Vacuum cleaning process | vacuum coating machine cleaning process requirements
Vacuum cleaning is generally defined as the process of removing unwanted material from the workpiece or system material surface prior to a vacuum process. It is necessary to clean the surface of vacuum parts, because the gas and steam source caused by pollutants will make the vacuum system can not get the required vacuum degree. In addition, due to the presence of pollutants, vacuum components will affect the connection strength and sealing performance.
A. Vacuum heating cleaning
Place the workpiece under normal pressure or vacuum heating. To promote the evaporation of 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 retention time in vacuum, heating temperature, types of pollutants and workpiece materials. The principle is to heat the workpiece. The desorption of water molecules and various hydrocarbon molecules adsorbed on the surface is enhanced. The degree of desorption enhancement depends on temperature. At ultra-high vacuum, the surface must be heated above 450 degrees for atomic-scale cleaning. Heating cleaning method is particularly effective. But sometimes, this treatment can have side effects. As a result of heating, certain hydrocarbons may aggregate into larger aggregates and decompose simultaneously into carbon slag.
B. UV cleaning
The hydrocarbons on the surface are decomposed by ultraviolet irradiation. For example, exposure to air for 15 hours can produce a clean glass surface. If a properly pre-cleaned surface is placed in an ultraviolet source that generates ozone. A clean surface (process cleaning) can be formed in a few minutes. This suggests that the presence of ozone increases the rate of cleaning. The cleaning mechanism is as follows: under ultraviolet irradiation, dirt molecules are stimulated and dissociated, while ozone is generated and exists to produce highly active atomic oxygen. The excited dirt molecules and free radicals produced by the dissociation of the dirt react with atomic oxygen. 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 equipment used the most. Use a hot wire or electrode as an electron source. The gas desorption of ion bombardment and the removal of certain hydrocarbons can be achieved by applying negative bias pressure to the surface to be cleaned. The cleaning effect depends on the electrode material, the geometry and the relationship with the surface. It depends on the number of ions per surface area and the energy of the ions. It depends on the effective electrical power. A vacuum chamber is filled with an inert gas (typically Ar gas) of appropriate partial pressure. The purpose of cleaning can be achieved by ion bombardment from a glow discharge at low pressure between two suitable electrodes. In the method. The inert gas is ionized and bombards the vacuum chamber wall, other structural parts of the vacuum chamber and the coated substrate, which can prevent certain vacuum systems from being baked at high temperature. Better cleaning of certain hydrocarbons can be achieved if oxygen is added to the charged gas. Oxygen is easily removed from the vacuum system because it oxidizes certain hydrocarbons to a volatile gas. The main components of impurities on the surface of stainless steel high vacuum and ultra high vacuum vessels are carbon and hydrocarbons. In general, the carbon is not volatile. After chemical cleaning, it is necessary to introduce Ar or Ar+O2 mixture gas for glow discharge cleaning, so that the impurities on the surface and the gas bound on the surface due to chemical action can be removed. In glow discharge cleaning. The most important parameters are the type of applied voltage (ac or dc), discharge voltage, current density, type of gas filled, and pressure. The most important parameters are the type of applied voltage (ac or dc), discharge voltage, current density, type of gas filled, and pressure. The most important parameters are the type of applied voltage (ac or dc), discharge voltage, current density, type of gas filled, and pressure. The duration of the bombardment. The shape of the electrode and the material and position of the parts to be cleaned
D. Gas flushing
1. Nitrogen flushing
When nitrogen is adsorbed on the material surface, the adsorption time is extremely short due to the small adsorption energy. Even if it is adsorbed on the wall of the device, it is easy to be removed. The pumping time of the system can be greatly reduced by using this property of nitrogen to flush the vacuum system. For example, before the vacuum coating machine is put into the atmosphere, dry nitrogen is first filled into the vacuum chamber and then filled into the atmosphere, then the pumping time of the next pumping cycle can be shortened by nearly half, the reason is that the adsorption energy of nitrogen is much smaller than that of water gas molecules. After nitrogen is filled into the vacuum, nitrogen molecules are first absorbed by the vacuum chamber wall. As the adsorption site is fixed, it is first filled with nitrogen molecules, and the adsorbed water molecules are very small, thus shortening the extraction time. If the system is contaminated by the spatter of the diffuser pump oil, the contaminated system can also be cleaned by nitrogen flushing method. Generally, the oil pollution can be eliminated by heating the system while cleaning the system with nitrogen.
2. Reaction gas flushing
This method is particularly suitable for internal cleaning (removal of hydrocarbon contamination) of large ultra high vacuum stainless steel systems. For the vacuum chamber and vacuum components of some large-scale ultra-high vacuum systems, the standard methods to eliminate the surface pollution are chemical cleaning, vacuum furnace roasting, glow discharge cleaning and original energy baking vacuum system, etc., in order to obtain the clean surface of atomic state. The above cleaning and degassing methods are often used before and during the installation of the vacuum system. After the installation (or operation) of the vacuum system, it is difficult to degasse the various parts in the vacuum system since they have been fixed. Once the system is (accidentally) contaminated (mainly by molecules with large atomic number, such as hydrocarbons), it is usually disassembled, processed and then installed again. The reaction gas process can be used for in-situ on-line degassing. Its cleaning mechanism: oxidizing gas (O2, N0) and reducing gas (H2, N H3) are quoted in the system to perform chemical cleaning on the metal surface to eliminate the pollution, so as to obtain the clean metal surface in atomic state. The rate of surface oxidation/reduction depends on the condition of contamination and the material on the metal surface. The rate of surface reaction is controlled by adjusting the pressure and temperature of the reaction gas. For each substrate, precise parameters are determined experimentally. These parameters are different for different crystalline orientations.
Vacuum coating machine cleaning process requirements
Vacuum material should be cleaned before vacuum process to remove contaminants from workpiece or system material surface; It is also very necessary to clean the surface of vacuum parts, because the gas and steam source caused by pollutants will not only make the vacuum system cannot obtain the required vacuum degree. In addition, due to the presence of pollutants, it will also affect the strength and sealing performance of vacuum parts connection.
Pollutants can be defined as "any useless substance or energy". According to the physical state of pollutants, they can be divided into solid, gas and liquid, which exist in the form of film or loose particles. In terms of its chemical properties, it can be in an ionic or covalent state, it can be inorganic or organic.
Surfaces exposed to air are the most susceptible to contamination. There are many sources of pollution. Adsorption, chemical reactions, leaching and drying processes, mechanical processes, and diffusion and segregation processes all contribute to the increase of surface contaminants in various components.
According to the vacuum coating processing plant, the common pollutants on the surface of vacuum materials are as follows:
1. Grease: lubricants, cutting fluids, vacuum grease and others contaminated during processing, assembly and operation;
2, acid, alkali, salt substances: residual substances during cleaning, hand sweat, minerals in water, etc.;
3. Surface oxides: surface oxides formed by materials placed in air for a long time or in humid air;
4. Water base: hand sweat during operation, water vapor during blowing, saliva, etc.;
5, polishing residue and dust and other organic matter in the ambient air.
The purpose of cleaning these pollutants is to improve the stability of the vacuum coating machine, so that the work can be more smoothly. According to the requirements, the cleaned surface can be divided into two types: atomic clean surface and technological clean surface.
In the vacuum coating machine before the use of coating materials coating materials to do a simple surface cleaning can extend the service life of the coating machine. Various pollutants not only make the vacuum system can not get the required vacuum degree, but also affect the strength and sealing performance of vacuum parts connection. Early in place cleaning work can reduce a lot of trouble, to avoid many small problems, the work efficiency, vacuum coating machine coating quality has a very positive role. It can greatly improve the working stability of all the wall and other component surfaces in the vacuum system under various working conditions.
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