Resistive Evaporation
The figure shows resistance systems used within the chamber. In this method for production of the heat use from the passing current through the source resistance, which is located inside the boat. In simple terms, the required heat to melt the material obtain by high electrical currents. Evaporated atoms, under vacuum condition, move toward the substrate and put on the substrate.
Figure 1: resistive thermal evaporation system.
The main equipment required in this method are:
1. Power Supply
Power supply use to evaporate the material, in current between 20 to 500 amps. Since, the electrical resistance of the source is low, is used normally of 1 to 3 kW power supply. In these systems is used commonly a switching mechanism to allow power supply feed a number of sources.
2. Electrode
Electrode is used to transferring the current from two copper electrodes, that these electrodes are connected to vacuum chamber floor, by two Feedthrough, as shown in Figure 1.
3. Boat
Boats, as shown in Figure 2, are used in a various forms such as filament, boat and a basket, depending on the type of material, shape and its evaporation volume. Their matter is often from refractory metals such as tungsten, molybdenum, and tantalum, and if there is the possibility of reacting with them, use dielectric boats such as alumina, zirconia and graphite. Table 1 shows melting point and vapor pressure of this boats at different temperatures.
Figure 2: boats with different shapes that used in resistive evaporation.
|
Melting point
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Vapor Pressure (in temperature)
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Boats
|
|
3380 C
|
C 2230torr in 10-2
|
W
|
|
C 3000
|
C 60 30torr in 10-2
|
Ta
|
|
C 2620
|
2530 C torr in 10-2
|
Mo
|
|
3700 C
|
2600 C torr in 10-2
|
Graphitic (C)
|
|
2030 C
|
C torr in 1900 10-2
|
Al2O3
|
|
C 2500
|
1600 C torr in 10-2
|
BN
|
The vacuum chamber
As indicated previously, evaporation should be done inside the vacuum chamber. After that, the vacuum reaches to desired value, (about 10-5 - 10-6 Torr), required heat can be adjusted by controlling the electric current according to the formula. Resistance evaporation method is used usually, for materials with a melting point less than 1500 °C. Between dielectric SiO, MgF2 and ZnS and between the metals, iron, nickel, aluminum, copper, tin, silver, gold and platinum, are evaporated usually with this way easily.
This method has a number of advantages and disadvantages, like many other coating methods. The main disadvantages are:
. According to this fact that, the energy of reached atoms to substrate have low energy. Layers which formed by this method have generally low density.
. The impossibility of coating for refractory material which is due to the limitation of generated heat in the boat.
. The possibility of combining the material with the boat, during the coating.
The main advantages of this method include:
. The use of this method, compared to other physical and chemical methods.
. The low cost of this system compared to other coating systems.