1.Introduction to Evaporative Coating
Vacuum evaporation coating refers to a technique of obtaining thin films by heating and evaporating a certain substance under vacuum conditions to deposit it on the surface of the substrate material. The substance that has been evaporated is called a evaporative material. Evaporative coating was first proposed by M. Faraday in 1857 and has become one of the mainstream coating technologies after over a hundred years of development.
Evaporative coating is a kind of PVD vacuum deposition, which is characterized by that under vacuum conditions, the material evaporates and condenses on the glass surface to form a film, and then after high temperature heat treatment, a film with strong adhesion is formed on the glass surface. Sputtering coating and evaporation coating are currently the two most mainstream PVD coating methods. The sputtering coating process has good repeatability and controllable film thickness, which can obtain a uniform thickness of thin film on a large area of substrate material. The prepared thin film has advantages such as high purity, good density, and strong adhesion with the substrate material, and has become one of the main technologies for preparing thin film materials. The vacuum evaporation coating technology has the characteristics of simple and convenient operation, fast film forming speed, etc., and is mainly applied to the coating of optical components, LEDs, flat panel displays, and semiconductor separators. From a manufacturing perspective, the manufacturing complexity of evaporated materials is much lower than that of sputtering targets, and evaporated coatings are commonly used for coating small-sized substrate materials. In addition, the selection range of evaporation coating materials is also quite large, with more than 70 elements, more than 50 inorganic compound and a variety of alloy materials available.
2.Principle of evaporation coating
The vacuum evaporation coating system generally consists of three parts: a vacuum chamber, an evaporation source or evaporation heating device, a substrate placement device and a substrate heating device. In order to evaporate the material to be deposited in vacuum, a container is needed to support or contain the evaporated material, and at the same time, it is necessary to provide evaporation heat to reach a sufficiently high temperature to generate the required vapor pressure. The physical process of evaporation includes: evaporation or sublimation of the deposited material into gaseous particles, rapid transport of gaseous particles from the evaporation source to the substrate surface, attachment of gaseous particles to the substrate surface for nucleation and growth into a solid film, reconstruction or chemical bond of film atoms.
Place the substrate in a vacuum chamber and heat the film material using methods such as resistance, electron beam, laser, etc. to evaporate or sublimate the film material, and vaporize it into particles (atoms, molecules, or atomic clusters) with a certain energy (0.1-0.3eV). Gaseous particles are transported rapidly to the substrate in a linear motion with basically no collision. Some particles arriving at the surface of the substrate are reflected, while the other part is absorbed on the substrate and diffused on the surface. Two dimensional collisions occur between the deposited atoms, forming clusters, and some may evaporate after a short pause on the surface. Particle clusters continuously collide with diffusing particles, adsorb single particles, or release single particles. This process is repeated, and when the number of aggregated particles exceeds a certain critical value, it becomes a stable nucleus, and then continues to adsorb and diffuse particles, gradually growing up. Finally, through the contact and merging of adjacent stable nuclei, a continuous thin film is formed.
3.Common evaporation materials and selection of evaporation equipment
Types of evaporation materials: according to chemical composition, they can be mainly divided into metal/non-metal particle evaporation materials, oxide evaporation materials, fluoride evaporation materials, etc.
Metal and non-metallic particles: aluminum steaming material, nickel steaming material, copper steaming material, silver steaming material, titanium steaming material, silicon steaming material, vanadium steaming material, magnesium steaming material, tin steaming material, chromium steaming material, indium steaming material, silver copper steaming material, gold steaming material, microcrystalline silver powder, etc.
Oxides: titanium tantalum alloy, zirconium titanium alloy, silicon aluminum alloy, aluminum trioxide, zirconia, titanium pentoxide, quartz ring, erbium oxide, lanthanum titanate, etc.
Fluoride: magnesium fluoride, dysprosium fluoride, lanthanum fluoride, etc.
| Evaporation Materials | Evaporation Mode | Evaporation Source Material |
| 1.Metal and non-metallic particles: aluminum steaming material, nickel steaming material, copper steaming material, silver steaming material, titanium steaming material, silicon steaming material, vanadium steaming material, magnesium steaming material, tin steaming material, chromium steaming material, indium steaming material, silver copper steaming material, gold steaming material, microcrystalline silver powder, etc. 2.Oxides: titanium tantalum alloy, zirconium titanium alloy, silicon aluminum alloy, aluminum trioxide, zirconia, titanium pentoxide, quartz ring, erbium oxide, lanthanum titanate, etc. 3.Fluoride: magnesium fluoride, dysprosium fluoride, lanthanum fluoride, etc. | Resistance evaporation: Materials with an evaporation temperature of 1000-2000°C can be heated by resistance as the evaporation source. After the heater resistance is energized, heat is generated to make the molecules or atoms of the evaporating materials obtain enough kinetic energy to evaporate. | Selection of evaporation source materials: 1. High melting point materials (melting point of evaporation source material>evaporation temperature) 2. Reduce pollution from evaporation sources (evaporation temperature of thin film materials<corresponding temperature of evaporation source materials at vapor pressure of 10-8 Torr); 3. The evaporation source material does not react with the thin film material; 4. The wettability of the thin film material to the evaporation source; 5. Commonly used evaporation source materials include: iridium crucible, tungsten crucible, molybdenum crucible, tantalum crucible, high-temperature resistant metal oxide, ceramic or graphite crucible |
| Molybdenum particles, tantalum particles, niobium particles, silicon (Si), magnesium fluoride (MgF2), titanium dioxide (TiO2), gallium tritelluride (Ga2Te3), aluminum oxide (Al2O3), tin oxide (SnO2), etc | Electron beam evaporation: The evaporation of refractory materials, achieving rapid evaporation at a higher power density to prevent alloy fractionation; Simultaneously placing multiple crucibles and evaporating multiple different substances simultaneously or separately; Most electron beam evaporation systems use magnetic focusing or magnetic bending electron beams, and the evaporated material is placed in a water-cooled crucible. Evaporation occurs on the surface of the material, effectively suppressing the reaction between the crucible and the evaporated material. It is suitable for preparing high-purity thin films and can be used to prepare thin film materials in the fields of optics, electronics, and optoelectronics. The kinetic energy of the evaporated molecules is large, and a stronger and denser film layer can be obtained than resistance heating. | The structure of electron beam evaporation source can be divided into three types: straight gun (Bulls gun), ring gun (electric deflection) and e-gun (magnetic deflection). Multiple collapses can be placed within an evaporation device, allowing for simultaneous or separate evaporation and deposition of various substances. |