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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.
+ 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.