Time-dependent Measurement of Ion Composition in Pulse-operated Ar/C₄F₈/O₂ Dual-frequency Capacitively-Coupled Plasma

In the HAR etching, transport of neutral radicals to the hole bottom becomes difficult due to large incident angle and high surface chemical reaction probability and ionic fluorocarbon becomes important as the precursor of etching reaction. In pulse-operated plasma, ion flux composition to the hole bottom might be time-varying during the pulse-on period and, as the C/F ratio influences the etching performance, control of ion composition during the pulse-on period is important. In this study, time-dependent ion composition in an Ar/C₄F₈/O₂ dual-frequency pulsed capacitively coupled plasma is measured by a mass spectrometer with an energy analyzer. For the precise time-resolved measurement, ion transit time through the mass spectrometer is carefully evaluated. By integrating energy distribution of each ionic species, ion flux to the mass spectrometer is evaluated. In the pulsed plasma, temporal variation of the ion composition, i.e., gradual increase of fluorocarbon ions after turning on the pulse, is observed. High electron temperature at a moment of plasma ignition in the pulse is considered as the origin of this phenomenon with higher Ar ionization rate rather than C₄F₈ ionization rate. Gradual change of the ion composition for ~100 ms is explained by ion diffusion loss from the plasma.