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The material must be cleaned before being coated by the vacuum coating machine, which is the vacuum cleaning process that we often say at present. The vacuum cleaning of the vacuum coating machine is generally defined as removing the surface of the workpiece or the system material before the vacuum process. The desired material process. The surface cleaning treatment of vacuum parts is necessary, because the gas and vapor sources caused by pollutants will make the vacuum system unable to obtain the required vacuum. In addition, due to the presence of contaminants, the strength and sealing performance of the joints of the vacuum components will also be affected.
1. Vacuum heating cleaning
Place the workpiece under normal pressure or vacuum to heat. Promote the evaporation of volatile impurities on its 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 contaminants and the material of the workpiece. The principle is to heat the workpiece. Promote the desorption of water molecules and various hydrocarbon molecules adsorbed on its surface to increase. The degree of desorption enhancement is related to temperature. Under ultra-high vacuum, in order to get an atomic level clean surface, the heating temperature must be higher than 450 degrees. The heating cleaning method is particularly effective. But sometimes, this treatment method can also produce side effects. As a result of heating, some hydrocarbons may polymerize into larger agglomerates and decompose into carbon residues at the same time
2. Ultraviolet radiation cleaning
Use ultraviolet radiation to decompose hydrocarbons on the surface. For example, exposure to air for 15 hours can produce a clean glass surface. If you put a properly pre-cleaned surface in a UV source that produces ozone. It takes a few minutes to form a clean surface (process cleaning). This indicates that the presence of ozone increases the cleaning rate. The cleaning mechanism is: under ultraviolet radiation, dirt molecules are excited and dissociated, and the generation and existence of ozone produces highly reactive atomic oxygen. The excited dirt molecules and free radicals generated by the dissociation of dirt interact with atomic oxygen. The formation of relatively simple and volatile molecules. Such as H203, CO2 and N2. The reaction rate increases with increasing temperature.
Three, discharge cleaning
This cleaning method is widely used in the cleaning and degassing of high vacuum and ultra-high vacuum systems. Especially used in vacuum coating machines. Use a hot wire or electrode as an electron source. Applying a negative bias relative to the surface to be cleaned can achieve gas desorption by ion bombardment and the removal of certain hydrocarbons. The cleaning effect depends on the electrode material, geometry and its relationship with the surface. That is, it depends on the number of ions per unit surface area and ion energy. It depends on the effective electrical power. Fill the vacuum chamber with an inert gas of appropriate partial pressure (typically Ar gas). The purpose of cleaning can be achieved by the ion bombardment generated by the glow discharge at low pressure between two appropriate electrodes. The method. The inert gas is ionized and bombarded the inner wall of the vacuum chamber, other structural parts in the vacuum chamber, and the substrate to be plated, which can prevent certain vacuum systems from being baked at high temperatures. If oxygen is added to the charged gas, a better cleaning effect can be obtained for certain hydrocarbons. Because oxygen can oxidize certain hydrocarbons to produce volatile gases, which are easily eliminated by the vacuum system. The main components of impurities on the surface of stainless steel high vacuum and ultra-high vacuum vessels are carbon and hydrocarbons. Under normal circumstances, the carbon in it cannot be volatilized alone. After chemical cleaning, Ar or Ar+O2 mixed gas needs to be introduced for glow discharge cleaning, so that impurities on the surface and gas bound on the surface due to chemical action are removed. In the glow discharge cleaning. The important parameters are the type of applied voltage (AC or DC), the size of the discharge voltage, the current density, the type of gas charged and the pressure. The duration of the bombardment. The shape of the electrode and the material and location of the parts to be cleaned.
Four, gas flushing
1. Nitrogen flushing
When nitrogen is adsorbed on the surface of the material, because the adsorption energy is small, the occlusion surface time is extremely short. Even if it is adsorbed on the wall, it is easy to be pumped away. 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, 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 far Smaller than water vapor molecules, nitrogen molecules are first adsorbed by the wall of the vacuum chamber after being filled with nitrogen under vacuum. Because the adsorption site is fixed, it is first occupied by nitrogen molecules, and the adsorbed water molecules are few, thus shortening the pumping time. If the system is contaminated by diffusion pump oil spray, nitrogen flushing method can also be used to clean the contaminated system. Generally, the system is baked and heated while flushing the system with nitrogen to eliminate oil pollution.
2. Reactive gas flushing
This method is especially suitable for the internal washing (removal of hydrocarbon pollution) of large ultra-high stainless steel vacuum coating machines. Generally, for the vacuum chamber and vacuum components of some large ultra-high vacuum systems, in order to obtain a clean surface in the atomic state, the standard method to eliminate surface pollution is chemical cleaning, vacuum furnace baking, glow discharge cleaning and the original energy baking vacuum system And other methods. The above cleaning and degassing methods are often used before and during the installation of the vacuum system. After the vacuum system is installed (or after the system is running), because the various parts in the vacuum system have been fixed, it is difficult to degas them at this time. Once the system is (incidentally) contaminated (mainly large atomic number) Molecules such as hydrocarbon pollution), usually have to be disassembled and reprocessed and then installed. With reactive gas technology, in-situ online degassing can be performed. Effectively remove the pollution of hydrocarbons in the stainless steel vacuum chamber. The cleaning mechanism: citing oxidizing gas (O2, N0) and reducing gas (H2, NH3) in the system to clean the metal surface by chemical reaction to eliminate pollution, In order to obtain an atomic state of clean metal surface. The rate of surface oxidation/reduction depends on the pollution and the material of the metal surface. The size of the surface reaction rate is controlled by adjusting the pressure and temperature of the reaction gas. For each substrate, the precise parameters must be determined through experiments. For different crystal orientations, these parameters are different.