Transcript 幻灯片 1 - ciac

Preparation of Metal Thin Films
- PVD, CVD & Sputtering
State Key Laboratory of Electroanalytical Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Oct.30, 2003
Steps & General Factors of Thin Film Growth & Nucleation
Target
Production of the
appropriate atomic,
molecular, or ionic
species
Oct.30, 2003
Medium
Substrate
Transport to the substrate
through a medium
e.g.: vacuum, electric field,
distance, stability of species,
concentration of supporting
electrolyte, etc.
Condensation on substrate
The nucleation density and
the average nucleus size
depend on a number of
parameters such as,
1.the energy of the aim species
2.the rate of attachment
3.The activation energy of
adsorption & desorption
4.Thermal diffusion
5.Temperature
6.Topography
7.Chemical nature of the
substrate.
Initial Nucleation & Growth
Island
Layer-by-Layer
Stranski-Krastanov
(Mixed Type)
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Effect of Substrate Temperature on the Lateral Grain Size
100 Å thick Au films deposited at 100, 200, and 300℃ by vacuum evaporation.
Oct.30, 2003
Microstructure & Growth Process
The small islands start coalescing with each other in an attempt to
reduce the surface area. This tendency to form bigger islands is termed
agglomeration and is enhanced by increasing the surface mobility of
the adsorbed species, as, for example, by increasing the substrate
temperature.
Except under special conditions, the crystallographic orientation and
the topographical details of different islands are randomly distributed,
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Formation of large-grain-sized epitaxial/single-crystal films
The conditions favoring epitaxial growth are: high surface mobility
as obtained at high substrate temperatures, low supersaturation, clean,
smooth, and inert substrate surfaces, and crystallographic
compatibility between the substrate and the deposit material
On the other extreme of thin film microstructures, highly disordered,
very fine-grained, noncrystalline deposits with grain size< 20Å and
showing halo-type diffraction patterns similar to those of amorphous
structures are obtained under conditions of high supersaturation and
low surface mobility. The surface mobility of the adsorbed species
may be inhibited, for example, by decreasing the substrate
teimperature, by introducing reactive impurities into the film during
growth, or by codeposition of materials of different atomic sizes and
low surface mobilities. Under these conditions, the film is
amorphouslike and grows layer by layer.
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Effect on Surface Roughness
A quantitative measure of roughness, the roughness factor, is
the ratio of the real effective area to the geometrical area.
Under conditions of low nucleation barrier and high
supersaturation, the initial nucleation density is high and the size of
the critical nucleus is small. This results in fine-grained, smooth
deposits which become continuous at small thicknesses.
High surface mobility, in general, increases the surface smoothness of
the films by filling in the concavities.
Low Tsubstrate ~ large RF (amorphous-like, but mirror-like)
High Tsubstrate ~ small RF (epitaxial & single-crystal-like)
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Adhesion
The adhesion of a film to the substrate is strongly dependent on the
chemical nature, cleanliness, and the microscopic topography of the
substrate surface.
Presence of contaminants on the substrate surface may increase or
decrease the adhesion depending on whether the adsorption energy is
increased or decreased, respectively.
The adhesion of a film can be improved by providing more nucleation
centers on the substrate, as by using a fine-grained substrate or a
substrate precoated with suitable materials.
Loose and porous deposits formed under conditions of high
supersaturation and poor vacuum are less adherent than the
compact deposits.
Oct.30, 2003
Classification of Techniques on Thin Film Deposition
Vapor Deposition
Physical Vapor Deposition
Vacuum Evaporatoin
Sputtering
Glow-Discharge Sputtering
Ion-Beam Sputtering
Chemical Vapor Deposition
Solution Deposition
Chemical Solution Deposition
Autocatalytic Reduction / Electroless Plating
Spray Pyrolysis
Conversion Coatings
Electrochemical Deposition
Electrodeposition
Anodization
Electrophoretic Deposition
Cathodic Conversion
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Physical Vapor Deposition (PVD)
The characteristic feature of PVD techniques is that the transport of vapors from the source
to the substrate takes place by physical means.
Here, in this way, the vapor species of a solid material is created by thermal evaporation,
or by mechanically knocking out the atoms or molecules, and then deposited directly on
substrate without any chemical reactions.
The mean free path (mfp) of the ambient gas molecules is greater than the dimensions of the
deposition chamber and the source-to-substrate distance. Under low-pressure ambient
conditions, the transport of the material from the source to the substrate occurs by
molecular beams.
Low Pressure
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High Pressure
Physical Vapor Deposition (PVD)
Mono-source evaporation/sputtering
Multi-source co-evaporation/co-sputtering
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Physical Vapor Deposition (PVD) - Vacuum Evaporation
As the name implies, the techniques consists of vaporization of the solid
material by heating it to sufficiently high temperatures and condensing it
onto a cooler substrate to form a film.
The simplest and the most common method is to support the material in a
filament-basket or boat, which is heated electrically, or is indirectly heated
in crucibles of insulating materials such as quartz, graphite, alumina, and
zirconia, which are supported in a metal cradle.
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Kinetics of Evaporation & Condensation
The rate of free evaporation of atoms or molecules from a clean surface
of unit area in vacuum is given by Langmuir-Dushman equation,
Ne  3.5131022 pe (1/ MT )1/ 2 molecules cm2 s 1
pe - the equilibrium vapor pressure (in Torr) of the evaporant under
saturated vapor conditions at a temperature T.
M - the molecular weight of the vapor species.
The rate of condensation of the vapors depends not only on the
evaporation rate but also on the source geometry, its position relative to
the substrate, the degree of vacuum, and the condensation coefficient.
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Necessary of Vacuum on PVD
Because of collisions with the ambient gas molecules, a fraction of the vapors,
proportional to exp(-d/) (, mean free path for air), is scattered and hence
randomized in direction within a distance d during their transfer through the gas.
Pressure
(Torr)
Mean free path
(cm)
Time for monolayer
deposition at 1Å/s
(sec)
10-2
0.5
4400
10-4
51
44
10-5
510
4.4
10-7
51000
0.044
10-9
5100000
0.00044
Thus, pressures lower than 10-5 Torr species and for substrate-to-source distance of
~10 to 50 cm in a vacuum chamber. Good vacuum is also necessary for producing
contamination-free deposits (also useful and required for oxidizable species).
Oct.30, 2003
Physical Vapor Deposition (PVD) - Sputtering
The vapor species are created by mechanically knocking out the atoms
or molecules from the surface of a solid material by bombarding it with
energetic, non-reactive ions. The ejection process, known as sputtering,
occurs as a result of momentum transfer between the impinging ions and
the atoms of the target being bombarded.
Target
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Medium
Substrate
Characteristics of the Sputtering Process
The sputtered species, in general, are predominantly neutral.
The sputtering yield, defined as the number of ejected species per incident ion, increases
with the energy and mass of the bombarding ions.
For higher energies, the yield approaches saturation, which occurs at higher energies for
heavier bombarding particles.
e.g.: Xe+ ~ 100KeV and Ar+ ~ 20KeV for saturation.
Sometimes, at very high energies of the bombarding ions, the yield decreases because of the
increasing penetration depth and hence increasing energy lossed below the surface, with the
consequence that not all the affected atoms are able to reach the surface to escape.
The yield increases as (cos)
with increasing obliqueness () of the incident ions. However, at
large angles of incidence the surface penetration effect decrease the yield drastically.
-1
The yield is rather insensitive to the target temperature except at very high temperatures where it
show an apparent rapid increase due to the accompanying thermal evaporation.
The energy of the ejected atoms shows a Maxwellian distribution with a long tail toward higher
energies.
The energies of the atoms or molecules sputtered at a given rate are about one order of
magnitude higher than those thermally evaporated at the same rate. However, since sputtering
yields are low and the ion currents are limited, sputter-deposition rates ae invariably one to two
orders of magnitude lower compared to thermal evaporation rates under normal conditions.
The sputtering process is very inefficient from the energy point of view, because most of the energy
is converted to heat which becomes a serious limitation at high deposition rates.
The sputtering process ensures layer-by-layer ejection from a multicomponent target and
results into a homogeneous film on the substrate.
Oct.30, 2003
Sputtering – Glow-Discharge Sputtering
Glow-Discharge Sputtering by different electrode configurations
Diode Sputtering
Cathode
Cathode
Target
Target
Bias Sputtering
----------------------------------
Ion Platting
++
Getter Sputtering
Magnetron Sputtering
+
+
++
+
++
Substrate
----------------Assisted or Triode Sputtering
RF Sputtering
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Insulator
Substrate
++++++++++++++++++++++++++++++++++++
Anode
Anode
Sputtering – Glow-Discharge Sputtering
A cheap and simple means of producing ions for sputtering is provided
by the well-known phenomenon of glow discharge, which occurs when
an electric field is applied between two electrodes in a gas at low
pressure (~ 10-2 Torr). The gas breaks down to conduct electricity, above
a certain voltage applied between the electrodes, The cathode dark space,
across which most of the applied voltage drops, is the most important
region for sputtering. Ion and electrons created at breakdown are
accelerated across this region. The energetic positive gas ions strike the
cathode to produce sputtering and cause emission of secondary electrons
which are essential for sustaining the glow discharge. The accelerated
electrons produce more ions by collision with gas atoms in the negativeglow region lying adjacent to the cathode dark space.
Oct.30, 2003
Sputtering – Glow-Discharge Sputtering
The energy of the bombarding ions depends on the accelerating cathode fall
(i.e., the voltage across the cathode dark space) and the thickness d of the
cathode dark space, which inversely proportional to the gas pressure p, that is
pd=const (Paschen’s law)
The number of ions striking the cathode current depends on the gas pressure
and the applied voltage. Initially, as the gas pressure is increased, the cathode
current increases due to increases of the number of the ions, also, at higher
gas pressures, the sputtered atoms are prevented from reaching the substrate
at the anode because of randomization due to the large number of collisions
with the gas molecules. Finally, an effective glow-discharge sputtering can
take place only within an optimum pressure range of 20 to 100 mTorr.
Since the sputtered species are diffusely scattered by ambient gas molecules
during their transit, they reach the substrate in randomized directions and
energies. As a result of the diffuse nature of material transport, the atoms
deposit at places not necessarily in the line of sight of the cathode. Also, note
that because of the collisions the energetic ions hit the cathode at high oblique
angles, which is actually helpful in increasing the yield.
Oct.30, 2003
Our BAL-TEC SCD 050
Au
Oct.30, 2003
eAr+
Procedures & Attention
Pre-sputtering of Cr Wafer (removing the oxide-layer)
Pump down properly in the E-3 range
Select process-vacuum (with Ar) 2.510-2 mbar
Shutter closed
Sputter-head cooling on
Raise the current from 70 mA to 150 mA continual (during 1 min)
Open the shutter for a while
Sputtering of Cr Layer
Select process vacuum (with Ar) 2.510-2 mbar
Shutter closed
Sputter-head cooling on
Raise the current from 80 mA to 120 mA continual (during 30 sec)
Open the shutter for a certain time
Sputtering of Au Layer
Change the target after Cr layer sputtering
Select process vacuum (with Ar) 2.510-2 mbar
Shutter closed
Sputter-head cooling on
Raise the current gradually and stay at 40 mA ~ 60 mA
Open the shutter for a setting time
Remarks:
Pre-sputtering at 100 mA: No chance to remove the oxide-layer!!
Often the plasma is not stable in immediately at high current, So please raise the sputter current smoothly (see above)!!
If the oxide-layer is thick (if the target is new or not used for a long time), then please remove it without specimens in the chamber!!
For the second run directly after that (with inserted samples now), no or only a very short pre-sputtering time is needed!!
Oct.30, 2003
Substrates Cleaning & Pre-coating
Piranha solution
Rinsed
dried
Cleaned Substrate
H2SO4:H2O2=7:3
1
H2O
Cr underlayer
Then good adhesion of Au can be obtained at substrate
5-15nm
2
Silanization
Improved adhesion of hydrophobic Au at glass
(fluxing in propyltrimethoxysilane/toluene solution)
a. Haller, I. J. Am. Chem. Soc. 1978, 100, 8050.
b. So, H.;Pope, M. T. Inorg. Chem. 1972, 11, 1441.
Oct.30, 2003
Cleaning & Regeneration of Sputtered Quartz
Solution Composition (only for 4 ~5 pieces):
6ml of Aqua Regia Solution (ratio in mole, HNO3 (65%):HCl(38%) = 1:3) + 0.5ml of H2O2
VHNO3 (65%) = 0.85 ml
VHCl (38%) = 5.15 ml
VH2O2 (ca.30%) = 0.50 ml
Cleaning under Microwave (only suitable for Ag, Au, Pd, Pt, etc., or their alloy, and total metal mass < 0.5g)
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250W 5min
400W 5min
650W 5min
250W 5min
Ventilation 7min
Cooling by water
washing thoroughly by distilled water
dry
Attention: Aqua Regia solution must be freshly prepared.
Oct.30, 2003
Thick Film Deposition Techniques
Liquid-Phase Epitaxy
Screen Printing (available in lab of polymer science, microcontact printing, CP)
The procedure for screen printing consists of dispersing
a paste (or ink) of the material to be deposited on a
mesh-type screen on which the desired pattern is
photolithographically defined such that open mesh areas
in the screen correspond to the configuration to be
printed. The substrate is placed a short distance beneath
the screen. A flexible wiper, called the squeegee, then
moves across the screen surface, deflecting the screen
vertically, bringing it into contact with the substrate, and
forcing the paste through the open mesh areas. On
removal of the squeegee, the screen regains its original
position, leaving behind the printed paste pattern on the
substrate. The substrate is allowed to stand at ambient
temperature for some time to enable the paste to
coalesce to form a coherent level film.
Melt Spinning
Dip Coating, Spinning & Solution Casting
Oct.30, 2003
Monitoring & Analyzing
Deposition Rate & Thickness
Mechanical Method
Gravimetric Method
Optical Methods (such as interferometric, optically absorbed, ellipsometric, etc.)
Radiation Methods (such as back-scattering of , -rays, x-ray fluorescence, etc.)
Structure & Morphology
Microscope (STM, AFM, SEM, TEM, Optical, etc.)
LEED (for structural analysis on the film surface)
Composition
ESCA (AES, UPS, XPS, etc.)
SIMS (Secondary ion mass spectrometer)
RBS (Rutherford back-scattering)
Oct.30, 2003
Further Readings
General
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K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York (1969).
K.L. Chopra, Thin Film Device Applications (1983).
Vacuum Evaporation
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W. Schlemminger & D. Stark, Thin Solid Films 137 (1986) 49.
C. Marliere, D. Renard & J.P. Chavineau, Thin Solid Films 201 (1990) 317.
D. Hecht & D. Stark, Thin Solid Films 238 (1994) 258.
Sputtering
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M. Matsui, H. Nagayoshi, G. Muto, S. Tanimoto, K. Kuroiwa & J. Tarui, Jpn. J. Appl. Phys. 29 (1990)
62.
T. Maruyama & T. Morishita, J. Appl. Phys. 77 (1995) 6641.
Y. Inoue, M. Nomiya & O. Takai, Vacuum 51 (1998) 673.
D. Lützenkirchen-Hecht & R. Frahm, J. Synchrotron Rad. 8 (2001), in press.
CVD - Electrochemistry
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Oct.30, 2003
V.I. Birss & C.K. Smith, Electrochim. Acta 32 (1987) 259.
O. R. Melroy, M. F. Toney, G. L. Borges, M. G. Samant, J. B. Kortright, P. N. Ross & L. Blum, J.
Electroanal. Chem. 258 (1989) 403.
O.M. Magnussen, J. Hotlos, R.J. Nichols, D.M. Kolb & R.J. Behm, Phys. Rev. Lett. 64 (1990) 2929.
B.M. Jovic, V.D. Jovic & D.M. Drazic, J. Electroanal. Chem. 399 (1995) 197.
S. Taguchi & A. Aramata, J. Electroanal. Chem. 396 (1995) 131.
B.M. Ocko, X.J. Wang & Th. Wandlowski, Phys. Rev. Lett. 79 (1997) 1511.
S. Wu, J. Lipkowski, T. Tyliszczak & A.P. Hitchcock, J. Phys. Chem. B 101 (1997) 10310.
B. Wohlmann, Z. Park, M. Kruft, C. Stuhlmann & K. Wandelt, Colloids and Surfaces A 134 (1998) 15.
The Latest Readings
• G. L. Fisher, A. E. Hooper, R. L. Opila, D. L. Allara, N. Winograd, J. Phys. Chem. B 104 (2000)
3267-3273.
• Mitsuo Kawasaki, Tomoo Sato, Takumi Tanaka, Kazunori Takao, Langmuir 16 (2000) 17191728.
• Vincent S. Smentkowski, Progress in Surface Science 64 (2000) 1-58.
• S. Bharathi, M. Nogami, S. Ikeda, Langmuir 17 (2001) 7468-7471.
• Debora Goncalves, Eugene A. Irene, Langmuir 17 (2001) 5031-5038
• Shane C. Street, A. Rar, J. N. Zhou, W. J. Liu, J. A. Barnard, Chem. Mater. 13 (2001) 3669-3677.
Oct.30, 2003