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Methods and T ehni ques in Surface Science

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 and T ehni ques in Surface Science

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  1. Methods andTehniquesin Surface Science Prof. Dumitru LUCA “Alexandru Ion Cuza” University, Iasi, Romania

  2. 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

  3. Echipaments for UHV A typical example of an UHV machine (TUE-Eindhoven)

  4. 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).

  5. Vapor pressure of usual metals as a function of temperature

  6. 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.

  7. Preliminary vacuum pumps. Rotary pump

  8. 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)

  9. High vacuum pumps. Diffusion pump • Advantages: • Working pressure: 10-3 – 10-7 mbar. • Drawbacks: • Back-streaming (gas, oil/Hg vapors. • Baffle.

  10. Turbomolecular pump

  11. 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.

  12. 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

  13. 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

  14. 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)

  15. 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

  16. 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.

  17. Hemispheric analyzer Capacitors system (4 plates) For surface scanning

  18. -V Specimen Electron gun Auger e- energy- selected Electron multiplier Cylindrical mirror analyzer (CMA)

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