Evaporation, or more specifically, Resistance Thermal Evaporation, involves the heating of the evaporation material within a vacuum environment. The materials used in vacuum evaporation are usually made into pellets, so they are also called evaporation pellets. These pellets are heated to a temperature above which the vapor pressure of the material exceeds that of the vacuum environment in which it is contained (more on this later).
The vacuum evaporation form is simple and the equipment required to produce the film is actually very small. First, like other PVD technologies, some type of vacuum system is needed to provide the right operating environment. Electrical energy must be supplied in the vacuum chamber, usually from a remote power source through a standard electrical feeder to the vacuum chamber, and then to the busbar via a busbar. A resistance source (evaporation source) is connected between the bus bar and the bus bar. The composition of the source may vary based on the particular material being evaporated. Typically, they are high-resistance wires or sheet materials shaped like “boat-shaped” (usually refractory metals) that are connected between the two electrodes to generate heat when powering the bus. By applying a current to the electrodes, the source material heats up and the vapor pressure increases. Once the vapor pressure of the material exceeds the vapor pressure of the background environment, vaporized molecules condense between the source material and the substrate and condense there to form a film.
Each material has a specific vapor pressure under any conditions. Generally, the higher the temperature (the power of the resistor source), the higher the deposition rate of the evaporated material. The vapor pressure of various materials is readily available in the literature and can be easily viewed via the Internet.
Unlike sputter coatings that eject a single atom or cluster at a time, thermal evaporation into a vapor stream can be quite robust, thus achieving high deposition rates in a short run time. Resistance evaporation is most suitable for elemental materials, which by definition have a single melting point at a given pressure and can form a uniform film.
In addition to the eutectic composition, the alloy typically does not have a single melting point, but as the material temperature rises above the liquidus, the alloy emits a non-uniform vapor stream of various amounts of each component. This will result in a non-uniform composition in the resulting film. For more information, please visit https://www.sputtertargets.net/.
Vacuum Evaporation Coating 