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 V T T H 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 P P 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 u 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 P P N h v 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
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