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