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What is vacuum evaporation (resistive evaporation and electron beam evaporation)?
18 May, 2021 2021
Author: Ali Kosari Mehr

Resistive evaporation and electron beam evaporation

In general, the vacuum evaporation process consists of two steps: First, the evaporation of materials, to be deposited, in a vacuum chamber with a pressure lower than 10-6 torr being present. Second, the condensation of evaporated materials on a substrate.

Being the most common heating source utilized for thin-film deposition, resistive heating is deployed to evaporate materials through either a filament, a boat, or a refractory crucible. It is noteworthy that these filaments are usually made of refractory metals such as Ta, Mo, and W with/without a ceramic coating. In this method, the quality of the process hinges upon two main considerations: First, it is essential that the evaporated materials form droplets adhering to the filament, owing to surface tension. Second, it is pivotal that the materials’ capability to be alloyed or to react with the filament is insignificant. It should be noted that the former case and the latter case have a rather opposing nature. Delineating this matter, one can affirm that the materials properly adhering to the filament have a greater tendency to be alloyed with the filament. It worth mentioning that metals mainly have limited capability to react with the filament chemically; the exceptions are alkali metals (i.e., potassium, sodium, etc.).

Given that refractory metals are not evaporated by means of a typical resistive heating source, an indirect source of heating is deployed for the evaporation of these metals, namely electron beam source. In this regard, crucibles typically made of quartz, graphite, alumina, beryllia, boron nitride, or zirconia are utilized for indirect heating. Moreover, there are two profound differences between electron beam sources and resistive heating sources: First, unlike resistive heating, thermal energy in electron beam evaporation is provided in the uppermost part of the materials to be deposited by means of the great kinetic energy of a high-current electron beam. Second, in electron beam evaporation, the materials to be deposited are placed in a cavity/crucible being cooled by water [1,2].

References:

  1. Wasa K, Kitabatake M, Adachi H (2004) Thin Films Material Technology: Sputtering of Compound Materials. Springer. Webpage
  2. E. B. Graper, D. Glocker, S. Shah (2017) Handbook of Thin Film Process Technology. Institute of Physics Publishing. Webpage