For precise control of a plasma process, the condition of the plasma must be monitored during the ongoing process. Generally, two methods are available:
- Spectroscopic monitoring:
A low-pressure plasma emits the colour typical for the process gas , i.e. the wavelength of the electromagnetic radiation. The colour changes with the increasing proportion of residual gases. This means that additional wavelengths (spectral lines) are observed in a spectroscopic measurement. The intensity of the radiation changes with the pressure of the low-pressure plasma.
It can be normal that the spectrum of a plasma changes over the course of the process because reaction products accumulate in the plasma. However, checks are required to ensure that the changes do not exceed the specified tolerances.
- Measurement and monitoring of pressure, gas flow, voltage, time, and other process parameters.