Physics of Thin Films :Physics of Thin Films Physical Vapor Deposition


Evaporation :Evaporation The Process 1.Source material>gaseous state 2.Transport source atoms to substrate 3.Deposit atoms on substrate 4. Binding on the surface of substrate


Evaporation-over view :Evaporation-over view Atoms to gas state Some sources sublime from solid Some sources evaporate from liquid Some sources break apart Metal alloy sources do not give same alloy in films


Physics of Thin films :Physics of Thin films Atoms to gas state


Requirement of vapor sources :Requirement of vapor sources Should have very high melting point Low vapor pressure at the deposition temperature Should not react with the evaporant materials Commonly used materials Tungsten, Molybdenum, Tantalum etc., Shape: should be able to hold the evaporant in the available form


Construction and use of vapor sources :Construction and use of vapor sources Hair pin and helix evaporants affixed to source Held by its surface tension Typically in the form of wire Evaporation takes place in all directions A A. Hair pin source B. Wire helix


Construction and use of vapor sources contd. :Construction and use of vapor sources contd. Wire basket -used to evaporate pellets/chips which do not wet the source on heating -if wetting occurs the turns of the basket are shorted and temp. of the source drops Metal foil dimpled -tungsten/moly./tantalum sheets -for small evaporant quantities -have capacities of grams -reduced width in the centre to concentrate heat in the area of evaporant A C. Wire basket D. Dimpled foil


Construction and use of vapor sources contd. :Construction and use of vapor sources contd. Boat/boat covered with perforations To be used with compounds which may break apart like SiO The evaporated particles are reflected before exit from the source


Evaporation of multi component materials :Evaporation of multi component materials PROBLEMS Components evaporate independently based on each separate vapor pressure Films rich in component evaporating faster/film not same as parent material Non stoichiometric film formation Solutions: Flash evaporation, 3 temp. method


Physics of Thin films :Physics of Thin films Special evaporation techniques


Flash evaporation :Flash evaporation Principle: An instant quantitative evaporation of a small amount of material on to a hot surface of a source Powder kept in a feeder is electro-magnetically vibrated to let the powder drop slowly on a heated boat used to deposit Best suited for: metal alloys and metal dielectric films Advantage: high deposition rate


Three temperature method :Three temperature method Compounds from 2nd 6th or 3rd and 5th groups(group of Mendeley system) Evaporation conducted from separate sources Each at different temperature Substrate itself is kept at elevated temperature Temperatures are of critical importance


Laser beam evaporation :Laser beam evaporation Suited for materials with high melting point Advantages: Energy source is outside the vacuum system Small spot has high temperature No interaction of the evaporant with the evaporation source High deposition rate Dis advantage: high price, complexity of apparatus, poor reproducibility of thickness


Electron bombardment :Electron bombardment Suited for: Evaporating materials which are deposited with great difficulty or cannot be deposited at all ADVANTAGE: Prevention of contamination by the evaporation source material Beam heats only the evaporant whereas the target holder is usually cooled No particle from the source(electron beam) reach the substrate


Advantages of the special techniques :Advantages of the special techniques Vapor particles will have high deposition rate Particles arriving at higher speed at the substrate are charged These influence the mechanism of film formation


Evaporation rate :Evaporation rate From kinetic theory


Transport to Surface :Transport to Surface Line of sight deposition Want to avoid collisions in gas long mean free path Good vacuum If h=source to substrate distance for h of 10-100cm , want P< 10-5 torr bigger h, lower P


Distribution of evaporant :Distribution of evaporant Depends on geometry of source Thickness in case of a point source


Distribution of evaporant contd. :Distribution of evaporant contd. Thickness in case of a surface source


Physics of Thin films :Physics of Thin films Deposition on to substrate Thickness distribution


Point source :Point source d0 and d are the thick nesses of deposits vertically from a source at a distance “h” and at a horizontal distance “l” Where d0=max. thickness which is


Surface source :Surface source


Uniformity in thickness :Uniformity in thickness decrease sample size increase source to substrate distance(h) Need bigger chamber Need better vacuum Wastes evaporant Put source and substrate on same sphere surface Move substrate during deposition Use rotating mask to reduce evaporant near centre


Film purity :Film purity Problem: contamination from source materials Solution: use pure materials (99.999%) Problem: contamination from source/substrate heaters/evaporant surface Solution: i. Use materials with low diffusion ii. Use shutter at the beginning of evaporation Problem:residual gas in the chamber gives two “sources” impinging Solution: better vacuum and higher deposition rate


References :References K.L.Chopra, Thin film phenomena L.Holland, Vacuum deposition of thin films L.Maissel and R.Glang, Handbook of thin film technology