Thin Films 1
THIN FILMS ARE VITAL IN MODERN TECHNOLOGY:
Protective coatings - Hard films
Optical coatings - Filters, mirrors, lenses
Microelectronic devices
Optoelectronic devices, Photonic devices
Electrode surfaces
Photoelectric devices, photovoltaics, solar cells
Xerography, Photography, Lithography
Catalyst surfaces
Information storage, magnetic, magneto-optical, optical memories
Synthesis of Thin Films
Thin Films 2
•Crystalline, Amorphous, Microcrystalline
•Monolayer, multilayer, superlattice, junctions
•Free-standing, supported
•Epitaxial (commensurate), incommensurate
Synthesis of Thin Films
Thin Films 3
Free-Standing Films
Fe(CO)5 + O2 → amorph. Fe2O3
Prepared as a film on a NaCl crystal support
Dissolution of NaCl in water = Free-standing film
Thin Films 4
Synthesis of Thin Films
FILM PROPERTIES DEPEND ON NUMEROUS CONSIDERATIONS:
Thickness
Surface : volume ratio
Structure, surface versus bulk, surface reconstruction, surface roughness
Hydrophobicity, hydrophilicy (Si-OH vs. Si-H)
Composition
Texture: single crystal, microcrystalline, domains, orientation: Si (100) vs. (111)
Form: supported or unsupported, nature of substrate
Thin Films 5
Surfaces
Surface energy [J m−2] a scalar
Surface stress [J m−2] a tensor
Same for liquids, different for solids
Surface tension [J m−2] the work done in creating unit area
of new surface (= Surface energy in one-component systems)
Thin Films 6
Surface Energy
Surface energy [J m−2] depends on:
•The distance of the face from the center of the crystal
•Miller indices
•Surface roughness
•The radius of curvature
Thin Films 7
Surfaces
The TSK (TLK) Model of a Surface (Kossel/Stranski)
Terrace
Step/Ledge
Kink
Vacancy
Adatom
Island
Thin Films 8
Surfaces
Screw dislocation on graphite
Spiral growth
Thin Films 9
Surfaces
AFM showing C atoms
within the hexagonal
graphite unit cells.
Image size 2×2 nm2.
Thin Films 10
Thin Films 11
Thin Films 12
Higher temperature = Faster diffusion
Surface Diffusion
Surface diffusion coefficient D
D = a2 ks a … effective hopping distance between sites
ks … site-to-site hopping rate of an adatom
ks = A exp(-Vs/kbT) Vs … potential-energy barrier to hopping from site
to site
T … substrate temperature
Thin Films 13
The dissociative collision of a CH4
molecule with a nickel surface
does not significantly perturb the
nickel atom at the impact point.
Thin Films 14
Si(111) Surface
Silicon "diamond lattice" structure
a = 5.463 Å
Si(111) = a set of atomic planes
One plane outlined with red
Si (111) etches more slowly than (001)
Si (111) oxidizes twice as rapidly as (001)
Thin Films 15
Si(111) Surface
A top view of the atomic
arrangement for the (111) plane
orange = the top layer
green = deeper layers
Thin Films 16
Si(111) Surface
Thin Films 17
Reconstruction
Relaxation = energy lowering, no chnge in symmetry
Reconstruction = the surface atoms rearrange to a more
energetically stable configuration.
Symmetry lowering
2D symmetry – 17 plane groups/ 230 bulk space groups
Thin Films 18
7x7 Reconstruction
When (111) surface of Si is heated to high temperatures under the Ultra-High
Vacuum conditions the surface atoms rearrange to a more energetically stable
configuration called 7x7 reconstruction.
STM image of
Si(111) surface
Thin Films 19
7x7 Reconstruction
Thin Films 20
7x7 Reconstruction
3D representation of the 7x7 STM image
The image area is 18x8 nm2, the height of the "bumps" is only about 0.04 nm
Thin Films 21
Si(100) Surface
ideal reconstructed
Thin Films 22
STM images of the silicon-silicon
dimers imaged with
(a) Vsample = -2.0 V
(b) Vsample = 2.3 V
The filled and empty states of these
highly ordered dimers can be probed
by biasing the surface in the opposite
directions
The dimensions of the figure are 2.3
nm x 7.7nm.
Thin Films 23
H-terminated Si(100) Surface
Thin Films 24
Thin Films 25
MAIN METHODS OF SYNTHESIZING THIN FILMS:
CHEMICAL, ELECTROCHEMICAL, PHYSICAL
Cathodic deposition, Anodic deposition, Electroless deposition
Thermal oxidation, nitridation
Chemical vapor deposition (CVD)
Metal organic chemical vapor deposition (MOCVD)
Molecular beam epitaxy, supersonic cluster beams, aerosol
deposition
Liquid phase epitaxy
Self-assembly, surface anchoring, SAM
Discharge techniques, RF, microwave
Laser ablation
Cathode sputtering, vacuum evaporation
Synthesis of Thin Films
Thin Films 26
CATHODIC DEPOSITION
Two electrodes, dipped into electrolyte solution
External potential applied
Metal deposition onto the cathode as thin film
Anode metal slowly dissolves
ELECTROLESS DEPOSITION
Spontaneous, No applied potential
Depends on electrochemical potential difference between electrode
and solution redox active species to be deposited
Both methods limited to metallic films on conducting substrates
ANODIC DEPOSITION
Deposition of oxide films, such as alumina, titania
Deposition of conducting polymer films by oxidative polymerization
of monomer, such as thiophene, pyrolle, aniline
Oxide films formed from metallic electrode in aqueous salts or acids
Synthesis of Thin Films
Thin Films 27
Example:
Anodic oxidation of aluminum in oxalic or phosphoric acid
Pt|H3PO4, H2O|Al
Al → Al3+ + 3e- anode
PO4
3- +2e- → PO3
3- + O2- cathode
2Al3+ + 3O2- → γ-Al2O3 (annealing) → α-Al2O3
overall electrochemistry:
2Al + 3PO4
3- → Al2O3 + PO3
3The
applied potential controls the oxide thickness and the rate at which it
forms, oxide anions from solution have to diffuse through an Al2O3 layer of
growing thickness on the reacting Al substrate, to attain an equilibrium
thickness of the alumina film
Porous Alumina Films
Thin Films 28
Self-organizing process observed,
whereby a regular array of size tunable
hcp pores form and permeate
orthogonally through the alumina film
Exceptionally useful process for creating controlled porosity
membranes, photonic gap materials, template for synthesizing
semiconductor nanostructures, host for synthesizing and organizing
aligned carbon nanotubes for high intensity electron emission
displays, and last but not least, fuel cell electrode materials
Porous Alumina Films
Thin Films 29
AFM picture of porous alumina film
Thin Films 30
AFM picture of porous alumina film
Thin Films 31
Porous Alumina Films
Thin Films 32
SYNTHESIS OF THIN FILMS
THERMAL OXIDATION
Oxides, metal exposed to a glow discharge
Al + O2 → (RT) Al2O3, thickness 3-4 nm
Similar method applicable to other metals, Ti, V, W, Zr etc
Nitrides, exceptionally hard, high temperature protective coating
Ti + NH3 → TiN
Al + NH3 → AlN
Thin Films 33
CHEMICAL VAPOR DEPOSITION
Pyrolysis, photolysis, chemical reaction, discharges, RF, microwave
Epitaxial films, correct matching to substrate lattice
EXAMPLES OF CVD
CH4 + H2 (RF, MW) → C, diamond films
Et4Si (thermal, air) → SiO2
SiCl4 or SiH4 (thermal, H2) → a-HSi
SiH4 + PH3 (RF) → n-Si
Si2H6 + B2H6 (RF) → p-Si
Synthesis of Thin Films
Thin Films 34
Thin Films 35
SYNTHESIS OF THIN FILMS
SiH3SiH2SiH2PH2 (RF) → n-Si
Me3Ga (laser photolysis, heating) → Ga
Me3Ga + AsH3 + H2 → GaAs + CH4
Si (laser evaporation, supersonic jet) Sin
+
(size selected cluster
deposition) → Si
Thin Films 36
METAL ORGANIC CHEMICAL VAPOR DEPOSITION,
MOCVD
Invented by Mansevit in 1968
Recognized high volatility of metal organic compounds as sources
for semiconductor thin film preparations
MOCVD PRECURSORS, SINGLE SOURCE MATERIALS
Me3Ga, Me3Al, Et3In
NH3, PH3, AsH3
H2S, H2Se
Me2Te, Me2Hg, Me2Zn, Me4Pb, Et2Cd
All toxic materials – a problem of safe disposal of toxic waste
Example - IR detectors:
Me2Cd + Me2Hg + Me2Te (H2, 500o
C) → CdxHg1-xTe
Synthesis of Thin Films
Thin Films 37
Specially designed MOCVD reactors
Controlled flow of precursors to single crystal heated substrate
Most reactions occur in range 400-1300o
C
Complications of diffusion at interfaces, disruption of atomically flat
epitaxial surfaces/interfaces occurs during deposition
Photolytic processes (photoepitaxy) help to bring the deposition
temperatures to more reasonable temperatures
REQUIREMENTS OF MOCVD PRECURSORS
RT stable, no polymerization, decomposition
Easy handling, simple storage
Not too reactive
Vaporization without decomposition at modest T < 100o
C
Low rate of homogeneous pyrolysis (gas phase) wrt heterogeneous
decomposition (surface)
HOMO : HETERO rates ~ 1 : 1000
Synthesis of Thin Films
Thin Films 38
Heterogeneous reaction on substrate
Greater than on other hot surfaces in reactor
Not on supports, vessel etc
Ready chemisorption of precursor on substrate
Detailed surface and gas phase studies of structure of adsorbed
species, reactive intermediates, kineticss, vital for quantifying film
nucleation and growth processes
Electronic and optical films synthesized in this way
Semiconductors, metals, silicides, nitrides, oxides, mixed oxides (e.g.,
high Tc superconductors)
Synthesis of Thin Films
Thin Films 39
CRITICAL PARAMETERS IN MATERIALS PREPARATION
FOR SYNTHESIS OF THIN FILMS
Composition control
Variety of materials to be deposited
Good film uniformity over large areas to be covered, > 100 cm2
Precise reproducibility
Growth rate, thickness control, 2-2000 nm layer thickness
Precise control of film thickness = accurate control of deposition,
film growth rate
Crystal quality, epitaxy
High degree of film perfection
Defects degrade device performance
Purity of precursors: usually less than 10-9
impurity levels
Stringent demands on starting material purity
Challenge for chemistry, purifying and analyzing at the ppb level
Demands exceptionally clean growth system otherwise defeats the
object of controlled doping of films for device applications
Interface widths
Abrupt changes of composition and dopant concentration required,
quantum confined structures
30-40 sequential layers often needed
Alternating composition and graded composition films
0.5-50 nm thickness required with atomic level precision
All of the above has been more-or-less perfected in the electronics
and optics industries
Thin Films 40
SYNTHESIS OF THIN FILMS
TECHNIQUES USED TO GROW SEMICONDUCTOR FILMS AND
MULTILAYERED FILMS
MOCVD
Liquid phase epitaxy
Chemical vapor transport
Molecular beam epitaxy
Laser ablation
Thin Films 41
PHYSICAL METHODS FOR PREPARING THIN FILMS
CATHODE SPUTTERING
Bell jar equipment
10-1
to 10-2
torr of Ar, Kr, Xe
Glow discharge created
Positively charged rare gas ions
Accelerated in a high voltage to cathode target
High energy ions collide with cathode
Sputter material from cathode
Deposits on substrate opposite cathode to form thin film
Multi-target sputtering creates composite or multilayer films
THERMAL VACUUM EVAPORATION
High vacuum bell jar > 10-6
torr
Heating e-beam, resistive, laser
Gaseous material deposited on substrate
Thin films nucleate and grow
Containers must be chemically inert:
W, Ta, Nb, Pt, BN, Al2O3, ZrO2, Graphite
Substrates include insulators, metals, glass, alkali halides, silicon
Sources include metals, alloys, semiconductors, insulators, inorganic
salts
Thin Films 42
Epitaxy
Epitaxial reactions = surface structure controlled reactions
Crystallographic orientation of the film is controlled by the substrate
Kinetic control – TD metastable phases
YMnO3
•hexagognal in bulk
•cubic perovskite film on NdGaO3 substrates
Homoepitaxy – same compound/orientation in substrate and film
Heteroepitaxy – different compounds in the substrate and film
Thin Films 43
MOLECULAR BEAM EPITAXY
Million dollar thin film machine, ideal for preparing high quality
artificial semiconductor quantum superlattices, ferroelectrics,
superconductors
Ultrahigh vacuum system >10-12
torr
Elemental or compound sources in shutter controlled Knudsen
effusion cells
Ar+
ion gun for cleaning substrate surface or depth profiling sample
using Auger analyzer
High energy electron diffraction for surface structure analysis
Mass spectrometer for control and detection of vapor species
e-gun for heating the substrate
Synthesis of Thin Films
Thin Films 44
MBE
Thin Films 45
SYNTHESIS OF THIN FILMS
PHOTOEPITAXY
Making atomically perfect thin films under milder and more
controlled conditions, Mullin and Tunnicliffe 1984
Et2Te + Hg (pool) + H2 (hν , 200 o
C) → HgTe + 2C2H6
MOCVD preparation requires 500 o
C using Me2Te + Me2Hg
Advantages of photoepitaxy
Lower temperature operation
Multilayer formation
Less damage of layers
Lower interlayer diffusion
Easy to fabricate abrupt boundaries
Less defects, strain, irregularities at interfaces
CdTe can be deposited onto GaAs at 200-250 o
C even with a 14%
lattice mismatch
GaAs is susceptible to damage under MOCVD conditions 650-750 o
C
Thin Films 46
SYNTHESIS OF THIN FILMS
EXTENSIONS OF PHOTOLYTIC METHODS - LASER
WRITING AND LASER ETCHING
Laser writing:
Substrate GaAs
Me3Al or Me2Zn adsorbed layer or gas phase
Focussed UV laser on film
Photodissociation of organometallic precursor:
Me3Al → Al + C2H6
Creates sub-micron lines of Al or Zn
Thin Films 47
SYNTHESIS OF THIN FILMS
Laser photoetching:
GaAs substrate
Gaseous or adsorbed layer of CH3Br
Focussed UV laser
Creates reactive Br atoms
CH3Br(g) (hν ) → CH3(g) + Br(g)
Br(g) + GaAs(s) → GaAs…Brn(ad)
GaAs…Brn(ad) → GaBrn(g) + AsBrn(g)
Adsorbed reactive surface Br atoms erode surface regions irradiated
with laser
Vaporization of volatile gallium and arsenic bromides from surface
creates sub-micron etched line
Thin Films 48
Pulsed Laser Ablation
Thin Films 49
Pulsed Laser Ablation
(a) Initial absorption of laser radiation (indicated by long arrows), melting and
vaporization begin (shaded area indicates melted material, short arrows
indicate motion of solid–liquid interface)
(b) Melt front propagates into the solid, vaporization continues and laser-plume
interactions start to become important
(c) Absorption of incident laser radiation by the plume, and plasma formation
(d) Melt front recedes leading to eventual re-solidification.
Thin Films 50
Porous Si
SEM of a porous silicon, the pores extending from the surface of the Si(100) wafer down into
the bulk, etched from n-type Si(100) (P-doped, 0.75-0.95 ø‚cm) at 77.2 mA cm-2 for 1 min
with a 1:1 solution of 49% HF(aq)/ EtOH. Scale bar is 10 μm.
Thin Films 51
Porous Si
HF:EtOH = 1:2.5
j = 10 mA/cm2
t = 30 min
Thin Films 52
Chemistry on Si Surface
Thin Films 53
Porous Si
500 600 700 800 900
0
1
2
3
4
2.4 2.2 2 1.8 1.6 1.4
PLIntensity[lin.u.]
Wavelength[nm]
Energy[eV]
T = 295 K
λexc = 457.9
nm
Luminiscence of p-Si
Thin Films 54
Thin Films 55
Chemistry on Si Surface
Thin Films 56
Hydrosilylation
Thin Films 57
Chemistry on Si Surface
Thin Films 58
Carbaanion LiR, RMgX
Thin Films 59
Thin Films 60
2+2 Cycloaddition
Thin Films 61
2+2 Cycloaddition
Thin Films 62
DA 4+2
Thin Films 63
Secondary Chemistry
Thin Films 64
Secondary Chemistry
Thin Films 65
Secondary Chemistry
Thin Films 66
Secondary Chemistry
Thin Films 67
SELF-ASSEMBLED MONOLAYERS
Self-assembly: spontaneous organization of molecules into stable,
structurally well-defined aggregates
Self-assembled monolayers (SAM): two-dimensional ordered
assemblies of long hydrocarbon chains anchored through chemical
bonds to surfaces of solid inorganic substrates
Alkanethiolates on gold and alkylsiloxanes on silicon dioxide belong
the most notoriously studied SAM systems
Thin Films 68
Metal surfaces Au, Ag, Cu, Pt, Hg, Fe,…
react with
Thiols M + RSH M-S-R + 1/2 H2
Disulfides 2 M + RSSR 2 M-S-R
Sulfides M + RSR M-S-R
Same products formed in all three reactions: thiolates. RSH are
more soluble and react 103
faster with Au than RSSR.
SELF-ASSEMBLED MONOLAYERS
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
Metal Surface
Substrates: gold polycrystalline films
on Si(SiO2), glass, mica.
Thickness 5-300 nm, sputtering, evaporation
Atomically flat
Thin Films 69
Tilt
Twist
Precession
Thin Films 70
Au surface = ccp
Thin Films 71
Thin Films 72
SELF-ASSEMBLED MONOLAYERS
Thermodynamics
Au does not form surface oxide layer
Reaction driving force:
Au-S bond energy 160-185 kJ mol-1
van der Waals attraction between alkyl chains
6-8 kJ mol-1
per CH2
In t
BuSH and n-C18SH competition reaction, the linear alkyl thiol
binds 300 – 700 times better.
Surface coverage 1014
molecules per cm2
C16 chain length ∼2.2 nm, 32-40° tilted, all-trans
Chemical stability: Cu/C18SH sustains HNO3
Thermal stability: Au/RSH loses sulfur at 170-230 °C
Thin Films 73
SELF-ASSEMBLED MONOLAYERS
Binding modes on Au(111)
On-top sites
Hollow sites – threefold, more stable by 25 kJ mol-1
Bridging sites – the most stable!! (QM calculations)
Au – S – C = 180°, sp
Au – S – C = 104°, sp3
, more stable by 1.7 kJ mol-1
barrier to interconversion 10.5 kJ mol-1
Thin Films 74
SELF-ASSEMBLED MONOLAYERS
Au(111)
Hexagonal array of S, S….S distance 4.97 Å, interchain distance in
crystalline paraffins 4.65 Å, tilt angles 25 - 30° to reestablish alkyl
chain contacts, hollow site binding, 21.4 Å2
per molecule
Ag(111)
Hexagonal array of S, S….S distance 4.41 Å, on-top site binding,
more tightly packed alkyl chains, no tilt
Thin Films 75
SELF-ASSEMBLED MONOLAYERS
Kinetics
Au(111) + RSH reactions proceed in two steps:
1. First step, fast (minutes), diffusion controlled Langmuir
adsorption, concentration dependent (1 mM ∼ 1 min, 1 μM ∼ 100
min)
2. Second step, slow (hours), disordered film orders to a 2D crystal,
surface crystallization, defect healing, trapped solvent expulsion.
Mechanisms: alkyl chain flipping, RSlateral
diffusion, equilibrium
with dissolved RSH, Au atom diffusion, Au in solution. Better
crystallinity of films in polar solvents: MeOH, EtOH,…
Thin Films 76
SELF-ASSEMBLED MONOLAYERS
Surface chemical derivatization
HS – (CH2)n – X
X = CH3, CF3, OH, NH2, SH, COOH, COOR, CN, CH=CH2, C≡CH,
Cl, Br, OCH3, SO3H, SiMe3, ferrocenyl, ….
Microfabrication
♠ Self-assembly, at thermodynamic minima, rejects defects, high
degree of perfection.
♠ Dimension in the range 1 nm to 1000 μm, too large for chemical
synthesis, too small for microlithography.
♠ High efficiency, spontaneous.
Thin Films 77
SiO2 Surfaces
SiO2 Surfaces
native oxide on Si
silicagel
Chemical derivatization methods are based on the reactivity of the surface hydroxyl
groups with various reagents
isolated vicinal geminal
Thin Films 78
[O3Si]⎯OH stands for the siliceous surface
1. Grafting
Reactions with trifunctional reagents, such as alkyltrichlorosilanes and
trialkoxyalkylsilanes, lead to the three-fold attachment of the SiR groups.
3 [O3Si]⎯OH + Cl3SiR {[O3Si]⎯O}3SiR + 3 HCl
3 [O3Si]⎯OH + (MeO)3SiR {[O3Si]⎯O}3SiR + 3 MeOH
SiO2 Surfaces
Thin Films 79
2. Chlorination/Displacement Method
The first step is the replacement of the Si-OH groups by more
reactive Si-Cl bonds by chlorination.
[O3Si]⎯OH + SOCl2 [O3Si]⎯Cl + HCl + SO2
[O3Si]⎯OH + CCl4 [O3Si]⎯Cl + COCl2 + HCl
In the subsequent step, the surface is treated with a Grignard or
organolithium reagent with the formation of strong Si-C bonds.
[O3Si]⎯Cl + RMgCl [O3Si]⎯R + MgCl2
[O3Si]⎯Cl + RLi [O3Si]⎯R + LiCl
400 °C
SiO2 Surfaces
Thin Films 80
3. Post Modification Method
The organic groups (R) covalently anchored to the siliceous surface
by the two previous methods can be subsequently chemically
modified. The most extensively developed is chemistry of 3aminopropyl(trimethoxy)silane.
A large number of chemical
transformations of the amino moiety to other functional groups are
known.
4. Hybrid sol-gel method (co-condensation)
A thin layer of a hybrid (organically modified) silica gel can be
deposited on the silica surface from a solution of TEOS and
(MeO)3SiR by controlled hydrolysis and condensation.
(MeO)3SiR + (MeO)4Si + 7 H2O [O3Si]⎯R + 7 MeOH
SiO2 Surfaces
Thin Films 81
5. Organometallic modification method
Organometallic reagents, such as metal alkyls, halides, amides, and
alkoxides can be used to deposit a monolayer of metal complexes on
the surface (MLn stands for an organometallic group, M for a metal,
L for a ligand, R” for a short alkyl chain, X for halogen).
[O3Si]⎯OH + R”−MLn R”H + [O3Si]⎯O−MLn
[O3Si]⎯OH + X−MLn HX + [O3Si]⎯O−MLn
[O3Si]⎯OH + Me2N−MLn Me2NH + [O3Si]⎯O−MLn
[O3Si]⎯OH + R”O−MLn R”OH + [O3Si]⎯O−MLn
SiO2 Surfaces
Thin Films 82
These organometallic moieties can serve as attachment points for
further modification with long chain alcohols, thiols, carboxylic
acids, phosphates, and diketonates.
[O3Si]⎯O−MLn + HOR [O3Si]⎯O−MLn-1−OR + HL
[O3Si]⎯O−MLn + HOOCR [O3Si]⎯O−MLn-1−OOCR + HL
SiO2 Surfaces
Thin Films 83
Thin Films 84
Manipulations with SAM
Thin Films 85
Manipulations with SAM
Thin Films 86
Manipulations with SAM
Thin Films 87
Manipulations with SAM
Thin Films 88
Manipulations with SAM