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A study of diamond thin film and diamondlike carbon film deposition using electron cyclotron resonance microwave discharges

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Order N umber 8117486
A stu d y of diam ond th in film an d diam ondlike carb o n film
deposition using electron cyclotron resonance m icrowave
discharges
Kuo, Szu-Cherng, Ph.D.
Polytechnic University, 1991
UMI
300N.ZeebRd.
Ann Arbor, M I 48106
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A STUDY OF DIAMOND THIN FILM
AND DIAMONDLIKE CARBON FILM DEPOSITION USING
ELECTRON CYCLOTRON RESONANCE MICROWAVE DISCHARGES
DISSERTATION
Subm itted in P artial Fulfillment
of the Requirements for the
Degree of
DOCTOR OF PHILOSOPHY (Electrical Engineering)
a t the
POLYTECHNIC UNIVERSITY
by
Szu-Cherng Kuo
January 1991
Approved:
Head of Departm ent
to f/o
.1 3
Jo
Copy No._________
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Approved by the Guidance Committee:
M ajor : Electrical Engineering
_____
Erich E. K u n h ard t (
Professor of Electrophysics
C hairm an of Guidance Committee
and Thesis Advisor
M em ber
fjrJl
______
B ernard R. Cheo
Professor of Electrical Engineering
M inor : Com puter Science
^ a m e s T. LaTourrette
Professor of Electrical Engineering
and Com puter Science
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Microfilm or other copies of th is dissertation
are obtainable from
University Microfilms
300 N. Zeeb Road
Ann Arbor, M ichigan 48106
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VITA
Szu-Chem g Kuo was bom in Taipei, Taiwan, Republic of China on May
21, 1958. He graduated from Tam kang U niversity, Taiw an w ith a B.S. degree
in Applied C hem istry in 1981. This was followed by two years of m ilitary
service. In 1985, he received the M.S. degree in Com puter Science from New
York In stitu te of Technology. In Ja n u ary , 1985 h e accepted a research
fellow ship from th e E lectrical E n g in eerin g D e p a rtm e n t of Polytechnic
U niversity to pursue th e Ph.D. degree in Electrical E ngineering u nder the
guidance of professors E. E. K unhardt, B. R. Cheo and J . T. LaTourrette. He
received the M.S. degree in System E ngineering in J a n u ary , 1989 an d the
Ph.D. degree in E lectrical Engineering in Ja n u ary , 1991 from Polytechnic
University.
The research reported in th is dissertatio n w as conducted from May,
1988 to August, 1990 a t Polytechnic University. This work w as supported by
G rum m an Aerospace Corp. from December, 1989 to A ugust, 1990 under the
Contract No. 20-58293.
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ACKNOWLEDGEMENT
I would like to take this opportunity to th an k th e m any people a t WRI
and in the EE departm ent who assisted me over th e last several years as I
pursued m y Ph.D. degree.
P a rtic u la r th anks a re due Professor E. E. K unhardt, my d issertatio n
advisor, who introduced me to the field and guided me throughout. T hanks are
also due Professor B. R. Cheo for theoretical and technical support in th e area
of microwave engineering, Professor J . T. LaTourrette for technical support in
th e com putational aspects of th e research , an d for th eir service on my
guidance committee.
I would like to th an k my brother, professor S. P. Kuo, for his guidance,
encouragem ent and support, Dr. A. R. Srivatsa for his invaluable help and
encouragem ent during th e la s t y ear, an d H ugh Jones who h as been a
wonderful lab-m ate throughout the last five years.
I would like to express my appreciation to my lovely wife, Shanli Tsui,
w ith whom I always share my feelings. L ast, b u t no t lea st of all, is my
gratitude to my m other for her constant encouragement.
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AN ABSTRACT
A STUDY OF DIAMOND TH IN FILM
AND DIAMONDLIKE CARBON FILM DEPOSITION USING
ELECTRON CYCLOTRON RESONANCE MICROWAVE DISCHARGES
by
Szu-Cherng Kuo
Advisor: Erich E. Kunhardt
Subm itted in P artial Fulfillm ent of the Requirem ents
for the Degree of Doctor of Philosophy (Electrical Engineering)
Jan u ary 1991
The electron cyclotron resonance (ECR) plasm a is very useful for th in
film technologies since it enables :
1. generation of a very dense plasm a with n e > 10n /cm3 a t f=2.45 GHz;
2. generation of a highly ionized plasm a (ionization degree > 1%);
3. generation of a plasm a in the low pressure regime (10'4 - 10'2 Torr);
4. g e n e ra tio n of a rad ia lly hom ogeneous p lasm a colum n w ith larg e
diam eters; and
5. acceleration of the plasm a in an inhomogeneous m agnetic field.
vi
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A larg e va rie ty of deposition techniques have been used to prepare
diam ond th in film s and diam ondlike carbon (DLC) film s. ECR plasm aa ssisted chemical vapor deposition (PACVD) is a new technique currently
receiving m uch in te rest. The ECR plasm a system offers a m ore complex
param eter space th a n the more conventional PACVD processes. These include
m agnetic confinem ent of the plasm a, independent source control over the
dissociation of reaction gases, independent substrate bias of DC or R F voltage,
independent su b stra te tem p eratu re control, dow nstream plasm a operation
a n d th e m agnetic m irror configuration which allows for th e extraction of
specific ion energies from the plasm a chamber.
In th is work we have set up a n ECR plasm a-assisted m ate ria ls
processing system . A Lisitano coil is used to effectively couple microwave
energy into the plasm a, and a divergent m agnetic field configuration is used to
push the plasm a out of the Lisitano coil. Langm uir probe m easurem ents and
optical emission spectroscopy were performed to characterize th e ECR plasma.
We have deposited hard DLC films on silicon substrates using this ECR
plasm a system . The deposition was operated a t a -200 V DC bias, substrate
tem perature T = 200°C, pressure P = 5*10'4 Torr using CH4 as the reaction gas.
The diam ond th in film deposition using ECR PACVD technique h as produced
some initial results. F u rth e r studies into th e effects of dense ion flux in the
ECR plasm a on diamond formation is needed.
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TABLE OF CONTENTS
ABSTRACT
vi
LIST OF FIGURES
x
I.
1
INTRODUCTION
I I . ELECTRON CYCLOTRON RESONANCE (ECR) PLASMA SYSTEM 8
A.
Microwave Energy Coupling in a Static M agnetic Field
B.
Design of an ECR Plasm a System
14
1. Lisitano Coils
15
C.
D.
2. M agnetic Coils
18
3.
Microwave Power Supplies
22
4.
An ECR Plasm a System
27
P lasm a Diagnostics
30
1.
Langm uir Probe M easurem ents
31
2.
Optical Emission Spectroscopy
32
3.
Properties of Our Hydrogen Plasm a
33
Pulse M odulated ECR Plasm a
46
1.
RF M agnetic Field M odulation
47
2.
High Voltage Pulse M odulation
48
I I I . DIAMONDLIKE CARBON (DLC) FILM DEPOSITION
A.
8
Properties of DLC Films
51
52
B.
Structure of DLC Films
52
C.
Growth of DLC Films
53
viii
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D.
1.
Experim ental Procedure
2.
R esults and Discussion
54
56
C haracterization of DLC Films
57
1.
Ram an Spectroscopy
57
2.
N ear and Extended X-ray Absorption Fine Structure
59
3.
Fourier Transform Infrared Spectroscopy
Analysis (NEXAFS)
(FTIR)
IV . DIAMOND THIN FILM DEPOSITION
09
A.
Structure of Diamond
B.
Properties of N atural and Low-Pressure Grown Diamond
72
C.
Growth of Diamond Thin Film a t Low Pressure
75
D.
V.
59
69
Experim ent
80
1.
Substrate Preparation
80
2.
Experim ental Procedure
81
3.
R esults and Discussion
82
CONCLUSION
APPENDIX I
85
Interaction of Electrons with Atomic & Molecular
87
Hydrogen
APPENDIX II
M easurements of the Balmer Line Ratios
Em itted
98
from a Hydrogen Thyratron Discharge a t High
C urrent Densities
REFERENCES
106
ix
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LIST OF FIGURES
Figure
Page
1.1
G raphite/diam ond equilibrium diagram.
4
2.1
Efficiency versus a and b.
12
2.2
Electron trajectory in a static magnetic field when E = 0.
14
2.3
Electron trajectory in a static magnetic field when E = E ^ 0*,
where co = coc and E 01 B.
14
2.4
A slotted cylinder (Lisitano, 1968).
16
2.5
Standing wave along a short-circuited slot.
17
2.6
E field along a bent short-circuited slot.
17
2.7
Effective electric field in the coupler.
19
2.8
A circular loop located at z which carries current I.
20
2.9
B field profiles in symmetrical & magnetic beach configurations.
23
2.10 B field profiles due to different magnetic coil currents.
24
2.11 The construction of a microwave power supply.
25
2.12 H a line intensity due to a 60 Hz half-wave form microwave
26
power which is the output from a commercial microwave oven.
2.13 D iagram of ECR-CVD apparatus and the magnetic flux density.
28
2.14 A circuit for obtaining the probe characteristic in a few psec.
34
x
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Figure
Page
2.15 A typical probe I-V characteristic.
35
2.16 Electron tem perature versus microwave power in hydrogen
plasm a.
36
2.17 Plasm a density versus microwave power in hydrogen plasm a.
37
2.18 Space potential & floating potential versus microwave power in
hydrogen plasm a.
38
2.19 Plasm a density profile along the r axis in hydrogen plasm a.
39
2.20 Plasm a density profile along the z axis in hydrogen plasm a.
40
2.21
Space potential & floating potential along the r axis in hydrogen
plasm a.
41
2.22 Space potential & floating potential along the z axis in hydrogen
p lasm a.
42
2.23
Optical emission spectroscopy setup.
43
2.24
Spectroscopic evidence for existence of selected species in
hydrogen plasm a.
44
2.25
B field profiles in a symmetrical configuration due to a RF
m agnetic modulation.
49
2.26
B field profiles in a magnetic beach configuration due to a RF
m agnetic modulation.
50
3.1
A six-member sp2 bonded carbon ring.
53
xi
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Figure
Page
3.2
Variation of the peak frequencies of the major R am an bands
as a function of excitation wavelength for the DLC film
samples A (0% hydrogen content) and C (30%), highly
oriented pyrolytic graphite (HOPG), pyrolytic graphite (PG)
and glassy carbon (GC). (From Yoshikawa et al.53)
58
3.3
The Ram an spectrum of a DLC film.
61
3.4
The Ram an spectrum of a CVD diamond th in film.
62
3.5
The core level spectrum of a DLC film.
63
3.6
The core level spectrum of a CVD diamond th in film.
64
3.7
The core level spectrum of a graphite.
65
3.8
The core level spectrum of an amorphous carbon film.
66
3.9
The core level spectrum of a Si substrate.
67
3.10 The FTIR spectrum of the relevant region from a DLC film.
68
4.1
Atomic A rrangem ent of Carbon Atoms in Diamond.
70
4.2
Atomic A rrangem ent of Carbon Atoms in G raphite.
72
4.3
Carbon Phase Diagram.
76
4.4
Free Energy Difference between Diamond and G raphite a t
298 K and 1 atm,
77
4.5
Dangling Bond Form ation between Hydrogen Atoms and th e
Uppermost Layers of the Carbon Atoms in the Diamond Lattice.
79
xii
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Figure
Page
4.6
Ram an Spectrum of Glassy Carbon Film.
83
4.7
B field profiles due to a m agnetic m irror configuration.
84
The elastic scattering cross section for e-H.
91
A l .l
A1.2 Energy levels of an atomic hydrogen.
92
A1.3 Spectral lines originated in transitions between energy levels
of an atomic hydrogen.
93
A1.4 Total 2s + 2p excitation cross section for H and ionization
cross section.
91
A1.5
Vibrational excitation cross section in H2.
94
A1.6
Total scattering cross section summed over all rotational and
vibrational final states.
94
A1.7
The potential energy curves in H2 for triplet and singlet states.
95
A1.8
Dissociative attachm ent in H 2, HD and D2 near threshold
96
A1.9
Dissociative attachm ent in H2> HD and D2
96
A1.10 Dissociation cross section for H 2
97
xiii
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1
CHAPTER I
INTRODUCTION
Gas discharge plasm as have been widely utilized for the synthesis of
m aterials and the modification of surface properties in recent years. Examples
of the use of plasm a assisted m aterials processing are
*
Microelectronics Fabrication (Etching and Deposition)
*
Photosensitive Thin Films (Photovoltaics, Photoreceptors)
*
Surface T reatm ent (Nitriding, Carbiding, Oxidation)
*
Protective Coatings (Diamond films)
*
Gas Phase Synthesis of Solids (Ceramics)
The plasm a usually used in th in film technologies is th e low gas
p ressu re DC, RF or microwave glow discharge which operates in different
gases a t pressures ranging from 10'4 to 10 Torr. Electron densities ne of such
plasm as are typically between 109 and 1012 cm'3, electron tem peratures Te are
betw een 1 and 10 eV and ion tem peratures Tj are considerably lower Tj < 0.1
eV. The plasm a of low pressure glow discharges is electrically quasi-neutral
and far from therm al equilibrium since the electron energies are higher th an
those of ions and neutral particles, i.e. atoms and molecules (Te >T j = Tn). The
p lasm a q uasi-neutrality is violated only in th e close vicinity of surfaces
bounding the plasm a or im m ersed in th e plasm a, i.e. also a t su b strate
surfaces. The region where the q uasi-neutrality condition is not satisfied is
called a plasm a sheath. Across this sh eath ions are accelerated from w ithin
th e plasm a to the surface. In these non-equilibrium plasm as, the collisions of
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2
high energy electrons and gas molecules resu lt in dissociative processes th a t
would norm ally occur only a t very high tem peratures, i.e., more th an 5,000 K
in th e case of th erm al equilibrium . Therefore, n on-isotherm al plasm as
facilitate the preparation of m aterials and compositions th a t are difficult to
obtain using conventional, therm ally activated chemical vapor deposition
(CVD).
An im p o rta n t developm ent in low -pressure an d lo w -tem p eratu re
microwave plasm a processing is th a t of the electron cyclotron resonance (ECR)
discharge1,2,3. In an ECR discharge, microwave energy is coupled via the
na tu ra l resonant frequency to the electron gas in th e presence of a static
magnetic field. This resonance occurs when th e electron cyclotron frequency
equals the microwave frequency.
The ECR plasm a is very useful for thin film technologies since it enables
*
generation of a very dense plasma w ith ne > 10n /cm3 a t f=2.45 GHz;
*
generation of a highly ionized plasma (ionization degree > 1%);
*
generation of a plasma in the low pressure regime (10*4 - 10*2 Torr);
*
generation of a radially homogeneous plasm a colum n w ith large
*
acceleration of the plasma in an inhomogeneous m agnetic field.
diam eters; and
Furtherm ore, any kind of reactive gas can be used to form th e plasm a, and the
ECR plasm a is intrinsically electrodeless.
A large variety of deposition techniques have been used to prepare
diam ond th in films and diam ondlike carbon (DLC) film s. EC R plasm aassisted chemical vapor deposition (PACVD) is a new technique currently
receiving m uch interest. The ECR plasm a system offers a m ore complex
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3
pa ra m e te r space th a n th e m ore conventional PACVD processes. T hese
param eters include m agnetic confinement of the plasm a, independent source
control over the dissociation of reaction gases, independent su b strate bias of
DC or RF voltage, independent su b strate tem p eratu re control, dow nstream
plasm a operation and m agnetic m irror configurations which allow for the
extraction of specific ion energies from the plasm a chamber.
In this work an ECR plasm a-assisted m aterials processing system has
been designed and set up. A Lisitano coil4,5 is used to effectively couple
microwave energy into th e p lasm a, and a d iv erg en t m ag n etic field
configuration is used to push the plasm a out of th e Lisitano coil. Langm uir
probe m easurem ents and optical emission spectroscopy were perform ed to
characterize the ECR plasma.
P u re diam ond contains only carbon atom s. The carbon atom s are
arran g ed in such a way th a t each one is surro u n d ed by fo u r n e a re s t
neighbors, forming a tetrahedron. These atoms are linked by covalent bonds
which are very strong indeed, and th is is why diamond is strong; to break the
diamond the bond m ust be broken. N atural diamond can be divided into two
main groups : Type I and Type II, due to the presence or absence of nitrogen6.
Approximately 98% of na tu ra l diamonds are the Type I.
The in d u strial in te rest in diamond (Type II) is driven by its extrem e
properties. Diamond stands alone as the densest (num ber density), strongest
(elastic modulus), and hardest known m aterial. In addition, it h as th e highest
room -tem perature therm al conductivity of any substance. It is tra n sp a re n t
from 250 nm in the ultraviolet to the infrared and, when doped, is a high bandgap semiconductor, structurally analogous to silicon.
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4
Graphite is another allotrope of carbon. The carbon atom s are arranged
in layers. In a layer, each carbon atom is surrounded by three other carbon
atom s, all a t the same distance of separation (1.42
A).
The distance between
adjacent layers is 3.41 A, too great a distance for a tru e bond to exist.
Diamond is therm odynam ically un stab le a t ordinary p ressures and
tem peratures. Carbon prefers to exist in the form of graphite a t atm ospheric
pressure and room tem perature, as shown in Fig. 1.1. I f th e tem perature is
raised, b u t not the pressure, diamond will revert to graphite a t approximately
1,700°C. Conventionally, diamond synthesis is operated in very high pressures
and tem peratures so th a t diamond can be form ed in its therm odynam ically
stable region. This process was first developed in the 1950s by General Electric.
100
80
E 70
0
500 1000 1500 20002500 3000
Fig. 1.1
G raphite/diam ond equilibrium d iag ram (R. B erm an, Oxford
University)
Over the p ast 40 years a parallel effort h as been directed toward the
growth of diamond a t low pressures7,8, where it is m etastable. Polycrystalline
diamond films can now be produced on a variety of substrates. In addition, the
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5
recognition of an entirely new class of solids, the so-called "diamondlike"
carbons has arisen from this work. The m ost successful low-pressure m ethod
has been th a t of the deposition of diam ond from hydrocarbon/hydrogen gas
m ixtures (e.g. 1% CH4 in H2) a t subatm ospheric pressures. In all processes a
carbon-containing gas is energetically activated to decompose the source gas
molecules. The m ethods differ m ainly in th e m eans used to decompose the
source gas, which can be classified into three different types, as described
below.
*
Therm al decomposition method : H eat and light decompose th e source
gas molecules, e.g., hot filam ent CVD9'10, electron assisted CVD11 and
oxyacetylene torch12 process.
*
Therm al plasm a m ethod : The tem peratures of neutrals and electrons
are approxim ately equal, e.g., DC plasm a je t13 PACVD and hollow
cathode14'15 PACVD.
*
Low tem perature m ethod : Electron tem peratures are m uch g reater
th an the ion and neutral tem peratures, e.g., RF16 PACVD, microwave17
PACVD and DC glow18 PACVD.
The low pressure process has th e ability to grow diamond as films a t
relatively moderate conditions, and this m akes diamond a candidate for m any
applications. Potential uses for vapor-grown diam ond include abrasives,
bearing an d w ear re s is ta n t surfaces, tool coatings, h e a t sinks, optical
coatings, optical windows and as active electronic device elements. Presently,
however, m ost of these applications have not yet been realized and aw ait
fu rth e r im provem ents in crystalline quality, surface smoothness, process
tem perature and in the economics of production.
Low -temperature deposition of silicon dioxide films from ECR plasm as
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6
has been demonstrated by H erak et al.19 The synthesis of diamond films at P =
100 m Torr and T = 600°C using m agneto-microwave plasm a CVD h a s been
dem onstrated by Suzuki et al.20 The ECR PACVD m ay allow th e reduction of
the diam ond deposition process tem perature. Growth of smooth diam ond films
by hot filam ent CVD a t P = 10'4 Torr has been reported by Swec et al.21 Thus,
lower pressure deposition processes m ight lead to smoother surfaces. In this
work, we would like to explore the feasibility of lower pressure (P = 1 mTorr)
diam ond thin film deposition using an ECR plasm a. Moreover, ECR plasm a
techniques offer a way to study th e effect of ion beams on diam ond film
deposition. This will aid in understanding the film growth mechanism.
Diamondlike carbon (DLC) films have recently a ttrac te d atte n tio n
because they possess m any of the useful properties th a t diamond films have.
T hey are optically tra n s p a re n t in th e in frared , have a hig h th erm al
conductivity, a low coefficient of friction, high electrical resistivity, and are
chemically inert and resistant to chemical attack. DLC films are expected to be
used as passivation films on optical or electronic devices.
DLC films (a-C:H) are formed when hydrocarbon molecular ions, e.g.,
GmH n+, with energies about 100 eV h it a substrate7,8. The films are smooth,
amorphous and hard. Chemically, the DLC film is composed of 25 - 40% of
hydrogen in carbon. Structurally, DLC films have been confirmed to consist of
a m ixture of tetrahedral (sp3) and trigonal (sp2) bonding stru ctu res. These
films have been prepared using a large variety of deposition techniques. These
include ion-beam deposition22; sputtering deposition of graphite targ ets23; and
PACVD with DC24,25, RF26, microwave27 and ECR28,29 plasm a. The films
prepared with ion beams, a-C, typically do not contain hydrogen and have been
shown to be diamondlike.
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7
An im portant application for DLC films is th a t of passivation layers for
integrated circuits26. This applicatio n relies on th e film 's hig h electrical
resistivity, resistance to chemical a ttack , low diffusivity, and high therm al
conductivity. The high therm al conductivity can effectively d rain away waste
h e a t generated in the devices and allow closer packing. D eposition of these
film s will occur tow ard th e end of th e device fab rica tio n line, a fte r
m etallization, and will prevent th e use of high processing tem p eratu res.
Therefore, a low -tem perature process such as ECR plasm a CVD is suited for
th is process.
In th is experim ent, h a rd DLC films were deposited th ro u g h ECR
plasm a decomposition of CH4 gas. Ion species gained energy from a m agnetic
beach configuration in the plasm a colum n a n d a negative DC bias on the
substrate. High energy ion species played an im p o rtan t role on th e growth of
DLC films.
C hapter II describes the design of our ECR p lasm a system and th e
characteristics of the ECR plasm a as m easured by Langm uir probe and optical
emission spectroscopy. A description o f the properties, stru ctu re, growth and
characterization of DLC films is p resented in chapter III. Following this, the
structure, properties, applications, grow th and ch aracterization of diam ond
films are presented in chapter IV. In addition, the role of atomic hydrogen in
low pressure diam ond film deposition is explored in th is chapter. Finally, a
conclusion is presented in chapter V.
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CHAPTER n
ELECTRON CYCLOTRON RESONANCE (ECR) PLASMA SYSTEM
The efficient coupling of electrom agnetic power into th e plasm a is
always a m ajor concern in the design of an electrical discharge system. The
microwave power dissipated into a plasm a can be found usin g Joule's law,
i.e., P abs = E J . In section A, we will use Joule's law to derive th e ECR
condition under which the power dissipation is m ost efficient. The design of
our ECR plasm a system based on the ECR condition is th en described in detail
in section B. L a n g m u ir probe m ea su rem e n ts a n d p lasm a em ission
spectroscopy of a hydrogen discharge in this system are shown in section C. A
discussion of the operations of a RF m agnetic field m odulated ECR plasm a and
a high voltage pulse m odulated ECR plasm a will follow in section D.
A.
Microwave Energy Coupling in a Static Magnetic Field133
Consider the case of a volume of plasm a existing in a sta tic m agnetic
field. The microwave power absorbed by the plasm a is the integral of P at,s = E J
over the entire volume. We would like to show th a t u nder th e ECR condition,
the microwave power dissipation is m ost efficient.
Assume th a t the electrons in the plasm a are free to move in a stationary
uniform background of ions and neu trals. An electron will lose m om entum
when it collides w ith background ions and neutrals. The equation of motion for
electrons (Langevin equation) is
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9
^ m u = - e (E + u x B) - muvm
(2.1)
where u is the drift velocity and vm is the collision frequency for momentum
transfer. This macroscopic equation describes the averaged electron motion
u n d e r th e influence of exte rn a l fields and collisions. C onsidering the
sinusoidal steady-state solution only, we have
jcomu = - e (E + u x B) - m uvm
(2.2)
L et the m agnetic field be in the z direction and E have components in any
direction. Equation (2.2) can be w ritten in three scalar equations.
(vm + jco) mux = - e (Ex + uyBz) ^
(vm + jco) muy = - e (Ey - uxBz) ^
(2.3)
(vm + jco)muz = - e E z
Solving ux, Uy and uz in term s of Ex, Ey and Ez, we have
Ux = ^
Vm+j°)
E x + -£----------------- ---- e v
m (Vm + j©)2 + C0c2
m (Vm + jco)2 + C0c2
u -1 3 .
“c
Ex + ^fi-------Vm+j- ---- E v
m (Vm+jC0)2 + C0c2
m (Vm + jco)2 + C0c2
uz = - ^ — 1
(24)
p-
1 Vm + JCO
where coc = eB/m is the electron cyclotron frequency.
The conduction current density is J = -neeu = ctE. Using equations (2.4), J
can be expressed as
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10
a
•°x
<r±
0
0
0
o il
_ n ee2
1
0 '
' a±
V
h =
Jz
m
.
Ex
Ey
.Ez
(2.5)
V m +jco
(Vm + jw )2 + COc2 t
=^
_____ “ s______
m
(Vm + jco)2 + (Dc2 f
CT// = neg2.
"
m
Vm + JCO
are the components of the tensor conductivity.
The tim e-average power density is
Pabs= l /2 R e [ E J * ]
= 1/2 Re
+ E / y* + EZJ Z*]
Using equation (2.5), and neglecting Ez term (which is small), we have
Pab9=iR«[<JjJ|Eil2
_ nee2vm T
_L_
4 m [vm2 + (w-0
1IEi|2
(2 .6 )
where |EX|2 = |EX|2 + |Ey|2 The influence of a n impressed static m agnetic field
on the microwave energy coupling process can be observed from equation (2.6).
Let a = vm/co and b = (0,/co. These two normalized variables represent the effects
of collision and static m agnetic field respectively, i.e., a oc pressure and h oc B.
Equation (2.6) becomes
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Efflctancyi^ ^ =a[ w F +^ F ]
M
Since |E±|2 is proportional to the in p u t power, equation (2.7) rep resen ts the
power dissipation efficiency. The plot of efficiency versus a and b is shown in
Fig. 2.1. This plot represents the relations between power dissipation efficiency
and pressure as well as static m agnetic field. There is a m axim um located at
th e b = 1 position, and another smaller maximum located a t th e a = 1 position.
These maxima can be realized by the following discussion.
If the microwave frequency approaches the electron cyclotron frequency
and the electron collision frequency is reasonably sm all w ith respect to the
microwave frequency (vm2 « co2), then
P d*-2^
4m
-j-J
j lE if
vm2 + (co-coc)2
(2.8)
The conductivity reaches a peak a t co = coc. This is shown as th e big m axim um
in Fig. 2.1. Thus, in a low pressure microwave discharge, th e microwave
power dissipation is most efficient if
*
co = (0C,
*
the tim e varying electric field is perpendicular to the static m agnetic
*
the electron collision frequency is reasonably small w ith respect to the
field and
microwave frequency (vm2 « co2).
The effect of pressure on ECR coupling can be observed in Fig. 2.1. The
power dissipation is inversely proportional to vm. Thus, a t higher pressures
the energy absorption process becomes collisional and the m agnetic field has
little influence on the heating of the electron gas. It is th en clear th a t ECR is a
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I
0
I -
Z- ^
(Aoum) o j j j ^ j S o /
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13
coupling technique for low-pressure discharges where the electrons can orbit
m any tim es between elastic and inelastic collisions.
In the case of hydrogen plasm a, for which vm is relatively independent
of electron velocity, from Brown34,
vm > 4.8*10® P (H 2)
where P is the gas pressure in Torr a t 300 K. If f = 2.45 GHz, then the pressure
P should be less th a n 30 m Torr in order to satisfy vm2 « o)2 requirem ent. As
pressure increases the power dissipation tra n sits from a purely ECR heating
to a collisional heating. At P = 3 Torr, the power dissipation efficiency in a
microwave discharge without a m agnetic field becomes hig h er th an th a t with
a m agnetic field. So, a t higher pressure (P > 3 Torr), an im pressed magnetic
field will reduce the power dissipation efficiency.
If (oc = 0, equation (2.6) becomes
Pabs=
- - 1- , IE i|2
2 m vm2 + (02
(2.9)
This equation describes the microwave power dissipation in th e absence of a
static m agnetic field. By solving dPabs/dvm = 0, the maximum of equation (2.9)
is found to be co = vm. This is shown as the small maxim um in Fig. 2.1. Thus,
good microwave energy coupling is directly related to a synchronization
betw een the combined electron-neutral and electron-ion collisional processes
and the excitation frequency to.
A physical picture of electron cyclotron resonance h e atin g is shown
below. An electron u nder the influence of a sta tic m agnetic field B will
undergo cyclotron m otion a t th e n a tu ra l frequency coc (electron cyclotron
frequency), as shown in Fig. 2.2.
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14
-ifcttttr
(B Field out of Page)
Fig. 2.2
Electron trajectory in a static magnetic field when E = 0.
Now, if a high frequency electric field E = E 0e3wt is impressed such th a t © = (0C
and E 0 1 B , the electron continuously gains energy from th e electric field,
therefore accelerating, resulting in an outward, spiraling trajectory along a
m agnetic field line, as shown in Fig. 2.3.
rr
-ECR Coupling
Front V iew
(B Field out of Page)
Fig. 2.3
Electron trajectory in a static magnetic field when EsE ^® *, where
co = (Dc and E0 1 B.
The high energy electrons subsequently collide w ith neutral particles thereby
generating a low pressure plasma.
B.
Design of a n ECR Plasm a System
In order to absorb microwave power efficiently, i t is necessary to
generate the microwave mode in which the electric field is perpendicular to
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15
the static m agnetic field. The devices needed to do th is are, for example,
cylindrical cavity resonators and Lisitano coils. The L isitano coil is more
compact and flexible th an the cylindrical cavity resonator, and it can achieve a
b etter spatial uniformity. In our system, we use a Lisitano coil to generate the
electric field.
For a microwave frequency f = 2.45 GHz, th e m agnetic field which
satisfies the ECR condition is 875 Gauss. In this section, th e design of the
Lisitano coil and magnetic coils will be discussed.
1.
Ligi tanQ C oil§4»5
As shown in Fig. 2.4, a Lisitano coil is a slotted m etal cylinder. The slots
are distributed over the surface of the metal cylinder. They are axially oriented
and are connected with each other in series and are term inated w ith a shortcircuited slot a t the end of the cylinder. A coaxial connector is connected to the
first slot. The specifications of the Lisitano coil are :
*
Length of slots : X/2. For f=2.45 GHz, the length of the slots is about 6 cm.
*
Number of slo ts: 10 to 12
*
Width of s lo t: 1.5 mm
*
Diameters of the slotted cylinders: 0.875" to 1.5"
*
Thickness of the cylinder w a ll: 1 mm
The structure of a slotted cylinder can be understood by th e following
explanation :
i)
ii)
A slot is analogous to a parallel plate waveguide.
If we apply microwave to a slot which is short-circuited a t one end, a
standing wave along the slot will be set up. Fig. 2.5 shows the standing
wave along a short-circuited slot which is one w avelength long.
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16
X./2
Microwave Input.
Fig. 2.4
A Slotted Cylinder (Lisitano, 1968)
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Fig. 2.5
iii)
Standing wave along a short-circuited slot.
Now if we bend a short-circuited slot 180° in every 7J2 position and th en
wrap it around, a slotted cylinder is formed. Fig. 2.6 shows th e electric
field along a bent short-circuited slot which is one wavelength long.
H -------------------- V2 ---------------------►
Fig. 2.6
E field along a bent short-circuited slot.
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18
iv)
The electric field lines inside the slotted cylinder are all oriented in the
same azim uthal direction, as shown in Fig. 2.7.
The AC E field configuration inside th e slotted cylinder is sim ilar to th a t
of th e TEn ll mode of a cylindrical cavity resonator, where n is equal to the
num ber of slots. Since the length of the slot is X/2, the maximum of standing
wave voltage is located in the central cross section of the cylinder.
Ard35 and Consoli36 have shown th a t without accounting for collisions a
ponderom otive force (radiation p ressu re) exerting on a plasm a h a s the
expression :
F - c “ p27E2
p
0 4oo (o-o)c)
This shows a resonance in th e neighborhood of a n electron cyclotron
frequency. From Fig. 2.7 we see th a t the E field in ten sity inside th e slotted
cylinder h a s an outw ard gradient, therefore th e ponderom otive force is
oriented toward the center of the slotted cylinder. This force which is exerted
on the plasm a inside the slotted cylinder can improve the spatial uniformity of
the plasm a.
2.
M agnetic Coils
The m agnetic field satisfying an electron cyclotron resonance (ECR)
condition where the electron cyclotron frequency is equal to a microwave
frequency can be found by this relation :
1
m
For f = 2.45 GHz, the magnetic flux density is B = 875 Gauss.
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19
COUPLER SLOTS
Fig. 2.7 EFECTIVE ELECTRIC FIELD
IN THE COUPLER
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20
In th e design of m agnetic coils, B io t-S av art's law is u se d for
determ ining the B field caused by a current I in the magnetic coils. Consider a
circular loop shown in Fig. 2.8.
Fig. 2.8
A circular loop located a t z which carries cu rren t I.
The B field a t an arbitrary position along x-axis caused by th is circular loop is
found to be
f z cos<|>X + z sin<t>y - (d - r cos<)>) z ,
I -------------- ^
;-----------1
[(d cos<J) - r) + (d sin<t>) + z2]3/2
Since a m irror sym m etry exists about z-axis, the B field a t a n a rb itra ry
position can be found using this formula. For r = 0, the B field along z-axis of
this circular loop is found to be
R - > Ir2
These two formulas were used to develop a computer program which solves for
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21
the B field, a t a n a rbitrary position, caused by a superposition of concentric
circular m agnetic coils with different currents.
In th e construction of m agnetic coils, th e lim itatio n of coil size, the
availability of DC power supplies and the m eans for cooling had to be tak en into
account. Also, in order to exert more control over th e m agnetic field gradient,
two identical m agnetic coils were constructed a n d u sed in a stack. The
specifications for these two m agnetic coils are :
*
Inner diam eter : 8"
*
L e n g th : 3.65"
*
Wire : AWG 8 square wire
*
Num ber of tu rn s : 286 (13 turns/layer * 22 layers)
*
Inductance : 4.8 mH
There are th ree layers of cooling w ater which are located on top, in between
and a t the bottom of these two stacked m agnetic coils. The cu rren t required to
produce a magnetic flux density B = 875 Gauss is about 36 A.
The DC m agnetic field was m easured u sin g a G auss m ete r w ith a
vertical probe. The m easured data is in agreem ent w ith th e calculated data.
Hereafter, the magnetic field will be found by computer calculation.
The m agnetic field generated by a m agnetic coil will diverge along the
axial direction. I f a charged particle is gyrating in th e divergent m agnetic
field, i t will experience a force in th e axial direction due to th e divergent
m agnetic field. The expression of this force is :
F# = - p V//B
where p. =
js
m agnetic moment of the gyrating particle. This force
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22
accelerates electrons faster th an ions, thereby generating a static E field in the
axial direction. Consequently, ions in the plasm a colum n are effectively
accelerated in th e axial direction. For the purpose of ECR plasm a-assisted
m aterials processing, we can control the ion energy incident on a substrate by
changing the m agnetic field profile. A m agnetic m irror configuration is a way
to achieve this.
Typical B field intensity in the axial direction are shown in Fig. 2.9. Note
th a t z = 0 is located on top of the coil. The center of th e Lisitano coil can be
positioned a t z = 1.9" which is the center of m agnetic coils. Due to the
sym m etry, th e plasm a produced inside th e Lisitano coil sees divergent
m agnetic fields in the upward and downward directions. Therefore, p a rt of the
plasm a moves upward and the rest moves downward. Alternatively, the center
of Lisitano coil can be positioned a t z = 3". In this way, Lisitano coil is located a t
a m agnetic "beach", i.e. in a magnetic field decreasing along the z direction.
The plasm a produced inside the Lisitano coil sees a divergent magnetic field in
th e downw ard direction. Consequently, m ost of th e p lasm a moves in the
downward direction. A m agnetic beach geometry is preferred in th e cu rren t
setup. Fig. 2.10. shows the B field profiles corresponding to different magnetic
coil currents.
3.
Microwave Power Supplies
An Em erson AT738 microwave oven is used as the basic building block
of our microwave power supply. It can generate 530 W full power w ith an
operating frequency of 2.45 GHz. Its m agnetron has a waveguide output. B ut
in our application, we need a coaxial output. Therefore, a rectangular cavity
resonator (TE102 mode) was constructed to convert the waveguide output to a
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o_,
cq
11 : top coil current
12 : bottom coil current
11=12=35.7 A
11=12=37.5 A
oo
20
40
60
z (*0.135 inch)
Fig.2.9 B field profiles in symmetrical & m agnetic beach configurations.
The vertical bars show the positions of the center of Lisitano coil.
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co­
co
11 : top coil current
12 : bottom coil current
11 =12= 35.7 A
11=45 a , 12=26 A
M=
m
CM
40
60
z (*0.135 inch)
80
TOo
Fig. 2.10 B field profiles due to different magnetic coil currents.
rectangular
cavity
magnetron
output probe
77 “
L/2
2L
magnetron
A -----------
fila m e n t
negative HV input
'
coaxial connector
L/2
The anode of the magnetron
is grounded.
L = 86.5 mm
Fig. 2.11 The construction of a microwave power supply
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26
coaxial output. This cavity was m ade using alum inum pieces. Fig. 2.11 shows
the construction of our microwave power supply. Note th a t L is determined by
the resonant frequency expression :
where m = 1, n = 0, p = 2, a = L, b = L/2 an d d = 2L in this case. The m agnetron
output probe antenna and the coaxial in p u t probe a n te n n a are located a t
positions 1/4 away from the short-circuited walls, w here th e E fields are
m ax im a.
Fig. 2.12 H a line intensity due to a 60 Hz half-wave form microwave power
which is the output from a commercial microwave oven. (5ms/div)
The microwave oven employs a half-wave rectifying circuit to supply
high voltage to the m agnetron and th e microwave output is a 60 Hz half-wave
form, as indicated by a hydrogen plasm a emission spectrum H a line intensity
shown in Fig. 2.12. For CW operation, we simply connect a high voltage DC
power supply to the magnetron. The output microwave power is controlled by
regulating the input high voltage, and it can range from 10 to 100W. We don't
use i t for higher power operations because : (i) the m agnetron is not designed
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27
for CW operation and (ii) the coaxial connectors an d cables m ay overheat.
Pulse mode operation can be easily achieved by replacing the high voltage DC
power supply with a high voltage pulse generator.
4.
An.ECR Pla sm a System
Fig. 2.13 shows the configuration of our ECR plasm a-assisted m aterials
processing system . T he overall system consists of a vacuum system , a
m icrowave system and a su b stra te control system . A closer exam ination
follows.
A four-way cross, PYREX cham ber is used as our vacuum chamber.
The inner diam eter is 6" and the length is 18". This chamber is connected via a
g ate valve to a 1200 liter/sec diffusion pum p. The vacuum p ressure is
m onitored by two thermocouple gauges and an ionization gauge.
The m icrowave system consists of a m icrowave power supply, an
isolator, a directional coupler, a double stub tu n er and a Lisitano coil which
a re interconnected with high power coaxial cables. The isolator prevents the
reflected power from dam aging th e microwave power supply. Two power
m eters are connected to the directional coupler and monitor the forward and
reflected powers. Maximum power tran sfer from the microwave power supply
to the plasm a occurs when the output impedance of the power supply is equal
to the in p u t impedance of the tuned circuit which includes the plasm a.
Impedance m atching is achieved using a double stub tuner which produces a
standing wave between the discharge and the tim er thus increasing coupling
to the discharge and reducing the reflected power. Therefore, im pedance
m atching is achieved when the reflected power is a minimum . To set up a
m agnetic beach configuration, the top of Lisitano coil is positioned in the
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28
double
stub tuner
gas
to power meters
gas
isolator
icoaxial cable
directional
coupler
type K thermocouple
rectangular
cavity
substrate positioner
magnetron
DCorRF
bias
magnetic
coils
ionization
Lisitano
coil
gas injecting
ring --------
substrate
heater
B (Gauss)
valve
to diffusion
pump
Fig. 2.13 Diagram of ECR-CVD apparatus and the magnetic flux density.
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29
center of m agnetic coils, where the highest m agnetic field intensity exists. The
m agnetic field inside the Lisitano coil diverges in the downw ard direction. By
a d ju stin g th e m agnetic coil cu rren ts, a m agnetic field o f 875 G auss is
generated in the center of the Lisitano coil to fulfill th e ECR condition. The
divergent magnetic field accelerates the plasm a tow ard th e substrate.
A cylindrically shaped, 1" diam eter, high tem p e ra tu re resistive h eater
serves a s th e su b s tra te holder. The surface of th e h e a te r is m ade of
molybdenum. A thin disc of graphite is placed on top of th e h e a te r to prevent
any deposition on its surface during experim ents. S u b stra te s of size up to
0.7"*0.7" can be clam ped on the heater. The h e a te r is held by a substrate
positioner. This su b stra te positioner is ad ju stab le in th re e dim ensions,
allowing the substrate to be placed a t any desired position. The su b strate
tem perature is controlled by a tem perature controller which employed a type K
thermocouple in mechanical contact w ith the top of th e su b stra te to m easure
the tem perature. The substrate is electrically floating allowing a DC or RF bias
to be applied to it to control the sh eath potential d u rin g experim ents. The
reaction gases can be introduced into the Lisitano coil, th e high microwave
energy region, through a gas tube. A lternatively, th e reactio n gases can be
introduced into the plasm a column below th e L isitan o coil th ro u g h a gas
injecting ring for downstream operations.
In operation, the deposition chamber is first evacuated down to 7*10‘7
Torr by a diffusion pum p. The su b stra te is th en h e a te d to th e desired
tem perature. After which the reaction gases are introduced into th e vacuum
cham ber through the top flange and/or a gas injecting ring. The flow rate is
controlled by variable leak valves or electronic m ass flow controllers, while the
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30
operating pressure is in tu rn controlled by th e flow rate and the opening of the
gate valve. A bias voltage is applied to the su b strate before the ECR plasm a
CVD begins.
The general operating param eters of th is ECR plasm a system are listed
h ere.
*
Microwave frequency : 2.45 GHz
*
Microwave pow er: 10 -100 W
*
Magnetic coil c u rre n t: 38 A (B = 875 Gauss)
*
Pressure : 10*4 - 10"2 Torr
*
Flow rate : up to 100 SCCM (cubic centim eter per m inute a t STP)
The ECR plasm a system offers a more complex p aram eter space th an
conventional p lasm a-assisted chemical vapor deposition processes. These
include :
*
magnetic confinement of the plasm a
*
independent source control over the dissociation of reaction gases
*
independent substrate bias of DC or RF voltage
*
independent substrate tem perature control
*
downstream plasm a operation
*
m agnetic m irro r configuration which allows for
th e e x tractio n of
specific ion energies from the plasm a cham ber
C.
P lasm a Diagnostics
A fter se ttin g up the system , we perform ed p lasm a diagnostics to
characterize the plasm a. These included Langm uir probe m easurem ents and
optical emission spectroscopy. From these diagnostics, we have been able to
extract inform ation about electron tem perature, spatial resolution of plasm a
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31
density, space potential, floating potential and excited species in the plasma.
1.
L angm uir Probe M easurem ents
An L-shaped L angm uir single probe was constructed and is shown in
Fig. 2.14. The cylindrical probe was made with tungsten w ire (diameter=0.02")
and is electrically insulated through the use of ceramic tubes and Varian Torr
Seal Resin. The probe tip is 0.04" long37,38. This probe was placed in the plasma
colum n in such a m anner th a t the probe tip could reach th e center of the
plasm a column and was perpendicular to the DC m agnetic field, i.e., the probe
tip is sitting in a horizontal plane. The probe can be moved up and down, and
ro ta te d .
Since th e
p lasm a
colum n h a s c y lin d rical
sy m m etry , th is
configuration can m easure all three dimensional probe characteristics.
The circuit for probe m easurem ents is also shown in Fig. 2.14. The
sweep g enerator is a component of a Tektronix 545A X-Y oscilloscope. The
sweep period was set to be 1 msec, during experiments. The X-mode read the
probe voltage while the Y-mode sim ultaneously read the resistor voltage. The
I-V characteristic of the probe can therefore be found in 1 msec. Imm ediately
after recording the I-V characteristic, the floating potential of the probe was
recorded using another oscilloscope (Tektronix 7104).
The param eters used for the Langm uir probe m easurem ents are :
*
Gas : hydrogen
*
Pressure : 1 m Torr
*
Flow rate : 30 SCCM
*
Microwave pow er: 20 -140 W
*
Langm uir probe location : varied
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32
A typical I-V c h a ra c te ristic is red raw n in Pig. 2.15. E lectron
tem peratures can be found using data a t region B (the transition region) and
th e following relation :
kTe
e
In
Vpi-VP2
(Vri / V r2)
w here Vpl and Vp2 are the probe voltages, VRl and VR2 are th e resistor
voltages. The plasm a density can be found with the following relation :
iig = _Yes_ |2 jL m j1'2
* RAelkTe)
where A = 4rl for a cylindrical probe in a strong magnetic field. For our probe,
r = 0.01” and 1= 0.04”. The space potential Va and the floating potential Vf are
also shown in Fig. 2.15.
In the first set of m easurem ents, the probe was located a t 3.2 cm below
the Lisitano coil, and the microwave power was varied from 20 to 140 W. A plot
of electron tem perature, plasm a density, space potential and floating potential
versus microwave power are shown in Fig. 2.16, 2.17 and 2.18 respectively. In
the second set of m easurem ents, the microwave power was fixed while the
probe location was varied. A plot of plasm a density, space potential and
floating potential versus probe location are shown in Fig. 2.19, 2.20, 2.21 and
2.22 respectively. As shown in Fig. 2.19, the plasm a density is quite uniform
for r <, 1.2 cm, about the radius of the Lisitano coil used.
2.
Optical Emission Spectroscopy
Fig. 2.23 shows our optical emission spectroscopy setup. In operation,
the plasm a light exited the 1" sapphire viewport of the vacuum chamber, was
focused by a lens, and then transm itted to a spectrom eter via a fiber optical
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33
cable. A computer driven spectrom eter scanned over the visible light region.
The output signals were amplified by a photom ultiplier an d pream plifier
before they were recorded by a chart recorder. Each signal were divided by the
transm ittance of the fiber optical cable and th e qu an tu m efficiency of the
photomultiplier (at its wavelength) to get the normalized intensity.
We detected atomic hydrogen and m olecular hydrogen spectra in a
hydrogen plasm a. The Balm er series Ha (6563A), Hp (486lA) and Hy (434lA)
lines of atomic hydrogen spectrum 56 are shown in Fig. 2.24. The highest
intensity was recorded for the H a line, as expected. The ratio of Ha /Hp is about
11, in agreem ent w ith the theoretical resu lt given by P e n e tra n te and
K unhardt39. The m olecular hydrogen spectrum dem onstrated strong lines
(intensities higher th an Hp line) a t wavelengths of 6020A, 6123A (shown in
Fig. 2.24), 6227A and 6329A. These lines correspond to th e trip le t sta te
tra n sitio n s of electronic excitation in m olecular hydrogen40. Electronic
excitation into triplet states is an effective dissociation process for m olecular
hydrogen39'41’42.
3.
Properties of Our Hydrogen Plasm a
i)
In our ECR microwave discharge, a sharply shaped cylindrical plasm a
column is produced. This suggests th a t the m agnetic field strongly
confines th e charged particle s (electrons and ions) an d reduces
ambipolar diffusion in the transverse direction. Therefore, th e hydrogen
plasma decays by three-body recombination and radiative recombination
inside the plasm a column as well as by surface recom bination on the
Lisitano coil, substrate and top flange.
ii)
Most m etal surfaces are good catalysts for th e recom bination of atomic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
34
20K Ohm
bias
-30V
Sw eep
vo ltag e
g e n e ra to r
X-Y
scope
0 - 140 V
L angm uir
single p robe
p lasm a colum n
Fig. 2.14 A circuit for obtainin g th e probe ch a ra c te ristic in a
few m icroseconds.
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35
Yes
~
Vpl
Fig. 2.15 A typical probe I-V characteristic.
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20
40
60
80
100
120
140
160
power (W)
Fig. 2.16
Electron temperature versus microwave
power in hydrogen plasma. P=1 mTorr, z= 3.2 cm
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37
o_
cd
20
power (W)
Fig. 2.17
Plasma density versus microwave power
in hydrogen plasma. P=1 mTorr, z=3.2 cm.
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38
Voltage
co-
20
40
power (W)
Fig. 2.18
Space potential & floating potential
versus microwave power in hydrogen plasma.
P - 1 mTorr, z = 3.2 cm.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(*10
density
plasma
0.0
0.5
2.0
2.5
r (cm )
Fig. 2.19
Plasma density profile along the r axis
in hydrogen plasma. P=1mTorr, Te=1.8eV
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40
N-
plasma
density
(*10
n-
z (cm )
Fig. 2.20
Plasma density profile along the z axis
in hydrogen plasma. P = 1 mTorr, Te = 1.8eV.
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41
CO-
Voltage
m-
Ei o □ a Y s
■N-
O -
0.0
0.5
2.0
2.5
r (cm)
Fig. 2.21
Space potential & floating potential
along the r axis in hydrogen plasma.
P = 1 mTorr, Te = 1.8 eV.
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42
CO-
Voltage
co-
a a a a Vg
O -
z (cm )
Fig. 2.22
Space potential & floating potential
along the z axis in hydrogen plasma.
P = 1 mTorr, Te = 1.8 eV.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
p lasm a co lu m n
fo cu sin g len s
fib e r o p tic a l
c ab le
SPEX 1871
s p e c tro m e te r
PMT
P re a m p lifie r
HV
p o w e r s u p p ly
CD2
C o m p u d riv e
Fig. 2.23 O ptical em issio n sp e c tro sc o p y s e tu p
C h art
reco rd er
i 11m r r rr r n ri t it i m r r] 11 T i t r m f r F frji
Fig. 2.24 Spectroscopic evidence for existence of selected species in hydrogen plasm a
"M lIiI I I I l i l l i J j 1.1 II. 11.1.1jH U U-.-l-lM-I-I-U44-
( • n • -s ) jCa ts uai ux
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45
hydrogen. On th e other hand, alum inum does not favor recombination
as strongly a s m ost m etals43 and th is point was experimentally verified.
I f a copper m esh was positioned across the plasm a column, th en there
was alm ost no plasm a un d er the m esh observed visually. However,
when th e copper m esh was replaced with a alum inum mesh, th ere was
a fairly strong plasm a un d er th e m esh. For th is reason, we made
Lisitano coils using alum inum to reduce surface recombination.
iii)
G lass cham ber walls also act as a catalyst for the recom bination of
atom ic hydrogen. Adding a small am ount (0.5%) of 0 2 can inhibit
surface recombination of atomic hydrogen on the walls43,44. Also, oxygen
can increase th e H atom concentration through a sequence of reactions
such as :
H + 0 2-> 0 H + 0
o + h 2~ > o h + h
o h + h 2 - > h + h 2o
OH + O H - > H 20 + 0
iv)
For the ECR microwave discharge, an ionization degree > 10‘3 is easily
achieved. The electron energy distribution converges to a Maxwellian
form as a re su lt of electron-electron collisions. Also, th e frequency of
electron-ion collisions become comparable to th a t of electron-neutral
collisions. As a consequence of this, th e electron tem perature decreases,
due to the increase in the total momentum tran sfer cross section.
v)
For P = 1 m Torr and microwave power = 80 W, th e electron tem perature
is about 3 eV, and the electron-molecule energy exchange is dominated
by vibrational and electronic excitation. These two processes lead to
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46
effective molecular dissociation into atomic hydrogen.
vi)
Excited atom s rep resen t th e prim ary source of electrons in highly
ionized hydrogen plasm as. Even though the num ber of excited atom s is
relatively small, th eir ionization cross sections a re larg e an d th e ir
io n iz a tio n
th re s h o ld
e n e rg ie s
a re
s m a ll.
C o rre sp o n d in g ly ,
recombination into excited atomic states represents th e prim ary loss of
charged particles. This stepwise ionization plays a significant role in
m aintaining the plasm a. Also, th e dom inant ion in th e hydrogen
plasm a is H+.
D.
Pulse M odulated EC R Plasm a
In our ECR plasm a, the electron density n e is about 1011/cm 3 and th e
density of atomic hydrogen nH is about 1013/cm3 for P = 1 mTorr. The lifetime of
atomic hydrogen Xjj is about 0.48 second43. The lifetime of the m ethyl radical
(CH3) xCHg is about 8.4*10'3 second45. On th e other hand, th e plasm a density
relaxation time x„e is in the order of some tens of jxsec46-57. Therefore,
*
in the tran sien t regime (before reaching steady-state), th e production
rate of atomic hydrogen is higher th an th a t of ionized species (because
nH > ne), and
*
in a steady state, the production rate of ionized species ne/x„e is higher
than th a t of atomic hydrogen nH/xH.
When in a CW mode operation, the ECR plasm a is in a steady state, and
so a n intense ion flux is generated in this case. This is desirable for the DLC
film deposition, for example. On the other hand, a higher production ratio of
atomic hydrogen over ionized species can be achieved using a pulse m odulated
ECR plasma. The pulse repetition rate is about 1 - 1 0 KHz and the pulse w idth
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47
is about 0.1 - 1 (xsec. RF magnetic field and high voltage pulse modulation are
considered here.
1.
RF M agnetic Field Modulation
A m agnetic coil having a diam eter larger th a n a Lisitano coil could be
placed concentrically about the Lisitano coil. This m agnetic coil will generate
a n RF m agnetic field if an RF current is im pressed through it. The m agnetic
field inside th e Lisitano coil is th en a superposition of a DC an d a n RF
m agnetic field. This RF magnetic field will move th e ECR layer in and out of
th e Lisitano coil a t the applied RF frequency. Since th e microwave power
dissipation occurs mostly within the ECR layer, the RF magnetic field in effect
m odulate the power absorption of the plasm a. The tim e interval within which
the ECR layer stays inside the Lisitano coil could be adjusted by varying the RF
m agnetic field intensity. Fig. 2.25 an d 2.26 show th e RF m agnetic field
m o d u la ted
B field profiles for a sym m etrical an d a m agnetic beach
configuration respectively.
The specifications of this m agnetic coil are listed below :
*
In ner diam eter : 1.25"
*
L e n g th : 2.75"
*
W ire : AWG 18 transform er wire
*
Num ber of tu r n s : 408 (68 turns/layer * 6 layers)
*
Inductance :7.6*10‘4 H
The circuit is composed of a RF power supply, a blocking capacitor of 0.1 |iF
and the m agnetic coil connected in series. The operating frequency is 20 KHz.
In operation, the RF m agnetic field induced a RF voltage drop in the DC
m agnetic coils, which in tu rn caused th e DC power supplies to oscillate.
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48
Therefore, stable operation is hard to reach using th is setup.
2.
Higfajfollage. Pulse Modulation
A pulse m odulated ECR plasm a system could be set up by replacing the
high voltage DC power supply for the m agnetron w ith a high voltage pulse
generator. The Velonex high voltage pulse generator could generate voltage
pulse of up to 20 KV with pulse w idths of 0.1 - 1 psec. and a pulse repetition rate
o fl-lO K H z .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
top coil current:35.7A
bottom coil current:35.7A
RF current:!A
DC+RF B FIELD
DC
B FIELD
DC-RF B FIELD
3 0 -
oO to -
20
40
z (*0.135
60
100
inch)
Fig. 2.25 B field profiles in a sym m etrical configuration due to a RF m agnetic
modulation. The vertical bars show the top, middle and bottom Df Lisitano coil.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
top coil current:37.5A
bottom coil current:37.5A
RF current: 1A
DC+RF
DC
DC-RF
FIELD
FIELD
FIELD
60
80
"CJ o — o<D '«■*+— o -
CD
20
40
100
z ( * 0 . 1 3 5 inch)
Fig. 2.26 B field profiles in a m agnetic beach configuration due to a RF m ag n etic
m odulation. The vertical bars show the top, middle and bottom ol Lisitano coil.
51
CHAPTER III
DIAMONDLIKE CARBON (DLC) FILM DEPOSITION
D iam ondlike carbon (DLC) films have recen tly a ttra c te d a tte n tio n
because they have m any of the useful properties possessed by diamond films.
F or exam ple, DLC films can be used as passivation lay ers in in te g rate d
circuits26. T his application relies on the film 's high electrical resistiv ity ,
resistance to chemical attack, low diffusivity, and high th erm al conductivity.
The high therm al conductivity can effectively drain away w aste h eat generated
in the devices and allow closer packing of elements. Deposition of these films
will occur toward the end of the device fabrication line, after m etallization, and
thus the use of high processing tem peratures will be avoided. Therefore, a lowtem perature process such as ECR plasm a CVD is suited for th is process.
In our experim ent, h a rd DLC film s w ere deposited th ro u g h ECR
plasm a decomposition of CH4 gas. Ion species gained energy from a m agnetic
beach configuration in the plasm a column and a negative DC b ias on the
substrate. H igh energy ion species played an im portant role in the grow th of
DLC films.
The properties and structure of DLC films are introduced in sections A
and B. The ECR plasm a CVD grow th conditions of DLC film s a re th e n
described in section C. The deposited films were characterized by R am an
Spectroscopy, N ear and Extended X-ray Absorption Fine S tructure (NEXAFS)
a n alysis an d F o u rie r T ransform In fra re d Spectroscopy (FTIR). T hese
characterizations are described in section D.
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52
A.
P roperties of DLC Films
Diamondlike carbon films have m any of the useful properties possessed
by diamond films, as described below.
*
Optically transparent in the infrared
X = 3 - 200 pm
*
Extrem e hardness
H ardness > 7 on the Mohs' scale. It can not be scratched by alum inum
oxide ceramics.
*
High electrical resistivity
Electrical resistivity > 10^ Q cm
*
Chemically inert and resistan t to chemical attack
*
High therm al conductivity
*
Low coefficient of friction
O ther properties of DLC films are listed below.
*
Refractive index n = 1.8 - 2.2
*
Optical band gap Eg = 1 - 2 eV
6.
Structure of DLC Films
The current belief is th a t the DLC films are composed of clusters of six
member sp2 bonded carbon rings th a t are separated by both amorphous sp3 and
sp2 bonded media47’48. A typical carbon ring is shown in Fig. 3.1. C lusters
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53
containing four rings have been proposed as the m ost likely structure48. Fink et
al.49 concluded th a t 2/3 of carbon atoms were sp3 bonded in a typical DLC film
having an optical gap of about 1.5 eV and th a t 1/3 were sp2 bonded.
Fig. 3.1
A six-member sp2 bonded carbon ring. Each vertex represents a
carbon atom.
The films contain 25 to 40% hydrogen7. The hydrogen, in the C-H bond,
can aid in stabilizing the amorphous stru ctu re and provides strain relief. An
excessive increase in the hydrogen content resu lts in a decrease in hardness
and a n increase of the polymeric structure in the film.
Because of the amorphous stru ctu re and th e hydrogen content in the
film, the symbol a-C:H is used to represent DLC films.
C.
Growth of DLC Filins
DLC films are form ed when hydrocarbon ions h it a su b stra te with
im pact energies in the range from fifty to several h undred eV7. The incident
ions (CmH n+) will undergo rapid neutralization and fragm entation. The power
density dissipated within the surface layer of the su b strate is very high. This
energetic mobile surface layer will be quenched by th e underlying substrate,
which has a m uch lower tem perature, and form a diamondlike carbon film.
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54
l.
E xp e r im e n ta l P roc ed ure
The ECR plasm a system is shown in Fig. 2.13, an d th e basic operating
procedure is described in section B.4, chapter II. In DLC film deposition, a
m agnetic beach configuration was adopted to accelerate positive ions toward
the su b strate which was located 5" below th e L isitano coil. The deposition
cham ber was first evacuated down to 7*10' 7 Torr by a 1200 1/sec diffusion
pump. The substrate was then heated to 200 °C. After which the reaction gas
(m ethane) was introduced into the Lisitano coil through th e top flange. The
flow rate was controlled by a variable leak valve, while th e operating pressure
was in tu rn controlled by the flow rate and the opening of the gate valve. A -200
V DC bias was applied to the su b strate before th e ECR p lasm a CVD was
initiated. The grow th conditions of DLC films are sum m arized in Table 3.1.
V ariation of the flow rate (from 1.4 SC CM to 90 SCCM) did not change the DLC
film quality. On the other hand, when the pressure was increased to 10"2 Torr,
or when lower su b stra te tem peratures were used, or when lower values of
negative bias voltage were applied, th e deposits become softer a n d appear
tra n sp a re n t in the visible. The film hardness was tested by scratching with a
piece of alum inum oxide ceramic28. W hen th e m eth an e flow ra te was 90
SCCM, an additional hydrogen flow rate of 15 SCCM did not change the film
quality.
An optical em ission spectroscopy study w as carried ou t durin g the
A),
A), C2(5625 A) and the hydrogen Balm er line series Ha (6563 A),
Hp(4861 A) and Hy(4341 A). The molecular hydrogen em ission spectrum was
deposition. The spectrum detected in the m ethane p lasm a were CH(4315
C2(5167
also detected.
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55
TABLE 3.1 Growth conditions of DLC films.
Substrate
Reaction gas
Microwave power
P re ssu re
P-type Si(100) polished wafer
m ethane
20 - 70 W
1*10'4 - 5*10"^ Torr
Flow rate
90 SCCM
Substrate tem perature
Substrate bias voltage
Deposition time
180-270 C
-150 to-200 V
2 hours
S tru c tu ra l inform ation on these films was obtained u sin g Ram an
Spectroscopy as well as N ear and Extended X-ray Absorption Fine Structure
Analysis (NEXAFS). While Ram an Spectroscopy provides average information
from the entire film, the NEXAFS technique provides inform ation about the
near-surface region of the film. A Krypton ion laser (wavelength = 4067
A) was
used as the excitation source for Raman Spectroscopy. NEXAFS analysis of the
samples was carried out a t a beamline in the U ltraviolet ring located a t the
National Synchrotron Light Source (NSLS) a t Brookhaven N ational Laboratory.
The absorption spectrum about the Carbon K edge was monitored. However,
these techniques do not provide detailed information regarding the presence of
hydrogen in the film. To achieve th is, F o u rie r T ran sfo rm In fra re d
Spectroscopy (FTIR) was carried out on these films. A Cygnus 1000 FTIR
system was used. The spectra were obtained in the tran sm issio n mode. A
reference spectrum was obtained from a bare silicon piece sliced off th e same
wafer as the substrates. Approximately 5000 scans were carried out alternately
on the film /substrate and reference piece. Signals from th e reference piece
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56
were su b tra c te d from signals from the film /su b strate and th e resu ltin g
spectrum was integrated to obtain information from the carbon film.
2.
Results and Discussion
Optim um deposition param eters for th e form ation of hard, diamondlike
carbon films are listed in table 1. Use of lower substrate tem peratures or lower
v alu es of negative b ias voltage resu lte d in softer film s w hich ap p ear
tra n sp a re n t in the visible. Many authors7,50’51 have indicated th a t th is kind of
polymerlike film forms because of an excessive increase of hydrogen content in
th e films. This suggests th a t high energy ion bom bardm ent and m oderate
substrate tem peratures are favorable for th e de-hydrogenation process. Also,
strong hydrogen emission lines in the m ethane plasm a were detected. From
these, we conclude th a t the de-hydrogenation process occurs in the gas phase
as well as during ion bombardment.
An increase in the am bient pressure from 5*10'4 Torr to 10‘2 Torr, while
all other param eters rem ained the same, led to a formation of the same kind of
polymerlike films. At P = 5*10'4 Torr, the ion m ean free p a th is in the order of
10 cm, which is about the same length as the plasm a column. Thus, ions
m ade ju s t a few collisions before bombarding the substrate. On the other hand,
a t P = 10'2 T orr, th e ions und erw en t 20 tim es m ore collisions before
bom barding the substrate, and th is therm alization effect reduced positive ion
im pinging energies. The positive ions which deposited on the insulating DLC
film s accum ulated there. Charge accum ulation stopped w hen positive ions
and electrons deposited in the same rate. At higher pressures, the electron
tem p e ra tu re is lower59 and m ore positive charges accum ulated on the
in su la tin g film s. Because of the th erm alizatio n an d charge accum ulation
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57
effects, the im pinging positive ions have lower energies a t higher pressures.
This resulted in the form ation of soft, polymerlike films. Ion beam deposition
of DLC films using m ass selected carbon ion beam s h as been achieved a t room
tem perature using ion energies of less than about 100 eV22. In our ECR plasm a
CVD, the ion impinging energy is probably below 100 eV at P = 10'2 Torr and 150 V bias voltage.
D.
C haracterization o f DLC Film s
The deposits were analyzed w ith R am an Spectroscopy, N ear and
Extended X-ray Absorption Fine S tructure Analysis and Fourier Transform
Infrared Spectroscopy.
1.
Ram an Spectroscopy
A thorough discussion of Ram an spectra of DLC films was given by
Yoshikawa et al.52>53 A summary of the current belief is listed below.
*
R am an spectra of DLC films could be well resolved into twob ands a t
about 1400 and 1530 cm'1 with Gaussian line shapes.
*
T hese b a n d s are o rig in ated from
carb o n
atom s w ith an
sp 2
configuration. The Raman bands a t around 1400 and 1530 cm'1 originate
prim arily from arom atic rings with large and small sizes, respectively.
*
The Ram an spectral profile varies w ith th e excitation wavelength. The
positions of these bands increase lin e a rly w ith an increase of th e
excitation frequency (decrease of the excitation wavelength) as shown in
Fig. 3.2. The relative intensity betw een 1400 an d 1530 cm'1 bands
decreases w ith an increase of the excitation frequency.
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58
The relative intensity between 1400 and 1530 cm'1 bands decreases as the
hydrogen content increases.
1600
1580
1560
1540
1520
1500
: 1420
1400
1360
1360
1340
1320
4000
6000
5000
WAVELENGTH(A)
7000
Fig. 3.2
V ariation of th e peak frequencies of the m ajor Ram an
bands as a function of excitation wavelength for the DLC film samples A
(0% hydrogen content) and C (30%), highly oriented pyrolytic graphite
(HOPG), pyrolytic g raphite (PG) and glassy carbon (GC). (From
Yoshikawa et al,53)
O ur Raman spectroscopy was carried out a t room tem perature w ith the
4067
A
line of a Krypton ion laser. A Ram an spectrum from a DLC film is
reproduced in Fig. 3.3. This spectrum could be resolved into two bands. A
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
59
relatively sharp band is centered a t around 1580 cm'1 and a broad shoulder
band is centered a t around 1410 cm '1. The positions of these two bands is in
good agreem ent w ith the extrapolated d a ta in Fig. 3.2. Such spectra are
characteristic of diamondlike carbon films52,53’28,29. No kink in th e spectrum
was observed around 1330 - 1350 cm'1. This suggests the absence of significant
a m ounts of m icrocrystalline g rap h ite or diam ond in th ese films. F or
comparison, the R am an spectrum of a CVD diamond th in film is shown in
Fig. 3.4.
2.
N ear a n d -E x te n d e d
X-rav A bsorption Fine S tru c tu re A nalysis
(NEXAFS)
While Ram an spectroscopy provides average information about th e film,
th e n ear surface structure can be probed by using the NEXAFS technique.
Core level spectroscopy provides accurate inform ation reg ard in g n e a re st
neighbour bonding types. A typical core level spectrum of a DLC film is shown
in Fig. 3.5. Unlike the core level spectra of crystalline diamond (Fig. 3.6) and
g raphite (Fig. 3.7) which exhibit characteristic sharp features w ith in th e
energy range shown in the spectrum, diamondlike carbon exhibits a smooth
broad peak spanning the energy interval 290 to 310 eV. This is sim ilar to the
reported NEXAFS spectra of DLC film s58. The core level sp ectra of an
amorphous carbon film and a Si substrate are shown in Fig. 3.8 and 3.9.
3.
Fourier Transform Infrared Spectroscopy (FTIR)
Both the R am an a nd NEXAFS sp ectra confirmed th e diam ondlike
na tu re of these films. In order to determine the extent and n ature of hydrogen
bonded to carbon in these films, FTIR studies were carried out. There are
about 9 different peaks, corresponding to different types of C*H bonding, a t
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
different wavenum bers between 2800 and 3100 cm '1. The FTIR spectrum of the
relevant region from a DLC film is reproduced in Fig. 3.10. I t should be noted
th a t this spectrum was obtained by accum ulating d ata from 5000 scans and
subtracting the background signal from a b are silicon surface sliced off from
the sam e w afer as did the substrate. As seen, th e absolute values of th e
absorbance are small, indicating the presence of sm all qu an tities of hydrogen
in these films. The broad spectrum combined w ith th e absence of any large
kinks precludes the unambiguous identification of the type of C-H bonding in
the DLC films.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
O
<
CD O
c
4-i
CD
CO 'H
E
C_ ID
0
CD
cr
2100
0 O
r-l CD
□ ..
o
1900
in ru o
2 3 0 0
61
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
of a DLC film.
The Raman spectrum
Fig. 3.3
1700
(cm -1)
1500
1300
S h ift
1100
Raman
900
700
500
Diamond-Like/Si
#7
Srivatsa
a. 0
o
C_ +>
DJ tO
I—I Q]
P
+*
Ll c
LJ
(0
c_ in
cu o
..
D. ID
CVD
Fig. 3.4
Diamond
on
Si
The Raman spectrum
#3
Snivatsa
0) o
of aCVD diamond thin film.
Raman
o
(0 -H
oas/s^unoo
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
63
<C ..--J
Q_ t t l
X Li.. IQ Q_ f_J i—i
U - US I— ZZ Q p j
1
I
j
3
•If
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
(DIAMOND)
T ub A p 17 1 9 9 0
PSJi3 i
(C
CF5
K - E O G E ) ( 2 G D E G ) ( 1 1 . 4 H ! 1 . 4 , 2 MRD ) 0 . 2 N A , B E 2 ! 1 MA
MODE
START 1 1 : 4 6 : 1 5
END 1 1 : 5 6 : 4 5
APR 1 7 9 0 . 0 1 0
G R A P H I TE.
(HOPB) REPEAT
WITH
CF'5 MODE:
IIOVEABLE E X I T
START i 1 { 2 7 : 2 5
S L IT .2 0 0 P A ,
B W 33H A
END 1 1 • 5 8 : 3 0
AAPPRR 1ifto
8 9 n0 . 0 0 9
FXEN=
25.00
HFUN=
0.00
T3TF-
1 .0 0
NPTS=
0C=2,
300
1CH
2C=0; 3 0 0
NSCN=
T0TSCN=
1.500L. COG
0.501
300
310
320
PHOTON ENERGY
Fig. 3.7
The core level spectrum of a graphite.
1
1
66
1
A
s
$
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SI SUBSTRATE (ARUM) (C K-EOBE) ( 1 1 . 4 X 1 1 . 4 ) 2 0 0 P A , RC-31BMA
Med Ap r 16 199 0 CFS MODE START 1 0 : 2 5 : 0 5 END 10 ; 34 •. 36 APR 18 9 0 . 0 0 4
25.00
Ht- UM=
3. SOC-
T0T3CN-
2 . yOC
i .800-
J
280
290
Fig. 3.9
300
310
320
PHOTON ENERGY
T he core level
of a Si
340
Wavenumber
s
Microns
tn
The FTIR spectrum
m
of the relevant region from
a DLC film.
68
Fig. 3.10
cn
- < x a t f l o c . . o r o c : o a >
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69
CHAPTER IV
DIAMOND THIN FILM DEPOSITION
The low -tem perature deposition of silicon dioxide film s from ECR
plasm as h a s been dem onstrated by H erak et al.19 The synthesis of diam ond
films a t P = 100 mTorr and T = 600°C using magneto-microwave plasm a CVD
has been dem onstrated by Suzuki et al.20 I t appears th a t ECR PACVD m ay
allow for a lower tem perature diamond deposition process. Growth of smooth
diamond films by hot filament CVD a t P = 10‘4 Torr has been reported by Swec
et al.21 Thus, a lower pressure deposition process m ight lead to a smoother
surface. In this work, we would like to explore the feasibility of lower pressure
(P = 1 m Torr) diamond th in film deposition using ECR plasm a. Moreover, the
ECR plasm a presents an opportunity to examine th e effect of ion beam s on
diam ond film deposition, a topic which needs to be studied in order to more
fully u n d e rsta n d the film growth mechanism.
T he structure, properties and potential applications of diam ond films
a re introduced in sections A and B. The strategy for diamond growth a t low
pressu re s an d th e role of atom ic hydrogen in diam ond grow th are th en
discussed in section C. The experim ental procedure and in itial resu lts are
described in section D.
A.
Structure of Diamond
P u re diam ond contains only carbon atom s. The carbon atom s are
a rra n g e d in such a way th a t each one is surro u n d ed by four n e a re s t
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70
neighbors, form ing a tetrahedron, a s shown in Fig. 4.1a. These atoms are
linked by sp3 hybrid
o
bonds (1.54
A).
These covalent bonds are very strong
indeed, and this is why diamond is strong; to break th e diamond th e bond m ust
be broken. This structure repeats itself, forming a g ian t molecule shown in
Fig. 4.1b, which is diamond.
a
Fig. 4.1
b
a. Tetrahedral arrangem ent of carbon atoms.
b. Atomic arrangem ent of carbon atom s in diamond.
N atural diamond can be divided into two m ain groups : Type I and Type
II, due to the presence or absence of nitrogen6. Approximately 98% of natu ral
diamonds are Type I.
Type la :
In m ost cases the nitrogen (0.1%) has diffused through the crystal
and formed into platelets. Diamonds containing nitrogen platelets are known
as Type la . These diam onds absorb UV lig h t b u t n o t visible light. W hen
irradiated in UV light, Type la diamonds fluoresce.
Type l b :
If the nitrogen atom s (0.1%) are dispersed a t random throughout
the crystal, th a t is Type lb diamond. A perfect diamond is colorless, but Type
lb diam ond, because of the n itro g en atom s, absorbs some of the blue
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71
component of visible light, and, therefore, has a green-yellow color. These
diam onds are extrem ely rare, accounting for about 0. 1% of all n a tu ra l
diam onds. B ut alm ost all synthetic diam onds which are synthesized using
very high pressures and tem peratures are of this form.
Type Ila :
Diamonds which do not have nitrogen platelets, and therefore do
not fluoresce in UV, are called Type II. Type Ila diamonds constitute only 2%
of all n a tu ra l diamonds. The therm al conductivity of Type I la diam ond is
much greater th an th at of Type I (2 to 3 times greater); a t room tem perature it
is about five tim es better th an copper. This property m ake Type II diamond
very useful in semiconductor devices.
Type lib :
These diam onds are semiconductors (p type), blue in color and
extrem ely rare. On the other hand, Type I and Ila diamonds are very good
electrical insulators.
Graphite is another allotrope of carbon. The carbon atoms are arranged
in lay e rs, as shown in Fig. 4.2. In each layer, th e carbon atom s are
surrounded by three other carbon atoms, all a t the same distance of separation
(1.42 A). Each carbon is bonded to its three neighbors by sp2 hybrid c bonds. The
rem aining 2p orbital is employed in k bonding to the same three atoms. The rc
bonds extend over the entire layer, and so n electrons are free to move from one
bond to the next. The distance between adjacent layers is 3.41
A,
too g reat a
distance for a true bond to exist. Instead, it is the Van der W aals force which
causes the w eaker interm olecular attractions. T h at is why g raphite layers
slide p ast one another so easily and are readily separated.
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72
Fig. 4.2
B.
Atomic Arrangem ent of Carbon Atoms in Graphite.
Properties of Natural and Low-Pressure Grown Diamond
Some mechanical and electrical properties of natu ral and low-pressure
CVD diamond are listed in table 4.17. Among these outstanding properties, the
hardness, m olar density a nd room tem p eratu re therm al conductivity of
diam ond are higher th an any other known m aterial; the compressibility and
bulk m odulus of diamond are lower th an any other known m aterial; and the
therm al expansion coefficient of diamond is lower th an Invar.
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73
Table 4.1
Some Properties of N atural and Low-Pressure CVD Diamond
Property
Type Ila
CVD diam ond
H ardness, GPa
90
80-90
M ass density, g/cm3
3.515
2.8-3.5
M olar density, g atom/cm3
0.293
0.23-0.29
M elting point, °C
> 3,700
Specific heat a t 300 K, J/g
6.195
Thermal conductivity a t 298 K,
W/cm K
20
10-20
Therm al expansion coefficient
a t 293 K, K 1
0.8*10'6
Friction coefficient (dry)
0.05 (111)
>0.05(100)
T ransm issivity
2,250A - far IR
Bulk modulus, N/m2
4.4 - 5.9* 1011
Compressibility, cm2/kg
Lattice constant,
A
1.7*10'7
3.567
Bandgap a t 300 K, eV
5.45
Dielectric constant a t 300 K
5.7
5.7
Refractive index a t 5893 A
2.41726
2.4
Resistivity, Q cm
>101G
Breakdown voltage, V/cm
>107
Hole mobility, cm2/V sec
1,600
Electron mobility, cm2/V sec
1,900
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74
The relationships betw een th e unique properties of diam ond and its
current or potential applications are listed in table 4.2.
Table 4.2
Kobashi)
Properties and Applications of Diamond Coatings (From K.
Properties
A pplications
Hard
Low friction
Abrasive coating for cutting tools and
bearings
Low thermal expansion
Electrical insulator
Heat sinks for electronic devices
High thermal conductivity
Heat resistive
High power microwave devices
Large band gap
Low dielectric constant
RF electronic devices
High hole mobility
High speed electronic devices
Acid resistive
Radiation resistive(to X-ray)
Electronic devices for severe
environments such as in space
or in nuclear reactors
Transparent (250nm to IR)
Window and lens materials
Large refractive index
Electro-optical devices
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75
C.
G row th of D iam ond T hin Film a t Low P ressu re
Diamond is therm odynam ically unstable a t ordinary p ressures and
tem peratures. Carbon would m uch prefer to exist in th e form a graphite at
atm ospheric p ressures and room tem p eratu re, as shown in Fig. 4.3. If th e
tem perature is raised, but not the pressure, diamond will revert to graphite a t
approxiamtely 1,700°C. Conventionally, diamond synthesis is operated in very
hig h p ressu re s an d te m p e ra tu re s so th a t th e diam ond form s in its
therm odynam ically stable region. This process was firs t developed in the
1950’s by General Electric. Almost all synthetic diamonds formed, using this
strategy, are of Type lb. On the other hand, the low pressure process h as the
ability to grow better quality Type II diamond30 and to grow diamond as films
a t relatively m oderate conditions.
At low p ressure, diam ond is in the therm odynam ically m etastable
phase while graphite is in the therm odynam ically stable phase. B ut the free
energy difference betw een them is very sm all, only 0.03 eV/atom. The
difference in energy betw een the solid phases and free carbon atom s is very
large, about 7 eV/atom (Fig. 4.4). Both graphite and diam ond are in deep
potential wells w ith a very large activation energy b a rrier betw een them .
Consequently, once diamond is formed it rem ains as a m etastable phase with
a negligible rate of transform ation to graphite unless heated to extremely high
tem peratures.
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120
r shock —q u e n ch .
Pressure, GPa
Liquid
Liquid
C atalytic, Gr -* Di
0
1000
Fig. 4.3
2000
300 0
4000
5000
Carbon Phase Diagram.
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77
free carbon atom
activat
energy
barrier
7 eV/atom
diamond
(metastable phase)
0.03 eV/atom
graphite
Fig. 4.4
Free Energy Difference between Diamond and G raphite a t 298 K
and 1 atm.
Formation of a m etastable phase depends on selecting conditions where
rate s of competing processes to undesirable products are low. The processes
com peting with diamond growth are nucleation and grow th of graphitic
deposits and spontaneous graphitization of the diamond surface. Is th ere a
way to minimize these two undesired processes ?
The etching rates of various carbon m aterials in a hydrogen plasm a are
shown in table 4.331. The etching rate s of artificial g rap h ite and vitreous
carbon are about 20 tim es higher th an th a t of diamond. These resu lts suggest
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78
t h a t the graphitic carbon deposited together with diamond in a low pressure
CVD process is easily regasified in the hydrogen plasm a. It is also significant
t h a t diam ond h a s a small etching rate. So, w ith the help of atomic hydrogen,
th e growth of graphitic carbon can be stopped while the diamond is growing.
Table 4.3
Etching Rate of Various Carbon M aterials in Hydrogen
P lasm a (Modified from N. Setaka31)
Carbon m aterials
Artificial graphite
Etching rate (mg/cm2 h)
0.13
Vitreous carbon
0.15
N atu ral diam ond
0.006
Hydrogen atom s are able to form dangling bonds w ith the upperm ost
layers of the carbon atoms in the diamond lattice, as shown in Fig. 4.5. These
carbon-hydrogen bonds hold the diam ond-crystal stru ctu re in place until the
n ext layer of carbon atom s deposits out, tak in g the place of the hydrogen
atoms. W ithout these bonds, the diamond lattice would quickly revert to the flat
six-m em bered p la n a r rin g stru c tu re typical of g rap h ite. Thus, atom ic
hydrogen prevents the spontaneous graphitization of th e diam ond surface
during the diamond growth process.
Atomic hydrogen plays an im portant role in low pressure diamond CVD
processes. I t prevents reconstruction of the diamond surface, suppresses the
form ation of graphitic nuclei and rem oves hydrogen from th e hydrogen
s a tu ra te d surface and from hydrocarbon m olecules to form reactive free
radical sites.
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79
Fig. 4.5
D angling Bond Form ation betw een Hydrogen Atoms and th e
U ppermost Layers of the Carbon Atoms in the Diamond Lattice.
The m ost successful low-pressure m ethod h as been th e deposition of
diam ond from hydrocarbon/hydrogen gas m ixtures. The diam ond form ation
reactions are largely independent of the choice of carbon containing molecule
used a s the source gas; however, th ird elem ents such as oxygen can greatly
influence th e process chem istry43’44*45. Commonly used hydrocarbon gases
include CH4, C2H 2, CH3OH, CO etc. The gas m ixture composition is typically
very rich in hydrogen, e.g. 1% CH4 in H2. This composition is close to the
eq uilibrium concentration of m ethane c alcu lated from th e equilibrium
constant32 of the reaction C(graphite) + 2H2 <==> CH4, although th e plasm a
environm ent is not in equilibrium.
S u b stra te tem p e ra tu re s are u sually 800 - 1000°C, a tem p e ra tu re
sufficiently high to give some adsorbed su rface species m obility on th e
diam ond surface.
In all low -pressure processes th e carbon-containing gas m ixture is
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80
energetically activated to decompose the hydrocarbon gas molecules. The
prim ary differences among the low-pressure processes lies m ainly in the
m eans used to decompose the source gas. They can be classified to three
different categories, as described below.
*
Thermal decomposition method : H eat and light decompose the source
gas molecules, e.g., hot filam ent CVD9*10, electron assisted CVD11 and
oxyacetylene torch12 process.
*
Therm al plasm a method : The tem peratures of n eu trals and electrons
are approxim ately equal, e.g., DC plasm a je t 13 PACVD and hollow
cathode14’15 PACVD.
*
Low tem perature method : Electron tem peratures are much g reater
th an the ion and neutral tem peratures, e.g., RF16 PACVD, microwave17
PACVD and DC glow18 PACVD.
D.
Experiment
In this work, we would like to explore the feasibility of lower pressure (P
= 1 mTorr) diamond thin film deposition using ECR plasma. Moreover, ECR
plasm a offers a way to study the effect of ion beam s on diam ond film
deposition. This is a topic which needs to be studied in order to more fully
understand the film growth mechanism.
1.
gHb&tr.ate.p r eparatipn
The nucleation rate of diamond is enhanced by polishing the substrate
with diamond powder before the deposition7,8. This polishing process in effect
provides a num ber of nucleation sites on th e su b strate. T he su b stra te
preparation procedure is listed below.
*
S u b stra te : p-type Si(100). 0.7"x0.7"
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81
*
Rough polished with #600 sand paper.
*
Fine polished with l|J.m diamond powder.
*
Ultrasonically cleaned with acetone for 5 minutes.
*
Dried with high pressure in ert gas.
2.
Experim ental Procedure
The ECR plasm a system is shown in Fig. 2.13, and the basic operating
procedure is described in section B.4, chapter II. In diam ond th in film
deposition, a m agnetic beach configuration was adopted to effectively push the
plasm a toward the substrate. The deposition chamber was first evacuated
down to 7*10-7 Torr with a 1200 1/sec diffusion pump. The substrate was then
heated up. After which the reaction gases were introduced into the Lisitano
coil through the top flange and/or introduced into the dow nstream plasm a
colum n through a gas injecting ring. The flow rate s an d gas m ixture
composition were controlled by two electronic m ass flow controllers, while the
operating pressure was in turn controlled by the flow rates and the opening of
the gate valve. A DC bias was applied to the substrate before th e ECR plasma
CVD began. The experimental param eters are listed below :
*
substrate location: 5" -12" below the Lisitano coil
*
substrate tem perature : 400 - 1000°C
*
reaction gas mixture : 0.5 - 4% CH4/H2; 2% CO/H2 (pre-mixed);
*
Pressure : 1-10 mTorr
2% CH3OH/H2 (pre-mixed); 0.5 - 4% C2H2/H2
*
DC bias voltage : 0 - 40 V
*
Microwave pow er: 20 -160 W
An optical emission spectroscopy study was carried out during the
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82
deposition. The hydrogen Balmer line series H a , H p and Hy were detected in
th e m ethane/hydrogen plasm a. The m olecular hydrogen emission spectrum
was also detected. Structural inform ation on the films were obtained using
R am an Spectroscopy and X-ray diffraction.
3.
BfifflBajmd.Discussion
For Tsub < 900 C, black/brown color soft film s were deposited on the
substrates. These films were characterized as macrocrystalline graphite. For
^sub > 900 C, grayish color hard films were deposited on th e substrates. These
films were characterized as glassy carbon54,55. A R am an spectrum of glassy
carbon film is shown in Fig. 4.6.
Strong atomic hydrogen em ission spectra were detected in th e ECR
plasm a. This indicates th a t extensive dissociation of m olecular hydrogen as
well as excitation and ionization of atom ic hydrogen occurred in th e ECR
plasm a. The ion density is about 1011 cm '3. The atom ic hydrogen concentration
is expected to be much higher than th a t due to the hot-filam ent CVD process.
B ut growth of diamond film at P = 0.1 m Torr using hot-filam ent CVD has been
reported21. The major difference between ECR p lasm a and hot-filament CVD is
the existence of the ion beam. Therefore, the ion beam in the ECR plasm a m ay
have a negative effect on diamond form ation. The ion energy and ion beam
density can be controlled by a DC bias and th e m agnetic field profile n e a r the
substrate, e.g. m irror or cusp magnetic field. These in terestin g topics needs to
be studied. A nother m agnetic coil needs to be add ed onto th e c u rre n t
experim ental ap p a ra tu s to set up m irro r or cusp m agnetic field n e a r th e
substrate. The B field profiles due to a m agnetic m irro r configuration (m irror
or cusp m agnetic field) are shown in Fig. 4.7.
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Fig. 4.6
Count T im e(sec) = 0 .5 0
of Glassy Carbon Film.
D a te = 1 2 -1 3 -1 9 8 9
#1127
Raman Spectrum
F I 3DEC89\ARUN.DAT
83
o
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o_
o l
top 2 coils current = 40 A
bottom coil current = 4.25 A
No superposition
Superposition
O"
o-
00 -
o-
30
z (*0.135 inch)
Fig. 4.7
B field profiles due to a magnetic mirror configuration.
CHAPTER V
CONCLUSION
We have set up a Lisitano coil excited ECR plasma system in order to
perform plasm a assisted m aterials processing. A commercial microwave oven
was modified to serve as a microwave power supply. We developed a computer
program which can calculate the th ree dimensional magnetic field intensity
generated by a m agnetic coil. This calculation m atched the G auss m eter
m easurem ents very well. An optical emission spectral system was also set up.
Langm uir single probe m easurem ents indicated th a t Te » 3 eV and n e »
10u cm'3 in a ECR hydrogen plasma.
H ard DLC films were deposited through ECR plasm a decomposition of
CH 4 gas. Ion species gained energy from a magnetic beach configuration in
the plasm a column and a negative DC bias on the substrate. The hardness of
the films is strongly dependent on th e positive ion impinging energy and th e
substrate tem perature. De-hydrogenation process occurred in the gas phase as
well as during ion bombardment. Positive ion accumulation on th e deposited
in su la tin g film s lowered the negative bias voltage and m ay cause the
form ation of soft films.
H ard glassy carbon films and soft m icrocrystalline graphite films were
deposited through ECR plasm a decomposition of the reaction gas m ixture.
Optical emission spectroscopy study indicated th a t extensive dissociation of
m olecular hydrogen as well as excitation and ionization of atomic hydrogen
occurred in the ECR plasm a. The ion beam in the ECR plasm a m ay have a
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86
negative effect on the diamond formation. The ion energy and ion beam density
can be controlled by a DC bias together w ith a suitable magnetic field profile
near the substrate.
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87
APPENDIX I
Interaction of Electrons w ith Atomic & Molecular Hydrogen
In a n ECR microwave discharge, electrons can rapidly gain energy
from microwave fields while n eu tral molecules an d ions rem ain relatively
unaffected. High energy electrons th a t collide w ith n e u tra l m olecules can
break chemical bonds, excite and activate th e working gas an d so in itiate
chemical reactions a t or near room tem perature. Since atomic hydrogen plays
a very im portant role in a diam ond th in film deposition, th e interaction of
electrons with atomic and m olecular hydrogen is considered in th is section.
The reader is referred to Penetrante and K unhardt39 as well as Drukarev41 for
a thorough discussion.
A.
Interaction Processes
1.
e-H collision processes and cross sections
*
elastic scattering
e + H (ls) --> e + H (ls)
The elastic scattering cross section for e-H is shown in Fig. A l.l.
*
excitation and de-excitation
e + H(m) --> e + H(n)
m *n
Energy levels of an atomic hydrogen are shown in Fig, A1.2. Spectral
lines originated in transitions between energy levels of atomic hydrogen
are shown in Fig. Al.3.
*
ionization
e + H(n) - > 2e + H+
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Total 2s + 2p excitation cross section for H and ionization cross section
are shown in Fig. A1.4.
elastic scattering
e + HgCX) --> e + H2(X)
rotational excitation
vibrational excitation
e + H 2( v = 0 ) -> e + H 2( v = 1 ,2 ,3 )
The rotational and vibrational excitation cross sections in H 2 are shown
in Fig. A1.5 and A1.6.
electronic excitation into triplet states
e + H2 --> e + H2 (triplet states)
electronic excitation into singlet states
e + H2 --> e + H2 (singlet states)
The potential energy curves for trip let and singlet states in H2 are shown
in Fig. A1.7.
ionization
e + H2(X) --> 2e + H2+
dissociative attachm ent
e + H2(X) --> H + H '
The cross section of dissociative attachm ent in H 2 is shown in Fig. A1.8
and A1.9.
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89
*
dissociative recombination
e + H2+- > H + H
The m ost im portant energy region for the incident electron is 10"2 - 1 0 1
eV.
*
dissociation through electronic excitation
H2 (triplet states) --> H + H
The dissociation can proceed th ro u g h trip le t s ta te s since they
correspond to repulsive p o ten tial energy in H 2 (Fig. A1.7). The
dissociation cross section for H2 is shown in Fig. Al.10.
3.
Radiative processes
*
radiative recombination
e + H+ --> H(n) + hv
*
spontaneous emission of radiation
H(n) ~> H(m) + hv
*
absorption of radiation
hv + H(m) --> H(n)
B.
Plasma chemistry
Consider a simple model of a hydrogen plasm a consisting of H 2, H, H +
and electrons. Below we will discuss the processes which contribute to the
dynamic behavior of this system, i.e.
e
e
H2 <==> H <==> H+
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90
1.
H2<==>H
The production of H occurs through dissociation of H2
e + H2 --> e + H2 (triplet states)
H2 (triplet states) --> H + H
e + H2(X) --> H + H*
The loss of H occurs through recombination into H2
2H + H2 --> 2H2
3 H - > H + H2
H + H (wall) - > H2
2.
H <==> H+
Ionization is by electron collision with H atom a t any state.
e + H(m) --> 2e + H+
The loss of H + occurs through three-body recombination into any state,
2e + H+ - > e + H (n ),
and radiative recombination
e + H+ - > H(n) + hv
3.
Transitions of H atom between any pair of states
Electron impact
e + H(n) -> e + H(m)
Spontaneous emission of radiation
H(n) - > H(m) + hv
Absorption of radiation
hv + H(m) -> H(n)
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0
t
8
12
16
E.eV
Fig. A l .l e - H scattering. Full line : m atrix variational method; Dashed line :
close coupling m ethod; circles : polarized orbital m ethod (Sinfailam and
Nesbet, 1972); : Experim ental data.
6 .n a
^ 1.0
200
a
E ,e V
E,e V
Fig. A1.4 Total 2s + 2p excitation cross section for H (fig. a) and ionization cross
section (fig. b). 1. Born approxim ation; 2. Classical binary calculation; 3.
Experim ental data.
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92
free electron
energy, J
energy, eV
excited states
ground state
Fig. A1.2
Energy levels of an atomic hydrogen.
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93
= 5= 3
n= 2
limit
Lyman
series
Fig. A1.3
Balmer
series
Paschen
series
Brackett
series
Spectral lines originated in transitions between energy levels of
an atomic hydrogen.
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94
nr
E,eV
Fig. A1.5 Vibrational excitation cross section in H 2 (Schulz, 1973).
J§
Va
2
3
if
5
S
7
8
9
E,eV
Fig. A1.6
Total scattering cross section summ ed over all rotational and
vibrational final states. Solid line : theory (Drukarev and Yurova, 1977).
Dashed line : experiment (Golden et al., 1966).
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95
Fig. A1.7 Potential energy curves in H2 . 1 : triplet state; 2 : singlet state.
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EeV
Fig. A1.8 Dissociative attachm ent in H2, HD and D2 n e a r threshold (Schulz,
1973). 1. (H2)*1022 cm2; 2. (HD)*1023 cm2; 3. (D2)*1024 cm2.
0-8
0-4
E lectron energy (ev)
Fig. A1.9 Dissociative attachm ent in H2, HD and D2. Full curves : calculations
(Bardsley et al., 1966). Broken curves : experiment (Rapp et al., 1965).
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97
6
IQ
E,eV
Fig. A1.10
D isso cia tio n
cross
se ctio n
for
H2
( C a rtw rig h t
and
K upperm ann, 1967).
1. Experim ental d a ta for the total dissociation cross section including
th e ionization of a molecule.
2. Reduced experimental curve. Contribution of ionization subtracted.
3. Calculated cross section.
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98
APPENDIX II
Measurements of the Balmer T.inp Ratios EmitteH from
a Hydrogen Thyratron Discharge at High Current Densities
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Thirty-Ninth Annual Gaseous Electronics Conference
October 7-10,1986
Madison, Wisconsin
MEASUREMENTS OF THE BALMER LINE RATIOS EMITTED FROM
A DISCHARGE IN A HYDROGEN THYRATRON AT HIGH CURRENT
DENSITIES.
,
J . F u h r, Th. Aschw anden, B.M. P e n e t r a n t e S. Kuo
and E .E . K u n h ard t
WEBER RESEARCH INSTITUTE - POLYTECHNIC UNIVERSITY
OF NEW YORK
A
high
resolution
monochromator/photomultiplier system was used to deter­
mine the Intensity ratios of the Balmer lines Ha, H^ and Hj
emitted by atomic hydrogen In a thyratron discharge.
Current densities up to iooo\A/cm2 have been Investigated.
The line ratios H a/H^ H^/Hj.and Ha/Hj do not show a
significant dependence on current density. This is In agree­
m ent with a theoretical calculation of the relative line ratios
[l]. However, the absolute values of the measured ratios
are generally higher than the calculated ones. Time
resolved measurements of the light emitted at the
wavelength of the corresponding atomic states have been
carried out and comparison was made with the time depen­
dent current density in the discharge.
* Work supported by the D efense Nuclear Agency.
[l] B. M. Penetrante and E. E. Kunhardt, J. Appl. Phys.
59, 3 383 (1 9 8 6 ).
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Fig. 1: Schematic cross section of a thyratron.
a) theorie
b > experiment
ARC
✓IFVf?
Fig. 2: Typical current and voltage signals
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
P.F.N.
MC
Fig. 3: Experimental set-up.
H.V. dc power supply : Vdc = 0
PMT : Photomultiplier
Charging coil : 112 fin
MC
Charging resistance :-200 KOhm
Pulse forming network :
pulse width = 4 jus
characteristic impedence Zo *>
Ct - 0.3 uF
lit
- llpH
Trigger pulse amplitude ** 200 -
: Thyratron grid - pulse
A
: Thyratron anode
C-
: Thyratron cathode
i
CT
: Thyratron heating current
: Transformer monitor
i(t): Discharge current
C
R|. — 5.6 Ohm
: Monochromator
G
: Voltage divider
u(t): Discharge voltage
R
: Load resistance
H.V.: Power supply
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8j u s
to r th
c u r r e n t /•
Ce-ftecWon
Z tines ti> secies)
v o lta je .
c c /ty s e .
Time re s o lv e d c u r r e n t, v o lta g e and e m itte d l i g h t s ig n a l s
f o r h ig h power s w itc h in g .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1860 mV
p
• Ha: 656.3 nm
656.
172 mV
p.
H/?: 486.1 nm
485.8
iSRlte
433.8
l,M m
11.1
434.4
Fig. 5: Intensity line profiles.
I = 1.8 kA, J = 360 A/cm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
lin e s .
Balmer
the
of
ra tio s
=a-
00f
in te n s ity
0 0 ‘5
F ig .
6: M easured
OO-fj
X
o
o
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106
Rkl<n:rtMf!liS
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