The Effect Of A Negative Direct-Current Voltage On Striated Structures And Electrical Parameters In A Capacitively Coupled Rf Discharge In Cf4

The effects of a negative direct-current (dc) voltage on the striated structures and the electrical parameters have been studied by multi-fold experimental diagnostics and particle-in-cell/Monte Carlo collision (PIC/MCC) simulations in a dc superposed rf (radio-frequency, 8 MHz) capacitive discharge in CF4. The electron density, the spatio-temporal distribution of electron-impact excitation rate and the electrical parameters measured by a hairpin probe, phase resolved optical emission spectroscopy (PROES) and a voltage and a current probe are compared with the corresponding simulation results. With the increase of the magnitude of the dc voltage, vertical bar V-dc vertical bar, all the experimental observations are well reproduced and analyzed by the simulations. It was found that the plasma bulk is compressed and the electron density decreases slightly at low vertical bar V-dc vertical bar, while the plasma bulk broadens at high vertical bar V-dc vertical bar when the ionization at the edge of the completely expanded sheath adjacent to the powered electrode is significantly enhanced due to the secondary electron emission (SEE). By increasing vertical bar V-dc vertical bar, the region of the strong ionization/excitation shrinks towards the edge of the collapsing sheath adjacent to the grounded electrode. And the ionization inside the bulk region is significantly suppressed during the collapsing phase of the sheath adjacent to the powered electrode. This is mainly attributed to the fact that the dc component of the bulk electric field and the spatial gradient of the electron density are simultaneously enhanced during the transition from the 'striation' mode to 'striation-gamma' hybrid mode when increasing vertical bar V-dc vertical bar. Besides, we found that increasing vertical bar V-dc vertical bar can somewhat suppress the rf power deposition at low |Vdc|, and enhance the rf power deposition at high vertical bar V-dc vertical bar. The magnitude of the dc current exhibits a complex dependence on vertical bar V-dc vertical bar, due to nonlinear change of the dc resistance of the plasma.