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Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects

1999

Studies in microwave and RF capacitively coupled excimer lamp

Joseph Divine Ametepe College of William & Mary - Arts & Sciences

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Recommended Citation Ametepe, Joseph Divine, "Studies in microwave and RF capacitively coupled excimer lamp" (1999). Dissertations, Theses, and Masters Projects. Paper 1539623954. https://dx.doi.org/doi:10.21220/s2-hzya-5811

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with with permission permission of the of copyright the copyright owner. owner. Further Further reproduction reproduction prohibited withoutprohibited permission. without permission. STUDIES IN MICROWAVE AND RF CAPACITIVELY COUPLED

EXCIMER LAMP

A dissertation presented to

The Faculty o f the Department o f Applied Science

The College o f W illiam & Mary in Virginia

In Partial Fulfillment

O f the Requirement for the Degree o f

Doctor o f Philosophy

By

Joseph D. Ametepe

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPROVAL SHEET

This dissertation is submitted in partial fulfillm ent

o f the requirements for the degree o f

Doctor o f Philosophy.

^ w l X j u l s ^ j ) >J Joseph D. Ametepe

Approved, July 1999

Dr. Dennis M. Manos

Dr. Michael J. Kelley I

I — Dr. Karl Schoenbach Old Dominion University

Dr. Fred Dylla Thomas Jefferson National Acceleration Facility

Dr. Brian Holloway &

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

Acknowledgements ...... v

List o f Tables ...... v ii

List o f Figures ...... v iii

Abstract ...... x

Chapter 1: Introduction ...... 1 1.1. Overview and Motivation ...... 1 1.2. Theory and formation o f excimers ...... 4 1.3. Different excitation techniques ...... 10 1.4. Advantages o f excimer lamps ...... 11 1.5. Commercially available lamps ...... 12 t .6. Goals o f this work ...... 16 1.7. Outline o f the thesis ...... 17

Chapter 2: Plasma Fundamentals 2.1. Plasma fundamentals ...... 22 2.1.1. Plasma Sheath ...... 26 2.2. Microwave and RF breakdown o f Gases ...... 29 2.3. Electron loss processes ...... 34 2.3.1. Diffusion ...... 34 2.3.2. Attachment ...... 38 2.3.3. Recombination ...... 39

Chapter 3: Characterization and Modeling o f a 2.45 GHz Microwave Xe excimer lamp 3.1. Experimental Facilities ...... 41 3.1.1. Lamp Cavity ...... 44 3.1.2. Lamp Cooling ...... 45 3.1.3. Safety Issues ...... 45 3.2. Lamp Characterization ...... 49 3.2.1. Intensity Calibration ...... 50 3.2.2. Wavelength-Calibration ...... 51 3.3. Discharge tube cleaning ...... 53 3.4. Results and Discussion

Chapter 4: Capacitively Coupled RF Driven Xe/Xe-Ar Excimer Lamp ...... 79 4.1. Introduction to 13.56 MHz RF excimer drive ...... 79 4.2. 13.56 MHz RF Experimental Facilities ...... 80 4.2.1. Capacitively coupled lamp ...... 80 4.2.2. 13.56 MHz lamp characterization ...... 85

iii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.2.3. DOE Experiment ...... 85 4.3. Results and discussion ...... 87 4.3.1. Xenon ...... 91 4.3.2. Xenon-Argon mixture ...... 91 4.4. Optical power output ...... 92

Chapter 5: A 2.45 GHz KrI Excimer Lamp ...... 102 5.1. KrI introduction ...... 102 5.2. KrI experimentatl facility ...... 103 5.3. K rI Results and discussion ...... 104

Chapter 6: Simulation o f 13.56 MHz rf Excimer Lamp ...... 118 6.2. Model description ...... 120 6.3. Model results and discussion ...... 129

Chapter 7: Conclusion and Future Work ...... 144 7.1. Conclusions ...... 144 7.2. Future works ...... 146

APPENDIXES Appendix 1 ...... 148 Appendix 2 ...... 149 Appendix 3a ...... 150 Appendix 3b ...... 151 Appendix 4 ...... 152 Appendix 5 ...... 153 Appendix 6 ...... 154

BIBLIOGRAPHY...... 156

iv

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

Special thanks go to Professor Dennis M. Manos (my advisor and principal

investigator o f this work) for believing and trusting in me and providing me with the

opportunity to learn and to develop not only as an experimentalist, but also as a leader.

During the past six years, I learned from him the values and importance o f time, hard

work, dedication, and good writing skills. Through his grants, he provided funds for my

education. I conducted this investigation under his guidance and I wish to thank him for

his patient guidance and criticism throughout the investigation. The process was long and

not without pain but it was worth it . . “ Professor Manos, is a mentor and a father to me.”

Next, I wish to thank Professor Michael Kelley (previously at DuPont but now

with the College o f William and Mary) for his careful reading and criticism o f my

manuscripts. Prof. Michael Kelley offered many valuable guidance and suggestions

throughout this investigation. He constantly looked over my shoulder to make sure that I

was doing the right thing and in the right direction. I w ill always remember him for the

academic and profession ethics he instilled in me. Also, I would like to thank the rest o f

my thesis committee members (Dr. Schoenbach, Dr. Dylla, and Dr. Holloway) for

reading over my work within such a short notice.

Next in line is Jessie Diggs. I extend special thanks to him for the countless and

stressful hours we spent working together to 1) meet report deadlines, 2) defend our

experimental findings, and 3) prepare for meetings. Further, I wish to thank the other

graduate and undergraduate students w ith whom I worked over the past six years: Chris

Nichols (now at Sandia National Labs., Albuquergue), Tom Venhaus (also at Sandia

V

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Laboratory, Livermore), Carol Kalil, Xiaming Tang, and Lingling Wu and all the

undergraduate students who worked with us. Special thanks go to Kathee Card and

Bernadette Kulas who indirectly contributed so much to this work. Kathee made sure

that my classes were registered for (each semester) and that the right paper work was

completed in order for me to continue receiving paychecks.

This work would not have been a success without the help of Jock Fugitt of

TJNAF. He offered technical assistance in designing and building our microwave

cavities and designing o f the 13.56 MHz rf drive. Also, special thanks to John Bensel,

Kirk Jacobs, and Mel Woods o f the physics department machine shop. They offered help

when it was needed. I appreciate their contribution to the success of this project. I would

also like to thank DuPont/TJNAF for providing funding for this project.

To my special friends Edwin, Peter, Kofi, and Cornelius, I wish to thank you all

for your support and help; it’s been fun being around you. Unfortunately, it is time for

me to move on, but I w ill always remember you. To my brother and sisters back in

Ghana, I thank you all so very much for your prayers, support, and encouragement.

My most special thanks go to my parents, to whom I dedicate this work.

Although they are not here with us, I would like to thank them for putting me through

school; I owe everything to them. I wish that they could be here to share in this

wonderful accomplishment. Their memory w ill always be alive in me.

Finally, I would like to thank my wonderful wife and son (Sharon and Joseph Jr.)

for their moral support. My life would not be complete without them.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

Table Page

1.1. Variability o f peak locations o f selected excimers ...... 3

1.2. Excitation and ionization energies o f the noble gases ...... 9

1.3. Different excitation techniques ...... 10

3.1. Oriel Hg-Ar calibration lamp, NIST, and our lab. X values ...... 55

4.1. List generated from design o f experiments ...... 89

vii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

Figure Page

1.1. Schematic o f molecular energy levels ...... 5

1.2-1.4 DBD UV planar sources ...... 19

1.5 DBD U V c y lin d rica l source ...... 20

1.6 Typical Microwave Source ...... 21

2 .1. Plasma Sheath ...... 28

2.2. Variation of electron concentration, tie with E-field ...... 30

2.3. Ionization efficiency vs. electron energy ...... 31

2.4. Excitation efficiencies for He, Ne, Ar, and H i ...... 32

2.5. Probability o f collision and collision cross-section ...... 33

3.1. Xe Excimer lamp experimental set-up ...... 43

3.2. 1 kW 2.45 GHz microwave lamp cavity ...... 47

3.3. Schematics o f the lamp cavity, Duct working, and blower ...... 48

3.4a-f. Oriel Hg-Ar calibration lamp spectra ...... 58

3.4g. D2 lamp spectrum down to 130 nm ...... 59

3.5. Xe spectrum in 2.45 GHz microwave drive ...... 73

3.5a-g. Schematics o f angular dependence analysis ...... 65

3.6a Effects o f Pressure on Xe continuum ...... 75

3.6b. Intensity versus pressure curve ...... 76

3.7. Cooling effects on Xe spectral output ...... 77

3.8. Normalized intensity model calculation versus expt measurements.. .78

4.1a. RF lamp set-up ...... 82

viii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.1b. Cross-sectional view o f lamp set-up...... 83

4.2. Circuit equivalent o f lamp set-up ...... 84

4.3. Experimental set-up o f 13.56 MHz lamp ...... 86

4.4. Spectrum of F-600 H Fusion bulb ...... 96

4.5. Xe Spectrum in 13.56 MHz lamp ...... 97

4.6a. Xe/Ar (500-torr) Spectrum in 13.56 MHz lamp ...... 98

4.6b. Xe/Ar (1000-torr) Spectrum in 13.56 MHz lamp...... 99

4.6c. Xe/Ar (10-torr) Spectrum in 13.56 MHz lamp ...... 100

4.6d. Xe/Ar (100-torr) Spectrum in 13.56 MHz lamp ...... 101

5.1. Oriel Hg-Ar/D2 Calibration lamp Spectrum ...... 108

5.2. Schematic Potential Curves for the noble-gas halide ...... 109

5.2a. KrI UV emission (10-torr) in 2.45 GHz microwave lamp ...... 110

5.2b. KrI UV emission (50-torr) in 2.45 GHz microwave lamp ...... I l l

5.2c. KrI UV emission (100-torr) in 2.45 GHz microwave lamp ...... 112

6.1a-c Electron density profiles along length o f discharge bulb ...... 130

6.2a. Variations o f the electron, positive ion, and total current ...... 135

6.3. Average electron energy and potential along length o f bulb ...... 139

6.4. The plasma current and power densities ...... 140

6.5a. PIC result, plasma density profile density distribution ...... 141

6.5b. PIC result, electron energy distribution ...... 142

ix

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

A lamp that emits strongly in the 180-200 nm region is desirable because o f the response o f organic materials in this wavelength range. Therefore there is the demand for high-powered, efficient, and low-cost UV and VUV sources. The purpose o f this work is to construct and study two novel (low-cost) UV sources and to understand the I) emission (180-200 nm) characteristics, 2) electron energy and 3) temperature distribution in the plasmas generated by these two novel excim er lamps. We designed, constructed and studied 1) a probe-coupled 2.45 GHz microwave arrangement to drive Xe and KrI excimer lamps and 2) a 13.45 GHz RF capacitively coupled arrangement to drive Xe/XeAr excimer lamps. In the 2.45 GHz microwave drive, the Xe electrical efficiency and output power in the 160-200 nm range both increased with pressure and input power up to 1500 torr and 600 W (42.5 W/cm3) respectively. For the KrI discharge, over the pressure range o f 50-100 torr, more than 80% of the emission was in the wavelength range 170 - 190 nm. Model calculation that takes into account the angular distribution o f intensity and experimental measurement o f the angular distribution of emission find considerable intensity well away from the surface normal. The calculated efficiency varied from 20 to 40% for the Xe and 8 to 20% for the KrI depending on pressure giving for the first time good agreement between theoretical calculations and experimental measurements o f excimer lamp performance. Over the pressure range studied, the highest output power was ~ 0.96 W/cm2. The 13.56 MHz lamp arrangement was used to produce a bright, halogen-free light in the 180-200 nm range. At input powers o f > 500 W and pressures > 500 torr, more than 80% o f the emission appears in the spectral region between 180 nm and 200 nm with a strong 193 nm emission due to energy transfer mechanism between A r and Xe. The estimated electrical efficiency is 15 - 20%, taking into account the angular distribution o f the light intensity. Output power increased with increasing pressure up to 1500 torr. Cooling with liquid nitrogen boil-off rather than room air more than doubled the optical output power for fixed input power. We used an RF fluid model to calculate the plasma electron density and electron temperature distribution along the length o f the discharge bulb. Our results indicate that the electron density and temperature distribution along the length o f the bulb is constant. Electron density is an important plasma parameter; it determines the rates o f production o f reactive species and ions and provides basis for monitoring and real-time control.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter I: Introduction i

CHAPTER 1: Introduction

1.1. Overview, Motivation, and Plasma fundamentals

Ultraviolet (UV) light is the name given to electromagnetic (EM) radiation in the

wavelength (X) range between visible light and X-rays (10 nm - 400 nm or frequencies

from 7.5E14 to 3E16 Hz). UV light is divided into long and short Arranges. The long A.-

range is further divided into UVA 315-400 nm, UVB 280-315 nm, and UVC 200-280

nm. The short Arrange (A. < 200 nm) is called vacuum ultraviolet (VUV) because air, in

particular the oxygen in air, strongly absorbs light at A.< 200 nm, w ith the result that work

with VUV light is usually performed in a vacuum.

The short-A. UV photons have energy > 6 eV sufficient to induce radical

formation in nearly all organics. Accordingly, they may be used to drive radical-

mediated reactions that lead to material surface modifications18*21. For example, UV light

from A.=354 nm to A.=126 nm (Table 1.1) can selectively generate certain radicals w ith a

wide range o f prospective industrial chemical and photo-physical84 processes. These

include: 1) UV curing of paints, metal, wood, paper, plastic parts, foils, compact discs,

glass fibers, varnishes and adhesives17, 2) photo-degradation o f a variety o f pollutants22*

2S, 3) photo-etching o f polymers26 4) micro-structuring o f large area polymer surfaces,

and 5) improve the adhesion properties o f coatings as well as the wettability and

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printability o f polymers by changing the morphology and chemical properties o f the

surface. UV photons can also be used for cleaning surfaces, for initiating metal organic

chemical vapor deposition (MOVCD), and for photo-oxidation.

The necessary conditions for a light source to make UV photo-chemistry

applicable in an industrial process are 1) high efficiency 2) low photon cost and 3) high

enough power to match process speeds. While a variety o f coherent UV sources (high-

power KrF - 248 nm and ArF - 193 nm) and incoherent lamps (low-pressure glow

discharge in mercury/rare gas mixtures and xenon flashes) are available, only a few high-

power UV sources have as yet become standard equipment for industrial application.

A common problem with mercury lamps is their broad emission spectra; i.e., in

addition to emitting the necessary k radiation, they also produce UV radiation of

unwanted wavelengths as well as visible light. Lasers are available at wavelength o f 157-

nm (F 2), 193-nm (ArF), 248-nm (KrF), 308-nm (XeCl), and 351-nm (XeF) with average

powers from a few watts to a few hundred watts. The lasers are used as research tools as

a convenient source o f UV. The principal applications are deep UV lithography, pulsed

laser deposition and polyimide micromachining (via cutting). Comprehensive reviews on

excimer lasers exist.115,116

In recent years, various kinds o f UV lamps1*16 capable o f delivering high power,

high efficiency, and narrow-band light have been commercialized. Such UV lamps are

now called “excimer lamps.” These new incoherent UV lamps produce their radiation

through the formation of unstable molecular complexes. They provide excellent

performance w ith high efficiency, narrow spectral o f radiation, and low photon cost.

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EXCIMER WAVELENGTH/nm

A t2* 126

K r2* 146

f 2* 158

Xe2* 172

C l2* 259

B r2* 289

I2* 342

K rI* 190

A rF * 193

A rC l* 175

K rC l* 222

X e l* 253

X eC l* 308

XeF* 354

Table l. l. The peak locations o f selected excimers. A large number o f different

excimer spectra can be obtained from rare gas dimers, halogen dimers, and rare gas

halide dimers, which are sometimes referred to as exciplexes.

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1.2. Theory and formation of Excimers

The first indications o f molecular structures in rare-gas were mentioned by

Goldstein31 and Curtis32 in 1913. In two separate papers, they reported molecular

structures in spectra of helium (He) discharges and later in the 1930’s, molecular

emission o f rare-gas mixtures was detected33. Extensive investigations into the properties

o f the He 2* excimer, were carried out by Hopfield34 and the first mention o f rare gas

excimer formation was by Tanaka35. In this section, we discuss excimer formation

mechanisms.

Consider a diatomic molecule Rg *2 with potential energy curves for the X 'zg+

state and excited 3£u+, 'Zu+ states. The X ‘l g+ state correlates to two ground state lSo and

'So separated atoms and the excited 3£UT, ' l u+ states correlates to one ground state *So and

one excited state 3P [,2 separated atoms (Figure l.l). The 'Zg+ (*So + lSo) state potential

energy curve has a shallow minimum and decreases smoothly as the inter-nuclear

distance r increases, yielding an unbound or “repulsive” state. In such a ground state,

there are no well-defined vibrational states. For heavy atoms, there may be a very

shallow minimum in the ground state permitting at most a few vibrational states to exist.

There are 3Po and *P| states lying 0.5 eV - 1.2 eV above the (3P 2, 3P|) states (Fig.

1.1). These do not form stable molecules. Radiation to the 'So ground state from the 'Pi

and 3P0 states are dipole forbidden. The 0.5 - 1.2 eV energy gap between the level pairs

(3P2, 3Pi) and (3Po, ‘Pi) makes non-radiative (collisional) transitions between them

negligible. In contrast, collisional equilibration between 3P 2 and 3P|, which are separated

by only 0.12 eV is fast. The long-lived (B,l£u+ and A,3£ u*) states are populated both by

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E(*V)

'P .+ 'S. 'Pi+'s,

1st Continuum

2nd Continuum

S* (Resonance line)

Intcrnuclcar distance (R)

Fig. 1.1. A simplified schematic potential energy diagram identifying the excited

atomic and molecular levels o f rare gases. In pure gases, continuous VUV emissions

arise from transitions between the weakly bound 3X„+ (3P2 + ’So) and ,I U+ (3Pi + ‘So)

molecular states and the dissociative ground state lXg+ ('So + 'So), generally repulsive.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 1: Introduction 6

direct excitation from the ground state and by radiative cascade from higher excited

atomic states or (ionic states after recombination). The potential energy curve for the

excited states (*5V and 3SU+) are bound, thus the molecule, Rg* 2, can temporarily exist in

them. These molecules (Rg* 2), after radiating to the (repulsive or weakly bound) ground

state, X'Eg*, dissociate within a few nanoseconds, either on their own or during a

collision with another molecule o f the gas mixture, giving o ff their binding energy in the

form o f electromagnetic radiation. The radiation from the 'Z u+ and 3X„+ states leads to the

appearance o f continuous emission bands in the VUV, irrespective o f the pumping

method65. There is also a resonance line that is ascribed to the transitions from the 3P| ->

‘So (and by collisional intermixing, 3P 2 3P|-> ‘So) (Figure 1.1). The high-energy

continuum (short X) transition is called the “first continuum” the lower energy transition

is called the second continuum as seen in Fig. 1.1. W ithin a homologous series of

excimers, eg. He 2*, Ne 2*, A r 2*, both continua shift toward shorter wavelengths as

atom ic mass dim inishes.

So for lamp development purposes, excimers may be regarded as diatomic

molecules (or complexes o f molecules) that disintegrate w ithin a few nanoseconds giving

o ff their binding energy in the form o f UV light. The spontaneous radiative emission

life tim e ( t) is generally proportional to the cube of the light wavelength (t~A.3). In

principle, all rare gas and rare-gas/halogen-gas mixtures can be used to form excimers.

These form bound excited dimers lying ~ 8 - 20 eV above the ‘Zg+ (‘So + ’So) state with

binding energies o f ~ 0.5 - 2 eV (Fig. 1.1). Their properties and kinetics o f formation

have been studied extensively3,7’ 8' 115, l29, " d ,3°.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 1: Introduction 7

To form excimers, the constituent excited atoms, Rg*, can be created by energetic

radiation such as X-rays or VUV radiation or by particle beams e.g., protons, alpha

particles, heavy ions or nuclear fragments. Electrons are well suited for dissociating

molecules and for forming Rg*. Accelerated to high speeds by an applied electric field,

they transfer their kinetic energy as excitation energy to the atoms and molecules, causing

gas breakdown. Equations 1.1 -1 .5, shows the main features o f the population dynamics

for rare gases.

e + Rg -> Rg’ + e...... (excitation ) (1.1)

Rg' + Rg + M -+ (Rg2 U + A / (association ) (1.2)

(RSi )„o + M -► ( Rg2* ) „ 0...... (relaxation ) (1.3)

( ^ 2*)k»o ~+2Rg + hv(UV) .... (emission and dissociation) (1.4)

Rg' -> Rg + h v ...... (deactivation ) (1.5)

Reaction (1.1) = electron impact process to create Rg*

Reaction (1.2) = 3-body process creating (Rg 2*) v*o

Reaction (1.3) = vibrational relaxation process to form (Rg 2*) v=o

Reaction (1.4) = dissociation step (UV emission)

High pressure is required to ensure that (1.2) is faster (higher rate) than any

collisional quenching or radiative transfer process that can deactivate Rg*. The

probability that reaction (1.2) occurs increases as pressure increases. In practice,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 1: Introduction 8

pressures > I bar are required. M in (1.2) represents a third collision partner that can be

an atom or molecule o f the gases involved or can be an added buffer gas. In reaction

(1.2), the metastable Rg*(3P 2) atoms combine with ground state atoms Rg('So) in three

body collision processes to form stabilized dimers in high vibrational ((Rg 2*)v*o) levels.

At low pressures, these dimers emit spontaneously, giving rise to the first continuum; at

high pressures, some relax rapidly into the lower vibrational levels by binary collisions

with the ground state atoms, and then emit to yield the second continuum. Other groups

have proposed that there exists a third continuum that corresponds to transitions between

the Rg 2 + <-> Rg+ states that can be initiated by rare-gas excitation with high-energy

protons, electrons, or alpha particle61'67.

In electro-negative gases, the recombination o f positive and negative ions can be

major paths leading to excimer formation. A typical example is the formation of the

XeCl* excimer (equations 1.6 -1.9).

e + Xe-*Xe++2e (1.6)

e + C /2 - * Cl + Cl~ (1.7)

Xe* + C r + M - > XeCl* + M (1.8)

XeCl' ->Xe + Cl+U V radiation (A = 308wn) (1.9)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 1: Introduction

Rare Gas Energy of lowest excited Ionization Energy (eV)

State (eV)

He 19.8 24.6

Ne 16.6 21.6

A t 11.6 15.8

K r 9.9 14.0

Xe 8.3 12.1

Table 1.2. Excitation and ionization energies o f the noble gases.

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13. Different Excitation Techniques

Excimers can be generated in a number o f ways. In the past decade, many

research groups have studied different excitation techniques for excimer sources

(Table 1.3).

Table 1.3: Different Excitation Techniques

Excitation Technique Reference Dielectric Barrier Discharge 1-3, 8 ,9 , 17, 36-38

(DBD) (spontaneous emission)

UV pre-ionization 39-43

(transverse discharges)

Microwave discharges 14,15,43-53

Pulsed longitudinal Discharge 37,49

Continuous longitudinal Discharge 54, 55

Nuclear excitation 56

Hollow Cathode Discharge (HCD) 57-60

Electron beams 61-64

Protons 65

Alpha particles 66-67

Heavy ions 68

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1.4. Advantages of Excimer Lamps

Excimer lamps, due to their sim plicity and reliability, present several advantages over

lasers, especially when large areas or large volumes have to be irradiated.

• An excimer lamp, with gas pressure lower than that o f an excimer laser, reduces self­

absorption of radiation from excimer molecules. This increases the operating

efficiency and allows adopting a simpler and lower cost electrical power supply

system. Theoretical efficiencies as high as 40-50% have been predicted for the VUV

radiation o f Xe 2 and K r 2 excimers2.

• Excimer lamps are compact and easier to handle and maintain. Laser systems are

relatively large with high capital costs, are expensive to operate and maintain.

• Excimer lamps produce incoherent radiation, as a result, processing over a large

sample area without speckle or interference fringes can be performed17.

• Collimators are not required.

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1.5. Commercial Available Lamps

The only commercial lamps currently available are made by 1) Heraeus/Asea Brown

Boveri (ABB), Corporate Research, CH-5405, Baden/Switzerland, 2) Fusion Systems,

7600 Standish Place, Rockville, Maryland 20855-2798, Tel: (301) 251-0300, and 3)

Ushio, 10550 Camden Drive, Cypress, CA 90630-4600, Tel: 1 800 218-7446. Heraeus

Nobelicht recently commercialized the ABB dielectric barrier-discharge (DBD) lamps at

172 nm, 222 nm, and 308 nm. Fusion Systems and Ushio offer microwave driven

excimer lamps commercially, but not in the deep UV. In general, these lamps are

operated without electrodes and comprise a UV bulb that contains a plasma-forming

medium, a coupling means to couple microwave or RF energy to the discharge envelope

and means to excite the plasma-forming medium.

The DBD was first introduced by W. Siemens69 in 1857 for “ozonizing” air with

detailed information provided later by Buss, FClemenc et al., Honda and Naito, Gobrecht

et al., Bagirov et al., Tanake et al. and Hirth70*77. Warburg180'181 and Becker182, 183 in

I I fiA 10 4 Germany, Otto in France, Briner et al. in Switzerland, Philippov and his group in

Russia, Devins187 in the United States, Lunt188 in England and Fuji et a l.189 in Japan are

among the early groups that studied ozone and nitrogen oxide formation in DBD lamps.

As o f today, the production o f ozone (known to be a potent bactericide and viricide) is

still the major industrial application of DBD’s with thousands of installed ozone

generating facilities based on this principle. For this reason the DBD is sometimes called

the ozonizer discharge. However, in modem years novel applications o f DBD’s for

pumping o f CO 2 lasers190*192 and excimer lamps193'198, flue gas treatment199*202, surface

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m odification203, pollution control204*208, H 2S decomposition209 and CO 2 hydrogenation210’

211 have been proposed.

The DBD discharge configuration is characterized by the presence o f at least one

dielectric barrier in the current path between the electrodes and the discharge space2

(Figs. 1.2-1.5). In general, gas spaces bounded by one or two dielectrics have

approximately the same breakdown voltage as if they were between metal electrodes.

Low-loss dielectric plates, or tubes, o f high breakdown strength, such as glass, quartz, or

ceramic, to which metal electrode coatings can be applied, are used. Metal electrodes

with dielectric coatings, e.g. steel tubes with enamel layers can also be used.

The presence of the dielectric(s) precludes dc operation. Although DBD

configurations can be operated between line frequency and microwave frequencies, the

typical operating range for most technical DBD applications lies between 500 Hz and 500

kHz98. The most interesting property o f DBD’s is that, in most gases near atmospheric

pressure, breakdown is initiated in a large number o f independent current filaments or

micro-discharge channels. Plasma parameters in the breakdown channels can be modeled

and consequently can be optimized for a given application106*144.

Several research groups99*103 have investigated discharges in microwave cavities since

the 1950’s. A typical microwave excimer lamp is shown in F ig .l.6. When a dc voltage of

approximately 4 kV is applied to the magnetron tube it oscillates, radiating microwaves

from 0.3 GHz to 30 GHz, corresponding to wavelength range o f 1 to 100 cm in free space

(A. = f7c). These microwaves propagate through the directional couplers, the stub tuners,

and a coupling hole into the resonant cavity generating a strong EM -field that produces a

discharge. Microwaves can be used for the excitation o f both low and high-pressure

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gaseous lasers and lamps in many types o f geometrical configurations over a wide range

o f power loading. Previous work on microwave lamp excitation has been published by

FCumagai et al.14, l5, Mendelsohn et al.47, Prelas et al56, and Christensen173, among

others43*53. Microwave discharge excitation provides a quasi-continuous excimer light

source that eliminates the discharge instabilities inherent in a DBD. Additionally, such

discharges are spatially uniform over larger volumes and at higher pressures than other

types o f dc self sustained discharges.

Presently available commercial lamps have the follow ing advantages:

• Because they are electrode-less, sputtering o f electrode materials that tends to blacken

the inside wall o f the discharge vessel, altering light transmission, is avoided. This is

particularly important for spectroscopic light sources and for plasma-assisted

chemical processing.

• Gas contamination by adsorbed/absorbed and reactive particles from the electrodes is

eliminated, prolonging bulb lifetime.

• The critical seal between metal and glass is avoided.

• Theoretical efficiencies of up to 60% have been reported in DBD. In microwave

lamps it is possible to couple over 90% o f the microwave power to the plasma46.

• In microwave cavity applicators, ignition o f the discharge is easy because high E-

fields are generated.

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Major disadvantages o f present commercial lamps are:

• To date, microwave lamps operating at frequencies above 1 GHz, require a well-

designed tuned resonant cavity for efficient coupling o f the microwave energy. Such

matching conditions make it difficult and expensive to increase the size o f the UV

source and to construct an efficient microwave lamp.

• The hazards resulting from microwave leakage has to be overcome.

Although, in the past decade, excimer lamps have been extensively studied, issues

such as improving lamp efficiency to over 20 %, effective cooling o f discharge tubes,

which is of great importance to the power density deposited into the lamp, and

measurements and modeling o f the discharge (plasma) parameters have received less

attention. Additionally, the available intensities from available excimer lamps, especially

in the VUV range, are moderate. While a variety o f UV lasers and excimer lamps are

available for laboratory testing, only a few high power sources have become standard

equipment for industrial applications.

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1.6. Goals of this work

The goals o f the work are

1) present two (low-cost) novel high-power excimer lamps,

2) through modeling obtain estimates o f plasma parameters such as the electron density

(nc) and temperature (Te) distributions to gain a better understanding of the

performance and energy coupling of these lamps. The optical power output,

efficiency, and branching ratios ail depend on the ne and Te.

3) ascertain the performance and efficiency o f these lamps.

This was accomplished by performing the following tasks:

• Developing prototype lamps using either 13.56 MHz or 2.45 GHz microwave energy

in bulbs having diameters between 9 mm - 13 mm, at pressures from a few torr to

several atmospheres and power levels from 50 W to several kW.

• Determining the optimal geometry and conditions for such high-pressure designs that

w ill provide the best UV emission with a large variety of applications.

• Studying those characteristics of these lamps that are relevant for large-scale

industrial UV application, such as radiative emission characteristics.

• Performing test experiments with these lamp designs with different fill gases to

determine their efficiency, and lifetime.

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• Studying the cooling effects on the emission characteristics o f these lamps.

1.7. Outline of the Thesis

Chapter 2 discusses the basic plasma physics o f gas discharges. This includes

fundamental plasma concepts, microwave and RF breakdown in gases, and electron loss

processes.

Chapter 3 discusses the design, construction and studies o f a novel probe-coupled

2.4S GHz microwave Xe excimer lamp. Electrical efficiency and output power in the

160-200 nm range (Xe second continuum) were measured as functions o f pressure and

input powers. The cooling effect of liquid Nitrogen boil-off rather than room air on

output was investigated.

Chapter 4 discusses the design, construction, and studies o f a 13.56 MHz excimer

lamp. We used 13.56 MHz RF in a novel, capacitively-coupled excimer lamp

arrangement to produce a bright, halogen-free light source in the 180-200 nm range. In

this arrangement, power was coupled to an electrode-less bulb using a tuner that is both a

part o f the power supply and the lamp cavity. Results o f experimental studies o f Xe and

Xe/Ar in this lamp are presented. The emission characteristics in the wavelength range o f

160 - 200 nm as function o f input powers and pressures are presented.

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Chapter 5 discusses a 2.45 GHz K rl microwave excimer lamp. We used 2.4S

GHz microwaves in a novel, probe-coupled arrangement to excite a K rI excimer lamp.

Over the pressure range o f 50-100 torr, more than 80% o f the emission was in the

wavelength range 170 - 190 nm. The electrical efficiency results as a function o f gas

pressure and input powers are also presented.

Chapter 6 presents results o f computations to predict the electron density, tie,

electron temperature, Te (in terms o f mean electron energy), and current densities in a

medium pressure (5 < p <20 torr) argon, krypton, and xenon from a 1-D fluid model o f

13.56 MHz capacitively-coupled lamp. The model is a 1-1/2 D RF simulation code that

solves the fluid model equations (continuity, momentum or d rift diffusion, and energy

equation) coupled to the Poisson equation. Results from model calculations were

compared to particle-in-cell calculations at higher pressure.

Chapter 7 presents conclusions. A discussion of future directions in building

microwave-driven flat panel designs for deposition, etching, and photolithography

experiments is presented. We also present possible improvements in lamp designs.

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High Voltage Electrode High ' '"‘A| >4^pielectric vo: r 0 Oiacharge Gap Barrier Generatornr Ground Electrode

Fig. 12

Fig. 1.3 Fig. 1.4

Figures 1.2, 1.3 and 1.4 are different geometries o f schematic diagrams o f a DBD

planar Excimer UV source. The pressure o f the fillin g gas varies in the range o f 1 bar.

The sinusoidal applied voltage peaks at about 20 kV and a frequency o f the order o f 1

kHz. [Kogelschartz, 1997].

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Oielectric Discharge Gap Barrier v

.NV

i\ v .sv

Ground Electrode

Figure 1.5. Schematic diagram o f a DBD cylindrical Excimer UV source. [Kogelschartz et al., 1997].

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THREE STUB TUNER DIRECTIONAL RESONANCE COUPLER CAVITY

MESH MAGNETRON 2.45 GHz

QUARTZ BULB

Figure 1.6. A typical microwave excimer lamp. It is comprised o f a waveguide,

directional coupler, stub tuners, and a resonant cavity. [Furusawa et al., 1995].

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Chapter 2

In this chapter, we discuss fundamental plasma concepts, microwave and RF

breakdown in gases, and electron loss processes.

2.1. Plasma fundamentals

In general, a plasma can be described as a gas composed o f neutral (atomic or

molecular) species, electrons and ions. A collection o f ions and electrons cannot coexist

in a volume indefinitely, because electrons w ill recombine with ions at a finite rate until

the charge is neutralized. Thus, to sustain a plasma, ions and electrons must be

constantly generated in the plasma by some external source o f energy. Fig. 2.1 shows a

plasma constrained in a volume and being generated by some outside radiation source,

say a microwave field.

The physics of discharge plasmas are almost completely dominated by the

behavior o f electrons. Because o f the large mass difference between the electrons and

ions or neutrals, very little kinetic energy is transferred through elastic collisions from the

hot electrons to the more massive species. The result is that the electrons quickly come

to equilibrium at a higher temperature than the ions and neutrals. A Maxwell-Boltzman

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(MB) distribution function is commonly used to characterize the electron energy. For an

MB distribution, the characteristic electron temperature, Te, is defined as:

mev2ihe _ 3kTe 2 2

where, vu,c is the electron thermal velocity. It is these energetic electrons with energies

above the levels shown in Table 1.2 that are responsible for producing most o f the

reactive species in the plasma. The ions also exchange energy efficiently with the

background gas through elastic and inelastic collisions. The net effect is that the

electrons, ions, and neutrals o f the discharge can be described by different equilibrium

temperatures, namely Te» T j = Tg, where the subscripts e, i, and g refer to the electrons,

ions and neutral background gas respectively (typically, Te = 2 to 4 eV, Tg = 0.02 to 0.05

eV,Tj = 0.05-0.5 eV).

For a quasi-neutral discharge plasma, the electric field, E (required to sustain

plasma), would be zero and the electric potential, , would be constant everywhere.

When ions and electrons hit the wall, they recombine and are lost. Because o f the much

higher thermal velocity of electrons compared with ions in a confined plasma,

(Vihe»Vu,„ where V u * = (Te/m«)l/2 and V u,i = (T j/m j) l/2 are the relative thermal velocities

o f electrons, and ions respectively), electrons are lost faster causing the plasma to accrue

a net positive charge (plasma potential, Vp) with respect to the walls; i.e., the wall

potential, Vw> is negative. The result is a thin layer (order o f several Debye lengths in

thickness) rich in positively charged ions (nj»ne) adjacent to all cold walls with which

the plasma is in contact. This leads to a potential profile that is positive with respect to

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the walls in the bulk o f the plasma, and falls o ff quickly to the wall potential (normally

walls are held at “ground” or some fixed potential).

Discharge plasmas have two defining characteristics: (a) electrons have a much

higher thermal velocity, than the ions or neutrals and (b) they are electro-neutral,

down to the scale o f tens o f hundreds o f microns, except near material boundaries.

The first characteristic means that any electrically floating material, such as a

metal bar, introduced into the plasma w ill charge up negatively due to the high m obility

o f the electrons and thus be at a lower potential than the plasma. The charging w ill stop

when the floating bar achieves a potential such that an equal current o f positive ions and

electrons reach the bar. This potential is called the floating potential, Vf. The difference

between the plasma potential, Vp, and the floating potential, Vr, is:

V-Vf = — Ln 2.2 p f 2e v 2.3 meJ

Eq [2.2] has been written to include the correction term for the ion current due to the

Bohm sheath criterion.

Such a plasma w ill adjust to seek a higher potential than anything placed within it.

The plasma potential, Vp, is dependent on a number o f factors including source geometry,

pressure, and method o f plasma creation. The magnitude o f the plasma potential, Vp, is

important because it determines the energy o f plasma particles bombarding surfaces with

which the plasma is in contact, which may include devices being processed.

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For the second condition, the Debye length, Xd, is the characteristic length beyond

which the electro-neutrality condition holds, where

(2.3)

Quasi-neutrality also means that plasmas have the ability to shield out time varying

electric fields with frequencies below a characteristic frequency, fpe. called the plasma

frequency. That is, electromagnetic waves launched at f < fpe into an unmagnetized

plasma w ill be cut-off, while waves with f > fpe can propagate through the plasma.

Expressions for values o f Xp and fpe, in units most often used by experimentalists are

given below:

2.4

fp, {Hz) = 8.98 x 103 2.5

0)^ =5.64xl04 -y/nj 2.6

The product o f X d and o)pe is on the order o f the thermal velocity o f electrons, V u * .

2.7

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2.1.1. Plasma Sheath

One o f the most important concepts in describing a plasma is the “dark space

sheath ” or simply the “ sheath” . The sheath is a thin, positively-charged layer adjacent to

all surfaces in contact with the plasma. The thickness o f the sheath is a small multiple o f

the Debye length, (Xd). The function o f a sheath is to form a potential barrier so that the

more mobile electrons, are confined electrostatically. The sheath size depends on the

applied voltage and the ionization density o f the plasma and can be calculated by the use

o f Poisson’s equation. When ions reach the sheath edge by diffusion or in response to

small potential gradients in the plasma, they are accelerated across the sheath o f width s

by the potential drop, Vp.

A positive space charge sheath can exist only if the ions approach the plasma-

sheath boundary at the ion-sound speed [Bohm, 1949]. In order for this to occur, there

must be a quasi-neutral transition region between the positive space-charge region and

the plasma, where the ions are accelerated from their thermal velocity to the sound speed.

This phenomena is known as the Bohm sheath criterion [Bohm, 1949]. The ion sound-

speed, Cs, is:

Cs (cm /sec) 2.8

where, y is the adiabatic index, which in mono-atomic gases is equal to 1 for isothermal

flows and 5/3 for adiabatic flows.

A plasma can be characterized by the two parameters ne and kTe, with 1 < ric <

ta 7 10 cm ', 0.1 < kTe £ 10 eV. These parameters are determined by the plasma power

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loading. From these parameters the electrical properties (discharge current density and

deposited power density) o f the plasma and the rates o f production o f reactive species and

ions can be deduced and provides a basis for process monitoring and real-time control. In

the present investigation o f the 13.56 MHz rf capacitively coupled discharge, a model

calculation is used to provide the plasma electron density in the pressures in the range o f

5 to rr to 20 torr.

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- V = O

e?

\ Dark Spaca S haath

V (V o lts)

Fig. 2.1. The dark space sheath is the region in which electrons have been

“rejected” by the electric field o f either an external applied voltage or potential o f the

plasma with respect to ground. The magnitude o f the plasma potential is determined by

the geometry o f the contacting volume. In general, the smaller the volume o f the

enclosing structure (or grounded surfaces in contact w ith the plasma), the larger Vp is.

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2.2. Microwave and RF breakdown of Gases

When an electric field is applied to a gas, the free electrons move in the direction

of the field, resulting in a current density, I. If the E-field is gradually increased from

zero, the gas appears to obey Ohm’s law until the field becomes large enough to impart

sufficient energy to some o f the electrons to produce secondary electrons by collision. If

the E-field is further increased, many secondary electrons are produced causing the gas to

become conductive. For a very small change in voltage or field near this value, the

electron concentration (n«) and current I, change by many orders o f magnitude, causing

the gas to breakdown (Fig.2.2). This is the Townsend breakdown criterion; a criterion for

dc discharges. In a gas discharge, electrons are produced by ionization and lost by

diffusion, recombination, and other processes. When the production rate is equal to the

loss rate, a steady state results. Near threshold however, the production rate needs to be

only very slightly greater than the loss rate for breakdown to occur, so the Townsend

breakdown criterion is customarily defined by saying that at breakdown the ionization

rate must equal the loss rate. However, the criterion has proved useful in considering

discharges produced by E-fields o f any frequency, including microwaves129. At a given

microwave frequency, different gas species w ill breakdown at different pressures. Thus,

microwave breakdown process depends on 1) the ionization potential o f the gas, 2) the

gas pressure and 3) the microwave frequency.

The ionization and excitation efficiencies for He, Ne, Ar, and H 2 are shown in

Figs. 2.3 and 2.4. The probability o f collision and cross-sections forNe, Ar, Kr, and Xe

are also shown if Fig.2.5.

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Fig. 2.2: Variation o f electron concentration, n«, with E-field. The flat portion of

the curve illustrates the fact that at breakdown n« may vary over a wide range, typically

from 103 to 108 electrons-cm*3, while the practical field remains constant.

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0.0/

0.06

0.0S

0.04

| 0.03

0.02

0.01

Electron energy (volts)

Fig. 2.3: Ionization efficiencies for a small range o f electron energy for He, Ne,

A r, and H 2. The data for He and Ne are those o f Fox119, the A r data are those o f Fox and

Bleakney119’ l2°, and the H 2 data are those o f Tate and [A.D. MacDonald, Microwave

Breakdown in Gases, John W iley and Sons, Inc., New York, p.26]

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0.07 0.06 0.05 0.04 0.03 0.02 0.01

8 10 12 14 16 18 20 22 Electron energy (volts)

Fig 2.4: Excitation efficiencies for He, Ne, Ar, and H 2. The data for He, Ne, and

Ar are those o f Leibnitz122. The H 2 data are those o f Ramien123 [A.D. MacDonald,

Microwave Breakdown in Gases, John W iley and Sons, Inc., New York. p.25].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 2: Plasma fundamentals 33

140

36 x 1 0 * 16 120

100 Pc

Electrpn energy (volts)

Fig.2.5. Probability of collision and collision cross-section for neon, argon,

krypton, and xenon. [A.D. MacDonald, Microwave Breakdown in Gases, John W iley and

Sons, Inc., New York, p.20]

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2.3. Electron Loss Processes

For all self-sustained plasmas the volume integrated and time-averaged rate o f

generation o f charged particles must be equal to their global rate o f loss. The main

electron loss processes in a gas discharge include diffusion, attachment, and

recombination.

2.3.1. Diffusion

The rate of diffusion, for free diffusion, may be specified by a diffusion

coefficient D. The diffusion potential is defmed as the product of the diffusion

coefficient, D, and the electron concentration, n. Eq. 2.9 (Fick’s law) is the rate

diffusion equation in terms of the diffusion potential (Dn) and current density, T, with

relating continuity equations (Eqs. 2.10 and 2.11).

r = -V(Z)«) (2.9)

—-v-r-p=o (2.10) dt

— = V 2 {Dn)+nv, (2.11) dt V 1

where

P = production rate o f sources or sinks within the volume considered

nv;= rate at which electrons are produced by some source and

Vi = net ionization rate per electron.

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I f the diffusion coefficient, D, and ionization rate, vj, are independent o f space and

time respectively, then in Eq. [2.11], it is possible to 1) separate variables and 2) set both

sides o f resulting equation to equal to a constant, -y (Eqs. 2.12, 2.13, 2.14, and 2.15).

1 dn _ D = — V 2n (2.12) n dt ' nn

(2.13)

(2.14)

n = n0e K-rV (2.15)

The solution o f Eq. 2.14 is 2.15 and that for Eq. 2.13 depends on the form

. 7t3t . 7ry.nz n = n0 sin — sin— sin— (2.16) 0 X Y Z

(2.17)

o f the Laplacian. The design o f most experiments is based on rectangular or cylindrical

geometry, therefore we consider the solutions in such systems. For rectangular geometry,

the solution and eigenvalues in rectangular coordinates to Eq. [2.13] are Eqs. 2.16 and

2.17 respectively.

Sim ilarly, in cylindrical coordinates, the solution and eigenvalues are

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• » , L -/v, r n = nQ s in — JQ 2.405— (2.18)

(2.19) { R ) U J D

with the condition that the electron concentration is zero at the boundaries. Solutions

involving higher harmonics o f these functions are also possible, but are not considered

here. In general, these functions are associated with high-order decreasing exponentials in

time, which damp the harmonics rapidly. Thus neglecting them is not a serious source o f

error in quasi-equilibrium calculations. In the above equations, X is the height and R is

the radius o f the cylinder. For large parallel plates the results can be approximated from

either o f these two solutions by considering the plates to be separated in the coordinate X

o f Eq. [2.17], with Y-> co and Z-> oo; or Eq. [2.19] by allowing R-> oo. In either case the

ratio y/D it2fX2. Because y/D has units o f reciprocal distance squared, it is convenient

to w rite

r (2.20) D A 2

where A is a characteristic diffusion length. For cylindrical geometries it is sometimes

convenient to make a small correction for the finite radius. Using Eq. [2.19], we rewrite

Eq. [2.15] as

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The rate o f change o f electron concentration in an ionized volume from which

electrons escape by diffusion depends on the difference between the ionization rate and

the ratio o f the diffusion coefficient to the square o f the characteristic diffusion length.

Solutions o f the spatial diffusion equation are often frequently used in breakdown studies,

but solutions124 o f the type,

n(x,t) = A(Dt)-U2e-

are useful in studying breakdown conditions when the time o f formation o f the plasma is

important. In very fast studies, higher harmonics including the full time dependent

summation expansions are required.

Ambi diffusion (presence of the interactions between electrons and ions) is

important in breakdown problems where there is a high ambient electron density. Under

ambiploar diffusion, the E-field is found by writing Eq. [2.9] as:

f = p,nE - D ,V n = ~ntnE - DeVn (2.23)

e D'~ D' Vn (2.24) M i+ M e »

T = V n - D ^ n Mi + Me

(2.25) Mi +Me

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This is Ficks law with a new diffusion coefficient, Da, called the ambipolar

diffusion coefficient.

u D + u D Da= M‘ e ' (2.26) M , + M e

where m, p* are the ion and electron mobilities and Dj, De the corresponding diffusion

coefficients. The transition from free to ambipolar diffusion has been studied in detail by

A llis and Rose125 and separately by A llis 126.

2.3.2. Attachment

When a neutral atom acquires a negative charge by attachment, an electron that

might otherwise have acquired large velocities from the field is replaced by a negative

ion which is at least two thousand times as heavy and which w ill therefore be accelerated

to proportionately lower velocities during a cycle of the E-field. Thus the ions can be

considered fixed in space and irrelevant to the developmental plasma dynamics except for

forming excimers A+ + B' -> AB*. Thus the process of attachment is dynamically

equivalent to a volume term representing disappearance o f electrons. A measure o f the

likelihood o f attachment is the electron affinity, values o f which have been tabulated by

B row n127.

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Quantitative measurements o f attachment have been given in the literature in a

number o f different ways. I f va is the attachment rate, i.e., the number o f attachments per

second, the ratio ha = va/vc is the attachment efficiency and is analogous to the excitation

and ionization efficiencies. This quantity is often called the probability o f attachment, as

reported by Bradbury [N.E. Bradbury, Phys. Rev., 44, 885 (1933)]. More recent data

have been reported in terms o f a quantity a/p, where a is the probability o f attachment

per centimeter o f travel.

2.33. Recombination

Electrons are produced by ionization. If an electron collides with a positive ion,

there is a certain probability that the two w ill recombine to form a neutral atom, leading

to the electron being removed from the discharge. Diffusion, recombination, and

attachment are the major electron loss processes. These are described by the following

process rate equations:

dn+ _ dn * = - a rn+ne (2.27) ~dT~~dt

dn -i . . . — = - a rn ' (2.28) dt

- = — + art (2.29) n nn

where ar is the recombination coefficient, n+, n« are concentrations o f positive ions and

electrons. In general, n+ = He = n.

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Biondi and Brown found recombination coefficients in the noble gases several

orders o f magnitude larger than would be explainable by radiative recombination. In

microwave systems, the E-field oscillates so rapidly that the force on an electron changes

direction before the electron can travel very far. This means that electrons are not swept

out o f the discharge area, as they are when lower frequency is used. Electrons that leave

the discharge areas can be assumed in low-order models to do so by the process of

diffusion or pressure driven convection.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3: Characterization and Modeling o f Microwave Xe Excimer Lamp 4 1

Chapter 3: Characterization and Modeling of a 2.45 GHz Microwave Xe

Excimer Lamp

Since many materials absorb radiation at wavelengths shorter than 250-nm, very

efficient UV and VUV sources are desirable for stimulating chemical processes.

Therefore, there is the demand for high-power, efficient, low-cost and large area UV

sources. The next three chapters describe novel high-power and efficient excimer lamps

for producing photons inexpensively.

This chapter describes a microwave drive in which the probe o f the magnetron is

directly coupled to the cavity without using wave-guides, directional couplers, or tuners.

Also, this chapter discusses 1) the spectral output o f Xe as a function o f pressure, 2)

cooling effects on the deposited power density, and 3) a new model that more accurately

estimates the efficiency o f light production in the system.

3.2. EXPERIMENTAL FACILITY

The lamp (Fig. 3.1) consists o f a 2.45 GHz magnetron, microwave cavity, and a

discharge bulb that passes into the cavity through two long, grounded cylindrical tubes

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designed to suppress leakage o f microwave energy. The probe o f magnetron is directly

coupled to the cavity. The gas in the lamp is not circulated.

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Resonant Cavity Excimer Lamp MgF2 Window

Spectrometer

To Pumping System

M agnetron with probe in resonant cavity

Fig. 3.1. Experimental Setup. The probe o f the magnetron is directly coupled

into the microwave cavity o f a 2.45 GHz microwave driven arrangement to drive a Xe

excimer lamp.

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3.2.1. Lamp Cavity

The microwave cavity has dimensions 20.5 cm x 25.5 cm x 13.5 cm with two

grounded cylindrical tubes (~ 10 - 13 cm long) attached to its ends to allow room air or

N2 b o il-o ff to flow along the length o f bulb (Fig. 3.2). The grounded cylindrical tubes are

~ 3.2 cm in diameter, sufficiently smaller than the microwave cut-off wavelength to yield

a field attenuation o f ~ 26 dB per diameter o f length. These cylindrical tubes permit no

significant r f leakage during lamp operation.

The microwave cavity and the grounded cylindrical tubes are made from aluminum (Al)

because 1 ) it is inexpensive and easy to work with, 2 ) it has the highest power reflection

coefficient, and 3) it has the lowest skin depth in the region o f 100 nm to 600 nm.

The skin depth o f A l is given approximately as:

5 = (2/(0|io

where

to = 27lf

po = 4tcx10 ' 7 (Henries/meter) magnetic permeability in free space.

a = 3.82 X 10 7 (Mhos/m) at 2.45 GHz.

The lamp can be operated over a wide pressure range (at least 10 to 1500 torr)

with continuous wave (CW) mode input power o f more than 6 kW. The discharge bulb is

an open-ended, cylindrical “Suprasil” quartz tube, 8 mm O.D by 50 cm long with a

radiating surface o f-125 cm 2 and volume o f 14.1 cm3. The bulb is held in position in the

grounded cylindrical cavities by 3 equally spaced ceramic screws.

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3.2.2. Lamp Cooling

Proper operation o f the lamp requires cooling o f the magnetrons, lamp cavity, and

bulb. A twin rotary fan is used to cool the magnetrons and a Fusion Systems 1250/F10T

blower (used in the “pull-only” mode) is used to cool the lamp cavity and bulb. The

minimum air-flow capacity o f the Fusion Systems blower is 290 cfin (8.2 m 3/min.). In

the “pull-only” mode, a single, large capacity blower pulls room air or N 2 b o il-o ff

through the grounded cylindrical tubes, along the length o f the bulb, through the lamp

cavity, out a duct leading to a roof vent (Fig. 3.2 and 3.3). The blower also exhausts hot

air and ozone produced during bulb operation.

3 2 3 . Safety Issues

We addressed three major safety issues: UV radiation, microwave leakage, and

ozone. To reduce UV exposure 1) the bulb is contained in the lamp cavity 2) the “ lamp

drive” is enclosed in a Faraday shield, and 3) a plexi-glass screen separates the lamp from

personnel. We used a RAHAM microwave meter (Model 2, General Microwave

Corporation) to monitor the leakage levels on the microwave cavity. The meter measures

power density levels from 2 mW/cm 2 to 200 mW/cm 2 in accordance w ith the standards

established by Occupational Safety and Health Standards and the Department o f Defense.

For technical specification o f this meter, see appendix 1. For modulated fields a power

density o f 10 mW/cm 2 as averaged over any six-minute period may not be exceeded.

Ozone (O 3) is a form of oxygen, which is formed by UV light. It has a

characteristic pungent odor and at > 0.1 PPM concentrations, it can be dangerous. We

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used an ozone detector meter (ENMET Corporation, PPM 0.03) to track ozone level

during bulb operation. A built-in audible alarm is triggered when the meter reads > 0.01

PPM. According to the American Conference o f Government Hygienists standards, the

allowable dosage for ozone is 0.1 PPM o f continuous exposure.

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D irection o f air flow

Duct work

Fig. 3.2. Cross-sectional view o f the 1 kW microwave excimer lamp cavity. The

horizontal arrows show the direction o f airflow. The air is drawn through the ends o f the

grounded cylindrical cavities and along the length o f the bulb. The gray circle is the

position for the duct, and the bulb is represented by the dotted line. The 8 mm diameter

bulb offer the benefit o f increase surface-to-volume ratio.

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Lamp Cavity Duct Work ower

Fig. 3.3. Schematics o f the Lamp Cavity, Duct Work, and Blower. The Blower

pulls air through the ends o f the grounded cylindrical tubes, along the length o f bulb, and

out the duct.

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33. LAMP CHARACTERIZATION

To characterize the lamp’s output, we constructed a spectrometer based on a 0.3

m McPherson Model 218 scanning monochromator. It is equipped with a 1200 lines/mm

plane grating blazed at 200 nm providing spectral resolution o f 0.06 nm. A turbo pump,

backed by a mechanical pump, evacuates the system to pressures below 10~5 to rr fo r

vacuum or controlled atmosphere operation. The detector is a Hamamatsu R928 PMT

tube with a sodium salicylate scintillator. Software is written in Lab-View for data

acquisition. For detailed spectrometer system description and operation, see appendix 2.

Light from the lamp reaches the spectrometer through a 4 mm i.d. nitrogen purged

tube to overcome the absorption o f oxygen below 200 nm. We have demonstrated,

through extensive experimentation, that purging such a small diameter tube with ultra

pure, high-speed nitrogen is more effective in eliminating absorption by oxygen and

water than by pumping. The highly restrictive conductance, high photo-irradiance, and

large heat load at the end closest to the bulb lead to out-gassing of oxygen containing

species which are much more effectively removed from the optical path by the pure

(-99.9% ) N2- flow than attempting to remove them by pumping at the farthest end. Also,

the method avoids the need to work out difficult seals for windows at the hot end o f the

light path, which contacts the bulb perpendicular to the bulb’s long axis. The other end o f

the tube touches the MgF 2 window at the entrance slit o f the spectrometer or power meter

(Fig. 4.3). The tube is 10 cm long and creates an effective 2.3° angular aperture for the

detector.

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3.3.1. Intensity Calibration

The spectrometer was intensity calibrated from 200 to 320 nm with an Oriel

Instruments deuterium lamp, model 63345, o f calibrated spectral irradiance (the radiant

flux per unit area onto a specified surface) F(Xj). This data, supplied by the calibration

lamp manufacturer was used to determine the detection efficiency o f the system as a

function o f wavelength, C (Xj), using

L(A.j) = C(Xj).F(Xi), (3.2)

where L (X j) is the observed response as a function o f wavelength from 200 - 320 nm. To

transfer the calibration to the 160 - 2 0 0 nm range, where manufacturer’s calibration data

is not available, we compared the measured spectral radiance (sum o f all angular

distribution of radiation over a specified solid angle) to the spectral irradiance data

supplied by the factory.

Radiant flux = 4>e (W) (3.3)

Radiance (Lc) = d 2(j>e / (dQ dA cos) (W /sr-m2) (3.4)

Irradiance (Ee) = dc / dA (W/m2) (3.5)

d A - the area o f a surface element (m2)

dfJ = the unit solid angle (Sr)

<|> = the angle between the ray and the normal to dA

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e = used here as any radiometric quantity

Since the quantities, radiance and irradiance have different units, the data sets are

1) arbitrarily normalized at 250-nm and 2) plotted on the same wavelength and intensity

scale. The root-mean-square deviation o f the two data sets is about 1%. Fig. 3.4 shows

the normalized spectral radiance compared to the spectral irradiance o f the D 2 lamp. Both

quantities have a sim ilar dependence on wavelength throughout the 200 - 320-nm region.

Since the wavelength distribution o f the spectral radiance is nearly the same as that o f the

spectral irradiance, we extrapolated the deuterium lamp spectral radiance curve down to

160 nm. This extrapolation technique is justified136*140. Ott has shown that this

procedure is in agreement with direct calibration using synchrotron, plasma blackbody,

and wall-stabilized arc, primary and secondary standards.

As a secondary check, our calibration results were applied to data obtained from

an Oriel Hg-Ar wavelength calibration lamp yielding the expected line ratios (1:16) for

the 18S nm and 253 nm Hg I.

3.3.2. Wavelength-Calibration

Wavelength calibration o f the spectrometer was performed using Hg line radiation

sources. Figure 3.4a shows Oriel’s Hg-Ar typical calibration lamp spectrum superposed

on a Fusion Systems F-600 H-bulb spectrum in our system. The fill gas in the latter bulb

is mainly Hg, but the bulb-jacket material suppresses the Hg I 185-nm line. There is a

strong agreement between the X positions o f the two spectra. Figure 3.4b is a typical

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Oriel Hg-Ar calibration lamp spectrum with X-values that are within ±0.01 nm o f the

factory-supplied values. Also, there is a strong agreement between our X-values and that

o f NIST values. The Hg 1185 nm line indicates that our system is sensitive to below 200

nm and is not affected by oxygen absorption. Table 3.1. shows our experimental

wavelength values compared to that o f NIST and Oriel Instruments.

Selected sections o f Figure 3.4b are plotted as a m ulti-part figure to determine

the \ accuracy (Figs. 3.4c-f). Fig. 3.4c shows the neutral (Hg I) 185.01 nm line, labeled

as 184.95 nm in the NIST database and 185 nm in the M IT X. table. The full-w idth at half

maximum (FWHM) o f our 185.01 nm line is ~1.5 Angstroms. Fig. 3.4d shows the singly

ionized (Hg II) 296.70, neutral (Hg I) 302.10, and neutral 313.12 lines. These lines are

nominally labeled as 296.70 nm, 302.10, and 313.15 nm respectively in the NIST

database. The ^.-values compared to that o f NIST in this Arrange is within ± 0.02 nm.

Figs. 3.4e, shows the neutral (Hg I) 365.02, 369.78, 404.65, and 407.78 nm lines, and

Fig. 3.4f shows the Hg I doublet nominal at 576.96 and 579.066. The doublet lines are

resolved out as two separate lines. We note that the wavelength accuracy o f our system

is such that all o f the approximately 3000 data points in each o f the spectra shown are

within an absolute error (±3 SD) o f 0.06-nm. We conclude that our spectrometer is not

only sensitive to radiation below 2 0 0 nm, it has a resolution close to the theoretical lim it

of the spectrometer and that our wavelength values are in good agreement with

wavelengths from published internet database sites212*217.

Fig. 3.4g shows our spectrum o f the D 2 calibration lamp down to 130-nm. This

spectrum is consistent with the Oriel published D 2 spectrum. As indicated above, the

effectiveness o f our N 2 elution technique is confirmed by the complete absence o f oxygen

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absorption (Schumann-Runge band, Shumann-Runge continuum, and Herzberg

continuum) which would be evident in the range 130-160 nm, 160-200 nm, and 200-240

nm respectively (See appendix 3) if any O 2 were present in the optical path. The D 2

spectrum we observed is within 1% o f the Oriel published spectrum. The structure at

around ~ 322 nm (Fig. 3.4g) is 2nd-order o f the 161 nm D 2 emission. The ratio o f the

latter to the former is ~ 16. Second order o f other lines is also evident. This would permit

us to make a quantitative correction for second order transmission, though as discussed

later, no such correction is necessary. The neutral (Hg I) 185 nm line and the onset o f the

first continuum o f the D 2 again indicates that the spectrometer is sensitive down to 160

nm.

3.3.3. Discharge Tube Cleaning

The discharge tube was cleaned with reagent isopropyl alcohol followed by 18

M ft-cm de-ionized water, then evacuated to ~ 5 x 10 ' 3 torr at ~450 °C under vacuum

before introducing research grade Xe gas. A general purity check o f the gas handling

system was performed by monitoring the vacuum UV emission spectra at ~200 torr,

observing no atomic emissions from impurity gases. The detailed experiments were

restricted to 160-320 nm. Data was obtained over the pressure range o f 100 - 1S00 torr;

a typical pressure range for excimer formation. In operation, the temperature o f the lamp

jacket is controlled by flowing air or cold nitrogen gas (boil-off from liquid nitrogen)

along the length o f the discharge vessel preventing the discharge bulb from failing. The

temperature o f the cold nitrogen gas when it enters the lamp housing is ~ 1 0 0 K .

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We used an International Light IL 1400A radiometer with a SEL 185 detector

head to measure optical power o f our spectral results. The radiometer is responsive from

170-225 nm (Fig. 3.5b) and is NIST traceable and therefore is the preferred device for

absolute power measurement. The spectral emission and power measurements were done

along the axis o f the bulb and also normal to the bulb axis.

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Factory X/nm O r ig in N IS T X7nm Observed X/nm

(O rie l) (This work)

253.65 Hg I 253.63

265.51 Hg I 265.51

270.31 Hg II 270.54

275.78 Hg I 275.97

296.73 Hg I 296.73 296.70

301.56 Arlll 301.0 301.2

313.18 Hg I 313.15 313.02

333.59 A r lll 333.61 333.60

365.02 Hg I 365.01 365.02

404.66 Hg I 404.65 404.63

435.84 Hg I 435.83 435.83

Table 3.1. Oriel Hg-Ar calibration lamp, NIST, and “our” laboratory X values.

There is a strong agreement between our k values and that o f NIST to within ± 0.06 nm,

the theory best o f the spectrometer.

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Hg-Ar F-600 H-Bulb

Hg-Ar Calbration lamp and Fusion F-600 H B ub Spectra

5 IA Date: 8/22/97 . m Tlmt: 7:30 pm .

4

9 5 , 3 i V* C

I 2

1

0

200 300 400 500 600 700 800

W avelength (nm)

Figure 3.4a. Comparison of the Hg-Ar calibration lamp to another Hg bulb (Fusion

Systems F-600 H bulb). There is strong agreement between the two data sets.

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Intensity (a.u) a strong agreement between our wavelength values and that o f N IS T to w ith in ± 0.05 nm. nm. 0.05 ± in ith w to T IS N f o that and values wavelength our between agreement strong a The 185 nm line indicates that our system is sensitive below 200 nm. 200 below sensitive is system our that indicates line nm 185 The 0 2 4 3 Lamp Excimer Xe f Microwave o Modeling and Characterization 3: Chapter 1 5 Figure 3.4b. T ypical H g-A r calibration lamp spectrum in “ our” system. There is is There system. our” “ in spectrum lamp calibration r g-A H ypical T 3.4b. Figure 0 30 0 50 0 70 800 700 600 500 400 300 200 Mercury-Argon CalibrationLamp Spectrum Vtavelength (nm)

ie 1210am Time: Date: 7/1/97 Date:

57 Chapter 3: Characterization and Modeling o fMicrowave Xe Excimer Lamp 58

■rayAloiCoMoilapSpoctai IbRHiMvi CMoi lapSpcta 1 ! 1 | I I T J r r- i | i i-i | 1 T . 5

— OafcJMT F ra : 1210 m

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—omm — OatK 771/97 tin e I210M T ig * 1210 a

3 4 3 « • M M e e a S 2 £2

5 Vj l iA.

a a a a a «o 410 sn a a oio a a m tentactfMt

Fig. 3.4c Fig. 3 .4 f

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced

effectiveness o f our N2 elution technique is confirm ed by the complete absence o f f o absence complete the by ed confirm bands. is Schumann-Runge from technique absorption elution oxygen N2 our f o effectiveness R*lativ* Intensity (a.u) Chapter 3: Characterization and Modeling o f Microwave Xe Excimer Lamp Excimer Xe f Microwave o Modeling and Characterization 3: Chapter 015 025 005 035 0.1 0 2 0 Fig. 3.4g. D2 lamp spectrum over the wavelength range o f 130 -3S0 nm. The The nm. -3S0 130 f o range wavelength the over spectrum lamp D2 3.4g. Fig. 5 250 150 Curve fit D2(091297/1145) A/A *-15.87, A*heightofptak 1,A*htfgMofpMk2. A*heightofptak *-15.87, A/A 200 tr nto herto A o . A to A, f o ratio e f th o ination eterm D W avelength (nm) avelength W 0 350 300

59 Chapter 3: Characterization and Modeling o f Microwave Xe Excimer Lamp 60

3.4. RESULTS AND DISCUSSION

3.4.1. Xenon

The generation o f excimer light has been modeled by Kogelschatz et al.3, Prelas et

al.141, and Ivanov et al.142. These models are specific to the DBD and pulse discharges

and predict efficiencies in the range o f 30-40%. What follows is more general and not

limited to any particular or specific discharge type.

The Xe second continuum (Fig.3.5), is a transition from the ( 3Z+U) excited state to

the ('ITg) state. This spectrum is similar to those observed in other types o f discharges.

The shape is mainly dependent on the population o f the radiating 3Z+U and ‘S+u states.

This population is governed by 1) the dependence o f the molecular excitation and de­

excitation processes on electron temperature and gas pressure, 2 ) the dependence o f the

vibrational relaxation rate on gas density, and 3) the probabilities o f the radiative

decay143. The molecular formation scheme 144 is summarized below:

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Jfe( 3P,)— Xe(% ) + h v(147 nm) ...... (1)

AS*7»,) + Xe(lSQ )-**-► XeCP2) + Xe{\)...... (2 )

XeCPJ + XeCS0)-^ X e /(‘O ...... (3 a )

XeCP2) + Xe('S0) - ± U X e 2''(% +)...... (3b)

Xe2vCZu*) + X e ^ - + X e 2CZu*) + Xe...... (4 a)

Xe2''(3Zu+) + X e-±!-+ X e2C K ) + Xe...... (4 b)

Xe2y(lZ /) —l-^-> X e 2(lI g+) + hy(l50nm) (5a)

Xel \ i z ; ) - ± l ^ X e 2(x'L ; ) + hv{\5Qnm) (56)

Xe2 v(' S /) ■> Xe2 ( ‘ 2 / ) + hv(l 74 nm ± 8 ) . . . ( 6 a )

Xe2* ( 3 Z .'-) l/r* >Ae 2 ( 112 / ) + h v(l74 nm ± 8 )...( 6 6 )

where the superscript v denotes a vibrationally excited diatomic molecule, ti is the

atomic lifetim e (pressure dependent due to radiation trapping), and Tsav, t$bv, t«, are

the molecular decay times for Xe 2v(lZu+), Xe 2v( 32 u+), Xe 2( 1Z„+)» and Xe 2( 3Zu+) states

respectively. The decay times i 5«v ~ x^and t 5bv ~ t 6 b for the heavy rare gases (for Kr and

Xe). For Ar and lighter rare gases, it is expected that Tfe < * 5av and t6b < tjb v. Thus

processes 5a and 5b w ill be more effective in argon than in krypton and xenon9,17.

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The intensity scales o f Figs. 3.5-3.7 are calibrated power measurements at the

detector using the calibrated deuterium lamp. As shown in figure 3.5, the second

continuum peaks at 176 nm with a spectral half-width o f 10-15 nm, consistent with

previous studies. Up to 80% o f the emission is confined to a wavelength region o f 160-

200 nm with 600 W input power at 1500 torr and flowing liquid nitrogen boil-off over the

bulb. The discharge length was independent o f pressure. However, the discharge column

constricts to form a narrow filamentary structure at pressures > 1500 torr and input

powers o f > 1000 Watts. A ll data presented here was taken from outside the filament-

forming pressure and power ranges.

Excimer molecules of xenon, form in pure Xe in a three body collisions with a

rate process that is approximately proportional to the square o f the gas pressure270:

Xe* + 2Xe->Xe2’ + Xe (3.6)

Xe22 —l/A-tZXe + Oiy (3.10)

where Xe* is the excited atom and Xe the ground-state atom. The steady state rate

equation for photon production is:

l*> r (3-ii) dt r r

[Xe] (cm'3) = the density o f ground state atoms

rr - the average radiative lifetime of Xe2*

k (c m 6 /s) = the rate constant o f three-body formation.

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Fig. 3.6a gives the measured intensity, I, o f the peak o f the second continuum o f

Xe 2* as a function o f pressure, P. A power fit to the data resulted in I qc P18. This is

within experimental error for an expected form o f P 2 dependence.

The density o f the ground-state atoms, [Xe], is directly proportional to pressure,

leading to a contribution proportional to the square o f the pressure. However the density

o f excited atoms, [Xe*], depends not only on pressure, but also on other plasma

parameters such as electron temperature and density, which also change with pressure.

The peak intensity and spectral half-width values o f the radiation depend on the

fill gas pressure (Figure 3.6b). A t high pressures, the width o f the 3XU+ to ‘Zg+ emission is

considerably broader. The position o f the peak maximum does not shift detectably with

pressure. A t low pressures, radiative decay (Eq. 1) competes with vibrational relaxation

(Eqs. 4a and 4b). At higher pressures, the relaxation rates increase significantly, through

a collision-assisted vibrational relaxation process, leading to increased populations in the

lower levels and to higher intensities o f the second continua. Spectral simulations o f this

band are in progress by another member o f the group (J. Diggs) to estimate the upper

state vibrational population o f emitters in the discharge. For a given lamp set-up, there is

an upper lim it pressure above which the deposited microwave power w ill be too low to

compensate for the elastic collision losses. A ll our data were collected below this lim it so

the output power and efficiency increased with pressure. Kumagai et al. reported 14' 15

average output power as a function o f pressure (SO, 100, ISO, 200 torr) in KrF and ArF

discharges from a microwave lamp. In their system, the peak efficiency (12%) o f the

KrF discharge is at SO torr, with an output power o f 120 W. At a given operating

pressure, both the output power and efficiency o f their system increased with increasing

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deposited microwave power, however, the efficiency decreased as pressure increases.

The observed difference in trend o f efficiency versus pressure in the two systems (ours

and Kumagai et al.’s) may be due to collisional quenching o f the buffer gases used in

Kumagai’s studies. Schoenbach et al.57, reported pressure dependence o f efficiency and

peak intensity in a Xe high pressure hollow cathode discharge excimer lamp. Their

results show a continuous increase in intensity with pressure, consistent w ith our results.

A maximum efficiency o f between 6% and 9% at 400 torr was observed.

3.5. Lamp Efficiency Calculation

A characteristic feature o f excimer light sources is that there is no self-absorption.

Consequently, to a first approximation, the radiation from any point in the discharge can

escape without attenuation or radiation trapping. Therefore the emission column length

determines the angular distribution of the emitted light. Detailed calculation of the

collection efficiency is elaborate. However, an in itia l estimate can be computed from the

relationship between emitter column length and collection angle. The calculation can be

set as a multiple integral over emitter volumes, or equivalently, as an integral sum o f

paths intersecting the bulb surface. In either calculation, we compute the ratio of rays

collected by our detector to all rays emitted by the lamp to determine an estimate o f the

total output power. Figs. 3.5a-3.5e, show the relationship between the light paths

intersecting the bulb surface with the azimuthal and polar collection angles.

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Fig. [3.5a]. End-view o f cylindrical bulb. Analysis o f the relationship between azimuthal

collection angle and rays intersecting bulb surface.

r/sin0j = Di / sin0 (3.12)

But 0 = 180 - 20, and r = D/2

sin 0 = sin(180 - 2 0 0

= sin 2 0 j

= 2sin0jcos0j (3.13)

From Eqs. [3.12 ] and [3.13 ]

Dj = rsin0 / sin0 = Dcos0j (3.14)

Since part o f the ray is always in the jacket (Suprasil) material Eq. [3.14] is

corrected as Dj = Dcos0| - 2t/cos0j if Dj is taken to represent the path length

through the plasma excited gas in the lamp.

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For the azimotfcil ■ogle, 0.

D

Di*Dx Cos6t

Fig. [3.5b]. End-view o f cylindrical bulb - A careful analysis o f the light rays that

intersect the bulb surface shows that 6 cannot be greater than ±48.7°.

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3 n i

0 - Sin'1 (%) - 48.6*

(-71/3.7) £ 0 £ (n/3.7)

I ~ exp{-at/ cos 6}

Fig. [3.5c] and [3.5d]. Analysis o f the length o f the light path in the attenuating

medium o f the bulb (Suprasil) as a function o f the azimuthal collection angle, ignoring

refraction.

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Side-view o f cylindrical bulb

Firlltpikriiiht

(Cross-sectional View)

i D I ~ exp{-at/cos

D rlV C *

Fig. [3.5d ] Fig. [3.5e]

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In the range o f pressure and input power studied, the electron density in the

plasma is low enough that the index o f refraction at visible and ultraviolet wavelengths is

nearly unity. Accordingly we do not consider the effect o f refraction. For a plasma index

o f refraction greater than unity, emission intensity w ill be displaced toward angles away

from the surface normal, potentially leading to radiation trapping by total internal

reflection.

The discharge appears to uniformly fill our bulb except as otherwise noted, so that

P in Eq. [3.9] below is a constant. Consider a reference frame at a point on the outer

surface, then the intensity into an angular element, dOdij>, is:

I = [P/cos()] [Dj/cosGj - 2t/cos0i]exp{ -at/cos(0)cos()} (3.9)

where

D = bulb diameter

P(r,0,(j>) = production o f light at a location within the bulb

t = bulb wall thickness

a (A.) = absorbance o f the bulb wall material at wavelength A.

<(> = polar angle

0 = azimuthal angle

The intensities along the length of the bulb, measured by the radiometer,

perpendicular to the bulb, at 15, 25, and 35 cm from one end are within ±2% o f each

other. In the calculation, t = 0.1 cm and a = 0.6 cm*1 near 172 nm32.

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The light collected by the radiometer at any location may be determined by

integrating the above equation over the detector’s angular aperture (2.3°). The total light

emitted by the bulb is calculated by integrating Eq. [3.12], using the integration lim its -n

/2.005 < <|»< 7r/2.005 (to account for the finite length o f the bulb) and -ti/3.7 < 0

(to account for only light rays that intersect bulb surface). The ratio of these two

integrals, which represents the ratio o f the light collected by the radiometer to the total

emitted light at a given location on the bulb surface, is 0.0036.

In an earlier published calculation, we approximated the bulb surface as a plane

sheet (Figs. 3.5f and 3.5g), assuming that emission at a ll locations on the surface o f the

bulb were the same (P = constant). In this case, the total light emitted by the bulb was

calculated by integrating Eq. [3.12] using the integration lim its -rc/2.1 < < n/2.1 (to

avoid the singularity at 7i/2) and -tc/2.005 < 0 < rt/2.005 to account for the finite length o f

the bulb.

Figure 3.8, shows the normalized model calculations and experimental

measurements o f intensity versus the radial angle in the plane containing the bulb axis.

The maximum optical power output is obtained when the radiometer is looking down the

axis of the bulb. There is a good agreement between the model calculations and

experimental measurements, justifying the neglect o f refraction effects earlier.

The longitudinal profile o f electron density, n«, is expected to correlate with the

brightness o f the light emission. We have no measures o f n« and Te, however, so it is

possible that the radial profiles o f these parameters are non-uniform. If the electron

densities are 10IS cm'3 or higher, the skin-depth (appendix 4) at these frequencies would

be less than one-fifth the radius, so that the discharge could be hollow. Visual

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observation indicates that the center (looking down the long axis o f the bulb) is slightly

brighter than the edge. This may not be reliable since most o f the light is in the

ultraviolet. We note that a non-uniform radial profile would not change the results o f our

model for the angular distribution, except to change the value of our (arbitrary)

normalization at 0 = 0°.

Our lamp geometry allows either air or cold nitrogen gas (boil-off from liquid

nitrogen) to flow along the length o f the discharge tube. The effect o f additional cooling

is quite important as seen in the comparison o f the curves o f figure 3.7. With the use o f

cold gas cooling, the FWHM of the continuum decreased slightly, which we take as

evidence o f lower vibrational excitation o f the 3I „ + and ‘l u+ states. The peak intensity

increases with no shift in the peak position.

We define efficiency as the optical output power divided by the electrical input

power. In defining our efficiency we make the assumption that once a plasma is ignited

in our bulb, the dominant sink for the forward microwave power is dissipation in the

plasma. Our magnetrons are directly coupled to the cavity, so there is no opportunity to

incorporate circulators or power meters to measure reflected power. We assume that the

reflected power is a small fraction o f the forward power and that this fraction does not

depend significantly on the pressure or the power level. This would certainly be untrue in

a tuned high-Q cavity operating on a specific mode. However, for our over-moded

cavity, the assumption is supported by observations o f the magnetron tube temperature

over our range o f pressure and power. The detailed mechanisms for wave dissipation in

the plasma, which include ohmic heating, stochastic heating, and other losses, are beyond

the present scope. I f efficiency were to be defined as light output divided by absorbed

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power then efficiencies measured under our “wall-plug” definition, at our highest values

o f power and pressure, would serve as lower bounds. Over the pressure range o f 670 torr

to 1500 torr, the measured efficiency o f this lamp varies monotonically from 20% - 40%,

consistent with estimates from theory. A t 600 W input power, the maximum efficiency

o f 40% is obtained at 1500 torr. We estimate the error in these measurements, dominated

by uncertainties in the radiometric integration to be o f the order o f 15%. For comparison,

the best experimental UV efficiencies for xenon reported3 in the DBD are close to 10%.

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4

3.5

3

E

2 £ (0 e S 1.5

1

0.5

0

160 180 200 220 240 260 280 300 320 Wavelength (nm)

Fig. 3.5a. Xe excimer lamp output power referred to the bulb surface; 600 W

input power, cooled by liquid nitrogen boil-off. Detector looking down the long axis o f

bulb.

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Spectral Responsivity 100

SED185 DEEP UV DETECTOR 9 0

80

7 0

6 0

SO

4 0

3 0

20

10

1 5 0 175 200 2 2 5 2 5 C wavelength (nm)

Fig. 3.5b. Spectral Response o f the power meter. The sensitivity curve o f the

power meter overlaps ~ 54 ± 5% o f our Xe 2* output curve.

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5

4

| 3 £ £ <0 c 8 2

1

0 160 180 200 220 240 260 280 300 320

Wavelength (nm)

Fig.[3.6a ]. Effect o f pressure on Xe excimer lamp output w ith 600 W input

power. Cooled by liquid nitrogen boil off, ( ___) 1500 Torr, ( ------) 1200 Torr, ( ..... )

875 Torr. At 600 W input power, the maximum efficiency o f 40% is obtained at 1500

Torr.

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5

4

3 to0 3 £ (A e 3 e 2

1

0 0 500 1000 1500

Pressure (Torr)

Fig. 3.6b. Intensities o f second continua o f Xe 2* as a function o f pressure. It

shows an approximately power law dependence on pressure w ith exponent o f 1.8 (see

text).

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5

4

3 E I £ (0 e « 2 c

1

0

160 180 200 220 240 260 280 300 320

Wavelength (nm)

Fig. 3.7. Effect o f cooling on Xe excimer lamp output power with 600 W input

power. (....) liquid nitrogen boil-off, (—) ambient air.

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‘ ■ I i • i i |------1------r— i ------1------1------1------1------1------1 - 1.2

° ' 8 E i £ 0.6 m e i 0.4

0.2

-100 -60 0 50 100

* (-90°to 85°)With 6 set to 0°

Fig. 3.8. Normalized calculated (—) and measured (♦ ) angular dependence o f

emission intensity in the plane containing the bulb axis (±90°) and the tube to the detector

(0°). Hg-Ar Calibration lamp Wavelength list compared to NIST wavelength Values

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Chapter 4: Capacitively Coupled RF Driven Xe/Xe-Ar Excimer Lamp

In this chapter, we 1) describe a novel 13.56 MHz rf Xe-Ar excimer lamp 2)

present results from experimental studies o f Xe and Xe-Ar excimers in the region 180 nm

to 320 nm from 10 torr to 1500 torr and 3) compare the results to microwave and DBD

excim er lamps.

4.1. INTRODUCTION

In recent years many research groups have investigated different electrical

discharge techniques to drive excimer emission (Table 1.3). So far as we are aware, the

use of capacitively coupled rf discharge to produce excimer emission has not been

reported though Alexandrovich et al.214 described capacitively coupled r f discharges for

electrode-less subminiature fluorescent lamps with Hg-Ar gas mixtures. Also, no

previous rf excimer lamp exists in which the tuner is both a part o f the “cabling” to the

power supply as well as an integral part of the lamp cavity. In previous rf designs,

configured to avoid electrodes internal to the plasma, the dominant capacitively or

inductively coupled designs employ external tunable inductors and/or capacitors to create

an impedance match between a tuned cavity and the power supply. The present design

eliminates this precise tuning requirement, resulting in a simple and compact system.

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A lamp that emits strongly in the 180 nm - 200 nm region is especially desirable

because o f the responsiveness of organic materials in this wavelength range. The

usefulness o f Ar/F mixtures in lamps to produce 193 nm is lim ited by fluorine attack on

the quartz from which such bulbs are usually made. Lamps based on the 190 nm K r/I

excimer, face the problem o f iodine condensation.245 Thus, we seek a candidate among

the inert gases and their mixtures. Though several groups have studied energy transfer

processes underlying UV emission in Ar/Xe mixtures39, 40 and measured their rate

constants, no excimer-lamp based on Ar/Xe mixtures has been reported.

4.2. EXPERIMENTAL FACILITIES

4.2.1 Capacitively Coupled Lamp

Figure 4.1a shows the lamp schematics. The lamp consists o f an rf generator,

impedance matching network, coupling means, and lamp bulb in a grounded cylindrical

cavity. Figure 4.1b shows the cross-sectional view o f the lamp set-up. The bulb, tuner,

and grounded cavity are concentric. Fig. 4.2 shows an equivalent electrical circuit

diagram. The r f generator (ENI Power System, Inc., model ACG-10) delivers up to 1 kW

o f 13.56 MHz into a 50Q load. The impedance matching network transforms the lamp

and cavity electrical load impedance (Z = R+jXc) into a constant 50 ohm of pure

resistance. It is a “pi” network, composed of two variable capacitors and a fixed

inductor, L. The coupling means (tuner) is a cylindrical conductor o f length 7.5 -12.5 cm

and diameter 5 - 7.5 cm, (see Figure 4.1a) placed concentric to the discharge cavity

providing an adjustable capacitance between the center conductor and ground. It is

outside the bulb, eliminating the problems associated with having an electrode in contact

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with the plasma and it does not obstruct the lamp’s light output. The bulb is a valved,

open-ended, cylindrical “Suprasil” quartz tube, 8 mm O.D (6 mm I.D.) by 50 cm long

with a radiating surface area o f-125 cm2 and a gas volume o f 14.1 cm3. The lamp set-up

is housed in a Faraday cage to reduce r f leakage.

When power is applied, the matching network tuner servo attempts to transform

the capacitive reactance into a resistive impedance. The tuner, however, cannot

transform a pure reactance into a non-zero (50 Q) resistance, therefore, it w ill search until

the bulb plasma ignites. Bulb ignition yields a needed dissipative resistance permitting

the tuner to rapidly lock-in. Additionally, plasma ignition is made easier by moving the

tuner within the cavity, to reduce reflected power and increasing the coupling

capacitance.

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Discharge bulb Sliding tuner Cylindrical cavity

♦To Pump

Match Box

Fig. 4.1a. RF lamp Set-up. Tuner is placed outside the bulb; it is a part o f the

cabling to the power supply as well as part o f the matching network.

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Sliding Tuner

Cylindrical cavity

RF Generator

Impedance matching

Fig. 4.1b. Cross-sectional view o f lamp Set-up. The bulb, tuner, and grounded

cavity are concentric.

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Inductor

o 1 u T n

> t i t CCP Applicator Coupler Load (Plasma)

Fig. 4.2. C ircuit equivalent o f lamp Set-up. Sections A, B, and C correspond to

the impedance matching network, sliding coupler and discharge (plasma) respectively.

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4.2.2. Lamp Characterization

In this experiment, data was taken at 20-25 micron slits so as to admit the

maximum signal. We re-calibrated the spectrometer at these new slit sizes using the same

procedure described in chapter 3. The theoretical lim it o f the spectrometer resolution is

0.6 Angstrom at 10 microns slit for the 1200/mm grating of our spectrometer. The

corresponding spectrometer resolution at 20-25 micron slits is 1.5 Angstroms. Light

from the lamp reaches the spectrometer through the N 2 purged tube arrangement (See

Fig. 4.3).

Because o f the large cross-section for electronic energy transfer in Xe-Ar mixture,

care is taken to ensure the purity o f the gases used. The cleaning o f the discharge tube is

the same as described in chapter 3. To ensure mixing o f the gases, small quantities Xe

are admitted first and allowed to diffuse throughout the discharge bulb. The bulb is then

back-filled with argon to the desired pressure. Reproducibility o f the data was checked

after several days w ith no detectable changes, confirming complete mixing.

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Nitrogen purged path MgF2 Window Discharge bulb

Spectrometer Cylindrical cavity

a Nitrogen I"

To Pump

Fig. 4.3. Experimental set-up o f 13.56 MHz rf capacitively coupled excimer lamp. Light

reaches MgF 2 window o f spectrometer through a nitrogen fille d tube.

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4.23. DOE Experiment

A design o f experiment (DOE) program is used for the design and analysis of

experiments. For experiments that require the treatment o f several factors at different

levels, DOE can be used to efficiently generate a reduced optimal set o f experimental

runs, spanning a given parameter space, and yielding the same results as though the

completed set o f experimental runs were carried out.

In our experiment, to efficiently examine the fu ll parametric variation o f Xe/Ar

mixtures, we used the DOE program to generate a matrix o f 25 experimental runs (table

4.1) spanning five choices o f values (levels) for three variables (factors): power, pressure,

and gas composition. The choice of parameter ranges was based on preliminary

experimental runs. These 25 experimental runs sought the best combination o f pressure,

gas composition, and power input giving the best optical power output in the 180 - 200

nm region. Discharge characteristics were found to be very similar over the pressure

ranges o f 0.2-10 torr and over the range 25-100 torr, hence the choice o f 10 and 100 torr

to represent very low and low-pressure operation. The upper lim it, 1500 torr, is set by

the mechanical strength of the thin-wall (1 mm) of the discharge bulb used. Using a

remote I.R temperature sensor, we estimate that the bulb surface reaches -1000° C at

input powers o f ~900 W. This upper lim it on the input power is set to prevent the bulb

from over-heating and rupturing. With better means o f cooling the bulb, the lamp could

be operated at several kW. The experiments were conducted in random order. Because

o f our cooling limitations in the present design, a 5-minute pre-conditioning period was

found to be necessary for the discharge to completely stabilize for each set o f parameters.

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Power and/or spectral data were then collected from 11-15 minutes. Each set of

parameters was repeated two or three times to check reproducibility. Occasionally, data

sets were further repeated after several days to ascertain reproducibility.

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Tabel 4.1. DOE Experimental Design for three major variables: Power, Pressure,

and Xe/Ar composition.

Pressure (Torr) Power (Watts) Composition (Xe % )

100 200 1.0

100 500 0.01

1500 200 5.0

100 300 5.0

1000 500 5.0

1000 300 1.0

1000 200 0.1

10 650 5.0

10 350 10

500 350 10

1000 650 10

1000 100 0.01

500 100 5.0

1500 650 0.1

100 500 10

1500 350 1.0

100 350 0.1

10 200 0.1

10 300 0.1

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10 500 1.0

500 650 1.0

500 300 0.01

1500 500 0.01

500 500 0.1

1500 300 10

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43. RESULTS AND DISCUSSION

43.2. Xenon

As noted earlier, the position o f the peak o f the Xe second continuum depends on

(1) the discharge excitation technique and (2) how the upper vibrational levels are

populated. The shape o f the continuum (*’ 3SU\ l u) -> ('2 g+, Og*) transition can be

estimated by calculating the quantum-mechanical dipole-operator integral in the Franck-

Condon approximation. The shape o f the continuum is approximately given by

\z E v A vs (a>e) = © vs I / Vv(R) i|/e(R) dR | 2 (4.1)

where o)ve = transition photon frequency between the discrete level v and the lower

electronic state.

iyv( R) = vibrational function o f the discrete level v o f the upper state

v|/e( R) = vibrational function o f the continuous spectrum o f the lower electronic

state.

Detailed calculations by another member o f the group (J. Diggs) are in progress.

Figure 4.5 shows a spectrum o f pure Xe at 1000 torr, 200 W input r f power. The

spectrum contains only the second continuum assigned as “ 172 nm” in the excimer

literature. The peak appears here at about 186 nm, a shift o f 14 nm toward longer

wavelength. This is near the long wavelength lim it o f the v = 2 wavefunction overlap

predicted by the Franck-Condon calculation. A t the time o f this writing we are unable to

explain why this large shift occurs in this system, since on semi-classical grounds, the

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overlap should shift to shorter wavelengths near the turning point o f the potential. We

speculate that some sort o f as-yet-undetermined perturbation is occurring in this system.

In our system, this second continuum o f xenon begins to appear as the pressure is

increased to ~ 200 torr with an input power of 350 W. The relatively low pressure at

which the second continuum is observed in the 13.56 MHz drive compared to other

systems (microwave and DBD lamps), suggests that the electron energy distribution

contains a large proportion of high energy ( » 10 eV) electrons, a major fraction of

which is deposited in the Xe excited dimer. It is restricted to 170 -200 nm and its

intensity increases with increasing pressure and input power. The spectral half-width is

~10 nm. No other radiation is detected between 160 and 400 nm for operating pressures

> 200 torr (within the input power range o f200 - 800 W).

4 3 3 . Xenon - Argon Mixture

Figure 4.6a shows the emission o f Xe/Ar mixture in our 13.56 MHz lamp at ~500

torr with 1% Xe concentration. The emission is dominated by a continuum between the

region o f 180 nm and 200 nm containing a sharp line at 192.92 nm (which we denote ~

193-nm). We believe the -193-nm emission is due to an energy transfer mechanism

between argon and xenon. It is necessary to rule out emission from potential impurities,

however, from the NIST data table, one possible impurity candidate emitting at this

wavelength is Carbon I (E, - E* = 10192.63 - 61981.82). However, in our spectrum,

there are no other Carbon I emissions, in particular very strong emissions known at

175.18-nm and 176.53-nm are missing. Mo, Si, Fe, N i, O, Cr, and Cu are the only other

possible emitter candidates, having weak line emission at 193, that could be in our

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system. Again no other known (strong) lines o f these species appear. The emissions at

216.6-nm, 247.3nm, and 307.7-nm we believe are due to Ar III (Ar2*). Over the

wavelength range studied, we do not observe any Ar I and Ar II atomic lines, possibly

with the exception o f the 193 nm, which was identified as a weak emission from A r II by

Johnson et al. It seems highly unlikely that the 193 can be attributed to Ar II (Ar*) here

Fig. 4.6b. shows the emission o f Xe/Ar mixtures at 1000-torr w ith 5% Xe. In both Fig. 6a

and 6b, there is a strong underlying continuum, which we attribute to the onset o f a third

continuum described by Boichenko et al40. It has been suggested that, the third

continuum can be formed by the transitions (see appendix 5)

e + Ar2* (A2I U*) -+ Ar2' (UZU*) + hv (4.2)

that represents photo-recombination processes40.

Figure 4.6c, shows the emission o f Xe/Ar mixture (0.1% Xe concentration) at ~10

torr, with 300 Watts input r f powers. We observed molecular bands at 230 and 270-nm;

Druyvesteyn33, Jongerius et al.216, Friedl217, and Kugler218 reported sim ilar emissions.

The emission appearing at 230 nm, is attributed by M iller219 and Dutuit220 to molecular

transitions arising from transition between the inner turning point o f higher vibrational

levels o f [‘S uY P i] and/or [3SU+][3P 2] states and the repulsive portion o f the ground state

o f Ar. As the pressure is increased to 100 torr (Figure 4.6d) with 1% Xe, the molecular

band at 270 nm diminishes in intensity with a relatively weak emission between 180 and

200 nm again having the narrow line superimposed on it at ~193 nm. In the pressure

range o f 10 to ~ 100 torr, increasing input power gives rise to other atomic and molecular

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emissions but does not increase emission from 180 - 200 nm. In all cases, at pressures >

200 torr, increasing the Xe concentration does not change the position o f the ~193 nm

emission.

A well-known emitter at 193-nm is the ArF excimer. We did not have any plastic

part, viton, or other polymeric parts such as O-ring in our gas handling system. Our gas

handling system has never been used to deliver either Cl or F. Additionally, we did not

use any cleaning agent containing HF or HC1. Neither did Heraeus, the Suprasil

manufacturers use HF or HC1 as a cleaning agent. The 193-nm emission therefore is

likely due to some energy transfer mechanism between argon and xenon possibly

involving the singly ionized Ar+ ion.

The production o f the -193 nm line and emission in the 180-200 nm range using

Xe/Ar mixture in our 13.56 MHz rf excimer lamp is unique. A combination o f high

operating pressure and rf input powers o f > 300 W favors efficient emission in 180-200

nm region. A t pressures greater than 500 torr, increasing input power enhances the 180-

200 nm emission. The greatest emission in this band is obtained at -1000 torr with input

powers o f greater than 350 W (Fig. 4.6b).

4.4. Optical Power Output

The total optical power output, for the Xe-Ar mixture experiments, varied from 50

W to - 200 W (integrating the area under the spectral distribution curve) at input powers

of between 200W to 1000W. We compared our results to the IL radiometer for wider

slits and the deviation was less than 2%. More than 80% o f the emission appears at k <

200 nm and the best experimental efficiency (total light output in the range o f 180-200

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4: Capacitively coupled RF excimer lamp 95

nm divided by the electrical input power) is 20%. We measured the output power using

the radiometer permitting a more accurate correction, using our previous model214, for the

angular distribution o f emitted intensity. The previous work215 indicates that cooling with

liquid nitrogen boil-off, rather than room temperature air, may increase light output

considerably. For comparison, the best experimental efficiency reported by Kumagai et

al.14, 15 is between 5-12% for ArF and KrF (microwaves), and 10% Xe (DBD) by

Kogelschatz et al.3 These groups considered the solid angle for the detection system and

the transmission o f the quartz envelope and estimated the true efficiency to be close to

30%.

From our DOE runs, operating the lamp at higher pressures and higher input

powers favored emission in the 160-200 nm region.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced

Intensity (a.u) 4 0 2 3 5 1 The data provided here is o f higher resolution than the published spectrum. The gas ercury. M The ainly m is spectrum. bulb the f published o the than position com resolution higher f o is here provided data The Chapter 4: Capacitively coupled RF excimer lamp coupled excimer RF Capacitively 4: Chapter Figure 4.4g. A spectrum o f an F-600 H Fusion Systems bulb in our spectrometer. spectrometer. our in bulb Systems Fusion H F-600 an f o spectrum A 4.4g. Figure 200 gi g S 5 t*> ! 300 < Spectrum of Fusion F-600 H-Bub F-600 Fusion of Spectrum n w I a csegh( ) m (n fcvslength W 400 500 Tfana: 7:30 pm Tfana: 7:30 D *» : 8/22797 : *» D 600

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced than 80% o f the emission is in the region 160-200 nm; 350 Watts input power and 1000 1000 and power input Watts 350 nm; pressure. 160-200 rr to region the in is emission the f o 80% than Chapter 4: Capacitively coupled RF excimer lamp excimer coupled Capacitively RF 4: Chapter Intensity (W/cm2) Fig. 4.5. A 13.56 M Hz r f capacitively coupled Xe excimer lamp spectrum. More More spectrum. lamp excimer Xe coupled capacitively f r Hz M 13.56 A 4.5. Fig. 0.5 2.5 1.5 6 10 0 20 4 20 8 30 320 300 280 260 240 220 200 180 160 Wavelength(nm) 97 Chapter 4: Capacitively coupled RF excimer lamp 98

051697/1225.dat ]

0.8

E <* 0.6 5

iv* C 4» 0 .4

0.2

160 180 200 220 240 260 280 300 320

Wavelength (nm)

Fig. 4.6a. A 13.56 MHz capacitively coupled Xe-Ar excimer lamp spectrum; 650

Watts input power, and 500 torr pressure. The gas composition is 1% Xe and 99% Ar.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced nu oe n 10 or rsue Tegscmpsto s5 n 5 r. A 95% and 5% is position com gas The pressure. rr to 1000 and power input Chapter 4: Capacitively coupled lamp excimer RF coupled Capacitively 4: Chapter Intensity (W/cm} F ig. 4.6b. A 13.56 M H z r f capacitively coupled X e -A r excim er lamp; 500 Watts Watts 500 lamp; er excim r -A e X coupled capacitively f r z H M 13.56 A 4.6b. ig. F 0.2 0.4 0.6 .8 0 0 1 160 180 56714.a I 051697/1345.dat 200 vlnt nm) m (n avelength W 220 40260 6 2 0 24 280 300 99 320 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced

Intensity (W orn2} at nu oe n 1 or rsue Tegscmpsto s01 ead9.%Ar. A 99.9% and Xe 0.1% is position com gas The pressure. rr to 10 and power input Watts Chapter 4: Capacitively coupled lamp coupled RF Capacitively excimer 4:Chapter 0.2 0.4 0.6 0.8 Fig. 4.6c. A 13.56 M H z capacitively coupled X e -A r m ixture lam p spectrum; 200 200 spectrum; p lam ixture m r -A e X coupled capacitively z H M 13.56 A 4.6c. Fig. 0 1 6 10 2 240 260 280 300 320 0 0 3 0 8 2 0 6 2 0 4 2 220 0 0 2 180 160 5 9n2&xlatJ x 20& 997n 050 vlnt (nm) avelength W 0 0 1 Chapter 4: Capacitively coupled RF excimer lamp 101

— ----- 051597/1440.dat |

1

0.8

0.6

0.4

0.2

0

160 180 200 220 240 260 280 300

Wavelength (nm)

Fig. 4.6d. Spectrum of Xe/Ar mixture at 100 torr from a 13.56 MHz rf

capacitively coupled lamp. The input power is 500 W. The gas composition is 10% Xe

and 90% Ar.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. chapter 5: KrI Excimer lamp 102

Chapter 5: A 2.45 GHz KrI Excimer Lamp

In this chapter, we discuss the design, construction, and study o f a potentially

useful new VUV photolysis source operating on the excimer bands in krypton iodide

(K rI) generated in a 2.45 GHz microwave lamp. The discharge characteristics over the

pressure range o f 5 to 100 torr are studied.

5.1. INTRODUCTION

Rare gases and rare-gas halogen mixtures are among the most extensively

explored excimers; they are used in commercial lasers116,246, 247. These excimers are

formed from reactions o f electronically excited argon (Ar), krypton (Kr), and xenon (Xe)

( 3Po^) atoms with the halogens or with a halogen containing molecule 2 48 ,249 . However,

fluorine (F) and Chlorine (Cl) attack the materials from which the bulb and connections

must be made (F attack >> Cl , so bromine (Br) and iodine (I) are preferred. Detailed

studies o f these metastable ( 3Poj) atoms as dilute mixtures o f Br 2, h, HBr, HI, CH2B r2,

CHBr3, CBr4, CH 3I, and CH 2I 2 in argon exist250. Previous theoretical work 251’253

indicates that K rI excimers are possible and K rI excimers spectra have been reported, but

only at low (< 25 torr) pressure2,25°. Although, our work on Xe-Ar mixture led to

emission at around 193-nm, no lamp, preferentially emitting in the neighborhood o f 190

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nm other than ArF has been published. Accordingly, we are motivated to investigate KrI

excimer emission in this study.

5.2. EXPERIMENTAL FACILITY:

The microwave drive and experimental set-up are described in detail in chapter 3

and elsewhere215. Briefly, the lamp is an open-ended, cylindrical “ Suprasil” quartz tube,

8 mm outer diameter ( 6 mm inner diameter) by 50 cm long with radiating surface area o f

~125 cm 2 and of volume 14.1 cm3. High purity (99.5%, Aldrich Chemical Co.) iodine

and research grade krypton gas (Air-Products and Chemicals, Inc.) were used. The lamp

characterization procedure is the same as in Chapter 3.

The lamp bulb is rinsed with reagent grade isopropyl alcohol followed by 18 MQ-

cm de-ionized water, then evacuated to ~ 5 x l 0 '3 torr at 450°C and cooled before

introducing the I and Kr. A charged weight o f the iodine is introduced into the bulb,

which is then back-filled with Kr gas to achieve a desired total pressure and I:K r (based

on the I charged weight) for the mixture. Occasionally, the gas handling system is heated

by a flame while pumping to remove condensed I. When the bulb is energized, a Kr

plasma is first generated which then sublimes the I to form a I/K r mixture. The lamp is

cooled by flowing room-temperature air or liquid nitrogen boil-off (T-100 K). A

discharge stabilization and mixing period o f ~ 2 0 minutes assures thorough mixing o f the

iodine and krypton gas. No features appear in the emission spectrum at ~ 5 torr that

cannot be accounted for by krypton and iodine atoms, indicating that impurities are at a

low level or absent. To better understand the K rI system, separate experimental runs were

conducted using pure iodine.

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As before Oriel Hg-Ar and Deuterium calibration lamps are used for the

wavelength and intensity calibrations. The spectrometer was calibrated before every KrI

experiment. Also, as before, we used an international Light IL 1400A meter and SEL 185

detector as a radiometer to measure integrated optical power. The apparent radiometer

reading was converted to real power reading, I r(>.),using

Ji a(>.)/S(X)=/i r(a.) (5.1)

where IA(X) came from the spectrometer and S(X) is the known radiometer sensitivity.

In the present experiments, data were obtained over the pressure range o f 10 torr -

100 torr, examining the spectral range from 160-320 nm. Experimental and simulation

work is in progress to extend the pressure range to > 1 0 0 to rr as a function o f input

power.

S3. RESULTS AND DISCUSSION:

The direct excitation o f the rare gas atom, the reaction o f the excited atoms with

iodine to form the rare gas halide, and the dissociation steps o f K rI* are summarized

below:

e + KrC'S0)-> Kr*( 3P0^) + e (5.2)

K r*( 3P0>2) + 12 -> K rI* +1 (5.3a)

Kr* + RI -> KrI* + R (5.3b)

K rI* - > K r + I + hv (5.4)

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Reaction (5.3a) is replaced with (5.3b) if iodine-containing molecules (RI) are

used instead o f I 2. In some cases, iodine molecules are preferentially excited through

direct electron impact or energy transfer from a primary excited rare gas atom or excimer.

Reaction (5.2) must be fast enough that energy is not preferentially channeled into the

iodine molecule. Operating conditions must be chosen such that the radiative dissociative

step (5.4) dominates over other loss channels. For Kr with iodine, the accessible I* states

are easily populated. KrI emission is restricted mainly to vibronic states lying below

these I* states; indicating that higher v levels o f K rI* are pre-dissociated to yield excited

I atoms, making K rI* emission d ifficult to observe2.

From a calculated potential energy diagram 250 (Schematics shown in Fig. 5.2) for

rare gas halogen molecules, there are three close-lying excited bound states labeled B, C,

and D. Calculations 251 252 show that these states can undergo transitions to X\a, A3/2, and

A t/2 lower states. The B->X and D->X transitions have the highest transition moments

and thus show the strongest emission2. The C-»A transition is the weakest.

Figs. 5.3a-c present spectra obtained at 10, 50, and 100 torr and I:K r o f ~ 0.1 -

0.001 (based on charged weights o f I). The emission appears mainly in the range o f 160-

190 nm and depends on the estimated I:K r used in the mixture. A t 10 torr (I:K r o f ~0.1),

we observed a structured broad emission between 160-180 nm, atomic emission at 184.48

and 187.57 nm, and a sharply peaked 206.16 nm atomic iodine emission. At this

pressure, the I 206 /I 187 ratio is -3.3 and increasing input power from 400 W to 600 W did

not change the shape or position o f these emissions, however, the optical power output

increased from ~ 30 W to 48 W. A t 50 torr (I:K r o f ~ 0.1), in addition to the 184.48 nm

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and 187.57 nm emissions, the spectrum shows vibrational band structures between 170

nm and 180 nm w ith I 206 /I 187 ratio ~ 2. As pressure is increased to 100 torr (I:K r o f ~

0.001), the vibrational band structures arising from Kr* and I (Fig. 5c) in the 165 - 190

nm regime are enhanced. A t all three pressures, there is unknown emission at -309 nm

that is shaded to longer wavelengths. W ithin the operating pressure regimes o f 10-100

torr, we established that for efficient generation of K rI emission, the mole ratio o f I:K r

must be < 0.01 based on the I charged weight (Fig. 5c).

Our results (185 nm < k < 215 nm) agree with published KrI spectrum by

Kogelschatz et al. taken in a barrier discharge system. In the wavelength o f 180-nm,

Kogelschatz et al. also observed atomic iodine emissions at 206.6-nm and 187.6-nm. As

is the habit o f that group, the displayed spectra are abruptly truncated at the edges o f the

features they describe. Nevertheless, though the lower lim it shown in their spectra was

~185-nm, the authors mention that they saw strong iodine lines below 185-nm. Casassa

et al. in studying the generation o f KrI using Kr with iodine, and with iodine containing

molecules such as CH 3I, and CH 2I 2, observed only atomic lines from energetically

accessible excited states o f iodine, populated by dissociative excitation o f the reagent

molecules. However, in mixtures o f Kr and HI, Casassa et al. observed three band-like

features, which he identified as belonging to KrI. He labeled these as (Bi/ 2- * X i/ 2) 180-

190-nm, (C 3/2-»A3/2) 190-200-nm, and (B 1/2—>A i/2) 210-235-nm. Casassa refers to these

K rI bands as main, secondary, and tertiary continua (MC, SC, and TC), pointing out the

parallel between these emission and three iodine band systems, previously labeled (MC,

SC, and TC) , observed in iodine discharge experiments by Tellinghuisen et al. and

Velazco et al. In our K r/I mixture work, the bands in the 165-210 nm region can be

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attributed to the MC and SC bands o f KrI. The TC band appears to be absent. Our

studies differ from the previous studies in: ( 1) the mode o f excitation, which governs the

population o f the accessible upper states, ( 2 ) the operating pressures, which determines

the collisional and vibrational relaxation processes and (3) the halogen containing

molecules used in forming the KrI mixture.

In the present study, a fraction of about 50% of the total emission appears

between 170 - 225 nm at 50 torr, rising to an 80% fraction in this range at 100 torr.

Taking into account the angular distribution o f emitted light, the optical output power

measured by the radiometer varied from 48 W to 120 W depending on the pressure. Over

the pressure range 1 0 - 1 0 0 torr, the measured electrical efficiency (light output in the

range of 170 - 225 nm divided by electrical input power) is between 8 and 20%,

consistent with estimates from theory. The highest experimental efficiency appeared at

an I:K r o f ~ 0.001, total gas pressure o f 100 torr, and input power o f 600 W. Electrical

efficiency as function o f pressure is shown in Fig.2. We previously observed215, a similar

electrical efficiency trend with pressure for Xe in our microwave lamp over the pressure

range o f 100 - 1500 torr. Accordingly, we expect that, at higher pressures (> 100 torr),

the electrical efficiency w ill be higher.

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D2 (0091297/1210,1240) Hg-Ar (091197/1405,1415)

Hg-Ar and D2 Calibration Lamp Spectra 5 02 data x 5

4

3

2

1

0

200 300 400 600500

Wavalangth(nm)

Figure 5.1. An Oriel Hg-Ar calibration lamp spectrum superposed on an Oriel D2

calibration lamp over the wavelength range o f 160 nm to 660 nm. The “our” wavelength

values for the Hg-Ar lamp are consistent with data supplied by the factory. Both spectra

indicate that our spectrometer is sensitive down to 160 nm. Also, no oxygen absorption

(170 < A. < 190 nm) is evident.

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PE

b

C(3/2'

A(l/2)

A(3/2]

X(l/2]

Intemuclear Distance

Fig. 5.2. Schematic potential curves for the noble-gas halides, AX. Eso, a (cm '1):

A r: 1432; K r: 5371; Xe: 10537. Eso. x (cm*1): Br: 3685; I: 7603. ( ---- ) Transitions from

B(l/2), transitions from C(3/2).

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081197/1145.dat I

KrI excimer emission in 2.45 GHz microwave lamp at 600 Vfetts input power 2

1.5

0.5

0 160 180 200 220 240 260 280 300 320

Wavelength (nm)

Fig. 5.3a. KrI UV emission in a 2.45 GHz microwave lamp at 600 Watts input

power. Operating pressure 10 torr and I:K r of ~0.1. The 206 iodine dominates the

spectrum. The emission between 170-190 nm can be attributed to the B to X transition.

Atomic iodine lines (I I) are superposed at 206.16, 187.6, and 184.48 nm. These

emissions are consistent with X values from the NIST atomic X database. The optical

output power is ~ 48 Watts.

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091597/1150.dat I

KrI excimer em ission in 2.45 GHz microwave lamp at 600 Watts input power

Pressure=50 torr

I m s M% m t I 0.4

160 180 200 220 240260 280 300 320

wavelength (nm)

Fig. 5.3b. KrI VUV emission in a 2.45 GHz microwave lamp at 600 W input

power. Operating pressure 50 torr and I:Kr o f ~0.1. The atomic iodine lines between

160 nm and 206.16 nm are clearly identified to within ±0.02 nm o f the NIST k values.

The optical power output is ~90 W for an electrical efficiency o f -15%.

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091597n210^atj|

KrI VUV e m issio n in 2.45 GHz m icro w av e Lam p

2.5

2

1.5

1

0.5

r>

0 200 250300 350

Wavelength (nm)

Fig. S.3c. KrI excimer emission in a 2.45 GHz microwave lamp at 600 Watts

input power. Operating pressure 100 torr and I:K r o f -0.001 (based on charge weight o f

I). The optical power output is 120 Watts yielding the best observed electrical efficiency

(- 2 0 %) in these studies.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced chapter 5: KrI lamp Excimer KrI 5: chapter increasing pressure. increasing Efficiency iue54 Iecmrefii yV.Pesr. h fcenc nrae t ith w increases cy n fficie e The Pressure. Vs. cy n fficie e excimer rI K 5.4. Figure 20 22 18 10 14 16 12 6 8 0 20 40 Pressure (torr) 080 60 100 120 113 chapter 5: KrI Excimer lamp 114

Iodine (10 Tonr) |

Iodine Spectrum in 2.45 GHz Microwave Lamp

* >

200 300 400 500 600

Wavelength (nm)

Fig. 5.5. Pure iodine spectrum over the range 160 nm to 660 nm. The structures

between 160 and 200 nm are the neutral (I I) 178.27,179.90,183.03,184.44 nm lines,

and a strong iodine atomic line at 206.16. Our wavelength values are consistent with

wavelength values from the NIST database.

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iodine (5-Toir)

Iodine Spectrum in 2.4S GHz Microwave Lamp

0.8

U.Q £ V* C

I 0.4 > I £ 0.2

- 0.2

160 170 180 190 200 210 220

Wavelength (nm)

Fig. 5.5a. Iodine spectrum (5-Torr) in 2.45 GHz microwave lamp. The lines

are pressure broadened.

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Iodine (10 Torr)

Iodine Spectrum in 2.45 GHz Microwave Lamp

> v 2 0.5 K

■0.5

170160 180 190 200 210 220

Wavelength (nm)

Fig. 5.5b. Pure iodine spectrum (10-Torr) in the 2.45 GHz microwave lamp. The

wavelength position o f the Iodine lines agrees with the NIST values. However, the lines

appear to be pressure broadened.

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Relative Intensity

8 190 180 Wavelength (nm) 200 210 117 Chapter 6. Simulation o f 13.56 MHz r fExcimer Lamp 118

Chapter 6: Simulation of 13.56 MHz rf Excimer lamp

This chapter discusses modeling and simulation o f the capacitively coupled lamps

using a 1-D r f hybrid code for calculating n«, np, current densities, and the mean electron

energies in discharges o f argon, krypton, and xenon over the pressure range o f 5 to 20

torr. We compare the results of this “low pressure” simulation to particle-in-cell

calculations o f a capacitively coupled lamp operating at 1 atmosphere.

6.1. Introduction

The first numerical models o f dc discharges220, “ ‘go back more than 30 years.

The need for accurate descriptions and predictions o f dc and rf glow discharges has

increased , due to the development of plasma processing in the micro-electronic

industry 224,225. Extensive literature exists on this subject. In the past decade, a number o f

1-D fluid models o f dc and r f discharges have been developed 226 ' 250 mainly for glow

discharges for low pressure (< 5 torr) plasma processes. Boeuf et al. and others 253 * 264

have also studied and modeled capacitively coupled discharges at low pressures (< 5-

torr). Nichols and Manos have described a 2-D hybrid fluid code for low pressure

transport in rf-driven plasmas. No one model works satisfactorily for all dc and rf

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discharges, due to the great sensitivity o f plasma to physical geometries and discharge

conditions, so although the Nichols and Manos code is available, it is not well-suited for

our geometry.

Our lamp geometry and bulb size also made it difficult to use Langmuir probe

techniques to measure ne and Te. The high rf fields, the very large heat fluxes, and the

propensity o f metallic devices to arc, render probes almost impossible in our geometries.

Building a microwave interferometer to measure n« and Te is expensive and difficult. In

constructing a microwave interferometer, factors to consider include cutoff frequency,

refraction o f the beam, phase sensitivity, and plasma size. The microwave frequency

must be high enough to permit the beam to propagate through the plasma (refer chapter 2 ,

plasma fundamentals, p.26) and small enough to achieve sufficient resolution. Also,

system costs increase rapidly with frequency. Thus w ithin the time and cost constraints

o f this program, such a system was not practicably achievable. Accordingly, we were

motivated to use model calculations to obtain n«, Te, current densities and E-field patterns

from the simulation of rf discharges in various gases and to compare our results to

experimental measurements by others in systems having comparable pressure and power

density.

We need to evaluate the electron density and temperature distributions in the

discharge bulb, because the total luminance (see appendix 6 ) depends on these

parameters and the spectral distribution depends on Te in a sensitive way though the

dependence o f branching ratios, excitation rates, and so on are implied by the kinetic

equations o f chapter 4. In this work, we present results o f the calculation o f electron

density, n«, electron temperature, Te, and axial electric field at pressures o f 5 < p <20 torr

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in argon, krypton, and xenon. These parameters also determine the plasma power

loading. The r f lamp geometry has been described in detail in chapter 4.

6.2. Model Description

We w ill briefly describe the approaches to modeling using particle-in-cell (PIC),

fluid, and Monte Carlo code. More detail on fluid, Monte-Carlo and hybrid codes is

contained in the paper o f Nichols & Manos and the references therein. The fundamental

equations, which we review below, are not d ifficu lt to code, but are not user-friendly and

flexible display is tedious. The simplest model is low-D fluid rf excited, capacitively

coupled model. We have been fortunate to have access to a version o f such a 2-D code,

from the Swedish group, which developed it266. The fluid code solves the fluid (particle

continuity, momentum transfer or drift diffusion, and energy) equations coupled to the

Poisson equation for the electron field until a periodic stable-state (A(x, t + T) = A(x,t),

where A is a target function describing a plasma parameter) is reached. The code

includes a data base o f ionization rates and electron and ion transport coefficients for IS

gases.

Code inputs including the discharge length, pressure p, applied rms voltage V*,

frequency co, and electrode secondary emission coefficient y. The code output include

ne, Te, and the E-field and rms current density. From these fundamental parameters, the

electrical properties (discharge current density and deposited power density) of the

plasma can be deduced. Table 1 shows the variable definitions used in the simulation.

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For comparison, PIC code was used to calculate the average electron energy and

density profile along the length o f the discharge tube. This code was developed by the

University o f California at Berkeley plasma group, formally headed by Ned Birdsall and

was generously provided to us by John Verboucover and his collaborators. The data used

in the PIC simulation include (1) electron/ion thermal velocities, (2) electron/ion collision

rate, electron-neutral collision frequency, ion-neutral collision frequency, electron/ion

gyro-frequency, electron/ion plasma frequency, electron/ion trapping rate, and (3) the ion

m obility and diffusion coefficient.

The detailed description of the particle-in-cell technique has been given in

Birdsall’s excellent text. The method accounts completely for all electron-mediated

microstate transitions, and is fully coupled in an iterative scheme to Maxwell’s equations

for E and B including internal currents, fields, and drifts.

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Table 1. Variables used in calculations.

Quantity Symbol

Electron density De

Positive ion density np

Negative ion density nn

Mean electron energy £

Electric field potential V

Electron, positive, negative source term Se, Sp, S„

Electric field strength E

Ionization rate Vl

Attachment rate Va

Diffusion coefficient De, Dp, Dn Electron, ion m obility Me. Mp Electron flux Oc Energy Loss term L(e) Recombination coefficient a

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The PIC code, when taken in a moment expansion at small values o f plasma

current in the absence o f external magnetic fields should be consistent with the fluid

equations below. We w ill concentrate on that set for this exposition.

The 2- D (one space, plus time) fluid equations for continuity, momentum, and

energy equations for the electrons are:

dn, 3 0 , _ — + — - = S, (6.1) dt dx

dne O e=-eptEne-D e (6 .2)

(6.3) dt 3 dx

Se — (Vj - Va)n« + Vdeuchnn — krecomlKe-i^np (6.4)

The detachment rate and electron-ion recombination coefficients are denoted by Vdetach

and krecomb

As shown below, a similar set applies to the ions (and neutrals), coupled through the

ionization rates and the requirement o f quasi-neutrality (n« ~ n,) in the bulb region away

from the electrode sheaths. The ionization (v,) and attachment frequencies (va) are

functions o f the electron mean energy &. The electron diffusion coefficient, De, and

m obility, are related to the average energy through the Einstein relation,

A. = M ^ = 2£ ( 6 5 ) Me e 3e

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where the electron temperature, Te, or mean energy

8c=(3/2)kBTe (6.6)

at each location and time is deduced from the electron energy equation. Similarly, under

the assumption o f ambipolar diffusion, the diffusion coefficients are deduced from the

Einstein relation, and the ion mobilities depend on the local reduced electric field E/p. In

the absence o f a thorough review o f the ambipolar assumption, it is sufficient to say that

the electrostatic coupling o f electrons and ions yields a mass (cross-section) averaged

diffusion, which applies to the gradient o f both. That is electrons drag the heavier ions

along, speeding the ions, slowing the electrons, to maintain bulk quasi-neutrality. The

energy flux and source term in the energy equation are written as:

(6.13) OX

Se =-ecE - n em (6.14)

where L(e), the loss term, depends on the local mean electron energy with the same

functional dependence as under equilibrium conditions (Se = 0 and L(e) = ejicE2).

Sim ilarly, the continuity and momentum equations for the positive ions are:

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dn„ 5 0 . — £-+ — P- = SB 6.7) dt dx p '

p= +eppEnp - D p- ^ - (6.8) a&t

Sp Vjllc — krecomb(e-i)nenp — k recom[>

and for the negative ions:

dn„ 5 0 . _ „ - i + — =l = S„ (6.10) dt dx

O n = -e p pEnn - D n^ (6.11) ax

Sn — Vjnc — krecomb(i-i)nnnp * V^nn ( 6 .1 2 )

These fluid Eqs. [6.1-6.14] are strongly coupled through Poisson’s Eq. [6.15] for the

electric potential, V,

^ L = . L t „ _ , 1 - 2 lnp ne nn\ (6 .1 5 ) O X £ q

E = - — (6.16) dx

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And asymptotically coupled by the requirement o f a quasi-equilibrium at long times such

that locally: dnc/dtl® = dnp/dt|m = dnn/dt|« = 0 when averaged over one period o f the r f

drive frequency.

Boundary Conditions

The electron flux normal (index n) and directed toward the tube walls is given by

re,n ('/iJncVeih (6.17)

where ve>(h is the electron thermal velocity, given by

1/2 (6.18)

The electron energy flux to the walls is set to

qe,n neve,th(2kBTe) (6.19)

The ion flux to the walls is assumed to dominated by the drift term computed from the

component o f the ion convective velocity directed toward the wall,

r p,n — n p(ip E n (6 .20)

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representing a flux o f ions accelerated through the edge sheath region, and is neglegible

otherwise.

Since the tube wall is dielectric, the total charge density, ct, per unit surface o f the

wall is obtained by assuming that electrons and ions recombine on first contact with the

(perfectly absorbing) boundary. The surface charge is therefore given by integrating

(6-2D

Assuming a perfect dielectric, the sheath electric field E„ at the point o f contact with the

boundary, is obtained from the surface charge a by

E„ = nE = o/eo (6.22)

This boundary value satisfies Poisson’s equation and provides the necessary starting

point of self-consistent iteration of the sheath. Clearly at long-times, in the lim it of

ambipolar fluxes, do/dt -> 0.

Assumptions

The assumptions made in writing the fluid equations are

1) The momentum equations are written in the drift-difiiision approximation, i.e., the

inertia terms (time derivatives) and the energy gradient terms have been neglected. This

approximation is reasonable except for very low pressures where the positive ion

transport through the sheath is not collisional.

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2) The diffusion term is written De(dn«/dx) and not d(Dene)/dx. This results is a

considerable numerical simplification, and in particular, it is this assumption that allows

the energy flux term in the energy equation to be cast into a form analogous to the flux

term in the electron continuity equation.

3) In the energy equation, the usual assumptions are made:

a. the pressure tensor is isotropic and diagonal,

b. the d rift energy is negligible with respect to the thermal energy, and

c. the heat flux is proportional to the electron temperature gradient.

Summary of solution technique:

A very efficient and rugged numerical algorithm is used in the Siglo-rf. The

essentials of the numerical method used are well described in Chapter 4 o f FCurata,

Numerical Analysis for Semiconductor Devices. The major points are:

1. A Crank-Nicholson estimate o f the time derivatives in the continuity and energy

equations is used.

2. For the spatial gradient terms, the exponential scheme o f Scharfetter and Gummel

(1969) for the discretization o f the flux is used.

3. The coupled fluid and Poisson’s equations are nonlinear, and the time integration is

done im plicitly. Thus, the fluxes in the continuity and energy equations are linearized

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with respect to the variables, and the linearized equations are solved using Newton’s

method.

Under our simulation conditions (13.56 MHz, argon, xenon, and krypton

pressures o f 5 -20 torr, input powers o f 10 - 300 W), the computational time is on the

order of a few rf cycles/minute of CPU (Pentium II, 450 MHz) time. Without

acceleration, harmonic steady state is reached in ~ 1 - 2 hours. At steady state, the

production-loss balance for each type o f particle is checked for convergence according to

the bulk quasi-neutrality condition and parametric functional harmonicity at ©rf described

above. The system usually reached equilibrium in about 1200 rf cycles..

Results and Discussion

The code was run at various pressures and input powers. The resulting spatial

variations o f the electron and positive ion density profiles are shown in Fig.6.1a, 6.1b,

and 6. lc for argon, xenon, and krypton at 20 torr w ith 300 Watts o f r f input power.

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0 Average Densities X 0.161+13 o-3, Power input ■ 300 R, pressure ■ 20 torr

Electron Positive Ion 0 . 8 -

0 .2-

0.0 0.0 10 20 30 40 50 Position

Fig. 6.1. The spatial variation o f electron and positive ion densities for Ar. The I ? central electron density is nearly constant in the plasma. The plasma density is ~ 10

cm„ -3 .

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Average Densities

Z 0.14S+15 ce-3, Power input * 300N, Pressure * 30 torr

Electron Positive Xon 0 .8 H

0.6H

0.4H

0 .2 H

0.0 0.0 10 20 30 40 SO Position

Fig. 6.1b. Electron/positive ion density profile in 20 torr of Kr at an rf input

power o f 100 W. The electron density (~2xl013 cm*3) is maximum at the center o f the

discharge.

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0 Avtraga Densities

X 0.191+15 g b- 3 , Power input * 300 W, Pressure * 20 torr

Electron Positive Zon 0. 8 -

0.6-

0.4-

0.2-

0.0 0.0 Position a t

Fig. 6.1c. Electron/positive ion density profile o f 20 torr Xe gas at rf input power

o f 300 W. The electron density is on the order o f ~1014 cm'3.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz r fExcimer Lamp 133

Figs. 6.1a, 6.1b, and 6.1c, show that the electron density profiles are almost flat

over two-thirds o f the tube diameter, predicting that the electron density distribution is

almost constant along nearly the entire length o f the bulb except in the sheath regions at

the ends. The electron and positive ion densities were strong functions o f the working gas

identity, ranging from 1012 to 1014 cm'3 at 20-torr and 300 W input power. The electron

densities for all o f the gases studied increased roughly linearly with pressure over the 5-

20 torr pressure range. I f this trend were to continue we would expect the code to predict

n« to be ~1015 cm'3 at 1 atmosphere, the plasma densities reported here are qualitatively

and quantitatively consistent with other computational and experimental work by

Gottscho252, Godyak253, and Kushner262, who reported plasma densities o f 109 - 1010 cm'3

at 100 m torr to 2 -to rr pressure range.

Near the walls, n« is not expected to be constant since the powered-sheath extends

from the electrodes well into the bulb toward the bulk (Figs. 6.1a, 6.1b, and 6.1c). Thus

there are “spikes”, as expected, in the electron/positive ion density profile close to the

wall of the bulb, with a greater positive net charge than electrons. Although, we did not

study frequency effects, it has been shown by Boeuf255, that at 10 MHz for Ar+ the ion

density does not respond to the time-varying field and is only sensitive to the time

averaged field. Thus at 13.56 MHz, the formation o f positive net charge density in the

sheaths (Figs. 6.1a-c) is entirely due to electron motion. The electron density is nearly

constant in the “bulk” plasma. The large ionization source in this collisional sheath

provides energy to the secondary electrons, by collisional transfer, and to the “plasma

electrons leading to spikes in the calculated mean energy near the walls (depending on r f

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phase). Such spikes affect the ionization and detachment rates. Usually, this is o f minor

significance because the number o f ionization or attachment events in this region is small.

In our lamp, the discharge is sustained mainly by the ionization produced by the nearly

thermalized plasma electrons. In other r f simulation works by Boeuf et al., the spatial

variation of the charged particle densities at 10 MHz, V * = 500 V revealed that np is

slightly greater than ne at bulb walls.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz r f Excimer Lamp 135

Ar Currents in Plasma

X 0.29E+02 mA/cm2

Electron 0 . 8- Positive Ion 0 . 6 - Total 0.4- 0 . 2-

0.0 0.2 0.4 0.6 0.8 time t/T (T= 73.75 ns)

Fig. 6.2a. Time variations o f the electron, positive ion, displacement, and total

(displacement and conductive) current densities ( f = 13.56 MHz, V #= 250 V, gas = Ar).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz r f Excimer Lamp 136

Xe Currents m Plasma

X 0.70E+02 mA/cm2

Electron 0 .8- Positive Ion 0 . 6- Total

0 .2-

0.0 0.2 0.4 0.6 0.8 time t/T (T= 73.75 ns)

Fig. 6.2b. Time variations o f the electron, positive ion, displacement, and total

(displacement and conductive) current densities ( f = 13.56 MHz, V * = 250 V, gas = Xe).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz r f Excimer Lamp 137

The positive ion convective and displacement (capacitive coupling) current

densities (Figs. 6.2a and 6.2b) are negligible in the plasma. The total current density

nearly the same as the electron current density. This is expected since, as discussed

above, ions respond almost entirely to the wear electric field, which is negligible in the

buck plasma.

The electron temperature, Te, is inferred from plots o f average electron energy (Sc

= (3/2)KTe). We note that the electron distribution function does not necessarily have to

be Maxwellian. The average electron energy and potential profiles are shown in Figs.

[6.3]. These profiles are flat, indicating that the electron temperature is almost constant

along the length o f the tube. The average electron energy is about 40 V. Comparing

Figs. 6.1 and 6.3, the maximum o f plasma density is associated with a maximum in the

plasma potential.

The plasma, current, and power densities in last 300 cylces is shown in Fig. 6.4.

The power is related to the current density and electric field through

We see that the current increases with power being constant, indicating either a

decreasing E-field, or a transition toward lower column resistance in the lamp as n« and

Te achieve their final (highest) value.

PIC calculations presented below have been performed267 in pure argon at a gas

pressure o f 1 atmosphere. The variation o f the electron mean energy as a function o f

time and the average electron density at r f input power o f 800 W are shown in Figs. 6.5a

and 6.5b. The figures predict average electron energy o f-12 eV and electron density o f

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz r fExcimer Lamp 138

~ 1015 cm'3. The electron density distribution profile is constant over V* o f the length of

the discharge tube. The results obtained from the PIC calculation are consistent with the

results we obtained with the SIGLO-rf calculations. Extrapolation o f SIGLO to such a

large value o f pressure is, o f course not acceptable, however the persistent observation o f

increasing light intensity with increasing pressure at a fixed temperature supports our

doing this.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz rf Excimer Lamp 139

xe Average Elec. Energy & Potential

X 0.91E+03 V X 0.91E+03 cm-3s-l

P o t e n t i a l Energy

0 . 8- - 0.8

1 ■■ 1 0 . 6 - 1 i ^0.6 " ! 1 1 1 0.4- r-0.4

0 . 2- - 0.2

f J 0.0 • 0.0 0.0 0.2 0.4 0.6 0.8

Position x 50 cm)

Fig. 6.3. Variation o f average electron energy and potential along length o f bulb.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz r fExcimer Lamp 140

j Plasma Densities, Current and Power vs cycle

Electrons Positive Ions 0 .8- X 0.22E+15 cm-3

0 . 6 - X 0.61E+02 mA/cm2

0.4- Power X 0.41E+04 mW/cm2

0 .2-

Last 300 Cycles

Fig. 6.4. The plasma, current, and power densities in last 300 cylcles.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6. Simulation o f 13.56 MHz r f Excimer Lamp

3 E + 1 5

2.5E+15

2 E + 1 5

1.5 E + 1 5

1E+15

5E+14

0.02 0 .0 3 0 .0 4 0 .0 5

Fig. 6.5a PIC results. Variation o f plasma density distribution along the length o f

the tube267.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced Chapter 6. Simulation o f f f 13.56 MHz Lamp Excimer r o Simulation 6.Chapter Avw*c*K£(«V) 12.1 12.2 12.3 12.4 12.5 12.6 i.65. I eut lcrneeg srbui a ucinoftme- tim f o function a as n utio istrib d energy electron f o results PIC 6.5b. Fig. E0 2E-05 1E-05 Seated tiM Seated 3E-05 4E-05 Chapter 6. Simulation o f 13.56 MHz r fExcimer Lamp 143

It is an important feature o f these RF discharges that the plasma parameters (lie,

Tc) can be influenced and thus optimized by the input RF power and operating pressure.

The mean electron energy is mainly controlled by the electric field. The presence o f such

high hot electrons and the presence o f Ar+, Ar2+, and Ar3+ (possibly), suggest that there

are a lot o f fast electrons opening up channels for these ionization states. The electron

distribution is most likely not Maxwellian.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 144

Chapter 7: Conclusion and Future Work

In this chapter, conclusions are drawn about the lamp performance including the

photon production as a function o f electron density and temperature along the length o f

the bulb. Finally, we discuss a proposed flat lamp development project.

7.1. Conclusion

We developed 1) a novel 13.56 MHz RF capacitively coupled lamp to produce Xe

and Xe/Ar excimers and 2) a 2.45 GHz microwave Xe and K rI excimer lamp. We studied

the emission characteristics of various gas fills, pressure and cooling effects. We

calculated the longitudinal electron density and temperature profiles along the length of

bulb.

• For both Xe and Xe/Ar discharges, > 80% of the emission was observed in a

wavelength region 160-nm to 200-nm. The Xe/Ar had a strong 193-nm emission at

pressures > 500-torr and input powers > 350 W. The Xe/Ar lamp may provide a

useful bright, selective halogen-free 180-200-nm light source.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 145

• For KrI, more than 80% o f the emission is in the 170 - 225-nm range with output

power o f 120 W.

• The electrical efficiency and power output of the Xe excimer increased with

increasing pressure and input power; consistent with what is observed in our

microwave lamp35 and the DBD lamp1'5.

• Talcing into account the angular distribution of the light intensity, we estimated an

electrical efficiency o f 1) 20-40% for Xe in the microwave drive, 2) 15-20% for Xe

and Xe-Ar mixtures, and 3) ~ 20% for KrI.

• Our results suggest that the electron energy distribution in this lamps, contains a large

fraction o f high-energy electrons ( » 10 eV) that populate accessible excited states o f

A r and Xe leading to emission in the neighborhood o f 190-nm.

The luminance o f the discharge bulb is dependent on the electron density, n«, and

temperature, Te, distribution, so in order to understand the lamp performance we used a

model calculation to estimate n« and Te along the length o f the bulb. Our results show

that

• n« and Te is almost constant along the bulb length. Assuming no to be constant also

(ignoring recombination coefficient), the photon production (light output), hv:

$hv = f(nc, Te, Oo)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 146

is constant along the length o f the discharge bulb. This result is in good agreement

w ith the light output measurements made along the length o f the bulb. From our

calculations, we project that the electron density o f our plasma over the pressure

range o f 500-torr to 2 atmospheres is ~ 1015 - 1016 cm'3. This determines how much

power is coupled to the plasma and also the cut-off frequency.

• The light intensity, I(X), depends on fe(vr vz), where vr and v2 are the radial and

longitudinal velocity profiles. Thus Te gives information as to whether there are fast

enough electrons to open channels o f Ar+ and Ar2+. We predict that Te is > 18 eV .

These energies are fast enough to open channels o f the Ar+ and Ar2* states.

7.2. Future Work

There is a growing interest in the deposition and processing of thin films at low

temperatures to eliminate the inherent problems associated with high temperature

processing. Photo-enhanced processing is one o f the techniques which has received

considerable interest. To date, one o f the commonly used UV sources is the low pressure

Hg lamp for large area UV processing. However in most o f the cases using Hg lamps,

there is the need to photo sensitize the reaction with Hg. This presents the drawback

effects o f health hazards and trace quantities o f contamination in the deposited films,

lim iting the types and quality o f film s deposited.

Here, we propose a program to develop tubular lamps with reflectors for

photolithography processes, material surface treatment, and film deposition. These lamps

are electrode-less and would be driven by 2.4S GHz microwaves and 13.56 MHz RF. We

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. have already demonstrated the production o f intense UV light in the cylindrical versions

o f such lamps.

We w ill use higher order (2 ,3-D) fluid models to study the electron and temperature

distribution o f the proposed flat panel discharge lamps.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX 148

A ppendix 1

Technical Specifications for General Microwave Corporation RAHAM Model 2

Broadband Electromagnetic Radiation Hazard Meter.

FEATURE DESCRIPTIVE DATA

Frequency Range 0.01 to 3 GHz

30 dB dynamic range. Three 10 dB ranges

Power Density Ranges with fu ll scale readings o f 2 mW/cm2, 20

mW/cm2, and 200 mW/cm2.

±1.25 dB 0.01 to 2.5 GHz

Frequency Sensitivity 2.5 dB, -0 dB 2.5 to 3.0 GHz

Calibration Accuracy ±0.5 dB

Average Power Overload I W/cm"

Peak Power Overload 30 W/cm2 (max)

Pulse Energy Density Overload 150 W-psec/cm2 (max)

Polarization Anisotropic

±0.5 dB (max) change in sensitivity due to

Ellipticity rotation about an axis through the handle

Noise <1% peak-to-peak on sensitive range

Response Time 1.5 seconds (approx)

Battery Operation 500 hours (expendable)

Recorder Output 0.124 V full scale into a nominal R(100K))

Operating Temperature Range 0°C to +55°C

•

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Appendix 2.

Lamp Operating Procedure

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced APPENDIX Appendix 3a: Appendix

Absorption eraaa Motion ( cm *) Absorption spectrum o f f O o spectrum Absorption -L U J . I . . 200 2 and O and i i - J QrfHvtlqf eonimual QrfHvtlqf ooot M Wonolonth might (Ini 3 at wavelength between 130 and 500-nm. and 130 between wavelength at 1 m m I i i i i i k I i i n liu 150 APPENDIX 151

Appendix 3b.

The transmittance o f UV through 1 cm o f atmosphere. The curve shows the

Schumann-Runge O 2 absorption strutures. The transmittance for the 195-nm line is 0.92

for 1 cm air path and 0.0003 for 1 m.

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Appendix 4

Skin depth calculation

where ojp = f p = 9000(n),/2/sec, and c = 3.0x10® m/sec, at to = 2.45 GHz

a!0 -3 r* _ 1 . For np = 0 cm , f p =

2 . For np = 0 11 cm'3, fp =

3. For np = 0 !2 cm'3, fp =

4. For np = 0 13 cm'3, fp =

5. For np = 0 14 cm'3, fp = 90 GHz, and 5 = 3.33 mm.

6. For np = 0 15 cm'3, fp = 285 GHz and 8 = 1.05 mm

7. For np = 0 16 cm'3, fp = 900 GHz and 8 = 0.33 mm

8. For np = 0 17 cm'3, fp = 2846 GHz and 8 = 0.105 mm

9. For np = 01® cm'3, fp = 9000 GHz and 8 = 0.003 mm

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Appendix 5.

c

Ar^SgP^+Ajj'So)

>CrVJ5JJP >* Ar<3P i j )

Ar*(3S*3P*) Art'S.)

Ar V Alt'So)

A r('P |j) + Ai<+1 So)

Potential Energy Curves for Ar gas molecules and Ar ions. The arrows

identify dimer transitions (I and II denote the first and second continua) and

transition in the third continua by III.

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Appendix 6

Some Photometric and Radiometric terms

Radiometry is a system o f language, mathematics, and instrumentation used to

describe and measure the propagation o f electromagnetic radiation, including the effects

on that radiation o f reflection, refraction, absorption, transmission, and scattering by

material substances in their solid, liquid, and gaseous phases. Photometry is a system

used for the same purpose when the radiation is to be detected by the human eye. In

radiometry and photometry, one is concerned with how radiation is distributed over the

electromagnetic spectrum. Such a distribution is called a spectral distribution or

spectrum.

There are four fundamental quantities each that are central to both radiometry and

photometry. Radiant flux, 4>, Irradiance, E, Radiant Intensity, L, and Radiance, L - for

radiometry. Luminous flux, Illuminance, Luminous Intensity, and Luminance - for

photometry.

1. Spectral Radiant energy, Qx, Q\ = dQ/dX (joules nm*1)

2. Radiant Flux (power), 0 ,0 = dQ/dt (joules s '). It is the rate o f flow o f radiant

energy.

3. Spectral radiant flux (power), = dQx /dt (W nm*1 or J.nnT'.s*1)

4. Irradiance, E, is the area density o f radiant flux: E = dO/dso (W.m*2), where dd> is an

element o f radiant flux and dso is an element o f area in the surface. It is a function o f

position on the specified surface. Irradiance is the most important quantity for

describing radiation incident on or leaving a surface when it is not essential to

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describe the angular or directional distribution o f the radiation in detail.

5. Spectral irradiance, Ex, is the radiant flux per unit area per unit A. interval at a point in

a specified surface. Ex = dE/dA. = d2Qx/ds0dt (W.m’2.nm '1 or J.m*2.nm*,.s*1)

6. Radiant intensity, I, is the solid angle density o f radiant flux. I = d is

the element o f flux incident on or emerging from a point within element do if solid

angle in a specified direction.

7. Spectral radiant intensity, Ix = dl/dA. = d2Qx/dodt (W .nm'l.Sr*1 or J.nm'1, s'1, Sr*1). It

is the radiant flux per unit solid angle and per unit A. interval at the wavelength

incident on, passing through, or emerging from a point in space in a given direction.

8. Radiance, L, is the area and solid angle density o f radiant flux, the radiant flux per unit

projected area and per unit solid angle incident on, passing through, or emerging in a

specified direction from a specified point in a specified surface. L = d2/dcodS, where

dS = dsocosd, projected area.

9. Spectral radiance, Lx, is the spectral concentration o f radiance, the radiant flux per unit

projected area, per unit solid angle, and per unit wavelength interval incident on,

passing through, or emerging in a specified direction from a specified point in a

specified surface. Lx = dL/dA. = d3/dcodsbcos0dA. (W.m*2.Sr*,.nm*1).

Note: Radiance and irradiance are quite different quantities. Radiance describes the

angular distribution o f radiation while irradiance adds up all this angular distribution over

a specified solid angle Q and lumps it together.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bibliography IS6

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V i t a

Joseph Divine Ametepe

Joseph D. Ametepe was bom in Accra, Ghana (West Africa). After graduating

from High School, he received the following degrees: B.Sc (with honors) in Mathematics

from the University of Science and Technology, Kumasi - Ghana (1989), M.S. in

Mathematics from Hampton University in Hampton, Virginia (1993), M.S. in Applied

Science from the College o f William and Mary in Williamsburg, Virginia (1995) and a

Ph.D. in surface/interface physics also from the College o f W illiam and Mary (1999). He

is married to Sharon N. Addy and they have a son, Joseph Jr. Currently, Joseph Ametepe

is an assistant professor o f Physics at Hollins University in Roanoke, VA.

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