Coating Machine and Tungsten Heater

The working principle of evaporation coating is that pump gas in coating chamber under 10-2Pa pressure firstly, and then heat the heater to evaporate the material, atomic or molecular of vapor would escape from the surface of the evaporation source and deposit onto the substrate to form a film, which including the basic processes of extraction, evaporation deposition. Coating machine is mainly composed of the coating chamber, a vacuum system, vacuum measurement system and circuit control system. Wherein the tungsten heater is mainly placed in the coating chamber as an evaporator heater.

We can use resistive heating evaporation source - spiral tungsten heater to prepare aluminum film. Spiral tungsten heater is equivalent to a resistor, after electrifying, it would generates heat and its resistivity will increase. When the temperature is about 1000℃, and the resistivity of the evaporation source is five times the room temperature, the evaporation sources of Joule heat is to be able to make aluminum atoms obtain the enough kinetic energy to evaporate.

When the coating chamber pumped to a high vacuum, use the bezel to cover tungsten wire to avoid aluminum would spray onto substrates when pre-melting. Then slowly electrifying and heat tungsten wire, so that aluminum will melt and adhere to the spiral tungsten wire, this process is called pre-melting. The aim is to eliminate impurities which absorb in aluminum, so aluminum would not generate a lot of bleed causing damage to vacuum when evaporation, on the other hand, it also ensures the purity of the film.

Melting temperature of aluminum is 670℃, filiform tungsten wire as evaporation source is because that aluminum has a wetting effect for wolfram wire, evaporation is from the large surface and relatively stable, this wettability is relevant to temperature of tungsten surface. If the temperature is too high, aluminum and tungsten will produce alloys at high temperatures, which can easily cause tungsten have serious distortion even blown.

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