Effects of 'step-like' amplitude-modulation on a pulsed capacitively coupled RF discharge: an experimental investigation

We present measurements of the time evolution of plasma and electrical parameters in a pulsed capacitively coupled argon discharge operated at a radio frequency of 12.5 MHz, whose amplitude is 'step-up' and 'step-down' modulated. The 'step-up (-down)' amplitude-modulated waveform consists of three segments, i.e., a low (high)-voltage, a high (low)-voltage, and a zero-voltage stage. Here, we focus on the effect of the ratio (zeta = V (L)/V (H) <= 1) of the low-(V (L)) to high-voltage (V (H)) amplitude (measured at the end of the respective segment) on the time evolution of discharge parameters. We monitor the behavior of the discharge by measuring (i) the optical emission intensity (OEI) of a selected Ar-I spectral line, (ii) the electron density at the center of the plasma (using a hairpin probe) as well as (iii) the electrical characteristics (by voltage and current probes). It is found that at relatively large zeta (i.e., at low disparity between the two voltage amplitudes), for both the 'step-up' and 'step-down' cases, these parameters evolve relatively smoothly with time upon changing the voltage amplitude, and the ignition process strongly depends on the duration of the zero-voltage period. At low zeta (i.e., at high disparity between the voltage amplitudes), an abnormal evolution of the parameters can be observed during the low-voltage period for both cases. Specifically, the voltage amplitude and the modulus of the system impedance increase to a higher value, while the relative phase, phi (vi), between the voltage and the current approaches 90 degrees, resulting in a reduction of the power deposition and the OEI. The enhanced voltage amplitude decreases to a steady-state value, accompanied by a decline of phi (vi), and an abnormal increase of the current amplitude and the electron density after some time, of which the duration increases with the decrease of zeta. The zeta-dependent evolution of the electron density during the low-voltage period was found to significantly affect the subsequent ignition process and electron power absorption mode at the beginning of the high-voltage period.