6.a.preparation of thin films

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preparation of thin films

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6.a.preparation of thin films

  1. 1. Preparation of thin films <ul><li>Methods of Evaporation </li></ul><ul><li>a) Resistive heating : Melt or Flash evaporation </li></ul><ul><li>b) Electron gun heating </li></ul><ul><li>c) Sputtering </li></ul><ul><li>d) Chemical methods </li></ul><ul><li>e) Pulsed laser Deposition </li></ul><ul><li>f) Molecular beam Epitaxy </li></ul><ul><li>g) Ion beam sputtering </li></ul><ul><li>h) Cluster deposition technique </li></ul>
  2. 2. <ul><li>Deposition Process involves: </li></ul><ul><li>Transition of a solid or liquid to a vapour/gas ions /plasma state </li></ul><ul><li>Vapour to reach the substrate from the source </li></ul><ul><li>Condensation of the vapour onto the substrate </li></ul><ul><li>forming a film. </li></ul><ul><li>Parts of a Vacuum system : </li></ul><ul><li>Vacuum pumps </li></ul><ul><li>Pressure gauges </li></ul><ul><li>Source from which material is evaporated </li></ul><ul><li>Substrate on which film is formed </li></ul><ul><li>Thickness monitor </li></ul>
  3. 3. <ul><li>Why vacuum? </li></ul><ul><li>To increase the Mean free path of the particles </li></ul><ul><li>To avoid oxidation of the films in case of non-oxide materials </li></ul><ul><li>Production of Vacuum : Atm pr. = 760 Torr </li></ul><ul><li>Rotary pump ~ 10 -3 Torr minimum pr. </li></ul><ul><li>Diffusion pump ~ 10 –6 Torr minimum pr. </li></ul><ul><li>Measurement of Pressure </li></ul><ul><li>Thermocouple gauge ( Pirani gauge) </li></ul><ul><li>Ionization gauge (Hot cathode) </li></ul><ul><li>3. Penning gauge (Cold cathode Ionization gauge) </li></ul>
  4. 5. Rotary pump Block diagram of a Thin film coating unit G1 G2 G3 G1, G2 - Pirani gauges G3 – Penning gauge (Bayard-Alpert) Diffusion pump LN 2 Trap Vacuum Chamber Air inlet Stator &Rotor Rotary pump
  5. 6. Diffusion Pump Silicone oil , heater Nozzles, Cooling water Addition of LN2 Cold Trap 1.Condenses some molecules 2. Reduces backstreaming 3.Improves vacuum by lowering pressure from 10 -5 to 10 -6 torr
  6. 7. High Vacuum Low Vacuum (after diffusion pump) (after rotary pump) Low Pressure guages
  7. 8. <ul><li>Deposition Monitoring </li></ul><ul><li>Quartz crystal Oscillator – Resonance frequency shifts as mass gets deposited on the AT-cut Quartz crystal </li></ul><ul><li>Optical monitors – reflectivity </li></ul><ul><li>Resistance monitor </li></ul><ul><li>Fizeau fringes – interferometry Tolansky method </li></ul><ul><li>Profilometer </li></ul>Source material : should posses high melting point and not react with the material under study. W - boats, filaments, Ta, Mo - boats, Al 2 O 3- crucibles For reactive materials, flash evaporation is used instead of thermal evaporation
  8. 9. l d d 0 h Source Substrate Uniformity of Film thickness - limited to a small area

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