Materials are cleaned before they are coated by a vacuum coating machine, which is commonly known as the vacuum cleaning process. The vacuum cleaning of a vacuum coating machine is generally defined as before the vacuum process, the process of removing unwanted material from the surface of a workpiece or system material. Cleaning the surface of vacuum parts is necessary because the gas and vapor source caused by pollutants will make the vacuum system can not obtain the required degree of vacuum. In addition, the presence of contaminants can also affect the strength and sealing performance of the vacuum component joints.

A: Vacuum heating cleaning

The workpiece is heated under normal pressure or vacuum. To promote the evaporation of the volatile impurities on the surface to achieve the purpose of cleaning,the cleaning effect of this method is related to the environmental pressure of the workpiece, the length of the retention time in the vacuum, the heating temperature, the type of pollutant and the material of the workpiece. The principle is to heat the workpiece. The desorption of water molecules and various hydrocarbon molecules on its surface is enhanced. The degree of desorption enhancement is related to temperature.

In order to obtain atomically clean surfaces in ultra-high vacuum, the heating temperature must be higher than 450 degrees. The method of heating and cleaning is particularly effective. But sometimes this treatment can have side effects. As a result of heating, some hydrocarbons may polymerize to form larger aggregates and decompose at the same time into carbon slag.

B: Ultraviolet radiation cleaning

Decomposition of hydrocarbons on the surface by ultraviolet irradiation. For example, exposure to air for 15 hours produces a clean glass surface. If the properly precleaned surface is placed in an ozone-producing UV source. A clean surface can be formed in just a few minutes. This suggests that the presence of ozone increases the cleaning rate. Its cleaning mechanism is: under the ultraviolet radiation, the contaminant molecule is stimulated and dissociated, and the formation and existence of ozone produces highly active atomic oxygen. The interaction of excited pollutant molecules and free radicals produced by the dissociation of pollutant with atomic oxygen. To form simpler, more volatile molecules. Such as H203, CO2 and N2. The reaction rate increases with the increase of temperature.

C: Discharge cleaning

This cleaning method is widely used in the cleaning and degassing of high vacuum and ultra-high vacuum systems. Especially in the vacuum coating machine used in the most. Use a hot wire or electrode as an electron source. Gas Desorption by ion bombardment and removal of some hydrocarbons can be achieved by applying a negative bias voltage relative to the surface to be cleaned. The cleaning effect depends on the electrode material, the geometry shape and its relation with the surface. That is, it depends on the number and energy of ions per unit surface area. Which depends on the effective electrical power. Fill the vacuum chamber with an inert gas (typically Ar gas) at an appropriate partial pressure.

Cleaning can be achieved by an ion bombardment produced by a glow discharge at low pressure between two suitable electrodes. In this method. The inert gas is ionized and bombarded against the walls of the vacuum chamber, other structural elements in the vacuum chamber, and the coated substrate, which can prevent certain vacuum systems from being baked at high temperatures. Some hydrocarbons can be cleaned better if oxygen is added to the filled gas. Because oxygen can oxidize certain hydrocarbons into volatile gases that are easily eliminated by the vacuum system.

The main components of impurities on the surface of stainless steel high and ultra-high vacuum vessels are carbon and hydrocarbons. In general, the carbon can not be vaporized by itself. After chemical cleaning, it is necessary to introduce Ar or AR + o 2 mixture gas to carry out glow discharge cleaning, so as to remove the impurities on the surface and the gas trapped on the surface due to chemical action. In glow discharge cleaning. The most important parameters are the type of applied voltage (AC or DC) , the magnitude of the discharge voltage, the current density, the type of gas charged and the pressure, the duration of the bombardment, the shape of the electrodes and the material and location of the components to be cleaned.

D:Gas flushing

Nitrogen flushing

When nitrogen is adsorbed on the surface, the adsorption energy is small, so the retention time is very short. Even if it's attached to the wall, it can easily be removed. Using this property of nitrogen to flush the vacuum system can greatly shorten the pumping time of the system.

If the vacuum coating machine is flushed with dry nitrogen into the vacuum chamber before being put into the atmosphere, the pumping time of the next pumping cycle can be reduced by nearly half, because the adsorption energy of nitrogen is much smaller than that of water gas, nitrogen Molecules are first adsorbed on the wall of the vacuum chamber after being filled with nitrogen under vacuum.

As the adsorption site is a certain, the first nitrogen molecules occupied, the adsorption of water molecules on the few, so that the pumping time is shortened. Nitrogen flushing can also be used to clean contaminated systems if they are contaminated by diffusion pump oil spills. General is the side of the system for baking heating, while washing the system with nitrogen, oil pollution can be eliminated.

Reactive gas flushing

This method is especially suitable for internal cleaning (removing hydrocarbon contamination) of large ultra-high stainless steel vacuum coater. In general, for the Vacuum Chamber and Vacuum Element of some large UHV system, the standard method to clean the surface of atomic state is chemical cleaning, methods of vacuum furnace roasting, glow discharge cleaning and original energy baking vacuum system, etc. . The above methods of cleaning and degassing are commonly used before and during installation of vacuum systems.

After installation (or operation) of the vacuum system, it is difficult to degassing the various parts of the vacuum system because they have already been fixed, once the system is contaminated (accidentally) , mainly by large-atom molecules such as hydrocarbons, it is usually dismantled and reprocessed for reinstallation. The in-situ on-line degassing can be carried out by using the reaction gas process. Effectively remove hydrocarbon contamination in stainless steel vacuum chamber.

The cleaning mechanism: The oxidizing gas (O 2, N 0) and the reducing gas (H 2, N H 3) are quoted in the system to clean the metal surface by chemical reaction to remove the pollution, in order to obtain the atomic state of the clean metal surface. The rate of surface oxidation/reduction depends on the contamination and the material of the metal surface. The rate of surface reaction is controlled by adjusting the pressure and temperature of the reaction gas. For each substrate, the exact parameters are determined experimentally. These parameters are different for different crystallographic orientations.