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1Physical Vapour Deposition of Thin Film
Coatings Part I Vacuum, plasma generation, vacuum
arc, laser ablation
WITOLD GULBINSKI
2- Outline
- Gas, vacuum and plasma basic terms
- Plasma generation methods for sputter deposition
applications - Low pressure DC discharge
- Low pressure RF discharge
- Low pressure microwave discharge electron
cyclotron resonance (ECR) - Vacuum arc and its application for thin film
deposition - Laser ablation
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4Number density n 1025m-3 1019cm-3
Collision frequency
Impingement flux
Mean free path ? m
Pressure pPa 1hPa 1mbar
GAS Ar, 1000hPa (1000mbar) at 300K (270C)
Temperature K
Mean velocity
Gas temperature measure of energy E kT
At 270C -gt kT 0.025 eV
5Maxwell-Boltzmann distribution function
T 0C
Oxygen 106 molecules
216km/h 60m/s
1080km/h 300m/s
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8Primary (low) vacuum (LV) 1000 10 Pa Medium
vacuum (MV) 10Pa 0.1Pa High vacuum (HV) 0.1-
10-3 Pa Ultra high vacuum (UHV) 10-3 10-7Pa
Sputtering, magnetron sputtering
Pressure
Vacuum arc, laser ablation, electron beam
evaporation
9Vacuum pump types
- Positive displacement pumps (LOW VACUUM) use a
mechanism to repeatedly expand a cavity, allow
gases to flow in from the chamber, seal off the
cavity, and exhaust it to the atmosphere (rotary,
piston, scroll pumps) - interaction between gas molecules substantial
(viscous flow) - Momentum transfer pumps (MV HV), also called
molecular pumps, use high speed jets of dense
fluid or high speed rotating blades to knock
gaseous molecules out of the chamber (diffusion,
turbo pumps) - interaction between molecules negligible
(molecular flow) - Entrapment pumps (UHV) capture gases in a solid
or absorbed state (ion pumps, Ti sublimation
pumps, cryopumps, sorption pumps)
10Primary (low) vacuum 1000 10 Pa
Rotary vane pump
Rotary pumps Atmospheric pressure 1Pa
Roots pump
11Medium vacuum 10Pa 0.1Pa
Sputtering, magnetron sputtering
Oil diffusion pump 1Pa 10-3Pa Pumping speed up
to thousands l/s Water or LN2 cooled back stream
trap (primary vacuum pump water cooling
necessary)
12 High vacuum (HV) 0.1- 10-3 Pa Ultra high vacuum
(UHV) 10-3 10-7Pa
Vacuum arc, laser ablation, electron beam
evaporation
Turbomolecular pumps 1Pa 10-5Pa Pumping speed
up to thousands l/s Classical, ceramic or
magnetic bearings Rotation speed up to 60 000
1/min (primary vacuum pump water cooling
necessary)
13- A pumpdown curve from air shows the three major
zone divisions based on residual gases - the volume zone dominated by air,
- the drydown zone dominated by desorbing water
vapor -gt ALWAYS WENT HOT CHAMBER! - the hydrogen zone dominated by hydrogen.
14Vacuum how low pressure is measured?
- Range down to 10-2 Pa
- Only for calibration purposes
- For gases not condensing at measurement
temperature
Mc Leod vacuummeter
15Vacuum how low pressure is measured?
Range from atmospheric pressure down to
10-2Pa Gas type independent measurement!
Capacitance gauge
16Vacuum how low pressure is measured?
- VA ? 100 - 150 Volts
- Vc ? -25 to -75 Volts
- Ie ? 10 ?A - 10 mA (stabilized)
- Gas type sensitive
17Thermocouple gauge
Cold cathode vacuum gauge
down to 1Pa
down to 10-1 Pa
18Pirani gauge 100hPa-1Pa
19- Cryopump (HVUHV)
- cold finger (20K)
- very high pumping speed
- cyclic regeneration necessary
- limited Hydrogen pumping
20- Ion pump (UHV)
- all gases
- no mowing parts
21Gas flow measurement and controll
22Trottle valves
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23Gate valves
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24www.matec.org/about/Prod/vendors/mks/matec.shtml
25Residual gas analysis (quadrupole mass filter)
DC voltage sweep superimposed with RF voltage
Only ions with m/Z ratio corresponding to
resonance condition pass the analyzer
26Low temperature (cold) plasma
- Quasineutrality condition (ne ni) - first used
by Irving Langmuir and Lewi Tonks, in 1929 - Debye length ?D or Debye sphere radius (distance
over which quasi-neutrality may break down) - ?D depends on plasma density (10-4m in glow
discharge tube) - high electrical conductivity (increasing with
electron density and temperature)
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28Opposed effects keep plasma in stable state
Energy level of equilibrium state depends
on pressure and type of gas/gas mixture manner
of excitation (DC, RF, MW) dissipated
power plasma-wall interaction
DC, RF, MW field interaction with
charged particles
29Ci and Ce denote ion and electron average speed,
respectively.
30- Plasma deposition process choice
- Let us assume that we need to do deposit thin
film coating by sputtering (Pt, Pd, W., high
temperature melting compound) - Material to be deposited is expensive
- High deposition rate is desirable (high
productivity) - High purity of deposited film is required
- Thermal load to coated substrate should not be
too high - The shape of item to be coated is complex
- Cost/performance ratio for your deposition system
should be high
The best choice? At least several compromise
solutions are possible!
31Two electrode DC glow discharge - is it useful
for sputtering?
Normal glow discharge region Discharge current
density 1mA/cm2 Discharge voltage 1-10kV
Vacuum arc discharge Discharge current density
103A/cm2 Discharge volatage 15-30V
32Two electrode DC glow discharge - is it useful
for sputtering?
Normal discharge region Discharge current
density 1mA/cm2 Discharge voltage 1-10kV
mysite.du.edu/jcalvert/phys/dischg.htm
33Two electrode glow discharge - is it useful for
sputtering?
Potential distribution and the main regions of
plasma emission between electrodes of DC glow
discharge in gas under low pressure
34a- number of ions created by one electron per
unit length of its drift path
Gas ionization coefficient a vs electron energy
Ee
35Disadvantages of Diode DC sputtering
- High voltage (several kV) necessary to initiate
and sustain the discharge - Low ion current density at the cathode (less than
1mA/cm2) low flux of sputtered atoms - High working gas pressure (5-10Pa), resulting in
low deposition rate due to scattering - It cannot be used for direct sputtering of
dielectric materials because of charging effect
leading to the extinction of discharge - Could be used for Ag or Cu sputtering, just to
show that it works
R
36- How to improve usefulness of two-electrode glow
discharge for sputtering purposes? - increase the probability of ionisation a (i.e.
change electron energy), - increase plasma density (ne ni)
It can be achieved by
- Injection of additional electrons into the
discharge by using hot filament as an electron
source. - Ionisation enhancement given by radio frequency
(RF) or microwave (MW) field excitation. - Use of constant magnetic field to increase the
path length of electrons before they are
collected on the anode - magnetrons
37Schematic view of DC triode sputtering system
lt 1kV
- Independent ionization source
- Enhanced plasma density
- Lower discharge voltage
- Lower gas pressure (1Pa)
- Higher deposition rate
- Cathode current density up to 250mA/cm2
38Radio frequency (RF) sputtering
Frequency 13.56 or 27.12 MHz Enhanced plasma
density Lower discharge voltage (Vpp lt 1kV) Lower
working gas pressure (0.1Pa) Higher deposition
rate Sputtering of dielectrics possible
Impedance matching problem RF radiation
39meltltMA
-?
40Self-bias of a dielectric surface
41Voltage distribution with blocking capacitor C
42Typical RF-matching circuit
43Output power (DC) vs load resistance
44Electron Cyclotron Resonsnce (ECR) enhanced MW
plasma discharge
Frequency 2.45 GHz
Resonance condition B 875 Gauss
45Electron Cyclotron Resonsnce (ECR) enhanced MW
plasma discharge
Argon plasma 0,13 Pa
Oxygen plasma 5x10-2 Pa
Distributed ECR (DECR) discharge
46Electron Cyclotron Resonsnce (ECR) enhanced MW
plasma discharge
- Plasma generated independently on sputtering
circuit - Plasma density independently regulated
- Low pressure plasma (pressure as low as 10-2Pa)
- Sputtering circuit (cathode substrate) immersed
in the MW plasma
47- How to improve usefulness of two-electrode glow
discharge for sputtering purposes? - increase the probability of ionisation a (i.e.
change electron energy), - increase plasma density (ne ni)
It can be achieved by
- Injection of additional electrons into the
discharge by using hot filament as an electron
source triode sputtering - Ionisation enhancement given by radio frequency
(RF) or microwave (MW) field excitation RF and
MW plasma sputtering - Use of constant magnetic field to increase the
path length of electrons before they are
collected on the anode.
48Trigger
Vacuum arc (cathodic arc) evaporation
- A vacuum arc can arise when the contact of metal
surfaces (electrodes) is broken in a good vacuum. - Once initiated, a vacuum arc can persist since
the freed ionized particles gain kinetic energy
from the electric field, - Metal surfaces are heated through high speed
particle (ions) collisions. - This process creates an incandescent cathode spot
which frees more particles, thereby sustaining
the arc. - Working pressure as low as 10-3Pa
- Discharge voltage 20-30V dependent on cathode
material - Discharge current hundreds of Amps
- Can be ignited by mechanical trigger, spark
discharge or laser pulse
Cathode (-)
Anode ()
49Vacuum arc (cathodic arc)
Crater traces left by cathode spot
High ionization degree of cathode material High
deposition rate Droplets! (0.1- 10µm) Reactive
deposition of compounds (TiN, CrN, TiC, TiAlN)
possible
50Vacuum arc (cathodic arc)
Droplets separation magnetic S-type
microparticle filter/separator
51Electron beam evaporation
Pressure less than 10-3 Pa Electron current up
to 10A Accelerating voltage up to 10kV Magnetic
field focusing and scanning/swiching High purity
and deposition rate Reactive deposition possible
52Electron beam evaporation
53Pulsed Laser Ablation
A laser beam of sufficient intensity is focused
and directed at a target located inside a vacuum
chamber.
- able to evaporate/deposit a large variety of
materials and mixtures - able to preserve the stochiometric ratio of the
target material - good adhesion between coating and substrate
- coating at room temperature.
54Pulsed Laser Ablation
- The properties of the thin films can be
controlled by - Laser beam intensity,
- substrate temperature,
- buffer gas pressure,
- incident angle of the coating plume
- Intensity (Watts/cm2) peak power (W)/focal spot
area (cm2) - Fluence (Joules/cm2) laser pulse energy
(J)/focal spot area (cm2) - Pulse duration ns, ps, fs
- Pulse repetition frequency up to hundreds of Hz
- Wave length UV (Excimer lasers), VIS, IR (Nd-YAG
(Y3AI5O12) , CO2)
55Pulsed Laser Ablation
- Excimer (excited dimer) lasers
- average power over 100W
- repetition frequency up to 1000Hz
- pumped by electrical discharge
56Pulsed Laser Ablation
57Pulsed Laser Ablation
- Ultrashort (picosecond and femtosecond) pulses
- ablate material via the rapid creation of a
plasma that absorbs the incident energy resulting
in direct vaporisation from the target surface. - this produces negligible heating and shock-wave
damage
Melted zone
58Pulsed Laser Ablation
- There are several key parameters to consider for
laser ablation - selection of a wavelength with a minimum
absorption depth to ensure a high energy
deposition in a small volume for rapid and
complete ablation - short pulse duration to maximize peak power and
to minimize thermal conduction to the surrounding
work material - pulse repetition rate - If the rate is too low,
all of the energy which was not used for ablation
will leave the ablation zone allowing cooling - beam quality, measured by the brightness
(energy), the focusability, and the homogeneity
59Sputter deposition methods discussed DC diode
sputtering DC triode sputtering RF diode
sputtering MW plasma sputtering
Plasma related PVD methods discussed Vacuum arc
deposition Laser ablation Electron beam deposition
What still needs to be discussed? MAGNETRON
SPUTTERING
60- Recommended literature
- Kelly, P.J., Arnell, R.D. (2000) Magnetron
Sputtering A Review of Recent Developments and
Applications, Vacuum, 56, 159-172. - Safi, I. (2000) Recent Aspects Concerning DC
Reactive Magnetron Sputtering of Thin Films a
Review, Surf. Coat. Technol. 127, 203-219. - Musil, J. (1998) Recent Advances in Magnetron
Sputtering Technology, Surf. Coat. Technol.
100/101, 280-286. - Bunshah, R.F. (1991) Handbook of Deposition
Technologies for Films and Coatings Science,
Technology and Applications, Second Edition,
Noyes Publ., New Jersey - Wasa, K., Hayakawa, S. (1991) Handbook of Sputter
Deposition Technology, Noyes Publ., Park Ridge,
New Jersey. - S. M. Rossnagel, J.J. Cuomo W.D. Westwood
(eds.) Handbook of Plasma Processing Technology,
Noyes Publ. (1990) Park Ridge, New Jersey