previous lecture - chalmers...lecture 6: vacuum & plasmas • what glow discharge / plasma is...

Previous Lecture

Electron beam lithoghraphy

Electrons are

generated in vacuum

e-

Electron beams

propagate in vacuum

• 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:

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.

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

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)

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.

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

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

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

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

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

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

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

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

Pump types

Positive

displacement

Momentum

transferEntrapment

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

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

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.

Rotary vane pump

+ High capacity

- Potential back streaming of oil into vacuum

chamber.

Atm - ~10-3 mbar

Scroll pump

• Moving scroll orbiting a fixed scroll.

• Compressed gas volume pushed towards

center outlet.

Gas inlet

Gas outlet

Scroll pump

+ Oil free

+ Reliable, low maintenance.

- Low to medium capacity

Atm - ~10-2 mbar

Diaphragm pump

+ Oil free

+ Reliable, low maintenance.

- Low capacity

Atm - ~1 mbar

Roots pump - Single stage boaster

• Counter rotating blades moves gas

volume.

• No contact between surfaces

→ oil free operation.

- 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

Roots pump - Multiple stage

• Multiple stage counter-rotating blades.

• No contact between surfaces

→ oil free operation.

- 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

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

Turbo pump

+ High capacity

+ Low maintenance

- Sudden large gas loads may cause severe,

expensive damage.

10-1 mbar - ~10-8 mbar

Turbo pump

Tool #404

September 2012

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.

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

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.

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.

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

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?

3600

Pumping speed diagram

At what Argon gas load [sccm] can we maintain a pump inlet pressure of 1x10-4 mbar?

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]

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

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

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

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

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:

• 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?

• 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.

• 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

• 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

DC-plasma reactor

Electrodes must have electrically conducting surfaces.

Pump

Gas

Pressure1 mTorr - 1 Torr

DC-plasma reactor

Anode

Cathode

Ionization

Secondary

electron emission

Low pressure glow discharge

Crooks

dark spaceFaraday

dark spaceAnode

dark space

~1mbar

~ 1kV

Aston

dark space

Cathodeglow

Negative glow

Positive glow

RF-plasma reactor

Electrically isolated electrode surfaces OK.

Pump

Gas

Pressure1 mTorr - 1 Torr

~13.5 MHz

Impedancematching

DC-bias

Velect.

0t

VDC-bias

~

Ion surplus

Electron surplus

DC-bias

V1 / V2 (A2 / A1)4

Area A1 Area A2

V2 A1

Area A1 Area A2

V1 A2

4

A1 > A2

V1

Magnetically confined plasma

Magnetron, commonly used for sputter deposition sources.

E

Inductively coupled plasma (ICP)

Process gas inlet

RF-gen

Z-match

Water

Water

Antenna

Electrostatic shield

Exhausts

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

Next Lecture

Vacuum & Plasma systems for

Dry etching