What is vacuum coating technology?

2021/4/9 20:09:37

Coating method and classification
Film formation under vacuum conditions has many advantages: it can reduce the collision between the atoms and molecules of the evaporation material and the molecules in the process of flying to the substrate, reduce the chemical reaction (such as oxidation, etc.) between the active molecules in the gas and the evaporation source material, and Reduce the amount of gas molecules entering the film as impurities during the film formation process, thereby improving the density, purity, deposition rate and adhesion of the film layer to the substrate. Generally, vacuum evaporation requires the pressure in the film forming chamber to be equal to or lower than 10-2Pa. For occasions where the evaporation source is far from the substrate and the film quality is very high, the pressure is required to be lower.

Mainly divided into the following categories:
Evaporation coating, sputtering coating and ion plating.
Evaporation coating: By heating and evaporating a certain substance to deposit it on the solid surface, it is called evaporation coating. This method was first proposed by M. Faraday in 1857, and it has become one of the commonly used coating techniques in modern times.

Evaporated substances such as metals, compounds, etc. are placed in a crucible or hung on a hot wire as the evaporation source, and the workpiece to be plated, such as metal, ceramic, plastic and other substrates, is placed in front of the crucible. After the system is evacuated to a high vacuum, the crucible is heated to evaporate the contents. The atoms or molecules of the evaporated substance are deposited on the surface of the substrate in a condensed manner. The thickness of the film can range from hundreds of angstroms to several microns. The thickness of the film is determined by the evaporation rate and time of the evaporation source (or the loading amount), and is related to the distance between the source and the substrate. For large-area coatings, a rotating substrate or multiple evaporation sources are often used to ensure the uniformity of the film thickness. The distance from the evaporation source to the substrate should be less than the mean free path of vapor molecules in the residual gas to prevent the collision of vapor molecules with residual gas molecules from causing chemical effects. The average kinetic energy of vapor molecules is about 0.1 to 0.2 electron volts.

There are three types of evaporation sources. ①Resistance heating source: use refractory metals such as tungsten and tantalum to make boat foil or filament, and apply electric current to heat the evaporated substance above it or in the crucible (Figure 1 [Schematic diagram of evaporation coating equipment]) resistance heating The source is mainly used to evaporate materials such as Cd, Pb, Ag, Al, Cu, Cr, Au, Ni and so on. ②High-frequency induction heating source: Use high-frequency induction current to heat the crucible and evaporate material. ③Electron beam heating source: suitable for materials with higher evaporation temperature (not less than 2000 [618-1]), that is, bombarding the material with electron beams to evaporate.

Compared with other vacuum coating methods, evaporative coating has a higher deposition rate, and can be coated with simple substance and compound films that are not easy to be thermally decomposed.

In order to deposit a high-purity single crystal film, molecular beam epitaxy can be used. The molecular beam epitaxy device for growing doped GaAlAs single crystal layer is shown in Figure 2 [Schematic diagram of molecular beam epitaxy device]. The jet furnace is equipped with a molecular beam source. When it is heated to a certain temperature under ultra-high vacuum, the elements in the furnace are ejected to the substrate in a beam-like molecular stream. The substrate is heated to a certain temperature, the molecules deposited on the substrate can migrate, and the crystals are grown in the order of the substrate lattice. Molecular beam epitaxy can be used to obtain a high-purity compound single crystal film with the required stoichiometric ratio. The film grows the slowest. The speed can be controlled at 1 single layer per second. By controlling the baffle, the single crystal film with the required composition and structure can be made accurately. Molecular beam epitaxy is widely used to manufacture various optical integrated devices and various superlattice structure films.

Sputtering coating: When high-energy particles are used to bombard the solid surface, the particles on the solid surface can gain energy and escape the surface, and be deposited on the substrate. Sputtering phenomenon began to be used in coating technology in 1870, and gradually used in industrial production after 1930 due to the increase in deposition rate. Commonly used two-pole sputtering equipment is shown in Figure 3 [Schematic diagram of two-pole sputtering]. Usually the material to be deposited is made into a plate-a target, which is fixed on the cathode. The substrate is placed on the anode facing the target surface, a few centimeters away from the target. After the system is pumped to a high vacuum, it is filled with a gas of 10-1 Pa (usually argon), and a voltage of several thousand volts is applied between the cathode and the anode, and a glow discharge is generated between the two electrodes. The positive ions generated by the discharge fly to the cathode under the action of an electric field and collide with atoms on the target surface. The target atoms that escape from the target surface due to the collision are called sputtering atoms, and their energy is in the range of 1 to tens of electron volts. The sputtered atoms are deposited on the surface of the substrate to form a film. Unlike evaporation coating, sputter coating is not limited by the melting point of the film material, and can sputter refractory substances such as W, Ta, C, Mo, WC, TiC, etc. The sputtering compound film can be sputtered by the reactive sputtering method, that is, the reactive gas (O, N, HS, CH, etc.) is added to the Ar gas, and the reactive gas and its ions react with the target atom or the sputtered atom to form a compound (such as oxide, nitrogen) Compound, etc.) and deposited on the substrate. The high-frequency sputtering method can be used to deposit the insulating film. The substrate is mounted on the grounded electrode, and the insulating target is mounted on the opposite electrode. One end of the high-frequency power supply is grounded, and one end is connected to an electrode equipped with an insulating target through a matching network and a DC blocking capacitor. After switching on the high-frequency power supply, the high-frequency voltage continuously changes its polarity. The electrons and positive ions in the plasma hit the insulating target during the positive half cycle and the negative half cycle of the voltage, respectively. Since the electron mobility is higher than the positive ions, the surface of the insulating target is negatively charged. When the dynamic equilibrium is reached, the target is at a negative bias potential, so that the sputtering of the positive ions to the target continues. The use of magnetron sputtering can increase the deposition rate by nearly an order of magnitude compared with non-magnetron sputtering.

Ion plating: The molecules of the evaporated substance are ionized by electron impact and deposited on the solid surface as ions, which is called ion plating. This technique was proposed by D. Matox in 1963. Ion plating is a combination of vacuum evaporation and cathode sputtering technology. An ion plating system is shown in Figure 4 [Ion Plating System Schematic Diagram]. The substrate stage is used as the cathode and the outer shell is used as the anode, and an inert gas (such as argon) is filled to generate glow discharge. The molecules evaporated from the evaporation source are ionized as they pass through the plasma zone. The positive ions are accelerated to the surface of the substrate by the negative voltage of the substrate stage. Unionized neutral atoms (about 95% of the vaporized material) are also deposited on the surface of the substrate or the vacuum chamber wall. The acceleration effect of the electric field on the ionized vapor molecules (the ion energy is about several hundred to several thousand electron volts) and the sputtering cleaning effect of argon ions on the substrate greatly improve the adhesion strength of the film. The ion plating process combines the characteristics of evaporation (high deposition rate) and sputtering (good film adhesion) process, and has good diffraction, which can be used to coat workpieces with complex shapes.
Talking about the advantages of vacuum coating
What are the characteristics of vacuum coating?

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