Determining an optimal ion energy for plasma processing of a dielectric substrate

An ion energy for plasma processing of a dielectric substrate (109) is determined by exposing the dielectric substrate (109) to a plasma discharge and applying a pulsed voltage waveform to the dielectric substrate. The pulsed voltage waveform comprises a sequence of pulses, each pulse comprising a higher voltage interval and a lower voltage interval having a voltage slope. Further, first pulses of the sequence having differing voltage slopes between one another are generated and applied to the dielectric substrate. For each one of the first pulses, the voltage slope (S) and an output current (I P ) corresponding to the voltage slope are determined. For each one of the first pulses, at least one coefficient (k, b) of a mathematical relation between the voltage slope and the corresponding output current based solely on the voltage slope and the output current determined for one or more of the first pulses is determined. A test function is applied to the at least one coefficient and an optimal voltage slope value corresponding to the at least one coefficient making the test function true is selected. An apparatus for plasma processing of a dielectric substrate implements the above method.