Spatial profiles of interelectrode electron density in direct current superposed dual-frequency capacitively coupled plasmas

We present experimentally determined spatial profiles of the interelectrode electron density (n(e)) in dual-frequency capacitively coupled plasmas in which the negative direct current (dc) bias voltage (V-dc) is superposed; in the experiment, 13 MHz (P-low) was applied to the lower electrode and 60 MHz (P-high) to the upper electrode. The bulk n(e) increased substantially with increases in the external power, P-high, P-low, and with increases in V-dc. When P-low was insufficient, the bulk n(e) decreased as the V-dc bias increased. The bulk n(e) increased due to its dependence on V-dc, especially for |V-dc| > 500 V. This may correspond to the sheath voltages (V-s) of the lower electrode. The n(e) values in front of the upper electrode were coupled with the V-dc: the V-dc dependence first decreased and then increased. The dc currents (I-dc) of the upper electrode were collected when a large P-low was applied. The value of I-dc at the threshold value of V-dc approximate to V-s (e.g. -500 V) increased with an increase in n(e). When |V-dc| exceeded the threshold, the spatial n(e) profile and the I-dc dependence were changed relative to the electrical characteristics of the dc superposition; this led to a change in the location of the maximum n(e), the width of the area of n(e) depletion in front of the electrodes, and a transition in the electron heating modes.