Thermal evaporation method as PVD coating method
Thermal vacuum deposition is one method for fabricating thin films under a high vacuum environment also addressed as "thermal evaporation method". With this method, an electron beam (e-beam) or resistive heating is usually used to evaporate the desired material inside the vacuum coating chamber, which then adheres to a substrate placed above it. This method can classified as a form of PVD, which stands for physical vapor deposition, and is suitable for fabricating high-quality thin films with thicknesses on the order of nanometers on glass, plastic, films, metals, and almost any other kind of material. An e-beam can be used to melt evaporation material with melting points even above 2000 degree, which are common for thin films on optical components, some of which consist of more than 300 layers.
By coating the substrate with thin layers of the evaporation material, the transmittance, reflectance, and absorption of specific wavelengths of light can be accurately controlled. The schematic above explains the working of anti-reflection coatings, which increase the transmittance of visible light.
The so-called "vacuum coater" apparatuses used for fabricating thin films are of two types: serial and batch. The latter requires the coating chamber to be opened each time the substrate or evaporation material is changed after every deposition process is complete. In contrast, the serial coaters are designed for continuous operation: so after one process, the substrate and evaporation material are automatically changed or refilled by a computer-controlled system.
The substrate holder used inside the coating chamber looks like a dome. Each substrate is set in a hole in this dome, which rotates during the deposition process. After setting the substrate on the holder, the inside of the chamber is kept at a high vacuum to ensure that the molecules of the evaporation material can attach to the substrate. There are many vacuum coaters used across the world but basic function of this thermal evaporation method are same.
In addition, to increase the adhesion to the substrate, the substrate, itself, must also be heated. The substrate temperature has a significant influence on the film structure and optical properties as well.
To coat substrates that cannot endure high temperatures, like plastic, for example, ion bombardment is used to increase the adhesion to the unheated substrate. With this method, the evaporation material is bombarded with ions to accelerate the molecules of the material and increase its adhesion to the substrate.
Evaporation materials are usually placed above the evaporation source, called the hearth, with a crucible or boat. The crucibles often used in this process are made of inert materials with high heat-resistance such as oxygen free copper, tungsten and PBN.
During deposition, the degree of vacuum inside the coating chamber must be kept high, but the vacuum is often compromised by gas emitted from the evaporation material, substrate, or other equipment inside the chamber, and also by any remaining moisture. This often results in low-quality thin films with unacceptable optical loss through absorption, scattering, or spectral shifting. So this "out-gassing" inside the chamber should be prevented to obtain a high-quality product.
Several methods have been developed for fabricating thin films as thermal evaporation methods, but generally, coating methods can classified into wet processes (liquid-phase methods) and dry processes (gas-phase methods). Typical liquid-phase methods are metallizing and the sol-gel method, which use chemical reactions for fabricating thin films. The former is suitable for large substrates or substrates with complicated shapes, but does not allow easy control of the film thickness and is also susceptible to poisoning from the environment.
Gas-phase methods can be classified into CVD and PVD. CVD, which stands for chemical vapor deposition, uses "gases" as the deposition materials and chemical reactions to fabricate the thin films. PVD, which stands for physical vapor deposition, uses solid deposition materials that are usually irradiated by high-energy sources that vaporize it to create thin films. Both methods require a high-vacuum environment, and so produce high-purity products. The two major PVD methods are sputtering and vacuum evaporation. Our evaporation materials are particularly suited to vacuum evaporation, i.e., thermal vacuum deposition.
This site mainly focuses on introducing useful information about thermal evaporation methods and its related theme.