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Thin films, surfaces and nanomaterials

What is the main reason vacuum evaporation is carried out in high vacuum (for example, around 10⁻⁴ Pa)?

  1. ATo make the mean free path much longer than the source-to-substrate distance, so evaporated atoms reach the substrate unscattered
  2. BLowering the pressure raises the melting point of the source, suppressing bumping during heating and keeping the evaporation rate steady
  3. CWithout convection in vacuum the substrate cools less easily, which improves the crystallinity of the film
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Answer: A. To make the mean free path much longer than the source-to-substrate distance, so evaporated atoms reach the substrate unscattered

The mean free path is inversely proportional to pressure; for air at room temperature it is a few millimeters at 1 Pa and tens of meters at 10⁻⁴ Pa. If the mean free path is much longer than the distance from source to substrate (tens of centimeters to about 1 m), evaporated atoms reach the substrate in straight lines with almost no collisions with residual gas. Contamination of the film by residual oxygen and water is also prevented. Lowering the pressure barely changes the melting point, and evaporation actually becomes easier because the surrounding pressure is lower. In vacuum there is no convective heat exchange, but improving film crystallinity is not the main purpose of high vacuum.

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