Boat & Filament Sources
Electrically heated evaporation sources are used for the deposition of the majority of thin films, with resistively joule heated sources being the most common. The advantage of these sources is they provide a simple means of thin film deposition – a current is passed through a resistive element containing the material to be deposited, causing it to heat. As the element heats up to temperatures which elevate the deposition material’s vapour pressure beyond that of the vacuum chamber, the material will begin to evaporate and deposit onto the substrate.
One challenge associated with resistive thermal evaporation is keeping the deposition material isolated from the crucible/boat material to prevent metal contamination. To achieve this, the vapour pressure of the deposition material must be much higher than that of the resistive boat at the temperature of evaporation.
Typically, resistive filaments and boats are made of refractory metals such as tungsten, tantalum, or molybdenum, and can be coated with various ceramics to improve material compatibility properties. These boats and filaments also come in different shapes and capacities, such as straight and coiled wires, dimpled plates, or boxes. When choosing a resistive boat for a given material, it is critical to ensure a high melting point differential and low solubility.