Why coating in vacuum?
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Under ordinary conditions of steaming coating material to form ideal film, in fact, such as the pressure is not enough under the condition of low (or high vacuum degree is not enough) also can not get good results, such as 10 2 aluminum evaporation under an order of magnitude, the film does not light, even hair grey, black, and poor mechanical strength, use the squirrel brush gently brush the aluminum layer can be damaged.
Evaporation must be carried out under certain vacuum conditions because:
(1) higher vacuum degree can ensure that the average free path of vaporized molecules is greater than the distance from the evaporation source to the base.
Due to thermal motion of molecules, molecular collisions between also very frequently, so although the speed of gas molecules high (up to several hundred meters per second), but because it in the process of forward to collisions with other molecules for many times, a molecule in the walking by the distance between two consecutive collision free path called it, and a large number of molecular free path statistical average value is called molecular mean free path.
Since the pressure of the gas is proportional to the number of molecules per volume, the average free path is also proportional to the pressure of the gas.
In the process of vacuum deposition of thin films, low vacuum deposition is called when the deposition distance is greater than the average free path of molecules, and high vacuum deposition is called when the deposition distance is less than the average free path of molecules. In high vacuum deposition, the collision between evaporated atoms (or molecules) and residual gas molecules can be ignored. Therefore, vaporized atoms fly to the substrate in a straight line, so that the vaporized atoms that maintain greater kinetic energy to the substrate can condense into a stronger film layer on the substrate. In low vacuum deposition, the speed and direction of vaporized atoms change as a result of the collision, and it is even possible to form a collection of vapor atoms in space -- similar to how water vapor creates fog in the atmosphere.
(2) the pollution of residual gas can be reduced at a high vacuum degree. If the vacuum degree is not too high, there are many residual gas molecules (oxygen, nitrogen, water and hydrocarbons, etc.) in the vacuum room, which can bring great harm to the plating of thin films. They collide with vaporized membrane molecules to shorten the average free path. They collide and react with surfaces that are forming membranes; They hide in the formed film and gradually erode the film; They are combined with evaporation source at high temperature to reduce their service life; They form an oxide layer on the surface of the evaporated film to prevent the evaporation process from proceeding smoothly... .