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viii Preface
The needfor plasmaprocesstools withmuch higherplasmadensities,
Ne> 1012 cm-3, was one ofthe major motivations in the search for novel
plasmasourceswithhigherionizationefficiency. Itwaswidelyacceptedthat
such plasmas could be generated in discharges excited at higher RF
(including microwave) frequencies, or in discharges employing power
coupling schemes which are more efficientthan capacitivecoupling, orby
utilizingconfinementand resonanteffects ofstaticmagneticfieldgenerated
by external means (coils, permanentmagnets).
Inthe lastfewyears,substantialprogresshas beenmade inthedesign,
research, and developmentofvarioustypesofplasmasources which could
beconsideredascandidatesforthereplacementofa"traditional"13.56MHz
capacitivelycoupledsource. Some ofthese sources, suchas ECRsources,
have a rather long history ofdevelopment (since the pioneering works of
Musil and Suzuki in the 1970s), and are successfully used in submicron
plasma etching (Hitachi ECRtool), while others (helical resonator) were
appliedto plasma processing recently and are still inthe R&D stage.
Thepresentbookcompriseseightchapterswhichdescribethephysical
principles, design features, plasma parameters, and process operation
characteristics ofRF and microwave high density plasmasources suitable
for use in lowpressure (1-100 mtorr), large area(D = 15-25 em), plasma
tools. During its preparation, the editor has endeavored to avoid two
extremes, i.e., making the book too academic (even too theoretical) or too
practical,bycompilingpublicationswhichmaygivethereader"mountains"
ofplasmaparametersandprocessdatabutlackthephysicalprincipleswhich
govern the source performance and control plasma parameters. Thus, he
invited contributors who are both designers and "applicators" of novel
plasma sources, and who also made considerable contributions to the
understandingofthe physics ofplasmasources.
The book starts with helicon plasmasources described inthe chapter
writtenbyFrancisF.Chen,oneofthepioneeringdesignersofheliconsources
andauthorofmanytheoreticalandexperimentalpapers inplasmasciences.
Hepresentsatheoreticalmodeloftheheliconwave excitation,propagation,
andabsorptioninmagnetizedplasmasatlowpressuresofO.I-50mtorr, and
at magnetic fields of30-1000G. He discusses the possiblemechanism of
wave absorption (Landau damping) believed to be "responsible" for the
generationofvery highplasmadensities (up to 1013cm'). Thethree most
promisingtypes ofheliconantennasandhelicon plasmasourcesoperatedat
Preface ix
RF frequencies of2-30 MHz are described and accompanied by detailed
plasma characterization. Some results ofhelicon source applications for
plasmaetchingarealso discussed.
The inductively coupled RF plasma sources are represented by two
sources-theplanarinductivesourceandtheinductivelycoupledRFplasma
source. The design features, RF power coupling scheme, plasmaparam
eters,andperformanceofplanarinductivesourcearedescribedbyDr. John
Forsterand Dr.JohnKeller. ThistypeofRFsource,developedbyIBM and
LamResearchCorp., isnowacommercialproductandissuccessfullyusedin
polysilicon,metal, and silicon oxideetching. The planarsourceistypically
operatedat a"traditional"RF frequencyof13.56 MHz,and atpressuresof
afewmtorr. WithRF powerof1-2kW, and plasmaconfinementby static
magneticfields,the plasmadensity ofafew 1012cm-3was achievedbothin
argon and in reactive gases. The authors also discuss the physics ofRF
power absorption and plasma density spatial profile. It should also be
noticedthatsincethischapterwaswritten,manypapershavebeenpublished
dealing with modeling, diagnostics, and process characterizationofplanar
plasmasources, often called TCP (Transformer Coupled Plasma).
The other type ofinductively coupled RF plasma source utilizing a
traditional solenoid (helical coil) wound around the plasmachamber with
dielectricwalls, ispresentedbyWayneJohnson. Hedescribesthe principle
and performanceofashieldedhelical resonatorplasmasourcewith acoilof
lengthequaltotheoddnumberofquarter-wavelengths(orhalf-wavelengths)
ofthe driving RF frequency. The operation ofRF discharge in the "pure"
inductive mode was achieved by using a slotted electrostatic shield which
suppressescapacitivecouplingbetweentheRFcoilandtheRFplasma. The
exclusion ofcapacitive coupling allows substantial improvement of RF
plasmaionizationefficiencyandavoidsthenegativeeffects oftheionsheath
phenomenonontheprocessquality. Theauthorpresentstheuniquefeatures,
plasma parameters, and operational conditions ofthe shielded RF source
(SRFS), and also illustrates the source capability by its performance and
results in anisotropic plasma etching ofpolysilicon and silicon oxide, and
photoresist stripping.
Capacitively coupled RF plasma sources are represented by a sym
metrical(two equal RFelectrodesfedfrom the push-pullRF generator)RF
discharge driven at frequencies much higher than 13.56 MHz. Michael
ColganandMeyyaMeyyappanpresentacomprehensivereviewandanalysis
x Preface
of the progress in research and development of very high frequency
capacitivelycoupledRFdischargesoperatingatfrequenciesof40-150MHz
andpressuresofl-500mtorr. TheydiscussRFpowerdissipativeprocesses,
includingstochasticelectronheatingontheplasma-sheathboundaryandthe
role ofRFelectrodesheath(s)for maintenanceofRFdischarge. Theyalso
showhow RF-driven frequency affects the discharge ionizationefficiency,
plasmadensity,andDC potentialbetween RF electrodeand RF plasma. By
using RF frequencies of50-100 MHz, the authors increasedthe portionof
RF powerabsorbed by plasmaelectronsto 30-50 %, resulting ina plasma
density of 1011 cm-3. The authors also present experimental results
illustratingthe capability ofvery high frequency capacitively coupled RF
dischargesforplasmaprocessing,inparticular,fordepositionofamorphous
siliconfilms. Imayaddthatwiththeemploymentofthestaticmagneticfield
ofafewtensgauss,theionizationefficiencycouldbefurtherincreasedtothe
level close to thoseofmicrowave and inductivelycoupledplasmas.
Anextensivereviewofsurfacewave(SW)plasmasourcesispresented
inthechapterwrittenbyMichelMoisan,JoelleMargot,andZenonZakrzewski.
TheuniquefeaturesofSWplasmasourcesarethepossibilityofthecomplete
separationofthedischargezonefrom theplasmaprocesszone, andthe wide
rangeoftheoperational"external"parameterssuchasexcitationfrequency,
3MHzto 10GHz,and gas pressure, 0.1 mtorrto 5atm. The authors have
made a considerable contributionto the design and developmentofsurface
wave plasmasources as well as to the studyofthe physics ofsurfacewave
excitationandpropagation,andtothepropertiesofplasmassustainedbythe
surface waves. Three major types ofSW launchers, matching and tuning
techniquesandcircuitry,and SWplasmasourcesoperationalconditionsare
given in detail.
Microwave plasma sources are represented by three chapters. The
chapterby Jes Asmussengives a detailed description ofthe whole class of
microwave plasma sources utilizing the tuned resonant cavity and the
excitation antenna. This class ofsources, calledMicrowave Plasma Disk
Reactors (MPDR), havebeen successfullyused inmany applications such
as plasma etching, diamond coating, ion beam etching, and silicon oxide
deposition. Also presented is the history ofMPDR development and the
description of major concepts and modifications of microwave plasma
sources and reactors including the multipolar ECR reactor. Microwave
mode excitation, matching problems, and power absorption efficiency are
Preface xi
discussedindetail as wellas the results ofnovelapplicationsofMPDRto
largearea(45emdiameter)diamondthinfilmdeposition,andGaAsetching.
AnElectron Cyclotron Resonance (ECR) plasma source employing
magnetic coils, commonly called NTT-type source, or divergent field
source, ispresentedbyJamesStevens. Hedevelopedamodelofpropagation
andabsorptionofwhistlerwavesinoverdenseECRplasmawhichexplains
the existence ofvarious ECR plasmamodes with different plasmadensity
radial and axial profiles. He also analyzes microwave power matching
conditions andpresents techniques whichprovidesgood coupling ofmicro
waveimpedancewiththat ofanECRplasmathat results in90%microwave
power absorption and plasma densities ofa few 1012 cm2. The author
discusses effects ofleft- and right-hand polarized wave propagation and
absorptiononECRplasmasourceperformanceandplasmaparameters. He
alsoreviewsdifferentmodificationsofdivergentmagneticfieldECRplasma
sources and analyzes plasma parameters relationships with microwave
power, source configuration, and gas pressure. The capability of ECR
plasma sources for plasma processing is illustrated by the results of
anisotropic etching performed using the ECR plasma source ofhis own
design, which employs only right-hand polarized waveofTEll mode.
The chapter written by Jacques Pelletier describes the Distributed
Electron Cyclotron Resonance (DECR) plasmasource. This type ofECR
sourceemploysseveral(typically eight)microwaveantennas symmetrically
surroundingtheprocesszone,andmultipolarmagneticplasmaconfinement.
Pelletier is the designer ofone ofthe first DECR plasma sources. This
chapterdescribes indetail the principles ofmultiple magnetic field plasma
confinementand,inparticular,trappingoffast(primary)electrons. Healso
discusses the importance ofperiphery ionization near the chamber walls.
DesignsofseveralplasmasourcesutilizingprinciplesofdistributedECRare
described together withtheir major characteristics. Also discussedare the
applicationsofDECRplasmasourcesinvariousareasofplasmaprocessing:
plasma etching, wafer cleaning, SiOzplanarization, silicon epitaxy, and
tungsten thin filmdeposition.
Itis impossible to include inthis book all novel high density plasma
sources whicharesuitableforlowpressureplasmaprocessing. Forinstance,
we"missed"ahollowanodeplasmasourcerecentlyintroducedtothemarket
byDrytek (Lam Research Div.), and aninductively coupled plasmasource
which was developed by Applied Materials Corp. Nevertheless, the editor
believes that this book presents a comprehensive survey and detailed
xii Preface
description and characterization of most advanced high density plasma
sources used inplasmaprocessing. Healso hopesthis book willbehelpful
for scientists and engineers working on plasma source design and process
development.
Woburn, Massachusetts OlegA. Popov
January, 1996
Contributors
OlegA. Popov, Editor
MatsushitaElectric Works
Research& DevelopmentLaboratory
Woburn, Massachusetts
Jes Asmussen, Jr. John H. Keller
DepartmentofElectricalEngineering IBM Microelectronic Division
Michigan State University Hopewell Junction, New York
East Lansing, Michigan
.loelle Margot
Francis F. Chen DepartmentofPhysics
Electrical Engineering Department University ofMontreal
UniversityofCalifornia,LosAngeles Montreal, Quebec, Canada
Los Angeles, California
MeyyaMeyyappan
Michael J. Colgan Scientific Research Associates, Inc.
Alimentrics, Inc. Glastonbury, Connecticut
Morris Plains, New Jersey
Michel Moisan
John C. Forster DepartmentofPhysics
Applied Materials Corporation University ofMontreal
Santa Clara, California Montreal, Quebec, Canada
Wayne L.Johnson Jacques Pelletier
Prototech Research, Inc. LEMD-CNRS
Tempe, Arizona Grenoble Cedex, France
xiii
xiv Contributors
James E. Stevens Zenon Zakrzewski
MicroelectronicsDevelopment Adadernie des Sciences de Poigne
Laboratory Gdansk, Poland
SandiaNational Laboratories
Albuquerque, NewMexico
NOTICE
Tothebest ofourknowledgetheinformationinthispublicationis
accurate;however thePublisherdoesnotassumeanyresponsibil
ityorliabilityfortheaccuracyorcompletenessof,orconsequences
arisingfrom,suchinformation.Thisbookisintendedforinformational
purposesonly. Mentionoftradenamesorcommercialproductsdoes
notconstituteendorsementorrecommendationforusebythePublish
er. Final determination of the suitability of any information or
product foruse contemplatedbyanyuser, and the manner ofthat
use, is the sole responsibility of the user. We recommend that
anyone intending to rely on any recommendation of materials or
proceduresmentionedinthispublicationshould satisfyhimselfas
tosuch suitability,and that he canmeet all applicable safety and
health standards.
1
Helicon Plasma Sources
Francis F. Chen
1.0 INTRODUCTION
1.1 BriefHistory
Low-frequency whistler waves are well known in ionospheric re
search because oftheir insensitivity to variations in plasma density and
magneticfield. When these waves are confined to a cylinder, they losetheir
electromagnetic character and become partly electrostatic, changing their
propagation and polarizationcharacteristics as well. These boundedwhis
tlers, called helicons, were first investigated and observed in solid state
plasmas)2][17][52][75] Upon the discovery by Lehane and Thonemannlv'I in
Englandthat helicons can exist in a gaseous plasma, a series oftheoretical
papers,l5]-[7][41][42][49][60] following ageneralformulationfor waves incylin
ders byWoods[89][90] laidthe foundation for helicon research. Experiments
were resumed in Australia by Davies and Christiansenlf-l and Boswell.[8]
Interest inthis academic subject subsided until Boswell!9][1O][l6] found that
helicon waves were unusually efficient in producing plasmas; in fact, the
absorptionofRF energy was morethan 1000times fasterthanthe theoreti
cal rate due to collisions. In1985, Chen[23] proposed Landau damping as
the reason for this discrepancy. The complete calculation appeared in
1991,[27] togetherwith the suggestionthatwave accelerationbythe Landau
1
2 High DensityPlasma Sources
mechanismcanbe usedto produceprimaryelectronsattheoptimumenergy
for ionization. Since 1985,a numberof papershave been writtento verify
the collisionlessdampingmechanism,[32][34][61][68][92]toclarifythephysicsof
helicon discharges,[21][34J[55J[56][77]-[79][82][83][85] to explore the possibilities of
heliconsources'p4][15][24]-[26][33][35][37][68][91]andtotesttheirworthinmanu
facturing applications.[13][18]-[20][54J[59J[70][73][74][80][81] In the period 1986
1992,themaingroupsengagedinheliconresearchwereBoswellet al.,!8][13]
[15][20][68][73][74][91][92] Chen et al.,£24]-[27][32]-[35][37][43][85] and Shoji et
aU18][19][54]-[56][61][70][77]-[82] By 1993, the helicon source had emerged as
oneofthe majoralternativestothe RIE(ReactiveIon Etching)dischargefor
high-throughputplasma-aidedmanufacturing.
1.2 Advantages ofHelicon Sources
To be a viable alternative to the RIE discharge, the ECR (Electron
Cyclotron Resonance) source, and the RFI (Radiofrequency Inductive) or
TCP (Transformer CoupledPlasma) discharge, the helicon source must be
a better practical tool. In contrast to the other plasma sources, helicon
discharges started from a solid theoretical background, but their perfor
mance in practical use is relatively undocumented. Optimization ofthe
configuration has not yet been extensively investigated, and computer
modeling in realistic geometries has only recently begun. The purported
advantages ofhelicon sources are as follows.
High Density. Typical helicon discharges inargon with 1-2 kW of
RF power have average plasma densities in excess of 1013 ern"and peak
densities oforder 1014cm', about two orders ofmagnitude higher than in
usual processingplasmas. However, densities ingasesotherthanargonare
considerablylower, the densitywill dropwhen spreadovera largearea,and
the quoted power is somewhat higher than normally used. When these
factors are taken into account, this advantageof heliconsources is reduced
to perhaps a factor of3-10, which is still an appreciablegain.
High Efficiency. The fact that helicon discharges produce more
plasma at given input power than other RF or DC discharges has been
knownsincethe early experimentsofBoswelJ.f8][lO] Thatthe densityjumps
dramatically when the discharge snaps from a nonresonant RF discharge
into the helicon mode has been seen by Boswell.l'Pl by Chen,[25] and by
Shoji.£82] Thekey to the helicon'sefficiencyisthe accelerationofelectrons
to ionizing energies by "surfing" on the wave, leading to rapid transfer of
wave energy to primary electrons.
