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EBE – Electron beam evaporation

EBE | TE

During electron beam evaporation, an electron beam is used to heat the coating material within a water-cooled crucible. The size and movement pattern of the electron beam can be optimized and controlled to ensure uniform evaporation of the molten material. The material vapour condenses on rotating substrates within a vacuum chamber, forming a layer. Additional oxygen gas is typically added to ensure full oxidation of the film. Substrates are usually heated to 150–350 °C to increase the mobility of condensing particles and enhance oxidation. Due to its effectiveness, electron beam evaporation (EBE) has been widely used for many years as one of the main coating technologies. Different metal oxides, fluorides, selenides, and sulfides can be successfully coated using this technology.

Thermal evaporation (TE) is an even older deposition technology that emerged at the end of the 19th century. Its principle relies on the direct heating of a material-filled boat (made from molybdenum, tantalum, tungsten, etc.) by a high electrical current. This causes the material to melt, evaporate, and condense on the substrates. TE is commonly used for the evaporation of metal fluorides and metals; it remains the primary technology for producing dielectric coatings in the vacuum UV (VUV) to deep UV (DUV) range, as well as for metallic mirrors.

  • Main advantages
  • Disadvantages
  • Cost effectiveness
  • Evaporation of metal fluorides (coatings for the DUV range), metal selenides, and sulfides (coatings for the IR range)
  • High LIDT in the UV spectral range
  • Low stress and porosity of coatings allow for a wide choice of substrate materials (including soft, sensitive, and exotic crystals)
  • Low energy (~0.1 eV) of condensing particles, resulting in considerably porous coatings, which can absorb moisture and be sensitive to environmental conditions (humidity and temperature).
  • The need to heat the substrates makes deposition on temperature-sensitive substrates challenging.
  • The typically polycrystalline microstructure and layer porosity can result in significant interface and surface roughness, leading to additional scatter losses.
  • Due to many process variables, changes in spatial vapour plume distribution, and the non-uniform vacuum-air shift of different coating layers, achieving coatings with complex spectral properties is limited.

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