Zhurin Viacheslav Zhurin I Industrial Ion...

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Zhurin

Industrial Ion Sources

Ion sources and the development of ion beams are produced by the creation of strong electrostatic fi elds in plasma. The Hall-current ion source operation is based on the physical principles of electron magne-tization and on the increase of plasma resistance and electron lifetime, during which electrons can interact with neutral particles and ionize them. This concept was implemented in the development of modern electric propulsion devices for space apparatuses, which were later transformed and are now used as ion sources.

From the contents: Hall-current ion sources Ion source and vacuum chamber infl uence of various effects on ion beam parameters Oscillations and instabilities in Hall-current ion sources Optimum operation of Hall-current ion sources Cathodes-neutralizers for ion sources Industrial gridless broad beam ion sources producers and the need for standardization Operation of industrial ion sources with reactive gases Ion beam and radiation impact on substrate heating Ion beam energy and current Plasma Optical Systems Ion and Plasma Sources for Science and Technology Ion Assist, and its different applications Magnetron with Non-Equipotential Cathode

Viacheslav V. Zhurin, PhD is President of Colorado Advanced Technology LLC (www.ion-plasma.com). Dr. Zhurin is an in-ternationally recognized specialist in Industrial Ion Sources and Electric Propulsion. Before coming to the USA in 1991 he was in several Academy of Science Research Institutes as a Head of Laboratories, Departments and Deputy Director. Since 1991 he is in the USA working on Ion and Plasma Sources and Electric Propulsion. He was at Kaufman & Robin-son Inc. (19912004) and Veeco Instruments (20042005).

Viacheslav Zhurin


Broadbeam Gridless Ion Source Technology

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Viacheslav V. Zhurin


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Viacheslav V. Zhurin


Broadbeam Gridless Ion Source Technology

The Author

Prof. Viacheslav V. Zhurin548 Charrington CourtFort Collins, CO 80525-5870USA

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Contents

Preface XI

1 Hall-Current Ion Sources 11.1 Introduction 11.2 Closed Drift Ion Sources 21.3 End-Hall Ion Sources 51.4 Electric Discharge and Ion Beam VoltAmpere Characteristics 191.5 Operating Parameters Characterizing Ion Source 24

References 26

2 Ion Source and Vacuum Chamber. Influence of Various Effectson Ion Beam Parameters 29

2.1 Introduction 292.2 Mass Entrainment 322.3 Charge-Exchange Influence on Ion Beam Flow 342.4 Doubly Ionized Particles and Their Role 362.5 Influence of Vacuum Chamber Pumping Rate 402.6 Dielectric Depositions on an Anode During Operation

with Reactive Gases 412.7 Estimation of Returned Sputtered Particles to Ion Source 432.8 Influence of Ion Source Heating on its Operation 472.9 Negative Ions and their Role 482.10 Conclusion 50

References 50

3 Oscillations and Instabilities in Hall-Current Ion Sources 533.1 Introduction 533.2 Oscillations and Instabilities 563.3 Types of Oscillations 563.3.1 Ionization Oscillations 563.3.2 Flight Oscillations 583.3.3 Contour Oscillations 58

V

3.3.4 Hybrid Azimuthal Oscillations 603.3.5 Oscillations Due to High Pressure 613.3.6 Oscillations Due to Ion Beam Underneutralization 613.3.7 Oscillations Due to Incorrect Operation 623.3.8 Oscillations Due to Presence of Water Vapors 623.4 Conclusions and What to Do About Oscillations 63

References 64

4 Optimum Operation of Hall-Current Ion Sources 674.1 Introduction 674.2 Regime of Nonself-Sustained Discharge and Optimum Operation

Conditions of End-Hall Ion Source 704.2.1 Discharge VoltAmpere Characteristics 704.3 Operation of End-Hall Ion Source with Excessive

Electron Emission 714.4 Ion Beam Energy of End-Hall Ion Source 734.5 End-Hall Ion Source Optimum Magnetic Field for Ion Beam

Current 764.6 Ion Beam Energy Distribution as a Function of Angle

With Various Emission Currents 814.7 Conclusion 82

References 83

5 Cathode Neutralizers for Ion Sources 855.1 Introduction 855.2 Ion Beam and its Practical Neutralization 875.3 Hot Filament Electron Source and Thermoelectron Emission 935.3.1 RichardsonDushman Formula for Thermoelectron

Emission Current Density 935.3.2 Recent Improvements in HF Design 1015.4 Hollow Cathodes 1055.4.1 Introduction 1055.4.2 Hollow Cathode Physics 1095.4.3 Hollow Cathodes for Industrial Ion Sources 1155.4.4 HC Modes of Operation 1215.4.5 Hollow Cathode Tip and Keeper 1235.4.6 General Conclusions about Hollow Cathodes 1255.4.7 Other Cathodes for Ion Sources 1265.4.7.1 Plasma Bridge 1265.4.7.2 Neutralizer with Closed Electron Drift 1285.4.7.3 Radio-Frequency Neutralizers 1295.4.7.4 Cold Cathodes 1345.4.7.5 Neutralization with Alternating Current 1355.4.7.6 Plasma Bridge Based on Magnetron Discharge Principles 1365.4.7.7 Ion Beam Neutralization with Magnetron Electrons 139

VI Contents

5.4.7.8 Ion Beam Neutralization with Electron Gun 1405.4.7.9 Microwave Discharge Neutralizer 1415.4.8 Cathode Erosion Rates 1415.4.9 Important Features of Cathode Neutralizers 1425.5 Conclusions about Cathode Neutralizers 142

Appendix 5.A: Web Addresses 144References 144

6 Industrial Gridless Broad-Beam Ion Source Producers, Problemsand the Need for Their Standardization 149

6.1 World Producers of Ion Sources 1496.1.1 Theoretical Consideration for Closed Electron

Drift Design 1546.2 Specific Designs of End-Hall-Current Ion Sources for Thin Film

Technology 1596.3 Nontraditional Broad Beam Ion Sources 1686.4 Linear Ion Sources 1786.5 Hall-Current Ion Sources Basic Operation Parameter

Problems 1836.6 The Need for Standardization of Ion Sources 1906.7 Conclusions 194

Appendix 6.A: Web Addresses 194References 195

7 Operation of Industrial Ion Sources with Reactive Gases 1977.1 Introduction 1977.2 Low- and High-Temperature Oxidation 1987.3 Ion Source Operation with Dielectric and Insulating Depositions

on an Anode 1997.4 End-Hall with Grooved Anode and Baffle 2037.5 End-Hall With Hidden Anode Area for Continuing Discharge

Operation 2057.6 Practical Operation of Hall-Current Ion Sources with

Reactive Gases 206References 208

8 Ion Beam and Radiation Impact on Substrate Heating 2098.1 Introduction 2098.2 Target-Substrate Heating By Radiation and Ion Beam 2118.3 Experimental Measurements of Ion Beam and Radiation Impact

on a Target-Substrate 2188.4 Conclusion 222

Appendix A.8: Web Addresses 222References 222

Contents VII

9 Ion Beam Energy and Current 2239.1 Introduction 2239.2 Ion Beam Energy Distribution 2259.3 Retarding Potential Probes 228

References 240

10 Plasma Optical Systems 24110.1 Introduction 24110.2 Plasma Optics Evolution 24210.3 Electrostatic Fields in Plasma 24310.4 Plasma Optical Systems with Equipotential

Magnetic Field Lines 24410.5 Plasma Lenses 24510.6 Practical Applications of Plasma Optical Systems

in Technology 24810.6.1 Ion Beam Focusing and Defocusing with Plasma Lens 24810.6.2 Ion Beam Soldering with Focused or Partially Focused

Ion Beam 249References 254

11 Ion and Plasma Sources for Science and Technology 25511.1 Introduction 25511.2 Vacuum Pump 25511.3 Commutating Properties of Gas Discharge in Magnetic Field 25611.3.1 Plasma Switch 25711.4 Hollow Cathode as Vacuum Valve 25811.5 Ion Source for Levitation 26011.6 Hydrogen Motion through Metal Membrane for MPD

Plasma Source 26111.7 Plasmaoptical Mass Separator 26211.8 Plasma Stealth and Other Effects in Modern Airdynamics 26311.9 Conclusion 266

References 266

12 Ion Assist, and Its Different Applications 26912.1 Introduction 26912.2 Ion Beam Sputtering 27012.3 Ion Assisted Deposition 27212.4 Biased Target Deposition 27812.5 Ion Assisted Magnetron Deposition with Magnetron Electrons

for Ion Beam Neutralization 28012.5.1 Ion Afflux and Ion Assist 28112.6 Ion Assisted Magnetron Discharge for Enhancement of Cathode

Sputtering 28312.6.1 Magnetron Discharge with Ion Beam Assist 283

VIII Contents

12.7 Conclusion 285References 285

13 Magnetron with Non-equipotential Cathode 28713.1 Introduction 28713.2 Short History of Magnetron Development 28813.3 Magnetron with Segments at Different Potentials 29213.4 The Phenomenology of a Magnetron Discharge with NEC 30413.5 Conclusion 306

References 307

Index 309

Contents IX

Preface

The ion source, according to general definition, is a device for obtaining directedflows of ions. Ion sources are utilized as accelerators of charged particles for thinfilm technology, mass separators, plasma current switches, plasma accelerators,vacuum pumps, and many other devices.

The main application of ion sources described in this book is for materialprocessing: cleaning, etching various targets and surfaces, assisting in depositionof thin films on substrates, and obtaining new combinations of materials that, insome cases, can only be done with ion beams.

Industrial broad beam Hall-current ion sources, or industrial ion sources, will bedescribed and discussed in detail in this book. These particular types of ion sourcesare widely utilized for various technologies at the industrial level. Many companiesuse these ion sources 5 to 7 days a week, 24 hours a day, withoutmajor interruptions.Interruptions aremainly caused by opening vacuum chambers in which ion sourcesare placed or removing processed parts, introducing new portions of parts forprocessing by ion beams of ion sources, or for technical repair or substitution ofion sources, their parts, targets, substrates, and so on. Some companies have largenumbers of ion sources (up to 30 to 50 devices) and, as a rule, do not make anychanges in the designs, magnetic fields, emission currents, and so on.

Research and development (R&D) laboratories in small and large companies, andin many technical universities, utilize a few ion sources for numerous physical andtechnical tasks or obtaining various materials with new or improved properties ofknown materials. Universities and small companies can introduce some changes indesign and operation procedures, which can change the main parameters of ionsources.

The purpose of this book is to offer assistance and support to the users as well asthe designers and developers of industrial ion sources. Many developers are stilltrying to improve the performance of existing ion sources with broader operationalparameters, with a certain specific range of parameters, or with non-traditionalworking gases.

The author will provide the data necessary for everyday work with ion sources, andwill offer advice on how to obtain certain features and how such features can beestimated that are true or to what extent they are true, in order to provide the bestpossible results inmaterial processing with ion sources. A detailed description of the

Preface XI

well-known and hidden problems of some industrial ion sources will also begiven.There will be no special chapters about plasma physics, as it is assumed that most

readers have a general knowledge of the subject. Technological Ion Sources by IanBrown [1] or Introduction in Plasmadynamics by Alexei Morozov [2] are excellentsources for information on the basics of plasma physics.It is known that efficiency, reliability, and longevity of various materials, parts,

details of devices, and machinery are determined by the surface, not volumetricproperties of materials. Plasma technologies in vacuum, based on accelerated ionflows, provide a wide range of possibilities for control of composition, structure,degrees of processing, and chemical and physical properties of various materials,especially their surfaces. The technologies that utilize ion sourcesmake it possible toprocess surface layers of materials with high operational properties that wereunknown before.Ion sources are based on the technology of plasma accelerators that were first

utilized for electric propulsion and controlled nuclear fusion problems. During thedevelopment of industrial ion sources, there are specific physical and technical taskscaused by the generation of ionized flows of various working gases with theutilization of numerous design and technological approaches the analysis of theregimes of processing materials and devices.This book will review a variety of sources that are accepted as classical devices, or

established typical designs that can be utilized for technological tasks. Certain basicoperating parameters of such devices will be discussed and advice will be given onhow to operate them in their optimum regime, that is, with high operational valuesbut without producing harm to the technological processes.This book will provide the data needed for daily work with ion sources and general

descriptions of the most well-known designs. It is hoped that the book will provideinformation on the selection of suitable ion sources and ion production methods forspecific applications. The book concentrates on practical aspects and introduces theprinciple functions of gridless Hall-current broad beam ion sources. Basic plasmaparameters will be defined and discussed. The working principles of various ionsources will be explained, and examples of each type of ion source will be presentedwith their operational data. Tables of ion currents and ion beam energies character-izing the performance of different ion sources will be presented. The ion source andits place in a vacuum chamber, and ion beam interaction with various parts of avacuum chamber are described. Space-charge, ion source and vacuum chamberinteraction effects, and numerous methods of ion beam neutralization are dis-cussed. Various methods for the measurement of current, ion beam energy, andradiation effects are estimated and compared. The author is hopeful that this bookwill be a valuable reference on the subject of industrial ion sources, and beneficial topractitioners, university professors, and graduate students interested in plasmaaccelerators, ion interactions with materials, and ion beam techniques.As noted above, there are numerous things industrial ion sources can do and

various problems accompanying their operation. In many cases there are answers tothese problems, but in some cases there are no straight solutions because this

XII Preface

subject is still in a developing stage andmany problems are under investigation. It isa very interesting yet challenging task to find answers in the fields of physics andchemistry, to find new horizons where industrial ion sources broaden our views.

The author thanks Dr. T. Randolph (Jet Propulsion Laboratory, Pasadena, CA),Prof. A.I. Bugrova (Russian Institute of Radio-Electronics, Moscow), Prof. K.N.Kozubsky (Fakel Enterprise, Kaliningrad, Russia), Prof. M. Kristiansen (TexasTech University, Lubbock), and Dr. V. Chutko (Vecor, Irvine, California) for theirdiscussions that helped in this work. Over the last several years, I worked withDr. E.V. Klyuev and Dr. A. I. Sidorov (Ion Sources & Technologies, Moscow region,Russia) on various improvements in ion sources; our work in this book is presentedwith many illustrations of new designs and unorthodox approaches. Chapter 9, IonBeam Energy and Current, was written with Dr. P.A. Tsygankov (Bauman TechnicalUniversity, Moscow), who makes unique multigridded probes for qualification ofion sources and magnetrons. Dr. A. I. Sidorov helped with many figures of thisbook. Chapter 13,Magnetron with Nonequipotential Cathode was written with Dr. P.A. Tsygankov and Dr. N.G. Elistratov (Bauman Technical University, Moscow). Manythanks to a friend of our family, J. Bell, for his generosity. I am very grateful toB. Davis for his support.

I want to thank my daughter Dasha for being smart and funny, with unusualtalents in tennis and yoga, growing fast, encouraging, and surprising us all; my sonDmitri for his cleverness and inventiveness; and my daughter Olga, who chose herown journey in this life and left warm memories.

Special thanks go to my lovely wife, Lyudmila, who always was and is a greatsupport and inspiration in my life with science and various inventions, all of whichwould be impossible to achieve without her.

References1. Brown, Ian G. (ed.) (2010) The Physics

and Technology of Ion Sources, Wiley-VCHVerlag GmbH & Co, KGaA, November 30,2004.

2. Morozov, A.I., (2006) Introduction intoPlasmadynamics, 2nd edn, Fizmatlit,Moscow, (in Russian).

Preface XIII

1Hall-Current Ion Sources

1.1Introduction

Ion sources and the development of ion beams are produced by the creation of strongelectrostatic fields in plasma. For quite a long period of time, before the 1950s,electrical discharges were studiedwithoutmagnetic fields. It was believed that strongelectrostatic fields in a plasma volumewere impossible to develop. At that time, it wasexperimentally found that strong electric fields could be observed only in thin layersof Debye-layer scale near electrodes, in places where quasineutrality is broken.

Then, in the 1950s, successful experiments confirmed the theoretical possibilityof magnetic field utilization providing magnetization of electrons, which sharplyhelped to increase plasma electrical resistance and to obtain large electric fields withthe development of Hall currents in crossed electric and magnetic fields in a plasmavolume.

TheHall-current ion source operation is based on the physical principles of electronmagnetization and on the increase of plasma resistance and electron lifetime, duringwhich electrons can interact with neutral particles and ionize them. This concept wasimplemented in the development of modern electric propulsion devices for spaceapparatuses, which were later transformed and are now used as ion sources.

In the ion source discharge channel, the electrons aremagnetized, ifvet 1 (ve isthe electron cyclotron frequency in a magnetic field; t is the average time betweenelectron collisions with other particles and the discharge channel walls). The ions areusually notmagnetized,vit 1 (vi is the ion cyclotron frequency in amagneticfield;t is the average time between ion collisions with other particles and dischargechannel walls) andmove under the applied electrical field between the anode and thecathode. During discharge in the magnetic field, electrons move to the anode not instraight lines, but rather in circles in crossed magnetic and electric fields; theyexperience collisions with working gasmolecules, ions, discharge channel walls, andalso due to oscillations. Ions are not influenced by themagnetic field, but move fromtheir places of origin, usually near to the anode into the cathode direction along theelectricfield.Moving from the ion source, an ionflow captures the necessary numberof electrons for neutralization and develops what is called an ion beam, though the

Industrial Ion Sources: Broadbeam Gridless Ion Source Technology, First Edition. Viacheslav V. Zhurin. 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

j1

ions are accompanied, in general, by electrons. Electrons drifting in the azimuthdirection neutralize the space charge of ions in the discharge channel. Due to thisphysical fact, inHall-current ion sources (contrary to gridded ion sources) there is nolimit for an ion beam current that exists in gridded ion sources caused by a spacecharge of ions. Because electrons move in circles in crossed magnetic and electricfields, the ion sources and thrusters use a principle known as ion source-thrusterswith closed electron drift.

Part of the regular operation ofHall-current ion sources is the existence of a varietyof oscillations of discharge current and voltage in a certain range of values, especiallyat low and high values. Operatingmagnetic fields depend on ion source dimensions,but as a rule, magnetic fields in a discharge channel of the cylindrical ion sources arenot very high: from about 100 G to a maximum of 500 G. It is necessary to note thatlinear ion sources with closed electron drift, which will also be discussed in the book,can have substantially higher magnetic fields, sometimes over 1000 G.

In the following sections, existing gridless ion sources and their designing featuresand differences will be described, followed by a discussion of the main physicalcharacteristics of such ion sources.

1.2Closed Drift Ion Sources

Closed drift ion sources (CDIS) were developed to a very high degree of efficiency asthrusters in the 1960s; then in 1972, Russian scientists launched a Meteor satellitewith the thruster based on the closed electron drift principle.

Currently, every third Russian satellite is equipped with a closed electron driftthruster. There are already over 200 thrusters in space; many CDIS are on Americansatellites too.

Besides the magnetization of electrons, one of the basic ideas in the successfuloperation of CDIS for a broad range of discharge voltages, currents, and a variety ofgases is that themagnetic field in a discharge channel increases from the anode to anion source exit. In otherwords,CDIS is an ion source with a positivemagnetic gradient ina discharge channel. The main operating magnetic field is a radial component. Thedischarge channel has an annular form.

In general, there are two types of CDIS: magnetic layer ion source (MLIS)(Figure 1.1) and two modifications of anode layer ion source (ALIS), shown inFigure 1.2a and b [1, 2] as schematics and three-dimensional pictures. Themain partsof CDIS anode, magnetic poles, andmagnetic coils developing themagnetic field are shown in these figures. Cathode neutralizers are not shown.

In Figure 1.2a and b, ALIS have a different placement of anode: in Figure 1.2a, it isinside the discharge channel; and in Figure 1.2b, some of the anodes surface isextended outside the magnetic poles.

Such an extended anode provides a slightly narrower range of operationaldischarge voltages, but it has the advantage of sharply reducing the erosion ofmagnetic poles in comparison with regular ALIS anode placement (Figure 1.2a.)

2j 1 Hall-Current Ion Sources

The dimensions of a cylindrical form CDIS are from about 20mm of the exit-plane diameter up to 290mm (recent thruster design). As ion sources, CDIS areused from about 50 to 100mm of exit diameter. Working gases are Ar, Xe, O2, N2,H2, CH4, and others.

The range of operation for discharge voltages is Vd 801000 V; for dischargecurrents, it is Id 0.115 A. The mean ion beam energies are about Ei 0.7 eVd(in eV); the ion beam current is about Ii (0.70.8) Id. The erosion rate of the anodeis negligible; the erosion rate of magnetic poles is substantial, though for ALIS, theerosion rate of poles with an extended anode area outside the discharge channel [2]is negligible. The ion beam divergence for MLIS is about 20 (for 7080%) of ionbeam flow; for ALIS, it is about 1520 [1]. The hollow cathode (HC) is utilized as asource of electrons for ion beamneutralization and ionization of neutral atoms;HCerosion is negligible.

In thin film technology, cylindrical CDIS are not widely utilized. However,Diamonex [3, 4] use cylindrical MLIS for DLC coating. Russian companies, Platarand MIREA use cylindrical MLIS for a variety of thin film tasks (etching, sputtering,ion beam assistance). ALIS modification without an external source of electrons isutilized extensively by many companies, mainly in the form of linear ion sources ofdifferent dimensions (up to 300 cm long).

CDIS advantages:

1) High transformation of a discharge current Id into an ion beam current Ii, Ii/Id0.70.8, with utilization of an external source of electrons and with adequatemagnetic field optimization.

Figure 1.1 Magnetic layer closed drift ion source.

1.2 Closed Drift Ion Sources j3

2) Wide range of discharge voltages (ion beam mean energies), from aboutVd 801000V, Ei 55700 eV.

3) Optimummagneticfields for cylindrical CDIS are in the range of 100Gto 600G;for linear ALIS, themagnetic fields are usually substantially higher, over 1000G.

4) Because of good magnetic field optimization, the cylindrical CDIS can operateup to about 1.5 kWof applied powerwithout awater-cooled anode.Hot discharge

Figure 1.2 (a) Regular anode layer closed drift ion source. (b) Anode layer closed drift ion sourcewith extended anode to reduce sputtering of magnetic poles.


plasma with an optimized magnetic field is well separated from the dischargechannel walls and anode.

Shortcomings:

1) Need for a variety of operating conditions in magnetic field optimization. Theratio of the ion beam current to the discharge current Ii/Id f(Hmax) is not alinear function of themagnetic field, and themaximumdepends on the workinggas, discharge voltage, and current. Optimization is provided by magnetic coilswith variablemagnetic fields. Permanentmagnets can only be used for a specificselected range ofVd and Id and aworking gas. In practice, notmany userswant toperform such optimization.

2) Operation of CDIS without an external source of electrons in the so-called self-sustained discharge [5] (discussed in subsequent chapters) produces low trans-formationof the discharge current into an ionbeamcurrent Ii (0.050.1) Id andhigh spread of ion beam mean energies Ei (0.40.5) eVd (eV); and in theion beam energy distribution, there are ions with low (from several eV) and high(up to twice eVd) distribution. The length of ALIS is usually in the range 1520 to100, 200, and even 300 cm.

1.3End-Hall Ion Sources

The discharge channel has a cylindrical form with a massive hollow conical anode.The cathode, serving as a source of electrons, is usually in the form of a hot filament(HF) or HC. Generally, electrons are only magnetized at the exit part, where themagnetic field has a radial component connecting to the externalmagnetic pole. Alsoat the exit, themagnetic field is quite low because the end-Hall ion source utilizes thepermanentmagnetsmagneticfield, which decreases from the place under the anodewhere a gas distributing system is usually located. The magnetic-field value on thepermanent magnet top (or on electromagnet) is about 12 kG; at the ion source exit(front flange), this magnetic field is reduced to about 50100 G. Due to this, the end-Hall ion source can be considered as a source with a negative gradient of magnetic field.The next series of figures show a variety of end-Hall-type ion sources with the

following main parts labeled: anode, insulators, body (magnetic path), reflector-gasdistributor, working gas, permanent magnet, magnetic coil, magnetically soft iron,and conical insert. The cathode is not shown; it is either HF or HC. (Cathodes arediscussed later in Chapter 5.)Information about one of the first end-Hall-type ion sources was published in

1973 [6]. Figure 1.3 presents a schematic drawing and three-dimensional picture oftheHall-current ion source for the development of low-energy ions, indicating an exitarea for the neutralized ion beam; a front flange; a discharge channel made ofdielectricmaterial; an anode connected to a power supply (not shown); a backflange; acathode as an HF; a working gas; and a system of electromagnetic coils providing anon-uniform axial symmetric magnetic field distribution in a discharge channel.Amagnetic field in the discharge channel is sufficient formagnetization of electrons

1.3 End-Hall Ion Sources j5

(vete 1). At the same time, ions are not magnetized (viti < 1) in the same way asclosed drift ion sources-thrusters. The ion source was operated in stable conditionswith several working gases, such as hydrogen (H2), nitrogen (N2) and argon (Ar), atdischarge voltagesVd 150600Vandwith discharge currents Id 0.151.0 A. Also,it was reported that the ion beamangular divergencewas 16 and the ion beamenergywas close to the discharge voltage in eV.

The distinctive feature of this ion source is the presence of the cathode neutralizerinside the discharge channel. In all further designs, the cathode neutralizer is outsidethe discharge channel. The front flange, which has a small conical opening, doesnot allow the extraction of high ion beam currents. With the discharge currentsId 0.151.0 A, the ion beam currents were Ii 0.430mA. In other words, thedischarge current conversion into the ion beam was quite low.

The main design of an end-Hall ion source, which is still used by most producers,is presented in Figure 1.4. As shown, a hollow cone-shaped anode and a magnetic

Figure 1.3 One of the first Hall-current low-energy ion sources [5]. HF cathode is in the dischargechannel.

Figure 1.4 End-Hall ion source with permanent magnet on axis [7].


fieldmade by one or several permanentmagnets are placed on the sources axis underthe reflector. The permanent magnet is usually fabricated from Alnico, either 5 or 8;Alnico magnets are good for sustaining high temperatures up to about 540 C. Thisdesignwas suggested inEnd-Hall IonSource, byH.R.Kaufman andR.S.Robinson [7].It was developed and extensively studied at Kaufman & Robinson Inc. (K&R), with atleast three different dimensions (Mark I, Mark II, Mark III) and discharge currentsfrom under 1 A up to about 15 A and discharge voltages from about 50V up to 300Vwith various working gases. After its patenting in 1989, it was produced byCommonwealth Scientific Corporation (CSC) for about 10 years, and later in1999 by Consolidated Vacuum Corporation (CVC) and in early 2000 by VeecoInstruments. It is necessary to note that Veeco and K&R continue to provideimprovements to this design, which will be discussed later. The main design createsa simple and reliable device. Many foreign ion source producers, especially Chineseand S. Korean companies, made similar designs.

This and other varieties of end-Hall ion sources are characterized by the followingfeatures:

1) Overall dimension of the outer flange for the exit of an ion beam. Depending onthe geometry and outerflange, end-Halls are designed for application of a certainworking gasmassflow,which translates into a discharge current and then into anion beam current. The external flange, as a rule, is made of a soft magnetic ironand is part of a magnetic circuitmagnetic pole; the external shell of the ionsource is also fabricated of a soft iron.

2) Geometrical dimension of the anode, which is usually several centimeters inlength and diameter. Anodes can bemade of a variety ofmaterials that determinethe electrical conductivity, its participation in a thin film process (as a contam-inant, or chemical element that can be a part of a thin film), and in some cases,possible resistance to anode poisoning. Anode poisoning is usually adeposition of thin films of reactive gas compositions with the anode, dischargechannel, and vacuumchambermaterials. These dielectric depositions drasticallychange the operational characteristics of the ion source, decreasing majorparameters such as the ion beam current, and mean energy. This unpleasantfeature will be discussed in detail in Chapter 6.

3) Design of a working gas introduction into a gas discharge channel, in particular,how aworking gas is applied; how the area under the anode is designed; howwellmixed aworking gas is when applied into the anode area; if anyworking gas has apossibility to escape from a discharge channel before being ionized by an appliedelectric potential; how the electrical conductive plate (sometimes called as areflector) between the anode area and magnetmagnetic pole (where a workinggas usually is applied) is affected by an ion beam that (in many cases of end-Hallion sources) has a component directed into the side of a reflector causingsputtering, producing damage, and contaminating an ion beam with thereflector material.

4) Magnetic field value in a discharge channel and how magnetic field lines aredirected; is a magnetic field gradient positive (in certain cases, an end-Hall ion


source can be designed with a positive magnetic field gradient) or negative; howpermanent magnets or magnetic coils are placed; what kind of material per-manent magnets are made of because it is important not to apply high levels ofheat to magnets due to the possible threat of being demagnetized.

5) In almost all plasmadynamic systems in which discharge takes place in electricand magnetic fields, and especially in the presence of crossed electric andmagnetic fields, there are various types of oscillations and instabilities of maindischarge values: discharge current and voltage. The analysis of plasma para-meters in ion sources shows that the ion beam energy spread is determined bythe extended region of ionization and oscillations of electrical potential in adischarge channel. Development of ions with energy exceeding eVd shows thatoscillations play an important role and produce a significant impact on the ionbeam current and the energy of ions. Detailed description of various types ofoscillations and instabilities will be presented in Chapter 3.

Figure 1.5 shows the samedesign as Figure 1.4, where amagnetic coil [7] is utilizedinstead of a permanent magnet. A magnetic coil requires a separate power supply,but it allows changing magnetic field values in the discharge channel over abroader range. For those who want to use a magnetic coil, it is necessary to notethat the magnetic field distributions provided by a permanent magnet and amagnetic coil are slightly different, and the discharge behavior is slightly differentas well. Only scrupulous investigations show a different behavior of ion beamparameters.

Figure 1.6 shows an end-Hall of the S. Korean company, VTC-Korea [8]. Inthis design, a soft iron cylinder is inserted to continue amagnetic path close to a gasdistributor reflector in order to reduce the high-temperature impact on a perma-nent magnet.

The reason for inserting such a soft iron cylinder is the fact that manyproducers are trying not to use Alnico permanent magnets because they canget much higher magnetic field values with, for example, Nd-Fe-B magnets.

Figure 1.5 End-Hall ion source with magnetic coil on axis [7].


However, Nd-Fe-B magnets are significantly more sensitive to high temperaturesand their maximum operational temperature is about 150200 C, depending onthe magnets quality.

As shown in Figure 1.7,magnets are placed at the base of an ion source body [9]; asoft iron cylinder is placed where the permanent magnet is usually located on theion source axis, similar to Figure 1.4. This company [9] also utilizes regularplacement of a permanent magnet at the ion source base. This end-Hall ion sourcedesign is also equipped with a water-cooled anode and water-cooled magnetassembly.

Utilization of a soft-iron cylinder has another advantage: if an ion beam wouldpenetrate a reflector (and this happens quite frequently), it would not harm amagnet.

Figure 1.6 End-Hall with soft iron on top of permanent magnet to reduce temperature impact onmagnet [8].

Figure 1.7 End-Hall with magnets at base; soft iron is used instead of magnet; design reducestemperature impact on magnets [9].


As shown in Figure 1.8, the gas distribution area is substantially increased.Working gas is applied through holes that have a certain angle to provide a gasvortexflow for betterworking gas distribution [10]. This distribution increases the ionbeam current in comparison with the regular end-Hall design (Figure 1.4), whichtranslates to improved conditions for working gas ionization and, correspondingly,to a higher ion beam current than in a regular end-Hall.

Figure 1.9 shows another version of a gas distribution area with a straight-through working gas flow. According to Svirin and Stogny [11], in this design thegas distribution is arranged by the conical inserts and the hole. Here, theelectromagnet is utilized for finding the optimummagnetic field, and it is claimedthat such a design provides a higher ion beam current than in the regular end-Hall(Figure 1.4).

Figure 1.8 End-Hall with buffer area for improved gas distribution [10].

Figure 1.9 End-Hall with hollow insert under anode reduces inserts sputtering [11].


Figure 1.10 shows an end-Hall ion source with a discharge channel that is underthe anode potential, including a gas distributing area-reflector [12]. Our experimentswith a similar design showed the following features of such a design:

1) Reflector connected to the anode operates as the anode itself. An electron currentdelivered by an external source of electrons (neutralizer) becomes attracted tothe central part of a reflector; mainly a longitudinal magnetic field providesconfinement of a discharge area and directs straight to the center of a reflectoranode.

2) Such a design reduces the ion beam current compared to a reflector, which isunder a floating potential. For example, for argon working gas, a dischargevoltage Vd 50V and discharge current Id 5 A, and an ion beam current forend-Hall with a floating potential Ii 0.8 A; for the end-Hall ion source with areflector connected with anode, this value Ii 0.4 A. For Vd 100V, Id 5 A, anion beam current for a floating potential design Ii 1.2 A and for a reflectorconnected with an anode Ii 0.6 A.

In many cases, producers of end-Hall ion sources make the anode and reflectorfrom various materials, but mainly of stainless steel.

Here are some considerations about the utilization of the reflector and anodemade of stainless steel. The application of high currents and voltages leads to thestainless sputtering and development of magnetized flakes adjusted to a reflectorand standing on its top. Some flakes are several millimeters in length and cancreate an electrical connection between a reflector and an anode. In this case and asnoted above, an ion beam current will be reduced substantially. A reflector will bedamaged faster and an ion beam will be dirtier (more sputtering erosion of areflector area). To avoid such a situation, it is necessary to frequently inspect an ionsource discharge channel, and the reflector and anode must be cleaned regularly.Utilization of other materials, like Ta, Ti, Hf, and Mo, can be a good substitution

Figure 1.10 End-Hall with connected anode and reflector; working gas distributed through ashower cap [12].


for stainless steel. They are sputtered less and their components can participate incertain depositions of these materials.

Figure 1.11 shows an end-Hall design with the indirect water-cooled anodethrough a dielectric plate [13]. In order to have high ion discharge currents andvoltages (higher ion beam currents and energies) with high electric powers releasedinto a discharge channel, and not to overheat the main part of a discharge channelanode, it is necessary to water cool the anode because end-Hall-type ion sources havea low efficiency of transformation of the discharge current into the ion beam current.Water-cooled anode designs have been practically developed from the beginning ofthe end-Hall ion source introduction. In general, it is a water flow in the anode thathas a cavity with electrical separation of the anode potential through the insulators.The insulators must be clean, and the water should have no contaminating particles.That is why purifiedwater is sometimes utilized, and from time to time the insulatorsmust be cleaned of any contaminating residue.

Water-cooled anodesmake it possible to apply at least twice asmuch electric powercompared to radiation-cooled anodes.

Figure 1.11 [13] shows a schematic design of an unconventional anode-coolingsystemwhere the anode is cooled through a dielectric plate.Waterflows in a cavity of acopper plate under a dielectric plate. The anode, in such a case, is not directly cooled,but through the dielectric plate and at high applied electric powers of about 3 kW(Vd 200V, Id 15 A), it can be heated to very high temperatures of about 1070 C.With the direct water-cooled anode, it is heated to 500 C; at the same time, the gasdistributor reflector has decreased its temperature from 1050 C (direct watercooling) to 630 C. In a vacuum of about 105 103 Torr, the mean free path ofparticles is substantially longer than the dimensions of an ion source and there is noconvectional heat transfer. In the points of connection of any solidmaterial, there arevery limited areas of a contact. In such a case, the main heat transfer is realized byradiation only.

Figure 1.11 End-Hall with water-cooled anode through a dielectric plate; such design helps fastassembly [13].


However, such a design has certain advantages in comparison with the directwater-cooled anode:

1) Because the anode is not connected with a water flow, the whole design is verysimple. The discharge channel and anode can be assembleddisassembled in afew minutes if the source is cooled off.

2) In the problem of so-called anode poisoning [14] (discussed in Chapter 7), theradiation-cooled anodes and the anode design (Figure 1.11) in some dielectricand insulating depositions do not stick like a water-cooled anode surface due tothe high heated anode surface. Such end-Hall ion sources can operate longer inconditions of anode bombardment by dielectric and insulating particles.

However, the anode surface with indirect coolingmust be carefullymonitored andnot exposed to temperatures at which the anode surface couldmelt. Also, a sputteringeffect that continuously takes place by electrons increases with a surfacestemperature.

This design showed slightly better performance than a regular end-Hall designwith the discharge current transformation into the ion beam current with Ii 0.3 Id.

Figure 1.12 presents an end-Hall ion sourcewith not only awater-cooled anode, butwith water-cooled magnets. For certain technological processes that are highlysensitive to change of temperature regimes in the discharge channel, such a designserves very well. A soft iron cylinder completely substituted a permanent magnetwith a series of smaller dimension magnets placed at the lower flange base. As aresult, such a design can operate at a high applied electric powers of about 3 kW.Also, this design showed a very low sputtering rate of the gas distributor reflector.

Figure 1.13 presents the unconventional gas application into the discharge channelwith a regular type end-Hall ion source, similar to that shown in Figure 1.4. It did not

Figure 1.12 End-Hall with water-cooled anode and magnets helping stabilizing operatingparameters [15].


show any advantage in the discharge current transformation; it gave Ii 0.2 Id.However, a sputtering erosion of the reflector is substantially lower than for a regulargas application (Figure 1.4).

Figure 1.14 shows a working gas application through the anode. Despite thecomplexity, this gas introduction has certain advantages, such as improved ion beamenergy distribution, which is substantially narrower than with a regular gas appli-cation (Figure 1.4). A protective cap placed on a permanent magnet gives a signalwhen an ion beam goes through the reflector [10]. Also, in such a case, the gasdistributor reflector experiences significantly less sputtering about half as muchas the regular one.

Figure 1.13 End-Hall with working gas applied from top, between anode and upper flange; lowreflectors sputtering.

Figure 1.14 End-Hall with working gas through anode; protective cap over a magnet gives signalwhen ion beam goes through reflector [10].


Zhurin Viacheslav Zhurin I Industrial Ion...

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