2. Vorlesung

Techniken der Oberflächenphysik
(Techniques of Surface Physics)
2. VL im WS14/15, 28.11.2015
Prof. Yong Lei & Stefan Boesemann (& Liying Liang)
Fachgebiet 3D-Nanostrukturierung, Institut für Physik
Contact: [email protected]
[email protected]; [email protected]
Office: Gebäude V 202, Unterpörlitzer Straße 38 (tel: 3748)
www.tu-ilmenau.de/nanostruk
Vorlesung:
Übung:
Mittwochs (G), 9 – 10:30, C 108
Mittwochs (U), 9 – 10:30, C 108
Outline for today
0. Introduction and motivation
1. Chemical Vapor Deposition
2. Physical Vapor Deposition
3. Atomic Layer Deposition
Well-controlled pore-opening process to the barrier layer of UTAMs
realizing pore-opening and surface nanostructures within the quantumsized range
An alumina barrier layer between the pore bottom and the aluminum foil of asprepared PAMs. It has a hemispherical and scalloped geometry. Using acidic
etching solutions, the barrier layer can be thinned and finally removed.
Nanodots (top view, Pd)
Nanoholes (top view, Si)
Tuning of the shapes and sizes of
UTAM-prepared nanostructures
To control the structural parameters (shape, size and spacing) is
very important
Controllable sizes and shapes:
The pore diameters of the UTAMs can be adjusted from about 10 to
400 nm to yield nanoparticles of corresponding size.
Nanometer-sized discs, hemispheres, hemi-ellipsoids, and
conics (by changing the aspect ratio of the pores of the UTAMs,
and the amount of material deposited through the UTAMs).
(d)
(c)
24.08
[nm]
200.00 nm
500.00 x 500.00 nm
46.16
[nm]
0.00
200.00 nm
10min 1:1
500.00 x 500.00 nm
0.00
2-18mins edge
After 15 min etching in 5wt% H3PO4 solution (30 oC) After 18 min etching in 5 wt% H3PO4 solution (30 oC)
The pore diameter is about 10 nm
The pore diameter is about 5 nm
(f)
(e)
54.12
[nm]
40.98
[nm]
200.00 nm
500.00 x 500.00 nm
2-24mins
0.00
200.00 nm
500.00 x 500.00 nm
0.00
h20 30mins
After 24 min etching in 5 wt% H3PO4 solution (30 oC) After 30 min etching in 5 wt% H3PO4 solution (30 oC)
The pore diameter is about 22 nm
The pore diameter is about 17 nm
Highly ordered nano-hemisphere arrays
Highly ordered nano-hemisphere arrays. Pore diameter, cell size and thickness of
the UTAM are about 80, 105, and 240 nm, respectively. The aspect ratio of the
apertures of the UTAM is about 1:3. The average height and base diameter of the
nano-hemispheres are approximately 35-40 and 75 nm, respectively.
Metallic nanotube arrays (by ALD)
Cover almost all inner
surface of AAO
Positive electrode
SnO2/MnO2 NT array
o-SnO2/MnO2
etching
ALD
SnO2
Au
evaporation
MnO2
a)
b)
etching
MnO2
c-SnO2/MnO2
Grote, Lei, et al., Journal of Power Sources, 2014, 256, 37-42.
Grote, Lei, et al., Applied Physics Letters, 2014, 104.
The core material: Nanotube
opening
Partial etching and mechanical removal
Binary nanowire arrays realized by electrodeposition via template
TiO2/Ag
TiO2/Au
TiO2/Ni
Surface patterning using polystyrene (PS) sphere template
The diameter of PS spheres can be controlled within 200 nm - 4.5
μm
Fabrication of Ag NanoshellArrays
S. Yang, Y. Lei, et al., Adv. Funct. Mater., 2010, 20, 2527
3D Ordered Macro-mesoporous Mo:BiVO4
Synthesised by Polystyrene Spheres
 Adjustable
template with
interconnected
area
 Suitable
infiltration with
high infiltration
fraction
 Controllable dual
pore diameter in
resulting
architectures
 Applicable to
various attractive
materials
PS template
BiVO4
Mo:BiVO4
Methoden zur Herstellung von
Oberflächen Strukturen
•
•
•
•
•
•
•
•
•
•
•
CVD Chemical Vapor Depsosition
PVD Physical Vapor Deposition, Sputter coating
ALD Atmoic Layer deposition
Electrochemical deposition
Spin coating
Template assisted
Lithography EB-Lithography, Photolithography
Reactive-ion etching
Printing Technology
Molecular Beam Epitaxy
…
I. Chemical Vapor Deposition
• CVD
Types of CVD
• CVD: Chemical Vapor Deposition
• PE-CVD: Plasma Enhanced CVD
• MP-CVD: Microwave plasma-assisted CVD
• RPE-CVD: Remote plasma-enhanced CVD
•
•
•
•
•
•
MO-CVD: Metal Organic CVD
AA-CVD: Aerosol assisted CVD
DL-ICVD: Direct liquid injection CVD
AP-CVD: Atmospheric pressure CVD
LP-CVD: Low-pressure CVD
UHV-CVD: Ultrahigh vacuum CVD
 Different types of CVDs for different materials, precursors,
growthrates, substrates…
Thermal - CVD
SOURCE: http://www.azonano.com/images/Article_Images/ImageForArticle_3423(1).jpg
When a conventional 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.
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 catalytic CVD in CNT growth are in the range of 800–1500 K.
Reaction Process in CVD
•
•
•
•
•
•
•
Mass transport of the reactant
Gas-phase reactions
Mass transport to the surface
Adsorption on the surface
Surface reactions
Surface migration
Incorporation of film
constituents, island formation
• Desorption of by-products
• Mass transport of by-products
Reaction Process in CVD
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 lattice structure and
orientation identical 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
Epitaxial Growth
Homoepitaxy of Si on a Si substrate
SiCl4(g)+2H2(g = Si(s)+4HCl(g) at approx. 1000-1200 °C
b) Vapor-Liquid-Solid (VLS) growth
• Catalytic 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 by CVD
1D Zinc oxide (ZnO) nanowires and
nanorods fabricated by CVD.
• diameters from 20 to 300 nm
• Length 20 µm
Chang et al. Chem. Mater., Vol. 16, No. 24, 2004
Further Examples of CVD
•
•
•
•
Dielectrics: silicon dioxide, silicon nitride…
Metal: tungsten, copper, titanium, aluminium …
Semiconductors: epitaxial silicon, germanium …
Nitrides: TiN, TaN
• Many other nanostructures ,such as nanobelts, nanotube, SnO2
nanoboxes….
SnO2 nanoboxe
Carbon Nanotube (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
II. Physical Vapor Deposition
• Thermal evaporation
• Electron beam evaporation
• Sputtering
Physical Vapot Deposition - PVD
Gas Phase
Gas Phase
transport
evaporation
Condensed Phase
(mostly solid e.g.
Au)
condensation
Condensed Phase
(usually solid)
Thermal evaporation holder
Resistance heated evaporation sources
Alumina crucible with
wired basekt
used in Ilmenau
Thermal evaporation
• Simple and in widespread use
• Common evaporation materials:
- Au, Ag, Al, Sn, Cr, Sb, Ge, In, Mg, Ga …
- CdS, PbS, Cdse …
• Use W, Ta or Mo filaments to heat evaporation
source
• Typical filament currents are 200-300 Amperes
• Typical deposition rates are 1-20 Angstrom/sec.
• Can only achieve temperatures of about 1800°C
Electron beam evaporation
electron beam heated evaporation source
a) thermal evaporation
b) mass transport
c) condensation and layer growth
If particles collide with each other
or with other gas particles during
the transportation they can lose
a part of their energy which is
required for the later layer growth.
To reduce these energy losses the
process pressure has an importan
influence because it limits the
mean free path (λ) essentially. λ can vary from 68 nm at
atmospheric pressure to 105 km in ultra-high vacuum
•
•
•
•
•
•
Electron beam evaporation
More complex, but extremely versatile
Achieves temperatures up to 3000 °C
Typical emission voltage is 8 – 10 kV
Evaporation crucibles in a copper hearth
Typical deposition rates 0.2-100 Angstrom/second
Common evaporation sources
- all materials accommodated by the thermal evaporation
- Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo
Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2
E-beam
evaporation
in Ilmenau
Sputtering
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.
SOURCE: http://www.tcbonding.com/images/sput_diagram.gif
PVD
advantages
disadvantages
• Low substrate
temperature
• Conformal film
• Relatively fast process
• Comparatively low cost
• Excellent thickness
control
•
•
•
•
•
No stoichiometric films
By-products incorporated
Cracking
Peeling
No high aspect ratio
materials
Evaluation of layer thickness –
oscillating crystal
The thickness of a layer fabricated by
thermal
or
electron
beam
evaporation can be measured
continuously during the experiment
by an oscillating crystal.
The measuring method is based on
the frequency shift of the oscillating
crystal, which is caused by the
material being evaporated onto the
crystal. Thereby the resonance
frequency
is
decreased
with
increasing material being deposited.
used crystals
Evaluation of layer thickness –
oscillating crystal
1.
Δ𝑓
𝑓0
=
Δ𝑑
−
𝑑0
=
f0= frequency of the cristal
d0= thickness of the cristal
ρq= density of the cristal
A= area of the cristal
Δm= mass of deposited layer
Δ𝑚
−
ρ𝑞∗𝐴∗𝑑0
Under consideration of:
2. φ=
Δ𝑚
𝐴
and 3. 𝑁 = 𝑓0 ∗ 𝑑0
φ = mass coverage
N = frequency constant
This yields the frequency shift:
𝑓0∗φ
𝑓02
4. Δ𝑓 = −
=−
φ
𝑑0∗ρ𝑞
𝑁∗ρ𝑞
This is only valid, if Δ𝑚 ≪ mq; mq= mass of crystal
Evaluation of layer thickness –
oscillating crystal
The layer thickness d=Δd can now beeing
calculated, if the layer density ρs is known.
From:
5. 𝑑 =
Δ𝑚
ρs∗𝐴
under consideration of 2. and 4. follows
𝑁 ∗ ρq∗Δ𝑓
𝑑=−
𝑓02∗ρs
Layer thickness
Frequency shift for different materials
Frequency shift
III. Atmoic Layer Deposition
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 affecting ALD surface reactions
•
Growth rate in ALD is typically
1Å/cycle or less.
◦ Cycle time varies
◦ Higher growth rates indicate
in most cases the CVD growth
•
ALD surface reactions can be affected by
◦ Reactivity of the precursor
 Reaction mode (ligand exchange, dissociation, agglomeration)
◦ Reactivity of the ligand removal agent at the selected temperature
◦ Number of the reactive sites
 Reaction mode (monofunctional, bifunctional)
◦ Size of the precursor, i.e. steric hindrance
Reviews about ALD mechanisms




‘Atomic layer deposition: an overview’, Chemical
Reviews 110, 111 (2010)
‘Surface chemistry of atomic layer deposition: a case
study for the TMA/water process’, Journal of Applied
Physics 97, 121301 (2005)
‘Atomic layer deposition chemistry: recent
developments and futrure challenges’, Angewandte
Chemie, international edition 42, 5548 (2003)
‘Atomic layer deposition: from precursors to thin film
structures’, Thin Solid Films 409, 138 (2002)
Key advantages of ALD

Surface controlled (self-limiting) thin film
~100% conformal
Precise thickness control
Excellent uniformity
Pinhole-free films
Repeatable process
Low process temperature
Graded or mixed layers/nanolaminates
High aspect ratio materials
Multiple Materials
‘Atomic layer deposition of transition metals’, Nature Materials 2, 749 (2003)
Ultrathin layer for high k gate

As the size of electrical devices is scaled down continuously, it is said that the gate
thickness need to be down to 1.0 nm in the near future. At this point, ALD is the only
promising technique. (ZrO2, HfO2)
‘Self-aligned ballistic molecular transistors
and electrically parrallel nanotube arrays’,
Nano Letters 4, 1319 (2004)
‘Parralel core-shell metal-dielectricsemiconductor germanium nanowires for
high-current surround-gate FETs’, Nano
Letters 6, 2785 (2006)
Thanks for listening
Any questions?
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online gestellt
http://www.tu-ilmenau.de/nanostruk/teaching/

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