Vacuum thermal evaporation is a Physical Vapor Deposition (PVD) technique where a material is heated in a vacuum chamber until it vaporizes and then condenses onto a substrate.
This process is used to deposit thin films of material onto a substrate, typically in a controlled environment with low gas pressure to ensure the vaporized material reaches the substrate without collisions.
5 Key Steps Explained
1. Process Setup
The process begins with a vacuum chamber, usually made of stainless steel.
This chamber houses a crucible or boat made of refractory materials like tungsten or molybdenum.
The material to be deposited (evaporant) is placed inside this crucible.
2. Heating and Evaporation
The material is heated using electrical current or electron beams.
This heating continues until the material's surface atoms gain enough energy to leave the surface, turning into vapor.
The vapor pressure of the material must reach at least 10 mTorr for effective deposition.
3. Deposition in Vacuum
The vacuum environment is crucial as it ensures that the vaporized material travels to the substrate without colliding with gas molecules.
The pressure inside the chamber is maintained at a level where the mean free path of the vaporized particles is longer than the distance between the source and the substrate, typically between 10^-5 to 10^-9 Torr.
4. Substrate Coating
The vaporized material condenses onto the substrate, forming a thin film.
The substrate is usually positioned above the evaporating material, and the deposition is "line-of-sight," meaning the vapor follows a direct path from the source to the substrate.
5. Energy and Microstructure
Since the energy of the ions impinging on the substrate surface is low, the substrate is often heated to a high temperature (about 250°C to 350°C) to modify the microstructure of the deposited layer, which can differ significantly from the bulk material.
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