L06-Vacuum_Evaporation

L06-Vacuum_Evaporation - Vacuum Evaporation Introduction...

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Vacuum Evaporation
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Introduction ± The objective is to controllably transfer atoms from a heated source (which can be a liquid or a solid) to a substrate located a distance away to grow a film. ± The source is heated directly or indirectly until the point is reached where it efficiently sublimes or evaporates. ± When analyzing this method, we need to start from evaporation rates and vapor pressure. ± Evaporation is normally done in the ballistic regime ( Kn > 1). ± Other than pressure and temperature, the placement of the heater, source and substrate are important factors. Heat Source Substrate d
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Process Summary ± Place a suitable material (the source) inside the vacuum chamber with a heater. ± Seal and evacuate the chamber. ± Heat the source. When the temperature reaches the evaporation temperature, atoms or molecules start to leave the surface of the source and travel in a more or less straight path until they reach another surface (substrate, chamber wall, instrumentation). ± Since these surfaces are at much lower temperatures, the molecules will transfer their energy to the substrate, lower their temperature and condense. ± Since the vapor pressure at the new temperature is much higher, they will not re- evaporate and adhere to the substrate. ± The deposition thickness is a function of the evaporation rate, the geometry of the source and the substrate and the time of evaporation. Substrate Source filament Current source
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Vapor Pressure ± Vapor pressure is the pressure at which the vapor phase is in equilibrium with the solid or the liquid phase at a given temperature. ± Below this pressure, surface evaporation is faster than condensation, above it it is slower. ± Theoretically, the vapor pressure can be found by the Clausius-Clapyeron equation. ± Over a small temperature range, the equation can be simplified as: ±² VT TH dT dP ' ' where ' H is the change in enthalpy, and V is the change in volume between the solid (or liquid) and vapor phases ¸ ¹ · ¨ © § ' ³ RT H PP e exp 0 where H e is the molar heat of evaporation
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Vapor Pressure of Elements ± In reality, empirical formulas and experimental data are more useful to find the vapor pressure of an element. ± For example, the vapor pressure of liquid Al is given by: ±² T T T torr P 6 10 52 . 3 log 999 . 0 409 . 12 15993 log ³ ³´ ³ Main Terms Smaller Terms
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Evaporation Rate ± The basic equation for evaporation flux is given by: ± Maximum flux is obtained when D e = 1 and P h = 0 ± This can also be put in mass units by multiplying flux with the atomic mass: ± ² MRT PP N hv A e e S D 2 ³ ) where ) e is the evaporation flux, D e is the coefficient of evaporation (0 < D e < 1), P v is the vapor pressure and P h is the ambient pressure.
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L06-Vacuum_Evaporation - Vacuum Evaporation Introduction...

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