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Methods and T ehni ques 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 )
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Methods andTehniquesin 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, t ~ 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, H2).
Joint types Elastomer gasket (O-ring 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.
Sorption pumps • 6 = Molecular sieve (zeolite) with huge specific area - 2500 m2/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: • Back-streaming (gas, oil/Hg vapors. • Baffle.
Ion pump UAK= 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 O2 si N2, H2, but small fot noble gases.
Gas flow regimes • 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 (O2, N2 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 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. Lüth in the reference list)
Magnetic lenses 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
-V Specimen Electron gun Auger e- energy- selected Electron multiplier Cylindrical mirror analyzer (CMA)