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Evaporation

Thermal Evaporation, E-beam and Ion Assistance

Evaporation is one of the oldest PVD method ever used in coating technology especially the thermal evaporation that it consists mainly in transferring heat to a material in a vacuum environment (10-6 mbar or lower), so that the material get to the vapur phase and spread all over the chamber condensing on the surrounsing surfaces.

The heat is usually transferred applying a current to a metal crucible(usually named boat) that transfers the heat to the material that is inside by Joule Effect.

In the e-beam evaporation the heat transfer is given by an electron beam focused on the material to be melted; e-beam evaporation is selected in for high temperature melting materials.

In the e-gun evaporation the material "evaporation speed" (evaporation rate) should be kept stable to better seek a compromise between speed and film quality. To maintain an adequately constant deposition rate, it is necessary to control it in real time, and to use the measured one as a feedback for adjusting the e-beam current (higher currents correspond to higher rates).

In order to measure the evaporation rate a quartz microbalance is introduced as a controller. The device consists in a piezoelectric quartz crystal, vibrating at its background frequency. Specifically the oscillation frequency will drop as the crystal's mass is increased by the material being deposited on it. This correlation is used as a feedback towards the ebeam power supply to drive the current for regulating the rate.

The evaporated material diffuses into the deposition chamber following linear trajectories. This property imposes to handle n a well designed manner the substrates in order to expose them uniformely to the evaporation source. Typical substrate holders are rotating Domes (consisting in sections of a sphere) or Planetary carousels with double rotation.

In applications requiring a good adhesion and higher density coating, the evaporation system is also equipped with an Ion Gun.  The Ion Gun is generating a plasma cone directed to the substrate. It can either be used for etching the substrate before the evaporation process (to improve the adhesion) or during it (for higher density). 

 

Evaporation system components