Nonthermal plasma chemistry and physics

Plasma Physics

Nonthermal Plasma
Chemistry and Physics

Nonthermal Plasma
Chemistry and Physics
edited by

In addition to introducing the basics of plasma physics, Nonthermal Plasma
Chemistry and Physics is a comprehensive presentation of recent developments in the rapidly growing field of nonthermal plasma chemistry. The book
offers a detailed discussion of the fundamentals of plasma chemical reactions
and modeling, nonthermal plasma sources, relevant diagnostic techniques,
and selected applications.

Jürgen Meichsner
Martin Schmidt
Ralf Schneider
Hans-Erich Wagner

Features
•Includes a compact introduction in the nonthermal plasma
physics and plasma–surface interaction
•Classifies the plasma sources and chemical plasma reactors,
and provides important similarity parameters
•Overviews experimental methods in plasma diagnostics and
surface (thin film) analysis
•Presents detailed research results with modeling and applications
•Promotes strategies in plasma modeling and provides specific
methods, including examples
Elucidating interconnections and trends, the book focuses on basic principles and illustrations across a broad field of applications. Expert contributors

address environmental aspects of plasma chemistry. The book also includes
selected plasma conditions and specific applications in volume plasma
chemistry and treatment of material surfaces such as plasma etching in
microelectronics, chemical modification of polymer surfaces and deposition
of functional thin films. Designed for students of plasma physics, Nonthermal
Plasma Chemistry and Physics is a concise resource also for specialists
in this and related fields of research.

59165
ISBN: 978-1-4200-5916-8

90000

9 781420 059168

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9/28/12 10:20 AM


Nonthermal Plasma
Chemistry and Physics

© 2013 by Taylor & Francis Group, LLC


© 2013 by Taylor & Francis Group, LLC


Nonthermal Plasma

Chemistry and Physics
edited by

Jürgen Meichsner
Martin Schmidt
Ralf Schneider
Hans-Erich Wagner

Boca Raton London New York

CRC Press is an imprint of the
Taylor & Francis Group, an informa business

© 2013 by Taylor & Francis Group, LLC


Cover design by Sascha Meichsner and Carsten Desjardins.

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Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Contributors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Chapter 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Chapter 2

Nonthermal Plasma Chemical Processes of General Interest . . . . . . . . . . 7

Chapter 3

Physics of Nonthermal Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Chapter 4

Nonthermal Plasma Chemical Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

Chapter 5

Elementary Processes on Surfaces in
Plasma–Wall Interaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

Chapter 6

Plasma Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Chapter 7

Surface and Thin Film Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

Chapter 8

Selected Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

Chapter 9

Modeling and Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407

Chapter 10

Trends and New Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539

v

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© 2013 by Taylor & Francis Group, LLC


Preface
Plasma processing is one of the key technologies worldwide, especially using nonthermal, low-temperature plasmas. Recently, the situation is characterized by the
fast-growing interest in the optimization of existing applications as well as the
development of new ones.
This book provides a basic introduction to nonthermal plasma chemistry and
physics for students of plasma physics, PhD students, and scientists. The fundamentals of plasma chemical reactions and its modeling, most importantly nonthermal
plasma sources, relevant diagnostic techniques, as well as selected applications, are
presented and discussed in a systematic manner. Interconnections are shown; trends
and new concepts are illustrated. The chapters discuss the basic principles and provide exemplary illustrations of the wide field of applications. Therefore, it is not the
aim of this book to give a complete overview of the state of the art in the research
areas. For this, the readers can refer to already existing excellent monographs and
topical reviews given in the references.

The book is based on contributions from internationally known experts in their
research fields, using examples from their own scientific activities to illustrate the
basic principles with applications.
After a short introduction to the field of nonthermal plasma chemistry with some
historical notes and its specific characteristics, topics of general interest in this field are
briefly presented, which illustrate the broad spectrum of applications. Dry air plasma
chemistry with ozone generation or lacquer stripping and ashing reactions are briefly
discussed. Plasma etching presents a key technology in integrated circuit production.
Methane gas reformation as well as diamond deposition are important topics of
hydrocarbon plasma chemistry. The formation of pre-biochemical compounds is also
observed in nonthermal plasmas. Thin film generation of plasma polymers, of metallic
compounds, and silicone-based cells are products of plasma chemical processes.
The fundamentals, sources, and diagnostics of nonthermal plasmas are discussed
next. The basic concepts of plasma physics for thermal and nonthermal plasmas,
including collisional processes, plasma kinetics, and macroscopic transport equations,
are introduced. Due to the importance of surface processes in many applications, the
plasma-wall boundary is also considered. The basic physics of different nonthermal
plasmas of electric discharges and the realizations for technical plasma sources are
presented at the end of this chapter.
Nonthermal plasma reactors are characterized in terms of the principles of
chemical quasi-equilibria, macroscopic kinetics, and plasma chemical similarity.
Plasma–surface interaction is one of the fastest-growing branches in plasma
physics and has got an important issue in the field of applied surface science. Its basic
question concerns the mastering of an old problem: the contact of different states
of matter. The investigation and application of plasma–surface interaction plays an
essential role in low-temperature plasma processing such as etching, deposition, or
modification of surfaces as well as in fusion research. Therefore, such elementary
processes on surfaces in contact with plasmas are discussed. The particle and energy
balance at the surface determine the importance of the different mechanisms.
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© 2013 by Taylor & Francis Group, LLC


viii

Preface

According to the broad spectrum of plasma components, different tasks exist
for the investigation of the plasma to understand the processes and to control
chemical reactions characteristic of the various applications. Therefore, the fundamentals of probe measurements, microwave interferometry, emission and absorption
spectroscopy, laser-induced fluorescence spectroscopy, and gas chromatography are
discussed.
Complementary techniques needed for surface and thin film analysis are
presented next.
The first part of the next chapter presents examples of applications of volume
plasma chemistry. The reactions take place in the volume, as pure gas phase reactions, or in heterogeneous processes with participation of the surface of substrates,
electrodes, or walls, sometimes assisted by catalytic effects. The second part concerns applications of surface chemistry. Here the plasma chemical reactions result in
changes in surface properties. The reactions may involve volume processes, but the
essential reactions take place at the surface. Etching and thin film deposition as well
as surface functionalization up to plasma medical applications are presented.
Modeling and simulation provide an increasing number of tools to improve the
basic understanding of nonthermal plasmas and allow predictive studies for optimization of processes. The hierarchy of plasma models is explained at the beginning of the
next chapter, followed by a discussion of theoretical concepts for elementary volume
and surface processes in gas discharges. The chapter concludes with an example
of modeling, namely, the spatiotemporal dynamics in radio-frequency discharges of
oxygen and its comparison with experimental results.
The book concludes with a discussion of trends and new concepts in this
fascinating and dynamic research area.

© 2013 by Taylor & Francis Group, LLC


Acknowledgments
We would like to express our deep gratitude to all coauthors. They are the fundament
on which this work is based.
A very special thank you goes to Andrea Kleiber (Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, EURATOM Association, Greifswald) for her endless
patience and amazing support. The book would never have been completed without her uncountable contributions and her careful attention. Bert Krames helped as
emergency support in the final processing and transformed the impossible into reality.
We would also like to gratefully acknowledge the work of Marcel Beu (LeibnizInstitut für Plasmaforschung und Technologie e.V. (INP Greifswald)) for helping us
with the drawings.
This work was partly supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich Transregio 24. One of the editors (M. S.) appreciates the support
of the INP Greifswald.
Very special thanks go to Lance Wobus of Taylor & Francis Group for his valuable
advice and his patience during the preparation of this book. We gave him a hard time
with this project, but he was always giving us a backup whenever problems appeared.

ix

© 2013 by Taylor & Francis Group, LLC


© 2013 by Taylor & Francis Group, LLC


Editors
Jürgen Meichsner
Institut für Physik der Ernst-Moritz-Arndt-Universität Greifswald, Felix-HausdorffStr. 6, D-17489 Greifswald, Germany
Martin Schmidt
Leibniz-Institut für Plasmaforschung und Technologie e.V. (INP Greifswald) FelixHausdorff-Str. 2, D-17489 Greifswald, Germany

Ralf Schneider
Recent address:
Institut für Physik der Ernst-Moritz-Arndt-Universität Greifswald, Felix-HausdorffStr. 6, D-17489 Greifswald, Germany
Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, EURATOM Association, Wendelsteinstr. 1, D-17491 Greifswald, Germany
Hans-Erich Wagner
Institut für Physik der Ernst-Moritz-Arndt-Universität Greifswald, Felix-HausdorffStr. 6, D-17489 Greifswald, Germany

xi

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© 2013 by Taylor & Francis Group, LLC


Contributors
Name

Institute

Kurt H. Becker
Ronny Brandenburg
Franz X. Bronold
Paul B. Davies
Andreas Dinklage
Kristian Dittmann
Jörg Ehlbeck
Holger Fehske
Rüdiger Foest
Mario Hannemann

Hans-Jürgen Hartfuß
Rainer Hippler
Holger Kersten
Kirill V. Kozlov
Boris P. Lavrov
Detlef Loffhagen
Jürgen Meichsner

PU Brooklyna
INP Greifswaldb
IfP Greifswaldc
DoC Cambridged
IPP Greifswalde
IfP Greifswaldc
INP Greifswaldb
IfP Greifswaldc
INP Greifswaldb
INP Greifswaldb
IPP Greifswalde
IfP Greifswaldc
IEAP Kielf
MSU Moscowg
FoP St.-Petersburgh
INP Greifswaldb
IfP Greifswaldc

Siegfried Müller

INP Greifswaldb

Andreas Ohl
Abha Rai
Jürgen Röpcke
Antoine Rousseau
Martin Schmidt

INP Greifswaldb
IPP Greifswalde
INP Greifswaldb
LPT Palaiseau Cedexi
INP Greifswaldb

Ralf Schneider

IfP Greifswaldc

†

INP Greifswaldb
INP Greifswaldb
INP Greifswaldb
RUB Bochumj
INP Greifswaldb
IfP Greifswaldc

Karsten Schröder
Hartmut Steffen
Dirk Uhrlandt
Achim von Keudell
Th. von Woedtke

Hans-Erich Wagner

Sections
Section 8.3
Section 8.2.2
Section 9.2
Section 6.3
Section 6.4
Section 9.4
Section 8.2.2
Section 9.2
Sections 8.2.2 and 8.2.3.4
Section 6.1
Section 6.2
Section 8.4
Sections 5 and 8.4
Section 8.1.1
Section 6.3
Sections 8.1.3 and 9.1
Chapters 1, 10, Sections 3.1
through 3.7, 7.1
Sections 8.1.2, 8.1.3, and
8.2.2.2
Section 8.2.3.4
Sections 8.2.4.1.1 and 9.3
Section 6.3
Section 6.3
Chapters 1, 6, 10, Sections 3.8,
6.5, 7.3, 8.1, and 8.2
Chapters 1, 4, 10,

Sections 8.2.3.1.1, 9.3, and 9.4
Sections 7.2 and 8.2.3.4
Sections 7.2 and 8.2.4.2
Section 9.1
Chapter 5
Section 8.2.3.5
Chapters 1, 4, 10, Section 6.6
(continued)

xiii

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xiv

Contributors

(continued)
Name
K.-D. Weltmann
Harm Wulff
Rolf-Jürgen Zahn
a
b

c

d

e

f

g
h

i

j
k

†

Institute
INP Greifswaldb
IfB Greifswaldk
INP Greifswaldb

Sections
Section 8.2.3.5
Sections 7.3, 7.5, and 8.2.4.2
Sections 8.1.2 and 8.2.3.3

Polytechnic Institute of New York University, Brooklyn, NY 11201, USA.
Leibniz-Institut für Plasmaforschung und Technologie e.V. (INP Greifswald), FelixHausdorff-Str. 2, D-17489 Greifswald, Germany.
Institut für Physik der Ernst-Moritz-Arndt-Universität Greifswald, Felix-HausdorffStr. 6, D-17489 Greifswald, Germany.
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2
1EW, U.K.
Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, EURATOM Association, Wendelsteinstr. 1, D-17491 Greifswald, Germany.

Institut für Experimentelle und Angewandte Physik der Christian-AlbrechtsUniversität zu Kiel, Leibnizstr. 19, D-24118 Kiel, Germany.
Moscow State University, Department of Chemistry, 119991 Moscow, Russia.
Faculty of Physics, St.-Petersburg State University, Ulianovskaya 2, 198904
St.-Petersburg, Russia.
Laboratoire de Physique et Technologie des Plasmas, Ecole Polytechnique, Route de
Saclay, F-91128 Palaiseau Cedex, France.
Ruhr-Universität Bochum, Universitätsstraße 150, D-44801 Bochum, Germany.
Institut für Biochemie der Ernst-Moritz-Arndt-Universität Greifswald, FelixHausdorff-Str. 4, D-17489 Greifswald, Germany.
Deceased.

© 2013 by Taylor & Francis Group, LLC


1 Introduction
CONTENTS
1.1
1.2
1.3
1.4

Plasma Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Historical Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Thermal and Nonthermal Plasma Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Specifics of Nonthermal Plasma Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 PLASMA CHEMISTRY
The plasma state of matter (also named fourth state) is characterized by the existence of free electrons, positive and negative ions, as well as excited species and
radicals in mixture with the other neutrals of gaseous systems (atoms, molecules) or
liquids. Well-known examples for natural plasmas are the interstellar matter, stars,
the ionosphere, lightnings, and flames.

Plasmas are generated by the input of thermal energy, electric field energy, radiation, or beams (lasers, UV photons, electrons, protons), respectively. The principles
of plasma generation are summarized in Figure 1.1. The focus of the book is on
technical plasmas in gaseous systems that are mostly initiated by applied electric
fields. Examples of such electrical gas discharges are the corona and barrier discharge, sparks, arcs, and plasma torches, operating usually at atmospheric pressure.
Technically important low-pressure plasmas are the different kinds of glow discharges
driven by dc and rf voltages or microwaves.
Gas heating by thermal energy and/or collisions of neutrals with free electrons
and photons of sufficiently high energy initiate the production of free radicals and
further charged species. The generated particles are the source for various chemical
reactions in the volume as well as on the plasma interfaces, forming new compounds,
depositing layers, and modifying the properties of materials.
The science and application of chemical conversions in plasmas, including
reactive processes at interfaces, is the subject of plasma chemistry.

1.2 HISTORICAL NOTES
The existence of chemical reactions in plasmas, initiated by the input of electrical
energy in gaseous atmosphere, is a very old experience of mankind, with even Homer
describing the smell of sulfur∗ in the Iliad in air after lightning.

∗

Sulfur was later identified as nitrogen oxides.

1

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2

Nonthermal Plasma Chemistry and Physics
Radiation
photons
Plasma

Beams
electrons
protons
…

Thermal energy
gas heating
compression
Chemical processes

Heating of electrons
Electric current in gas

Electrical
gas discharges

Breakdown

Electric fields

FIGURE 1.1 Principles of plasma generation.

Important historical milestones in the investigation and application of plasma
chemical reactions are as follows:
• The synthesis of H2 O in spark discharges operating in H2 /O2 mixtures

(H. Cavendish 1781)
• The fixation of air nitrogen in spark discharges forming NO

(H. Cavendish 1784, J. Priestley 1785)
• The discovery and application of the ozonizer

(W. v. Siemens 1857)
• The conversion of organic compounds in silent discharges

(M. Berthelot 1866)
• The industrial production of nitrogen oxides in the arc discharge

(Birkeland-Eyde process 1905)
• The industrial production of acetylene in plasmatrons

(Hüls process 1940)

1.3 THERMAL AND NONTHERMAL PLASMA CHEMISTRY
We have to distinguish two kinds of plasma chemical conversions, the thermal as
well as the nonthermal plasma chemistry. In the case of thermal plasma chemistry the
plasmas act primarily as generators of thermal energy. They operate typically in the
kW to MW power range. At high temperatures (range 103 –104 K) and high specific
enthalpies, the chemical compounds are decomposed. This process is called plasma
pyrolysis. The chemical reactions take place in/or nearby the thermal equilibrium,
characterized by the common temperature (the gas temperature) of all species. The
output of reaction products has to be optimized by a quenching procedure and their
separation from the gas mixture.

© 2013 by Taylor & Francis Group, LLC

3

Introduction
(a)
Processes in space
Pel

Reactants

A1

Active zone
Te >>Tg

Passive zone
Te ≈Tg

A1

Hot electrons

A2

A2

Ak

Stable

products

Elementary reactions
Dissociation
Recombination
excitation,
de-excitation,
etc.
etc.

Ak
Al

Pel
Active phase
Time τ

Passive phase

Processes in time

(b)

FIGURE 1.2 Operation scheme of a nonthermal plasma chemical flow reactor (a) and of a
closed reactor (b), with Pel electrical power, Te kinetic temperature of electrons, and Tg gas
temperature.

The subject of this book is the plasma chemistry in nonthermal plasmas. In this
case, the reaction mixture is far from the thermal equilibrium. The chemical conversions are initiated by the high temperature (Te ≥ 104 K) of free electrons at relative low
gas temperature (Tg ≤ 103 K) (plasma electrical conversion). The processes take place

under highly nonequilibrium conditions of all plasma species. Hot electrons, energetic ions, cold excited species, free atoms, and radicals are produced in the so-called
active zone (phase) of the different kinds of nonthermal (electrical) gas discharges. In
the passive zone (phase), the electrons cool down fast. The unstable plasma components change to stable reaction products by volume and wall reactions. The operation
scheme of nonthermal plasma chemical reactors is illustrated in Figure 1.2.

1.4 SPECIFICS OF NONTHERMAL PLASMA CHEMISTRY
Two important advantages of the application of nonthermal plasma chemistry have
to be emphasized. First, because of the high electron temperature of 104 –105 K

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4

Nonthermal Plasma Chemistry and Physics

(about 1–10 eV mean energy), reactive processes that require an extremely high
activation energy can be realized. Therefore, nearly all plasma chemical processes
are practicable, including the synthesis of rare and new products. For example,
the effective synthesis of ozone succeeds only under nonthermal plasma conditions.
Second, as a result of the relative low gas temperature, there is no thermal dissociation
of reaction products and no quenching needed. Connected with this, the thermal stress
of the reactor walls as well as of the treated interfaces is minimal. Therefore, thermalsensitive materials can only be modified under nonthermal plasma conditions. This
fact was essentially important for the fabrication of microelectronic elements, and it
opens the window for future technical applications, e.g., in plasma medicine.
On the other hand, the selectivity of nonthermal plasma processes, the output,
and energetic efficiency of the reaction products is usually small, with the exception
of ozone synthesis. Therefore, the applications of nonthermal plasma processing are
dominated by reactive plasma-wall processes (modification, etching of targets, thin
film deposition, etc.). Important applications are summarized in Figure 1.3.

Nonthermal plasma chemical flow reactors are often operated under low pressure
conditions. This requires the installation of an expensive vacuum technique and
limits the introduction of plasma processes in technological lines. To overcome these
problems, the recent trend worldwide is to develop atmospheric pressure plasma
methods. The focus is directed on the application of numerous types of microplasmas.
Nonthermal plasma processes take place under highly nonequilibrium conditions
for all species. Therefore, microphysical modeling of the physics and chemistry in
nonthermal plasmas requires the knowledge of the energy distribution function of the

Process

Application

Industry

Volume chemistry
Ozone synthesis
Air and flue gas cleaning
VUV radiation sources
…

Purification
Environmental protection
Plasma displays
…

Microelectronics
Surface chemistry

Micromechanics

Hardening

Etching

Corrosion protection

Structuring

Wear resistance, wettability

Cleaning

Photovoltaics

Functionalization

Biocompatible layers

Thin film deposition

Heterogeneous catalysis

…

…

Electronic industry
Mechanical engineering
Optical industry
Vehicle construction

Textile industry …
Printing and packaging industry
Energy technology
Plasma medicine
…

FIGURE 1.3 Important applications of nonthermal plasma chemistry.

© 2013 by Taylor & Francis Group, LLC


Introduction

5

electron gas in the active reactor zone. Its simulation has to be solved simultaneously
with the complex system of master equations for heavy particles, including the
reaction products. The mass action law of the equilibrium chemistry is not applicable.
Generally, the solution of this complex problem is a challenge to computational
physics.
The realization and optimization of plasma chemical processes starts with the
selection of suitable plasma sources. Indispensable are a profound plasma diagnostics
and process control. This requires the knowledge of the discharge operation parameters (power input, pressure, flow, gas mixture, etc.) and its interconnection with
the most important plasma parameters (gas temperature, electron density, electron
energy, electron distribution, etc.) and the plasma chemical process itself (particle
densities, mass balance, surface properties, etc.).
To sum up, nonthermal plasma sources of technical relevance, important diagnostic methods, as well as the fundamentals of kinetic modeling of complex plasma
processes are presented in this book.

© 2013 by Taylor & Francis Group, LLC

© 2013 by Taylor & Francis Group, LLC


Plasma
2 Nonthermal
Chemical Processes of
General Interest
CONTENTS
2.1

2.2
2.3

2.4
2.5

Dry Air, Oxygen, and Nitrogen Plasma Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1 Ozone Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.2 Plasma Ashing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Plasma Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Hydrocarbon Plasma Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1 Methane Gas Reformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.2 Diamond Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.3 Origin of Prebiochemical Compounds on Earth . . . . . . . . . . . . . . . . . . . . . 12
2.3.4 Plasma Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.5 Thin Film Deposition of Metal Compounds . . . . . . . . . . . . . . . . . . . . . . . . . 13
Thin Film Silicon Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

In this chapter, typical topics of general interest are briefly presented, which illustrate
the broad spectrum of applications. Dry air plasma chemistry with ozone generation or
lacquer stripping and ashing reactions are briefly discussed. Plasma etching presents
a key technology in integrated circuit production. Methane gas reformation as well
as diamond deposition are important topics of hydrocarbon plasma chemistry. The
formation of pre-biochemical compounds is observed in nonthermal plasmas, too.
Thin film generation as plasma polymers, of metallic compounds and silicone-based
cells are products of plasma chemical processes. Detailed discussions of selected
topics are given in Chapter 8.

2.1 DRY AIR, OXYGEN, AND NITROGEN PLASMA CHEMISTRY
The main processes in nonthermal plasmas operating in oxygen, nitrogen, or dry air
plasma are dissociative collisions of molecules, resulting in the generation of the
reactive atoms (O, N) [1,2], the formation of excited atoms and molecules, as well
as positive or negative ions. The formation of negative ions is essential mainly for
electronegative gases such as oxygen. The dissociative attachment of electrons of
excited O2 molecules generates negative atomic ions as well as oxygen atoms. The
threshold energy of this process is essentially lower than electron impact dissociation
and dissociative ionization of ground state molecules [3]. The reaction probability of
7

© 2013 by Taylor & Francis Group, LLC


8

Nonthermal Plasma Chemistry and Physics
Air

e–

e–

O2 N2
O

O2*

N

O2

NO

NO2
NO3

O3

NO2
N2O5

O3

N

O3

N2*

O2

N

NO
O3
NO2

O2

N*
N

N2O

NO2
NO3

N2O4

FIGURE 2.1 Diagram of primary chemical reactions in dry air plasma induced by electron
impact. (According to Becker, K.H. et al., Air plasma chemistry, in Becker, K.H. et al. (eds),
Non-Equilibrium Air Plasmas at Atmospheric Pressure, IoP, Bristol, U.K., pp. 124–182, 2005.)

heavy particle reactions of electronically excited species can exceed the probabilities
of ground state reactions by orders of magnitude [4,5].
The air plasma chemistry, e.g., is responsible for producing Nx Oy compounds,
which have a key role in global environmental problems like acid rain. The scheme
in Figure 2.1 of dominant plasma chemical reactions in dry air demonstrates the

complexity of the processes [6].
The plasma chemistry in oxygen is also of practical importance, namely, for
the ozone generation and for plasma ashing. Augmented combustion is essentially
influenced by air plasma chemistry [7].

2.1.1 OZONE GENERATION
Ozone is a powerful oxidizing agent which is non-chlorine alternative applied for
water treatment, disinfection, and odor removal. The only economical method for
ozone production is by dielectric barrier discharge in atmospheric pressure oxygen
or air with its nonthermal plasma. Ozone is the sole substance which is generated in
volume processes in nonthermal plasma in industrial dimensions.
Ozone results in three-body collision processes
O + O2 + M → O∗3 + M → O3 + M,

(2.1)

where M is a third collision partner as O2 , O, also O3 or N2 . Oxygen atoms are generated by dissociative electron impact. The ozone formation is reduced by competitive
reactions like recombination of two O atoms to O2 or reactions of O atoms with
ozone molecules O + O3 + M → 2O2 + M [8]. In Section 8.1.1 the ozone synthesis
is discussed in more detail.

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Nonthermal Plasma Chemical Processes of General Interest

9

2.1.2 PLASMA ASHING
The interaction of an oxygen plasma with hydrocarbon compounds leads to CO2 and

H2 O. In microelectronic industry, e.g., the photoresist mask is removed (stripped)
by an oxygen plasma. Damage of the semiconductor material by high-energy ions
must be avoided by low ion energies and high fluxes of neutral radicals, i.e., oxygen
atoms to the resist surface. The low-temperature plasma ashing procedure is used
for preparation of samples for electron microscopy [9] and for quantitative analysis
of lignite [10]. Oxygen plasmas are applicable to precision cleaning of metallic
surfaces contaminated by organic substances such as grease or oil [11]. Hazardous
gaseous organic molecules as volatile organic compounds (VOCs) may be destroyed
by reactive species like O∗2 (a1 Dg ), O(1 D), O(3 P), H, OH, N∗2 (A3 S+u ), N∗2 (B3 Pg ), and
N best into CO2 or H2 O [12].

2.2 PLASMA ETCHING
Plasma etching is the key technology for patterning in every chip production in
the microelectronic industry. It enables nonisotropic etching in sub-μm range with
significant increasing of packed density of electronic elements in integrated circuits
applied in computer production. For this process a nonreactive gas is fed into the
plasma where it is activated. The interaction of this activated gas with a solid substrate
generates in a chemical reaction a volatile compound which contains atoms of the
substrate. Exemplary is the silicon etching by a fluorine compound feed gas as
CF4 . The plasma activation leads to generation of fluorine atoms by electron impact
dissociation of the CF4 molecule.
CF4 + e− −→ CF3 + F + e− .

(2.2)

The fluorine atoms react with silicon and produce volatile SiF↑4
Si + 4F → SiF↑4 .

(2.3)

The activation energy for desorption of the etch product SiF↑4 is transferred from the
plasma to the surface by ion bombardment. Because of the existence of fluorocarbon
radicals (CFn ) a polymer film is deposited on the silicon surface, also on sidewalls
of trenches. The sidewall protection is important for the anisotropy of trench etching
with high aspect ratio (ratio depth to width).
Fundamental starting processes of activating the etching gases are the electron–
molecule collisions. A critical review of data of electron collision processes for a lot
of fluorine and chlorine containing gases is given by Christophorou and Olthoff [13].
An extensive presentation of plasma etching can be found in Section 8.2.1.

2.3 HYDROCARBON PLASMA CHEMISTRY
Important reaction channels in hydrocarbons are induced by collisions of hot electrons
with gaseous molecules in a cold environment. It offers a broad spectrum of applications beyond standard organic chemistry. The spectrum covers from complicated

© 2013 by Taylor & Francis Group, LLC


10

Nonthermal Plasma Chemistry and Physics

reactions of the formation of bio-organic compounds in the early earth atmosphere,
natural gas reformation, thin polymeric film formation, and creation of higher hydrocarbons to deposition of diamond-like thin films and generation of pure carbon as
soot or even diamond.

2.3.1 METHANE GAS REFORMATION
Methane is a dominant part of natural gas. Other sources are petroleum processing
off-gas and biogas. It is an important energy carrier and initial compound of chemical
industry but also a dangerous greenhouse gas. Electron impact dissociation leads to
CH3 radicals and H atoms [14]

CH4 + e− −→ CH3 + H + e− .

(2.4)

Another pathway is the reaction of this molecule with hydrogen atoms (also generated
by electron impact processes) [17]
CH4 + H −→ CH3 + H2 .

(2.5)

The abstraction of further H atoms leading to CH2 , CH, and C is possible by H atoms
but also by electron collisions, especially in pure methane plasmas. The reverse
reaction, the addition of H atom to CH3 to form CH4 occurs at low temperature
[18]. H atoms may be generated by dissociative electron collisions of H2 molecules;
at higher gas temperatures thermal dissociation of hydrogen molecules becomes
dominant as studied in thermal plasma chemistry. Here, some processes may be more
effective, but the specific production sensitivity of nonthermal plasma chemistry is
lost due to generation of new compounds in a cold gaseous environment.
A reaction scheme is presented in Figure 2.2 for dissociative electron collisions
with CH4 and H2 molecules [15] as well as for the formation of CHx and C2 Hy
compounds controlled by collisions of hydrocarbon molecules with H atoms [16].
Concerning the variety of the processes in a H2 –CH4 plasma, including the electron
impact-induced reactions, see also [15].
The principle process scheme (see Figure 2.2) shows the formation of ethane,
ethylene, and acetylene. An investigation of methane conversion in a pulsed
microwave discharge (p = 30 mbar) yields a selectivity of acetylene generation near
70% with an energy input of 10 eV/molecule. Here the methane dissociation is
initiated by electron impact. The generated H atoms provide the source for further H
atom abstraction from the methane molecule [19].
The conversion of a CH4 /CO2 mixture into higher hydrocarbons or syngas

(CO/H2 ) in a hybrid catalytic plasma reactor is reviewed by Istadi [20]. The chemical
reactions are initiated by electron impact dissociation of CO2 and CH4 generating CO
and O as well as CH3 and H, respectively. An important research topic is the direct
conversion of methane and carbon dioxide to methanol [21,22]. The investigation of
the reaction products of methane–CO2 mixture in an atmospheric pressure dielectric
barrier discharge shows a small concentration of methanol, but a lot of other pure

© 2013 by Taylor & Francis Group, LLC