In order to develop charging-free plasma processing, we investigated the function of negative charge injection to a SiO2 wafer, during etching in a pulsed two-frequency capacitively coupled plasma under various external conditions in CF4/Ar at low pressures, by using emission-selected computerized tomography. The formation of a negatively-charged layer by an excess negative charge in the interface close to the wafer and the degree of acceleration of negative charges were experimentally estimated through the spatiotemporal characteristics of the net excitation rate of the short-lived Ar(2p1) used as a probe. A bias pulse operation with a single positive component at the wafer during the off-phase of very high frequency (100 MHz) plasma production resulted in the most efficient formation of the negatively-charged layer close to the wafer. The influence of the external plasma conditions on the negatively-charged layer is observed in detail.
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