Vapor-depositing film materials at normal pressure doesn't form ideal thin films. In fact, if the pressure isn't low enough (or the vacuum isn't high enough), good results won't be achieved either. For example, when vapor-depositing aluminum at 10 2 Torr, the resulting film isn't only dull, but can even appear gray or black. Its mechanical strength is extremely poor, and a light brushing with a squirrel-hair brush can damage the aluminum layer.
Vapor deposition must be performed under certain vacuum conditions for the following reasons:
1. A high vacuum ensures that the mean free path of the vaporized molecules is greater than the distance from the evaporation source to the substrate. Due to the thermal motion of gas molecules, collisions between them are extremely frequent. Therefore, despite the high speed of gas molecules (up to several hundred meters per second), they collide with other molecules multiple times as they move forward. The distance a molecule travels between two consecutive collisions is called its free path, and the statistical average of the free paths of a large number of molecules is called the mean free path.
Since gas pressure is proportional to the number of molecules per unit volume, the mean free path is also proportional to gas pressure. During vacuum thin film deposition, when the deposition distance is greater than the mean free path of the molecules, it is called low vacuum deposition, while when the deposition distance is less than the mean free path of the molecules, it is called high vacuum deposition. During high vacuum deposition, collisions between evaporated atoms (or molecules) and residual gas molecules are negligible, so the vaporized atoms fly in a straight line toward the substrate. This allows the vaporized atoms, which reach the substrate with high kinetic energy, to condense on the substrate to form a relatively strong film layer. During low vacuum deposition, collisions can cause the vaporized atoms to change their speed and direction, potentially even forming a collection of vapor atoms in space-similar to the formation of fog by water vapor in the atmosphere.
2. A higher vacuum level can reduce residual gas contamination. At lower vacuum levels, the vacuum chamber contains numerous residual gas molecules (such as oxygen, nitrogen, water, and hydrocarbons), which can pose a significant threat to thin-film deposition. These gases collide with vaporized film molecules, shortening their mean free path; they collide with and react with the surface of the film being formed; they lodge within the already formed film, gradually eroding it; they react with the evaporation source at high temperatures, reducing its service life; and they form an oxide layer on the surface of the evaporated film, disrupting the deposition process.
Therefore, the evaporation vacuum coating machine must be in a vacuum state to coat the film layer, so that the film layer will be firm and free of pollutants.
