Schematic of ARE process

• Metal is heated and melted by a high-accelaration-voltage e-beam
• Metal has a thin plasma sheath on top of it
• Low-energy secondary electrons from the plasma sheath are pulled into the reactive zone by an interspace electrode placed above the pool
• The electrode is biased to allow positive dc or ac potential (typically 20 to 100 V)
• Low-energy electrons have a high ionization cross section, thus ionizing or activating the metals and gas atoms.
• In addition, collisions between ions and neutral atoms result in charge exchange processes yielding energetic neutrals and positive ions.
• It is believed that these energetic neutral atoms, "activate" the reaction, thus increasing the reaction between the reactants

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Plasma

• Enhance the reaction between the reactants
• Cause ionization of both the coating metal and gas atoms in the vapor phase
• It is generated by :
  • when using e-beam as evaporation source : an interspace positively biased electrode
  • when using resistance-heated source : low-energy electrons injected to the vapor phase from a thermionically heated filament

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• Type of coating materials: almost any, e.g. AL2O3, SiO2
• Kinetic energy : 5 to 20 eV
• Deposition pressure: 10^-2 to 1 Pa or 10^-4 to 10^-2 torr
• Deposition temperature : 200 to 1600 C
• Coating thickness range : 0.1 to 1000 E-6m
• Deposition rate : 1 to 75 E-6m/min
• Relative adhesion : Fair to good at elevated temperature
• Substrate heated for a good adhesion and microstructure

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Advantages:

• Extremely high deposition rates, variety of coating compositions
• Precise control of stoichiometry
• Better adhesion
• Denser microstructure than direct evaporation

Disadvantages:

• High substrate temperature
• Addition of an extra electrode, slightly complicated compared to evaporation
• Substrate must generally be rotated for uniform coating

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