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Previous Lecture
Electron beam lithoghraphy
Electrons are
generated in vacuum
e-
Electron beams
propagate in vacuum
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• What vacuum is and what it is used for
• Basic vacuum theory
• Basic parts of a vacuum system
• Generation of vacuum: Pumps
• Measuring vacuum: Gauges
Lecture 6: Vacuum & plasmas
Objectives
From this “vacuum” lecture you will learn:
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General definition
• vacuum = empty space, from vacuus = [Latin] empty
What is vacuum?
Scientific definitions
• A gas pressure lower than atmospheric.
• A space where the pressure is significantly lower than atmospheric.
• A condition in which the quantity of atmospheric gas present is
reduced to the degree that, for the process involved, its effect can be
considered negligible.
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Ideal gas law
• Experimentally found by Robert Boyle and published 1662.
p = pressure
V = volume
n = number of gas molecules
R = universal gas constant
T = temperature
• Works well for sub atmospheric pressure and normal temperature.
nRTpV
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qnRTpV
Ideal gas law
• Experimentally found by Robert Boyle and published 1662.
p = pressure
V = volume
n = number of gas molecules
R = universal gas constant
T = temperature
• Works well for sub atmospheric pressure and normal temperature.
• For better accuracy use a correction factor q(p,T). (gas specific)
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Kinetic gas theory
• …are treated as hard spheres.
• …are many, small, and far apart compared to their size.
• …collide elastically with walls and each other.
• …move randomly with constant speed between collisions.
• …obey Newton’s laws of motion.
The gas molecules…
Collisions Pressure
Daniel Bernoulli explained (1738) gas pressure from a
molecular point of view.
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Gas molecule speed distribution
kTmv
evkT
mvP 22
23
2
24
Derived from kinetic gas theory
v = gas molecule speed
m = gas molecule mass
k = Boltzmann’s constant
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Gas molecule speed & mean free path
l = mean free path
d = gas molecule diametervrms = rms velocity
pd
kT22
l m
kTvrms
3
N2 at room temperature
~ 500 m/s~ 7 cm between collisions
@ 10-3 mbar
Derived from kinetic gas theory
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Why use vacuum ?
mbar
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
Food preservation
Plasma processes
TV-tubes, etc.
Force
Scientific instruments
e.g. Electron microscope,
Mass spectrometer
Long mean free path
O2 free
Evaporation
Suction pads
Plastic forming
Packing
Thin-film coating
Freeze drying
Light bulbs / tubes
mean-free path @RT
~1 m
500 km altitude
Sea level
Mt. Everest
100 km altitude
Space begins
335 mbar
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Vacuum quality
mbar
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
Low vacuum
Fore vacuum
High vacuum
Ultra-high vacuum1 km
100 m
10 m
1 m
1 dm
1 cm
1 mm
N2 mean free path
@ 6·10-5 mbar
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Gas flow regimes
Viscous
flow
• Mean free path << wall-to-wall distance
• Flow limited by molecule-molecule collisions
• Gas flows like “virtually weightless liquid”
Molecular
flow
• Mean free path >> wall-to-wall distance
• Flow limited by molecule-wall collisions
• High conductance requires free line-of-sight over large solid angle
10-1 mbar
P
110-210-310-410-5
10-2
10-1
1
Dm
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Gas flow rates
P
60 sccm = 1 mbar l/s
Pp
Q
Q = Gas flow
P = Pressure
Pp = Pump inlet pressure
C = Conductance
C =Q
(P-Pp)
1 l/s = 3.6 m3/h
Sp = Pumping speed
Sp =
Q
Process gas flow [sccm]
Gas leaks [mbar l/s]
Fore vacuum pumps [m3/h]
High vacuum pumps [l/s]
Commonly:
Q
Sp
Q
Pp
Q
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Vacuum system
Electrical feedthroughCeramics
Flange sealElastomer O-ring
Metal seal
Motion feedthroughMetal bellows
Magnetically coupled
Elastomer O-ring
Ferro-fluidic
WindowBorosilicate glass
Quartz
Sapphire
MgFChamber wallStainless steel
Aluminum
Ceramics
Pump
Gauge
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High vacuum pump
Fore vacuum pump
High vacuum
<10-5 mbar
Atmospheric pressure
Exhausts~ 10-2 mbar
Large
absolute
pressure
difference
Small absolute
pressure
difference
100 kN/m2
Generation of vacuum
10 ton/m2
1 N/m2
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Pump types
Positive
displacement
Momentum
transferEntrapment
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High vacuum
<10-5 mbar
Atmospheric pressure
Exhausts~ 10-2 mbar
Generation of vacuum
High vacuum pumpTurbo
Diffusion
Fore vacuum pumpRotary vane
Scroll
Multiple stage roots
Diaphragm
Positive
displacement
Momentum
transfer
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High vacuum
<10-5 mbar
Atmospheric pressure
Exhausts~ 10-2 mbar
Generation of vacuum
High vacuum pumpCryo
Ion
Fore vacuum pumpRotary vane
Scroll
Multiple stage roots
Diaphragm
Entrapment
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Rotary vane pump
AB
A
BA
B
A
B
• Very common fore vacuum- and general
vacuum pump.
• Typically 1 or 2 stage configuration.
• Gas is moved by rotating vanes.
• Oil is used as seal, lubricant, and coolant.
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Rotary vane pump
+ High capacity
- Potential back streaming of oil into vacuum
chamber.
Atm - ~10-3 mbar
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Scroll pump
• Moving scroll orbiting a fixed scroll.
• Compressed gas volume pushed towards
center outlet.
Gas inlet
Gas outlet
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Scroll pump
+ Oil free
+ Reliable, low maintenance.
- Low to medium capacity
Atm - ~10-2 mbar
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Diaphragm pump
+ Oil free
+ Reliable, low maintenance.
- Low capacity
Atm - ~1 mbar
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Roots pump - Single stage boaster
• Counter rotating blades moves gas
volume.
• No contact between surfaces
→ oil free operation.
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- Works well only together with fore vacuum pump.
Roots pump.
Fore vaccum pump
+ High capacity from 10 mbar to ~10-4 mbar.
+ Oil free
Roots pump - Single stage boaster
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Roots pump - Multiple stage
• Multiple stage counter-rotating blades.
• No contact between surfaces
→ oil free operation.
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- Moving parts don’t seal higher ultimate pressure
+ Medium capacity
+ Oil free
Roots pump - Multiple stage
~5∙10-2 mbar
Atm - ~5∙10-2 mbar
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Turbo pump
• Fast moving rotor (30k to 90k rpm) with
several stages and many blades per stage.
• High efficiency in the molecular regime
where gas molecules collide with rotor
blade and not each other.
• Some pumps have magnetic, non-
contact, bearings.
• Best pump capacity for
heavy (slow) gas molecules.
Rotor
blade
Stator
blade
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Turbo pump
+ High capacity
+ Low maintenance
- Sudden large gas loads may cause severe,
expensive damage.
10-1 mbar - ~10-8 mbar
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Turbo pump
Tool #404
September 2012
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Diffusion pump
• Hot dense oil vapor is forced through
central jets angled downward to form a
conical curtain of vapor.
• Gas molecules are knocked downwards
and eventually reach the fore vacuum
pump.
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Diffusion pump
+ Simple pump without moving parts.
+ High capacity
+ Low maintenance
- Needs cooled baffle to reduce oil contamination of
vacuum chamber.
10-2 mbar - ~10-8 mbar
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Cryo pump
Helium gas expender
Cool head with several plates (stages).
The metal top side of the cool (12K)
plates traps gas molecules by
cryocondensation.
Helium gas compressor
The bottom side of the plates are
coated with active charcoal and traps
gas molecules by cryoadsorption.
The cooling is done with a Helium
filled refrigerator loop.
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Cryo pump
Gas Pumping speed (Ø20cm pump)
[ l/s ]
Water vapor 4000
Air 1500
Hydrogen 2500
Argon 1200
+ Very High capacity down to ~10-9 mbar.
+ No contamination.
- Pump saturates fast if exposed to high pressure or
continuous high gas flow.
- Need periodic regeneration (heating) of cool head.
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Ion pump
Array of steel tubes
Titanium plate
Magnet
• Free electrons move in helical trajectories
towards the anode, ionizing gas molecules
upon collisions.
• Gas ions strike the Ti cathodes and some gets buried.
• Sputtered Ti deposits inside the tubes and getters gas
molecules through chemical reactions.
B
U
Ti
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Ion pump
+ Simple pump without moving parts.
+ Can work at very low pressure ~10-11 mbar.
+ Oil free.
- Not suitable for gas loads.
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Pumping speed diagram
At what Argon gas load [sccm] can we maintain a pump inlet pressure of 1x10-4 mbar?
3600
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Pumping speed diagram
At what Argon gas load [sccm] can we maintain a pump inlet pressure of 1x10-4 mbar?
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Measuring vacuum
Bourdon
10210010-210-410-610-810-1010-12
T/C
Pirani
Capacitance manometer
McLeod
Penning
Schultz-Phelps Ion gauge
Bayard-Apert Ion gauge
Invert Magnetron
Residual Gas Analyzer
[mbar]
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Pirani vacuum gauge
• A wire resistor in a gauge tube, heated with an electrical current.
• A second wire resistor in a closed reference tube.
• The two wire resistors are 2/4 of a Wheatstone bridge.
• Higher pressure cools the wire and the resistance drops.
• The pressure is measured from the
unbalanced bridge.
• Pirani gauge works well for pressure
101 to ~10-5 mbar.
Gaugetube
Referencetube
Filaments
Meter
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Capacitance manometer (CM)
• The unknown pressure Px decide the position of the
metal membrane electrode relative a fixed second
electrode in a closed volume.
• The electrode capacitance can be converted to
pressure.
• Overlapping CM gauges works well for atmospheric
pressures to ~10-5 mbar.
• Each CM gauge covers a
pressure range of 4 orders of
magnitude.
• True reading for all gases.
• Rugged
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Penning vacuum gauge
• Penning gauge often cylindrical in shape.
• DC discharge generated by ~ 2 kV.
• Pressure converted from discharge current.
• Penning gauge works well for pressure 10-2 to ~10-9 mbar.
B
U
I
~ 2kV
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Ion vacuum gauge
• Electrons are emitted from a hot filament.
• Electrons are attracted by the positive
grid but pass several times before captured.
• Collisions with gas molecules creates ions
that are collected on negative pin.
• Pressure is converted from current Ig.
• Ion gauge works well for pressure
10-4 to ~10-10 mbar.
Ig
I
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Lecture 6: Vacuum & plasmas
• What glow discharge / plasma is
• What we use glow discharges for
• Different types of glow discharges: DC, RF
• High density plasmas: Magnetically confined, ECR, ICP
Objectives
From this “plasma” lecture you will learn:
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• Glow discharge is luminous plasma.
• Plasma is partially ionized gas.
• The glow is excess electromagnetic energy
radiating from excited gas atoms and molecules.
What is glow discharge?
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• Accelerated inert ions are used for:
Ion milling
Sputter deposition
• Accelerated reactive ions are used for:
Reactive ion beam etching (RIBE)
Reactive ion etching (RIE)
• Accelerated ions can be filtered and counted
Residual gas analysis (RGA)
How use glow discharge?
• Neutral particles are difficult to accelerate.
Ions and electrons can be extracted from a
glow discharge and easily accelerated.
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• Radicals from a plasma is used for:
Chemical vapor deposition (PECVD)
Plasma etching
How use glow discharge?
• The electromagnetic radiation from a plasma is used for
General illumination (light tubes, …)
Light sources for optical lithography
LASERs
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• Dissociation
e* + AB A + B + e
Glow discharge processes
* exited state
• Atomic ionization
e* + A A+ + e + e
• Molecular ionization
e* + AB AB+ + e + e
• Atomic excitation
e* + A A* + e
• Molecular excitation
e* + AB AB* + e
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DC-plasma reactor
Electrodes must have electrically conducting surfaces.
Pump
Gas
Pressure1 mTorr - 1 Torr
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DC-plasma reactor
Anode
Cathode
Ionization
Secondary
electron emission
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Low pressure glow discharge
Crooks
dark spaceFaraday
dark spaceAnode
dark space
~1mbar
~ 1kV
Aston
dark space
Cathodeglow
Negative glow
Positive glow
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RF-plasma reactor
Electrically isolated electrode surfaces OK.
Pump
Gas
Pressure1 mTorr - 1 Torr
~13.5 MHz
Impedancematching
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DC-bias
Velect.
0t
VDC-bias
~
Ion surplus
Electron surplus
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DC-bias
V1 / V2 (A2 / A1)4
Area A1 Area A2
V2 A1
Area A1 Area A2
V1 A2
4
A1 > A2
V1
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Magnetically confined plasma
Magnetron, commonly used for sputter deposition sources.
E
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Inductively coupled plasma (ICP)
Process gas inlet
RF-gen
Z-match
Water
Water
Antenna
Electrostatic shield
Exhausts
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Electron cyclotron resonance (ECR)
mTT
T
efmB
9009.0
106.1
103.91054.22
2
19
319
m
eB0
2.45 GHz
Microwave power
B
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Next Lecture
Vacuum & Plasma systems for
Dry etching