Transcript Metode şi tehnici de studiu a suprafeţelor
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