§4 Fundamental Experimental Techniques

§4.1 Experimental environments and methods: Vacuum

Vacuum in nature 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)

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

Turbo molecular pump (10-8 ≤ P ≤ 10-2 Pa)Principles• 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

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 =

8kBT

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

Scanning Tunneling Microscope (STM)

"Tunneling through a controllable vacuum gap", G. Binnig, H. Rohrer, et al., Appl. Phys. Lett. 40, 178 (1982); "Surface Studies by Scanning Tunneling Microscopy", G. Binnig, H. Rohrer, et al., Phys. Rev. Lett. 49, 57 (1982)

piezo scanner

tip

sample surfacetunnel current

equi-tunnel current line

"for their design of the scanning tunneling microscope"Nobel prize in Physics 1986

Measure quantum mechanical tunnel current between an atomically sharp tip and a solid surface separated only by a nanometer (nm) and applied a bias voltage across them. By scanning the tip, one can image topology and electron density of states of the sample surface with spatial resolution of atomic size.

piezo motor (coarse)

liquid He Pb bowl (superconducting)

Pt sampleW-tip

piezo scanner (fine)

permanent magnet

Gerd Binnig Heinrich Rohrer

Progress in STM technologyR

(W

) I (A)

105

106

107

108

10-6

10-7

10-8

10-9

displacement of tip (1div = 0.1 nm)

tip-sample distance dependence of tunnel current

surface topology of CaIrSn4(110)p gy 4(

10 nm × 10 nm T = 1.5 K, V = +50 mV, I = 0.1 nA

charge density wave in NbSe2

by T. Hanaguri (RIKEN)

superconducting energy gap in NbSe2

T = 52 mK, V = -6.0 mV, I = 2.0 nA Vmod = 20 μV

H. Kambara et al., Rev. Sci. Instrum. 78, 073703 (2007)

BCS theory

presentG. Binnig and H. Rohrer (1982)

Ultra low temperature STM/STS T = 30 mK, B = 13 T, P << 10-8 Pa multi extreme conditions

理１号館西棟B208号室

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