Electron and negative ion densities in a CW and pulsed 100 MHz capacitively coupled plasma discharge*

Capacitively coupled plasma (CCP) discharges operating at a very high frequency, 30 -300 MHz, are becoming very popular now a days due to enhanced plasma processing rates and lower damage to the substrate. This is mainly achieved due to higher plasma densities and lower electron temperature produced at higher driving frequencies. Moreover, pulsing of the discharge system is known to deliver charging-free plasma processes which is highly desirable for high-aspect-ratio plasma etching. In this study, we present electron and negative ion densities in a CW and pulsed 100 MHz CCP discharge produced in O$_{\mathrm{2}}$ and Ar/O$_{\mathrm{2}}$/C$_{\mathrm{4}}$F$_{\mathrm{8}}$ gas mixture. Electron density is determined by the Hairpin probe and negative ion density is determined by the pulse laser photo-detachment combined with Hairpin probe. Photo-detachment is performed at 532, 355 and 266 nm laser wavelengths in order to selectively photo-detach different negative ions present in the discharge. Experimental results are presented for several power (100-500 W), pressure (1-10 Pa) conditions and for several duty ratios (25 -- 75{\%}) for 1 KHz pulse repetition frequency. In CW O$_{\mathrm{2}}$ plasma, we observed a similar trend in electron and negative ion density vs power, whereas, in Ar/O$_{\mathrm{2}}$/C$_{\mathrm{4}}$F$_{\mathrm{8}}$ gas mixture an opposite trend is observed in electron and negative ion density.