Structural devices
Structural devices

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Structural devices

7.3.3 Plasmas

More control can be achieved in vapour deposition if a plasma is generated. A plasma is simply a gas where a proportion of the molecules have been ionised. The ions remain in an uneasy equilibrium with the electrons they have released, prevented from recombining only because the electrons are hot and fast-moving, and so are difficult to trap.

Plasmas are widely used in materials processing, with pressure ranging from 10−3 mbar to 1 mbar and typically up to 1% of the molecules ionised. Power must be continuously supplied to sustain this ionisation (either as a DC current or a radio-frequency field), adding a layer of complexity and expense to the equipment. However, this can easily be justified for one reason: charged ions can be accelerated by an electric field. Even 100 V of bias takes them to speeds equivalent to more than 1 000 000 °C of heating, so they slam into the biased surface and perform stunts of physics and chemistry that could not be achieved in any other way.

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