Methods and Tehniques in Surface Science

Prof. Dumitru LUCA “Alexandru Ion Cuza” University, Iasi, Romania

UHV systems

• Limit between HV and UHV: 10 -7 - 10 -8 mbar.

• Small leaks – crucial role in establishing the limit pressure.



= 3.2/p

(  s) (

p

– mbar

)

• Example: p = 10 -10 mbar,  ~ 2 - 3 h! UHV particularitaties. Operation modes.

    Leak rates and wall desorption rates – very low (OFHC).

Appropriate measures to diminish the back-stereaming.

Baking.

He testing

Echipaments for UHV

A typical example of an UHV machine (TUE-Eindhoven)

Vapor pressure for usual gases. Residual Gas Analysers

Most of the gases, including the light-mass ones can be pumped out by cryo pumps working at liquid He temperature (except for He, Ne, H 2 ).

Vapor pressure of usual metals as a function of temperature

Elastomer gasket (O-ring

Joint types

Metal gasket (OFC) CONFLAT • re-usable, • cheap, • up to 10 -7 mbar, but max. 150 °C.

• non-reusable, fragile knives.

• expensive (high-purity materials Cu Ag), • up to 10 -13 mbar.

Preliminary vacuum pumps. Rotary pump

Sorption pumps

6 = Molecular sieve (zeolite) with huge specific area - 2500 m 2 /g, LN-cooled (5).

- reactivation at 200 °C.

- pressure range: 1 atm – 10 -3 mbar.

- low efficiency for noble gases, oxygen and si hidrogen.

- cheap, contaminant-free (oil)

High vacuum pumps. Diffusion pump

Advantages: Working pressure: 10 -3 10 -7 mbar.

– Drawbacks: 1.

Back-streaming (gas, oil/Hg vapors.

Baffle.

Turbomolecular pump

Ion pump

U

AK = 1-10 kV

B

: 0.3 Tesla.

The molecules of the residual gas are ionized, then accelerated towards K, where they are “embedded” in the cathode (Ti). Simultaneously, the sputtered Ti forms a getter layer.

Intense magnetic field: increase in the apparent pressure.

Pressure range: 10 -4 -10 -11 mbar Safe in operation, oil-free.

High pumping speed for O 2 but small fot noble gases.

si N 2 , H 2 ,

Gas flow regimes

• • • • • • • A.

Viscous flow Pressure > 10 -4 mbar Mean free path – short Collision processes: molecule molecule Transfer of momentum between molecules Significant pressure gradients Flow: laminar, viscous (Poiseuille), turbulent

K

< 0.01

B.

Molecular flow • Presure < 10 -4 mbar • Mean free path – long • Collision processee: molecule-wall are much more frequent as compared to molecule-molecule • Pressure gradients - negligible.

• Pumping effect by collisions with pumping surfaces •

K

> 1

K =



/a

, Knudsen number

0.01 < K < 1 flow in the so-called intermediary (Knudsen) regime

Getter pumps/sublimation pumps

Hot Ti filament Ti evaporates in vacuum, thus forming a getter layer at surface.

High affinity for active residual gases (O 2 , N 2 etc).

Base pressure: 10 -9 – 10 -11 mbar.

Advantages: simplicity, low cost.

Drawbacks: low pumping speed, gas-dependent pumping speed

Analyzers for charged particles.

Electrostatic lenses

sin sin   

v

2

v

1 

n

2

n

1

mv

2 2 2 

mv

1 2 2 

eU eU

0 sin sin   

mv

1 2 2  1 

U U

0 Deviation of an electron beam in an uniform electric field.

Rem. The e/m does not occur in the upper equation!

Electrostatic lens (for practical design, see H. L üth in the reference list)

Magnetic lenses

AC

 2 

mv

cos 

eB

Focussing the charged particle beam in the magnetic field:

Up:

all the particules entering in A are focussed in C.

Down:

An example of a magnetic lens

Dispersive elements. Cylindrical sectors analyzer

Cylindrical sectors analyzer (  max =118.6

 - Herzog aperture) Biasing circuit The outer cylinder, negatively biased, repels the electronis, thus ensuring their energy-dependent dispersion.

Electrons with a certain energy are selected via bias browsing.

Hemispheric analyzer

Capacitors system (4 plates) For surface scanning

Cylindrical mirror analyzer (CMA)

Specimen Electron gun Auger e energy selected Electron multiplier

T

0 

eU p

0.77 ln

b a

-V

E

 

U p r

ln

b a

,

mv

0 2  

eE

0 2 

eU p r

0 ln

b a T

0 

mv

0 2 2 

eU p

2 ln

b a

