Fachgebiet*3D-Nanostrukturierung,*Ins7tut*für*Physik*!

Contact:!yong.lei@tu1ilmenau.de,!yang.xu@tu1ilmenau.de!!Office:!Heliosbau!1102,!Prof.!Schmidt1Straße!26!(tel:!3748)!

www.tu1ilmenau.de/nanostruk!!

Vorlesung: ! Mittwochs (U), 9 – 10:30, C 108!Übung: ! ! Mittwochs (G), 9 – 10:30, C 108!

Yong Lei & Yang Xu

Techniken der Oberflächenphysik (Technique of Surface Physics)

Methods!for!the!preparaRon!of!surface!nanostructures!•  Chemical!vapor!deposiRon!(CVD)!•  Physical!vapor!deposiRon!(PVD)!•  Atmoic!layer!deposiRon!(ALD)!•  Electrochemical!deposiRon!•  Spin!coaRng!•  Template1assisted!!•  E1beam!lithography,!photolithography!•  ReacRve!ion!etching!(RIE)!•  PrinRng!technology!!•  Molecular!beam!epitaxy!(MBE)!

Chemical!Vapor!DeposiRon!(CVD)!

!!

Types!of!CVD!

•  Chemical!vapor!deposiRon!•  Plasma!enhanced!CVD!(PECVD)!•  Metal!organic!CVD!(MOCVD)!•  Atmospheric!pressure!CVD!(APCVD)!•  Low1pressure!CVD!(LPCVD)!!•  Ultrahigh!vacuum!CVD!(UHVCVD)!•  Aerosol!assisted!CVD!(AACVD)!•  Direct!liquid!injecRon!CVD!(DLICVD)!•  Microwave!plasma1assisted!CVD!(MPCVD)!•  Remote!plasma1enhanced!CVD!(RPECVD)!!!!

Thermal!CVD!

Example!Carbon!Nanotubes:!!Hydrocarbons!or!CO!are!used!as!precursor.!A!typical!growth!process!involves:!1st:!purge!reactor!with! inert! gas;! 2nd:! gas! flow! is! switched! for! specified! growth! period;! 3rd:! gas!flow! is! switched! back! to! inert! gas! while! the! reactor! cools! down.! For! growth! on!substrates,!catalysts!need!to!be!applied!on!substrate!before! loading! it! inside!reactor.!Typical!temperatures!for!catalyRc!CVD!in!CNT!growth!are!in!the!range!of!800–1500!K.!

When!a!convenRonal!heat!source!(e.g.,! a! furnace)! is! used,! the!technique! is! called! thermal! CVD.!It! consists! of! a! quartz! tube!inserted! into! a! tube! furnace! and!has!a!gas!inlet!on!one!side!and!a!gas!outlet!on!the!other!side.!The!sample! is! placed! onto! a! quartz!boat!inside!the!tube.!!

ReacRon!Process!in!CVD!•  Mass!transport!of!the!reactant!!•  Gas1phase!reacRons!•  Mass!transport!to!the!surface!•  AdsorpRon!on!the!surface!•  Surface!reacRons!!•  Surface!migraRon!•  IncorporaRon!of!film!

consRtuents,!island!formaRon!•  DesorpRon!of!by1products!•  Mass!transport!of!by1products!!

a)!Epitaxial!Growth!The!term!epitaxy!describes!an!ordered!crystalline!growth!on!a!monocrystalline!substrate.!Because!the!substrate!acts!as!a!seed!crystal,!the!deposited!film!takes!on!a!lajce!structure!and!orientaRon!idenRcal!to!those!of!the!substrate!

Homoepitaxy:* a! crystalline! film! is!grown! on! a! substrate! or! film! of! the!same! material.! This! technique! can!grow! more! purified! films! than! the!substrate,! can! fabricate! layers! with!different! doping! levels! and! layers! of!different!isotopes.!!

Heteroepitaxy:* a! crystalline! film! is!grown!on! a! substrate! or! film,! but! the!materials! are! different! from! each!other.! This! technique! is! used! to! grow!e.g.! GaN! on! Sapphire! or! AlGaInP! on!GaAs!

Homoepitaxial growth of Si on a Si substrate

SiCl4(g)+2H2(g = Si(s)+4HCl(g) at approx. 1000-1200 °C

b)!Vapor1Liquid1Solid!(VLS)!growth!•  CatalyRc!nanodots!on!substrate!(e.g.!UTAM!technique)!•  Equilibrium!vapor!pressure!of!the!catalyst!must!be!small!so!that!the!droplet!does!not!vaporize!

•  Catalyst!must!be!inert!!

Nanostructures!prepared!by!CVD!

Chang&et&al.&Chem.&Mater.,&Vol.&16,&No.&24,&2004!

1D!ZnO!nanowires!and!nanorods!with!the!diameters!from!20!to!300!nm!and!length!of!20!µm.!!

•  Dielectrics:!silicon!dioxide,!silicon!nitride…!•  Metal:!tungsten,!copper,!Rtanium,!aluminium!…!•  Semiconductors:!epitaxial!silicon,!germanium!…!•  Nitrides:!TiN,!TaN!•  Many!other!nanostructures,!such!as!nanobelts,!nanotube,!

SnO2!nanoboxes….!

SnO2!nanobox!(SEM)! Carbon!Nanotubes!(SEM)! Carbon!Nanotube!(TEM)!

CVD!*Advantages:!•  high!growth!rates!possible!•  can!deposit!materials!which!are!hard!to!evaporate!•  good!reproducibility!•  can!grow!epitaxial!films!!!Disadvantages!•  high!temperatures!•  complex!processes!•  toxic!and!corrosive!gasses!

Physical!Vapor!DeposiRon!

•  Thermal!evaporaRon!•  Electron!beam!evaporaRon!•  Spuoering!

Physical!Vapor!DeposiRon!1!PVD!

Condensed!Phase!(mostly!solid!e.g.!

Au)!

Gas!Phase!

Condensed!Phase!(usually!solid)!

Gas!Phase!

evaporaRon!condensaRon!

transport!

Spuoering!

The! substrate! is! placed! in! a! vacuum!chamber!with! the! source!material,! named!a! target,!and!an!inert!gas!(such!as!argon)!is!introduced!at!low!pressure.!A!gas!plasma!is!struck!using!an!RF!power!source,!causing!the!gas!to!become!ionized.!The!ions!are!accelerated!towards!the!surface!of!the!target,!causing!atoms!of!the!source!material!to!break!off!from!the!target!in!vapor!form!and!condense!on!all!surfaces!including!the!substrate.!!

Thermal!evaporaRon!holder!Resistance)heated)evapora0on)sources)

Alumina!crucible!with!wired!basekt!!

Thermal!evaporaRon!

•  Simple!and!widely!used!•  Common!evaporaRon!materials:!

1!Au,!Ag,!Al,!Sn,!Cr,!Sb,!Ge,!In,!Mg,!Ga!…!1!CdS,!PbS,!Cdse!…!

•  Use!W,!Ta!or!Mo!filaments!to!heat!evaporaRon!source!!•  Typical!filament!currents!are!2001300!A!•  Typical!deposiRon!rates!are!1120!Angstrom/second!•  Can!only!achieve!temperatures!of!about!1800°C!

Electron!beam!evaporaRon!Electron)beam)heated)evapora0on)source)

•  More!complex,!but!extremely!versaRle!•  Achieves!temperatures!up!to!3000!°C!•  Typical!emission!voltage!is!8!–!10!kV!•  EvaporaRon!crucibles!in!a!copper!hearth!•  Typical!deposiRon!rates!0.21100!Angstrom/second!•  Common!evaporaRon!sources!

1!all!materials!accommodated!by!the!thermal!evaporaRon!1!Ni,!Pt,!Ir,!Rh,!Ti,!V,!Zr,!W,!Ta,!Mo!1!Al2O3,!SiO,!SiO2,!SnO2,!TiO2,!ZrO2!

PVD!

Advantages!•  Low!substrate!

temperature!•  Conformal!film!•  RelaRvely!fast!process!•  ComparaRvely!low!cost!•  Excellent!thickness!

control!

Disadvantages!•  No!stoichiometric!films!•  By1products!incorporated!•  Cracking!•  Peeling!•  No!high!aspect!raRo!

materials!

EvaluaRon!of!film!thickness!–!oscillaRng!crystal!The! thickness! of! a! film! fabricated! by!thermal! or! electron! beam! evaporaRon!can! be! measured! conRnuously! during!the!experiment!by!an!oscillaRng!crystal.!!The!measuring!method!is!based!on!the!frequency!shiq!of!the!oscillaRng!crystal,!which! is! caused! by! the!material! being!evaporated! onto! the! crystal.! Thereby!the! resonance! frequency! is! decreased!with! increasing! materia l! being!deposited.!!

Frequency!shiq!for!different!materials!Flim

!thickness!

Frequency!shiq!

Atomic!Layer!deposiRon!!Introduced!with!a!name!of!Atomic!Layer!Epitaxy!in!1974!by!Dr.!T.!Suntola!(Picosun!Board!Member)!

Mr. Sven Lindfors (Picosun CTO) and the early ALD reactor in 1978

Picosun!ALD!in!Ilmenau!

Principles!of!ALD!ALD is a chemical gas phase thin film deposition method based on alternate, saturative, surface reaction

The!ALD!process!window!

Factors!affecRng!ALD!surface!reacRons!•  Growth!rate!in!ALD!is!typically!!1�/cycle!or!less.!!◦  Cycle!Rme!varies!◦  Higher!growth!rates!indicate!in!most!cases!the!CVD!growth!!

•  ALD!surface!reacRons!can!be!affected!by!◦  ReacRvity!of!the!precursor!

!  ReacRon!mode!(ligand!exchange,!dissociaRon,!agglomeraRon)!◦  ReacRvity!of!the!ligand!removal!agent!at!the!selected!temperature!◦  Number!of!the!reacRve!sites!

!  ReacRon!mode!(monofuncRonal,!bifuncRonal)!◦  Size!of!the!precursor,!i.e.!steric!!hindrance!

Reviews!about!ALD!mechanisms!

!  ‘Atomic!layer!deposiRon:!an!overview’,!Chemical&Reviews!110,!111!(2010)!

!  ‘Surface!chemistry!of!atomic!layer!deposiRon:!a!case!study!for!the!TMA/water!process’,!Journal&of&Applied&Physics&97,!121301!(2005)!

!  ‘Atomic!layer!deposiRon!chemistry:!recent!developments!and!futrure!challenges’,!Angewandte&Chemie,!internaRonal!ediRon!42,!5548!(2003)!

!  ‘Atomic!layer!deposiRon:!from!precursors!to!thin!film!structures’,!Thin&Solid&Films&409,!138!(2002)!

Advantages!of!ALD!Surface!controlled!(self1limiRng)!thin!film!

•  ~100%!conformal!•  Precise!thickness!control!•  Excellent!uniformity!•  Pinhole1free!films!•  Repeatable!process!•  Low!process!temperature!•  Graded!or!mixed!!layers/nanolaminates!•  High!aspect!raRo!materials!

MulRple!Materials!

‘Atomic layer deposition of transition metals’, Nature Materials 2, 749 (2003)