Nonthermal plasma chemistry and physics

The book offers a detailed discussion of the fundamentals of plasma chemical reactions and modeling, nonthermal plasma sources, relevant diagnostic techniques, and selected applications.

Nonthermal Plasma Chemistry and Physics

edited by Jürgen Meichsner Martin Schmidt Ralf Schneider Hans-Erich Wagner

ISBN: 978-1-4200-5916-8

9 781420 059168

9 0 0 0 0 59165

Nonthermal Plasma Chemistry and Physics

In addition to introducing the basics of plasma physics, Nonthermal Plasma

Chemistry and Physics is a comprehensive presentation of recent

develop-ments 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

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

Nonthermal Plasma Chemistry and Physics

CRC Press is an imprint of the

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Nonthermal Plasma Chemistry and Physics

edited by

Jürgen Meichsner Martin Schmidt Ralf Schneider Hans-Erich Wagner

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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

Plasma processing is one of the key technologies worldwide, especially using thermal, low-temperature plasmas Recently, the situation is characterized by thefast-growing interest in the optimization of existing applications as well as thedevelopment of new ones

non-This book provides a basic introduction to nonthermal plasma chemistry andphysics for students of plasma physics, PhD students, and scientists The fundamen-tals of plasma chemical reactions and its modeling, most importantly nonthermalplasma sources, relevant diagnostic techniques, as well as selected applications, arepresented and discussed in a systematic manner Interconnections are shown; trendsand new concepts are illustrated The chapters discuss the basic principles and pro-vide exemplary illustrations of the wide field of applications Therefore, it is not theaim of this book to give a complete overview of the state of the art in the researchareas For this, the readers can refer to already existing excellent monographs andtopical reviews given in the references

The book is based on contributions from internationally known experts in theirresearch fields, using examples from their own scientific activities to illustrate thebasic principles with applications

After a short introduction to the field of nonthermal plasma chemistry with somehistorical notes and its specific characteristics, topics of general interest in this field arebriefly presented, which illustrate the broad spectrum of applications Dry air plasmachemistry with ozone generation or lacquer stripping and ashing reactions are brieflydiscussed Plasma etching presents a key technology in integrated circuit production.Methane gas reformation as well as diamond deposition are important topics ofhydrocarbon plasma chemistry The formation of pre-biochemical compounds is alsoobserved in nonthermal plasmas Thin film generation of plasma polymers, of metalliccompounds, and silicone-based cells are products of plasma chemical processes.The fundamentals, sources, and diagnostics of nonthermal plasmas are discussednext 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, theplasma-wall boundary is also considered The basic physics of different nonthermalplasmas of electric discharges and the realizations for technical plasma sources arepresented at the end of this chapter

Nonthermal plasma reactors are characterized in terms of the principles ofchemical quasi-equilibria, macroscopic kinetics, and plasma chemical similarity.Plasma–surface interaction is one of the fastest-growing branches in plasmaphysics and has got an important issue in the field of applied surface science Its basicquestion concerns the mastering of an old problem: the contact of different states

of matter The investigation and application of plasma–surface interaction plays anessential role in low-temperature plasma processing such as etching, deposition, ormodification of surfaces as well as in fusion research Therefore, such elementaryprocesses on surfaces in contact with plasmas are discussed The particle and energybalance at the surface determine the importance of the different mechanisms

vii

According to the broad spectrum of plasma components, different tasks existfor the investigation of the plasma to understand the processes and to controlchemical reactions characteristic of the various applications Therefore, the funda-mentals of probe measurements, microwave interferometry, emission and absorptionspectroscopy, laser-induced fluorescence spectroscopy, and gas chromatography arediscussed.

Complementary techniques needed for surface and thin film analysis arepresented next

The first part of the next chapter presents examples of applications of volumeplasma chemistry The reactions take place in the volume, as pure gas phase reac-tions, or in heterogeneous processes with participation of the surface of substrates,electrodes, or walls, sometimes assisted by catalytic effects The second part con-cerns applications of surface chemistry Here the plasma chemical reactions result inchanges in surface properties The reactions may involve volume processes, but theessential 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 thebasic understanding of nonthermal plasmas and allow predictive studies for optimiza-tion of processes The hierarchy of plasma models is explained at the beginning of thenext chapter, followed by a discussion of theoretical concepts for elementary volumeand surface processes in gas discharges The chapter concludes with an example

of modeling, namely, the spatiotemporal dynamics in radio-frequency discharges ofoxygen and its comparison with experimental results

The book concludes with a discussion of trends and new concepts in thisfascinating and dynamic research area

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 physik, Teilinstitut Greifswald, EURATOM Association, Greifswald) for her endlesspatience and amazing support The book would never have been completed with-out her uncountable contributions and her careful attention Bert Krames helped asemergency support in the final processing and transformed the impossible into reality

Plasma-We would also like to gratefully acknowledge the work of Marcel Beu Institut für Plasmaforschung und Technologie e.V (INP Greifswald)) for helping uswith the drawings

(Leibniz-This work was partly supported by the Deutsche Forschungsgemeinschaft, derforschungsbereich Transregio 24 One of the editors (M S.) appreciates the support

Son-of the INP Greifswald

Very special thanks go to Lance Wobus of Taylor & Francis Group for his valuableadvice and his patience during the preparation of this book We gave him a hard timewith this project, but he was always giving us a backup whenever problems appeared

ix

Kurt H Becker PU Brooklyn a Section 8.3

Ronny Brandenburg INP Greifswald b Section 8.2.2

Franz X Bronold IfP Greifswald c Section 9.2

Paul B Davies DoC Cambridge d Section 6.3

Andreas Dinklage IPP Greifswald e Section 6.4

Kristian Dittmann IfP Greifswald c Section 9.4

Jörg Ehlbeck INP Greifswald b Section 8.2.2

Holger Fehske IfP Greifswald c Section 9.2

Rüdiger Foest INP Greifswald b Sections 8.2.2 and 8.2.3.4

Mario Hannemann INP Greifswald b Section 6.1

Hans-Jürgen Hartfuß IPP Greifswald e Section 6.2

Rainer Hippler IfP Greifswald c Section 8.4

Holger Kersten IEAP Kiel f Sections 5 and 8.4

Kirill V Kozlov MSU Moscow g Section 8.1.1

Boris P Lavrov FoP St.-Petersburg h Section 6.3

Detlef Loffhagen INP Greifswald b Sections 8.1.3 and 9.1

Jürgen Meichsner IfP Greifswald c Chapters 1, 10, Sections 3.1

through 3.7, 7.1 Siegfried Müller INP Greifswald b Sections 8.1.2, 8.1.3, and

8.2.2.2 Andreas Ohl INP Greifswald b Section 8.2.3.4

Abha Rai IPP Greifswald e Sections 8.2.4.1.1 and 9.3

Jürgen Röpcke INP Greifswald b Section 6.3

Antoine Rousseau LPT Palaiseau Cedex i Section 6.3

Martin Schmidt INP Greifswald b Chapters 1, 6, 10, Sections 3.8,

6.5, 7.3, 8.1, and 8.2 Ralf Schneider IfP Greifswald c Chapters 1, 4, 10,

Sections 8.2.3.1.1, 9.3, and 9.4

† Karsten Schröder INP Greifswald b Sections 7.2 and 8.2.3.4

Hartmut Steffen INP Greifswald b Sections 7.2 and 8.2.4.2

Dirk Uhrlandt INP Greifswald b Section 9.1

Achim von Keudell RUB Bochum j Chapter 5

Th von Woedtke INP Greifswald b Section 8.2.3.5

Hans-Erich Wagner IfP Greifswald c Chapters 1, 4, 10, Section 6.6

(continued)

xiii

K.-D Weltmann INP Greifswald b Section 8.2.3.5

Harm Wulff IfB Greifswald k Sections 7.3, 7.5, and 8.2.4.2 Rolf-Jürgen Zahn INP Greifswald b Sections 8.1.2 and 8.2.3.3

a Polytechnic Institute of New York University, Brooklyn, NY 11201, USA.

b Leibniz-Institut für Plasmaforschung und Technologie e.V (INP Greifswald),

Felix-Hausdorff-Str 2, D-17489 Greifswald, Germany.

c Institut für Physik der Ernst-Moritz-Arndt-Universität Greifswald,

Felix-Hausdorff-Str 6, D-17489 Greifswald, Germany.

d Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2

1EW, U.K.

e Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, EURATOM

Associa-tion, Wendelsteinstr 1, D-17491 Greifswald, Germany.

f Institut für Experimentelle und Angewandte Physik der

Christian-Albrechts-Universität zu Kiel, Leibnizstr 19, D-24118 Kiel, Germany.

g Moscow State University, Department of Chemistry, 119991 Moscow, Russia.

h Faculty of Physics, St.-Petersburg State University, Ulianovskaya 2, 198904

St.-Petersburg, Russia.

i Laboratoire de Physique et Technologie des Plasmas, Ecole Polytechnique, Route de

Saclay, F-91128 Palaiseau Cedex, France.

j Ruhr-Universität Bochum, Universitätsstraße 150, D-44801 Bochum, Germany.

k Institut für Biochemie der Ernst-Moritz-Arndt-Universität Greifswald,

Felix-Hausdorff-Str 4, D-17489 Greifswald, Germany.

† Deceased.

1 Introduction

CONTENTS

1.1 Plasma Chemistry 1

1.2 Historical Notes 1

1.3 Thermal and Nonthermal Plasma Chemistry 2

1.4 Specifics of Nonthermal Plasma Chemistry 3

1.1 PLASMA CHEMISTRY

The plasma state of matter (also named fourth state) is characterized by the exis-tence 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, radi-ation, 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 dis-charge, 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

Electric fields

Heating of electrons Electric current in gas Breakdown

Electrical gas discharges

Thermal energy gas heating compression Chemical processes

FIGURE 1.1 Principles of plasma generation.

Important historical milestones in the investigation and application of plasmachemical reactions are as follows:

• The synthesis of H2O in spark discharges operating in H2/O2mixtures(H Cavendish 1781)

• The fixation of air nitrogen in spark discharges forming NO

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 103–104K) and high specificenthalpies, the chemical compounds are decomposed This process is called plasmapyrolysis The chemical reactions take place in/or nearby the thermal equilibrium,characterized by the common temperature (the gas temperature) of all species Theoutput of reaction products has to be optimized by a quenching procedure and theirseparation from the gas mixture

Active phase Time τ

Passive phase Processes in space

Dissociation excitation, etc.

FIGURE 1.2 Operation scheme of a nonthermal plasma chemical flow reactor (a) and of a

closed reactor (b), with P el electrical power, T e kinetic temperature of electrons, and T g gastemperature

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

conver-sions are initiated by the high temperature (T e≥ 104K) of free electrons at relative low

gas temperature (T g≤ 103K) (plasma electrical conversion) The processes take placeunder highly nonequilibrium conditions of all plasma species Hot electrons, ener-getic ions, cold excited species, free atoms, and radicals are produced in the so-calledactive zone (phase) of the different kinds of nonthermal (electrical) gas discharges Inthe passive zone (phase), the electrons cool down fast The unstable plasma compo-nents change to stable reaction products by volume and wall reactions The operationscheme 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

(about 1–10 eV mean energy), reactive processes that require an extremely highactivation energy can be realized Therefore, nearly all plasma chemical processesare 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, sensitive materials can only be modified under nonthermal plasma conditions Thisfact was essentially important for the fabrication of microelectronic elements, and itopens the window for future technical applications, e.g., in plasma medicine

thermal-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 aredominated by reactive plasma-wall processes (modification, etching of targets, thinfilm deposition, etc.) Important applications are summarized in Figure 1.3

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

Electronic industry Mechanical engineering Optical industry Vehicle construction Textile industry … Printing and packaging industry Energy technology

Air and flue gas cleaning

VUV radiation sources

…

Microelectronics Micromechanics Hardening Corrosion protection Wear resistance, wettability Photovoltaics

Biocompatible layers Heterogeneous catalysis

…

FIGURE 1.3 Important applications of nonthermal plasma chemistry.

electron gas in the active reactor zone Its simulation has to be solved simultaneouslywith the complex system of master equations for heavy particles, including thereaction products The mass action law of the equilibrium chemistry is not applicable.Generally, the solution of this complex problem is a challenge to computationalphysics.

The realization and optimization of plasma chemical processes starts with theselection of suitable plasma sources Indispensable are a profound plasma diagnosticsand process control This requires the knowledge of the discharge operation param-eters (power input, pressure, flow, gas mixture, etc.) and its interconnection withthe most important plasma parameters (gas temperature, electron density, electronenergy, electron distribution, etc.) and the plasma chemical process itself (particledensities, mass balance, surface properties, etc.)

To sum up, nonthermal plasma sources of technical relevance, important nostic methods, as well as the fundamentals of kinetic modeling of complex plasmaprocesses are presented in this book

diag-2 Nonthermal Plasma

Chemical Processes of General Interest

CONTENTS

2.1 Dry Air, Oxygen, and Nitrogen Plasma Chemistry 72.1.1 Ozone Generation 82.1.2 Plasma Ashing 92.2 Plasma Etching 92.3 Hydrocarbon Plasma Chemistry 92.3.1 Methane Gas Reformation 102.3.2 Diamond Deposition 122.3.3 Origin of Prebiochemical Compounds on Earth 122.3.4 Plasma Polymerization 132.3.5 Thin Film Deposition of Metal Compounds 132.4 Thin Film Silicon Solar Cells 142.5 Summary 14

In this chapter, typical topics of general interest are briefly presented, which illustratethe broad spectrum of applications Dry air plasma chemistry with ozone generation orlacquer 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 Theformation of pre-biochemical compounds is observed in nonthermal plasmas, too.Thin film generation as plasma polymers, of metallic compounds and silicone-basedcells are products of plasma chemical processes Detailed discussions of selectedtopics 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 airplasma are dissociative collisions of molecules, resulting in the generation of thereactive 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 forelectronegative gases such as oxygen The dissociative attachment of electrons ofexcited O2molecules generates negative atomic ions as well as oxygen atoms Thethreshold energy of this process is essentially lower than electron impact dissociationand dissociative ionization of ground state molecules [3] The reaction probability of

7

O O2* N N2*

O2 N2Air

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 NxOy 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 thecomplexity of the processes [6]

The plasma chemistry in oxygen is also of practical importance, namely, forthe ozone generation and for plasma ashing Augmented combustion is essentiallyinfluenced by air plasma chemistry [7]

Ozone is a powerful oxidizing agent which is non-chlorine alternative applied forwater treatment, disinfection, and odor removal The only economical method forozone production is by dielectric barrier discharge in atmospheric pressure oxygen

or air with its nonthermal plasma Ozone is the sole substance which is generated involume processes in nonthermal plasma in industrial dimensions

Ozone results in three-body collision processes

where M is a third collision partner as O2, O, also O3or N2 Oxygen atoms are ated by dissociative electron impact The ozone formation is reduced by competitivereactions like recombination of two O atoms to O2 or reactions of O atoms withozone molecules O + O3+ M→ 2O2+ M [8] In Section 8.1.1 the ozone synthesis

gener-is dgener-iscussed in more detail

2.1.2 PLASMAASHING

The interaction of an oxygen plasma with hydrocarbon compounds leads to CO2and

H2O In microelectronic industry, e.g., the photoresist mask is removed (stripped)

by an oxygen plasma Damage of the semiconductor material by high-energy ionsmust be avoided by low ion energies and high fluxes of neutral radicals, i.e., oxygenatoms to the resist surface The low-temperature plasma ashing procedure is usedfor preparation of samples for electron microscopy [9] and for quantitative analysis

of lignite [10] Oxygen plasmas are applicable to precision cleaning of metallicsurfaces contaminated by organic substances such as grease or oil [11] Hazardousgaseous organic molecules as volatile organic compounds (VOCs) may be destroyed

by reactive species like O∗2(a1Dg), O(1D), O(3P), H, OH, N∗2(A3S+u), N∗2 (B3Pg), and

N best into CO2or H2O [12]

2.2 PLASMA ETCHING

Plasma etching is the key technology for patterning in every chip production inthe microelectronic industry It enables nonisotropic etching in sub-μm range withsignificant increasing of packed density of electronic elements in integrated circuitsapplied in computer production For this process a nonreactive gas is fed into theplasma where it is activated The interaction of this activated gas with a solid substrategenerates in a chemical reaction a volatile compound which contains atoms of thesubstrate Exemplary is the silicon etching by a fluorine compound feed gas as

CF4 The plasma activation leads to generation of fluorine atoms by electron impactdissociation of the CF4molecule

of trenches The sidewall protection is important for the anisotropy of trench etchingwith 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 electronswith gaseous molecules in a cold environment It offers a broad spectrum of appli-cations beyond standard organic chemistry The spectrum covers from complicated

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

Methane is a dominant part of natural gas Other sources are petroleum processingoff-gas and biogas It is an important energy carrier and initial compound of chemicalindustry but also a dangerous greenhouse gas Electron impact dissociation leads to

CH3radicals and H atoms [14]

Another pathway is the reaction of this molecule with hydrogen atoms (also generated

by electron impact processes) [17]

The abstraction of further H atoms leading to CH2, CH, and C is possible by H atomsbut also by electron collisions, especially in pure methane plasmas The reversereaction, 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 H2molecules;

at higher gas temperatures thermal dissociation of hydrogen molecules becomesdominant as studied in thermal plasma chemistry Here, some processes may be moreeffective, but the specific production sensitivity of nonthermal plasma chemistry islost due to generation of new compounds in a cold gaseous environment

A reaction scheme is presented in Figure 2.2 for dissociative electron collisionswith CH4 and H2 molecules [15] as well as for the formation of CHx and C2Hy

compounds controlled by collisions of hydrocarbon molecules with H atoms [16].Concerning the variety of the processes in a H2–CH4plasma, including the electronimpact-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 near70% with an energy input of 10 eV/molecule Here the methane dissociation isinitiated by electron impact The generated H atoms provide the source for further Hatom abstraction from the methane molecule [19]

(CO/H2) in a hybrid catalytic plasma reactor is reviewed by Istadi [20] The chemicalreactions are initiated by electron impact dissociation of CO2and CH4generating COand O as well as CH3 and H, respectively An important research topic is the directconversion of methane and carbon dioxide to methanol [21,22] The investigation ofthe reaction products of methane–CO2mixture in an atmospheric pressure dielectricbarrier discharge shows a small concentration of methanol, but a lot of other pure

(CH3+M)(M)

+e −

FIGURE 2.2 Reaction schemes for the electron collision of CH4 induced CH3, CH4, and

CH formation (According to Röpcke, J et al., J Phys D: Appl Phys., 34, 2336, 2001.) H2

dissociation and of the transformation of C1Hxand C2Hyspecies by reactions with H atoms indiamond plasma CVD (According to Ma, J et al., Exploration of the gas phase chemistry in

microwave activated diamond depositing plasmas by laser spectroscopy, in ISPC 2007 Peking

CD, ISPC, Peaking University, Beijing, China, 2007.) M indicates the action of the wall.

and oxygenated hydrocarbons A carbon chain growth is supposed to occur mainly

by the reaction

According to the practical application an essential problem is the bad selectivity

of the plasma process [22,23] Products of the methane conversion include syngas,gaseous products as ethylene, acetylene, and propylene, liquid hydrocarbons, plasmapolymers, and oxygenates The complexity of hydrocarbon plasmas is demonstrated

in [24] by review of plasma chemical reactions in low-pressure acetylene plasmas

A spectacular example of application of nonequilibrium plasma chemistry is thedeposition of diamond films

2.3.2 DIAMONDDEPOSITION

Plasma-assisted chemical vapor deposition enables the formation of diamond atmoderate temperature and low pressure with hydrocarbons as starting compounds onnon-diamond substrates This allows the extended industrial use of the outstandingproperties of the diamond, such as extreme hardness, high thermal conductivity,broad optical transparency (deep UV to far IR), and wide band gap (5.4 eV) In gasphase chemistry of a CH4/H2 plasma the CH4 molecule is activated which leads tocarbon deposition with sp3(diamond) or sp2(graphite) bonding The hydrogen atomsgenerated in the plasma etch the deposited carbon producing volatile compounds CHn

(n= 1 − 4) Because the approximately 100 times higher etch rate of graphite [25]diamond remains on the substrate

For the diamond synthesis the principle growth species are CH3, C2, and H [26]

In the conventional H2rich plasma in H2/CH4mixtures the CH3radical is responsiblefor the diamond growth In plasmas with lower H2 concentration, as in Ar/H2/CH4

gas mixtures C2controls the diamond deposition

The radicals C2, CH, H, CH3 are observed by tunable infrared diode laserabsorption and emission spectroscopy, respectively [16,27]

The impact of the H atoms is manifold for the diamond deposition process.Besides the already mentioned etching of the graphite phase and the importance for

CH3generation the H atoms act with bonded hydrogen and create reactive sites forthe reaction of CH3radicals and serve for the addition of a C atom into the diamondlattice [26]

Hot filament chemical vapor deposition process and microwave plasma CVDwith low or higher energy input were the most successfully applied methods for dia-mond deposition [25] High substrate temperatures (typically>700◦C) ensure gooddiamond quality For various industrially important applications such as deposition onmicroelectronic substrates lower substrate temperatures are necessary The decrease

of the deposition rate with decreasing substrate temperature could be compensatedusing other starting gases such as halogenated compounds like C2H5Cl [28] or CO[29] and CO2[30] with H2admixtures

The deposition of well-ordered nanostructures as nanotips and nanotubes,nanowalls, graphene, and ultra nanocrystalline diamond is also observed under plasmaconditions using as source material mixtures of carbon-carrier gases such as hydro-carbons, fluorocarbons, etc [31–35] Fullerenes are generated successfully in thermalplasmas [31]

A more detailed discussion of the plasma-assisted deposition of a-CH films,diamond and erosion of graphite by H atoms is given in Section 8.2.4.1.1

One exciting and still not resolved question is the origin and the development oflife on earth The first step may be the formation of organic molecules, e.g., aminoacids Miller [36] investigated spark discharges in a mixture of methane, ammonia,hydrogen, water vapor, and liquid water In particular, under the action of UV radiationthe formation of various organic compounds were observed, e.g., hydrogen cyanide,amines, aldehydes, acrylonitrile In the aqueous solution, amino acids were formed

The products in the gas phase were generated by reactions between free radicalsand ions.

The state of the early earth atmosphere is discussed by Abelson [37] An N2–CO–H2 atmosphere is supposed and HCN and H2O were the principal products of agas discharge, beside small amounts of CO2and CH4 HCN in aqueous solution canlead to other organic compounds

Also the plasma chemistry in CH2–H2S [38] and CH4–PH3[39] atmospheres isstudied to detect prebiochemical substances

These experiments show the formation of organic compounds in plasma chemicalexperiments, but the further reaction steps in the direction of formation of livingstructures are largely speculative

Plasma polymerization is a process of thin film deposition on electrodes, walls, orsubstrates under the action of plasmas in an organic molecules containing atmosphere.The term “polymerization” is misleading because this product is not a polymerconsisting of equal components Plasma polymer films consist of highly cross-linked,brittle material with good dielectric properties They are pinhole-free, have low sol-ubility, pronounced chemical inertness, and good adhesion to the surface A broadspectrum of organic compounds such as hydrocarbons, fluorocarbon, and siliconorganics was applied for plasma polymerization In contrary to chemical polymeriza-tion these starting compounds in the feed gas can be free of double or multiple bonds

or cyclic structures The starting gas is activated in the plasma by electron collisions

or by collisions with other energetic plasma components as H atoms Ionic or neutralradicals are created The target surface is activated by ion bombardment The radicalsdiffuse (neutral) or drift (ionic) to the surface where they are bonded to the surface.The starting material can also move directly to the surface where a plasma-inducedpolymerization is possible

Section 8.2.4.1.2 gives more detailed information on plasma polymerization

A method of thin film deposition of simple metal compounds (e.g., oxides, nitrides)

is the plasma-enhanced chemical vapor deposition with metal organic starting gases.The advantage of this method is the low substrate temperature and is therefore usefulfor deposition on temperature sensitive materials [40] A study of deposition of TiNusing Tetrakis(diethylamine)titanium (TDEAT) shows the importance of H-radicals

in the H2 plasma for the stripping of TDEAT The formation of TiN requires N2

addition to the process [41]

The deposition of thin films of pure metals or simple metal compounds is possible

by sputtering in low-pressure discharges inert gas An example of reactive sputtering

is the deposition of TiN films The nitride formation is a surface process of the freshlydeposited Ti with plasma-activated nitrogen

Plasma-enhanced atomic layer deposition (PEALD) of metals or metallic pounds is based on a sequential use of self-terminating plasma-enhanced gas solid

com-reactions which leads to a coverage of the target surface by monolayer films withgood conformality [42].

2.4 THIN FILM SILICON SOLAR CELLS

The development of alternative energy sources to adverse fossil fuels is an importanttask for science and technology in the twenty first century Photovoltaic is a promisingcandidate in future renewable energy technology Thin Si-films are expected to besuccessful material for effective solar cells [43,44] Beside other methods plasma-enhanced chemical vapor deposition is widely used for generation of amorphoussilicon (a-Si:H) and microcrystalline (μc-Si:H) films Feed gas for plasma-assistedsilicon deposition is mainly pure monosilane SiH4 or SiH4/H2 mixtures Inside theplasma silane is dissociated by electron impact into SiH3, SiH2, SiH, Si, H2 and H,and H2 in H atoms Also SiH+x, SiH−x, and H+x ions are generated Secondary ionmolecule reactions and reactions between neutral species also have to be taken intoaccount, too Data on silane plasma chemistry are reviewed in [45]

generates dangling bonds or recombines with another SiH3 to Si2H6 SiH4 and

Si2H6 are desorbed from the surface The formation of a Si–Si bond occurs byreaction of SiH3 radicals with dangling bonds, the Si film is growing [43] Thelow-temperature plasma-enhanced deposition process (150◦C–300◦C) enables thedeposition on temperature-sensitive substrates as polymer foils The fabrication ofhomogenous large-area a-Si:H films with high deposition rate is important for applica-tions from an economical viewpoint Parallel plate rf-reactors operating at 13.56 MHzare usually used Deposition rates of 0.2–0.3 nm/s are observed Rates of 2 nm/s wereachieved by higher operating frequency (70 MHz) The deposition rate of μc-Si:Hcould be increased by application of a narrow gap discharge at higher pressures [46]

2.5 SUMMARY

Selected examples of the various possibilities of plasma chemical processes werepresented Most applications concern surface processes, such as thin film deposi-tion, etching, and cleaning Only one volume process for the generation of largeramounts of material has reached technical maturity and commercial importance, theozone synthesis Some processes and procedures are specific for plasma processing.Examples are the micropatterning in microelectronics, or the deposition of plasmapolymers on various substrates

The aim of future technical developments must be to enhance the selectivityand energy efficiency of plasma chemical processes, and research may lead to newmaterials with exciting properties

Content 173.1.1.2 Ionization Degree 193.1.1.3 Quasi-Neutrality and Debye Shielding 193.1.2 Debye Shielding Length 203.1.3 Plasma Frequency 223.1.4 High-Frequency Conductivity and Permittivity of Plasmas 243.1.5 Ambipolar Diffusion of Charged Particles 27

Magnetic Field 293.1.6.1 Cyclotron Motion 293.1.6.2 Drift Velocity Perpendicular to the Magnetic Field

due to Additional External Forces 313.1.6.3 Influence of Collisions on Charged Particle Transport

in Magnetized Plasmas 333.2 Thermal and Nonthermal Plasmas 333.2.1 Plasma in Thermodynamic Equilibrium 333.2.1.1 Complete Thermodynamic Equilibrium 333.2.1.2 Local Thermodynamic Equilibrium 353.2.2 Nonequilibrium Plasmas 363.3 Elementary Collision Processes and Cross Sections 383.3.1 Introduction and Overview 383.3.1.1 Dynamics of Elastic and Inelastic Collisions 393.3.2 Collision Processes in Nonthermal Plasmas and Their

Classification 423.3.3 Collision Cross Section 483.3.3.1 Gas Kinetic Cross Section 483.3.3.2 Polarization Cross Section 493.3.3.3 Coulomb Scattering 533.3.3.4 Electron Attachment Cross Section 553.3.3.5 Electron Impact Excitation and Ionization

Cross Section 553.3.3.6 Cross Section of the Neutral–Neutral Reaction at

Thermal Velocity with Threshold Energy 59

15

3.4 Plasma Kinetics 603.4.1 The Boltzmann Equation 603.4.2 Electron Energy Distribution Function in Nonthermal Plasmas 623.5 Averaged Quantities, Transport Equations, and Rate Coefficients 643.5.1 Weighted Average of Physical Quantities 643.5.2 Macroscopic Transport Equations 653.5.3 Analytical Calculation of Rate Coefficients

and Collision Frequencies 683.5.3.1 Gas Kinetic Neutral–Neutral Collision Frequency and

Mean Free Path Length 683.5.3.2 Rate Coefficient for Ion–Molecule Reactions with the

Langevin Capture Cross Section 693.5.3.3 Rate Coefficient for Neutral–Neutral Reactions with

Threshold Energy 693.5.4 Rate Coefficient for Electron Impact Ionization 703.5.5 Rate Coefficient of Three-Particle Collisions 723.5.6 Reaction Rate Coefficient of Chemical Reactions 733.5.7 Order of Chemical Reactions 743.6 Boundary Plasma-Surface, Plasma Sheaths 763.6.1 Stationary Plasma Sheath and Bohm Sheath Criterion 773.6.1.1 Generalized Bohm Sheath Criterion 803.6.1.2 Plasma Sheath in Electronegative Gases 803.6.1.3 Multi-Species Plasmas 813.6.1.4 Plasma Sheath in Front of the Floating Surface 823.6.2 High-Voltage Plasma Sheaths 823.6.2.1 Matrix Sheath 833.6.2.2 Child–Langmuir Sheath 833.6.2.3 Plasma Sheath with Ion Collisions 843.6.2.4 Ion Energy Distribution at the Surface 863.6.3 Nonstationary Plasma Sheaths 863.6.3.1 Collisionless Sheath in Capacitively Coupled RF

Plasma(e · |ϕ sh |  k B · T e ) 87

RF Plasma 893.7 Nonthermal Plasmas of Electric Gas Discharges 913.7.1 Introduction and Overview 913.7.2 Electric Breakdown in Gases and Townsend Discharge 933.7.2.1 Townsend Discharge 983.7.3 Glow Discharges 1003.7.3.1 Transition from the Townsend Discharge to the Glow

Discharge 1003.7.3.2 Glow Discharges 101

Anomalous Glow Discharges 1023.7.3.4 Stable Positive Column 1053.7.4 Arc Discharges 107

3.7.5 High Voltage Breakdown at Atmospheric Pressure, Corona andBarrier Discharge 1073.7.5.1 Barrier Discharge 1093.7.6 Glow Discharges at Alternating Electric Field, RF

and Microwave Plasmas 1103.7.6.1 Heating of Electrons in the Bulk Plasma by the

High-Frequency Electric Field 1123.7.6.2 Radio Frequency Plasmas 1133.8 Technical Plasma Sources 1153.8.1 Introduction 1153.8.2 Plasma Generation by Electric Fields 1153.8.3 Low-Pressure Plasma 1163.8.4 Direct Current Discharges 1163.8.5 Radio Frequency Discharges 1173.8.6 Capacitively Coupled Discharges 1183.8.7 Inductively Coupled Discharges 1203.8.8 Microwave Discharges 1213.8.9 Atmospheric Pressure Discharges 1253.8.10 Dielectric Barrier Discharge 1253.8.11 Microdischarge 1283.8.12 Cold Plasma Jets 1303.8.13 Beam-Generated Plasmas 1323.8.14 Properties of Plasma Sources for Plasma Chemistry 1333.8.15 Conclusions 135

3.1 BASIC CONCEPTS OF PLASMA PHYSICS

The name plasma defines a quasi-neutral many-particle system (total charge zero) consisting of sufficient high concentration of free positive (q+) and negative (q−)charged species, e.g., positive ions, negative ions, and electrons The system maycontain additionally neutral atoms, molecules, radicals, and excited species Theclassical plasma is characterized by higher mean kinetic energy of electrons or otherplasma species (1–105eV) compared to usual matter, and exceptional electromagneticphenomena like the shielding of the far-ranging Coulomb interaction and collectiveoscillations of the electron or ion ensemble Due to the presence of free chargedparticles the plasma is electrically conductive Furthermore, plasmas reveal nonlinearbehavior and have specific dielectric properties, and in interaction of charged plasmaparticles with an external magnetic field the plasma shows an anisotropic behaviorconcerning the charged particle transport and electromagnetic wave propagation

3.1.1.1 Classification of Plasmas Concerning Their Energy Content

The plasma state of matter in astrophysics, terrestrial environment, and under ficial conditions in laboratory stretches over wide ranges in electron concentrationand mean kinetic energy or temperature of charged species Therefore, the plasma

Inertial fusion

Magnetic fusion reactor Magnetic fusion experiments

Gas discharge plasmas

Flames

Solar corona Solar wind

Ionosphere

Interstellar gas

Degenerate quantum plasmas

Nonideal plasmas

Ideal (classical) plasmas

FIGURE 3.1 Temperature–density plot of the plasma world (With courtesy to A Könies.)

state is classified with respect to their energy content and physical-based collectiveinteractions of the charged particles into

• Ideal or classical plasmas

• Relativistic plasmas

Whereas relativistic plasmas are characterized by the mean electron energy k B ·

defined for the case that the mean translational energy exceeds the electrostaticinteraction energy between two charged species (e.g., positive ion with charge number

z and electron, Figure 3.1) By use of the coupling parameter () the ideal (classical)

plasmas are distinguished from nonideal plasmas

  1 (ideal plasma),  < 1 (weakly ideal plasma)

 > 1 (strongly nonideal plasma)

r ∝ n −1/3

e is the mean particle distance

n eis the electron density

z is the ion charge number

In the special case that the mean particle distance is lower than the thermal de Broglie wave length the quantum effects have to be considered in the field of nonideal

plasmas, and the quantum or degenerate plasmas have to be distinguished Thedegenerate quantum plasma is defined if the Fermi energy exceeds the translationalenergy of the electrons according to the degeneration parameter > 1

The ideal or classical plasmas are defined when the translational energy of

charged particles is the dominant energy (  1) like in the ideal gas approximation Within the region of ideal plasmas the nonequilibrium and nonthermal plasmas,

respectively, of electric gas discharges will be the focus and of interest

3.1.1.2 Ionization Degree

The degree of ionization χ gives information about the content of charged particles

in relation to the total particle concentrations In the simplest case, considering the

plasma of single charged positive ions, electrons, and neutrals, the degree of ionization

χ is defined as

where n e , n nare the particle concentrations of electrons and neutrals

The degree of ionization in plasmas observed in nature and laboratory varies over

a wide range between 10−8 and 1 From that point of view two important types ofplasmas can be distinguished:

1 Partially or weakly ionized plasmas with χ 1

2 Completely or strongly ionized plasmas with χ∼ 1

The considered nonthermal plasmas are exclusively associated to partially or weakly ionized plasmas (n e  n n) with ionization degrees of χ≈ n e /n n= 10−4–10−6

3.1.1.3 Quasi-Neutrality and Debye Shielding

The demand of quasi-neutrality (3.4) in plasmas has to be fulfilled over the

macroscopic length scale L of plasma dimension

z

z · e · n z+− e · n e − e · n−= 0 for L > λ D, (3.4)

where

z is the charge number of positive ions

n z+, n−, n e are the particle concentrations of positive ions with charge number z,

single charged negative ions and electrons

The electromagnetic interaction between charged species in the plasma and theirresponse to internal and external impacts lead to following deviations from thequasi-neutrality:

1 Debye shielding (charge shielding length): On the microscopic scale the tistical deviations from the quasi-neutrality in the electron and ion ensembleresult in the shielding of the far-ranging Coulomb interaction over a charac-teristic length scale, the Debye length λD, see Section 3.1.2 In other words,within regions with a dimension smaller than the Debye length the quasi-neutrality is fulfilled time-averaged, only Therefore, the plasma is defined ifthe deviation from the quasi-neutrality in relation to the characteristic length

sta-scales L of the plasma dimension is in the limit:

electric fields due to the polarization drift of charged species In particular,

the electrons are shifted out of the mean equilibrium position because oftheir lower inertia in respect to the heavy positive ions, see Section 3.1.4

3 Ambipolar diffusion: In spatially inhomogeneous plasmas with plasma

den-sity gradients over the length scale L λDspace charges are generated due tohigher mobility and diffusion coefficient of electrons against the heavy ions.The transport of positive ions and electrons is determined by the ambipo-lar electric field and described by the ambipolar diffusion coefficient, seeSection 3.1.5

4 Plasma boundary sheath: Plasmas in contact to condensed matter such asdischarge electrodes, surrounding solid walls or liquids, immersed probesfor plasma diagnostics and materials for surface processing are characterized

by a transition between the quasi-neutral plasma bulk and the interface to

the condensed matter: the plasma boundary sheath or plasma sheath The

electric potential and the charge of the interface are shielded against theplasma potential due to the self-consistent electric field of macroscopicspace charges in the plasma sheath, i.e., an electric double layer is formed,see Section 3.6

Considering the plasma as a continuum, the plasma can be described by

macro-scopic quantities In that way the Debye shielding length can be calculated by taking

into account the polarization effect around a selected positive and negative charged

particle as reference point, respectively The distribution of electrons (n e),

respec-tively, positive ions with state of charge z around the reference point is given by the Boltzmann distribution with electron temperature (T e ) and ion temperature (T+),respectively:

n e0 = n+0is the charged particle concentration without polarization

By use of the space charge density ρ and the linear approximation of the exponential

function for the condition e · ϕ/k B · T e,+ 1, the Poisson equation (3.8) can be solved

potential

Coulomb potential

FIGURE 3.2 Coulomb potential in comparison with the Debye shielding potential ϕ(r), see

(3.9), for two selected electron Debye lengths λDe, see (3.12)

Taking into consideration the total number of electrons N ein a sphere with radius

of the Debye length, it follows

The temporal deviation from the quasineutrality in nonthermal plasmas is mainly

determined by the oscillation frequency of the electron ensemble—the electron

equi-librium position relative to the positive ion leads to the formation of macroscopicspace charges The resulting electric space charge field initiates an opposite electronmovement and vice versa: The electron ensemble performs collective oscillations.Considering the plasma as a continuum again the frequency of electron oscillationscan be calculated by the

Particle balance equation ∂ρ

The calculation of the divergence of (3.16) and its time derivative results in

In a similar procedure the ion plasma frequency ωp+of the collective oscillation of

positive ions with charge number z can be found according to

where A+[u] is the ion mass in unified atomic mass units with 1u = 1.66538921(73) ·

10−27kg It follows for the ratio between both plasma frequencies

about ωp+∼ 20.9 MHz (radio frequency range)

3.1.4 HIGH-FREQUENCYCONDUCTIVITY ANDPERMITTIVITY OFPLASMAS

In this section the forced oscillation of electrons in a high frequency harmonic electric

field E (t) = E0· exp(−iωt) is described to understand the basic physics concerning

the electric conductivity and electromagnetic wave propagation in ideal and coldplasmas without external magnetic field Concerning the behavior of the electrons ininteraction with high-frequency electric field of an electromagnetic wave, the quasi-homogeneous field approximation is used That means the amplitude of electronoscillation is small compared with the wavelength [1] Taking into calculation theelectric field force and the Langevin term for considering the elastic collisions ofelectrons with neutrals by use of the mean collision frequency νe, the 1D equation of

motion in z-direction for the electrons can be written as

Using the electric power P e = j e · E, the real part Re{P e = j e · E} corresponds to the

absorbed power of the electrons from the electric high-frequency field

Re{Pe} = n e · e2· νe

2 eff= σ(ω) · E2

Considering the resulting polarization P pol due to the shift of electrons in the

high-frequency electric field by the distance z against the static positive ion ensemble, it

By use of the Maxwell relation (3.40) the dispersion function k (ω) of the

electro-magnetic wave can be obtained without exact solution of the Maxwell equations inthat special case

ε is the complex permittivity

c0is the speed of light

c ph is the phase velocity, and k is the wavenumber

According to this simplified calculation it follows for the dispersion function k (ω),

ω

• Temporal and spatial change of wave amplitude

• Strong absorption→ resonances in dispersion function, k(ω) → ∞, c ph= 0

• Reflection→ zero points in the dispersion function, k(ω) = 0, c ph→ ∞