Method of monitoring the discharge in a plasma process and monitoring device for monitoring the discharge in a plasma

A Method of monitoring the discharge in a plasma process comprises the steps: a. supplying a plasma process with a periodic power supply signal (71a); b. determining a first power supply signal waveform within a first time interval (”t) within a first period of the power supply signal; c. determining a second power supply signal waveform (31, 51) within a second time interval (”t) within a second period of the power supply signal, wherein the second time interval (”t) is at a position in the second period which corresponds to the position of the first time interval (”t) within the first period; d. comparing the second power supply signal waveform (31, 51) of the second time interval with the first power supply signal waveform of the first time interval or a reference signal waveform (50, 60), and determining a first comparison result; e. if the first comparison result corresponds to a given first comparison result, time shifting one of the second power supply signal waveform (31, 51) in the second time interval (”t), the first power supply waveform in the first time interval (”t) or the reference signal waveform (50, 60), and comparing the time shifted signal with one of the signal waveforms that were not time shifted, thereby obtaining a second comparison result.