Thermal evaporation, also known as vacuum evaporation, requires maintaining a very low pressure, typically around 10^-5 Torr.
This low pressure is essential to prevent collisions between evaporated molecules and gas molecules in the chamber.
Such collisions could alter the path of the evaporated molecules, negatively impacting the quality of the deposition.
What is the Pressure for Thermal Evaporation? (4 Key Points Explained)
1. Equilibrium Vapor Pressure (EVP)
The equilibrium vapor pressure mentioned in the reference is 10^-2 Torr.
This is the pressure at which the rate of molecules leaving the surface equals the rate of molecules returning to the surface, indicating a state of equilibrium.
However, for effective thermal evaporation, the pressure needs to be significantly lower to ensure a clear path for the evaporated molecules to the substrate without interference from residual gas molecules.
2. Importance of Low Pressure
In thermal evaporation, the process is conducted in a vacuum to increase the mean free path of the evaporated molecules.
At a pressure of 10^-5 Torr, the mean free path length of molecules is about 1 meter.
This length is sufficient to allow the evaporated molecules to travel from the source to the substrate without significant collisions, ensuring a high-quality deposition.
3. Effect of Pressure on Deposition Rate
The reference notes that the deposition rate of evaporation materials increases with higher temperatures (and thus higher power to the resistance source).
However, maintaining a low pressure is equally important for achieving high deposition rates.
A lower pressure environment allows for a more robust vapor stream, which can lead to faster and more efficient deposition.
4. Application in Thin Film Evaporation
In applications such as thermal separation via distillation, maintaining a low pressure helps in reducing the evaporation temperature and duration of thermal stress on the product.
This is particularly important for sensitive organic substances that can degrade at higher temperatures.
By reducing the operating pressure, the quality of the final product can be significantly improved.
In summary, the pressure for thermal evaporation is typically set at very low levels, around 10^-5 Torr, to facilitate efficient and high-quality deposition of materials.
This low pressure environment ensures that evaporated molecules can travel to the substrate without unwanted collisions, which is essential for the integrity and uniformity of the deposited films.
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