One strategy to getting more evaporation material into the chamber is to employ crucible heaters. In contrast, a resistive boat heats only from the bottom, and has a low capacity. A thermal evaporation crucible holds more material and acts like a furnace, heating the material from all sides, allowing for thicker films as well as fewer pumpdown cycles.
High capacity thermal evaporation sources are cylindrical containers fabricated from high temperature materials such as refractory metals, oxides, nitrides and graphite. These crucibles are encased by a resistive heater either as a coiled wire or a metallic sheet in form of a cylindrical basket. Together, the basket-shaped heater and cylindrical crucible assembly is referred to as crucible heater.
For crucible heaters to be an efficient source, they must be made of materials that can attain temperature of the evaporant material without having any significant vapor pressure of their own. Additionally, these crucibles should not react or alloy with the evaporant to cause any contamination. Their design allows heat to concentrate on the deposition materials uniformly. These heaters come with thermal shields which reduce the loss of radiative heat and keeps the components of the chamber at a adequately low temperature.