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

Abstract

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