2
4
3
5
FUNDAMENTAL ABLATION OF ARGON-FLOURIDE EXCIMER LASER ON
POLYMETHYL METHACRYLATE BY INTERFEROMETY TECHNIQUE
HANANI BINTI YAHAYA @ JAAFAR
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Physics)
Faculty of Science
Universiti Teknologi Malaysia
MEI 2006
6
iii
To my beloved husband, parents, and families
iv
ACKNOWLEDGEMENT
First of all, in humble way I wish to give all the Praise to Allah, the
Almighty God for His mercy that gives me the strength, keredhaanNya and time to
complete this work. With His blessing may this work be beneficial for the whole of
humanity.
I wish to express my sincere gratitude and appreciation to my main thesis
supervisor, Associate Professor Dr Noriah Bidin for encouragement, guidance,
opinion, and enjoyable discussion throughout this study. I am also very thankful to
my co-supervisor, Associate Professor Dr Mohamad Khairi Saidin for his support
and suggestions. Without their continued support and interest, this thesis would not
have been the same as presented here.
I am also indebted to the Government of Malaysia through IRPA scholarship
and Universiti Teknologi Malaysia for funding my study. Without this financial
support, this project would not possible.
In loving memory, Allahyarham En. Nyan Abu Bakar should also be
recognised for his kindly help and assisting in carrying out experimental works. My
sincere appreciation also extends to all my colleagues, friends, and others who have
provided assistance at various occasions. Their views, concerns, tips, and
encouragements are useful indeed. Unfortunately, it is not possible to list all of
them in this limited space.
Last, but not least, I am very grateful to my beloved family especially my
beloved husband for their prayers, continuing support, patience, valuable advices,
and ideas throughout the duration of this study.
v
ABSTRACT
A fundamental study is carried out to fabricate or in other words to ablate an
optical material by using a single pulse of ultraviolet light. In this case, argon
fluoride excimer laser has been used as the ultraviolet light source while polymethyl
methacrylate (PMMA) sample is used as the optical material. The laser ablation was
conducted in between 1 to 20 pulses as a fundamental ablation. The ablation effects
on the PMMA sample were analysed by using interferometry method. Straight line
and equidistance fringes pattern is an indicator for smooth and flat surface. The
effect of ablation was quantified by measuring shifted distance, intensity changes,
and spacing reduction of fringes. The initial fringes shifted were notified as ablation
threshold. This occurred after 9 pulses of exposures with corresponding threshold
fluence of 3.18 mJ/mm2. The fringes pattern becomes peculiar and difficult to trace
at higher exposures. This contributed the increasing in shifted distance and
fluctuating in the fringes intensity. High degree of surface roughness is indicated by
the large fraction number of half wavelength and speckle existence. The resolidified of removal particles are possibly responsible to cause the refractive index
of the tested region become fluctuated in the range of 1.43 to 1.49. By increasing
number of laser pulses from 10 pulses to 20 pulses after ablation threshold, the
ablation depth on the tested region was estimated and found that the depth varied in
between 40 nm to 800 nm. The corresponding laser fluence had given were in
between 3.6 mJ/mm2 to 7.3 mJ/mm2. Hence the fundamental study succeeds to
ablate the PMMA material by using ultraviolet light of excimer laser.
vi
ABSTRAK
Satu kajian telah dijalankan bagi memfabrikasi atau dalam kata lain
mengablasi bahan optik menggunakan satu denyut cahaya ultraungu. Dalam kes ini,
laser eksimer argon-florida telah digunakan sebagai sumber cahaya ultraungu
manakala sampel polimethil methakrilat (PMMA) digunakan sebagai bahan optik.
Pengablasian laser telah diuji pada asasnya menggunakan 1 hingga 20 denyutan
laser. Kesan ablasi ke atas sampel PMMA telah dianalisis menggunakan kaedah
interferometri. Garis lurus dan jarak pinggir interferens yang sama menunjuk suatu
permukaan yang rata. Kesan ablasi telah dikuantitikan dengan mengukur perubahan
jarak anjakan, keamatan dan kelebaran jalur pinggir. Anjakan pertama yang berlaku
pada pinggir dikenalpasti sebagai takat ablasi. Ia berlaku pada 9 bilangan denyut
laser iaitu bersamaan dengan 3.18 mJ/mm2 takat ablasi. Ini meliputi pertambahan
dalam anjakan dan perubahan pada keamatan pinggir. Darjah kekasaran permukaan
yang tinggi dapat dilihat berdasarkan kepada nombor pecahan separuh panjang
gelombang yang besar dan kewujudan bintik laser. Pengerasan semula zarah bahan
yang terkeluar berkemungkinan menjadi punca kepada perubahan indeks biasan
yang berlaku pada kawasan ujian di mana ia telah berubah di antara 1.43 hingga
1.49. Dengan meningkatkan bilangan denyut dari 10 denyut ke 20 denyut selepas
takat ablasi, kedalaman ablasi pada kawasan ujian telah dianggarkan dan didapati
berada di antara 40 nm ke 800 nm. Tenaga per unit kawasan yang terlibat ialah di
antara 3.6 mJ/mm2 hingga 7.3 mJ/mm2. Oleh itu, kajian asas berjaya mengablasi
bahan PMMA dengan menggunakan cahaya ultraungu daripada laser eksimer.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
TABLE OF CONTENTS
vii
LIST OF TABLES
x
LIST OF FIGURES
xi
LIST OF SYMBOLS
xiv
LIST OF APPENDICES
xv
INTRODUCTION
1
1.1
Overview
1
1.2
Laser Machining
2
1.3
Interferometric Observation
5
1.4
Research Objective
6
1.5
Research Scopes
6
1.6
Thesis Outline
7
THEORY
9
2.1
Introduction
9
2.2
Laser Ablation
9
2.3
Optical Material
12
2.4
Refractive Index
14
2.5
Interference - Optical Phase
16
viii
2.6
3
21
METHODOLOGY AND MATERIAL
22
3.1
Introduction
22
3.2
Excimer Laser System
22
3.2.1
Internal Structure
23
3.2.2
External Triggering
25
3.3
Energy Measurement
26
3.4
Beam Profile
28
3.5
Sample Preparation
30
3.6
Interferometer
31
3.6.1
Magnification
34
3.6.2
Fringes Analysis
36
3.7
4
Summary
Summary
37
CHARACTERIZATION OF EXCIMER LASER
BEAM
39
4.1
Introduction
39
4.2
Energy and Peak Power
39
4.2.1
Pumping Energy
40
4.2.2
Number of Pulses
42
4.3
Excimer Laser Beam Profile
4.3.1
Beam Profile at Various Discharged
Voltage
4.3.2
43
45
Beam Profile at Various Working
Distance
48
4.4
Energy Per Unit Area
51
4.5
Summary
52
ix
5
DIAGNOSE THE ABLATION EFFECT BY
INTERFEROMETRY METHOD
54
5.1
Introduction
54
5.2
Reference Fringes
55
5.3
Ablation Threshold
57
5.4
Ablation Effect
59
5.4.1
Shifted Fringes
63
5.4.2
The Changes in Fringes Intensity
67
5.4.3
Fringes Spacing
71
5.5
6
7
76
DETERMINATION OF ABLATION DEPTH
78
6.1
Introduction
78
6.2
Brewster Technique
79
6.3
Refractive Index Changes
80
6.4
Ablation Depth
85
6.6
Summary
93
CONCLUSIONS AND SUGGESTION
94
7.1
Conclusions
94
7.2
Problems and Suggestions
97
REFERENCES
PUBLICATIONS
APPENDIX
Summary
x
LIST OF TABLES
TABLE NO.
4.1
TITLE
PAGE
Single pulse energy and peak power of the beam upon
discharged voltage
40
Pulse energy and peak power of the laser beam at
different number of pulses
42
4.3
Area of intensity distribution upon discharged voltages
47
4.4
Area of intensity distribution upon working distance
50
4.5
Energy per unit area
51
5.1
Data of shifted distance for all tested fringes (F1 to F7)
64
5.2
Data of fringes intensity for all tested fringes (F1 to F7)
67
5.3
Data of intensity changes for all tested fringes (F1 to F7)
68
5.4
Data of fringes spacing for all tested fringes (F1 to F7)
72
5.5
The flatness for all tested fringes (F1 to F7) in term of
ë/2
73
6.1
Power at different angle on the exposed material
81
6.2
Refractive index of the exposed material
84
6.3
Results of calculated depth, d for 10, 12, and 14 pulses
exposures
88
Results of calculated depth, d for 16, 18, and 20 pulses
exposures
89
4.2
6.4
xi
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
Hypothetical steps in the interaction of a laser beam with
a polymer surface (Srinivasan, 1993)
11
2.2
Structure of polymethyl methacrylate
12
2.3
Schematic of a UV-treatment system (Garbassi et al.,
1998)
13
A wave reflecting and refracting at an interfaced (Hecht
& Zajac, 1989)
15
2.4
2.5
Interference produced by division wavefront (Parker,
1988)
(a) Young’s two-pinhole interferometer (b)
Lloyd’s mirror
17
2.6
Interference produced by division of amplitude (Parker,
1988)
18
2.7
Formation of fringes of equal inclination by reflection in
a plane-parallel plate (Hariharan, 1985)
18
3.1
An electrical discharge exciting a gas laser (Hecht, 1992)
3.2
Internal energy of R-H molecule in excited and ground
state (Hecht, 1992)
24
3.3
An excimer laser connected to a function generator for
external triggering
25
3.4
Ophir Optronics energy meter and its display
27
3.5
Experimental set up for excimer laser beam calibration
27
3.6
CCD camera Beamstar Profiler and its software
28
3.7
Option screen for Beamstar CCD laser beam profiler
29
3.8
Option windows for beam size measurement
30
23
xii
3.9
Schematic diagram of sample for ablation work
3.10
Schematic diagram of interferometer setup over excimer
laser system
33
3.11
Option windows for ruler calibration
35
3.12
Option windows for calibration set-up
36
4.1
Pulse energy of the beam versus discharged voltage
41
4.2
Pulse energy of the laser pulse versus number of pulses
43
4.3
Beam profiles of ArF excimer laser in 3D view
44
4.4
Beam profiles of ArF excimer laser in top view (2D)
(Magnification 15X)
44
4.5
Beam profiles at various discharged voltages of the ArF
excimer laser (Magnification 15X)
46
4.6
The laser spot area versus discharged voltage
48
4.7
Beam spot at various working distance of the ArF
excimer laser (Magnification of 15X)
49
The laser spot area versus working distance. Laser
operated at discharged voltage of 12 kV with repetition
rate of 20 Hz
50
4.9
Energy per unit area versus number of laser pulses
52
5.1
The image of reference interference pattern
(Magnification 2.5X)
55
The ‘option windows’ of Matrox Inspector software
56
The fringes pattern of PMMA and its spectrum profile
before ablation
57
5.4
Fringes profile at threshold energy
59
5.5
Ablation interferograms and their line profiles after
exposed by (a) 10 pulses (b) 12 pulses (c) 14 pulses
61
5.6
Ablation interferograms and their line profiles after
exposed by (a) 16 pulses (b) 18 pulses (c) 20pulses
62
5.7
Graph shifted distance of fringes upon number of pulses
4.8
5.2
5.3
31
66
xiii
5.8
Graph intensity changes upon fringes for 12, 14, and 16
pulses exposures
69
Graph intensity changes upon fringes for 18 and 20
pulses exposures
70
5.10
Option windows for spacing measurement
72
5.11
Histogram of flatness upon fringes after exposed with 10,
12, and 14 pulses
74
5.12
Histogram of flatness upon fringes after exposed with 16,
18, and 20 pulses
75
5.9
6.1
Schematic diagram of Brewster’s angle measurement
80
6.2
Power of the laser beam versus incidence angle for 10,
12, and 14 pulses exposures
82
6.3
Power of the laser beam versus incidence angle for 16,
18, and 20 pulses exposures
83
6.4
Refractive index versus number of pulses
85
6.5
The example of fringe pattern shifts by an amount of ∆x
(Pedrotti and Pedrotti, 1993)
86
6.6
Histogram of ablation depth upon fringes after 10, 12,
and 14 pulses
90
6.7
Histogram of ablation depth upon fringes after 16, 18,
and 20 pulses
91
6.8
Depthness as a function of number of pulses
92
xiv
LIST OF SYMBOLS
A
-
Area of high intensity distribution
[A]
-
Amplitude of wave
c
-
Speed of light in vacuum
d
-
Ablation depth
E
-
Pulse energy
[E]||
-
Light with electric vector
[Er]||
-
Reflected light
[Et]||
-
Transmitted light
F
-
Flatness
I
-
Intensity
L
-
Working distance
m
-
Integer
M
-
Order of interference
N
-
Number of pulses
n
-
Refractive index
P
-
Power
∆p
-
Optical path difference
t
-
Thickness
∆t
-
Changes thickness
V
-
Discharged voltage
W
-
Normal spacing (before exposure)
w
-
Fringes spacing after exposure
∆x
-
Shifted fringes
-
Incidence angle
-
Refraction angle
-
Brewster’s angle
-
Optical phase
-
Wavelength
èi, è1
èt, è2
èp

ë
xv
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
Specification of excimer laser
105
B
Specification of energy meter
106
C
Specification of BeamStar CCD Profiler
108
CHAPTER 1
INTRODUCTION
1.1
Overview
Micromachining technology is an exciting and emerging area of modern
technology because it allows compact and minifeature sizes to be fabricated. It has
been widely applied to various micro-systems, micro-sensors, micro-actuators, and
micro-optics. Miniaturising devices using micro-optics revolutionise many electrooptical systems such as video cameras, video phones, compact disk data storage,
robotics vision, optical scanners and high definition projection displays (Jay and
Stern, 1994). Both higher accuracy and lower cost microlens array fabrication
methods (Naessens et al., 2003, and Pan and Shen, 2004) are needed to meet the
rapid growth for these commercial devices.
The traditional mechanical approaches of cutting, drilling, and shaping
materials are no longer satisfactory for fabricating micronscale structures. Instead,
fabrication using beam techniques based on photons, electrons, and ions are used to
produce high-resolution structures. Lasers have been proven as effective tools in
micromachining. They have been used to solve fine machining problems in
numerous fields such as medical devices, telecommunication, microelectronics, fibre
optics, data storage, instrumentation, and micro optics. Although lasers battle with
other technologies for micromachining applications, they are suitable for those
application that demand more precision, speed, and “direct-write” capability. Lasers
can also work on most materials and environmentally friendly (Chang and Molian,
1999).
2
Laser machining or laser ablation is a surface structuring technique based on
the interaction of intense laser pulses with a material. It is extremely well suited for
fabricating the microstructures on polymer surfaces. Laser machining has been
proven to be a versatile technique for producing high accuracy dimension and
repeatability of features in a wide diversity of materials. Due to the non-contact and
direct-write nature of the process, the fabrication of the microstructure can take
place in a very late stage of a heterogeneous assembly. This makes laser ablation
very attractive for fabricating micro-optical components on opto-electronic
assemblies in comparison to other fabrication technique like injection molding,
embossing, and standard micro-electronic manufacturing process (Naessens et al.,
2000).
1.2
Laser Machining
High-power short-pulse laser ablation has been extensively applied in micro
machining technology in recent years. Laser ablation (also known as laser
machining) is most usually associated with the use of excimer lasers. Excimer laser
ablation is a valuable micro-fabrication technology, which is particularly well-suited
for surface structuring of polymers because of their excellent UV absorption and
highly non-thermal ablation behaviour (Naessens et al., 2003). It is now twenty
three years since first appeared on using UV excimer lasers to ablate the material
such as polymer (Dyer, 2003). Dyer has reviewed the basic mechanistic aspects of
the UV laser-polymer interaction; photophysical and photochemical mechanisms.
The interaction also accompanied by a visible plume, that is, the expanding cloud of
volatile products that are expelled from the irradiated surface by each laser pulse.
Particularly, laser ablation was started by Srinivasan and Mayne-Banton
(1982). They reported that the surface of polymer like PET (polyethylene
terephthalate) could be ablated and etched by radiation of 193 nm (ArF excimer
laser) wavelength without any subsequent processing. Since that time, the
interaction of pulsed, ultraviolet laser radiation from an excimer laser (193 nm, 248
nm, 308 nm, or 351 nm) with organic polymer surface has been the subject of
3
intense research activity (Dyer, 1992). Until now, many research efforts are carried
out to have a better understanding of excimer laser ablation of polymer.
Oldershaw (1993) has observed two features of excimer laser ablation.
Firstly, the interaction between the UV radiation and polymer is much localised, so
that patterns of high definition and controlled depth can be etched in the surface.
Because of the strong absorption of many polymers at 193 nm ArF laser, the depth
of etch for a single laser pulse is very small, usually less than 1 m. Secondly, there
is threshold laser fluence before it can ablate the surface. In discussing the UVpolymer interaction, Srinivasan (1993) has considered the power density of the
radiation at the polymer surface instead of the fluence as it is commonly done.
In 1996, Dyer’s group have done research on excimer laser ablation of low
and high absorption index polymers. They have used High Speed Shadow
Photography (HSSP), Probe Beam Deflection (PBD), and Time Resolved
Interferometry (TRI) techniques to provide information about the ablation process.
What has become clear from this activity is that laser parameter such as wavelength,
pulse duration, and fluence, each of them has contributing effects to the ablation
process and not all polymers exhibit the same ablation characteristics with a given
set of irradiance conditions. A given polymer can exhibit a variety of ablation
characteristics depending on the irradiance parameters applied (Dyer et al., 1996).
Beside that, it is shown that the acoustic waves generated during laser
ablation can be used to determine the ablation threshold and the ablation rate for
different fluences and depths, and also to characterize the different regions of the
process (Efthimiopoulus et al., 1998). The ablation process was also analysed
experimentally in terms of material removal rate, optical emission of the laserinduced plasma, hole geometry, debris production at the hole edge, and chemical
changes in the polymer induced by the laser irradiation (Wesner et al., 1999).
Optics for high power laser systems in fusion research requires very precise
control of phase distribution for obtaining uniform irradiation intensity on the pellet
4
target. In the conventional optical fabrication, the control of the phase in the optics
has been made by polishing the optics until the phase error is less than ë/8. This
requirement causes very expensive prices of optical components for high power
lasers. But then, Jitsuno (1999) has introduced a phase compensation technique,
which is a cheap and rapid fabrication process of precise optical elements. This
approach provides the way to control the phase of the optics, which is very
important for many other applications in optics.
In order to fabricate micro channels, Wagner and Hoffman (1999) have
performed multiple pulse laser ablation of stretched polymer with an ArF excimer
laser (193 nm). The surface structure remaining after ‘scanning ablation’, is
compared to the known results upon ‘static ablation’. Yang and Pan (2003) used a
numerical simulations technique to predict the profile of analogous microstructures
following excimer laser ablation to obtain desired micro-optical components.
Naessens et al. (2003) have applied a fabrication technique for microlenses
and arrays of microlenses using excimer laser ablation process. The process is based
on scanning a polymer surface with a pulsed excimer beam along well-chosen
multiple concentric contours and in this way microlenses of arbitrary shape can be
realized. Once again, numerical simulation was used to predict the profile of a
three-dimensional aspherical microlens and a microprism array (Pan and Shen,
2004). Based on the simulated results, the desired micro-optical lens profile was
obtained using excimer laser ablated polymer. Recently, Jensen et al. (2005) has
demonstrated a new method for forming microlenses or microlens arrays, which
utilizes excimer laser degradation of PMMA followed by a thermal treatment.
From all research efforts, laser ablation and processing are applied in many
area which covering fabrication of optical material, material removing, surface
modification, surface cleaning and many more. It gives us an interest to study and
characterize the fundamental of laser ablation. Although the application is wide, our
interest study is lay on interaction of excimer laser with polymer material and our
research in this field focusing towards the optical fabrication.
5
1.3
Interferometric Observation
Optical techniques are powerful for deformation analysis. Contour
measurement by interferometry is used widely to determine the shape of a surface.
Information about the surface of a static object can be obtained from interference
fringes whose contour lines or fringes characterize the surface on which they are
formed. There are two basic methods for measurement of surface shape or height
profile by optical contouring: phase shifting and the Fourier transform.
In 1995, Jitsuno’s group have made some basic fabrication experiments on
the measurement of the ablation rate. They made on a different kinds of optical
plastics and in situ interferometric observation of the surface figure of a PMMA
plate. A new approach for mitigate some important disadvantages of optical plastics
such as the bulk non-uniformity, the index drift due to the humidity absorption and
the strong birefringence have been proposed using the plastic-glass hybrid
component to reduce the thickness of plastic.
Scully et al. (1999) used a Fabry-Perot interferometer method to measure
small changes in optical path length in the sample as a function of laser power,
fluence, repetition rate and total accumulated energy of number of pulses.
They were using 3 mm thick PMMA slab as a Fabry Perot etalon for measuring the
optical path length changes at a selection of UV wavelength. When the UV
irradiated was illuminated with a green He-Ne laser at 548 nm, shifts in reflected
interference fringes were observed. These shifts indicate changes in sample
thickness, refractive index, and penetration depth in agreement with other
researchers.
The dynamic processes during laser ablation of polymers also have been
studied by Hauer et al. (2003). They were using nanosecond-interferometry and
shadowgraphy to compare the influence of the two absorption sites in the same and
the two different polymers. Both methods have the potential to give strong
indicating for the underlying mechanism.
6
Dennis et al. (2001) report on the technique for determining the change in
the refractive index of photosensitive glass. They demonstrated interferometerbased technique on fibre perform and bulk glass samples, achieving an optical-pathdifference (OPD) repeatability of 0.2 nm. But their technique was found to be
insensitive to the effects of photodarkening and material compaction. Karaalioglu
and Skarlatos (2003) have observed the surface profile of an aluminum (Al) thin
film and its thickness by electronic speckle pattern interferometry. The Michelson
interferometer was used as their basic interferometric system to obtain interference
fringes on a CCD camera. These interference fringes depend on the path difference
due to the surface contours of thin film. Then, they analyzed the interference fringes
with the fast Fourier transform method.
Regarding to some of the papers reviewed, it encourages us to observe the
result of fabrication process by using interferometry technique. Interferometer is a
sensitive detector, which is suitable to observe any changes on the surface shape
even though by a fundamental shot of excimer laser ablation.
1.4
Research Objective
This research was carried out in order to achieve the following objectives:-
1.5
1.
to diagnose the ablation source
2.
to determine the ablation threshold
3.
to characterize the ablation effect by interferometry method
4.
to estimate the ablation depth
Research Scopes
The ablation was conducted using UV light of ArF excimer laser. The laser
was triggered externally in order to vary the number of pulses from 1 to 20 pulses.
The optical material of polymethyl methacrylate (PMMA) was chosen as a
7
specimen. The multi exposure from single pulse was directed on the same spot. The
ablation was verified based on the laser parameters including pumping voltage,
working distance, and number of pulses. The ablation effect was measured and
calculated using interferometry method. The quantifying involved, including
shifted, intensity, and spacing of the fringes. Depthness of the sample at the ablated
area was estimated by knowing its refractive index.
1.6
Thesis Outline
This thesis is divided into seven chapters. The first chapter is introducing the
application of micromachining technology and its advantages upon traditional
approaches. Some previous researches which are related to the laser machining or
laser ablation and interferometric technique are reviewed. This chapter also
emphasize the aim of this research.
Chapter 2 discuss the theories that are related to the research. These include
mechanism of laser ablation, optical material and its affect on UV light, optical
properties of the material that is refractive index, and also about the interference
phenomenon.
Chapter 3 describes about methodology and equipments employed in this
research such as excimer laser system, which include internal structures and external
triggering. Measurement of pulse energy of excimer laser beam and its beam
profiles were discussed. This chapter also describes about sample preparation and
interferometer set up.
Characterization the source of ablation that is an excimer laser beam is found
in Chapter 4. Firstly, it involved the pulse energy and peak power of the beam upon
various laser parameters such as discharged voltage and number of pulses.
Secondly, the beam profiles of the laser beam upon working distance and discharged
voltage are described. Then, how to calculate the laser fluence are explained. This
8
is done in order to determine the appropriate laser energy or power needed during
ablation works.
Chapter 5 explains about laser ablative figuring. The ablation effects
occurred on the PMMA surface can be traced using interference method. The first
disturbance detected was referred as ablation threshold. Then the deformation
fringes were analyzed and quantified based on shifted, intensity, and spacing
changes.
The estimation of ablation depth on PMMA sample is obtained in chapter 6.
Prior to calculate the depth, the changes of refractive index of the exposed material
was measured using Brewster angle. The depth was estimated based on refractive
index, shifted fringes, and normal spacing.
Finally, chapter 7 conclude of the whole project. These provided with the
problems involved during perform the project. Last but not least, some works to be
carried out in the near future are suggested.
CHAPTER 2
THEORY
2.1
Introduction
Laser machining involves ablation process. In laser ablation, UV light from
excimer laser is used to ablate the target material that is polymethyl methacrylate
(PMMA), one type of polymers. Therefore, it is better to understand about
mechanism of laser ablation and optical material behaviour with respect to the UV
light illumination. The Brewster angle theory to obtain refractive index will be
discussed in this chapter. The phenomenon of interference is also included to figure
out the optical phase changes.
2.2
Laser Ablation
The name “laser ablation” is generally used to describe an explosive laser-
material interaction. Laser-material interactions involve coupling of optical energy
into a solid, resulting a process of material ejection in the form of species such as
atoms, ions, molecular species, and clusters (Chang and Molian, 1999). The
macroscopic effects of ablation include plasma, acoustic shocks, and crater of the
surface. Srinivasan and Mayne-Banton (1982) reported that when pulsed UV laser
radiation falls on the surface of a polymer, the material at the surface is
spontaneously etched away to a depth of 0.1 ìm to a few microns. Soon after their
first report about laser ablation of polymers, the discussions about laser ablation
10
mechanism started. The suggested mechanisms range from thermal, over
photothermal to photochemical (Dyer, 1992).
The interaction of pulsed, ultraviolet laser radiation with organic polymer
surface usually associated with the use of excimer laser beam (193 nm, 248 nm, 308
nm, or 351 nm). Srinivasan (1993) proposed that the absence of significant thermal
damage in polymers ablated with deep UV lasers could be explained by the fact that
incident photons have sufficiently high energy (e.g. 4.9 eV for the KrF, 6.4 eV for
the ArF laser) to directly break main chain bonds. At power densities greater than 1
MW/cm2 and using laser pulses of < 1 s pulse width (FWHM), these interactions
lead to the etching of the polymer. The result is an etch pattern in the solid with a
geometry that is defined by the light beam. The principal advantages in using
ultraviolet laser radiation rather than visible or infrared laser radiation for this
purpose lie in the precision (2000 Å) with which the depth of the cut can be
controlled and the lack of thermal damage of the substrate to a microscopic level.
Figure 2.1 shows the three successive steps of ablation process (Srinivasan,
1993). These steps are presumed to be the result of a single pulse interacting with
the surface. The stream of photons from laser pulse falls on the polymer and is
absorbed by a polymer surface. The first few photons reaching the polymer surface
do not cause the etching process, but they only melting the polymer surface (Rabek,
1996). Then, the energy is increased until an exposed area is shown to be ejected
from the surface, leaving an etched groove behind.
At the same time the fragments of laser ablation are ejected, a bright plume
was formed from the polymer surface. That is the expanding cloud of volatile
products that are expelled from the irradiated surface by each laser pulse. The
plume composition has been found to be quite complex, with species ranging from
low mass volatile gases to large polymer fragments and, in some cases, carbon-rich
clusters (Dyer,1992).
11
Laser beam
Mask
Irradiation
Absorption
Long-chain
molecules
Bond
breaking
Ablation
Figure 2.1 : Hypothetical steps in the interaction of a laser beam with a polymer
surface (Srinivasan, 1993)
Two specific models of bond-breaking mechanism can be considered,
photochemical and thermal ablation (Dyer, 2003). In the photochemical view of
ablation, termed “ablative photodecomposition”, decomposition reactions would
take place mainly from electronically excited states and repulsive forces between
species would lead to their rapid expulsion from the surface. Energy absorbed in
this bond-breaking process would restrict the temperature rise and the extent of
thermal damage to the substrate. In terms of thermal ablation, electronically excited
states are assumed to efficiently undergo internal conversion, leading to
randomization of the absorbed energy amongst the various degrees of freedom of the
molecule on a time scale that is short compared with the laser pulse. Bond breaking
will occur principally from the ground electronic state of the molecule.
Laser irradiance (power density) and the thermo-optical properties of the
material are critical parameters that influence the ablation process. Beside that, it
emphasized that a better understanding of the time scale of the ablation process is
fundamental for an understanding of the physical chemistry of this phenomenon
(Hauer et al., 2003).
12
2.3
Optical Material
Nowadays polymer-based materials are becoming ubiquitous in a variety of
high-tech applications such as specialty coatings, automotive, aerospace,
semiconductors, composites, optics, etc. (Diakoumakos and Raptis, 2003). The
involvement of polymers in modern technological processes is unique and
indispensable to the evolution of advanced products. One type of polymers is
polymethyl methacrylate or PMMA. PMMA is a crystal-clear thermoplastic with
excellent weatherability (Elias, 2003). It was first produced in 1933 by Rohm and
Haas in Germany.
PMMA is also known as acrylics or Perspex offer high light transmittance
with a refractive index of 1.49 and can be easily heat-formed without loss of optical
clarity. Prolonged exposure to moisture, or even total immersion in water, does not
significantly affect the mechanical or optical properties of PMMA. This type of
polymers are easily sawed, drilled, milled, engraved, and finished with sharp
carbide-tipped tools (Boedeker Plastics, Inc., 2003). PMMA is a thermoplastic
polymer with a chemical structure such as shown in Figure 2.2. The brackets
indicate the repeat unit.
H
CH3
H
CH3
H
CH3
C
C
C
C
C
C
H
COO.CH3
H
COO.CH3
H
COO.CH3
Figure 2.2 : Structure of polymethyl methacrylate
Material with light-induced refractive index changes are considerable interest
for applications in the fields of optical storage integrated optics. In many cases thick
samples are required. A promising material is PMMA, which may be produced
relatively simply in the form of large blocks. Further advantages of the polymer are
its excellent optical quality, its transparency in the visible spectral region and its
13
thermal stability against UV light (Kopietz et al., 1984) makes it the polymer of
choice for outdoor signs, lamps, airplane windows, dentures, etc. PMMA is a
versatile polymeric material that is well suited for many imaging and non-imaging
microelectronic applications. It is most commonly used as a high resolution positive
resist for direct write e-beam as well as x-ray and deep UV microlithographic
processes.
Photons, usually those with short wavelength, are energetic species which
are used to activate many chemical reactions (Garbassi et al., 1998). A typical
example of UV action on polymer surfaces is their degradation by sun exposure.
UV lamps are widely used for the treatment of polymer surfaces and the apparatus
involves essentially a lamp and simple illumination devices, such as the possibility
of selectively irradiating tiny areas (masks: microelectronics) or moving the sample
below the photon source (rollers: printing industries). Most applications involve the
photon-activated cross-linking (negative resists, paper coatings) or fragmentation
(positive resists) of polymer coatings.
Figure 2.3 shows schematic of a UV-treatment system, relative to
photolithography. A UV source (1) irradiates a substrate with a photosensitive
coating (2) through a mask (3). The coating is either depolymerised (positive resist)
or cross-linked (negative resist) by the radiation. Further wet treatments are used for
stripping the degraded positive resist (4) or the unaltered negative resists (5).
Figure 2.3 : Schematic of a UV-treatment system (Garbassi et al., 1998)
14
2.4
Refractive Index
A plane wavefront, going from a medium in which its phase velocity is v into
a second medium where the velocity is v’, changes direction at the interface. By
geometry, it can be shown that sin èi /sin èt = v/v’, where èi and èt are the angles of
incidence ray and the angle of transmittance ray, respectively. It can also be shown
that the path ray between any two points in this system is that which minimizes the
time for the light to travel between the points (Parker, 1988). This path ray would
not be a straight line unless èi = 90˚ or v = v’. Snell’s law states that:
ni sin èi = nt sin èt
(2.1)
where n t is the index of refraction of the medium. It follows that the refractive index
of a medium is nt = c/v’, because the refractive index of a vacuum, where v = c, has
the value 1.
However, Snell’s law is not appropriate to determine the changes of
refractive index without moving the material from the target in the experimental set
up. One way to measure the dynamic refractive index of the material is by using
Brewster’s angle technique. Visible light and all electromagnetic radiation are
transmitted in the form of transverse wave. It can be polarized by absorption,
refraction, diffraction and reflection. During the incident, reflected and transmitted
rays will all lay on the same plane that used as a reference.
When the light illuminated onto the sample surface [E]||, a portion will be
reflected while a certain amount will be transmitted such as depicted in Figure 2.4
(Hecht and Zajac, 1989). Light with the electric vector that is reflected from the flat
surface of material [Er]|| is vibrating in a plane that parallel to the surface and the
transmitted light [Et]|| is orthogonal with reflected light and polarized perpendicular
with the plane of incident. The plane that contains the incident, reflected, and
refracted waves is known as the plane of incidence.
15
[Er]||
[Ei]||
ni
èi
èr
nt
èt
[Et]||
Figure 2.4 : A wave reflecting and refracting at an interfaced (Hecht & Zajac, 1989)
Based on the Brewster’s angle theory, if the total value of angle of reflection
and angle of transmittance equals to 90˚ (èr + èt = 90˚), the reflected wave would
vanish entirely. The particular angle of incidence for which this situation occurs is
designated by èp and referred to as the polarization angle or Brewster’s angle,
whereupon èp + èt = 90˚. Hence, from Snell’s law,
ni sin èp = nt sin èt
(2.2)
and the fact that èt = 90˚ - èp, it follows that
ni sin èp = nt cos èp
(2.3)
tan èp = nt / ni
(2.4)
and
This is known as Brewster’s law after the man who discovered it empirically, Sir
David Brewster (Hecht and Zajac, 1989).
16
2.5
Interference – Optical Phase
The luminous phenomenon called interference is a direct consequence of the
wave nature of light. Using the interference of light, it can make interferometers,
which are instruments used to measure very accurately many physical parameters
(Malacara, 1988).
Optical interferometers based on both two-beam interference and multiplebeam interference of light are extremely powerful tools for metrology and
spectroscopy. A wide variety of measurements can be performed, ranging from
determining the shape of a surface to an accuracy of less than a millionth of an inch
and to determining the separation, by millions of miles, of binary stars (Parker,
1988). By using lasers in classical interferometers as well as holographic
interferometers and speckle interferometers, it is possible to perform deformation,
vibration, and contour measurements of diffuse objects that could not previously be
performed.
To obtain interference fringes, the phases of the two interfering waves must
be synchronized, that is, they must be coherent. Before the advent of lasers, this was
possible only if both waves originated from the same light source, either a division
of the wavefront or division of its amplitude (Malacara, 1988). Figure 2.5 (Parker,
1988) shows two arrangements for obtaining interference produced by division of
wavefront. This class of interference is produced when the two interfering
wavefronts are taken from different portions of the original wavefront.
In the Young’s double pinhole interferometer (Figure 2.5a), the light from a
point source illuminates two pinholes. The light diffracted by these pinholes gives
the interference of two point sources. For the Lloyd’s mirror experiment (Figure
2.5b), a mirror is used to provide a second image S2 of the point source S 1, and in the
region of overlap of the two beams, the interference of two spherical beams can be
observed.
17
Figure 2.5 : Interference produced by division wavefront (Parker, 1988)
(a) Young’s two-pinhole interferometer (b) Lloyd’s mirror
Figure 2.6 shows one technique for obtaining division of amplitude. This
class of interference occurs when both interfering beams are obtained by division of
the amplitude of the original wavefront by means of a partially reflecting optical
surface. Then, both beams travel different paths, and interference occurs when they
are recombined (Malacara, 1988). The visibility of the resulting interference fringes
is maximum when the amplitudes of the two interfering beams are equal. Typical
examples are Newton rings and the Michelson interferometer.
18
Figure 2.6 : Interference produced by division of amplitude (Parker, 1988)
A beam of light is actually a propagating electromagnetic wave (Hariharan,
1985). If two monochromatic waves propagating in the same direction and
polarized in the same plane are superposed at a point P, the total electric field at this
point is:[E] = [E1] + [E2]
(2.5)
where [E1] and [E2] are the electric fields due to the waves. If the two waves have
the same frequency, the intensity at this point is:I = |[A1] + [A2]| 2
(2.6)
where [A1] = a1 exp (-1) and [A2] = a2 exp (-2) are the complex amplitudes of the
waves. Accordingly,
I = [A1] 2 + [A2] 2 + [A1 ][A2] * + [A1] * [A2]
(2.7)
I = I1 + I2 + 2 (I1I2)-1 cos ∆
(2.8)
where I1 and I2 are the intensities at P due to the two waves acting separately, and
∆ = 1 – 2 is the phase difference between them.
19
If ∆p is the corresponding difference in the optical paths, the order of
interference is M = ∆p/ë. The intensity has its maximum value Imax when
M = m, ∆p = më, ∆ = 2m
(2.9)
where m is an integer, and its minimum value Imin when
M = (2m + 1) / 2,
∆p = (2m + 1) ë/2,
∆ = (2m + 1) 
(2.10)
Consider a transparent plane-parallel plate illuminated, as shown in Figure
2.7, by a point source of monochromatic light S. Any point P on the same side of
the plate as the source receives two beams of nearly equal amplitude from it, one
reflected from the upper surface of the plate and the other from its lower surface.
t
Figure 2.7 : Formation of fringes of equal inclination by reflection in a planeparallel plate (Hariharan, 1985)
A case of particular interest is when the plane of observation is at infinity.
This is the situation when the fringes are observed in the back focal plane of a lens.
20
In this case the two interfering rays AL and CL’ are parallel and are derived from the
same incident ray. Let the thickness of the plate be t and its refractive index n2,
while that of the medium on both sides of it is n 1. If 1 and 2 are respectively the
angle of incidence and refraction angle at the upper surface, then
and
AB = BC = t / cos 2
(2.11)
AC = 2t tan 2
(2.12)
AD = AC sin 1 = 2t tan 2 sin 1
(2.13)
Accordingly, the optical path difference between the two rays should be
p = n2 (AB + BC) – n1 AD
(2.14)
p = 2 t n2 – 2 t n1 tan 2 sin 1
cos è2
(2.15)
According to Equation (2.1) and cos2 è + sin2 è = 1, therefore,
p = 2 n2 t cos 2
(2.16)
However, by taking into account an additional phase shift of  introduced by
reflection at one of the surfaces, the optical path difference between the interfering
wavefronts is, therefore, actually:-
p = 2 n 2 t cos 2 ± /2
(2.16)
A bright fringe corresponds to the condition:2 n 2 t cos 2 ± /2 = m
(2.17)
where m is integer, while a dark fringe corresponds to the condition:2 n2 t cos 2 ± /2 = (2m + 1) /2
(2.18)
21
Equation 2.16 shows that for a given value of t, the phase difference between
the wavefronts depends only on the angle 2. This makes it possible to use an
extended monochromatic source instead of a point source. The interference fringes
produced in the back focal plane of L by any other point S’ on an extended source
are identical with those produced by S, so that their visibility is unaffected.
Similar phenomena can also be observed in transmission. In this case, the
directly transmitted beam interferes with the beam formed by two internal
reflections. Since the net phase shift introduced by the reflections at the two
surfaces of the plate is either zero or 2, the optical path difference between the
beams is:-
p = 2 n2 t cos 2
(2.19)
The fringes are, therefore, complimentary to those seen by reflection.
However, since the relative amplitudes of the two beams are usually very different,
the visibility of the fringes is low.
2.7
Summary
In summary, laser ablation theory explains about how materials interact
when exposed to laser light. In involves two bond breaking mechanisms,
photochemical and thermal ablation. Polymethyl methacrylate (PMMA) was used in
this research because this type of polymer has thermal stability against UV light.
UV light is the main source of ablation in this project. Therefore, PMMA is the
perfect sample for this research.
In order to determine the changes of optical properties (refractive index) of
PMMA, light polarization principle was used. The refractive index of PMMA was
measured by using Brewster’s law technique. Interference technique is the main
part to analyse the ablation results. In this research, interference was produced by
division of amplitude. Fabry Perot interferometer was used because it is most
suitable technique compared to others.
CHAPTER 3
METHODOLOGY AND MATERIAL
3.1
Introduction
In this chapter, sample preparation and equipments used in this experiment
will be described. The main machine used for ablation is excimer laser, whereas the
optical material employed as specimen is PMMA. The techniques used to analyse
the effect of ablation are including image processing, beam profiling, and
interferometer.
3.2
Excimer Laser System
Excimer lasers are a family of gas lasers in which light is emitted by a shortlived molecule made up of one rare gas atom (e.g. argon, krypton, or xenon) and one
halogen atom (e.g. fluorine, chlorine, or bromine). The name “Excimer” is the
abbreviation of “excited dimer”, a description of a molecule consisting of two
identical atoms, which exists only in an excited state (Hecht, 1992). When the
molecule drops to the ground state, which is the lower laser level, the molecule falls
apart. The most important excimer molecules are rare gas halides, compounds such
as argon fluoride, krypton fluoride, xenon fluoride, and xenon chloride, which can
be produced by passing
23
an electric discharge through a suitable gas mixture. All emit powerful pulses
lasting nanoseconds or tens of nanoseconds at wavelengths in or near the ultraviolet.
3.2.1 Internal Structure
The basic elements of excimer laser structures comprise of a tube, which can
be filled with the desired gas mixture, a suitable excitation source, a fully reflective
rear mirrors, and an uncoated output mirror that reflect a few percent of the beam
back into the cavity and transmit the rest. In discharge excitation, electric current
flows through the laser medium, typically ranging from a kilovolt (kV) to well over
tens of kilovolts delivers energy to the laser gas. A generic gas laser is shown in
Figure 3.1.
Laser medium (gas in a tube)
Laser beam
-
+
Rear
mirror
Discharge
Output
mirror
High-voltage source
Figure 3.1 : An electrical discharge exciting a gas laser (Hecht, 1992)
Excimer laser is excited by passing a short, intense electrical pulse through a
mixture of gases containing the desired rare gas and halogen. Normally, 90% or
more of the mixture is a buffer rare gas (typically helium or neon) that does not take
part in the reaction. The mixture also contains a small percent of the rare gas (argon,
krypton, or xenon) that becomes part of the excimer molecule, and a smaller fraction
of molecules that supply the needed halogen atoms. The halogen atoms may come
24
from halogen molecules such as F2, Cl2, or Br2, or from molecules that contain
halogens such as nitrogen trifluoride (NF3).
Figure 3.2 shows the energy levels of a typical rare-gas halide as a function
of the spacing between the two atoms in the molecule, R (the rare gas) and H (the
halide). The dip in the excited-state curve shows where the molecules are
metastable and the dip in the ground-state curve indicated that the molecules fall
apart.
Energy
Potential well
RH* (excited)
Laser transition
Ground state
R+H
Atomic distance
(between R and H)
Figure 3.2 : Internal energy of R-H molecule in excited and ground state (Hecht,
1992)
When electronically excited, the two component atoms attract each other to
form a stable molecule. The energy is at minimum when the two atoms are at a
certain distance apart, trapped in a potential well. When they are in that potential
well, they can occupy several vibrational levels as well. However, in the ground
state the two atoms are mutually repulsive or in some cases weakly bound. Thus,
when an excimer drops from the excited state to the ground state, the force between
the two atoms changes from attraction to repulsion and the molecule breaks-up.
Because of the ground state essentially does not exist, there is a population inversion
as long as there are molecules in the excited state. This process is performing again
and again; this is how the pulse excimer laser is trigger out.
25
3.2.2 External Triggering
An argon-fluoride (ArF) excimer laser model EX5-200/100 (it specification
is shown in Appendix A) manufactured by GAMLaser (2003) was employed as a
source of energy for the ablation works. The laser possesses a fundamental
wavelength of 193 nm, which produced ultraviolet laser radiation. The pulse energy
of this excimer laser is 12 mJ with pulse duration of 10 ns. The means output beam
dimension for this model is 6 mm x 4 mm. The size also depends on the output
energy of the laser beam.
The repetition rate of the excimer can be varied in the range of 20 to 200 Hz.
The discharged high voltage of the system can be varied in between 10 kV to 15 kV.
This excimer laser system was controlled by a personal computer card with 32 bit
Windows based software application to determine the desirable variable parameters
(GAMLaser Inc., 2003). Figure 3.3 shows picture of the excimer laser head, which
couple to an arbitrary function generator.
Figure 3.3 : An excimer laser connected to a function generator for external
triggering
The internal trigger of this laser can produce pulses in the range of 100 to
1000 pulses. The Sony Tektronix arbitrary function generator model AFG 310 was
employed to trigger the excimer laser from external. It can be used to trigger single
pulse and other number less than 100 pulses. In order to control the number of
26
pulses, the function generator is set up at ‘5 volts’, operated at ‘pulse’ output
function, and with ‘burst’ mode (Sony Tektronix Inc., 2003). The frequency of the
function generator is synchronised with the excimer laser system.
3.3
Energy Measurement
The pulse energy of excimer laser was calibrated based on various
parameters. Initially, the pulse energy is tested upon working distance in between
the excimer laser and the sensor. The energy produced by the laser is also verified
according to parameters of laser including discharged voltage and number of pulses.
The beam energy was detected by using Ophir Optronics energy meter model
Thermal Volume Absorbers 3A-P and the reading was displayed by Ophir Nova
Display as depicted in Figure 3.4. Details of the energy meter and its display
specification is shown in Appendix B. From the energy reading, the peak power of
the beam was calculated by dividing the energy over pulse width of the beam, which
is 10 ns.
Power = Pulse energy = Pulse energy
Pulse width
10 ns
A schematic diagram of excimer laser beam calibration is illustrated in Figure 3.5.
1 mW Helium-Neon with 632.8 nm productions of Melles Griot Inc. (1997) was
coaxial to the excimer laser. It was employed to ease the experimental alignment.
(3.1)
27
Figure 3.4 : Ophir Optronics energy meter and its display
Personal Computer
(controlled excimer
laser)
Personal Computer
(controlled
BeamStar CCD)
Function
Generator
Beam
Splitter
BeamStar
CCD
Ex5 Excimer
Laser
He-Ne
Thermal
Absorber
ArF gas
Nova
Display
Optical line
Electronic line
Gas line
Figure 3.5 : Experimental set up for excimer laser beam calibration
28
3.4
Beam Profile
A Beamstar CCD Laser Beam Profiler, production of Ophir Optronics
(2003) was used for diagnosing the profiles of the excimer laser beam (as shown in
Figure 3.5). It comprise of a video camera and a personal computer card with
software for imaging, capturing, storing, and performing two- and three-dimensional
intensity distribution of the laser beams. Figure 3.6 shows CCD Beam Profiler with
its example beam image displayed on the monitor. Detail of its specification is
shown in Appendix C.
Figure 3.6 : CCD camera Beamstar Profiler and its software
The spectral response of a standard Beamstar CCD camera is in the range of
193 nm to 266 nm and excimer laser has wavelength of 193 nm. A BeamStar U
telescope is used to reduce the beam size. The laser spot area was captured and
recorded by the CCD camera. The intensity distribution of the beam was analysed
by using Video Test-Size 5.0 software. Figure 3.7 shows an example of the option
screen of Beamstar CCD laser beam profiler software.
29
Figure 3.7 : Option screen for Beamstar CCD laser beam profiler
The beam profile can be chose either in two dimensions (2D) or three
dimensions (3D). At the left-hand side of Figure 3.7, it shows the image of laser
beam profile in 3D. The two images at the right side of the figure illustrated the
vertical profile (top part) and horizontal profile (bottom part) of the laser beam
respectively.
The 2D image of beam profile can use to measure the size of beam spot.
This can be done by capturing and storing the image and then view the image in
Video Test-Size 5.0 software. Figure 3.8 shows an example of option windows for
measuring beam size of excimer laser. Before start the measurement, the program
was set up with proper calibration. This is done by viewing a ruler image and
calibrates its scale in pixels. Then, at beam profile image, the high intensity area
(red colour) was highlighted and the program would automatically show the value of
the area measurement.
30
Figure 3.8 : Option windows for beam size measurement
3.5
Sample Preparation
Optical material used as a specimen in this research is a type of polymers. It
is a polymethyl methacrylate, denote as PMMA or Perspex. Some advantages of
PMMA are its excellent optical quality, transparency in the visible spectral region
and thermal stability against UV light. During ablation process, a PMMA plate with
thickness of 1.38 mm was placed 50 cm in front of the excimer laser system. The
system was coaxial with 1 mW Melles Griot Helium-Neon laser with wavelength of
632.8 nm. The PMMA plate has been cut in dimension of 60 mm x 50 mm, thus it
ease to place the plate on the holder. A schematic dimension of the sample is
depicted in Figure 3.9.
31
60 mm
1.38 mm
50 mm
Figure 3.9 : Schematic diagram of sample for ablation work
The ablation effect at the exposed area was analysed by measuring the
changes on the sample surface. This is done by calculating the ablation depth that
occurred on the sample surface after ablation, based on ablation depth equation.
One of the equation’s variables is refractive index of the sample which is measured
by using Brewster’s law technique. This will be discussed more detail in the
Chapter 6.
3.6
Interferometer
A Fabry-Perot interferometer was set up by using 5 mW, 632.8 nm Melles
Griot Helium-Neon lasers as a light source due to its long coherence length and high
irradiance (Noriah, 2002). Figure 3.10 shows a schematic diagram of interferometer
allignment.
The diameter of the output beam from the He-Ne laser is about 1 mm. The
beam was incident at an angle of 18˚. To ensure the beam can cover the whole of
target area, a bi-convex lens with focal length of 50 mm was employed. PMMA
plate was used as the target material. A bi-convex lens with focal length of 100 mm
was conducted.
A Fabry-Perot interferometer composed of two parallel mirrors. In this
particular set-up, the PMMA plate was acted as the two mirrors. The top surface
32
stand as the first mirror, while the back surface stand as the second mirror. The
reflection beams from each surface were combined and interfere on the screen. The
interference pattern was visualised on a screen. A Pulnix (TMC-67DSP) couple
charge device (CCD) colour camera was employed for permanently record the
interference patterns. The images were directly processed and analysed by using
Matrox Inspector 2.1 imaging software.
A personal computer was used for internally triggered the excimer laser,
whereas a function generator was employed to trigger the excimer laser from
external. He-Ne laser behind the excimer laser was used to ease an alignment
between PMMA sample and excimer laser. The whole experimental set up is
depicted in Figure 3.10.
33
Optical line
PMMA
sample
Electronic line
18˚ 18˚
360 mm
Personal
Computer
(controlled
CCD camera)
830 mm
Lens (100 mm)
L = 40 cm
CCD
camera
Ex5
Excimer
Laser
Lens (50 mm)
170 mm
1000 mm
He-Ne
laser
Screen
Personal Computer
(controlled excimer
laser)
He-Ne
laser
Arbitrary
Function
Generator
Figure 3.10 : Schematic diagram of interferometer setup over excimer laser system
34
3.6.1 Magnification
In order to compute the magnification of the image, two parts have been
considered. Firstly is between the PMMA sample and the screen, while the second
part is between the screen and CCD camera (referred to Figure 3.10). Both
magnifications were determined separately. The total is obtained by adding the two
of them.
The first magnification can be estimated by measuring object and image
distance. In this experiment (as depicted in Figure 3.10), lens with focal length of
+100 mm was used to focus the beam. The distance in between target that is
PMMA material and the lens is 360 mm. This is known as the object distance. The
distance between lens and screen, which is known as the image distance is 1000
mm. Thus, the magnification is obtained by dividing these two distances as
followed:-
Magnification, M = Distance of image
Distance of object
M = 1000 mm
360 mm
M = 2.78
For magnification between displayed screen and CCD camera, the
calibration of the image was done by capturing a ruler image at the screen as shown
in Figure 3.11. Three lines marker were made at 1 cm scale of ruler image, which is
L1, L2, and L3. From ‘measurement table’ at the left-bottom part of the figure, it
shows 59 pixels at the ‘info’ column for all three lines marker. This indicates that at
1 cm scale on the interference’s displayed screen is same with 59 pixels in the
computer or monitor screen.
35
Figure 3.11 : Option windows for ruler calibration
From this information, then the set-up was made at ‘calibration windows’,
which is shown at the right-bottom part in Figure 3.12. At the ‘aspects ratio’ part in
the windows, X and Y axis were set-up at 59 pixels with 1 cm at real field. From
this calibration set-up, all the measurement value will refer according to this scale.
36
Figure 3.12 : Option windows for calibration set-up
3.6.2 Fringes Analysis
The interference pattern is distorted when the target was exposed by the
excimer laser pulses. An interference pattern image for undamaged PMMA sample
was recorded as an indicator for a flat or undamaged material. Beside that, this
image also referred as a reference interference pattern.
37
In this study, the PMMA was exposed to numbers of pulses, which in the
range between one pulse to 20 pulses. The image of interference pattern for each
exposure target was immediately recorded. The intensity and shifted distance of
each fringe are estimated. This is carried out by comparing the spectrum of exposed
material with the reference fringes’ spectrum. Hence the effect or damage obtained
after ablation was quantified based on difference intensity and the shifted distance of
fringes.
Besides that, to investigate threshold ablation or minimum energy required to
damage the PMMA sample, an experiment was carried out by using single pulse
operation. Further explanation regarding this topic will be found in Chapter 5.
3.7
Summary
In summary, when developing laser ablation system, a few steps need to be
prepared. Firstly, it should have an energy source of ablation process. The source
employed in this research is high-power pulse laser from an argon-fluoride (ArF)
excimer laser model EX5-200/100 manufactured by GAMLaser ArF with 193 nm
wavelength, which the beam is in the ultraviolet region. A Sony Tektronix arbitrary
function generator model AFG 310 was connected to the excimer laser system as
external triggering for producing singles pulse or few pulses as needed.
Secondly, the laser beam was characterized by measuring its pulse energy
and beam profile. The measurement of pulse energy was made up by using Ophir
Optronics energy meter model Thermal Volume Absorbers 3A-P and Ophir Nova
Display, while the beam profiles were analysed by using a Beamstar CCD Laser
Beam Profiler, production of Ophir Optronics.
The sample material used was polymethyl methacrylate (PMMA) or widely
known as Perspex. It is one kind of polymer that gives a good availability of
interaction with excimer laser. The properties of the sample material during the
38
ablation worked were analysed, which is refractive index as optical properties and
surface shape changes as physical properties. This is done by using Brewster’s
technique upon measurement of refractive index and calculates the ablation depth on
the sample surface.
The important step is observation of ablation effects by using interferometry
technique. A Fabry Perot interferometer was used to produce the image of
interference during ablation works, where the PMMA sample was used as its etalon.
Then, the sample surface was analysed based on the changes of the fringes position
after the ablation works.
CHAPTER 4
CHARACTERIZATION OF EXCIMER LASER BEAM
4.1
Introduction
Excimer laser has been used as an energy source in the ablation works.
Before utilise this laser system, it is better to characterize the output of the laser
beam. The pulse energy of excimer laser beam is calibrated upon discharged
voltage, and number of pulses. By knowing the pulse energy for every working
parameter, power can be estimated. Beam profile of the laser upon working distance
and discharged voltage were captured and recorded. The intensity distribution of the
images are then analysed by using the beam profiles images.
4.2
Energy and Power
The pulse energy, E was tested upon various parameters. These included
working distance, L that is the distance between the excimer laser and PMMA plate,
discharged voltage of the laser system, V and number of the excimer laser pulses, N.
The discharged voltage, V was varied in between 10 kV to 15 kV. The variable
numbers of pulses, N are studied from 1 pulse to 20 pulses using external trigger in
conjunction with a function generator.
40
4.2.1 Pumping Energy
A discharged voltage was used to pump an ArF gas in active medium of the
excimer laser system. It has been controlled by a personal computer card and its
software. During laser operation, L was fixed at 40 cm. The laser was triggered
externally using a function generator. Table 4.1 tabulated the experimental results.
The energy of a single pulse, E was measured at various discharged voltages, V. V
was set up from low to high voltage and the corresponding output power was
measured. It was varied in the range of 10.0 kV to 15.0 kV. The energy, E was
taken three times then the average was calculated. The power, P was estimated by
dividing the energy with the pulse duration of 10 ns.
Table 4.1 : Single pulse energy and power of the beam upon discharged voltage
Energy, E ( 0.01 mJ)
Voltage, V
Power, P
(kV)
I
II
III
Average
(MW)
10.0
2.50
1.40
1.57
1.82
0.182 ± 0.019
10.5
3.46
2.78
2.86
3.03
11.0
4.55
3.93
3.79
4.09
11.5
5.60
4.75
4.83
5.06
12.0
6.18
5.50
5.75
5.81
12.5
7.17
6.15
6.11
6.48
13.0
7.69
6.74
7.11
7.18
13.5
8.00
7.36
7.35
7.57
14.0
8.08
7.46
7.41
7.65
14.5
8.07
7.34
7.51
7.64
15.0
8.12
7.33
7.45
7.63
0.303 ± 0.031
0.409 ± 0.042
0.506 ± 0.052
0.581 ± 0.059
0.648 ± 0.066
0.718 ± 0.073
0.757 ± 0.077
0.765 ± 0.078
0.764 ± 0.077
0.763 ± 0.077
41
The pulse energy of the excimer laser beam upon the discharged voltage was
plotted such as shown in a graph of Figure 4.1. From the graph, the curve obtained
shows that initially E is drastically increased with respect to V from 10.0 kV to 13.0
kV. After that, the curve remains almost constant from 13.5 kV until 15.0 kV. For
this particular studied, when V is increased, thus E is also increased. This indicates
that the more input energy given to the gas, the more energetic of the output laser.
However, that case is not last long. At certain limit, after achieved 13.5 kV, the
output energy remain almost the same even more energies are injected and is said to
become saturated. At below than 10.0 kV of discharged voltage, the pumping
energy cannot produce the output laser.
9.00
8.00
Pulse Energy (mJ)
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
Discharged Voltage (kV)
Figure 4.1 : Pulse energy of the beam versus discharged voltage
15.5
42
4.2.2 Number of Pulses
The duration of laser exposure is determined by the number of pulses
parameter, N. Table 4.2 shows the result for this experiment.
Table 4.2 : Pulse energy and power of the laser beam at different number of
pulses
Energy (mJ)  0.1 mJ
Number of
Power
pulses
I
II
III
Average
(MW)
1
6.8
6.6
7.0
6.8
0.68 ± 0.08
2
13.6
13.2
13.7
13.5
3
20.0
20.1
20.2
20.1
4
26.1
26.2
26.9
26.4
5
32.5
32.8
32.3
32.5
6
38.6
38.7
39.2
38.8
7
45.0
45.1
45.7
45.3
8
48.1
48.6
48.0
48.2
9
54.8
53.9
55.2
54.6
10
60.1
61.2
62.0
61.1
11
66.3
65.8
66.9
66.3
12
74.0
73.2
73.8
73.7
13
80.8
78.6
78.9
79.4
14
86.2
86.2
86.3
86.2
15
92.3
90.8
91.9
91.7
16
98.3
97.5
98.9
98.2
17
105.4
104.3
105.3
105.0
18
109.4
109.3
111.1
109.9
19
116.7
117.0
118.2
117.3
20
123.4
123.1
123.5
123.3
1.35 ± 0.15
2.01 ± 0.21
2.64 ± 0.27
3.25 ± 0.34
3.88 ± 0.40
4.53 ± 0.46
4.82 ± 0.49
5.46 ± 0.56
6.11 ± 0.62
6.63 ± 0.67
7.37 ± 0.75
7.94 ± 0.80
8.62 ± 0.87
9.17 ± 0.93
9.82 ± 0.99
10.50 ± 1.11
10.99 ± 1.11
11.73 ± 1.18
12.33 ± 1.24
43
In this particular experiment, the working distance was fixed at 40 cm, while
the laser parameters such as discharged voltage and repetition rate were operating at
12.0 kV and 20 Hz, respectively. In this experiment, the system was triggered
externally using function generator. The numbers of pulses, N were varied from 1
pulse to 20 pulses.
Figure 4.2 shows the graph of pulse energy, E versus number of pulses, N.
The curve is linear, thus the longer the period of exposure, the more energies were
accumulated. This indicates that E is proportional to N.
140.0
Pulse energy (mJ)
120.0
100.0
80.0
60.0
40.0
20.0
0.0
0
2
4
6
8
10
12
14
16
18
Number of pulses
Figure 4.2 : Pulse energy of the laser pulse versus number of pulses
4.3
Excimer Laser Beam Profile
The Beamstar CCD Laser Beam Profiler was used to capture and record the
laser beam of excimer laser. The images of intensity distribution for the excimer
laser beam are demonstrate in two- and three-dimensional. Figure 4.3 and Figure
4.4 show the beam profiles of the excimer beam in 3D view and top view (2D),
respectively.
20
44
The topological image shows that the beam is in a rectangular shape. The
Gaussian beam in 3D manifested by different colours, which represent the degrees
of the intensity distribution. The red colour indicated the hottest stage of intensity,
followed by yellow, green, light blue, and finally the dark blue represents the less
heat stage of the intensity.
vertical
axis
horizontal axis
Figure 4.3 : Beam profiles of ArF excimer laser in 3D view
Figure 4.4 : Beam profiles of ArF excimer laser in top view (2D) (Magnification
15X)
45
The area of high intensity distribution, A which represented by red colour
was measured upon working distance, L and discharged voltage, V of excimer laser.
L was varied in between 45 to 70 cm, while V was adjusted in the range of 10 kV to
15 kV. This red colour was chosen since it is consider having an optimum energy
and quite significant area to be quantified. Hence A was measured based on this
colour only. It is better to note that the beam profile was detected with the aid of
beam telescope. It is used to reduce the beam size.
4.3.1 Beam Profile at Various Discharged Voltage
The excimer beam profiles (top view) have been studied at various V. In this
experiment, some of laser parameters are kept constant. This include repetition rate,
which is set at 20 Hz. The laser was placed at constant L, which is 50 cm. This is
because the beam profile cannot detect the image which is placed less than 45 cm.
The captured images are arranged in the increasing order as shown in Figure 4.5.
The data for the areas of high intensity distribution (red and yellow area), A
measurement are listed in Table 4.3.
46
(a) 10.0 kV
(b) 10.5 kV
(c) 11.0 kV
(d)11.5 kV
(e) 12.0 kV
(f) 12.5 kV
(g) 13.0 kV
(h) 13.5 kV
(i)14.0 kV
(j) 14.5 kV
(k) 14.7 kV
(l) 15.0 kV
Figure 4.5 : Beam profiles at various discharged voltages of the ArF excimer laser
(Magnification 15X)
47
Table 4.3 : Area of intensity distribution upon discharged voltages
Area, A (mm2)
Discharged
Voltages, V (kV)
I
II
III
Average
10.0
11.58
11.89
12.88
12.12 ± 0.51
10.5
14.55
14.70
13.88
11.0
15.35
15.35
14.40
11.5
17.49
16.99
19.83
12.0
19.71
18.56
18.34
12.5
21.22
20.42
20.97
13.0
22.69
24.75
22.51
13.5
25.34
26.09
25.12
14.0
22.77
25.81
23.88
14.5
24.03
27.93
25.43
15.0
23.25
24.74
24.82
14.38 ± 0.33
15.04 ± 0.42
18.10 ± 1.15
18.87 ± 0.56
20.87 ± 0.30
23.32 ± 0.96
25.52 ± 0.38
24.15 ± 1.10
25.80 ± 1.42
24.27 ± 0.68
A graph of high intensity distribution area, A is plotted versus discharged
voltage, V such as depicted in Figure 4.6. It shows that A is gradually increased with
respect to V. This occurs in the range of 10.0 kV to 13.0 kV. However beyond 13.0
kV, the A almost remain constant. This is similar to the results obtained in Section
4.2.1, which after the pumping energy of 13.5 kV, the excitation of the excimer
molecule become saturated. Hence the output pulse energy remained the same.
Even though more input energy was given, A was not increases.
48
30
Laser spot area (mm*mm)
25
20
15
10
5
0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
Discharged voltage (kV)
Figure 4.6 : The laser spot area versus discharged voltage
4.3.2 Beam Profile at Various Working Distance
In this experiment, the profile of the beam was studied at variable working
distances, L within 45 cm to 70 cm. This is because the beam profile cannot detect
the image which is placed less than 45 cm. In this case, the excimer laser was
operated at constant discharged voltage at 12 kV and the frequency was set at 20 Hz.
However the laser was conducted in a single mode using external trigger. The laser
beam was captured using beam profiler with the aid of beam reducer telescope. The
typical beam profile recorded for this experiment is shown in Figure 4.7. The
profile is arranged in the increasing order of distance.
49
It seems that all images, from 45 cm to 70 cm, have almost the same profile,
which is nearly the same size of rectangular form. At L less than 55 cm, the dark
blue area cannot be seen clearly, meaning that the distribution of the beam is not
uniform. Each image comprised with various colours. The centre part and the
majority colour of beam profile are red, which indicates the highest intensity region
of the laser beam.
(a) 45 cm
(b) 50 cm
(c) 55 cm
(d) 60 cm
(e) 65 cm
(f) 70 cm
Figure 4.7 : Beam spot at various working distance of the ArF excimer laser
(Magnification of 15X)
The spot area of the beam, A which is comprised of red and yellow colour
was measured. The collected data are listed in Table 4.4. The spot of the laser
beam, A is plotted against working distance, L. The graph is shown in Figure 4.8.
The result shows that A is almost constant. The value of A is around 17.0 mm2.
50
Table 4.4 : Area of intensity distribution upon working distance
Area, A (mm2)
Working Distance,
(L ± 1) cm
I
II
III
Average
45
17.87
16.65
16.76
17.09 ± 0.52
50
17.85
17.63
16.29
55
17.65
17.03
14.67
60
18.38
15.58
15.68
65
19.74
18.02
18.04
70
15.76
18.36
17.04
17.26 ± 0.64
16.45 ± 1.19
16.55 ± 1.22
18.60 ± 0.76
17.05 ± 0.87
20
Area of intensity distribution
(mm*mm)
19
18
17
16
15
14
13
12
11
10
40
45
50
55
60
65
70
Distance (cm)
Figure 4.8 : The laser spot area versus working distance. Laser operated at
discharged voltage of 12 kV with repetition rate of 20 Hz
75
51
4.4
Energy Per Unit Area
Energy per unit area or also known as fluence was calculated using data in
Tabble 4.2. Area of the beam at 12.0 kV is 17.0 mm2 (by referring to Section 4.3.2).
This parameter is important in order to analyse the energy needed for ablation within
particular area. Table 4.5 tabulate results of energy per unit area according to the
number of laser pulses.
Table 4.5 : Energy per unit area
Number of
pulses
Energy, E (mJ)
Energy / Area, E/A
(mJ/mm2)
1
6.8
0.40
2
13.5
0.79
3
20.1
1.18
4
26.4
1.55
5
32.5
1.91
6
38.8
2.28
7
45.3
2.66
8
48.2
2.84
9
54.6
3.21
10
61.1
3.59
11
66.3
3.90
12
73.7
4.34
13
79.4
4.67
14
86.2
5.07
15
91.7
5.39
16
98.2
5.78
17
105.0
6.18
18
109.9
6.46
19
117.3
6.90
20
123.3
7.25
52
Figure 4.9 shows a graph of energy per unit area corresponding to number of
laser pulses. It shows that the laser fluences are increasing proportionally to the
number of pulses, which is almost exactly like Figure 4.2. This occurrence might be
caused by the parameter of area used was constant, while the variable parameter that
is energy has the same value as Section 4.2.2. Therefore, energy per unit area would
rise according to the increment of number of pulse.
Energy / Area (mJ/mm*mm)
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0
5
10
15
20
25
Number of pulses
4.5
Summary
In this diagnostic, the output characteristics of the excimer laser beam were
determined. These consist of pulse energy and beam area. The pulse energy has
been tested corresponding to three working parameters, which involved discharged
voltage, working distance, and number of pulses. The obtainable results indicate
that some parameters have been chosen for future ablation work. In this case,
discharged voltage of 12.0 kV is considered as the best choice since it produced
more stable pulse energy. Furthermore greater than 13.0 kV, the pulse energy
became saturated. Other parameter showed that the pulse energy is increasing with
respect to the number of pulses.
53
The output of excimer laser can be diagnosed by using beam profiler. The
beam can be displayed in 3D and 2D. The intensity distribution of laser beam is
visualized with different colours. The highest intensity region is indicated by the red
colour and followed by yellow, green, and blue. The topographic map indicates that
the beam spot was found to be in rectangular shape. Based on pulse energy and
beam spot area results, energy per unit area of laser beams were calculated and it
shows that the energy per unit area increased when raising the number of pulses.
CHAPTER 5
DIAGNOSE THE ABLATION EFFECT BY
INTERFEROMETRY METHOD
5.1
Introduction
Polymethyl metahacrylate (PMMA) is a transparent material of polymer.
The result of excimer laser ablation on PMMA surface is difficult to detect by naked
eye. One way to detect the effect of ablation is by using interferometry technique.
In this particular experiment, Fabry Perot interferometer was set-up. The
interference occurs due to the superposition of wavefronts from the front and the
back surface of the PMMA sample. The interference pattern from the PMMA
sample was used to quantify the ablation mechanism. The strike of excimer laser
pulses on the PMMA sample causes optical phase change. The deformation fringes
of the sample occur due to the laser ablation at the front surface of the sample.
In this method, the PMMA sample was exposed to the excimer laser beam,
which means the sample was ablated by varying the number of pulses. The ablation
threshold was achieved when the first changes of the deformation fringes was
detected. The ablation effect can be analysed according to the changes in fringe
position, fringe intensity, and fringe spacing after the ablation threshold. All the
altering parameters are used to describe the ablation effect.
55
5.2
Reference Fringes
Reference fringes are the fringes of interference pattern that appeared from
target material. They formed before the target sample was exposed to the laser
pulses. If the surface under test is perfectly flat, the fringes will be straight and
equidistant, otherwise they will be curved and the distance between them will vary
(Francon, 1966). The reference fringes are important for comparison with
deformation fringes pattern that occurred after the laser ablation process. The
comparison results are needed for further analysis upon ablation effect on the target
material.
The typical interference pattern of the PMMA target area before the excimer
laser exposure is shown in Figure 5.1. The reference fringes are considered as
straight line and almost equidistant. This indicates that the surface of the sample is
almost flat.
Figure 5.1 : The image of reference interference pattern (Magnification 2.5X)
The Matrox Inspector 2.1 software was utilised to analyse the fringes under
the measured area. For analysing purposes, a horizontal ‘line profile’ was drawn
across the fringes. This is done by using ‘image analysis’ option in the Matrox
Inspector 2.1 software. An example of the ‘option windows’ of the software is
shown in Figure 5.2.
56
Figure 5.2 : The ‘option windows’ of Matrox Inspector software
The fringes within the tested line are numbered from left to right as F1 to F7
as shown in Figure 5.3. The analysis is made by referring to the spectrum profile
image, which is shown at the bottom image in Figure 5.3. In the spectrum profile,
horizontal axis (X-axis) indicates the location of the fringes, whereas the vertical
axis (Y-axis) represents the intensity of the fringes. Each peak of the spectrum
profile and its location are determined by pixel number. Any changes in the
spectrum profile after the laser ablation can be computed by deducting the pixel
number between the deformation fringes and the reference fringes.
57
F1 F2 F3 F4 F5 F6 F7
Intensity
Distance
Figure 5.3 : The fringes pattern of PMMA and its spectrum profile before ablation
5.3
Ablation Threshold
Laser ablation causes materials being removed from the polymer surface.
Essentially, a distinct threshold ablation must be exceeded for significant material
removal to occur. This is happened when the incident photons from excimer laser
have sufficiently high energy to directly break main chain bonds in ablated polymer
(Dyer, 2003). Normally, the ablation threshold is determined in terms of threshold
fluence that is pulse energy per unit area, P/A (Kobayashi, 1999 and Wang et al.,
2001). For this experiment, the ablation threshold was defined by finding the
minimum energy of excimer laser pulses that responsible to deform the fringes
58
pattern. Then, the ablation threshold was calculated by dividing the minimum
energy of excimer laser over beam size area.
In order to determine the minimum energy required to deform the
interference fringes, an experiment was carried out by using single pulse operation.
Initially the interference pattern of unexposed material was recorded by using CCD
video camera. Thus, the image was utilised for reference purposes. After that, the
target area was exposed by a pulse and the image of interference pattern was
recorded for analysing. Then the same target area was exposed again by adding
more pulses. Similarly, the same procedure was followed by recording the event.
The observation initially visualised in real time. This gives an advantage to monitor
immediately the occurrence of ablation. The number of pulses was continuing
increases until the tested straight line fringes showed significant changes.
In this particular experiment, the fringes were started to give a significant
changes after the target being exposed by 9 pulses. This can be identified from the
movement of the fringes. The fringes started shifted toward the left-hand side. This
particular amount of pulses is state as the ablation threshold for that particular
sample. Figure 5.4 showed the comparison fringes profile (a) before and (b) after
the target has been exposed by 9 pulses. Noticing that the energy of 9 pulses laser
beam at 12.0 kV discharged high voltage was measured as 54 mJ (referred to Table
4.2).
From the result in Table 4.3 (area of intensity distribution), at 12.0 kV
discharged voltage, the beam area is around 19 mm2. Therefore the ablation
threshold is as below:-
Ablation threshold = Energy / Area
= 54 mJ / 19 mm2
= 2.84 mJ / mm2
59
This value is referred as the ablation threshold or also known as fluence threshold to
deform the fringes. This is determined by the first appearance of disturbance on
interference pattern.
F1
F2
F3 F4
F5
F6
F7
(a) 0 pulse
Intensity
(b) 9 pulses
Intensity
Distance
Figure 5.4 : Fringes profile at threshold energy
5.4
Ablation Effect
During ablation process, excimer laser pulses delivered photon with high
energy to the target material. The molecules in the material will experience the
bond-breaking mechanism, both the excited electrons and vibrational transitions of
molecules transfer energy to the phonons, resulting in thermal effects or the photon
energy breaks the bonds providing kinetic energy to the atom or molecule. This will
60
lead to their rapid expulsion from the surface (Chang and Molian, 1999). As a
result, the refractive index of the material is subject to change.
Interferometry technique is a dynamic and very sensitive sensor. In this
ablation effect study, the interference pattern will change even due to very weak
laser pulses energy. As the refractive index of the target material changes, the
fringes will be shifted from the original position.
The typical interferogram recorded due to the result of laser ablation are
shown in Figure 5.5 and Figure 5.6. The frames are arranged in the increasing order
for number of pulses. A line profile across seven fringes (marked as F1 until F7)
was drawn in each interferogram associated with its own graph. Figure 5.5 (a)
shows slightly changes occurred over the fringes pattern as compared to the
reference fringes of Figure 5.3. The ablation due to 10 pulses exposure produced
only little distortion. The line profiles from higher number of exposure such as
shown in Figure 5.5 (b) and (c) start indicating some significant changes especially
in position, intensity, and spacing of the fringes.
Figure 5.6 shows another disturbance of fringes pattern. Figure 5.6 (a)
shows interferogram after 16 pulses exposure, where fringe F7 begin to form
speckle. In Figure 5.6 (b), the speckle is obviously seen in fringes F5 and F6. In
Figure 5.6 (c), the fringe split and the speckles are spreading. Speckle pattern is
produced due to the interference formation by light from a rough surface. The
existence of speckle is an indicator of surface roughness (Parker, 1988). The surface
becomes rougher when there is more speckle formation exists.
In order to quantify this disturbance, the fringes position, intensity, and the
fringes width or spacing from all profiles were measured. Overall, the interference
patterns are changed for every additional number of pulses and can obviously seen
due to the shifted fringes, ∆x, intensity change, ∆I, and the broadening or the
reduction of fringes spacing, ∆w.
61
F6
F2
F1
F1 F3 F5
F2 F4
F4 F5
F7
F3
F7
F6
(a) 10 pulses exposure
F4
F5
F6
F3
F7
F1 F2
(b) 12 pulses exposure
F3
F6
F7
F4 F5
F1 F2
(c) 14 pulses exposure
Figure 5.5 : Ablation interferograms and their line profiles after exposed by
(a) 10 pulses (b) 12 pulses (c) 14 pulses
62
F5
F6
F7
F4
F1 F2
F3
(a) 16 pulses exposure
F4
F7
F6
F5
F1
F2
F3
(b) 18 pulses exposure
F5
F6
F1 F2 F3
F7
F4
new split fringe
3.4 mm at PMMA
(c) 20 pulses exposure
Figure 5.6 : Ablation interferograms and their line profiles after exposed by
(a) 16 pulses (b) 18 pulses (c) 20pulses
63
5.4.1 Shifted Fringes
In this study, the PMMA was exposed by different number of pulses, N,
which was varied in between one to twenty pulses. The image of interference
pattern after every exposure was immediately recorded. The spectrum profile for
each image was then compared to the reference. Then the shifted distance, ∆x for
each fringe under tested line was measured. Hence the ablation effect in this section
was quantified based on the measurement of ∆x occurred on the interference pattern.
After exceeded the threshold ablation at 9 pulses, the rest number of pulses
exposed to the target responsible to cause the interference pattern to become more
distortion and getting difficult to trace. The existence of peculiar pattern indicated
that the target experience a severe ablated. Typical deformation fringes are shown
in Figure 5.5 and Figure 5.6. Each interferogram is associated with its own
spectrum profile.
All spectrum profiles were quantified by measuring the shifted position
experienced by each fringe. The obtainable data were deducted with reference
fringes position. The results of deduction position are tabulated in Table 5.1. The
data are used to plot graph such as depicted in Figure 5.7.
64
Table 5.1 : Data of shifted distance for all tested fringes (F1 to F7)
Shifted distance, ∆x (mm)
Number of
pulses, N
F1
F2
F3
F4
F5
F6
F7
1
0.00
0.01
0.01
0.03
0.00
0.01
0.00
2
0.00
0.01
0.01
0.03
0.00
0.01
0.00
3
0.00
0.01
0.02
0.01
0.02
0.01
0.02
4
0.00
0.00
0.00
0.01
0.00
-0.01
0.00
5
0.03
0.02
-0.02
0.03
0.00
0.00
0.01
6
0.03
0.01
-0.01
0.00
-0.01
0.02
0.01
7
0.00
0.01
-0.01
0.01
-0.03
-0.01
0.00
8
0.01
-0.02
-0.02
-0.03
-0.04
-0.04
-0.02
9
0.02
-0.03
-0.06
-0.09
-0.10
-0.12
-0.05
10
-0.05
-0.08
-0.13
-0.13
-0.14
-0.19
-0.07
11
-0.06
-0.14
-0.19
-0.18
-0.22
-0.23
-0.15
12
-0.10
-0.15
-0.25
-0.24
-0.28
-0.29
-0.20
13
-0.09
-0.19
-0.28
-0.27
-0.31
-0.34
-0.26
14
-0.12
-0.23
-0.32
-0.34
-0.38
-0.44
-0.32
15
-0.13
-0.27
-0.36
-0.41
-0.51
-0.53
-0.42
16
-0.19
-0.32
-0.44
-0.52
-0.59
-0.61
-0.53
17
-0.22
-0.35
-0.51
-0.61
-0.68
-0.71
-0.67
18
-0.24
-0.42
-0.55
-0.68
-0.77
-0.83
-0.92
19
-0.30
-0.46
-0.61
-0.76
-0.88
-0.98
-1.05
20
-0.30
-0.49
-0.71
-0.81
-0.98
-1.12
-1.25
Note: Negative means the direction of shifted toward left side from the original
position.
65
Figure 5.7 showed a graph of shifted fringes, ∆x after the target has been
exposed by a series number of laser pulses, N. Overall, the trend each of the graph
is almost similar. After the ablation threshold at 9 pulses (2.84 mJ/mm2), some of
shifted distance ∆x decreased gradually upon N and some very drastically. Negative
value means the fringes are shifted to the left side while positive value indicates the
fringes are shifted to the right side from the original position (reference).
The fringe F1 experienced the shortest shifted distance compared to the other
tested fringes. It followed by the fringe F2 and the rest of other tested fringes in the
ablation works. In Figure 5.5 that is between 10 pulses and 14 pulses exposures,
most fringes at the right-hand side of the frame that is from F3 to F7, experienced
the most changes in fringes position. This can be seen obviously in a graph as
shown in Figure 5.7.
The fringe F6 has the longest shifted for laser exposure less than 18 pulses.
But after 18 pulses exposure, the fringe F7 took place and experienced the longest
shifted, followed by the fringes F6 and F5. This indicates that all those fringes,
which are located at the right-hand side of the frame, have experienced the most
shifted than other fringes.
The shifted fringes results show that the right-hand side of the frame is the
most affected area after the ablation works. This is proven by the longest shifted
experience by the fringes. This indicates that the right-hand side of the target area
suffering severe damage. Those results can be related to the thickness changes at the
exposed area. The thickness changes because of heating process occurred during
ablation works. Thus it removes particles from the surface, makes the thickness of
the sample became thinner at the right hand side. Therefore the longest shifted
happened at the right hand side fringes. The measurement of ablation depth within
the exposed target area will be discussed further in Chapter 6.
66
0.20
0.00
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14
15 16 17 18 19 20
Shifted fringes (mm) .
-0.20
F1
-0.40
F2
F3
-0.60
F4
F5
F6
-0.80
F7
-1.00
-1.20
-1.40
Number of pulses
Figure 5.7 : Graph shifted distance of fringes upon number of pulses
67
5.4.2 The Changes in Fringes Intensity
Intensity of the fringes, I is another parameter that could be measured from
Figure 5.5 and Figure 5.6. The changes of intensity, ∆I could result either the fringe
become high contrast or blur. The parameter is related to the coherence length of
the beam. It shows a good contrast or high intensity if the path length difference is
less than coherence length. The reverse condition occurs if the path length
difference getting greater, where the fringes tend to be blurred or the intensity is
getting poor. Instead, if the path length difference is greater than coherence length
of the beam, the interference pattern will be diminished and sooner disappear
(Parker, 1988).
The changes in path length involve the thickness changes on the sample.
The ablation of excimer laser could affect the thickness of the sample. Thus it
affects the path length of the beam. If the sample becomes thinner, therefore the
fringes will experience high contrast. Otherwise, if the sample becomes thicker due
to the overlapping from debris, the fringes will become blur. In order to confirm
whether the thickness of the sample change in this ablation study, the intensity from
the line profile for each fringe is measured and compared to the reference
interference profile. The collected data from the measurement are tabulated in Table
5.2 (where a.u. is an arbitrary unit).
Table 5.2 : Data of fringes intensity for all tested fringes (F1 to F7)
Number of
pulses, N
Intensity, I of different fringes (a.u.)
F1
F2
F3
F4
F5
F6
F7
0
111
120
130
123
128
134
132
12
102
98
121
138
148
138
139
14
104
102
162
126
132
154
148
16
98
102
83
111
159
168
154
18
115
107
91
173
142
171
171
20
96
104
91
78
159
135
141
68
The changes of fringes intensity, ∆I after exposure were calculated by
deducting intensity of fringes after exposure with before exposure (0 pulses). The
calculated data are tabulated in Table 5.3. The positive and negative values indicate
that the fringes have high contrast and low contrast of intensity, respectively. The
high contrast represents that the immediate intensity is higher than before being
exposed or ablated. Meanwhile, low contrast indicates that the immediate intensity
is lower than the reference intensity.
Table 5.3 : Data of intensity changes for all tested fringes (F1 to F7)
Intensity Changes, ∆I for different fringes (a.u.)
Number of
pulses, N
F1
F2
F3
F4
F5
F6
F7
12
-9
-22
-9
15
20
4
7
14
-7
-18
32
3
4
20
16
16
-13
-18
-47
-12
31
34
22
18
4
-13
-39
50
14
37
39
20
-15
-16
-39
-45
31
1
9
The data in Table 5.3 are used to plot graphs such as shown in Figure 5.8 and
Figure 5.9. Figure 5.8 (a) shows ∆I upon all fringes (F1 to F7) after 12 pulses
exposure. This is followed by ∆I for 14 pulses exposure in Figure 5.8 (b) and 16
pulses exposure in Figure 5.8 (c). Figure 5.9 shows ∆I after the material has been
exposed by 18 and 20 numbers of pulses.
In Figure 5.8 (a) that is after 12 pulses exposure, the intensity of the fringes
F1 to F3 started changing to lower intensity. Those fringes remain with low
intensity from 12 pulses to 20 pulses except that the intensity of fringes F1 and F3
are found slightly higher than before exposure as shown in Figure 5.9 (a) and Figure
5.8 (b), respectively. This indicates that the left-hand side of the frame having poor
contrast of fringes.
Intensity Changes (a.u.) .
69
50
30
10
-10
F1
F2
F3
F4
F5
F6
F7
F6
F7
F6
F7
-30
-50
Fringes
Intensity Changes (a.u.) .
(a) 12 pulses exposure
50
30
10
-10
F1
F2
F3
F4
F5
-30
-50
Fringes
Intensity Changes (a.u.) .
(b) 14 pulses exposure
50
30
10
-10
F1
F2
F3
F4
F5
-30
-50
Fringes
(c) 16 pulses exposure
Figure 5.8 : Graph intensity changes upon fringes for 12, 14, and 16 pulses
exposures
Intensity Changes (a.u.) .
70
50
30
10
-10
F1
F2
F3
F4
F5
F6
F7
F6
F7
-30
-50
Fringes
Intensity Changes (a.u.) .
(a) 18 pulses exposure
50
30
10
-10
F1
F2
F3
F4
F5
-30
-50
Fringes
(b) 20 pulses exposure
Figure 5.9 : Graph intensity changes upon fringes for 18, and 20 pulses exposures
The intensity of fringe F4 experiences unstable condition, which become
high and low contrast in certain exposure. This might be due to the debris from
ablation process that appears in that area, thus sometime the area is thicker and
sometime thinner. While the intensity changes of the fringes F5 to F7 remains high
contrast for all exposures especially in Figure 5.9. This indicates that the right-hand
side of the material possibly becomes thinner than before exposures.
71
5.4.3 Fringe Spacing
Straight line fringe pattern indicate that the wavefronts are collimated. Small
deformation in the reflected rays tends to introduce curvature in the fringes pattern.
If the normal spacing and the affected spacing of the fringe are W and w
respectively, the surface is prescribed a flatness of (w/W) ë/2 (Sirohi, 1985). It is
very common that the flatness of the mirror is described as ë/20, ë/10, and so on.
The flatness of the surface becomes smoother if the number of divider is larger. If
the fringes have irregular shape, this indicates that the surface is not uniform.
From Figure 5.6, the interference patterns become entirely different compare
to the reference or even with other number of pulses. Some of the spacing tends to
be very broad and the others become narrow. This also indicates that the fringes are
squeezing, means that some are compress and the rest will be rarefaction. The
accurate spacing of the fringe can be measured through the spectral width. Assume
that the negative value means the fringe is become narrow and the positive value
means the fringes is become wider than the reference value.
In this experiment, the flatness change is measured by comparing the fringes
spacing before and after exposure for each tested fringes. The fringes spacing in this
experiment is the measurement of the width of bright fringes. Figure 5.10 shows an
example for measuring the spacing. Two cursor lines were used to measure the
spacing. From the distance between those two lines, it shows that the fringe spacing
of the fringe F4 is 0.26 mm (where the calibration of the distance has been set up
before doing the measurement).
72
Figure 5.10 : Option windows for spacing measurement
The measurements of the width are carried out for the all fringes from F1 to
F7 corresponding to the number of pulses. The collected data obtained are tabulated
in Table 5.4.
Table 5.4 : Data of fringes spacing for all tested fringes (F1 to F7)
Number of
Fringes spacing of different fringes, w (mm)
pulses, N
F1
F2
F3
F4
F5
F6
F7
0
0.22
0.22
0.26
0.26
0.28
0.32
0.34
10
0.20
0.22
0.26
0.24
0.26
0.34
0.36
12
0.18
0.20
0.24
0.22
0.24
0.30
0.36
14
0.18
0.18
0.20
0.16
0.28
0.34
0.42
16
0.22
0.14
0.12
0.14
0.16
0.28
0.46
18
0.20
0.14
0.12
0.16
0.14
0.20
0.58
20
0.18
0.16
0.10
0.08
0.14
0.12
0.46
73
Based on the data of fringes spacing in Table 5.4, the spacing changes after
ablation, ∆w was calculated by deducting the fringes spacing after exposure, w with
fringes spacing before exposure, W, where ∆w = w - W . Then the calculated
spacing changes were used to determine the flatness of the ablated area by using
equation (5.1), (Sirohi, 1985):w   
 
W 2
Flatness, F =
(5.1)
The calculated flatness for the fringes F1 to F7 after ablation threshold are
tabulated in Table 5.5. Histogram of flatness in term of ë/2 against all fringes is
shown in Figure 5.11 and Figure 5.12.
Table 5.5 : The flatness for all tested fringes (F1 to F7) in term of ë/2
Number of
Flatness, F (ë/2)
pulses, N
F1
F2
F3
F4
F5
F6
F7
10
-0.09
0.00
0.00
-0.08
-0.07
0.06
0.06
12
-0.18
-0.09
-0.08
-0.15
-0.14
-0.06
0.06
14
-0.18
-0.18
-0.23
-0.38
0.00
0.06
0.24
16
0.00
-0.36
-0.54
-0.46
-0.43
-0.13
0.38
18
-0.09
-0.36
-0.54
-0.38
-0.50
-0.38
0.71
20
-0.18
-0.27
-0.62
-0.69
-0.50
-0.63
0.38
The flatness is prescribed in term of the fraction of spacing. If the fraction
number becomes higher, the surface is considered rough. On the other hand, the
smaller the fraction number, the smoother the surface. In this case, the calculated
flatness of all seven tested fringes represents the surface condition only within each
fringe area. This is because all fringes show difference spacing for every exposure
and even though for the same exposure, all fringes did not have the same spacing.
The negative value indicates that the spacing is narrowing, while the positive value
indicates that the spacing is broadening.
74
Flatness (lambda/2) .
0.8
0.6
0.4
0.2
0
F1
-0.2
F2
F3
F4
F5
F6
F7
-0.4
-0.6
-0.8
Fringes
(a) 10 pulses exposure
Flatness (lambda/2) .
0.8
0.6
0.4
0.2
0
-0.2
F1
F2
F3
F4
F5
F6
F7
F5
F6
F7
-0.4
-0.6
-0.8
Fringes
(b) 12 pulses exposure
Flatness (lambda/2) .
0.8
0.6
0.4
0.2
0
-0.2
F1
F2
F3
F4
-0.4
-0.6
-0.8
Fringes
(c) 14 pulses exposure
Figure 5.11 : Histogram of flatness upon fringes after exposed with 10, 12, and 14
pulses
75
Flatness (lambda/2) .
0.8
0.6
0.4
0.2
0
-0.2
F1
F2
F3
F4
F5
F6
F7
F6
F7
F6
F7
-0.4
-0.6
-0.8
Fringes
(a) 16 pulses exposure
Flatness (lambda/2) .
0.8
0.6
0.4
0.2
0
-0.2
F1
F2
F3
F4
F5
-0.4
-0.6
-0.8
Fringes
(b) 18 pulses exposure
Flatness (lambda/2) .
0.8
0.6
0.4
0.2
0
-0.2
F1
F2
F3
F4
F5
-0.4
-0.6
-0.8
Fringes
(c) 20 pulses exposure
Figure 5.12 : Histogram of flatness upon fringes after exposed with 16, 18, and 20
pulses
76
Figure 5.11 (a) shows that the area within fringes F2 and F3 experienced no
changes in flatness, while the rest fringes have fraction number of flatness less than
0.1 (ë/2). This indicates that after 10 pulses exposure, the flatness of the target
surface remains the same as before ablation. After 12 pulses exposure, the flatness
for most fringes started to show significant changes. Figure 5.11 (c) shows the
configuration of flatness after 14 pulses exposure. Fringe F4 possesses the
narrowest spacing with fraction number of flatness -0.38 (ë/2). The negative sign
also indicate that the level of the surface is lower than the original.
Figure 5.12 shows the higher degrees of flatness changes. The levels of the
flatness are obviously further down from the level of surface. In other word, the
spacing of the fringes is further narrower than the normal spacing. However the
level is fluctuated for every fringe. Nevertheless the fringe F7, from the lower
number of exposure until the maximum exposure in this test, shows the spacing is
always wider and broadening compare to the normal spacing. In fact, after 18 pulses
(Figure 5.12 (b)) shows the spacing achieved the maximum spreading.
5.5
Summary
The interaction between UV light from ArF excimer laser and optical
material like PMMA will involve the physical and optical properties of the material
changes. However the optical material is transparent, the changes are very difficult
to trace. Hence, the interferometry is the appropriate technique to detect a very
weak and sensitive change.
Through interference method, an ablation threshold was afforded to measure
with threshold ablation of 2.84 mJ/mm2, which is corresponding to 9 numbers of
pulses. The ablation was focused only on the same spot but the numbers of pulses
were continuing added up to 20 pulses.
77
After the ablation, the bond breaking occurred in the polymer molecules
responsible for changing the refractive index of the target material. This is shown
by the shifted of fringes at the exposed area. The erosion or the removal of the
particle from the target material has caused the thickness changes. This is
represented by the intensity or the contrast of the fringes. The increasing contrast
indicates the thinner part, whereas the blur or lower intensity indicates the thicker
part due to the effect re-solidified of debris. Meanwhile the creation of speckle and
the change in fringes spacing indicate the roughness of the surface.
Hence the ablation of UV light is confirmed to damage the optical material
of PMMA. The interference method is successfully traced the occurrence of
ablation effect at the right side of the tested frame. The thickness and the flatness
change are corresponding to the shifted fringes, the changes of the fringes intensity
and fringes spacing.
CHAPTER 6
DETERMINATION OF ABLATION DEPTH
6.1
Introduction
When the excimer laser beam strikes the PMMA material, it causes high
energy transfer from the photons beam onto molecules in the material. This brings
decomposition reaction in the molecules, where bond breaking mechanism and
thermal effect would take place in this ablation process. Some of the species would
expulsive from the surface as polymer fragments and etc. The phenomenon has
affected the properties of the material such as its physical, thermal, mechanical,
electrical and optical properties.
In order to analyse the changes occurred on the PMMA sample after the laser
ablation works, we only concern the optical property and physical property of the
sample. In optical property, the refractive index of the sample has been measured.
The Brewster’s law will be applied in order to measure the changes in refractive
index of the PMMA sample. Meanwhile the physical changes occurred on the
sample surface was carried out by estimating its ablation depth. This is done by
referring to the fringes of equal thickness method, where the shifted fringes, normal
spacing, and refractive index information have been used for ablation depth
calculation.
79
6.2
Brewster Technique
Brewster’s angle was used to determine the polarization of light based on the
reflection from a surface. As explained in Section 2.4, the unpolarized beam
represented by two perpendicular vibration components, which is refer as
perpendicular component, [Er]|| (transverse electric mode) and parallel component
[Et]|| (transverse magnetic mode) (as shown in Figure 2.4). When the beam strike
the incidence plane, it will reflect and refract at the plane, where the reflected beam
is partially polarized with a predominance of the [Er]|| component present and the
refracted beam is partially polarized and richer in the [Et]|| component. The angle of
incidence that produces a linearly polarized beam by reflection is called the
polarizing angle or Brewster’s angle, èp (Pedrotti and Pedrotti, 1993).
The experimental set up for this measurement is shown in Figure 6.1. 5 mW
He-Ne laser was employed as a source of collimated beam. The beam was incident
onto the sample surface and the power of reflected beam was detected by a power
meter. Protector is employed to examine the angle of incident and reflected beam.
èi is the incident angle of the beam and èr is the angle of the reflection beam.
During the ablation works, the PMMA sample and excimer laser system
were fixed position. In this case He-Ne laser and power meter were rotated to adjust
the angle èi and èr whereby èi is always equal with èr. Then, by detecting the power
of the reflected beam, P, thus èp was identified as P is minimum or zero (Hughes,
2002).
Brewster’s law has been used in this research for determining the refractive
index of the exposed area on the PMMA sample. Before ablation works, Brewster’s
angle, èp was measured. Then, the refractive index of the sample, n was determined
by using Brewster’s law as Equation 2.4. After that, the same spot area was exposed
to 10 until 20 pulses of excimer laser beam. After every exposure, èp of the sample
was measured. Consequently, the refractive index of the sample for each exposure
was determined using the same technique by assuming Brewster’s law is still valid
to use.
80
Power
meter
Protector
èr
PMMA
sample
Excimer
laser
system
èi
90˚
He-Ne
laser
Figure 6.1 : Schematic diagram of Brewster’s angle measurement
6.3
Refractive Index Changes
The PMMA sample was ablated or exposed by the excimer laser beam
within the range of 10 until 20 pulses. For every exposure, the power of He-Ne laser
beam, P was detected for angle in between 40˚ to 80˚, but more concentrate at angle
in between 50˚ and 60˚. This is because most of the plastic and glass have
Brewster’s angle, èp within that area. èi with the minimum power, Pmin was
considered as èp. The power measurement of P at different angles for each exposure
is tabulated in Table 6.1.
81
Table 6.1 : Power at different angle on the exposed material
Incidence angle
Power, P at different number of pulses, N (mW)
èi ± 1˚
10
12
14
16
18
20
40
0.447
0.457
0.460
0.440
0.430
0.430
45
0.387
0.383
0.397
0.390
0.377
0.363
50
0.353
0.353
0.363
0.363
0.340
0.327
51
0.347
0.343
0.367
0.360
0.340
0.327
52
0.343
0.340
0.353
0.360
0.333
0.320
53
0.337
0.337
0.353
0.350
0.323
0.317
54
0.327
0.330
0.350
0.347
0.317
0.313
55
0.327
0.330
0.330
0.340
0.312
0.297
56
0.317
0.323
0.340
0.333
0.310
0.307
57
0.323
0.330
0.343
0.340
0.320
0.313
58
0.330
0.327
0.337
0.337
0.317
0.307
59
0.323
0.330
0.350
0.343
0.323
0.303
60
0.323
0.327
0.337
0.340
0.330
0.323
70
0.330
0.330
0.340
0.343
0.327
0.330
80
0.383
0.410
0.423
0.410
0.390
0.383
By using data in Table 6.1, graphs of the beam power versus incidence angle
at different number of pulses are plotted as shown in Figure 6.2 and Figure 6.3,
respectively. Overall all graphs are curvature. The angle at minimum power is
taken as Brewster’s angle. Typically Figure 6.2 (a) shows that the PMMA which
exposed by 10 number of pulses having Brewster’s angle at 56˚. The rest of curves
in Figure 6.2 and Figure 6.3 also indicate almost similar value. Except that Figure
6.2 (c) and Figure 6.3 (c) having slightly lower Brewster’s angle of 55˚.
82
0.500
Power (mW)
0.450
0.400
0.350
0.300
0.250
0.200
30
40
50
60
70
Incidence angle
80
90
80
90
80
90
(a) 10 pulses exposure
0.500
Power (mW)
0.450
0.400
0.350
0.300
0.250
0.200
30
40
50
60
70
Incidence angle
(b) 12 pulses exposure
0.500
Power (mW)
0.450
0.400
0.350
0.300
0.250
0.200
30
40
50
60
70
Incidence angle
(c) 14 pulses exposure
Figure 6.2 : Power of the laser beam versus incidence angle for 10, 12, and 14
pulses exposures
83
0.500
Power (mW)
0.450
0.400
0.350
0.300
0.250
0.200
30
40
50
60
70
Incidence angle
80
90
80
90
80
90
(a) 16 pulses exposure
0.450
Power (mW)
0.400
0.350
0.300
0.250
0.200
30
40
50
60
70
Incidence angle
(b) 18 pulses exposure
Power (mW)
0.450
0.400
0.350
0.300
0.250
0.200
30
40
50
60
70
Incidence angle
(c) 20 pulses exposure
Figure 6.3 : Power of the laser beam versus incidence angle for 16, 18, and 20
pulses exposures
84
The collected data of Brewster’s angle for every exposure are tabulated in
Table 6.2. From Equation 2.4:tan èp = nt / ni
If ni = 1, therefore,
tan èp = nt
where n t = n, which is referred as refractive index of the PMMA sample. The
calculated refractive indexes of the PMMA sample are listed in the table below:-
Table 6.2 : Refractive index of the exposed material
Number of pulses, N
Brewster’s angle, èp (˚)
Refractive Index, n
10
56
1.483
12
56
1.483
14
55
1.428
16
56
1.483
18
56
1.483
20
55
1.428
Figure 6.4 shows a graph of refractive index of the exposed sample upon
number of pulses. A nonlinear graph is obtained. This indicates the refractive index
is fluctuated upon number of exposure. As the numbers of pulses are increasing, the
refractive index becomes unstable. It rises up and down to higher number of pulses.
The re-ablation on the debris or the re-melted polymer might contribute to the
fluctuation of the results.
85
Refractive index .
1.6
1.5
1.4
1.3
1.2
8
10
12
14
16
18
20
22
Number of pulses exposure
Figure 6.4 : Refractive index versus number of pulses
6.4
Ablation Depth
The ablation of excimer laser over the PMMA sample involved some species
expulsion from the sample surface. This causes physical changes on the surface,
where the thickness of the sample within the exposed area will vary at least in
micrometer depth. In other words, the depthness occurred on the sample surface due
to the laser exposure is referred as ablation depth, d. One way to determine the
sample thickness variation is by using fringes of equal thickness method. These
fringes are primarily used to test the optical phase changes.
The thickness can be estimated by using the equation discussed in Section
2.5. There are three parameters including thickness, t, angle of incidence, è, and
refractive index of the sample, n that can be varied (Sirohi, 1985). For the purpose
of present discussion, only the fringes that involved due to variation in t and n would
be studied, while è is constant.
86
The translation of one fringe system relative to the other provides a means of
determining d, as follows. For nearly normal incidence, 1 = 2 ≈ 0, therefore cos 2
= 1 and bright fringes satisfy Equation (2.10) and (2.11),
p = 2 n t = më
(6.1)
where t represents the sample thickness of the exposed area at particular point. If
the sample thickness at the exposed area has changes by an amount ∆t = d, the order
of interference m changes accordingly, therefore,
2n (∆t) = 2nd = (∆m) ë
(6.2)
where n is refractive index of the sample. Increasing the thickness t by ë/2n, where
changes the order of any fringe by ∆m = 1, that is the fringe pattern translates by one
whole fringe (Pedrotti and Pedrotti, 1993). For a shift of fringes of magnitude ∆x
(as shown in Figure 6.5), the change in m is given by ∆m = ∆x/X, resulting in
 x   
d    
 X  2 n 
(6.3)
Figure 6.5 : The example of fringe pattern shifts by an amount of ∆x (Pedrotti and
Pedrotti, 1993)
87
Since both fringe spacing, X and fringe shift, ∆x can be measured from an
interferogram, thus the ablation depth, d can be estimated. The data of fringe
spacing and fringe shifted would be taken from Chapter 5 for further calculation of
ablation depth.
The ablation depth of the sample was calculated after been exposed by 10 to
20 pulses. This is done by using the data of shifted fringes, ∆x from Table 5.1,
fringes spacing, X at zero pulse (before exposure) from Table 5.4, and refractive
index of Table 6.2. The wavelength of the He-Ne laser beam is 632.8 nm. An
example of the calculation depth of fringe F1 after 10 pulses is shown as follow,
where its ∆x, X, and n are -0.05 mm, 0.22 mm, and 1.4826, respectively:  0.05  632.8 
d 
 nm

 0.22  21.4826 
(6.4)
d = - 48.51 nm
The calculation results for every exposed material are tabulated in Table 6.3
and Table 6.4. The data from each table then are used to plot histogram such as
shown in Figure 6.6 and Figure 6.7. A comparison graph is plotted such as shown in
Figure 6.8. The depthness of the sample is plotted against the number of pulses
exposed on the PMMA. Negative value indicates that the depthness of the sample
within ablated area is getting deeper while positive value is vice versa.
88
Table 6.3 : Results of calculated depth, d for 10, 12, and 14 pulses exposures
(a) 10 pulses exposure, n = 1.4826
Fringes
∆x ± 0.01 mm
X ± 0.01 mm
(d ± 0.01) nm
F1
-0.05
0.22
-48.51
F2
-0.08
0.22
-77.60
F3
-0.13
0.26
-106.71
F4
-0.13
0.26
-106.71
F5
-0.14
0.28
-106.71
F6
-0.19
0.32
-126.71
F7
-0.07
0.34
-43.94
(b) 12 pulses exposure, n = 1.4826
Fringes
∆x ± 0.01 mm
X ± 0.01 mm
(d ± 0.01) nm
F1
-0.10
0.22
-97.00
F2
-0.15
0.22
-145.51
F3
-0.25
0.26
-205.20
F4
-0.24
0.26
-196.99
F5
-0.28
0.28
-213.41
F6
-0.29
0.32
-193.40
F7
-0.20
0.34
-125.54
(c) 14 pulses exposure, n = 1.4281
Fringes
∆x ± 0.01 mm
X ± 0.01 mm
(d ± 0.01) nm
F1
-0.12
0.22
-120.85
F2
-0.23
0.22
-231.62
F3
-0.32
0.26
-272.68
F4
-0.34
0.26
-289.72
F5
-0.38
0.28
-300.68
F6
-0.44
0.32
-304.63
F7
-0.32
0.34
-208.52
89
Table 6.4 : Results of calculated depth, d for 16, 18, and 20 pulses exposures
(a) 16 pulses exposure, n = 1.4826
Fringes
∆x ± 0.01 mm
X ± 0.01 mm
(d ± 0.01) nm
F1
-0.19
0.22
-184.31
F2
-0.32
0.22
-310.41
F3
-0.44
0.26
-361.16
F4
-0.52
0.26
-462.82
F5
-0.59
0.28
-449.69
F6
-0.61
0.32
-406.81
F7
-0.53
0.34
-332.67
(b) 18 pulses exposure, n = 1.4826
Fringes
∆x ± 0.01 mm
X ± 0.01 mm
(d ± 0.01) nm
F1
-0.24
0.22
-232.81
F2
-0.42
0.22
-407.42
F3
-0.55
0.26
-451.44
F4
-0.68
0.26
-558.15
F5
-0.77
0.28
-586.88
F6
-0.83
0.32
-553.53
F7
-0.92
0.34
-577.46
(c) 20 pulses exposure, n = 1.4281
Fringes
∆x ± 0.01 mm
X ± 0.01 mm
(d ± 0.01) nm
F1
-0.30
0.22
-302.11
F2
-0.49
0.22
-493.45
F3
-0.71
0.26
-605.00
F4
-0.81
0.26
-690.21
F5
-0.98
0.28
-775.43
F6
-1.12
0.32
-775.43
F7
-1.25
0.34
-814.52
90
Ablation depth (nm) .
0
F1
F2
F3
F4
F5
F6
F7
F5
F6
F7
F5
F6
F7
-200
-400
-600
-800
-1000
Fringes
(a) 10 pulses exposure
Ablation depth (nm) .
0
F1
F2
F3
F4
-200
-400
-600
-800
-1000
Fringes
(b) 12 pulses exposure
Ablation depth (nm) .
0
F1
F2
F3
F4
-200
-400
-600
-800
-1000
Fringes
(c) 14 pulses exposure
Figure 6.6 : Histograms of ablation depth upon fringes after 10, 12, and 14 pulses
91
Ablation depth (nm) .
0
F1
F2
F3
F4
F5
F6
F7
F5
F6
F7
F6
F7
-200
-400
-600
-800
-1000
Fringes
(a) 16 pulses exposure
Ablation depth (nm) .
0
F1
F2
F3
F4
-200
-400
-600
-800
-1000
Fringes
(b) 18 pulses exposure
Ablation depth (nm) .
0
F1
F2
F3
F4
F5
-200
-400
-600
-800
-1000
Fringes
(c) 20 pulses exposure
Figure 6.7 : Histograms of ablation depth upon fringes after 16, 18, and 20 pulses
92
Figure 6.6 shows the histograms of depthness after been exposed by 10 until
14 numbers of pulses. Only shallow depth obtained after 10 pulses exposure (Figure
6.6 (a)). Fringes F1 and F7 seem posses the shallowest depth in most of the
histograms in this Figure 6.6 (a – c). Fringes F2 until F6 possesses almost similar
depth through out the exposure from 10 to 14 pulses, of course they are increasing
the depth with respect to the number of pulses.
Figure 6.7 illustrates the depth of ablation after been exposed by 16 to 20
numbers of pulses. The configurations of the depth seem to be a bit different. Here
fringe F1 maintain as the shallowest depth in most histograms (Figure 6.7 (a – c)),
compared to fringe F7. Initially, it seem to be the second lower compared to the rest
of other fringes (Figure 6.6 (a)), but it becomes drastically changes after 18
exposure.
Figure 6.8 shows the inclination of depth towards the right hand side of
fringes. As discussed in Section 5.4.1, as the number of pulses increased, the fringes
at the right hand side (F5 to F7) were shifted to the left side. Therefore the area was
shifted to the centre of the exposed area and constitutes the deepest depth.
0
Depthness (nm) .
-100 8
10
12
14
16
18
20
22
-200
F1
-300
F2
F3
-400
F4
-500
F5
-600
F6
-700
F7
-800
-900
Number of pulses
Figure 6.8: Depthness as a function of number of exposures
93
6.6
Summary
The ablation depth can be estimated by knowing the refractive index
changes. In this case, the exposed PMMA after threshold energy were employed.
This including the exposed material of 10 to 20 numbers of pulses. Brewster’s angle
method was used to measure the refractive index of the exposed material. The result
showed that the Brewster’s angle was obtained in the range of 55˚ to 56˚ which is
corresponding to the refractive index of 1.43 to 1.48. In general, the refractive index
of the exposed material is independent with respect to the number of pulses.
With this range of refractive index together with the information of shifted
fringes and normal spacing of the fringes obtained previously, the ablation depth
was estimated. The calculation results indicate that in the early exposure within 10
to 14 pulses, the depth is concentrated in the centre of the tested region. However at
higher number of exposure that is 16 to 20 pulses, the material depth was more
toward the right-hand side of the test region. Overall the ablation depth is depending
on the number of exposure. The higher the number of pulses impact on the PMMA,
the deeper the ablation depth was found.
CHAPTER 7
CONCLUSIONS AND SUGGESTION
7.1
Conclusion
A fundamental laser ablative figuring was successfully studied. An Argon
Flouride (ArF) excimer laser was utilized as source of energy. A polymer of
Polymethyl methacrylate (PMMA) was employed as specimen. The fundamental
ablation effect was detected by using an interferometry technique.
As a preliminary, an ArF excimer laser was conducted to produce single
pulse or other number of pulses below than 100 pulses. This is done by connected
an arbitrary function generator to the excimer laser system as an external triggering.
The laser beam then characterized in terms of pulse energy and beam profile. By
determining the characterization of the laser beam upon the specific parameters, we
can trace the performance trend of excimer laser prior to utilize the laser beam upon
the sample.
Pulse energy of the laser beam was studied at various parameters, including
pumping energy and number of pulses. Single pulse energy was measured upon the
pumping energy of excimer laser system by varying its discharged voltage. The
results showed that the pulse energy of the beam increased between 1.8 mJ to 6.5 mJ
corresponding to the discharged voltage of 10.0 kV to 13.0 kV. It remains almost
constant around 7.6 mJ for discharged voltage in range of 13.0 kV to 15.0 kV.
95
However the pulses energy of the beam was found proportional to the number of
pulses. The pulses were increased in the range of 1 to 20 pulses.
The Beamstar CCD Laser Beam Profiler was used to characterize the profile
of the excimer laser beam. It shows the beam profile in two dimensional (2D) and
three dimensional (3D) views. From 2D view, the excimer laser beam possesses a
rectangular symmetry, while from 3D view, it shows that the excimer laser beam
have a uniform intensity distribution. The beam size has been measured upon
discharged voltage and working distance. This is done by measuring the area of the
beam spot. For characterizing upon discharged voltage, the results obtained show
that the trend of the altering laser spot area is almost similar to the altering trend of
pulse energy upon discharged voltage. The values vary from 12 mm2 to 24 mm2.
But for characterizing upon working distance, it seems to be stable and posses
around 17.0 mm2. Regarding to the pulse energy and beam size results, the energy
per unit area were determined. By varying the number of pulses, the laser fluence
was varied in between 0.4 mJ/mm2 to 7.25 mJ/mm2.
The PMMA is a transparent material; therefore it is very hard to detect the
fundamental ablation effect upon the material by naked eye. The interferometry
technique was chosen to analyse the physical properties of the ablated sample. It is
well known as very sensitive detection through the interference pattern. The Fabry
Perot interferometer was developed. The PMMA have double function. It acted as
a specimen as well as an etalon for the interferometer. The ablation effects on the
sample surface were realized by the appearance of distortion in the interferograms.
During ablation works the PMMA sample was exposed over number of
pulses, which is varying in between 1 pulse to 20 pulses. The interference patterns
for every exposure were permanently recorded. The first disturbance of the fringes
pattern was claimed as the achievement of ablation threshold. This occurred after
the PMMA received 9 pulses of excimer laser beam, which with pulses energy of 54
mJ and energy per unit area of 3.18 mJ/mm2. Immediately after the threshold point,
the fringes pattern was continuing distorted. The disturbances of the fringes were
quantified by measuring the position, the intensity, and the spacing of the fringes.
96
All these altering parameters are used to describe the physical and the optical
properties changes of the target material.
Prior to the laser exposure, the interference pattern of unexposed material
was taken as a reference. Straight line and equidistance fringe was obtained, which
is as indicator of smooth and flat surface. After the exposure, fringes was found
shifted, the contrast of fringes change, and the spacing of the fringes were also
affected.
The shifted of the bright fringes were measured according to the position of
the pixel. The deduction of the pixel compared to the reference was taken as the
value of shifted. The right-hand side of fringes experienced more shifted compare to
the left side of the test region. This indicates that the laser ablation has changed the
physical condition within this region.
The ablation effect also quantified based on the intensity of the fringes. If
the value of intensity after ablation is higher than before material has been exposed,
the material is getting thinner. In the other hand, if the value of intensity after
ablation is lower than before ablation, the material become thicker. This may due to
the re-solidified of the debris in that area. From this experiment, it shows that the
intensity of the right fringes is getting higher compared to the left-hand side fringes.
Therefore, it shows that the fringes at the right side are getting thinner.
Other parameter to determine the ablation effect is the fringes spacing. The
width of the fringe was measured in this aspect. The flatness of the material can be
estimated by dividing the present width with the original and multiplied with ë/2 as
shown in Equation 6.3. The greater the fraction numbers of flatness, the rougher the
surface of the ablated area is expected. On the other hand, if the fraction number of
flatness is small, thus the roughness is less. The results show that the fringes at the
right-hand side are rougher than the left-hand side as posses by the higher fraction
number of half wavelength.
97
The effect of ablation on the PMMA sample was also quantified by
calculating the depth of the sample. Prior to that calculation, the changes in
refractive index were measured. Brewster’s angle method was employed in this
measurement by considering it is still valid through out the experiment. The results
obtained show that the refractive index is independent with number of pulses. The
fluctuation was expected due to the re-solidified of the removal material. However
the refractive index was found in the range of 1.43 to 1.48.
The physical properties of the sample was analysed by measuring the depth
of ablation impact on the sample surface. The estimation is made upon the shifted
fringe, normal spacing, and the refractive index. The results from the calculation
showed that the depth of ablation upon the exposed material was found deeper with
respect to the increasing number of exposure. Most of the fringes at the right-hand
side experienced more impact of ablation than the other side.
As a conclusion, the study of fundamental ablation effect on a polymer
material by using ArF excimer laser is a preliminary effort towards advancement
technique of microfabrication. This can be expanding to more complex system of
microfabrication technology.
7.2
Problems and Suggestions
The main problem which is affected the result is contributed from the
collection of debris that occurred from the incompletion burning of polymer’s
fragment. The debris has affected the exposed target area during the ablation works,
thus it affected the results of the ablation effect study. To avoid the problem, the
ablation interaction should be performed inside the vacuum chamber or under
halogen gas environment. A powerful jet of nitrogen blow perpendicular to laser
beam and across the surface to sweep away the debris as soon as it is formed (Mohd.
Hazimin, 2003).
98
During the measurement of refractive index, the experimental set up has
employed a protector for determining the angle. The uncertainty of the angle, which
is the minimum unit of incidence angles, is only one degree. This is not so accurate
in measuring the Brewster’s angle. In addition, the uncertainty of parallax might be
happen in this experiment. Therefore it should use other experimental set up or
instrument, which can give more accurate Brewster’s angle value. Otherwise, in
future study, an instrument that can give direct value of refractive index should be
applied.
The analyzing of deformation fringes has been made with the help of Matrox
Inspector software. It takes time to measure fringes in a large area. For further
study using interference pattern, we suggest to develop a program for fringes
measurements, which can automatically measure shifted, intensity, and spacing of
the fringes. Besides that, the program also should be able measure the speckle
formation in the interferograms, which is also can be considered to quantify the
flatness of the sample.
Instead of that, the ablation effect has been studied only in one area, which
only in a small region. For a large region of fabrication, a scanner for moving the
sample during ablation process should be utilized. This scanner can be
computerized to ensure the sample is moving in smooth and precise.
As mention earlier, this study is just an initial stage of gaining the
fundamental knowledge about laser ablation towards the optical fabrication.
Therefore, further studies should be carried out in order to practically fabricate using
multiple exposures on unpolished material. This can be achieved by study on
energy per unit area changes. Hopefully, all the efforts and experimental works in
this study will be a good reference for future work and come out with new bright
ideas.
99
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PRESENTATION
1. Hanani Yahaya @ Jaafar, Mohammad Khairi Saidin, and Noriah Bidin,
Study the Excimer Laser Beam Profiles, Annual Fundamental Science
Seminar 2004 (AFSS 2004), 14 – 15 June 2004, Skudai, Johor.
2. Hanani Yahaya @ Jaafar, Mohammad Khairi Saidin, and Noriah Bidin,
Qualitative Study of Ablation Process on PMMA by Using
Interferometer Technique, Malaysian Science and Technology Congress
2004 (MSTC 2004), 5 – 7 October 2004, Sri Kembangan, Selangor.
3. Hanani Yahaya @ Jaafar, Mohammad Khairi Saidin, and Noriah Bidin,
Study the Effect of Excimer Laser Ablation on PMMA by Using
Interferometry Technique, The XXI Regional Conference and Workshop
on Solid State Science and Technology (RCWSST 2004), 10 – 13 October
2004, Kota Kinabalu, Sabah.
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APPENDIX A
Specification of excimer laser
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APPENDIX B
Specification of energy meter
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APPENDIX C
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