Thin Film Technology Lecture 2 Vacuum Surface Engineering Jari Koskinen 2013

1. Thin Film Technology Lecture 2 Vacuum Surface Engineering Jari Koskinen 2013

2. Vacuum surface engineering • Vacuum technology • Surface phenomena • Surface energetic ion interaction

3. Vacuum surface engineering • Vacuum technology • Surface phenomena • Surface energetic ion interaction

4. Vacuum system VACUUM GAUGE PUMP RESUDUAL GAS FLANCE VACUUM CHAMBER

6. Large surfaces, upscaling www.scheuten.com

7. Residual gas • Pumping of: • Residual gas: • Adsorption – Desorption • Diffusion of dissolver or trapped gas • Permeation trough materials • Leaks diffusion of dissolved molecules desorption pump • permeation • leak

8. Units of pressure Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

9. Sources of residual gas • High vacuum • pumping speed • leak • Good High vacuum • desorption from walls • baking • Ultra high vacuum • impurities • internal leaks • material selection • diffusion • permeation

10. Average mean free path (distance between collission) in nitrogen residual gas <λ> Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

11. Average mean free path (distance between collission) in nitrogen residual gas <λ> Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

12. Phases of residualgas • d = diameter of chamber • Viscotic <λ> < d/100 • Intermediate • Molecular <λ> >> d Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

13. Phases of residualgas • d = diameter of chamber • Viscotic <λ> < d/100 • Intermediate • Molecular <λ> >> d Filament does not oxidise Ultra Good High High Inter-mediate Rough Laminar flow Molecular flow clean surface Total pressure of residual gasses

14. Time to form one molecular layer on surface Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

15. Vapour and liquid in vacuum pump balance: condensation = evaporation balance: pumping = evaporation pressure constant until all liquidis pumped

16. Critical temperatures and pressures for some residual gasses Gas or vapor pump Helium Hydrogen Nitrogen Carbon monoxide Argon Oxygen Methane Carbon dioxideChlorine Ether Ethanol Carbon tetraclor. Water above Tc no liquid

17. Vacuum pumps • Positive displacement (mechanical pumps) • Momentum transfer (molecular pumps) • Entrapment

18. Mechanical pumps Rotary vane Roots Diafragma

19. Mechanical pumps Scroll pump

20. Momentum transfer Turbo molecular Oil diffusion pump

21. Entrapment cryo pump ion pump

22. Pumps and vacuum ranges

23. Vacuum gauges • Mechanical – diaphragm • Electronic • Piezoresitive (strain gauge) • Capacitive • Magnetic • Piezoelectric • Optical • Potentiometric • Resonant • Thermal conductivity – Pirani • Ionzation gauge • Hot cathode • Cold cathode (Penning)

24. Gauges Pirani ionization gauge hot filament

26. Residual gas analyser SRS RGA100 Residual Gas Analyzer

27. Vacuum systems

28. Vacuum systems

29. Gas flow in vacuum systems Q gas troughput [pressure*volume/s] Q = C(P1 – P2) in series: P1 P2 C conductance l/s in parallel:

30. Conductance of various geometries M. Ohring

31. Vacuum systems

32. Vacuum system design

33. Vacuum surface engineering • Vacuum technology • Surface phenomena • Surface energetic ion interaction

34. Surface energy γ surface tension = dW work needed to form surface dA In thermodynamic equilibrium:

35. Contact angle Young equation S solid L liquid G gas Spreading parameter S Complete wetting when S ≈ 0 non-wetting when S ≈ -2 ΥLG

36. Surface reconstruction

37. Surface structure and defects

38. Adsorption • Physisorption • Chemisorption

39. Adsorption • Physisorption • Chemical bonding: • polaroization (van der Waals) • Bonding energy ≈ 0.001 – 0.5 eV • Bond length ≈ 3 – 10 Å • For example: nobel gas or molecules on materials • Possibly precursion state before chemisorption

40. Adsorption • Chemisorption • Chemical bonding: • charge exchange • Bonding energy ≈ 0.5 – 5 eV • Bond length ≈ 1 – 3 Å • For example: H, O, N, CO on metals • Dissociation of molecule • Final absorption

41. Desorption • Adsorbed molecule receives energy ED in order to leave surface • thermal • radiation • photons • electrons • ions • electric field

42. Balance of absorption - desorption • collisions of molecules (gas) • S sticking coefficient • ED energy for desorption • P pressure • Coverage • ≈ • High P, low T more adsorption • ED large, full coverage • very little adsorption in UHV

43. Surface diffusion http://iramis.cea.fr/spcsi/Phocea/Vie_des_labos/Ast/astimg.php?voir=60&type=groupe

44. Surface diffusion • Diffusion is thermally activated random movement of adsorbed atoms • D = D0 e-Eact/kT • Eact large -> slow diffusion • T high – fast diffusion Eact Surface diffusion of Cu on Cu(111) http://iramis.cea.fr/spcsi/Phocea/Vie_des_labos/Ast/astimg.php?voir=60&type=groupe

45. Work function • Work function ϕ • EF Fermi energy • D dipole potential

46. Work function of some metals Adsorbed atoms alloying effect work function

47. Solubility of gasses into metals

48. Vacuum surface engineering • Vacuum technology • Surface phenomena • Surface energetic ion interaction

49. Energetic ion surface interactions

50. Secondary electrons