§4 Fundamental Experimental Techniques Pr

essu

re

10-10

(Pa)

10-8

10-6

10-4

10-2

100

102

104low vacuum

medium vacuum

high vacuum

ultra high vacuum

extremely high vacuum

vacuum packing

vacuum drying

mean free path (l) ≈ 10 cm

surface monolayer formation ≈ 30 s

surface monolayer formation ≈ 104 s

surface science

ordinary experiments

atmospherevacuum cleaner

daily life usage

space (10-19 Pa}

molecular device

accelerator, synchrotron radiation facility

vacuum evaporation (thin films)

vacuum metallurgy (protect from surface oxidation)

4.1 Experimental environments: Vacuum

Why vacuum environment ? Isolation from external noises (thermal, mechanical and acoustic vibrationsReduce scatterings with gas molecules for experimental objects (particles, electromagnetic waves, etc.) Obtain clean surfaces

1 Pa = 1 N/m2 = 10-5 bar 1 mbar = 100 Pa = 1 hPa1 torr = 133.322 Pa

1 atm = 1.01325 bar = 101325 Pa = 760 torr (mHg) 1 psi = 0.06895 bar = 6895 Pa

Various vacuum pumps and gauges

vacuum gauge

hot cathode ionization gauge

Bourdon gauge

diaphragm gauge

cold cathode ionization gauge

Pirani gauge

10-10 (Pa)10-8 10-6 10-4 10-2 100 102 104

vaga

n gauge

ugeg

1 Pa = 10-2 mbar

Pressure

ion pump

rotary pump

turbo molecular pump

cryo pump

10-10 (Pa)10-8 10-6 10-4 10-2 100 102 104

oil diffusion pump

vacuum pump

p

v

low vacuummedium vacuumhigh vacuumultra high vacuumextremely

high vacuum

Oil sealed rotary vane pump (10-2 ≤ P ≤ 103 Pa)Principles• Rotator with a vane introduces gas to a cylinder from inlet and push it

out to outlet

• Oil with low vapor pressure serves as a sealant between the vane and the cylinder and as a coolant was well.

Advantages• Can start even from atmosphere with large pumping speed.• Standard as back up pump for other pumps for higher vacuum.

Cautions• Contamination of vacuum chamber by back flow of vapor of heated oil

Use oil mist trap (adsorbent) Use oil free (dry) pumps: diaphragm pump (103-104 Pa),

scroll pump (1-10 Pa) • Mechanical vibrations and acoustic noises

S = fV0f : rotation speed of rotor V0 : gas volume introduced to cylinder

ULVAC, DIS-251 250 L/minscroll pump

Edwards, E2M18 280 L/minRotary vane pump

10-8 ≤ P ≤ 10-2 PaPrinciples• A multi-stage, turbine-like rotor with bladed disks

rotates with a high speed (> 1×104 rpm), which is not negligible compared to the thermal velocity of gas molecule, in a housing. Interposed invertedly between the rotor disks are bladed stator disks having similar geometries. Gas molecules can be compressed (exhausted) directionally in the rotor/stator pair.

Advantages• Perfectly or nearly oil free• Wide operation pressure range

Cautions• Sometimes broken when vacuum suddenly

degrades (P > 1 Pa) • Lower compression ratio for lighter molecules

• high frequency vibrational and acoustic noisesv ∝1 m

Molecules coming from left can pass through more easily compared to those from right

Pfeiffer, HiPace 300 (260 L/s)

Oil diffusion pump (10-6 ≤ P ≤ 10-1 Pa)Principles• Ultrasonic jet of oil vapor from nozzles, which is boiled at

the bottom and up-flows through a chimney, compresses gas molecules down to the bottom outlet

• The oil jet is collected back to the bottom boiler and circulates.

Advantages• High pumping speed with cheap price• Simple structure (no moving parts) , stable operation

Cautions• Oil contamination of vacuum chamber

Use water OR liq. N2 cold trap • Usually needs cooling water (possible water leak accident)• Should not exceed critical back pressure (10 Pa). Consider

best matched pipe diameter and pumping speed of rotary pump as a back one.

Agilent,VHS-4 (750 L/s)

Pumping speed curves

10010-110-210-310-410-5 P (Pa)

Pfeiffer, HiPace 300 (260 L/s)

Agilent, HS-2 (160 L/s)

ULVAC, DIS-251 (250 L/m) ULVAC, GLD-280A (280 L/m)

Pirani gauge, Thermocouple gauge (1 ≤ P ≤ 102 Pa)

Principles• By measuring the temperature of a resistive

conductor whose Joule heat is conducted by gas molecules. The thermal conductivity (κ) is given by:

, where γ ≡ Cp/Cv .

Advantages• Simple to use, cheap, durable

Cautions• Output is nonlinear with respective to pressure.• Gas dependent sensitivity

κ =1

2

γ +1

γ −1

R

2πMTP

Total pressure gauge• Diaphragm gauge

Partial pressure gauge• quadrupole mass spectrometer

Leybold Vacuum, Fundamentals of Vacuum Technology

Calibration curves of a thermocouple gauge for various gases

Ionization vacuum gaugePrinciples• Measure ionization current ( ), which is created by the collision

between thermal electrons emitted from the cathode and gas molecules, flows into the anode (grid).

Advantages• High sensitivity, covering wide pressure range • The cold cathode ionization gauge has much longer life time.

Cautions• The filament can easily be burned out in too high pressures.• Gas dependent sensitivity

Ii = SIeP

Relative sensitivity of ionization vacuum gauge for various gases

Quadrupole mass spectrometer

quadrupole electric field

Principles• Only ionized molecules with particular mass can pass along the

quadrupole electric field.

Advantages• Nearly ideal partial pressure gauge He leak detector • High sensitivity

Cautions• Operable only in high vacuum where the mean free path of gas

molecule should be longer than the detector length.• Rather expensive

Alcatel, ASM182 TD

He leak detector

mass spectroscopy of a vacuum grease

SI prefix (Metric prefix) SIA unit prefix to indicate a decadic multiple or fraction of the basic unit of measure in the International System of Units (SI)

10n prefix symbol

10-1 deci ( ) d

10-2 centi ( ) c

10-3 milli ( ) m

10-6 micro ( ) µ

10-9 nano ( ) n

10-12 pico ( ) p

10-15 femto ( ) f

10-18 atto ( ) a

10-21 zepto ( ) z

10-24 yocto ( ) y

10n prefix symbol

1024 yotta ( ) Y

1021 zetta ( ) Z

1018 exa ( ) E

1015 peta ( ) P

1012 tera ( ) T

109 giga ( ) G

106 mega ( ) M

103 kilo ( ) k

102 hecto ( ) h

101 deca ( ) da

f v( ) =4

π

m

2kBT

⎛

⎝ ⎜

⎞

⎠ ⎟ 3 2

v2exp −

mv 2

2kBT

⎛

⎝ ⎜

⎞

⎠ ⎟

m Tf (v)

N v v + dvdN dN = Nf(v)dv f (v)

n λ

λ =1

2πd2n

d = 0.375 nm

λ cm( ) =0.66P Pa( )

f (v

)

λ <<

λ

f v( )∫ dv = 1

vM =2kBT

m

v

v = vf v( )0∞∫ dv =

8kB

T

πm

v2 = v 2 f v( )0

∞∫ dv =3kBT

m

Q

S

d L C C = 1.4×106 d4 (P1 - P2)/2L

= 121 d3 /L

V C SS P

1S

=1S0

+1C

VdP

dt= −S P t( ) −P0{ } +Q

P t = ∞( ) = P0 +Q

S

P t( )− P0 −Q

S= P t = 0( )− P0 −

Q

S⎧ ⎨ ⎩

⎫ ⎬ ⎭ exp −

St

V

⎛ ⎝

⎞ ⎠

τ = τ0 expEdkBT

⎛

⎝ ⎜

⎞

⎠ ⎟

ττ0 ≈ 10-13 s (10-12 - 10-18 s) Ed

Ed

Student Reportpu

mpi

ng s

peed

(L/

s)

pressure (Pa)10-2 10-1 100 101 102 103 104

(Pa)

101

100

102

RP1RP2

RP3

10-1

103

pum

ping

spe

ed (

L/m

)

pressure (Pa)

Question-1 You want to evacuate a vacuum chamber (30 liter volume) from 1 atm at least down to P = 1×10-4 Pa with an appropriate combination of a rotary pump and an oil diffusion pump choosing each from RP1, RP2, RP3, DP1, DP2 and DP3 shown below. The higher pumping speed is, the more expensive the price is. Which is the cheapest combination should you take? And why? Neglect the finite conductance of pumping line (pipe) and gas desorption from the chamber. Assume the critical back pressure, above which diffusion pump cannot work properly, is 60 Pa for these three diffusion pumps.