Thermal energy is used to vaporize the target material so it can be used to deposit thin film onto the substrate. Deposited materials may include pure metals, non‑metals, oxides, and nitrides. This process offers a high level of control over film properties like thickness, adhesion, stress, and grain structure. Compared to other PVD and CVD techniques, thermal evaporation has one of the highest deposition rates.
A typical thermal evaporation system consists of the following components:
- Substrate(s) and target
- Heat source
- Vacuum pump
- Pressure controller / Gate valve
- Deposition rate monitor
- RGA (optional)
Thermal evaporation requires a very high vacuum of about (1×10-6 to 1×10-9 Torr). The required vacuum level depends on purity needs and the required mean free path. While differing in some specifics such as level of vacuum, the following three steps are common to all thermal evaporation setups:
- Vaporization: The target material is first placed into a crucible at the bottom of the vacuum chamber. A heat source (tungsten filament or electron beam) is then used to sublime or boil the target material into a vapor.
- Transport from target to substrate: The vaporized target material forms a vapor plume which travels to the substrate, installed directly above the target. Maintaining a stable, high vacuum level ensures the environment is free from contamination, and a mean free path ensures a virtually collision‑free journey of the vapors from the target to the substrate.
- Deposition and nucleation: Since the substrate’s surface is at a relatively low temperature, the vapors condense when they come in contact with it. Condensation is followed by nucleation, creating the first thin film layer. This process is carried out until the desired film thickness is reached.
Thermal evaporation methods
Thermal evaporation also has subtypes, differentiated by method of vaporizing the target material. Methods include molecular beam epitaxy (MBE), electron beam deposition, flash evaporation and resistive evaporation.
Common applications
Thermal evaporation is most often used to deposit electrically conductive metallic layers on solar cells, OLED displays and thin‑film transistors. It is also used in the manufacturing process of aluminum PET films.