Mechanical Reconstruction of an Industrial 915MHz Microwave Cavity Plasma Reactor System for Chemical Vapor Deposition Diamond Processes
©2003
Diplomarbeit
112 Seiten
Zusammenfassung
Inhaltsangabe:Zusammenfassung:
Synthetische Diamanten sind in Industrie und Wissenschaft sehr attraktiv, schaffen sie doch den Kompromiss zwischen einzigartigen Materialeigenschaften und geringen Anschaffungskosten. Mit der Verfeinerung von geeigneten CVD-Methoden und der intensiven Entwicklung von Reaktoren und Anlagen konnten die Herstellungskosten weiter gesenkt und vor allem enorme Fortschritte bei den möglichen Anwendungen und Abmessungen erzielt werden.
Diese Diplomarbeit beschäftigt sich mit dem Wiederaufbau und der Integration einer unbekannten industriellen 915MHz MCPR-Anlage (microwave cavity plasma reactor), für die Herstellung von CVD-Diamanten. Dabei wird umfangreich über den Stand der Technik bei relevanten CVD-Reaktoren, Anwendungsmöglichkeiten, Synthese und Materialeigenschaften von synthetischen Diamanten eingegangen. Weiterhin wird der Aufbau und die Wirkungsweise der MCPR-Reaktoranlage beschrieben.
Neben der Analyse der Ausgangsbedingungen wird auf die Umsetzung der Teilprobleme wie den Wiederaufbau des Mikrowellensystems, Vakuumanlage, Prozessgassystem und das Wasserkühlsystem eingegangen, wobei spezifische Probleme (Design, Funktion, Fehler, notwendige Änderungen) analysiert und Lösungen besprochen werden. Dabei werden Dimensionierungen von Kühlleistungen, Gasbedarfe (Prozessgase) und Einstellungen bei unbekannten Systemeigenschaften beschrieben. Weiterhin werden Dimensionierung und Auswahl von einem Kühlaggregat und Gaskühlströmen, die Konstruktion und Dimensionierung einer Hebevorrichtung und Kammergrößenskala und adäquate Systemparameterwahl erläutert, wobei auf jeweilige (un-) bekannte Randbedingungen eingegangen werden. Mit der Entscheidung von geeigneten Methoden (Helium-Leck-Test, Mikrowellenstrahlung u.a.) wurden die Teilsysteme auf Funktion und Sicherheit überprüft. Mit geeigneten Berechnungen konnten notwendige Reinheiten im Vakuumbereich (Leckratenbeurteilung, Prozessgaswechsel) erwiesen werden.
Abschließend werden Funktionstests und Auswirkungen auf den gewählten Aufbau der Anlage beschrieben und Ausblicke für weitere Modifikationen und Verbesserungen gemacht.
Die Arbeit zeigt mit 29 Abbildungen, 10 Tabellen und 24 Anlagen (Skizzen, Tabellen, u.a.) unterschiedliche Problemlösungen beim Wiederaufbau der Reaktoranlage.
Inhaltsverzeichnis:Table of Contents:
Assignment (Aufgabenstellung)ii
Bibliographical Delineation (Bibliographische Beschreibung und Referat)iii
Declaration […]
Synthetische Diamanten sind in Industrie und Wissenschaft sehr attraktiv, schaffen sie doch den Kompromiss zwischen einzigartigen Materialeigenschaften und geringen Anschaffungskosten. Mit der Verfeinerung von geeigneten CVD-Methoden und der intensiven Entwicklung von Reaktoren und Anlagen konnten die Herstellungskosten weiter gesenkt und vor allem enorme Fortschritte bei den möglichen Anwendungen und Abmessungen erzielt werden.
Diese Diplomarbeit beschäftigt sich mit dem Wiederaufbau und der Integration einer unbekannten industriellen 915MHz MCPR-Anlage (microwave cavity plasma reactor), für die Herstellung von CVD-Diamanten. Dabei wird umfangreich über den Stand der Technik bei relevanten CVD-Reaktoren, Anwendungsmöglichkeiten, Synthese und Materialeigenschaften von synthetischen Diamanten eingegangen. Weiterhin wird der Aufbau und die Wirkungsweise der MCPR-Reaktoranlage beschrieben.
Neben der Analyse der Ausgangsbedingungen wird auf die Umsetzung der Teilprobleme wie den Wiederaufbau des Mikrowellensystems, Vakuumanlage, Prozessgassystem und das Wasserkühlsystem eingegangen, wobei spezifische Probleme (Design, Funktion, Fehler, notwendige Änderungen) analysiert und Lösungen besprochen werden. Dabei werden Dimensionierungen von Kühlleistungen, Gasbedarfe (Prozessgase) und Einstellungen bei unbekannten Systemeigenschaften beschrieben. Weiterhin werden Dimensionierung und Auswahl von einem Kühlaggregat und Gaskühlströmen, die Konstruktion und Dimensionierung einer Hebevorrichtung und Kammergrößenskala und adäquate Systemparameterwahl erläutert, wobei auf jeweilige (un-) bekannte Randbedingungen eingegangen werden. Mit der Entscheidung von geeigneten Methoden (Helium-Leck-Test, Mikrowellenstrahlung u.a.) wurden die Teilsysteme auf Funktion und Sicherheit überprüft. Mit geeigneten Berechnungen konnten notwendige Reinheiten im Vakuumbereich (Leckratenbeurteilung, Prozessgaswechsel) erwiesen werden.
Abschließend werden Funktionstests und Auswirkungen auf den gewählten Aufbau der Anlage beschrieben und Ausblicke für weitere Modifikationen und Verbesserungen gemacht.
Die Arbeit zeigt mit 29 Abbildungen, 10 Tabellen und 24 Anlagen (Skizzen, Tabellen, u.a.) unterschiedliche Problemlösungen beim Wiederaufbau der Reaktoranlage.
Inhaltsverzeichnis:Table of Contents:
Assignment (Aufgabenstellung)ii
Bibliographical Delineation (Bibliographische Beschreibung und Referat)iii
Declaration […]
Leseprobe
Inhaltsverzeichnis
ID 7588
Fricke, Jörg: Mechanical Reconstruction of an Industrial 915MHz Microwave Cavity
Plasma Reactor System for Chemical Vapor Deposition Diamond Processes
Hamburg: Diplomica GmbH, 2004
Zugl.: Technische Fachhochschule Wildau, Technische Fachhochschule, Diplomarbeit,
2003
Dieses Werk ist urheberrechtlich geschützt. Die dadurch begründeten Rechte,
insbesondere die der Übersetzung, des Nachdrucks, des Vortrags, der Entnahme von
Abbildungen und Tabellen, der Funksendung, der Mikroverfilmung oder der
Vervielfältigung auf anderen Wegen und der Speicherung in Datenverarbeitungsanlagen,
bleiben, auch bei nur auszugsweiser Verwertung, vorbehalten. Eine Vervielfältigung
dieses Werkes oder von Teilen dieses Werkes ist auch im Einzelfall nur in den Grenzen
der gesetzlichen Bestimmungen des Urheberrechtsgesetzes der Bundesrepublik
Deutschland in der jeweils geltenden Fassung zulässig. Sie ist grundsätzlich
vergütungspflichtig. Zuwiderhandlungen unterliegen den Strafbestimmungen des
Urheberrechtes.
Die Wiedergabe von Gebrauchsnamen, Handelsnamen, Warenbezeichnungen usw. in
diesem Werk berechtigt auch ohne besondere Kennzeichnung nicht zu der Annahme,
dass solche Namen im Sinne der Warenzeichen- und Markenschutz-Gesetzgebung als frei
zu betrachten wären und daher von jedermann benutzt werden dürften.
Die Informationen in diesem Werk wurden mit Sorgfalt erarbeitet. Dennoch können
Fehler nicht vollständig ausgeschlossen werden, und die Diplomarbeiten Agentur, die
Autoren oder Übersetzer übernehmen keine juristische Verantwortung oder irgendeine
Haftung für evtl. verbliebene fehlerhafte Angaben und deren Folgen.
Diplomica GmbH
http://www.diplom.de, Hamburg 2004
Printed in Germany
Contents
Assignment (Aufgabenstellung)
ii
Bibliographical Delineation (Bibliographische Beschreibung und Referat)
iii
Declaration (Selbst¨
andigkeitserkl¨
arung)
iv
List of Symbols and Abbreviations
1
1
Introduction
3
2
Diamonds and Chemical Vapor Deposition
5
2.1
Morphology and Properties of Diamond . . . . . . . . . . . . . . . . . . . . .
5
2.2
Overview of Diamond Applications . . . . . . . . . . . . . . . . . . . . . . . .
7
2.3
Chemical Vapor Deposition of Diamonds . . . . . . . . . . . . . . . . . . . . .
8
2.4
Methods for CVD-Diamond Production . . . . . . . . . . . . . . . . . . . . .
11
3
Microwave Cavity Plasma Reactor 915 MHz System
14
3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
3.2
General Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
3.3
MCPR Subsystems and Operation Breakdown
. . . . . . . . . . . . . . . . .
15
3.3.1
The Microwave Cavity Reactor . . . . . . . . . . . . . . . . . . . . . .
15
3.3.2
Microwave Power Supply and Waveguide-Transmission Subsystem . .
17
3.3.3
Gas Flow Control and Vacuum Pump Subsystem . . . . . . . . . . . .
18
3.3.4
The Computer Monitoring and Control Subsystem . . . . . . . . . . .
19
4
System Analysis and Rebuild Planning
20
4.1
System Inventory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
4.2
Rebuild Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
v
Contents
vi
5
Mechanical Setup and Function Control
25
5.1
Maintenance and Match of the Microwave System
. . . . . . . . . . . . . . .
25
5.1.1
Microwave Components Condition Control
. . . . . . . . . . . . . . .
25
5.1.2
Applicator Mount and Lifting Rig . . . . . . . . . . . . . . . . . . . .
26
5.1.3
Applicator Purge Modification . . . . . . . . . . . . . . . . . . . . . .
29
5.1.4
Applicator Adjustments and Reading Device . . . . . . . . . . . . . .
30
5.1.5
Microwave Component Check and Leak Test . . . . . . . . . . . . . .
32
5.2
Design, Reconstruction and Modifications of the Gas System
. . . . . . . . .
33
5.2.1
Process Gas Breakdown and Source Choice . . . . . . . . . . . . . . .
33
5.2.2
Source Gas Dimensioning . . . . . . . . . . . . . . . . . . . . . . . . .
34
5.2.3
General Gas System Structure Design . . . . . . . . . . . . . . . . . .
36
5.2.4
Source Line Design and Safety Precautions . . . . . . . . . . . . . . .
38
5.2.5
Exhaust and Applicator Purge Cycles and Adjustments . . . . . . . .
39
5.2.6
Source Cylinder Exchange Procedures . . . . . . . . . . . . . . . . . .
41
5.3
Vacuum System Set Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
5.3.1
Chamber Dome Structure and System Preparation . . . . . . . . . . .
42
5.3.2
Vacuum System Leak Test . . . . . . . . . . . . . . . . . . . . . . . . .
43
5.3.3
Calculation and Evaluation of the Vacuum System Leak . . . . . . . .
45
5.4
Design and Reconstruction of the Water Cooling System
. . . . . . . . . . .
47
5.4.1
Water System Condition Control and Base Plate Ring Maintenance
.
47
5.4.2
Applicator Purge Heat Exchanger Evaluation . . . . . . . . . . . . . .
49
5.4.3
Chiller Dimensioning and Choice . . . . . . . . . . . . . . . . . . . . .
50
5.4.4
Water Cooling System Adjustments and Monitoring . . . . . . . . . .
51
6
MCPR 915 MHz Handling and First Deposition Run
54
7
Prospects and Improvements
56
7.1
General Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
7.2
Mass Flow Controlled Exhaust Purge
. . . . . . . . . . . . . . . . . . . . . .
56
7.3
Diffusion Pump for Vacuum Purity . . . . . . . . . . . . . . . . . . . . . . . .
57
7.4
Temperature and Temperature Distribution at the Quartz Bell Jar . . . . . .
58
7.5
System Necessity for NanoCrystalline Diamond Deposition . . . . . . . . . .
58
7.6
Decision on the Cooling Medium for the Applicator Purge . . . . . . . . . . .
59
8
Conclusion
60
Bibliography
62
Technische Fachhochschule
Wildau
November 2003
Contents
vii
List of Figures
64
List of Tables
66
List of Appendixes
67
Appendix
Technische Fachhochschule
Wildau
November 2003
List of Symbols and Abbreviations
Item
Description
Unit
a
cavity radius
cm
A
s
shearing area
mm
2
c
speed of light
m s
-1
c
H
2
O
specific heat capacity
kJ kg
-1
K
-1
D
m
thread major diameter
inch
D
p
thread pitch diameter
inch
F
t
tensile stress
N
f
0
frequency
Hz, s
-1
h
thread hight at pitch diameter
inch
L
C
cavity length
m, mm
L
P
coupling probe length
m, mm
m
a
applicator weight
lb
m
a
mass of air
kg
m
p
process gas mass flow
kg s
-1
m
u
useful gas mass flow
kg s
-1
n
t
number of theoretically stressed threads
-
p
thread pitch
inch
P
power
W
p
e
pressure at cylinder exchange
psi
p
s
pressure at full cylinders
psi
p
0
pressure at standard conditions (760 Torr)
Torr
Q
heat flow
J s
-1
, kJ s
-1
R
m
tensile strength
N mm
-2
R
s
specific gas constant
J kg
-1
K
-1
R
z
Ten Point Hight
µm
T
Temperature
C, K
1
List of Symbols and Abbreviations
2
t
p
pressure rising test period
s
t
w
cylinder operation time
s
T
0
temperature at standard conditions (0
C)
K; C
V
C
chamber volume
m
3
V
C
cylinder volume
m
3
v
L
volume flow rate of leak
sccm
v
max
maximum process gas volume flow
m
3
s
-1
x
np
np 'th zeros of the Bessel function
-
0
permittivity (dialectric constant)
A s V
-1
m
-1
PS
degree of efficiency for the microwave power supply
wave length
m
µ
0
permeability
V s A
-1
m
-1
b al
allowed maximum shear stress
N/mm
2
Abbreviations
Description
amu
atomic mass unit
AR
anti reflection
CVD
chemical vapor deposition
gal
gallon
HFCVD
hot filament chemical vapor deposition
HPHT
high pressure high temperature
IR
infrared
MCPR
microwave cavity plasma reactor
MCVD
microwave (enhanced) chemical vapor deposition
MFC
mass flow controller
MFM
mass flow meter
MFT
mass flow table
MPCVD
microwave plasma chemical vapor deposition
NPT
national pipe thread
RF
radio frequency
sccm
standard cubic centimeter per minute
TM
transverse magnetic
UNF
unified national fine thread
VCB
valve card board
Technische Fachhochschule
Wildau
November 2003
1 Introduction
Fraunhofer USA Center for Coatings and Laser Applications (CCL) is working in the field of
coatings and laser applications. Up to now, the centers coating division focused on coating
technologies such as physical vapor deposition and laser enhanced coatings. A new coopera-
tion with the Electrical and Computer Engineering Department (ECE) of the Michigan State
University resulted in the relocation from Peoria, Illinois into its Engineering Research Com-
plex in East Lansing, Michigan. The laboratory of this new partnership are devoted to the
field of synthesized diamonds via microwave enhanced chemical vapor deposition (MCVD)
and is equipped with different experimental CVD-reactors. With a disassembled industrial
microwave-CVD reactor system on site new vistas opened up for newly formed partnership
in the field of diamond synthesis via chemical vapor deposition.
Superior material properties of diamond offer a wide range of applications in research and
industry, especially for optical grade diamond applications. Furthermore, the technology of
microwave enhanced chemical vapor deposition constitutes a method of fabrication of com-
mercial competitive diamond products. These are only few reasons, so that Fraunhofer USA
CCL and the Michigan State University decided to rebuild this system for diamond coating
research and developments.
The Microwave Cavity Plasma Reactor (MCPR) 915 MHz was built in 1993-94 by Wavemat,
Inc. for industrial production of large diamond wafers and coating insert cutting tools. The
system represents a prototype, which was at the time the biggest of its kind worldwide. For
economical reasons the system was not operated for long and Wavemat, Inc. did produce
only one other similar to this type, which is not ready for use and operating. Therefore,
the MCPR 915 MHz represents a unique machine and its operating characteristics are hardly
known at the university and Fraunhofer.
Rebuilding and restarting the system after a long system shut down requires several tasks,
including the mechanical set-up and the modernization of the systems controller and the con-
trol software. This thesis describes the planning and execution of the mechanical set-up of
the disassembled system. The set-up focuses on the projected deposition of thick polycrys-
talline diamond films for optical applications. Therefore, a gas source system was designed
and combined with the existing gas system on the CVD machine. After the gas system was
3
4
completely the machine was tested for leaks. Also, a new chiller was dimensioned and the
water cooling system had to be rebuilt. The microwave system including the microwave power
generator and other mechanical components had to be prepared and checked.
The behavior and requirements of the system were unknown and obtained data or expe-
riences by the former operating company Norton Diamond Film were not available. Due
to an expected different plasma behavior in a large scale CVD system only marginal possi-
bilities of comparison to smaller experimental CVD-systems could be made. That is why,
decisions by experiences, estimations or calculations for reconstruction tasks were made and
are described in this thesis. Required and executed modifications to current circumstances
are shown. Furthermore, a literature research about microwave-CVD and synthesized poly-
crystalline diamonds is given at the beginning of this thesis. Finally, changes and suggestions
for further improvements of the systems capabilities are discussed.
Drawings of the current water cooling system and gas system are attached. Parameters
recorded during the first deposition runs are listed. Instructions on how to start and shut down
the system and changing gas cylinders were prepared. Furthermore, adjustments made are de-
scribed and listed, so that this thesis is the first document with details to the MCPR 915 MHz
handling and characteristics, utilizable and assistant for Fraunhofer USA CCL.
Throughout the text, the physical units do not always follow the SI-units (Syst`
eme Inter-
national d'unit´
es). This is unconventional, but due to the use of different unit systems in
the United States of America conversion into the SI system were not practical. Additionally,
some measurement devices in this microwave-CVD system, spare part distributors and work
shops work with different physical units. Because of the practical background of this thesis,
the original units are used and a conversion table to SI-units is given in the appendix.
Technische Fachhochschule
Wildau
November 2003
2 Diamonds and Chemical Vapor Deposition
2.1 Morphology and Properties of Diamond
There are two materials consisting solely of carbon, diamond and graphite. Yet, the two
materials have very different properties. That is based only on the different carbon structure.
The graphite lattice is composed of planar sheets of relatively strong-bonded six-member
carbon rings. The diamond structure though, consists of a lattice of tetrahedrally-bonded
carbon atoms. Both structures are shown in Figure 2.1. In graphite the bond strength between
Figure 2.1:
Carbon lattice of diamond and graphite
carbon atoms in the sheets is stronger than between the layers. That leads to the structure
with lubricant characteristics. The diamond atoms are covalently bonded to a strong lattice
in all directions. This different is caused by the hybridization type (arrangement of electrons
in the electron shells), sp
2
for graphite and sp
3
for diamond.
With the special structure diamond possesses remarkable physical properties, yet it is very
rare in nature. A selection of some properties and their values is given in Table 2.1. A
popular property is the hardness which has the highest value of all materials. Also diamond
has the highest thermal conductivity at room temperature, is the stiffest material, is least
5
2.1. MORPHOLOGY AND PROPERTIES OF DIAMOND
6
compressible and has an outstanding chemical inertness.
Table 2.1:
A selection of Diamond properties
Property
Value
mechanical hardness
ca. 90 GPa
compressibility
8.3
· 10
-13
m
2
N
-1
thermal conductivity (at room temperature)
2.0
· 10
3
W m
-1
K
-1
thermal expansion coefficient (at room temperature)
1.0
· 10
-6
K
Young's modulus
900 - 1050 GPa
optical transparency from the deep ultra violet to the far infrared
-
absorption coefficient (at wavelength 10.6 µm)
0.03 - 0.1 cm
1
refractive index (at wavelength 10.6 µm)
2.38
high resistivity (insulator)
breakdown voltage
ca. 10
7
Vm
-1
The optical properties are among the most interesting of the physical properties of diamond.
They include the widest spectral optical transmission range of all solid materials from the
deep ultra violet to the far infrared. This range starts at 225 nm and only a higher absorption
band between 2.5 to 7 µm wavelength disturbs the perfection of this property in the infrared
region. The value for transmission above 7 µm is about 71% . Also, for a transparent material
diamond has an unusually high index of refraction.
The very best synthetic optical diamond samples show many optical properties of natural
diamonds, yet their polycrystalline structure which is the most used in optical applications
can lead to scattering losses. The polycrystalline structure consists of many small crystals
grown together to form a film or bulk diamond. The concomitant grain boundaries raise the
issue of optical scattering. For transparent synthetic diamond windows of high quality an
optical scattering of 0.15 % at 10.6 µm wavelength (2.5
- 70
measuring angle) [1] was shown.
The major part of transmission losses is due to reflection. At present it is possible to
create anti-reflection (AR) coatings to increase the transmission. The unique properties of
diamond prefer the use in harsh environments thus limiting the range of coatings applicable.
Yttrium oxide (Y
2
O
3
) films have been successfully applied as anti-reflection coatings with
transmittance-values of about 90% at 10.6 µm [1]. Another methode to decrease reflection is
the creation of certain surface structures via laser treatment.
Technische Fachhochschule
Wildau
November 2003
2.2. OVERVIEW OF DIAMOND APPLICATIONS
7
2.2 Overview of Diamond Applications
Todays technological progress would not be possible without materials with outstanding prop-
erties. Today, highest mechanical, thermal and chemical resistances are often required at the
same time as well as high performance optical quality e.g. precision and little losses in certain
optical applications. The unique attributes of diamond make it an obviously useful material
for industry and science.
With its high hardness diamond is finding applications as abrasive and as coating on cutting
tool inserts, for example coated drills, reamers and countersinks to increase the life time and
cutting speed as well as providing a better finish. The creation of extremely sharp cutting
edges and very low-friction surfaces allow the machining of very sensitive parts like lenses in
the optical industry. Parts of gearboxes, engines, transmissions and so forth are coated for
a wear resistant finish. The high electrical resistivity (electrical insulator), high saturated
current velocity and high thermal conductivity make diamond an excellent semiconductor
material for high-power electronics, high temperature electronics and smart sensors in harsh
environments. Finally, the rapid progress in the computer technology would not have been
realized without diamond in electronic devices.
In optical applications the high hardness, high transmission at appropriate wavelength, high
thermal conductivity and chemical resistance make diamond a material of obvious interests.
A unique example represents the optical port window for an infrared radiometer experiment
on the Pioneer Venus probe in 1978. This 18.2 mm diameter window with a thickness of
2.8 mm resisted earth atmosphere on launch, the cold and vacuum of space and the Venus
atmosphere consisting partly of various acids (including sulfuric acid) at temperatures of
800 K and pressures of ca. 9.12
· 10
6
Pa [2].
In industry high power lasers such as carbon dioxide lasers are used mainly for material and
medical processing. Optical power with continuous waves of up to 10 kW are used for cutting
or welding of metal. This high optical power requires special properties on the output coupler
window and other optical components of the laser used to form the laser beam, e.g. focusing.
Until today the common material for these components is zinc selenide (ZnSe) with a very low
absorption coefficient of 0.0005 cm
-1
at the wavelength of 10.6 µm[1], the specific one in CO
2
lasers. However, because of the high optical powers, heat will be generated, which can lead to
thermal lensing and crack the ZnSe material. That is why, zinc selenide is the limiting factor
for the laser power. Diamond has a higher absorption coefficient (see tabel 2.1), but with the
high thermal conductivity diamond has clearly a better potential to withstand high optical
powers. With the chemical resistance and the high hardness diamond optical components can
also be used in harsh environments. A diamond window for CO
2
lasers and a radio frequency
Technische Fachhochschule
Wildau
November 2003
2.3. CHEMICAL VAPOR DEPOSITION OF DIAMONDS
8
(RF) window are shown in Figure 2.2 [3].
Figure 2.2:
Diamond application samples (a) radio frequency (RF) window 90 mm in diameter
mounted on a double flange assembly; (b) mounted laser exit window for a 10.6 µm CO
2
laser
In addition to laser components diamond is becoming more and more attractive in a wide
range of optical applications. Some of these are listed below.
Infrared windows of airborne IR-sensors
X-ray windows and X-ray lithography mask membranes
High power microwave windows
Miniature endoscope windows in the medicine
Windows for process control in harsh enviroments
High vacuum windows
Synchrotron windows
The notorious high costs and the limited size of natural diamond has been a significant barrier
for a more widespread use for a long time. Many applications especially optical components
are only possible using synthesized polycrystalline diamonds in larger sizes than natural ones
can possess.
2.3 Chemical Vapor Deposition of Diamonds
The structure of diamond has been known for a long time. Since graphite is the thermo-
dynamically stable allotrope it is extremely difficult to synthesize artificial diamond. To
overcome this problem researchers realized that conditions are needed where diamond is the
more stable phase.
Technische Fachhochschule
Wildau
November 2003
2.3. CHEMICAL VAPOR DEPOSITION OF DIAMONDS
9
Today three procedures producing pure diamond are known:
1. mined natural diamond
2. high pressure high temperature synthesis
3. chemical vapor deposition
Naturally, diamond is basically created by heating carbon under high pressure. This process
forms the basis for the high-pressure high-temperature (HPHT) technique which has been
marked by General Electric and has been used to produce industrial diamonds for several
decades. In this process graphite is compressed in a hydraulic press to tens of thousands of
bars, heated to over 2000 K in the presence of a suitable metal catalyst and left until diamond
crystallizes. However, HPHT synthesis just duplicates the natural HPHT process with the
same result of size ranges from nanometers to millimeters in the form of single crystals.
There is another perspective regarding the formation of diamond. In 1958 Eversole and
Deryagin carried out an experiment in which thermal decomposition carbon-containing gases
under reduced pressure were used to grow diamond on the surface of natural diamond crystals
heated to 900
C. This first experiment was a result of the idea to produce diamond by
Figure 2.3:
Phase diagram of carbon, showing regions utilized for synthetic diamond growth by HPHT
and CVD techniques
adding carbon atoms one-at-a-time to an initial template, in such a way that a tetrahedrally
bonded carbon network results. This experiment has been used the process of chemical vapor
Technische Fachhochschule
Wildau
November 2003
2.3. CHEMICAL VAPOR DEPOSITION OF DIAMONDS
10
deposition (CVD), basically defined as a material processing technology via chemical reaction
from the gas or vapor phase. The chemical reaction needs activation energy like resistance
heat or electric discharge. CVD is widely used, for example thin film coatings, high purity bulk
materials or powders and nearly all materials (pure or in compounds) have been deposited by
CVD. In terms of diamond, with CVD it is possible to synthesize diamond under conditions of
lower pressure and temperature. Figure 2.3 shows the phase diagram of carbon with regions
utilized for synthetic diamond growth by HPHT and CVD techniques [4].
The chemical and physical processes during diamond CVD are illustrated in Figure 2.4
[5] with the reactivation via hot filament. The mixed process gases (in this case methane
and hydrogen) which are inserted into the chamber pass an activation region, which provides
energy to the gaseous species. The activation causes molecules to fragment into reactive
radicals and atoms, creates ions and electrons, and heats the gas up to temperatures of a
few thousand Kelvins. After the activation region, these fragments undergo a complex set
of chemical reactions until they strike the substrate surface. If all conditions are suitable
diamond starts growing. The CVD synthesis for high purity diamonds works basically with a
gases in
H
2
CH
4
H
2
H
2
H
2
CH
4
H
2
H
2
H
H
H
H
H
H
H
H
H
2
CH
4
CH
3
CH
CH
CH
2
H
2
H
2
filament
flow and
reaction
reactants
activation
free radicals
diffusion layer
substrate
H
Figure 2.4:
Schematic representation of the physical and chemical processes occurring during dia-
mond CVD
carbon source gas like methane (CH
4
), carbon oxide (CO) or acetylene (C
2
H
2
) and hydrogen
(H
2
). In the CVD process graphite will be co-deposited with diamond leading to impure
mixed phases. That is why H
2
is added during deposition, leading to preferential etching of
the graphite, rather than diamond. Also hydrogen stabilizes the diamond surface (prevention
Technische Fachhochschule
Wildau
November 2003
2.4. METHODS FOR CVD-DIAMOND PRODUCTION
11
of graphite deposition), and generates carbon radicals and ions for the carbon deposition. To
ensure diamond growth faster than graphite the carbon source gas, e.g. CH
4
is diluted in a
excess of hydrogen in a typical mixing ratio of about 1% volume. Also, the temperature of
the substrate is usually between 700 and 1000
C [5][6] to ensure the formation of diamond
rather than amorphous carbon.
2.4 Methods for CVD-Diamond Production
In the past few years the potential applications of diamond and diamond films have stimulated
a great deal of research activity in various synthesis technologies. Basically all methods require
means to create an activated gas phase carbon-containing molecules. This activation source
might:
1. thermal methods (e.g. hot filament)
2. electric discharge (e.g. radio frequency or microwave)
3. combustion flame (e.g. oxyacetylene torch)
Figure 2.5 [5] illustrates hot filament CVD, microwave plasma assisted CVD and arc jet CVD,
the more common methods of CVD-diamond production. Hot filament CVD (Figure a) uses
a vacuum chamber with a typical gas mixture of 1% CH
4
in H
2
and a total process gas flow of
500-1000 sccm (standard cubic centimeter per minute). Throttle valves maintain the chamber
pressure to 1-80 Torr, while a substrate heater is used to bring the substrate temperature up to
600-1200
C [7]. The substrate to be coated, typical Si- or Mo sits a few millimeters beneath
the filament, which is electrically heated to temperatures of about 2200
C. The filament
material is usualy tungsten or tantalum and its temperature serves the chemical reaction
source. The Hot Filament CVD (HFCVD) is relatively cheap and it is possible to shape the
process for three-dimensional substrate coatings, for example tool coatings. This methode is
useful to produce reasonable quality polycrystalline diamonds of growth rates of 1-10 µm/h,
but there are also some disadvantages. The hot filament is particulary sensitive for oxidizing.
This limits the process gas selection. It is also very difficult to prevent contaminations of the
diamond film with filament material which leads to reduced diamond quality.
Plasma Jet, Arc Jet or Plasma Torch are promising alternatives of diamond CVD. The most
commonly used plasma jet is the DC (direct current) Arc Jet (see Figure d). In this method
gases with a relatively high flow rate (few thousands sccm) passes a high power electrical
discharge to form a jet of ionized particles, atoms and radicals. In the process chamber the
jet strike the substrate to be coated at a high velocity. The DC plasma jet method is a
Technische Fachhochschule
Wildau
November 2003
2.4. METHODS FOR CVD-DIAMOND PRODUCTION
12
(a)
filament
heater
pump
process
gases
process
gases
substrate
(b)
microwave
generator
waveguide
pump
electrodes
+
-
discharge
plasma jet
water
cooling
process gases
plasma
tuner
substrate
heater
(c)
(d )
microwave
generator
waveguide
pump
plasma
quartz
window
tuning
antenna
substrate
heater
Figure 2.5:
Examples of some of the more common types of low pressure CVD reactor. (a) Hot
filament, (b) NIRIM-type microwave plasma reactor, (c) ASTEX-type microwave plasma
reactor, and (d) DC arc jet (plasma torch)
high pressure apparatus with working pressures of 100-760 Torr [5]. The main features of this
process is the high growth rates, typically greater than 100 µm/h, the highest reported growth
rate is 930 µm/h [7] [5]. Yet, the main drawback of such high power high rate system is the
substrate cooling, since maintaining uniform substrate temperature is difficult. Furthermore,
the thermal shock, when the jet is ignited and extinguished prevents the use of a variety of
substrates. Silicon often shatters, so usually Mo is used as substrate material.
Microwave Plasma CVD (MPCVD) reactors are now among the widely used techniques for
diamond production and works under similar conditions as HFCVD. In a microwave reactor
microwave power is coupled into the chamber through a dielectric window, usually made of
quartz, in order to create a discharge. The microwaves couple their energy into the gas phase
electrons, they are accelerate and collide with the gas molecules. The gas phase molecules
Technische Fachhochschule
Wildau
November 2003
2.4. METHODS FOR CVD-DIAMOND PRODUCTION
13
will be heated and format active species like radicals, ions and more electrons which lead
to some chemical reactions and finally to a deposition on a pre-treated (e.g. grinding with
diamond powder) substrate. The two most common types of microwave plasma reactors are
the NIRIM-type reactor (Figure b) and the ASTEX-type reactor (Figure c).
In 1983 Japanese researchers at the National Institute for Research in Inorganic Materi-
als (NIRIM) developed a new microwave plasma reactor, called NIRIM-type reactor. The
NIRIM-type reactor consists of a quartz or silicon tube which is rectangularly arranged to
the waveguide appropriate for the propagation of the microwaves. The arrangement is such
that the electric field maximum is centered in the middle of the tube and the exact position
can be adjusted by using tuner. The major advantages of the NIRIM are the simple reactor
design with low set-up costs, easy and cheap replacements of damaged reactor tubes and a
possible variation of the substrate position relative to the plasma for better process tuning.
Unfortunately the NIRIM reactor has also disadvantages such as substrate size limits of ap-
proximately 1-2 cm
2
[1], too small for industrial applications. In addition to that the plasma
is generated very close to the reactor wall, which leads to etching and redeposition of the tube
material, the reason for contaminations of the growing film. For NIRIM-type reactors, the
microwave power level are no more than 1.5 kW at pressures of < 80 Torr resulting in growth
rates of 0.5-3 µm/h [8].
The other common reactor was designed in 1988 by
Bachmann and was then commercial-
ized by Applied Science and Technology, Inc. (ASTEX) a commercial manufacturer of plasma
systems based in the US. In the ASTEX-type reactor the microwaves are coupled into a water
cooled metal cavity through a quartz window. An antenna is used to convert the microwave
waveguide mode to a reasonable cavity mode to create a resonating electromagnetic field for
the plasma discharge. The substrate sits on top of a heated stage beneath the plasma ball.
The microwave power of up to 5 kW increases the growth rates to 4-14 µm/h [8] and substrates
as large as 10 cm diameter [1][8] can be coated in such reactors. Other advantages are the
wide range of process gases that can be used for the process, especially high concentrations of
oxygen (improves optical transmission in the visible spectrum). As well as under well defined
deposition conditions (like pressure and microwave power), these systems run very stable and
can deposit diamond with very high quality continuously for several days to allow thicknesses
of 0.5-1 mm.
When contamination free high quality diamonds are required, microwave plasma CVD is
a suitable method. Especially in optical and electronic applications MPCVD has been prove
for a long time and has found a lot of success.
Technische Fachhochschule
Wildau
November 2003
3 Microwave Cavity Plasma Reactor 915 MHz
System
3.1 Introduction
Many different Microwave Plasma Reactors for diamond deposition are used in science and
industry for various research and commercial purposes. Starting in 1986 and continuing
through 1997 the Michigan State University and Wavemat Inc. developed in collaboration
with Norton Company a series of Microwave Cavity Plasma Reactors (MCPR). This reactor
is also equipped with water cooled cavity walls, like the ASTEX-type reactor, but different
in the shape of the cavity and the discharge region (quartz bell jar). One object of effort was
Figure 3.1:
Reconstructed Microwave Plasma Reactor System 915 MHz
to build a system for the production of high purity diamonds in large sizes up to 7-10 inch
(2-3 inch are used often). In addition, thicknesses of some microns up to 1 mm and more for
prolific manufacturing of diamond films for a variety of applications includes cutting tools,
wear surfaces and optical parts. The MCPR 915 MHz prototype, shown in Figure 3.1, was
14
3.2. GENERAL STRUCTURE
15
built in 1993-94 for Norton Diamond Film and was operated until the Norton Company closed
its diamond thin film division. The MCPR 915 MHz system was brought to the Engineering
Research Complex at the Michigan State University in 2000 and several years later Fraunhofer
USA CCL and the university Department of Electrical and Computer Engineering (ECE)
resolved to rebuild the system for research and development of polycrystalline diamond films
for optical applications.
A short explanation of the structure and operation principle for the MCPR 915 MHz system
is given in this chapter.
3.2 General Structure
The generic structure of the MCPR 915 MHz system is shown in Figure 3.2. The subsystems
include the gas flow rate control system, the vacuum pumping and pressure control system,
the microwave power supply with waveguide and the discharge chamber with the applicator.
These subsystems work together, generally under computer control to provide the deposition
environment and to prevent damages of the system at incorrect operations.
Appli-
cator
Chamber
Microwave Power
Supply
Vacuum Pumping
and Pressure
Control
Exhaust
Waveguide
Process Gas
Gas Flow Rate
Control Subsystem
Figure 3.2:
General MCPR 915 MHz system structure
3.3 MCPR Subsystems and Operation Breakdown
3.3.1 The Microwave Cavity Reactor
The microwave discharge is produced inside a quartz dome which is located at one end of
the microwave cavity. The cavity is formed by a cylindrical water cooled wall and a water
cooled movable short which is electrically connected to the cavity wall via finger stocks. This
Technische Fachhochschule
Wildau
November 2003
3.3. MCPR SUBSYSTEMS AND OPERATION BREAKDOWN
16
short is used to adjust the proper cavity hight for the single discharge loaded resonant wave
mode inside the cavity, which is the TM
013
mode. A stub tuner on top of the applicator helps
to reduce reflected power for a better performance. The cavity bottom is located on a base
plate ring with flowing water inside to cool the dome foot region where the vacuum sealing is
located. Microwave power is coupled into the cavity applicator through an adjustable coaxial
power input port which consists of the power coupling probe and the outer conductor. The
movable coupling probe converts the waveguide mode to the TM
013
mode and forms the
end-section of the waveguide.
Process (reactive) gas is flowing through the base plate ring and the dome foot ring to
enter in the discharge region where a plasma ball is formed above the substrate. Substrate
Figure 3.3:
Schematic drawing of the microwave cavity plasma reactor
Technische Fachhochschule
Wildau
November 2003
3.3. MCPR SUBSYSTEMS AND OPERATION BREAKDOWN
17
and water cooled substrate holder are located beneath a quartz dome, the dielectric window
for the microwaves and outer cover of the vacuum system. To prevent that microwave energy
radiate out of the applicator a aluminum ring package is mounted beneath the dome foot
ring with finger stocks. These finger stocks works as a connector between separate parts to
enable the electromagnetic field. Through holes in the bottom aluminum ring and the finger
stocks, the exhaust gas can exit the reaction zone to the mechanical vacuum pump to leave
the system. In- and outputs for the applicator purge for dome cooling are located in the
cavity foot region.
3.3.2 Microwave Power Supply and Waveguide-Transmission Subsystem
The microwave power delivery subsystem is used to generate and transport the microwaves
to the plasma discharge. A schematic draft of the microwave circuit is shown in Figure 3.4.
The 915 MHz microwave is generated by a magnetron in the power supply. The microwave
propagates through the circulator and the dual-directional power coupler and into the cavity
915 MHz Microwave
Power Supply
Dummy
Load
Applicator
Circulator
Dual-Directional
Power Coupler
Microwave
Power Meter
Figure 3.4:
Waveguide power supply and wave transmission subsystem
applicator. Some of the incident power will be reflected back from the applicator and travels
in the opposite direction from the incident power. This reflected power propagates the dual-
directional power coupler and is directed to an attached dummy load by the circulator. In
the dummy load the microwave will be absorbed and dissipated as thermal energy. This
interception of reflected power prevent the propagation back into the power source, where
it may cause damage. The incident and reflected power is measured by microwave power
meters, which are also used to measure the reflected power during applicator adjustment
(short, coupling probe and stub tuner).
The wave transmission is carried by the rectangular shaped waveguides in the power supply
(gray colored). Between power supply and applicator, coaxial non-flexible formed pipes ensure
the microwave transmission (white colored).
Technische Fachhochschule
Wildau
November 2003
3.3. MCPR SUBSYSTEMS AND OPERATION BREAKDOWN
18
3.3.3 Gas Flow Control and Vacuum Pump Subsystem
Gases are used for the source of the diamond deposition. Additionally gases are used for
system tasks like pneumatic actuator for valves control, chamber venting, exhaust diluting,
applicator and waveguide purge. The gas flow control and vacuum pump subsystem are shown
in Figure 3.5.
The process gas usually consists of hydrogen, a carbon containing gas (methane, carbon
dioxide etc.)
and possibly argon.
The process source gases are feed through mass flow
controller, which are mounted on the mass flow table (MFT), to control the separate source
gas shares. After the mass flow controller the separate gas lines are fitted together to mix
the gases in the process gas line, which is connected with the base plate ring, the link to the
plasma discharge region. Valves on the mass flow table and in the vacuum pump line (shut
Figure 3.5:
Schematic drawing of the gas flow control and vacuum pump subsystem
off valve and vent valve) need pressure to switch. This pressure actuation is controlled via
control units (valve card board and pneumatic card), which transform electrical signals into
actuation pressure. Nitrogen lines supply the control units and thus the valves with pressure.
The mechanical roughing pump and the adjustable throttle valve are used to pump the
reacted gas from the deposition chamber in a continuous manner and to maintain the de-
sired pressure in the chamber. A control system automatically adjust the throttle valve to
achieve the desired pressure. The process pressure is measured by MKS Baratrons 2, 200 and
1000 Torr full scale pressure gages. A shut off valve (normally closed), directly mounted after
the process chamber separate the chamber and the exhaust line when the system is shut off. A
safety precaution included in the pumping system is the dilution of the hydrogen dominated
Technische Fachhochschule
Wildau
November 2003
3.3. MCPR SUBSYSTEMS AND OPERATION BREAKDOWN
19
exhaust gas at the exit of the mechanical pump. For it, a nitrogen purge, measured by a
mass flow meter is used. The mix rate of 9 (or more) times of the total hydrogen gas flow is
used, so that the resultant exhaust gas mixture is not flammable. Also the chamber will be
vented solely by nitrogen to prevent explosion when opening the chamber. Applicator and
waveguide purge is supplied by nitrogen for dome cooling and cooling the flange between two
waveguide sections, heated up by microwave power.
3.3.4 The Computer Monitoring and Control Subsystem
A computer monitor and control system are used to monitor the operation conditions and
to design and control the deposition processes. The MCPR 915 MHz was provided with a
Programmable Logic Controller (PLC) Model
9100e from Honeywell. A new control software
is built for PC monitoring and data input. Also new input/output components are installed,
because the new software could not work together with the old input/output components.
The general working principle of the computer controlling is shown in Figure 3.6. Read
and set data are send between computer control and deposition system. New set points can
be given and actual data can be read by the monitoring and input device. Read data are
separate gas flows, chamber pressure, valve state, temperatures, cooling fluid flow, incident
and reflected microwave power, substrate holder position and applicator adjustment (short
and coupling probe). Set data are separate gas flows, chamber pressure, valve actuation,
incident microwave power, substrate holder position and applicator adjustment.
Figure 3.6:
Working principle of the computer monitoring and control subsystem
Interlok functions are integrated in the control software and hardware and not operable for
the user. This functions shut down a running system if dangerous conditions exists. These
interlok conditions for example are open chamber door, no exhaust nitrogen flow or unreached
chamber pressure. The state of the system and other informations like warnings for process
endangered conditions are also shown on the monitor.
Technische Fachhochschule
Wildau
November 2003
Details
- Seiten
- Erscheinungsform
- Originalausgabe
- Jahr
- 2003
- ISBN (eBook)
- 9783832475888
- ISBN (Paperback)
- 9783838675886
- DOI
- 10.3239/9783832475888
- Dateigröße
- 1.8 MB
- Sprache
- Englisch
- Institution / Hochschule
- Technische Hochschule Wildau, ehem. Technische Fachhochschule Wildau – Ingenieurwesen/Wirtschaftsingenieurwesen
- Erscheinungsdatum
- 2004 (Januar)
- Note
- 1,0
- Schlagworte
- anlagenbau diamantsynthese gassystem mirkowellenreaktor
