Thermal evaporation is a method of physical vapor deposition (PVD) where a material is heated in a high vacuum chamber until it vaporizes, forming a vapor that then condenses onto a substrate to form a thin film.
This process is particularly effective for materials with low melting points and is known for its simplicity and low power consumption.
1. Process Setup
Thermal evaporation begins with a vacuum chamber typically made of stainless steel.
Inside this chamber, a crucible or boat made of refractory materials like tungsten or molybdenum holds the material to be deposited, known as the evaporant.
The vacuum environment is crucial as it allows the vapor to travel without reacting or scattering against other atoms, ensuring a clean deposition process.
2. Heating and Vaporization
The material is heated using a resistive heat source.
This heating continues until the material reaches its boiling point and vaporizes, producing vapor pressure.
Even a relatively low vapor pressure is sufficient in the vacuum environment to create a vapor cloud.
The importance of vapor pressure lies in its ability to facilitate the formation of a vapor stream that can travel across the chamber.
3. Deposition onto Substrate
The evaporated material, now in the form of a vapor stream, traverses the vacuum chamber and reaches the substrate.
Upon contact, it condenses and adheres to the substrate, forming a thin film.
This process is gentle, with evaporated particle energies around 0.12 eV, suitable for delicate materials and substrates.
4. Advantages and Limitations
Thermal evaporation is favored for its simplicity and the ability to deposit metals with low melting points.
However, it has limitations in handling high-melting materials due to the temperature constraints of the crucible materials.
The choice of crucible and heating method (electrically heated wires or conductive crucibles) depends on the material's properties and the desired quality of the layer.
5. Vacuum Requirements
The base pressure in the chamber is maintained at a very low level, typically between 10^-7 to 10^-5 mbar, to ensure the quality of the deposition.
This low pressure is necessary to prevent contamination and to allow the vapor to travel freely to the substrate without interference.
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