1. Basic knowledge of vacuum
"Vacuum" means the space of any gaseous state under which pressure is below the atmospheric pressure. The gas is thinner in this space, at least thinner than the atmosphere, but the absolute "vacuum," that is, the space without any gaseous particles, cannot be found, and ten thousand kilometers away from the earth, there are 3~4 cubic centimeters per cubic centimeter of molecules.
As the degree of the vacuum is the problem of the size of the gas pressure and the unit of measuring pressure is the unit of measuring the degree of vacuum. The international standard unit is “Pa”, which can also be measured in "Torr," "bar" and "mbar". The relationship between them is given by the following formula:
1 standard atmospheric pressure = 1.01 x 105 Pa = 760 Torr = 1.01 x 103 mbar
2. Why do you need to coat in a vacuum?
The evaporation coating material under atmospheric pressure to form the ideal film, in fact, as in the low enough pressure (or vacuum is not enough high) under the condition of the same cannot get good results, such as the 102 support order under the steamed, the film not only somberness, even gray, black, and the mechanical strength is poor, with a brush to gently brush the squirrel will destroy the aluminum layer. The evaporation must be carried out in a certain vacuum conditions, because of:
(1) The higher vacuum degree ensures that the average free path of the vaporizer is greater than the distance from the source to the base.
Due to the thermal motion of gas molecules, the collision between molecules is extremely frequent, the movement speed of gas molecules is quite high (up to several hundred meters per second), but as it moves forward more often than other molecules Collision, the distance a molecule moves between two successive collisions is called its free path, and the statistical mean of the free path of a large number of molecules is called the mean free path of the molecule.
Since the pressure of the gas is proportional to the number of molecules per unit volume, the average free range is proportional to the pressure of the gas. In the vacuum deposition thin films process, when the deposition distance is greater than the molecular mean free path, it is called low vacuum deposition, and when the deposition distance is smaller than the molecular mean free path, it is called high vacuum deposition. In high vacuum deposition process, the evaporation of atoms (or molecules) and the collision of residual gas molecules is negligible, so that the vaporized atoms fly in a straight line toward the substrate, thus keeping the larger kinetic energy reaching the vaporized atoms of the substrate and the substrate condenses into a more solid film. In low vacuum deposition, the speed and direction of vaporized atoms change with the result of collision, or even the formation of steam atom aggregates in space. The reason is similar to that of water vapor in the atmosphere.
(2) Residual gas pollution can be reduced at higher vacuum
In the case of the vacuum is not too high, the vacuum chamber contains a large number of residual gas molecules (oxygen, nitrogen, water and hydrocarbons, etc.), which can bring great harm to the plating of the thin film:
● They collide with vaporized film molecules such that the mean free path becomes shorter.
● They collide with and react with the surface being film-formed.
● They are hiding themselves in the formed film and gradually attack the film; they combine with the evaporation source to reduce their useful life.
● They formed on the surface of the evaporated film oxide layer so that the evaporation process cannot be carried out smoothly.
The development of modern vacuum technology has made it possible to measure the vacuum pressure from atmospheric pressure up to 10-12pa, and it is not possible to use only one kind of vacuum gauge to perform the measurement work in such a wide range, and there are several types of vacuum measurement of vacuum gauge respectively to complete their range:
(1) Thermocouple Vacuum Gauge and Pirani Vacuum Gauge
Within a certain range, the thermal conductivity of the gas is a function of pressure, which is the measurement principle of the thermocouple and Pirani gauges.
Thermocouple vacuum gauge adopts the method of measuring temperature change directly to work. When the heating current is maintained constant, if the gas pressure decreases, then the gas thermal conductivity decreases, the working temperature of the thermocouple increases, the corresponding thermoelectric force that is to increase, through the relationship between pressure and electromotive force to determine the system pressure.
Pirani vacuum gauges and thermocouple gauges are also commonly used as medium vacuum range measurements. Pirani vacuum gauge has a glass or metal shell, the shell is equipped with high temperature resistivity of the wire (such as platinum, tungsten, etc.), with the pressure drop, the filament temperature and resistance will increase, according to the measured the resistance value, the pressure value can be indirectly obtained.
(2) Cold-cathode Ionization Gauge
The cold cathode ionization vacuum meter is also called the Penning vacuum meter. When the discharge tube is operated at pressures lower than 102 - mbar, since the gas molecules from the cathode to anode few electron collision with the gas molecular process rarely, so the current is not related to the pressure of the gas. If the magnetic field is strengthened in the vertical direction of the electron motion, then the trajectory of the electron motion becomes spiral, so that the opportunity of electron ionizing gas molecules increases greatly. Even at very low gas densities, a large amount of gases can be ionized. When the anode voltage and magnetic field strength are constant, for a certain gas composition, the ionic current is directly related to the pressure, which is the working principle of the cold cathode vacuum gauge.