Structural devices
Structural devices

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Structural devices

7.3.2 Evaporation

The simplest vacuum deposition technique is to heat an ingot of metal in a crucible under vacuum, releasing metal vapour that coats everything in its path. This can be done either under high vacuum (< 0.1 mbar), in which case only surfaces in a line of sight from the source will be coated, or in a low-pressure atmosphere, when the vapour is scattered by gas atoms and can go around corners, making batch processing of multiple wafers possible.

This rather simple technique is fast and cheap, and with wafer rotation and a well-designed source can give good uniformity. Therefore, it is commonly used for optical coatings and similar continuous thin metal films. However, as with all techniques where material is deposited from a distant source, step coverage is imperfect.

Evaporation is difficult to control, as vapour pressure depends exponentially on temperature around the boiling point, and this is one of several reasons that it is not suited to automation. It can, obviously, be used only for materials that can be evaporated and recondensed without changing their composition, which in practice means only pure metals with accessible boiling points, such as aluminium and gold.

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