What is vacuum?

Lecture 6: Vaccum & plasmas
Outline
• What is vacuum?
• Why vacuum?
• Basic vacuum theory
• Overview of vacuum system & components
• Generation of vacuum:
Vacuum pumps
• Measuring vacuum:
Vacuum gauges
• Practical vacuum advice
• What is a glow discharge or plasma?
• Why glow discharge?
• Types of glow discharges: DC, RF
• High density plasmas: Magnetically confined, ECR, ICP

What is vacuum?
General definition
• vacuum = empty space, from vacuus = [Latin] empty
Scientific definitions
• A pressure lower than atmospheric, in an enclosed area.
• A space in which the pressure is significantly lower than
atmospheric pressure.
• 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.

Why vacuum?
• Control a chemical reaction.
Reaction rate, concentration, etc.
• Create suitable condition for plasmas.
~mbar
• Long mean free path.
Physical vapor deposition, cathode ray tube (CRT), etc.
• Cavity free manufacturing.
Vacuum mould, vacuum cast, vacuum package, etc.
• Create forces and flows.
Vacuum pick-up, vacuum cleaner, etc.

Ideal gas law
• Experimentally found by Robert Boyle and published 1662.
pV =
nRT
p = pressure
V = volume
n = number of gas molecules
R = universal gas constant
T = temperature
• Works well for sub atmosphere pressure and normal temperature.
• For better accuracy use a correction factor q(p,T). (gas specific)

Kinetic gas theory
A theory that could explain Robert Boyle’s experimental results.
The gas molecules…
• …are treated as hard spheres.
• …are many, small, and far apart compared to their size.
• …collide elastically with walls and each other.
• …moves randomly with constants speed between collisions.
• …obey Newton’s laws of motion.

Gas molecule speed distribution
P
Derived from kinetic gas theory
() v
⎡
= 4π
⎢
⎣
m
2πkT
⎤
⎥
⎦
3
2
v
2
e
−mv
v = gas molecule speed
m = gas molecule mass
k = Boltzmann’s constant
2
2kT

v rms
=
Gas molecule speed & mean free path
Derived from kinetic gas theory
3kT
m
v rms = root mean square velocity
λ =
kT
2π
d
2
p
λ = mean free path
d = gas molecule diameter

Ultra-high vacuum
High vacuum
Fore vacuum
Low vacuum
General vacuum chart
Mean
free path
Air
pressure
[mbar]
1km
100 m
10 m
1m
1dm
1cm
1mm
10 -13
10 -12
10 -11
10 -10
10 -9
10 -8
10 -7
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
1
10
Altitude
500 km
200 km
100 km
50 km
Application
Advanced scientific research
1000 km Space simulation
High vacuum vapor deposition
Industrial hard coating
Space begins
Incandescent lamp manufacturing
Vacuum packaging
100 11 km Commercial jet(250 mbar)
8848 m Mt. Everest (320 mbar)
1000 0m Sea level (1013 mbar)

m
1
10 -1
10 -2
D
10 -5
Molecular
flow
10 -4
10 -3
Intermediate
Gas flow regimes
10 -2
• Mean free path < wall distance
• Flow limited by molecule-molecule collisions
• Gas is “pushed” around corners
Viscous
flow
P
10-1 1 mbar
• Mean free path > wall distance
• Flow limited by molecule-wall collisions
• High conductance requires free line-of-sight over large solid angle

C = Q / (P – P p )
S p = Q / P p
3.6 m 3 /h = 1 l/s
P
P p
Gas flow rates
Q = 60sccm = 1 mbar l/s
Q
Q = Gas flow
P = Pressure
P p = Pump inlet pressure
C = Conductance
S p = Pumping speed
Commonly:
Process gas flow [sccm]
Gas leaks [mbar l/s]
Fore vacuum pumps [m 3 /h]
High vacuum pumps [l/s]

Electrical feedthrough
Ceramics
Chamber walls
Stainless steel
Aluminum
Vacuum system
Motion feedthrough
Metal bellows
Magnetic coupled
Elastomer O-ring
Ferro-fluidic
Windows
Borosilicate glass
Quartz
Sapphire
MgF
Ceramics Flange seal
Elastomer O-ring
Metal seal
Pump
Gauge

Not shown
Intermediate pump
Roots
Fore vacuum pump
(Backing pump)
Rotary vane
Scroll
Diaphragm
Generation of vacuum
High vacuum
10 -5 -10 -11? mbar
Process gas inlet
Fore vacuum
10 0 -10 -3 mbar
High vacuum pump
Turbo
Cryo
Diffusion
Ion
Atmospheric pressure
Exhausts

B
A
A
Rotary vane pump
• 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.
B
A
B
B
A

Rotary vane pump
+ High capacity from 10 3 to ~10 -2 mbar.
- Potential back streaming of oil into vacuum
chamber.

Scroll pump
• Moving scroll orbiting a fixed scroll.
• Compressed gas volume pushed towards
center outlet.

+ Oil free
+ Reliable, low maintenance.
Scroll pump
- Low to medium capacity (10 3 to ~10 -2 mbar)

Diaphragm pump
+ Oil free
+ Reliable, low maintenance.
- Low capacity (10 3 to ~1 mbar)

Roots pump
• Counter rotating blades moves gas
volume.
• No contact between surfaces → oil free
operation.
• Runs very hot without fore vacuum
pump.

Roots pump
+ High capacity from 10 to ~10-4 mbar.
(Medium capacity from 1000 to ~10 mbar)
+ Oil free
- Works best together with fore vacuum pump.

• Best pump capacity
for heavy (slow) gas
molecules.
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 modern pumps have magnetic,
non-contact, bearings.
Stator
blade
Rotor
blade

Turbo pump
+ High capacity from 10 -3 to ~10-8 mbar.
+ Low maintainance.
- Sudden large gas loads may cause severe,
expensive damage.

Cryo pump
Cool head with several plates (stages).
The metal top side of the cool (12K)
plates traps gas molecules by
cryocondensation.
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.
He gas expender
He gas compressor

Cryo pump
+ Very High capacity down to ~10-9 mbar.
+ No contamination.
- Pump saturates if exposed to high pressure or
continuous gas flow.
- Need periodic regeneration of cool head.
Gas Typical pumping speed
[l/s]
Water vapor 9000
Air 3000
Hydrogen 5000
Argon 2500

Diffusion pump
• Hot dense oil vapor is forced through
central jets angled downward to give a
conical curtain of vapor.
• Gas molecules are knocked downwards
and eventually reach the fore vacuum
pump.

Diffusion pump
+ Simple pump without moving parts.
+ High capacity from 10-3 to ~10-8 mbar.
+ Low maintenance.
- Needs cooled baffle to reduce oil contamination of
vacuum chamber.

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
Ti
U

Ion pump
+ Simple pump without moving parts.
+ Can work at very low pressure ~10 -11 mbar.
+ Oil free.
- Not suitable for gas loads.

Pumping speed diagram
At what Argon gas load [sccm] can we maintain a pump inlet pressure of 1x10-4 mbar?
= p ⋅ P S Q
p
= 3500⋅10
−4
mbar ⋅l
=
s
0.
35
mbar ⋅l
=
s
0.
35⋅
60 sccm
=
21sccm

Measuring vacuum
10 -12 10 -10 10 -8 10 -6 10 -4 10 -2 10 0
[mbar]
10 2
Bourdon
T/C
Pirani
Capacitive membrane
McLeod
Penning
Schultz-Phelps Ion gauge
Bayard-Apert Ion gauge
Invert Magnetron
RGA

Pirani vacuum gauge
• A heated wire resistor in a gauge tube.
• 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 resistance drops.
• The pressure is measured from the
unbalanced bridge .
• Pirani gauge works well for pressure
10 1 to ~10 -5 mbar.

Capacitive membrane gauge (CM)
• The unknown pressure P x 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.
• Gauge is usually calibrated at a pressure

Penning vacuum gauge
• Penning gauge often cylindrical in shape.
• DC discharge generated by ~ 2kV.
• Pressure converted from discharge current.
• Penning gauge works well for pressure 10 -2 to
~10 -9 mbar.
B
Magnet
U
I
~ 2kV

I
I g
Ion vacuum gauge
• Electrons are emitted from a hot filament.
• Electrons are attracted towards 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 I g .
• Ion gauge works well for pressure 10 -4 to
~10 -10 mbar.

Vacuum advice
• The walls of a vented chamber can host a
large amount of condensed matter. Mainly
water.
When the chamber is evacuated, the
condensed matter evaporates from the walls.
This process can prevent good vacuum for
weeks.
• Keeping the chamber warm when vented
reduces the condensation on the walls.
• Heating the walls of a evacuated chamber
speed up evaporation rate x2 per 10ºC.
• Do not try to compensate vacuum leaks with a
larger pump. Find the leaks and fix them!

What is a glow discharge?
• Glow discharge also called plasma
• Plasma is partially ionized gas.
• The glow is excess electromagnetic energy
radiating from excited gas atoms and molecules.

Why glow discharge?
• Neutral particles are difficult to accelerate. Ions
and electrons can be extracted from a glow
discharge and easily accelerated.
• 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)

Why glow discharge?
• Radicals from a plasma is used for:
Chemical vapor deposition (PECVD)
Plasma etching
• The electromagnetic radiation from a plasma is used for:
General illumination (light tubes, …)
Light sources for optical lithography
LASERs

Glow discharge processes
• Dissociation
e* + AB ⇔ A + B + e
• 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
* is exited state

DC-plasma reactor
Electrodes must have electrically conducting surfaces.
Pressure
1mTorr – 1Torr

Ionization
DC-plasma reactor
Anode
Cathode
Secondary
electron emission

Glow, charge, & field distribution

RF-plasma reactor
Electrically isolated electrode surfaces OK.
13.56 MHz
Pressure
1mTorr – 1Torr

Area A 1
DC-bias
V 1 / V 2 ≈ (A 2 / A 1 ) 4
Area A 2

Magnetically confined plasma
Magnetron, commonly used for sputter deposition sources.

Water
Water
Inductively coupled plasma (ICP)
Process gas inlet
Antenna
RF-gen
Z-match Electrostatic shield
Exhausts

B =
e fm 2π
0.
09
T
Electron cyclotron resonance (ECR)
9
2π
⋅2.
54⋅10
⋅9.
3⋅10
−19
1.
6⋅10
=
ω0
=
eB
m
=
= 90 mT
− 31
T
=
2.45 GHz