Relationship between formation of surface-reaction layers and flux of dissociated species in C4F8/Ar plasma for SiO2 etching using pulsed-microwave plasma

Abstract

The mechanism of highly selective etching of SiO2 using pulsed-microwave electron-cyclotron-resonance plasma was investigated by analyzing the relationship between plasma dissociations and fluorocarbon layers formed on surfaces during etching with a cyclo-C4F8/Ar gas mixture. Dissociated molecules of CxFy and CFx species were measured without fragmentations using ion attachment mass spectrometry, and both thicknesses and atomic concentrations of reaction layers formed on etched surfaces were analyzed using x-ray photoelectron spectroscopy. Thus, the impact of CxFy molecules on the formation of fluorocarbon layers were analyzed using this measurement system. The authors found that the process window of highly selective etching of SiO2 over Si was enlarged by using pulsed-microwave plasma because a thinner fluorocarbon layer was formed by controlling C4F8 dissociation by changing the duty cycle of the pulsed-microwaves. With conventional continuous plasma, an etch stop occurred at low wafer bias conditions because a thicker fluorocarbon layer, which protects the SiO2 surface from the ion bombardment, was formed on the SiO2 surface. The thicker fluorocarbon layer was formed from a large amount of CxFy species, such as C2F2, which were generated in the highly dissociated continuous plasma. On the contrary, with pulsed plasma, a thinner fluorocarbon layer was formed due to the lower flux of CxFy species because the dissociation of C4F8 was controlled by reducing the duty cycle of the pulsed-microwave plasma. As a result, the process window was enlarged to the low wafer bias conditions using the pulsed-microwave plasma.