Handbook of Sputtering Technology

639 Pages·2012·24.778 MB·English

1
Thin Films and Nanomaterials
Hideaki Adachi and Kiyotaka Wasa
ChapterOutline
1.1 ThinFilmsandNanomaterials 4
1.2 ThinFilmDevicesandMEMS 10
1.2.1 ThinFilmDevices 10
1.2.2 ThinFilmMEMS 12
1.3 ThinFilmDeposition 14
1.3.1 PhysicalVaporDeposition,PVD 14
VacuumDeposition 14
PulsedLaserDeposition 16
MolecularBeamEpitaxy 17
Sputtering 17
MiscellaneousPVDProcesses 22
1.3.2 ChemicalVaporDeposition,CVD 23
ThermallyActivatedCVD 23
Plasma-AssistedCVD 24
1.4 CharacterizationofThinFilms 25
1.5 SputteringasaNanomaterialEngineering 28
References 36
Thin films are fabricated by the deposition of material atoms on a substrate. A thin
film is defined as a low-dimensional material created by condensing, one-by-one,
atomic/molecular/ionic species of matter. The thickness is typically less than sev-
eral micron-meters. The thin films are typically deposited by a thermal evaporation
of source materials in vacuum. Figure 1.1 shows the typical evaporation process
forthedepositionofthinfilms.
Historically, thin films have been used for more than a half century in making
electronic devices, optical coatings, instrument hard coatings, and decorative parts.
In 1960s, thin film transistors (TFT) and/or thin film integrated circuits (ICs) were
extensively studied. These trials were not used in practice due to the drift of the
TFT. After the developments of Si-(MOS) (Metal Oxide Semiconductor) IC in
1970s, thin film materials were used only for passive devices. The real market of
thinfilmscouldnotbedeveloped likeSi-IC. Avarietyofnew materialswas devel-
oped in academic phase such as a diamond-like carbon (DLC), high-T supercon-
c
ductors. The thin film technology is a well-established material processing
technology. However, the thin film technology is still being developed on a daily
HandbookofSputterDepositionTechnology.
©2012ElsevierInc.Allrightsreserved.
4 HandbookofSputterDepositionTechnology
Figure1.1 Typicalthinfilmdeposition
systeminvacuum.
Substrate
Vacuum
Thin films
Evaporated atoms
Heating source materials
basis, since it is a key in the twenty-first century development of new functional
materialssuchasnanometermaterialsand/orman-madesuperlattices.
Thin film materials and devices are also available for minimization of toxic
materials, sincethe quantity used is limitedonly toasurface and/or thin film layer.
Thin film processing also saves energy consumption in production and is consid-
eredtobeanenvironment-benignmaterialtechnology.1
Thin film technology is both an old and a current key material technology.
Thin film materials and deposition processes have been reviewed in several
publications.2(cid:1)19 Among the earlier publications, the Handbook of Thin Film
Technology4 is still notable, even though 40 years have passed since the book was
published and many new and exciting developments have occurred in the interven-
ingyears.
At the beginning of this chapter, the features of the thin films are reviewed
in relation to their applications for thin film devices and/or thin film micro-
electromechanical systems (MEMS). Secondly, fundamentals of thin film fabrica-
tionprocessesareexplainedforabetterunderstandingofthesputteringdeposition.
1.1 Thin Films and Nanomaterials
Thin films are grown by the deposition of material atoms on a substrate. Typical
thin film growth process on a substrate by the deposition material atoms is shown
inFig.1.2.
Thethinfilmgrowthexhibitsthefollowingfeatures:
1. The birth of thin films of all materials starts with a random nucleation process followed
bynucleationandgrowthstages.
2. Thenucleationandgrowthstagesaredependentuponvariousdepositionconditions,such
asgrowthtemperature,growthrate,andsubstratesurfacechemistry.
3. The nucleation stage can be modified by external agencies, such as electron or ion
bombardments.
ThinFilmsandNanomaterials 5
Evaporated atom
Removing atom
Adsorb on substrate
Trapped atoms
Brownian motion
Nucleation and
film growth
Substrate
Figure1.2 Thegrowthmodelsofthethinfilms.
4. Filmmicrostructure,associateddefectstructure,andfilmstressdependonthedeposition
conditionofthenuclearstage.
5. Crystal phase and crystal orientation of the thin films are governed by the deposition
conditions.
The basic properties of thin films, such as film chemical composition, structural
properties, film thickness, are controlled by the deposition conditions. The thin
filmsexhibituniquepropertiesthatcannotbeobservedinbulkmaterials:
1. Unique material properties resulting from the atomic growth process on the growing
substrates.
2. Sizeeffectsincludingquantumsizeeffectscharacterizedbythethickness,crystalorienta-
tion,andmultilayeraspects.
Bulk materials are usually sintered from powder of source materials. The parti-
cle size of the powder is of the order of 1μm in diameter. Thin films are synthe-
sized from ultrafine particles, i.e., atoms or a cluster of atoms. Ultrauniform
compound materials are possibly synthesized from the atomic collisions between
theadatomsonasubstratesurface.
Anotherconsequenceofthethinfilmgrowthprocessisthephenomenonofsolu-
bility relaxation. The atomistic process of growth during codeposition allows
doping and alloying of films. Since thin films are formed from individual atomic,
molecular,orionicspecies,whichhavenosolubilityrestrictionsinthevaporphase,
the solubility conditions between different materials are considerably relaxed. This
allows the preparation of multicomponent materials, such as alloys and compounds
over an extended range of compositions as compared to the corresponding bulk
materials. It is thus possible to have tailor-made materials with desired properties,
which adds a new and exciting dimension to materials technology. An important
example of this technology of tailor-made materials is the formation of hydroge-
natedamorphousSifilmsforuse insolar cells.Hydrogenationhas madeitpossible
to vary the optical band gap of amorphous Si from 1 to about 2eV and to decrease
the density of dangling bond states in the band gap so that doping a (n and p) is
madepossible.20
Thepropertiesofthinfilmsare governed bythe depositionmethod. Thethermal
evaporation is a well-known process. The deposition process using the irradiation
of energetic species is known as sputtering. Bunsen and Grove first observed
6 HandbookofSputterDepositionTechnology
sputtering phenomena in a gas discharge tube over 150 years ago. The cathode
electrode was disintegrated by the discharge. Since that time, the basic level of
understanding of the sputtering process has been fairly well developed. It was
known that the disintegration of the cathode materials was caused by irradiation
of the cathode surface by highly energetic ions. The removed particles, called
sputtered species, were composed of highly energetic atoms. Their energy ranges
were 1(cid:1)10eV, which was higher than those of the other deposition processes such
as the thermal evaporation in vacuum. The energetic sputtered species lower the
synthesis temperature. Typical example of lowering the growth temperature is dia-
mond growth at room temperature.21 The known bulk diamonds are synthesized at
high pressure (B50,000psi) and high temperature (2000(cid:3)C). The deposition of
energetic carbon ions (B10(cid:1)100eV) enables the growth of cubic diamond crystal-
litesatroomtemperatureasshowninFig.1.3.aItispossibletosynthesizeahexag-
onal diamond at room temperature. Natural diamonds are cubic phase which is
stable on the earth. Hexagonal diamonds cannot not be grown under thermody-
namic equilibrium conditions; rather they are grown under nonthermal equilibrium
conditions.22Thegrowthofhexagonaldiamondssuggeststhatthethinfilmprocess
providesexoticmaterialsofnonthermalequilibriumphase.
Similar to the growth of hexagonal diamonds, varieties of exotic materials are
synthesized based on the thin film processes such as superconducting cuprate of
layered oxide perovskite compounds with high-transition temperature, T 23,24 spin-
c
dependent tunneling magnetoresistance (TMR) effect,25 stressed perovskite ferro-
electric thin films with high-Curie temperature,26,27 thickness and/or crystalline
size effects on dielectric constants of perovskite ferroelectrics,28,29 layered ferro-
electric perovskite thin films with a giant permittivity,30 and/or with pseudopyro-
electric effects.31 The stress affects the superconducting critical temperature for
both metal superconducting thin films and the high-T cuprate thin films.32,33 Thin
c
films further exhibit variety of interesting performance including intrinsic
Josephson junctions in the high-T curates34 and giant magnetoresistance (GMR)
c
effectsinmultilayer.35(cid:1)37,b
Figure1.3 Crystalstructureofroom
temperaturegrowthofdiamonds.
ThinFilmsandNanomaterials 7
Thethinfilmprocessisalsoavailableforthefabricationofthenanometermate-
rials. Nanomaterials are defined as follows: materials or components thereof in
alloys, compounds, or composites having one or more dimensions of nanometer
size(1nm51029m510A).Thenanomaterialsareclassifiedintothreetypes:
1. Zero-dimensional nanomaterials have all three dimensions of nanometer size (e.g., quan-
tumdots).
2. One-dimensional nanomaterials have two dimensions of nanometer size (e.g., quantum
wires).
3. Two-dimensional nanomaterials have one dimension of nanometer size (e.g., thin films,
superlattices).
The phenomenological dimensionality of nanomaterials depends on the size rela-
tive to physical parameters such as quantum confinement regime (#100 atoms),
mean free path of conduction electron (,10nm), mean free path of hot electron
(#1nm), Bohr excitation diameter (Si58.5nm, CdS56nm, GaAs5196nm), de
Broglie wavelength (,1nm).38 The three types of nanomaterials have been success-
fully synthesized by the thin film deposition processes such as codeposition, layer-
by-layer deposition in an atomic scale, and nanolithography.39 The typical structure
of the nanometer superlattices produced by the thin film process are shown in
Fig.1.4.
002 Surface
200
PT/PLT multilayers
PLT
<001>
PT
(001) MgO
10nm 50nm
Figure1.4 Typicalstructureofthenanometersuperlatticesproducedbythethinfilm
process.
8 HandbookofSputterDepositionTechnology
The current progress in thin film research is much indebted to the atomic obser-
vation technology including the scanning tunneling microscope (STM) developed
byBinnigandRohrer.40
Table 1.1 summarizes the interesting phenomena expected in thin film materials
anddevices.
Table1.1 InterestingPhenomenaExpectedinThinFilmMaterials
Increaseofresistivity,ρ,inmetal,ρ /ρ B(4/3)[γln(1/γ)]21
F B
ReducedT ,α,inmetal,α /α B[ln(l/γ)]21
cr F B
Sizeeffects Reducedmobility,μ,inmetal,μ /μ B[ln(l/γ)]21
F B
γ5t/l{1 Anomalousskineffectathighfrequenciesinmetal
t:filmthickness Reducedthermalconductivity,K,inmetal,K /K B(3/4)[γln(1/γ)]
F B
l:meanfreepathof Enhancedthermoelectricpower,S,inmetal,
electrons S /S B11(2/3)[(lnγ 21.42)/(lnγ20.42)]
F B
Reducedmobilityinsemiconductor,
μ /μ B(111/γ)21
F B
Quantumsizeeffectsinsemiconductorsandsemimetal,att,λ,de
Brogliewavelength:thickness-dependentoscillatoryvariationof
resistivity,Hallcoefficient,Hallmobility,andmagnetoresistance
GalvanomagneticsurfaceeffectsonHalleffectand
magnetoresistanceduetosurfacescattering
Fieldeffects Conductancechangeinsemiconductorsurfacebymeansofelectric
field,insulated-gateTFT
Spacechargelimited SCLCthroughinsulator,J;
current(SCLC) J510213μ εV2/t3(A/cm2)
d
(one-carriertrap-freeSCLS)
μ :driftmobilityofchargecarriers,ε:dielectricconstant
d
V:appliedvoltage
Tunnelingeffects Tunnelcurrentthroughthininsulatingfilms,voltage-controlled
negativeresistanceintunneldiode
Tunnelemissionfrommetal,hotelectrontriodeofmetal-base
transistor
Electroluminescence,photoemissionofelectrons
Tunnelspectroscopy
Tunnelcurrentbetweenislandstructureinultrathinfilms
Ferroelectricity IncreaseofCurietemperatureT byfilmstress
C
ΔT 52ε C(Q 12Q )σ(cubic-tetragonal)
C 0 11 12
C:Curieconstant;Q :cubicelectrostrictiveconstants;σ:
ij
hydrostaticstress
Thicknessdependenceofdielectricconstant
Crystallinesizeeffects
Giantpermittivity
Chargepumping,pseudopyroelectriceffects
Superconductivity Superconductivity-enhancement:increaseofcriticaltemperature,
T ,inmetalwithdecreasingthickness,t,ΔT BA/t2B/t2and/
C C
orcrystallitesize
(Continued)
ThinFilmsandNanomaterials 9
Table1.1 (Continued)
Stresseffects:tensilestressincreasesT ,compressivestress
C
decreasesT inmetal
C
Proximityeffectsinsuperimposedfilms:decreaseofT inmetal
C
causedbycontactofnormalmetal
Reducedtransitiontemperature,T ,(T /T )251(cid:1)1/(0.210.8t),
S S C s
wheret,ratioofthicknessofsuperconductingfilmsanda
s
criticalthicknessbelowwhichnosuperconductivityisobserved
foraconstantthicknessofnormalmetalfilms
Increaseofcriticalmagneticfield,H ,
C
atparallelfield,
H /H BO24λ/t,
CF CB
λ,penetrationdepthduetoGinzburg(cid:1)Landautheory
attransversefield,
H /H 5O2K,
CF CB
K,Ginzburg(cid:1)Landauparameter
Reducedcriticalcurrent,J ,
C
J /J Btanh(t/2λ)
CF CB
Supercurrenttunnelingthroughthinbarrier,Josephsonjunction,
andtunnelspectroscopy,intrinsicJosephsonjunctionsinhigh-T
C
cuprates
Magnetics Increaseinmagneticanisotropy:
Theanisotropiesoriginateinashapeanisotropy,
magnetocrystallineanisotropy,strain-magnetostriction
anisotropy,uniaxialshapeanisotropy
Magneticfreeenergy(E)isexpressedasE5Kusin2Φ2M(cid:1)H
whereK :magneticanisotropyconstant,M:magnetization,H:
u
magneticfield,Φ:anglebetweenMandeasyaxis
Increaseinmagnetizationandpermeabilityinamorphousstructure,
and/orlayeredstructure
GMReffectsinmultilayers:
MR5(ρ 2ρ )/ρ
AF F F
whereρ :antiparallelresistivity,ρ :parallelresistivity
AF F
GMRmultilayeronV-groovesubstrate
σ 5σ cos2θ1σ sin2θ
CAP CIP CPP
whereσ :conductivityforcurrentatanangletoplane,
CAP
σ :conductivityforcurrentinplane,σ :conductivityfor
CIP CPP
currentperpendiculartoplane,θ:angleofV-groove
Spin-dependentTMReffects:
TMR52P P /(12P P )
1 2 1 2
whereP ,P :spinpolarization
1 2
Exchangecouplingattheinterfacebetweenferromagnetic(FM)
andantiferromagnetic(AF)layers
(cid:1) Increaseofcoercivefield(H )
C
(cid:1) ShiftofM-Hcurve(exchangebias)
ElectronTransportPhenomena(F5film,B5bulk).
10 HandbookofSputterDepositionTechnology
1.2 Thin Film Devices and MEMS
1.2.1 Thin Film Devices
Since the latter part of the 1950s, thin films have been extensively studied in rela-
tion to their applications for making electronic devices. In the early 1960s, Weimer
proposed TFT composed of CdS semiconducting films.41 He succeeded in making
a256-stageTFTdecoder,drivenbytwo16-stageshiftresistors,fortelevisionscan-
ning, and associated photoconductors, capacitors, and resistors. Although these thin
film devices were considered as the best development of both the science and tech-
nology of thin films for an integrated microelectronic circuit, the poor stability
observed in TFTs was an impediment to practical use. The bulk silicon carbide
(SiC) MOS devices were successfullydeveloped at theendof1960s.42Thus,inthe
1960s, thin film devices for practical use were limited to passive devices such as
thin film resistors and capacitors. In the 1970s, several novel thin film devices
were proposed, including thin film surface acoustic wave (SAW) devices,43 and
integrated thin film bulk acoustic wave (BAW) devices,44 and thin film integrated
optics.45
A wide variety of thin film devices were developed. Of these, one of the most
interesting areas is a thin film amorphous silicon (a-Si) technology proposed by
Spear.46 This technology achieved low-temperature doping of impurities into a-Si
devices and suggested the possibility of making a-Si active devices such as a-Si
TFT and a-Si solar cells.47,48 In the 1980s, rapid progress was made in a-Si tech-
nology. Amorphous Si solar cells have been produced for an electronic calculator
although the energy conversion efficiency is 5(cid:1)7% and is lower than that of crys-
talline Si solar cells. In the middle of the 1980s, high-quality a-Si technology has
led to the production of a liquid crystal television with a-Si TFT. Due to the
improvement of a-Si thin film, the energy conversion efficiency of the a-Si solar
cells has been improved and the efficiency is as high as 12%.49 The a-Si/poly-Si
stacked cell shows the efficiency of 21(cid:1)23%,50 which is the same order of magni-
tude as the efficiency of single crystal Si solar cells. The processing temperature is
as low as 300(cid:3)C for the a-Si thin film solar cells. The thin film technology for the
high-efficiency a-Si solar cells with small processing energy will be a key technol-
ogy for the production of the clean energy, since the single crystal bulk Si solar
cellsconsumemuchenergyfortheproductionofthesolarcells.51
Other interesting thin film devices are ZnO thin film SAW filters for a color
television, mobile telephone, and a variety of communication systems.52,53 The
SAW devices act as a solid-state band-pass filter, which cannot be replaced by
a-Si-IC, and are composed of a layered structure of ZnO thin piezoelectric film on
a glass substrate. The high-quality growth techniques available for ZnO thin films
have made possible the large-scale production of these devices. This type of thin
film device is used in a higher frequency region of GHz band for CATV, satellite
TV, and personal telephone. The SAW devices are provided in a form of small tip-
like ceramic capacitors. Thin film SAW device is essential for a fabrication of RF-
MEMS.
ThinFilmsandNanomaterials 11
SiC thin film high-temperature sensors54 are another attractive thin film device
produced inthe 1980s. They suggestthe possibility of high-accuracy, low-tempera-
ture synthesis of high melting point materials by thin film growth processes.
The SiC thin film devices are now developed as a high-power semiconducting IC
and/or radiation resistance semiconducting devices. The nanometer multilayered
structure provided by δ-doping process realized the high mobility SiC MOS
devices.TheSiCMOSdeviceshaveahighpotentialforsavingenergyinconsumer
electronicssystems.55
Magnetic heads having a narrow magnetic gap for video tape recording systems
and for computer disk applications were produced by thin film processing. In the
productionofthemagneticgap,anonmagneticspacerwasformedfromglassmate-
rial. Prior to the use of thin film technology, the spacer manufacturing process was
quite complex. For instance, magnetic head core material was first immersed in a
mixed solution of finely crushed glass, then taken out and subjected to centrifuga-
tion so that a homogeneous glass layer was deposited on the opposing gap surfaces
of the core members. After forming a glass film on the core surfaces by firing the
deposited glass layer, the two opposing gap faces are butted against each other
with the glass layer sandwiched in between and then fused together by a heat treat-
ment to form the desired operative gap. Since the width of the magnetic gap was
around 0.3μm, these methods were difficult to use in production because of the
difficultyincontrollingthefilmthicknessofthefiredglass.
Thin film deposition technology overcomes these problems and realizes the pro-
duction of the magnetic head with a narrow gap length of 0.3μm.56 The narrow-
gap forming technology is based on the thin film deposition process in atomic
scale. The thin film technology with the precise deposition develops the layered
new materials including GMR magnetic materials. The spin-dependent TMR
effectsprovideahigh-densitymemorydiskofupto200Gbit/inch2.
At present, various kinds of thin film materials are used for the production of
the electronic devices including high precision resistors, SAW filters, optical disks,
magnetic memories, sensors, and active matrix for liquid crystal TV. Thin films of
the high-T superconductors are used for the fabrication of superconducting planar
c
filters with GHz band.57 The integrated acousto-optic and magnetooptic devices
have been further developed for optical information processing by Tsai.58 Recent
progress of these thin film devices is owed to the developments of Si-large-scale
integration (LSI) technology including thin film growth process, micro-fabrication,
and analysis technology of both the surface and interfaces of the thin films. It is
noted that the ferroelectric dynamic random access memory (FEDRAM) is devel-
oped and now used in practice. The ferroelectric thin films were used in the past
for the high capacitive electronic components and/or pyroelectric sensors.59 The
development of FEDRAM has owed to the integration of the LSI technology and
the ferroelectric thin film technology. After the development of FEDRAM, several
new thin film memory devices have been proposed, including a magnetoresistance
dynamic random access memory (MRDRAM) and a bi-stable resistance memory
(BRDRAM). Although these memory devices are not widely used yet, these new
memorydeviceswillbeakeyLSItechnologyforthenextgeneration.60(cid:1)65
12 HandbookofSputterDepositionTechnology
Figure 1.5 shows a photograph of a building with thin film solar cells on the
roof. Thin film solar cells are important energy sources in this century. At present,
the efficiency of the thin film solar cells is not acceptable. An advanced thin film
materialtechnologywillresultinhigh-efficiencysolarcells.c
1.2.2 Thin Film MEMS
MEMS are based on micro-mechanical engineering. The design and the fabrication
of MEMS are initially based on the combination of the micro-mechanical technol-
ogy with the micro-fabricating technology established in the Si-IC technology. The
Si-IC technology includes deposition and etching of dielectrics such as thin films
of SiO and/or Si N , and metal electrodes such as thin films of Al and Pt. Several
2 3 4
typesofMEMS areproposed.
The first stage of MEMS includes the active elements such as micro-actua-
tors, which are fabricated by the integration of bulk functional devices such as
piezoelectric ceramic cantilevers with a conventional electromechanical system.
The further miniaturization of MEMS could be achieved by the integration of
thin film functional devices instead of the bulk functional devices.
Figure1.5 Photographofthinfilmsolarcellsontheroof.ResearchInstituteofTechnology
InnovationsfortheEarth(Kyoto).