Cleaning process before vacuum coating

Vacuum cleaning is generally defined as the process of removing unwanted substances from the surface of the substrate or system material before the vacuum process. It is necessary to clean the surface of vacuum parts because the gas and vapor source caused by pollutants will make the vacuum system unable to obtain the required vacuum degree. In addition, due to the presence of pollutants, it will also affect the strength and sealing performance of the vacuum parts.

1、 Vacuum heating cleaning

The cleaning effect of this method is related to the environmental pressure of the workpiece, the retention time in a vacuum, the heating temperature, the type of pollutants, and the material of the workpiece. Its principle is to heat the workpiece to enhance the desorption of water molecules and various hydrocarbon molecules adsorbed on its surface. The degree of desorption enhancement is related to temperature. In order to get an atomic clean surface under an ultra-high vacuum, the heating temperature must be higher than 450 degrees. The heating cleaning method is especially effective. But sometimes, this treatment can also have side effects. As a result of heating, some hydrocarbons may polymerize into larger aggregates and decompose into carbon slag at the same time

2、 UV irradiation cleaning

Ultraviolet radiation is used to decompose hydrocarbons on the surface. For example, exposure to air for 15 hours produces a clean glass surface. If a properly pre-cleaned surface is placed in an ozone-producing UV source, a clean surface can be formed in a few minutes (process cleaning). This indicates that the presence of ozone increases the cleaning rate. The cleaning mechanism is: under UV irradiation, the pollutant molecules are stimulated and dissociated, and the formation and existence of ozone produce highly active atomic oxygen. The excited pollutant molecules and the free radicals produced by the dissociation of sewage react with atomic oxygen to form simple and volatile molecules, such as H203, CO2, and N2. The reaction rate increases with the increase of temperature

3、 Discharge cleaning

 This cleaning method is widely used in cleaning and degassing of high vacuum and ultra-high vacuum systems, especially in vacuum coating equipment. Using a hot wire or an electrode as an electron source and applying a negative bias voltage to the surface to be cleaned, the gas desorption by ion bombardment and the removal of some hydrocarbons can be realized. The cleaning effect depends on the electrode material, geometry, and its relationship with the surface, i.e., the number of ions per unit surface area and the ion energy, and thus the effective electric power. The vacuum chamber is filled with inert gas (typically Ar gas) with appropriate partial pressure. The purpose of cleaning can be achieved by ion bombardment generated by glow discharge at low pressure between two appropriate electrodes. In this method, the inert gas is ionized and bombarded on the wall of the vacuum chamber, other structural parts in the vacuum chamber, and the substrate, which can make some vacuum systems free from high-temperature baking. If oxygen is added to the charged gas, a better cleaning effect can be obtained for some hydrocarbons. Because oxygen can oxidize some hydrocarbons into volatile gases, which can be 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 hydrocarbon. After chemical cleaning, AR or Ar + O2 gas mixture should be introduced for glow discharge cleaning to remove the impurities on the surface and the gas bound 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 discharge voltage, current density, and so on The type and pressure of the charged gas, the duration of the bombardment, the shape of the electrode and the material and location of the parts to be cleaned, etc

4、 Gas flushing

  1. Nitrogen flushing

When nitrogen adsorbs on the surface of the material, the adsorption energy is small, so the adsorption time is very short. Even if the nitrogen 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 first filled with dry nitrogen into the vacuum chamber for washing before it is put into the atmosphere, 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 in a vacuum, the nitrogen molecules are first absorbed by the vacuum chamber wall. Because the adsorption site is fixed, the nitrogen molecules are occupied first, so the water molecules adsorbed are very few, so the pumping time is shortened. If the system is polluted by the spray of diffusion pump oil, the polluted system can also be cleaned by the nitrogen flushing method. Generally, the system is baked and heated while the system is flushed with nitrogen to eliminate oil pollution.

  2. Reaction gas flushing

This method is especially suitable for the internal cleaning of the large-scale ultra-high stainless steel vacuum system. In order to obtain an atomic clean surface, the standard methods to eliminate surface contamination are chemical cleaning, vacuum furnace roasting, glow discharge cleaning, and primary energy baking vacuum system. 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), it is very difficult to digest the various components in the vacuum system because they have been fixed. Once the system is contaminated (mainly by molecules with large atomic numbers, such as hydrocarbons), it is usually necessary to remove and re-treat the system before installation. The reaction gas process can be used for in-situ on-line degassing The cleaning mechanism is: in the system, oxidizing gases (O2, N0) and reducing gases (H2, NH3) are cited to carry out chemical reaction cleaning on the metal surface to eliminate pollution, so as to obtain clean metal surface in atomic state. The rate of surface oxidation/reduction depends on the pollution and the material of the metal surface. The surface reaction rate is controlled by adjusting the pressure and temperature of the reaction gas. For each substrate, the exact parameters are determined by experiments. For different crystal orientations, these parameters are different