th
coating beco
apered region
this are deta
ensor. Reprint
anging from
MMI sensor.
itive coating
The authors
ter of the fib
he same pri
he device is
optical fibre
apered sectio
ry small wai
he cladding.
omes the ne
n is able to s
ailed by Lacr
ted from [46], ©
m -20-80oC.
Changes
g and the fib
also demon
bre. The sen
nciples as
generally m
e. Zhu dem
on coated in
ist diameter
Now, the c
ew cladding.
support mult
roix [47].
© (2011) with
The detecti
in temperat
bre itself, an
nstrated that
nsor results c
an MMI w
much differen
monstrated
n thermally
, eventually
cladding bec
Due to thi
tiple modes.
permission fro
on mechani
ture will ch
nd these chan
the sensitivi
can be seen
42
waveguide
nt due to
an MMI
sensitive
the core
comes the
is sudden
. A more
om SPIE.
sm is the
hange the
nges will
ity of the
in figure
b)
Fig. 3
) Output power
.21.: a) Spectru
r of the device
um of the devic
for temperatur
pe
ce output for te
res ranging fro
ermission from
emperatures ra
om -20-80oC. R
m SPIE.
anging from -2
Reprinted from
1.6-78.8oC
m [46], © (2011
43
1) with
4
It wa
subst
numb
mass
and s
senso
detec
accom
It wa
becau
an in
optic
The s
silico
light
schem
Hydro
as decided th
trate. While
ber of advant
ive silicon e
such a move
or. Another
cting differen
mplish the sa
as also decid
use is it gene
nterferometri
al, Pd-based
sensors were
on, 3 µm bur
through the
matic of one
ogen Se
hat the hydro
e this platform
tages. The u
electronics in
would open
advantage i
nt analytes
ame function
ed that the d
erally much
c or spectra
d hydrogen s
e fabricated
ried SiO2 lay
e sensor, an
of the sensi
Fig. 4.1.:
Hydrog
ensor De
ogen sensor w
m is not as i
use of silicon
ndustry. Th
n up a wide
is that it is e
or environm
nality on a fi
detection me
more inexp
l change. T
ensor that us
on silicon-o
yer (H), and
nd a thin fil
ng elements
Schematic of t
WT
H
(a)
WT
H
(a)
Chapteren Sens
esign
would be fab
investigated
n waveguide
his is a long-
array of use
easier to be a
mental varia
ibre, especia
ethod would
pensive to de
This would a
ses intensity
on-insulator
silicon subs
lm of pallad
s in question
the Pd-coated h
d
L
d
L
r 4 sor Desig
bricated on a
d or as matur
es allows for
-term goal o
es and offer
able to integ
ables togethe
ally with inte
be intensity
etect a chang
also be the fi
y changes as
substrates w
strate. Ridge
dium was u
can be seen
hydrogen sens
gn
an integrated
re as fibre te
r the potentia
of the silicon
increased fu
grate multipl
er on an SO
ensity based
y based. Thi
ge in intensi
first demonst
a detection m
with a 220 nm
e waveguide
sed as the s
n in figure 4.
or waveguide.
Silicon
Palladium
Silica
Silicon
Palladium
Silica
d silicon-on-
echnology, i
al integrated
n photonics
unctionality
le sensors ca
OI chip than
detection.
is decision w
ity than it is
trate of an in
method.
m top layer
es were used
sensing elem
1.
44
-insulator
t offers a
d with the
industry,
to such a
apable of
n it is to
was made
to detect
ntegrated
(T+d) of
d to guide
ment. A
As m
the m
expan
cause
hydro
be de
detail
4.1
The t
and th
In or
mode
the fo
Fig.
In ch
taken
wave
proce
mentioned in
metal. In thi
nsion of the
es decreased
ogen is absor
etected at th
led below.
Waveg
two paramet
he waveguid
der to do th
e interaction
ollowing con
4.2.: Structure
oosing the w
n into accoun
eguide width
ess, the actua
section 1.2
is case, as h
e metal. Th
d attenuation
rbed into the
he output.
guide Wi
ters to consi
de pedestal h
at, the dime
n with the pa
nfiguration, a
e of hydrogen s
waveguide w
nt. The firs
h was restric
al etched wa
, as palladiu
hydrogen ato
his volume e
n due to decr
e palladium
The design
dth and
ider in the d
height. The
nsions of th
alladium film
as shown in
sensor wavegui
width and ped
st restriction
cted from 6
aveguide wid
um absorbs h
oms are abs
expansion re
reased scatte
thin film, th
n details of
Pedesta
design of the
goal is to ma
he waveguide
m. The simu
figure 4.2.
ide used in Lum
for easy viewi
destal height
n is single m
00-700 nm.
dths will be
hydrogen, it
orbed into t
esults in a r
ering from f
he transmissi
the differen
al Height
e waveguide
aximize the
e must be se
ulated struct
merical MODE
ing).
t, there are c
mode operati
Since reac
lower than
t changes th
the palladium
reduced elec
free electron
ion is going t
nt parameter
e itself are t
detection re
elected in or
ture was a ri
E Solutions (Pd
certain restric
ion. In orde
ctive ion etc
the designed
he electron d
m, there is a
ctron densit
ns [6]. Ther
to change, w
rs of this se
the wavegui
esponse of th
rder to maxi
idge wavegu
d thickness exa
ctions which
er to ensure
ching is an
d widths. T
45
density of
a volume
ty, which
refore, as
which can
ensor are
ide width
he sensor.
imize the
uide with
aggerated
h must be
this, the
isotropic
his is the
reaso
that t
reaso
In ord
using
propa
nm.
wave
the c
The r
As ca
peak
follow
on that the 6
the light mus
on, the pedes
der to maxim
g Lumerical
agation meth
The pedesta
eguide (with
alculated m
results of the
Fig. 4.3.: Simu
an be seen f
with a ped
wing simula
00-700 nm
st be confine
stal height ha
mize the mo
MODE Solu
hod. A swe
al height wa
out metal fil
etal loss is
e simulation
ulation results
from the sim
destal height
ted mode pr
range may s
ed within the
as been pract
de interactio
utions, whic
ep was cond
as varied be
lm) is theore
a good mea
can be seen
showing the si
mulation resu
t of approxi
rofiles, seen
seem a bit h
e waveguide
tically limite
on with the p
ch includes b
ducted over
tween 0-120
etically loss
asurement fo
in figure 4.3
imulated metal
ults, the mod
imately 80-
in figure 4.4
high, theoret
e and not lea
ed to around
palladium la
both an eige
the wavegu
0 nm, in 20
sless since it
or the mode
3.
l losses for diff
de interactio
100 nm. T
4.
tically. The
ak into the ad
d 120 nm.
ayer, simulat
nmode solve
uide widths o
nm increme
t has perfect
interaction
fferent wavegui
on with the m
This can be
e second lim
djacent slab.
tions were c
er and a 2.5
of 600, 650,
ents. Since
tly smooth s
with the me
ide configurati
metal layer r
explained
46
mitation is
For this
onducted
D FDTD
, and 700
the bare
sidewalls,
etal film.
ions.
reaches a
from the
In lo
same
mode
mode
is ap
horiz
Note
sidew
on th
layer
show
Fig. 4.4.: S
oking at the
waveguide
e and the de
e interaction
pproximately
zontal dimen
that the pre
walls. This i
he waveguide
s. Nonethe
wn in figure 4
Simulated TE m
e mode prof
with a 90 n
eposited pall
can be seen
y equal to th
sion).
evious simul
is a fairly sa
e sidewalls u
less, a simu
4.5.
mode profiles f
files for a P
m pedestal,
ladium on t
n to coincide
he center of
ations were
fe assumptio
using electro
ulation was
for waveguides
Pd-coated wa
it is clear th
the pedestal.
e with the ge
f the mode
done assum
on, as it wou
on-beam eva
conducted a
s with a) no pe
aveguide wi
hat there is in
. From thes
eometry whe
(the portion
ming zero me
uld be difficu
aporation and
assuming ful
edestal and b) 9
ith no pedes
ncreased int
se mode pro
ere the pedes
n of the mo
etal depositio
ult to get a n
d sub 10 nm
ll sidewall t
90 nm pedestal
stal compare
teraction bet
ofiles, the m
stal is at a he
ode with th
on on the w
noticeable d
m thickness p
thickness de
47
l.
ed to the
tween the
maximum
eight that
he largest
waveguide
eposition
palladium
eposition,
Fig.
It is c
differ
layer
great
sidew
4.5.: Simulatio
clear that if
rent effect. I
actually dec
est interacti
walls on both
Fig. 4.
on results show
the metal c
f there is a c
creases with
ion when th
h sides of the
6.: Simulated T
wing the simula
full sidew
ompletely c
complete sid
pedestal hei
here is no p
e mode, seen
TE mode profi
ated metal loss
wall deposition
oats the side
dewall coatin
ight. This is
pedestal, sinc
n in figure 4.
ile with no ped
ses for differen
n of the metal.
ewalls, the p
ng, the intera
s expected, a
ce there wi
.6.
destal and full s
nt waveguide co
pedestal heig
action of the
as it is clear
ill be 220 n
sidewall metal
onfigurations a
ght has a co
mode with t
that there w
nm palladium
deposition.
48
assuming
ompletely
the metal
will be the
m coated
49
Since the effect of pedestal height is completely different for coated and uncoated sidewalls, the
actual geometry of the palladium deposition can later be verified experimentally. To further
confirm results, a scanning electron microscope can be used.
4.2 Palladium Layer Dimensions
Since it is clear that a larger detection response will coincide with a larger mode interaction with
the palladium layer, in order to increase the detection response, it is clear that the palladium layer
must be made thicker and longer. The downside with this approach is the increased metal losses.
When the metal losses combined with the intrinsic waveguide losses are great enough that the
signal at the output becomes very difficult to detect, then the increased detection response is
probably not worth the very poor signal-to-noise ratio. Another factor to take into consideration
is the increased response time that comes with a palladium layer of increased thickness.
Because of these reasons, there are no optimal dimensions for the palladium layer, and any
dimensions that are chosen will be the result of a trade-off between different factors (output
SNR, detection response, and response time). For a hydrogen safety sensor, speed would be the
most important metric to measure. Because of this, the thinnest palladium layer possible would
likely be chosen (that displays a detection response large enough to be detected). In order to
further explore the effect that different palladium dimensions have on the response time and
detection response however, the palladium layers were deposited in a series of lengths (8 µm, 12
µm, 16 µm, and 20 µm). A range of different palladium thicknesses were also used. From here,
the corresponding detection response and response times were determined experimentally, as
will be discussed in Chapter 6.
5
The
fabric
of sin
the p
the S
used
Final
subst
All th
wave
the-ar
metal
clean
5.1
Wave
devel
using
below
Hydro
fabrication
cation, lift-o
ngle mode op
attern was d
SOI substrate
to expose t
lly, electron
trate.
he fabricatio
eguide fabric
rt Vistec EB
l deposition
nroom and an
Waveg
eguide fabr
lopment foll
g commercia
w in figure 5
H
ogen Se
process for
off preparatio
peration, ele
defined and d
e. For lift-o
the areas of
beam evap
on steps were
cation step w
BPG5000+
n steps wer
n adjacent C
guide Fa
ication is l
lowed by pa
al CAD soft
.1.
Hydrogen
ensor Fa
r the pallad
on, metal de
ectron beam
developed, re
off preparati
f the wavegu
poration wa
e completed
was complet
Electron Be
re complete
lass 10000 e
brication
largely a tw
attern etching
tware called
Chaptern Sensor
abricatio
dium waveg
eposition/lift
lithography
eactive ion e
ion, a metal
uides where
s used to d
d at facilities
ted in a Clas
eam Lithogr
ed in a Cl
etching/depo
n
wo-step pro
g. To begin
d L-Edit. A
r 5 r Fabrica
on
guides is a
t-off. In ord
y was chosen
etching was
llized mask
e the palladi
deposit the p
available at
ss 100 clean
raphy System
lass 1000 p
osition clean
ocess consis
n, a pattern f
A high level
ation
three step
der to create
n for the patt
used to etch
and UV ph
ium metal w
palladium m
t the Univer
nroom which
m. The lift
photolithogr
nroom.
sting of pa
for the wave
view of the
process: w
waveguides
tern definitio
h the wavegu
hotolithograp
was to be d
metal layer
sity of Toro
h contains a
t-off prepara
raphy/wet c
attern defini
eguides was
e design can
50
waveguide
s capable
on. After
uides into
phy were
deposited.
onto the
nto. The
state-of-
ation and
chemistry
ition and
designed
n be seen
F
The d
is ide
and
prepa
ident
In or
sectio
a des
due t
midd
Fig. 5.1.: CAD
wav
design is arr
entical to on
800 nm we
aration and w
ify the 600 n
rder to reduc
ons must be
igned width
to sidewall
dle of the de
layout showin
eguides of diff
ranged into t
ne another ex
ere used. T
will be expl
nm section o
ce the inser
kept to a mi
h of 2 µm. T
roughness w
esigned devic
ng a section of t
ferent widths.
three separat
xcept for the
The red cro
lained in sec
of the chip.
rtion loss of
inimum. To
This larger w
when compa
ce, the 2 µm
the hydrogen s
Crosses are us
te cells of w
e waveguide
osses are us
ction 5.2. T
f the device
o accomplish
waveguide se
ared to the
m waveguide
sensor. Each c
sed for alignme
waveguides, a
e width. Thr
sed for alig
The blue ver
, the overal
h this, the ou
ection will ha
nano-waveg
es will taper
cell (separated b
ent during lift-o
arranged hor
ree designed
gnment purp
rtical bars ar
ll length of
uter ends of
ave much lo
guide sectio
r down to th
by crosses) con
off stage.
rizontally. E
d widths of 6
poses during
re simply m
the nano-w
the wavegui
ower scatteri
ons. Rough
heir designe
51
ntains
Each cell
600, 700,
g lift-off
markers to
waveguide
ides have
ng losses
hly at the
ed widths
(600,
minim
Fig.
B
5.1.
This
ZEP-
taper
the 2
based
Since
using
the s
consi
adjac
both
acros
Follo
agitat
, 700, or 80
mize losses.
5.2.: CAD lay
Blue indicates
1 Patte
defined patt
-520A positi
and nano-w
µm wavegu
d on work do
e a positive r
g hydrofluori
substrate is
iderably redu
cent to the de
ensures that
ss the trench.
owing exposu
tion followe
0 nm) over
This design
out of the desig
an area of low
ern Expos
tern was the
ive resist. A
waveguide se
uide sections
one in etchin
resist must b
ic acid, whic
actually th
uce the e-be
esired waveg
t the e-beam
. The trench
F
ure, the sam
d by a statio
a length of
ned taper can
gned taper. Re
resolution exp
sure and D
en exposed o
A fracture res
ections and a
s. A dose o
ng similar wa
be used for S
ch attacks th
e negative
eam write tim
guide sectio
write time i
h design can
Fig. 5.3.: CAD
mples were d
onary 30s in
f 50 µm, wh
n be seen in
ed indicates the
posure and gree
Developm
onto an SOI
solution of 1
a resolution o
of 250 µC/cm
aveguides [4
SOI substrate
he oxide laye
of the patt
me, it was d
ons. The tren
is low and th
be seen in f
layout of a sec
developed in
an MIBK/IP
hich ensures
figure 5.2.
e non-exposed
en indicates an
ment
I substrate w
10 nm (5 nA
of 25 nm (35
m2 was used
48].
es (HSQ, a n
er in SOI), t
ern that we
decided that
nch width w
hat the optic
figure 5.3 (sh
ction of waveg
n a ZED-N5
PA solution.
a taper ang
d area where the
n area of high r
which has be
A beam curre
5 nA beam c
d in both cas
negative res
the area that
e wish to d
thin trenche
was chosen t
cal waveguid
hown in gree
guide.
50 solution f
Finally, the
gle of less th
e waveguide is
resolution expo
een spin-coa
ent) was use
current) was
ses and was
ist, must be
t must be ex
define. In
es would be
o be 1.15 µm
de mode will
en).
for 60s with
e samples w
52
han 1o to
s defined.
osure.
ated with
ed for the
s used for
obtained
removed
xposed on
order to
e exposed
m, which
l not leak
constant
were dried
with
befor
5.1.2
Follo
reacti
bomb
proce
purpo
An et
In ord
to be
rangi
A sch
Fig.
a nitrogen g
re overexpos
2 Patte
owing expos
ive ion etch
bard the sub
ess. The etc
oses of etchi
tching recipe
der to fabric
e adjusted. T
ing from 100
hematic draw
5.4.: Diagram
gun for 40s.
sure was witn
ern Etchin
sure and de
hing system.
bstrate to slo
cher is capab
ing and cool
e was obtain
RIE RF Pow
Chamber Pr
Etching Gas
Backside Co
Tab
cate wavegui
The recipe s
0-220 nm.
wing of the e
showing the p
An exposu
nessed.
ng
evelopment,
RIE uses c
owly remove
ble of supply
ling. The R
ned from [48
wer
ressure
s Flow Rates
ooling Gas F
ble 5.1.: Parame
ides with dif
shown in tab
etching proce
rocess of resist
ure of 60s w
the sample
hemically re
e desired are
ying SF6, C
RIE power an
] and consis
s
Flow Rate
eters used for t
fferent pedes
ble 5.1 was
ess is shown
t application an
was found to
es were etch
eactive plasm
eas of silico
HF3, CF4, O
nd chamber
sts of the par
80 Watt
80 mTorr
25 sccm SF6
10 sccm He
the RIE etching
stal heights,
used to fab
n in figure 5.
nd pattern etch
o be the opt
hing using
ma, where h
on as define
O2, and other
pressure ca
rameters sho
6, 12 sccm O
g recipe.
the time in t
bricate samp
.4.
hing. a) Un-etc
imal amoun
a Trion Ph
highly energ
d by the lith
r inert gasse
an also be co
own in table
O2
the etch cha
ples with etc
ched SOI wafe
53
nt of time
antom II
getic ions
hography
es for the
ontrolled.
5.1.
amber has
ch depths
r. b) Un-
etched
5.2
In or
a lift-
with
expos
be de
The
cross
photo
In the
The w
areas
the w
depos
withi
be ali
d SOI wafer w
wafer after
Lift-Off
rder to be ab
-off process
a thin layer
sed and deve
eposited.
challenging
es defined i
omask and U
e photomask
white areas
. In this cas
waveguide la
sit the pallad
in the 100 µm
igned with th
with positive res
r etching. d) S
f Prepara
ble to deposi
must be per
r of photore
eloped, creat
part of this
n the pattern
UV photolith
Fig. 5.5.: CAD
k diagram, th
are pure qu
se, notice th
ayer. This a
dium metal s
m long nano
he etched sa
sist that has bee
SOI wafer after
ation
t thin pallad
rformed. A
esist and the
ting window
s process is
n come into
hography. Th
D layout of the
he red area r
uartz and all
hat the crosse
allows the m
strips exactly
o-waveguide
ample is show
en applied, exp
r etching and re
dium layers o
lift-off proc
en the areas
ws to the und
s the alignm
play. The
he layout of
e lift-off mask.
epresents a l
low transmi
es on the ma
mask to be pr
y where they
e section. A
wn in figure
posed, and dev
esist removal s
onto the desi
cess is one w
s where the
derlying silic
ment, and th
lift-off layer
f the lift-off m
Red areas are
layer of chro
ission of UV
ask layer ali
recisely alig
y need to be
diagram sho
5.6.
veloped into the
showing the wa
ired location
where an etch
metal depo
con substrate
hat is where
r was define
mask can be
e chrome (opaq
ome, which
V light in or
ign perfectly
gned with th
, which is ac
owing how
e desired patter
aveguide ridge
ns of the wav
hed sample
osition is de
e where the m
the aforem
ed using a m
e seen in figu
que).
blocks UV r
rder to expo
y with the cr
he sample in
cross the wa
the lift-off m
54
rn. c) SOI
e.
veguides,
is coated
esired are
metal can
mentioned
metallized
ure 5.5.
radiation.
ose those
rosses on
n order to
aveguides
mask will
Fig.
layo
superi
The l
Step
1
2
3
4
5.6.: CAD lay
out of a section
imposed on top
for metal
lift-off recipe
. Spin Coat
. Spin Coat
90s
. Soft-Bake
. Expose, s
µm mask
yout of the wav
n of the wavegu
p of the wavegu
l deposition. T
e uses S1811
t P20, 5000 r
t S1811, 500
e, 100oC, 90
oft exposure
to sample d
veguide and lift
uide mask. b)
uide mask. Th
The square bloc
1 resist, and
C
rpm, 90s
00 rpm,
s
e, 5s, 100
istance
t-off masks to i
CAD layout of
he thin strips al
ck is used as a m
is detailed i
Comments
- P20 s
silico
- S181
defin
illustrate how
f a section of th
lign perfectly w
marker and for
in table 5.2.
serves as an
on substrate
11 is the acti
ne the windo
the lift-off pro
the lift-off mas
with the waveg
r thickness test
adhesion lay
and S1811 r
ve resist lay
ows for meta
ocedure works.
k. c) The lift-o
guides, creating
ting purposes.
yer between
resist
yer to be expo
al deposition
55
a) CAD
off mask
g a window
n the
osed to
56
5. Develop, MF321, 90s with
agitation
- Develop time can be tuned within 60-90s for
desired results
6. Rinse, DI water, 60s
7. Hard Bake, 60oC, 30 mins
Table 5.2.: Details of the lift-off recipe used for hydrogen sensor fabrication.
Once this process is completed, the sample is ready for metal deposition.
5.3 Metal Deposition and Lift-Off
A BOC Edwards Auto 306 Electron Beam Evaporator is used for metal deposition. Electron
beam evaporation works by heating a target material with an electron beam contained in a
vacuum chamber. As the target materials heats, it converts to the gaseous phase and then
precipitates in the chamber, coating everything within line of sight of the target with a thin layer
of solid target material.
The target material in this case was a small amount of palladium pellets contained within a 4 cc
Al2O3 crucible. After inserting the samples into the chamber and pumping it down to near-
vacuum, the electron beam gun power is increased to 11 mA to allow the target to pre-bake.
Note that the shutter over the target is kept closed during pre-bake so that the samples aren’t
exposed to any errant target material. After a pre-bake of 4 minutes, the beam current is
increased to 22 mA and then the shutter is opened. This current gives a very slow deposition rate
of approximately 0.5-1 Å/s.
After the metal deposition is complete, the only step remaining is to remove the remaining lift-
off resist. In order to do this, the sample is allowed to soak in ZDMAC stripper for 15 minutes at
a temperature of approximately 60oC and then placed in an ultrasonic bath for 1 minute. If the
resist has hardened and isn’t completely removed at this point, the sample can be allowed to soak
in ZDMAC overnight.
A schematic showing the metal deposition and resist removal can be seen in figure 5.7.
Fig. 5
expo
depo
5.4
After
determ
As ex
proce
the d
the d
of the
.7.: Diagram sh
osed, and devel
osited only with
T
Hydrog
r fabrication
mine the etc
xpected, the
ess, as can b
evice. Even
evice are sti
e etch can be
Fig. 5.8.: SEM
howing the me
loped. Resist c
hin the window
The end result is
gen Sens
was comple
ch quality as
ere is a dec
be seen in fig
n though the
ill within an
e further imp
M image of the
etal deposition
creates a windo
ws created by th
s a ridge waveg
sor Fabr
eted on the h
well as the t
ent amount
gure 5.8. B
surface and
acceptable
proved by m
two adjacent tr
process. a) E
ow for metal de
he resist. d) W
guide with a st
rication R
hydrogen sen
take a look a
of surface
Both of these
d sidewalls ap
range, as wi
more fine tuni
roughs that we
Etched SOI waf
eposition. c) W
Wafer after met
trip of metal co
Results
nsor sample
at the metal
and sidewa
e contribute
appear to be
ill be discus
ing of the etc
ere etched in or
fer. b) SOI wa
Wafer after me
tal deposition a
oating a section
es, SEM ima
layers.
all roughness
to the overa
quite rough,
sed in chapt
ch recipe.
rder to create a
afer after resist
etal deposition.
and final resist
n of it.
ages were ac
s due to the
all insertion
, the optical
ter 6. The ro
a single wavegu
57
is applied,
Metal is
removal.
cquired to
e etching
losses in
losses of
oughness
uide.
Anoth
SEM
appro
slante
samp
As fo
and 2
depos
her unwante
M image of t
oximately 10
ed sidewalls
ple with deep
or the pallad
20 µm. See
sited on top
ed effect that
the wavegu
0o by atomic
s cannot be p
p reactive ion
Fig. 5.9
dium metal, i
e figures 5.1
of waveguid
t RIE produ
ide etch pro
force micro
prevented.
n etching (D
.: Zoomed-in S
it deposited
10 and 5.11
des.
uces is slante
ofile. The
oscopy. Unf
One way to
DRIE).
SEM image of
very cleanly
for SEM i
ed sidewalls.
angle of th
fortunately, s
o have more
f a single etched
y and quite e
images of p
. See figure
he sidewalls
since RIE is
e vertical sid
d waveguide
evenly for le
alladium me
e 5.9 for a zo
s was verifi
an isotropic
dewalls is to
engths betwe
etal layers t
58
oomed in
ied to be
c process,
o etch the
een 8 µm
that were
Figg. 5.10.: SEM i
Fig. 5.11.:
image of a 2 nm
Zoomed-in SE
m thick deposit
EM image of a
tion of palladiu
waveguides
2 nm thick pal
um (of lengths
s.
lladium deposi
8 µm and 12 µ
ition onto a sin
µm) onto two s
ngle waveguide
59
separate
e.
6
6.1
All sa
seen
wave
out o
loss m
Polar
the p
coupl
dimen
Resea
An o
light
To co
from
Hydro
Experi
amples were
in figure 6.1
elength, 60 n
of the sensor
measuremen
rization padd
polarization
ling rig supp
nsions 38.5
arch System
optical chopp
at a frequen
Fig.
ontrol the g
the two cy
ogen Se
mental S
e tested in an
1. Light fro
nm bandwid
r using New
nts). A JDS E
dles and a ha
beam cube,
porting the s
cm 18.5
ms SR830 loc
per was plac
cy of 364 Hz
6.1.: Schemat
gas composit
ylinders cont
Sensor
ensor Ch
Setup
n optical cha
om a Thorlab
th, and outp
wport 60X ob
Erbium-dop
alf-wave pla
which was
sample and o
5 cm 12.5
ck-in amplifi
ced in the s
z.
tic of the exper
tion inside t
taining com
Chapterr Charac
haracte
aracterization
bs S5FC sup
put power of
bjectives len
ed fibre amp
ate were used
s used to co
objective len
5 cm. To m
iers were use
setup after th
rimental setup f
the box con
mpressed nitr
r 6 cterizatio
erization
n lab. A dia
perluminesce
f approxima
nses (a Thor
plifier was u
d to maximi
ontrol the in
nses were co
minimize the
ed for both i
he alignmen
for hydrogen s
ntaining the
rogen and 4
on
n
agram of the
ent diode wi
ately 21 mW
rlabs tunabl
used to amp
ize the amou
nput polariz
overed by a p
e effects of
input and ou
nt mirrors to
sensor characte
sample, the
4% hydrogen
e optical setu
ith 1531.6 n
W was couple
le laser was
plify the inpu
unt of power
zation. The
polycarbona
noise, two
utput signal d
o modulate t
erization.
control of
n (balanced
60
up can be
nm center
ed in and
used for
ut power.
r through
end fire
ate box of
Stanford
detection.
the input
gas flow
by 96%
61
nitrogen) are controlled by two Brooks 5850S digital mass flow controllers (MFC) using a
Brooks 0154 control unit. The individual flow rates are computer controlled and the outputs lead
to a T-junction to mix the gasses to the desired hydrogen concentration. The output of the T-
junction then leads to a union connection on the box where it is connected to a stainless steel
tube which directs the gas flow directly onto the sample. The tube to sample separation was
about 5 mm. A commercial hydrogen sensor from Nova Analytical Systems was used to verify
the hydrogen gas concentration in the gas mixture.
6.2 Losses
In order to measure the linear losses of the waveguides, the Fabry-Perot method was used [49].
Since each waveguide behaves as a cavity, they will exhibit a resonant effect that will be
different based on the losses within the cavity.
The losses for uncoated waveguides with a 90 nm pedestal ranged from 0.9 – 2.1 dB with an
average value of 1.48 dB (for waveguides that are not visibly damaged).
Metal-coated waveguides were also measured in order to determine the metal losses per micron
length of palladium. Losses were directly measured for Pd thicknesses of 1, 2, and 3 nm.
Results are shown in figure 6.2.
The s
the ab
6.3
The
transm
featur
prope
In 19
pallad
conce
Speci
thickn
which
drasti
temp
samples with
bove plot tho
Tempo
detection m
mission with
re of palladi
erties with re
985, Ming-W
dium-hydrog
entration iso
ifically, it w
ness approa
h is depende
ic change in
oral respons
Fig. 6.
h 5 nm of de
ough, if a lin
oral Resp
mechanism o
h respect to
ium thin film
espect to the
Way was one
gen system w
otherms of
was noticed
ches a certa
ent on the su
n certain pro
se, the most
2.: Metal losse
eposited Pd
near fit is ass
ponse
of the hydro
exposed hy
ms, in contr
eir thickness.
of the first t
with respect
the palladiu
that the th
in threshold
ubstrate mat
operties whe
notable of
es of the variou
were too lo
sumed, the lo
ogen sensor
ydrogen gas
rast to bulk p
.
to investigat
t to thicknes
um-hydroge
hin films be
d value of ar
terial, the pa
en compared
these prope
us Pd-coated w
ossy to obtain
osses can be
rs that have
s concentrati
palladium, i
te the pressur
ss [50]. Wha
en system a
ehaved much
round 50 nm
alladium-hyd
d to bulk pa
erties is the
waveguides.
n accurate m
e estimated a
e been teste
ion. An int
is that the fi
re concentra
at he found w
actually cha
h like bulk
m. Below th
drogen syste
alladium. W
effect that g
measurement
around 2.96 d
ed is the ch
teresting and
ilms exhibit
ation isotherm
was that the
ange with th
palladium
his specific th
em begins to
When speaki
grain bounda
62
ts. From
dB / µm.
hange in
d notable
different
ms of the
pressure
hickness.
until the
hickness,
o show a
ing about
aries and
63
defects have on the ability of the metal to absorb hydrogen. As the palladium layer gets thinner,
the effect that grain boundaries have on the film’s surface area to volume ratio become more
pronounced, and for a thinner film, this will reduce the amount of available hydrogen sites per
palladium atom [51]. What this implies is that as the film thickness is reduced, the localized size
of and locations of grain boundaries can have a profound effect on the amount of hydrogen that
is locally absorbed. This idea will be discussed in further detail as the different palladium
thicknesses and detection response results are discussed.
6.3.1 Results for Palladium Depositions Ranging from 2-5 nm
The temporal detection response experiments were carried out by first exposing a sample to a
constant flow of pure nitrogen gas to establish a baseline response. Next, the gas flow was
suddenly changed from pure nitrogen to 4% hydrogen (with a balance of nitrogen). Once the
sensor response reached a maximum, it was allowed to settle for a brief amount of time, and then
the sensor was flushed with pure nitrogen gas to reestablish the baseline response. This
procedure was repeated a number of times and the results were averaged. This test was
repeatable over hundreds of cycles with similar results. A typical response is shown below, in
figure 6.3, and is the temporal response of a waveguide with 700 nm width, 90 nm pedestal
height, 3 nm Pd thickness, and 8 µm Pd length. In order to ensure that the response arises from
the presence of hydrogen rather than some other factor, an uncoated waveguide was also tested.
The uncoated waveguide showed no response to hydrogen other than a momentary (~1 second)
and very limited (< 1% change in transmission) change in transmission when the hydrogen was
activated (due to a sudden change in gas flow rate onto the sample). For this reason, we can be
assured that the demonstrated responses are the result of the introduction of hydrogen gas into
the system.
As ca
result
lattic
For a
variab
pallad
great
shoul
these
incre
theor
issue
detec
the tw
alway
Fig. 6
an be seen, w
t of the redu
e.
a given palla
ble remaini
dium film, th
er amount o
ld cause a gr
extremely t
ased effect
rized that the
. While som
ction respons
wo variables
ys demonstr
6.3.: Temporal
when hydrog
ced electron
adium thickn
ng was pal
he greater th
of palladium
reater chang
thin films, h
that defect
e somewhat v
me of the in
se values fo
s. Another fa
rated an incr
response of a P
gen gas is int
n density of t
ness and wav
lladium film
he sensor res
. This mean
e in transmi
owever, that
s and grain
varying dete
ndividual wa
r different p
fact that supp
reased detec
Pd-coated wav
troduced, the
the metal aft
veguide dim
m length. I
ponse, since
s that a chan
ssion for a lo
t is not alwa
n boundaries
ection respon
aveguide res
pedestal heig
ports this the
ction respons
veguide when e
e transmissio
ter hydrogen
mensions (wid
It would se
e the wavegu
nge in the ab
ong strip tha
ays the case.
s play on v
nse values (s
sponses can
ghts, it is cle
eory is that s
se as the pa
exposed to hyd
on increases
n is absorbed
dth and pede
eem logical
uide mode w
bsorptive pr
an it should
. Revisiting
very thin pa
seen in figur
vary, if we
ear that ther
sensors with
alladium leng
drogen gas.
s. This incre
d and expand
estal height)
that the lo
would be exp
roperties of t
for a short s
g the early id
alladium fil
re 6.4) arise
e look at the
re is a trend
h a 13 nm film
gth increase
64
ease is a
ds the
), the one
onger the
posed to a
the metal
strip. For
dea of the
ms, it is
from this
e average
between
m almost
ed, which
woul
small
Fig.
with
The t
seen,
decre
small
which
conce
is qu
lower
depen
d be consiste
ler effect sin
. 6.4.: a) Detec
h different ped
temporal det
the magnit
eases. This
ler concentra
h will cause
entration of
uite high at t
r than this,
nding on the
ent with the
nce the 13 nm
ction response v
estal heights b
tection respo
tude of the
is consisten
ation gradie
e a smaller
hydrogen th
this point an
effectively
e detection eq
theory that g
m film has a
values for diffe
b) Average dete
p
onse for one
detection r
nt with theor
nt between
amount of
hat the senso
nd that the s
making the
quipment av
grain bound
much larger
erent lengths o
ection respons
alladium thick
e of the wav
response dec
ry, as a low
the ambient
hydrogen to
ors were test
sensor was n
e detection
vailable.
daries and de
r volume tha
f palladium fil
e of waveguid
kness).
veguides is
creases as t
wer hydrogen
t atmosphere
o be absorb
ted in was 0
not able to
limit of the
efects on the
an the thin 2-
lms (2 nm thick
es for varying
shown in fig
the hydroge
n gas concen
e and the pa
bed into the
.5%, but the
clearly dete
e sensor som
surface hav
-5 nm films.
kness) and wav
pedestal heigh
gure 6.5. A
en gas conc
ntration will
alladium me
metal. Th
e signal to no
ect any conc
mewhere ne
65
ve a much
.
veguides
hts (2 nm
As can be
centration
l create a
etal layer,
he lowest
oise ratio
centration
ear 0.5%,
Fig.
6.3.2
When
the s
pallad
6.5.: Detection
2 Resu
n the palladi
sensor is ex
dium films).
n response for v
p
ults for Pa
ium film thi
xposed to h
. This senso
varying hydrog
alladium thick
alladium D
ckness is re
hydrogen (as
or response c
gen concentrat
kness of 3 nm a
Deposition
duced to 1 n
s opposed t
can be seen i
tions for a wav
and length of 2
n of 1 nm
nm, a decrea
to an incre
n figure 6.6.
veguide with pe
0 µm.
ase in transm
ase in trans
.
edestal height o
mission is se
smission fo
66
of 90 nm,
een when
r thicker
The t
thin t
the fi
of dis
expos
form
film,
This
optic
is po
volta
resolu
order
resolu
under
senso
theor
theorized rea
that effectiv
ilm is not ab
screte pallad
sed to hydro
a continuou
which oppo
change in re
ally as well.
ssible that th
ge rating is
ution) or tha
r to verify i
ution SEM
r SEM. W
or correspon
rized to be th
Fig. 6.6.:
ason for this
ely turns the
ble to deposi
dium islands
ogen, the dis
us film [52]
oses and has
esistance rep
Although i
he gap geom
s required t
at the surfac
island forma
or to cut int
While island
nd very well
he reason for
Temporal resp
s change in
e sensor into
t evenly. A
s forming. A
screte pallad
. This actua
a greater ef
presents a ch
island forma
metry is too
to obtain go
ce is continu
ation, it wo
to the metal
formation w
to conventi
r the negativ
ponse of a wave
response is
o a nanogap
As a result, in
As investigat
dium islands
ally results
ffect than the
hange in ma
ation was not
small to see
ood contras
uous while t
ould be nec
l with a foc
was not visu
ional nanoga
ve sensor resp
eguide coated
due to the f
p sensor. A
nstead of a c
ated by Lee,
s expand in
in a decreas
e increase in
aterial prope
t able to be v
e (possible a
st for Pd on
there are vo
essary to im
cused ion be
ually verifie
ap sensor pr
ponse.
in 1 nm of Pd.
fact that a 1
nanogap sen
continuous fi
when these
volume and
se in resistan
n resistance t
erty which s
verified by c
as a relativel
n SOI, whi
olume gaps w
mage the sa
eam and ima
ed, the resul
resented in l
nm deposit
nsor is creat
film, we have
nanogap se
d connect to
nce of the p
that normall
hould manif
conventiona
ly low electr
ich results
within the m
ample with
age the cros
lts presented
literature, an
67
tion is so
ted when
e a series
nsors are
gether to
palladium
y occurs.
fest itself
l SEM, it
ron beam
in lower
metal. In
a higher
ss section
d by this
nd this is
6.4
Due
obser
note t
H-H
break
conce
bond
transm
that a
from
same
must
Fig.
To ex
wave
pallad
heigh
conce
from
Hyster
to the fact t
rved, as seen
that palladiu
bonds are br
k a Pd-H bo
entration gra
[7]. This e
mission duri
a certain cha
3% to 3.5%
state it was
be reduced
6.7.: Plot show
xamine the h
eguides were
dium thickn
ht of 85-90
entration is
4-0% in 0.
resis
that a pure P
n in figure 6
um can exist
roken and P
ond than it
adient requir
xplains why
ing absorptio
ange in transm
%. Once at
s in when o
to approxim
wing the hyster
hysteresis eff
e chosen that
ness. The w
nm, and p
gradually in
.5% decrem
Pd film was
6.7. To exp
in two diffe
d-H bonds a
does to br
red to break
y there is a h
on and desor
mission can
3.5% conce
riginally exp
mately 2%.
resis effect pre
fect of the pa
t are designe
waveguide d
palladium le
ncreased, fir
ments. At ea
s used, a sig
plain this phe
erent phases.
are formed.
eak a H-H
a Pd-H bond
hysteresis eff
rption. For e
be achieved
ntration thou
posed to 3%
sent in a typica
alladium-hy
ed to be iden
dimensions
ength of 8 µ
rst from 0-4
ach increme
gnificant hys
enomenon, w
. As the pall
Since it tak
bond, the a
d is greater t
ffect in the tr
example, if w
d from increa
ugh, in orde
% hydrogen
al Pd-coated w
ydrogen syste
ntical in all r
are: wavegu
µm. To ob
4% in 0.5%
ent, the tran
steresis effe
we look bac
ladium lattic
kes a greater
absolute val
than that req
ransmission
we look at f
asing the hyd
er to return t
concentratio
waveguide when
em on the SO
respects exc
uide width o
btain these
increments
nsmission is
ct was expe
ck to section
ce enters the
amount of e
lue of the h
quired to brea
when we lo
figure 6.7, w
drogen conc
the metal lay
on, the conc
n exposed to h
OI waveguid
cept for the d
of 800 nm,
plots, the h
s and then d
allowed to
68
ected and
n 2.1 and
β-phase,
energy to
hydrogen
ak a H-H
ook at the
we can see
centration
yer to the
centration
hydrogen.
des, three
deposited
pedestal
hydrogen
decreased
stabilize
befor
at eac
6.4.
The r
6.8.
Note
exper
clarit
transm
diffic
uncoa
metal
The m
pallad
effect
re the concen
ch concentra
1 Resu
resultant hys
Fig.
that the r
riments) in
ty). It has
mission valu
culty of prec
ated wavegu
l deposition.
most notewo
dium thickn
t that relates
ntration is ch
ation level is
ults for 2-5
steresis loop
6.8.: Hysteresi
esults are a
order to en
been deter
ues is largely
cisely aligni
uides in orde
.
orthy featur
ness is incre
s the tensile
hanged to th
extracted.
5 nm Pall
ps for pallad
is loops for pal
adjusted, as
sure that th
rmined from
y a result of
ing nano-wa
er to ensure t
e to note he
ased. This
e stiffness of
he next incre
adium De
dium thickne
lladium thickne
ssuming a l
he starting a
m experimen
f power drif
aveguides.
that it wasn’
ere is the in
can be exp
f a film to i
ement. From
eposition
esses of 2, 3
esses of 2,3, an
linear powe
and ending v
nts that the
ft due to rela
The presen
’t due to an e
ncreasing siz
plained by th
its thickness
m this plot, t
3, and 5 nm
nd 5 nm Pd dep
er draft (fa
values are t
e different
axing stage
nce of this d
effect arising
ze of the hy
he clamping
s. Specifica
the average
m are shown
positions.
airly consist
the same (in
starting and
micrometers
drift was ve
g from the p
ysteresis loop
g effect, wh
ally, it states
69
response
in figure
tent with
ncreasing
d ending
s and the
erified on
palladium
ps as the
hich is an
s that the
70
thinner the film, the higher the tensile stiffness [53]. For very thin films, this high tensile
stiffness limits the amount of volume expansion the film can undergo, which in turn limits the
amount of hydrogen that the film can absorb. These effects were demonstrated in much greater
detail by Lee [7], who determined that thinner films always demonstrated smaller hysteresis
loops, but generally also showed a lower response. Note that the measurements conducted by
Lee were resistance measurements on very large films (in dimensions orthogonal to thickness).
For the results presented here, note that the hysteresis effect was never completed eliminated
(unlike Lee, who was able to demonstrate a 5 nm film that exhibited no hysteresis). The
discrepancy is likely due to the fact that the palladium strips on the waveguides had dimensions
on the order of microns, while the palladium layers tested by Lee were deposited onto a 12.5 mm
x 12.5 mm piece of silicon, which likely had a much greater adhesion to the substrate.
6.4.2 Results for 1 nm Palladium Deposition
As can be seen in figure 6.9, a 1 nm deposition of palladium on the sensor still exhibits a
noticeable hysteresis effect. The reason for this effect is the same as was explained in the
previous section, as the palladium is still demonstrating a phase transition during hydrogen
exposure.
Due t
gas s
conce
hyste
6.5
The r
imple
respo
based
speed
optic
6.5.
The
incre
achie
to the presen
afety sensor
entration in
eresis effect w
Respo
response tim
ementation.
onse time is
d hydrogen
d, and even
al hydrogen
1 Resu
main proble
ases as the
eves a respon
Fig. 6
nce of hyste
r rather than
real-time. I
will be discu
onse Time
me of a senso
The Unite
<1s for mea
sensors hav
fewer over
sensor that m
ults for 2-5
em with m
hydrogen c
nse time of 5
6.9.: Hysteresis
eresis, the se
a sensor tha
In section 8
ussed.
e
or is one of
ed States De
asurement ra
ve been able
r the measur
meets those
5 nm Pall
ost palladiu
concentration
5 seconds fo
s loop for palla
ensor would
at is able to
8.2.1, some s
the most im
epartment of
anges from
e to demons
rement rang
goals and is
adium De
um-based hy
n decreases.
or 4% hydrog
adium thicknes
be better se
continuously
strategies th
mportant met
f Energy tar
0.1-10% [54
strate respon
ge of interes
s also robust
eposition
ydrogen sen
. Because
gen may dem
ss of 1 nm.
erved as an o
y monitor am
hat can be u
trics to cons
rget for hyd
4]. So far,
nse times ev
st. Creating
t is quite a ch
nsors is tha
of this, a h
monstrate a
on-off type h
mbient hydr
used to elimi
sider in any
drogen safet
very few pa
ven approac
g a palladiu
hallenge.
at the respo
hydrogen sen
response tim
71
hydrogen
rogen gas
inate this
practical
ty sensor
alladium-
ched that
um-based,
nse time
nsor that
me that is
ten ti
they
that t
an in
conce
Anoth
demo
in pe
hydro
reduc
hydro
effect
comp
[7], a
down
mes as long
don’t compl
the response
ntermediate
entrations m
her interesti
onstrates a ri
eak location
ogen system
ced, there is
ogen concen
t of palladiu
pletely suppr
and it appea
nward, as sho
for 0.5% hy
letely show
time values
peak, with
measured (0.5
Fig. 6.10.: Re
ing property
ightward shi
n is due to
m as palladiu
a reduction
ntration at a
um has been
ressing the p
ars as if red
own in figur
ydrogen. An
this trend. F
for palladiu
the lowest
5% and 4%,
esponse time da
y that the r
ift. It is theo
the changi
m films get
n in the over
fixed temp
n shown to
phase transit
ducing the
re 6.11.
n interesting
From experi
um films mea
response tim
respectively
ata for palladiu
results show
orized that t
ing pressure
thinner [50
rall width o
erature. Co
be eliminat
tion by reduc
film thickne
g property of
imental data
asuring 5 nm
mes near th
y).
um thicknesses
w is that th
the intermed
e-compositio
]. As the th
of the mixed
ombine this
ted for extre
cing the crit
ess shifts th
f very thin pa
a, seen in fig
m or thinner
he highest a
s of 2,3, and 5 n
he response
diate peaks a
on isotherm
hickness of t
d α and β ph
with the fac
emely thin
tical tempera
he overall p
alladium film
gure 6.10, it
actually dem
and lowest h
nm.
time peak
and the appa
ms of the pa
the palladium
hase (with r
ct that the h
films (as a
ature of the
phase diagra
72
ms is that
is shown
monstrate
hydrogen
actually
arent shift
alladium-
m film is
respect to
hysteresis
result of
material)
am curve
Fig. 6
As ca
transi
we n
conce
in the
in the
So fa
system
scien
6.5.2
Whil
than
comp
appro
10 se
6.11.: Theorize
(a
an be seen i
ition phase i
notice that
entrations, a
e response ti
e curve likely
ar, no work
m for thickn
nce front in o
2 Resu
e the magni
a 1% chan
pared to the
oximately 15
econds for m
d phase diagra
approaching bu
in figure 6.1
is extended.
the α-phas
and the phase
ime curves i
y correspond
has been d
nesses 5 nm
order to fully
ults for 1 n
itude of the
nge in transm
thicker pall
5 seconds wh
much of the h
ams for thick an
ulk) b) Theor
11a, for thick
Compare t
e is now e
e transition p
is due to the
ds to the pha
one to exten
m and under,
y explain the
nm Pallad
detection re
mission), th
ladium film
hen exposed
hydrogen exp
nd thin palladiu
rized phase diag
k films at 5
this to the th
extended to
phase is sho
se changes i
ase transition
nsively stud
so more wo
se effects.
dium Depo
esponse for
hey do exhib
m depositions
d to 2% hydr
posure spectr
um films. a) P
agram for very
0oC, the α-p
heorized pha
o encompas
ortened. It is
in the phase
n.
dy the prope
ork would n
osition
1 nm pallad
bit the faste
s. The sens
rogen and h
rum between
Phase diagram
thin palladium
phase is ver
ase diagram
ss a wider
s theorized t
e diagram. S
erties of the
need to be d
dium sensor
est overall
sor has a m
has a respons
n 0.5-4%, sh
m for thick palla
m film.
ry thin and t
in figure 6.
range of h
that the shift
Specifically,
hydrogen-p
done on the m
rs is rather l
response tim
maximum res
se time of lo
hown in figu
73
adium film
the phase
.11b, and
hydrogen
fting peak
the peak
palladium
materials
low (less
me when
sponse of
ower than
ure 6.12.
Even
due to
relati
n with the hy
o its large re
ively fast res
Fig. 6.12
steresis limi
esponse to hy
sponse time.
2.: Response ti
itation, the se
ydrogen gas
ime data for pa
ensor shows
concentratio
alladium thickn
s great poten
ons below th
ness of 1 nm.
ntial as a hyd
he lower exp
drogen safety
plosive limit
74
y sensor
and
Th
7
7.1
One o
multi
splitt
which
differ
onto
Some
mono
type
was e
Of th
temp
was c
sensit
hydro
chara
7.2
The f
direct
advan
he Optica
The OTempe
The Op
of the main
iple sensors
er at the inp
h would be
rent coating
a single dev
e coatings th
oxide detecti
of resonant
explored in c
he possible
erature sens
chosen as th
tivity and it
ogen senso
acterization o
Tempe
first design d
tional coupl
ntages or dis
al Nose:
Optical erature
ptical No
advantages
can be inte
put, it is easy
able to dete
or device s
ice.
hat would b
ion or Al2O3
structure suc
chapter 3.
additional s
or was chos
e sensing str
is relatively
r. The f
of this tempe
erature S
decision to b
ler or a mult
sadvantages.
Design a
Nose: Sensor
ose
of using an
egrated onto
y to have a
ect a differen
tructure wou
be of interes
3 for humidit
ch as a ring
sensing elem
sen for demo
ructure as it
y easy to fab
following s
erature senso
Sensor D
be made is th
timode-inter
Chapterand Impl
Senso
Designr
SOI platform
the same d
single input
nt environm
uld make it
st would be
ty [57]. A p
resonator, g
ments that co
onstrational
offers excel
bricate and i
sections wi
or.
esign
he coupling
rference (MM
r 7 lementat
or
and Im
m for a hydr
device. Usin
t source feed
mental param
possible to
e either NiO
possible devi
grating, or ca
ould be add
purposes. A
llent flexibil
integrate on
ill explore
method. Th
MI) coupler
tion of a
mpleme
rogen gas se
ng a simple
d into multip
meter. For e
integrate a
Ox [55] or S
ice that could
avity for tem
ded to an op
An MMI-co
ity in terms
nto the same
the design
he two choic
r. Each cou
Temper
entation
nsor is ease
e 1 x N MM
ple channels
ach channel
multitude o
nO2 [56] fo
d be used wo
mperature se
ptical nose
oupled ring r
of sensing r
SOI platfor
n, fabricati
ces are eithe
upling metho
75
rature
of a
at which
MI power
s, each of
l, using a
f sensors
or carbon
ould be a
ensing, as
sensor, a
resonator
range and
rm as the
ion, and
er using a
od has its
76
A directional coupler has more versatility in terms of controlling the coupling. The amount of
light coupled into the ring can be controlled quite precisely with the ring-to-bus gap. Because
there is no need for an extended coupling section (as there would be with an MMI coupler),
much higher quality factors and free spectral ranges can be obtained for very small ring radii.
There are a couple of major disadvantages with directional couplers. The first is the difficulty of
fabrication. Because of the typically very small ring-to-bus radius (around 100-200 nm), it is
very difficult to be able to etch a high quality gap of those dimensions, especially with RIE. This
plays directly into to the second disadvantage, which is the sensitivity of the device to
dimensional variations. The smaller the ring radius gets, the device will become more sensitive
to variations in the group index of the waveguide, which change as the dimensions of the
waveguide change. Since is it very difficult to create multiple devices with identical dimensions,
this makes a sensor utilizing directional couplers challenging to reproduce in large quantities.
MMI couplers are almost the opposite in terms of advantages and disadvantages when compared
to directional couplers. They are constrained in terms of the possible power splitting ratios that
can be used (50/50 and 85/15 are commonly used). The additional length that they introduce
into the resonant area (equal to the length of the coupler multiplied by two) makes it very
difficult to achieve high quality factors or free spectral ranges [58]. On the other hand, they are
very easy to fabricate, and the additional length added into the resonant area makes the devices
much less sensitive to dimensional variations. The coupling itself it also much less sensitive to
dimensional variations when compared to a directional coupler, where a small difference in ring-
to-bus distance can make a big difference in coupling ratio.
For these reasons discussed above, an MMI coupler was chosen for the design. This will allow
for relatively easy on-site fabrication and is a more repeatable and easily testable configuration.
To achieve critical coupling, the losses in the resonant area of the device must be balanced out
with the coupling into the ring [58]. Since the exact propagation losses are not easily simulated,
in order to ensure that one of the fabricated devices will come close to achieving critical
coupling, a series of different couplers were designed and fabricated. 50/50 couplers were
designed with widths of 3.3 and 4.5 µm and 85/15 couplers were designed with widths of 2.2 and
3.0 µm. A schematic showing the relevant dimensions involved in the design of an MMI coupler
is shown in figure 7.1. For an 85/15 coupler, the center-to-center separation of the input and
outpu
separ
Fig
(equa
When
appro
the M
the co
One d
two i
reach
pedes
to a v
For e
to tun
desig
Final
obtain
temp
simul
ut waveguid
ration must b
g. 7.1.: Diagram
al to the silicon
n designing
opriate lengt
MMI coupler
orrect couple
design detai
input wavegu
hes a distanc
stal height o
value of appr
each coupler
ne the desire
gned coupler
lly, in order
ned from [
eratures of 2
lated sensitiv
es must be e
be equal to o
m showing the
n top layer heig
g an MMI
th is directly
r, a series of
er lengths w
il to keep in
uides also d
ce of 300 nm
f 90 nm. Si
roximately 2
r length, a se
ed FSR and
r.
to determine
59] for silic
250 K and 30
vity of the d
exactly half
one third of t
relevant dimen
ght minus the e
iii) MM
coupler, a
y related to t
f simulations
were calculate
mind is as t
ecreases. O
m, there wil
nce this is an
2.2 µm for 85
eries of diffe
d extinction.
e an approxi
con for a s
00 K. This
evice was de
of the total
the coupler w
nsions when de
etch depth) ii)
MI width iv) M
desired wid
the width. In
s were condu
ed for a serie
the coupler
Once edge to
ll be some c
n undesired
5/15 coupler
erent ring rad
Ring radii
imate sensiti
series of wa
data was use
etermined to
coupler wid
width.
esigning an MM
Center-to-cen
MMI length.
dth is typic
n order to ch
ucted in Lum
es of widths.
width decre
edge separa
coupling bet
effect, the c
rs and 3.3 µm
dii will also
of 5, 10, an
ivity of the d
avelengths r
ed in a Lum
o be approxim
dth and for a
MI coupler. i)
nter input/outpu
cally chosen
hoose the op
merical MO
.
eases, the sep
ation of the
tween the tw
coupler width
m for 50/50
be fabricate
nd 15 µm w
device, refra
ranging from
merical MOD
mately 64 pm
a 50/50 coup
Height of the
ut waveguide s
n first and
ptimal dimen
DE Solution
paration bet
adjacent wa
wo waveguid
h is therefor
couplers.
ed in order to
were chosen
active index
m 1.2 to 3
DE simulation
m/K.
77
plers, the
coupler
separation
then the
nsions of
ns, where
tween the
aveguides
des for a
re limited
o be able
for each
data was
3 µm for
n and the
7.3
The
param
expos
nA b
param
RIE e
the M
Tempe
temperature
meters as th
sed using 5
eam current
meters for li
etched using
MMI coupler
erature S
e sensor w
he hydrogen
nm resolutio
t for the tape
ithography e
g the same re
r and overall
Fig
Sensor Fa
was patterned
sensor wav
on and a 1 n
er and active
exposure and
ecipe and par
device can b
g. 7.2.: SEM im
abricatio
d using ele
veguides ex
nA beam cur
e section of
d developme
rameters des
be seen in fi
mage of a 4.5 µ
on
ectron beam
xcept for the
rrent as oppo
the hydroge
ent were the
scribed in se
igures 7.2, 7
µm long MMI c
m lithograp
e fact that t
osed to a 10
en sensor wa
e same. Th
ection 5.1. S
.3, and 7.4.
coupler.
hy using t
the ring sec
nm resoluti
aveguides.
he devices w
Some SEM i
78
the same
ction was
ion and 5
All other
were then
mages of
Figg. 7.3.: Zoomed
Fig
d-in SEM imag
g. 7.4.: SEM im
ge of the input
mage of MMI c
waveguide gap
coupled ring re
p on an MMI c
sonator.
coupler.
79
7.4
7.4.
The
polar
entire
The s
nm b
a sin
silver
are u
samp
using
At th
and i
is col
For te
of a t
locate
Tempe
1 Expe
experimenta
rizers, detect
ety in figure
Fig. 7.5.
source is a T
andwidth, an
gle mode fi
r-coated mir
used for pola
pler is then u
g a Newport
he output, lig
s fed into an
llected by a c
emperature c
thermo-elect
ed in the cop
erature S
erimental
al setup for
tors, couplin
7.5.
: Schematic sh
Thorlabs S5
nd output po
ibre into a f
rrors and pa
arization con
used in order
Ultralign en
ght is couple
n Ando AQ6
computer thr
control, the
tric cooling
pper block w
Sensor C
Setup
r the temp
ng rig, optica
howing the exp
FC superlum
ower of appr
fibre-to-free-
ssed through
ntrol along w
to determin
nd-fire coup
ed back into
6317B optic
rough a GPI
sample is m
unit with di
where a 10 k
Character
erature sens
al spectrum
perimental setup
minescent di
roximately 2
-space collim
h a half-wav
with polariz
ne the input p
pling stage w
a fibre usin
cal spectrum
IB connectio
ounted on to
imensions 6
kΩ thermisto
rization
sor characte
analyzer, an
up for temperatu
iode with 15
21 mW. Ligh
mator. The
ve plate and
zation paddl
power. Ligh
with a pair o
ng a translat
m analyzer. D
on using a M
op of a copp
6 mm x 10 m
or is placed.
erization co
nd camera an
ure sensor cha
531.6 nm ce
ht from the
beam is th
d a polarizin
les on the in
ht is then cou
of Newport 6
ting stage an
Data from th
Matlab script.
per block, wh
mm x 1 mm
Excess spa
onsists of a
nd can be se
aracterization.
enter wavele
source is fed
hen aligned w
ng beam cub
nput fibre).
upled into th
60x objectiv
nd 20x objec
he spectrum
hich is locate
m. There is
ace within th
80
a source,
een in its
ength, 60
d through
with two
be (which
A beam
he sample
ve lenses.
ctive lens
analyzer
ed on top
a groove
he groove
is fil
Light
Using
using
7.4.2
In tes
coupl
and f
Fig
The s
aroun
losse
senso
lled with th
twave LDC-
g feedback f
g the TEC un
2 Resu
sting the fab
ling sections
free spectral
g. 7.6.: Transmi
sensor demo
nd 7.7 nm.
s are quite h
or’s perform
hermal paste
-3916 laser
from the the
nit mounted
ults
ricated samp
s, and 5 µm
range. The
ission spectrum
onstrates exti
The low me
high. This is
ance. In fac
e. The TEC
diode contro
ermistor, this
underneath t
ples, it was f
ring radii d
transmission
m for one of the
coupling
inction ratio
easured Q-fa
sn’t a major
ct, having a
C unit and
oller, which
s device is a
the copper b
found that th
emonstrated
n result of on
e ring resonato
g section, 5 µm
os of approxi
actor (~1000
problem tho
low Q actua
thermistor
h is used for
able to contro
block.
he devices w
d the best pe
ne of these d
or temperature
m ring radius).
imately 7-9
0) indicates t
ough as a hig
ally enables
are then co
r its TEC co
ol the tempe
with 50/50 co
erformance in
devices is sh
sensors (50/50
dB with a f
that the wav
gh Q is not r
the possibili
onnected to
ontroller cap
erature of th
ouplers, 4.5
n terms of e
hown in figur
0 coupler, 4.5 µ
free spectral
veguide and
really releva
ity of intens
81
an ILX
pabilities.
he sample
µm wide
extinction
re 7.6.
µm wide
range of
coupling
ant to this
ity-based
sensin
discu
The s
shifti
1532
With
nm p
match
comp
expan
Anoth
sensit
sensin
ng for this d
ussed.
sensor was
ing resonanc
nm). The re
Fig. 7
increasing t
per 10oC in
hing up clo
pared to the
nsion as wel
her property
tivity and th
ng range of
device, which
tested in tem
ce peaks, onl
esults can be
.7.: A single re
temperatures
ncrease in te
osely with s
simulated re
ll as fabricati
y of the sen
he free spectr
the device i
h will be exp
mperatures
ly one of the
e seen in figu
esonance peak
s, the resona
emperature,
simulated re
esults can lik
ion imperfec
nsors that w
ral range of t
f a waveleng
plained in a
ranging from
e resonance p
ure 7.7.
of the sensor f
ance peak ex
giving the
esults. The
kely be attrib
ctions.
was examine
the device.
gth based se
moment aft
m 20oC to
peaks was fo
for temperature
xperiences a
sensor a s
increased
buted to the
ed was the e
These two p
ensing schem
ter the sensit
50oC. In o
focused on (t
es varying from
red shift of
sensitivity v
sensitivity o
shift in reso
effect of the
properties de
me is used. T
tivity of the
order to illus
the peak loc
m 20-50oC.
approximate
value of 74.
of the actua
nance due to
e ring radiu
etermine the
The sensing
82
sensor is
strate the
ated near
ely 0.748
.8 pm/K,
al device
o thermal
us on the
effective
g range of
the de
an am
The e
ring r
Fig. 7
As ca
An in
the e
depen
than
drasti
100 K
evice will be
mount equal
experimenta
radii of 5, 10
7.8.: Free spect
an be seen f
ncreased sen
xpense of a
nd on the sp
an increased
ically on a s
K, which is s
e limited by
to the free sp
al results for
0, and 15 µm
tral range (a), s
from the resu
nsitivity can
reduced FS
pecific applic
d sensitivity,
scale of sing
suitable for m
the total tem
pectral range
the free spe
m are shown
sensitivity (b),
ults, there is
be achieved
SR, which g
cation of the
, as the prop
gle degrees.
most indoor
mperature ch
e of the devi
ectral range,
in figure 7.8
and Effective
s a clear trad
d by having
ives a reduc
e device, bu
perties of the
This speci
or outdoor a
hange that ca
ice and can b
.
, sensitivity,
8.
sensing range
deoff betwee
a longer res
ced sensing
ut in general
e hydrogen p
ific device h
applications.
auses a speci
be expressed
, and effecti
(c) of the devic
en sensitivit
sonant lengt
range. The
a larger ran
palladium sy
has a sensin
.
ific resonanc
d as:
ive sensing r
ce for varying
ty and sensin
th, but this w
e optimal de
nge is more
ystem do no
ng range app
83
ce to shift
(7.1)
range for
ring radii.
ng range.
will be at
esign will
desirable
ot change
proaching
One f
max)
wave
has b
In ord
half o
Fig
In thi
half w
temp
and t
and th
equal
which
7.9, t
make
decre
sensin
woul
final interest
allows the
elength based
been develop
der to have a
of one of the
g. 7.9.: Intensity
single resonan
is case, the
way between
erature. If t
he transmiss
he transmiss
l to the spect
h becomes la
this distance
e the effecti
easing the Q
ng would re
d be inheren
ting thing wo
e detection
d. This wou
ped and wou
an intensity b
e resonance p
y-based sensin
nce peak show
b) Normaliz
wavelength
n the resona
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sion would i
sion would d
tral distance
arger for sm
e would be a
ive sensing
Q-factor of th
equire more
ntly less accu
orth mention
mechanism
uld make the
uld reduce th
based detect
peaks, as sho
g scheme for th
wing the transm
zed transmissio
of the sourc
ance peak an
mental tempe
increase. If
decrease. Th
between the
maller Q devi
about 2.5 nm
range 33.6
he resonator
precise and
urate.
ning is that t
m of the sen
e sensor mor
he cost and
tion mechan
own in figur
he ring resonat
mitted intensity
on of the senso
ce would be
nd the start o
erature is in
f the tempera
he effective
e resonance
ices. For the
m. With a d
6 K. This
. The down
frequent cal
the low Q of
nsor to be
re compatib
complexity
ism, the sen
re 7.9.
tor temperature
at a specified w
or as a function
e chosen suc
of the resona
ncreased, the
ature is decr
sensing rang
peak and the
e specific res
device sensi
could be tu
nside of this
libration tha
f the device (
intensity b
ble with the h
of the over
nsor would h
e sensor. a) E
wavelength for
n of temperatur
ch that the s
ance dip for
e resonance p
reased, the p
ge in this det
e beginning
sonance peak
itivity of 74
uned to eve
approach is
an wavelengt
(high full-wi
ased as opp
hydrogen se
rall detection
ave to opera
Experimental da
r each tempera
re.
sensor would
r the desired
peak would
peak would b
tection mech
of the reson
k illustrated
4.8 pm/K, th
en higher v
s that intens
th-based sen
84
idth half-
posed to
ensor that
n system.
ate within
ata for a
ature
d operate
d baseline
red-shift
blue-shift
hanism is
nance dip,
in figure
his would
values by
ity-based
nsing and
8
8.1
An in
platfo
relati
below
phase
use o
as wi
The c
nose
temp
togeth
was d
sensit
demo
eleme
8.2
In ord
first
hyste
comp
of the
Concl
Summ
ntegrated, op
orm. The m
ively fast (<
w the explos
e change pre
of the sensor
ill be discuss
configuration
type device,
erature, hum
her with the
designed and
tivity and ra
onstration of
ents on the s
Future
der to furthe
is to furthe
eresis effect
pletely and th
ese ideas wil
C
usions
ary of Co
ptical, intens
agnitude of t
30s for a 2 n
ive limit of 4
esent as hydr
to more of a
sed in section
n of hydroge
, which wou
midity, etc.) i
hydrogen se
d implement
ange, which c
f this device
same chip.
Work
er improve th
er investigat
from the se
horough stud
ll be discuss
Conclusio
and Fut
ontributio
ity-based hy
the detection
nm thick pal
4%. A notic
rogen concen
an on-off typ
n 8.2.1.
en sensor pre
uld be able to
in a single de
ensor on the
ed using a ri
can both be
lays the grou
his technolo
te and expe
ensor respon
dy of the stru
ed in the fol
Chapterons and F
ture Wo
ons
ydrogen sens
n response is
lladium film)
ceable hyster
ntration incr
pe device un
esented wou
o sense multi
evice. To de
same chip,
ing resonato
adjusted dep
undwork for
gy, there are
eriment with
nse. The ot
ucture and p
llowing secti
r 8 Future W
ork
sor has been
s large (> 20
) over the ra
resis effect i
reases. This
nless modific
uld be very e
iple environm
emonstrate a
an optical ri
or structure.
pending on t
r the future i
e two major
h palladium
ther thing th
properties of
ions.
Work
demonstrate
0%) and the
ange of hydro
is present du
effect would
cations are m
easy to integr
mental facto
a device that
ing resonator
The sensor
the desired a
integration o
things that s
alloys in o
hat needs to
f very thin pa
ed on an SO
response tim
ogen concen
ue to there be
d limit the p
made to elimi
rate into an o
ors (different
t could be in
r temperatur
demonstrate
application.
of multiple se
should be do
order to rem
o be done is
alladium film
85
OI
me is
ntrations
eing a
ractical
inate it,
optical
t gases,
ntegrated
re sensor
ed good
The
ensing
one. The
move the
s a more
ms. Both
86
8.2.1 Palladium Alloys to Reduce Hysteresis Effects
An excellent way to remove the hysteresis effect that palladium experiences upon exposure to
hydrogen is to alloy the palladium with a metal such as nickel. By alloying palladium with
nickel, the α to β phase transition is suppressed, which for a sufficient amount of nickel content
allows the sensor to operate solely in the α-phase for a desired range of hydrogen concentrations
[60].
Operating exclusively in the α-phase and avoiding the phase transition to the β-phase has the
result of preventing the hydrogen atoms from chemically bonding with the palladium atoms. As
the forming and breaking of bonds is the reason for the hysteresis effect, preventing these bonds
from forming completely eliminates the effect, allowing the sensor to operate in a fully linear
regime. The tradeoff involved in this is that the sensor will have a greatly reduced detection
response magnitude. As the bulk of the refractive index and absorption change is due to the
phase transition into the β-phase, by operating completely in the α-phase, the detection response
will be reduced substantially. An approximate reduction (from experiments) is on the order of a
~5 times decrease in detection response.
8.2.2 Extensive Characterization of Thin Palladium Films
In order to completely understand the effects that these thin films exhibit when exposed to
hydrogen gas, it is necessary to fully understand the palladium-hydrogen system for very thin (<
10 nm) film thicknesses. While some work has been done to characterize thin films of
palladium, there has really been no extensive work done in characterizing films with thicknesses
below 10 nm. In order to accomplish this, a first step would be to cut into one of the films with a
focused ion beam in order to etch away a thin line of the film so that the full cross section can be
studied with a scanning electron or transmission electron microscope.
87
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