Cleaning steps of workpiece before coating in vacuum coating machine

Summary:  In order to improve the adhesion and smoothness of the plated film on the surface of the substrate...
  In order to improve the adhesion and smoothness of the plated film on the surface of the substrate, as well as the compactness of the film, before the substrate is hung in the vacuum coating machine, a preliminary cleaning step must be carried out to remove the oil stains, stains, Dust, to ensure that it is in a clean state, and then coating.
   1. Vacuum heating cleaning
       The workpiece is heated under normal pressure or vacuum. 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 ambient 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 enhanced desorption of water molecules and various hydrocarbon molecules adsorbed on its surface. The degree of desorption enhancement is temperature dependent. Under ultra-high vacuum, in order to obtain atomically clean surfaces, the heating temperature must be higher than 450 degrees. The heating cleaning method is particularly effective. But sometimes, this approach can also have side effects. As a result of heating, it may occur that some hydrocarbons aggregate into larger agglomerates and at the same time decompose into carbon residues
       2. Ultraviolet irradiation cleaning
       Uses UV radiation to decompose hydrocarbons on the surface. For example, exposure to air for 15 hours produces a clean glass surface. If properly pre-cleaned surfaces are placed in an ozone-generating UV source. A clean surface can be created in minutes (process clean). This indicates that the presence of ozone increases the cleaning rate. The cleaning mechanism is: under ultraviolet irradiation, the dirt molecules are excited and dissociated, and the generation and existence of ozone produces highly active atomic oxygen. Excited dirt molecules and free radicals generated by dirt dissociation interact with atomic oxygen. Simpler and more volatile molecules are formed. Such as H2O3, CO2 and N2. The reaction rate increases with increasing temperature.
      3. 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. A hot wire or electrode is used as the electron source. Applying a negative bias to the surface to be cleaned can achieve gas desorption by ion bombardment and removal of certain hydrocarbons. The cleaning effect depends on the electrode material, geometry and its relationship to the surface. That is, it depends on the number of ions and ion energy per unit surface area. Thereby it depends on the available electrical power. The vacuum chamber is filled with an inert gas (typically Ar gas) at an appropriate partial pressure. Cleaning can be achieved by ion bombardment by glow discharge at low voltage between two suitable electrodes. in this method. The inert gas is ionized and bombards the inner wall of the vacuum chamber, other structural parts in the vacuum chamber and the substrate to be plated, which can make some vacuum systems exempt from high temperature baking. Better cleaning results for some hydrocarbons can be obtained if oxygen is added to the charged gas. Because oxygen can oxidize certain hydrocarbons to form volatile gases that are easily removed 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. In general, the carbon in it cannot be volatilized alone. After chemical cleaning, it is necessary to introduce Ar or Ar+O2 mixed gas for glow discharge cleaning, so that impurities on the surface and gases bound to the surface due to chemical action are removed. in glow discharge cleaning. 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 parts to be cleaned, etc.
      4. Gas flushing
       (1) Nitrogen flushing
       When nitrogen is adsorbed on the surface of the material, due to the small adsorption energy, the surface retention time is extremely short. Even if it is adsorbed on the wall of the device, 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. For example, before the vacuum coating machine is put into the atmosphere, first fill the vacuum chamber with dry nitrogen to flush it and then fill it into the atmosphere, the pumping time of the next pumping cycle can be shortened by nearly half, because the adsorption energy of nitrogen is far Smaller than water vapor molecules, after being filled with nitrogen under vacuum, nitrogen molecules are first adsorbed by the vacuum chamber wall. Since the adsorption site is fixed, it is filled with nitrogen molecules first, and there are very few water molecules adsorbed, thus shortening the pumping time. If the system is polluted by the oil splash of the diffusion pump, nitrogen flushing method can also be used to clean the polluted system. Generally, while baking and heating the system, flushing the system with nitrogen gas can eliminate the oil pollution.
      (2) Reactive gas flushing
       This method is particularly suitable for internal washing (removing hydrocarbon contamination) of large ultra-high stainless steel vacuum coaters. Usually, for the vacuum chambers and vacuum components of some large ultra-high vacuum systems, in order to obtain atomically clean surfaces, the standard methods to eliminate surface contamination are chemical cleaning, vacuum furnace roasting, glow discharge cleaning and original energy roasting vacuum systems and other methods. The cleaning and degassing methods described above are commonly used before and during assembly of a vacuum system. After the vacuum system is installed (or after the system is in operation), since the various components in the vacuum system have been fixed, it is difficult to degas the various components in the vacuum system. Once the system is (accidentally) contaminated (mainly large atomic numbers) Molecules such as hydrocarbon contamination) are usually dismantled and reprocessed before installation. With the reactive gas process, in-situ online degassing can be performed. Effectively remove the pollution of hydrocarbons in the stainless steel vacuum chamber. Its cleaning mechanism: In the system, oxidizing gas (O2, N0) and reducing gas (H2, N H3) are quoted in the system to carry out chemical reaction cleaning on the metal surface to eliminate pollution, in order to obtain atomically clean metal surfaces. The rate of surface oxidation/reduction depends on the contamination 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 precise parameters are determined experimentally. These parameters are different for different crystallographic orientations.

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