6.a.preparation of thin films
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The document discusses several methods for preparing thin films including evaporation, sputtering, pulsed laser deposition, and molecular beam epitaxy. It describes the deposition process which involves transitioning material to a vapor state, transporting it to a substrate, and condensing it into a thin film. Key components of a vacuum system used for deposition include vacuum pumps, pressure gauges, a source, and substrate. Vacuum is needed to increase the mean free path of particles and avoid oxidation. Rotary and diffusion pumps are used to attain low and high vacuums, respectively, which are measured using gauges like pirani, ionization, and penning gauges. Thickness and uniformity are monitored during deposition.
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6.a.preparation of thin films
- 1. Preparation of thin films Methods of Evaporation a) Resistive heating : Melt or Flash evaporation b) Electron gun heating c) Sputtering d) Chemical methods e) Pulsed laser Deposition f) Molecular beam Epitaxy g) Ion beam sputtering h) Cluster deposition technique
- 2. Deposition Process involves: Transition of a solid or liquid to a vapour/gas ions /plasma state Vapour to reach the substrate from the source Condensation of the vapour onto the substrate forming a film. Parts of a Vacuum system : Vacuum pumps Pressure gauges Source from which material is evaporated Substrate on which film is formed Thickness monitor
- 3. Why vacuum? To increase the Mean free path of the particles To avoid oxidation of the films in case of non-oxide materials Production of Vacuum : Atm pr. = 760 Torr Rotary pump ~ 10 -3 Torr minimum pr. Diffusion pump ~ 10 –6 Torr minimum pr. Measurement of Pressure Thermocouple gauge ( Pirani gauge) Ionization gauge (Hot cathode) 3. Penning gauge (Cold cathode Ionization gauge)
- 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
- 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
- 7. High Vacuum Low Vacuum (after diffusion pump) (after rotary pump) Low Pressure guages
- 8. Deposition Monitoring Quartz crystal Oscillator – Resonance frequency shifts as mass gets deposited on the AT-cut Quartz crystal Optical monitors – reflectivity Resistance monitor Fizeau fringes – interferometry Tolansky method Profilometer 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
- 9. l d d 0 h Source Substrate Uniformity of Film thickness - limited to a small area