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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.
p = pressureV = volumen = number of gas moleculesR = universal gas constantT = temperature
nRTpV =
• 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
Derived from kinetic gas theory
( ) kTmv
evkT
mvP 222
32
24
−
⎥⎦⎤
⎢⎣⎡=π
π
v = gas molecule speedm = gas molecule massk = Boltzmann’s constant
Gas molecule speed & mean free path
Derived from kinetic gas theory
pdkT
22πλ =
mkTvrms
3=
λ = mean free pathd = gas molecule diameter
vrms = root mean square velocity
General vacuum chart
Airpressure[mbar]
Altitude
100
10
110-110-210-310-410-510-610-710-810-9
10-1010-1110-1210-13
1000 0 m8848 m
50 km
Incandescent lamp manufacturing
100 km
Industrial hard coating
High vacuum vapor deposition
Meanfree path
Vacuum packaging
Space begins
Application
200 km
500 km
1000 km Space simulation
Advanced scientific research
(1013 mbar)Mt. Everest (320 mbar)Sea level
11 km Commercial jet (250 mbar)
1 mm
1 cm
1 dm
1 m
10 m
100 m
1 km
Low vacuum
Fore vacuum
High vacuum
Ultra-high vacuum
Gas flow regimes
Viscousflow
• Mean free path < wall distance• Flow limited by molecule-molecule collisions• Gas is “pushed” around corners
Molecularflow
Intermediate
10-1 mbar
P
110-210-310-410-510-2
10-1
1
Dm
• Mean free path > wall distance• Flow limited by molecule-wall collisions• High conductance requires free line-of-sight over large solid angle
Gas flow rates
P
Q = 60sccm = 1 mbar l/s
Pp
C = Q / (P – Pp)
Sp = Q / PpQ3.6 m3/h = 1 l/s
Q = Gas flow
P = Pressure
Pp = Pump inlet pressure
C = Conductance
Sp = Pumping speed
Process gas flow [sccm]Gas leaks [mbar l/s]Fore vacuum pumps [m3/h]High vacuum pumps [l/s]
Commonly:
Vacuum systemWindowsBorosilicate glassQuartzSapphireMgF
Electrical feedthroughCeramics
Motion feedthroughMetal bellowsMagnetic coupledElastomer O-ringFerro-fluidicChamber walls
Stainless steelAluminumCeramics Flange seal
Elastomer O-ringMetal seal
GaugePump
Generation of vacuum
High vacuum pumpTurboCryoDiffusionIon
Fore vacuum100 - 10-3 mbar
High vacuum
10-5 - 10-11? mbar
Process gas inlet
Not shownIntermediate pumpRoots
Fore vacuum pump(Backing pump)Rotary vaneScrollDiaphragm
Atmospheric pressure
Exhausts
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.
AB
A
BA
BA
B
Rotary vane pump
+ High capacity from 103 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.
Scroll pump
+ Oil free
+ Reliable, low maintenance.
- Low to medium capacity (103 to ~10-2 mbar)
Diaphragm pump
+ Oil free+ Reliable, low maintenance.
- Low capacity (103 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.
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.
Rotor bladeStator
blade
• Best pump capacity for heavy (slow) gas molecules.
Turbo pump
+ High capacity from 10-3 to ~10-8 mbar.+ Low maintainance.
- Sudden large gas loads may cause severe, expensive damage.
Cryo pump
He gas expender
Cool head with several plates (stages).
The metal top side of the cool (12K) plates traps gas molecules by cryocondensation.
He 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.
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 9000Air 3000
Hydrogen 5000Argon 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 pumpArray of steel tubes
Titanium plate
Magnet
• Free electrons move in helical trajectories towardsthe 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
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 diagramAt what Argon gas load [sccm] can we maintain a pump inlet pressure of 1x10-4 mbar?
sccm21sccm6035.0s
lmbar35.0s
lmbar103500 4 =⋅=⋅
=⋅
⋅=⋅= −pp PSQ
Measuring vacuum
BourdonT/CPiraniCapacitive membraneMcLeodPenningSchultz-Phelps Ion gaugeBayard-Apert Ion gaugeInvert MagnetronRGA
10210-12 10-10 10-8 10-6 10-4 10-2 100
[mbar]
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 101 to ~10-5 mbar.
Capacitive membrane gauge (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.
• Gauge is usually calibrated at a pressure << CM gauge pressure range.
• CM gauge works well for pressure 103 to ~10-4 mbar.
• True reading for all gases.
• Rugged
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
U
I
Magnet
~ 2kV
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 Ig.
• Ion gauge works well for pressure 10-4 to ~10-10 mbar.
Ig
I
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 millingSputter 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 lithographyLASERs
Glow discharge processes
• Dissociatione* + AB ⇔ A + B + e
• Atomic ionizatione* + A ⇔ A+ + e + e
• Molecular ionizatione* + AB ⇔ AB+ + e + e
• Atomic excitatione* + A ⇔ A* + e
• Molecular excitatione* + AB ⇔ AB* + e
* is exited state
DC-plasma reactor
Electrodes must have electrically conducting surfaces.
Pressure1mTorr – 1Torr
DC-plasma reactor
Anode
Cathode
Ionization
Secondary electron emission
Glow, charge, & field distribution
RF-plasma reactor
Electrically isolated electrode surfaces OK.
Pressure1mTorr – 1Torr
13.56 MHz
DC-bias
V1 / V2 ≈ (A2 / A1)4
Area A1 Area A2
Magnetically confined plasma
Magnetron, commonly used for sputter deposition sources.
Inductively coupled plasma (ICP)Process gas inlet
AntennaWater
RF-genZ-match Electrostatic shield
Exhausts
Water
Electron cyclotron resonance (ECR)
mTT
T
efmB
9009.0
106.1103.91054.22
2
19
319
==
=⋅
⋅⋅⋅⋅
==
−
−π
π
meB
=0ω2.45 GHz