Electron beam evaporation vacuum coating equipment is commonly used for depositing AR/AF coatings, including hard films, decorative films, ITO films, bandpass filters, and HR films. It boasts advantages such as high efficiency, stronger production capacity, and lower production costs. For AR films (substrate glass transmittance > 91.5%), in the 420-680nm wavelength band, single-sided average transmittance > 95% and reflectance < 0.5% (average). Double-sided average transmittance > 98% and reflectance < 0.5% (average). It has wide market applications.
Vacuum evaporation coating technology involves placing the evaporating material in a water-cooled steel crucible and directly heating it with an electron beam. The evaporating material vaporizes and then condenses onto the substrate surface to form a film. This is an important heating method and a developing trend in vacuum evaporation coating technology. Electron beam evaporation overcomes many drawbacks of conventional resistance heating evaporation and is particularly suitable for fabricating high-melting-point and high-purity thin film materials.
Vacuum evaporation technology, which relies on electron beam bombardment evaporation, can be further classified into several types based on the form of the electron beam evaporation source, including ring guns, straight guns, E-type guns, and hollow cathode electron guns.
A ring gun emits an electron beam from a ring-shaped cathode. After being focused and deflected, the beam strikes a crucible, causing the metal to evaporate. Its structure is relatively simple, but its power and efficiency are low, making it primarily a laboratory device; it is no longer used in production facilities.
A straight gun is an axisymmetric linear accelerator where electrons are emitted from the filament cathode, focused into a fine beam, accelerated by the anode, and then strike a crucible, melting and evaporating the coating material. Straight guns range in power from several hundred watts to several hundred kilowatts; some are used for vacuum evaporation, while others are used for vacuum smelting. The disadvantages of straight guns are that the evaporated material can contaminate the gun structure, causing operational instability. Additionally, sodium ions escaping from the filament can also contaminate the coating. Recently, a West German company developed an improved version of the straight gun by adding a deflecting magnetic field at the electron beam exit and incorporating an independent evacuation system at the filament. This not only completely eliminates filament contamination of the coating but also improves the gun's lifespan.
The e-type electron gun, with a 270-degree deflection, overcomes the shortcomings of straight-gun electron evaporation and is one of the most widely used electron beam evaporation sources. The e-type electron gun can generate high power densities, melt high-melting-point metals, and produces high-energy evaporation particles, resulting in a strong bond between the film and the substrate and good film quality. The disadvantages are that the electron gun requires a high vacuum and negative high voltage, which leads to complex equipment structure, poor safety, difficulty in maintenance, and high cost.
The hollow cathode electron gun utilizes a plasma electron beam generated by a low-voltage, high-current hollow cathode discharge as its heating source. It uses a hollow tantalum tube as the cathode, a crucible as the anode, and an auxiliary anode near the tantalum tube. During vapor deposition using the hollow cathode electron gun, the generated evaporation ions have high energy and high ionization rate, resulting in high-quality films. The hollow cathode electron gun requires a lower vacuum level than the e-type electron gun and operates at low voltage, making the equipment relatively simpler, safer, and less expensive. Currently, both e-type and hollow cathode electron guns have been successfully applied in vapor deposition and ion plating equipment in our country. The gun power can reach tens of thousands of kilowatts, and various thin films have been deposited for industries such as machinery and electronics.
The advantages of the evaporation source in electron beam evaporation vacuum coating equipment are:
1) The electron beam bombardment heat source has a high beam current density, achieving a much higher energy density than resistance heating sources. It can evaporate materials up to 3000 degrees Celsius with a high evaporation rate;
2) Since the material being evaporated is placed in a water-cooled crucible, evaporation of the container material and reactions between the container material and the evaporation material are avoided, which is crucial for improving the purity of the coating;
3) Heat can be directly applied to the surface of the evaporation material, resulting in high thermal efficiency and minimal losses through heat conduction and radiation.
