Investigations of surface reactions during C2F6 plasma etching of SiO2 with equipment and feature scale models

During fluorocarbon plasma etching of SiO2, a polymer passivation layer is generally deposited on the surface of the wafer. The polymer layer regulates the etch by limiting the availability of activation energy and reactants, and providing the fuel for removal of oxygen. To investigate these processes, a surface reaction mechanism for fluorocarbon plasma etching of SiO2 has been developed. The mechanism describes the polymerization process as resulting from neutral sticking, ion sputtering, F atom etching, and low-energy ion assisted deposition. The etch mechanism is a multistep passivation process which results in consumption of both the polymer and the wafer. The surface mechanism was incorporated into an equipment scale simulator to investigate the properties of SiO2 etching in an inductively coupled C2F6 discharge, and predicts that the SiO2 etch rate saturates at high substrate biases due to the depletion of passivation. Experimental results for SiO2 etch rates and selectivity of SiO2 over Si as a function of substrate bias were well reproduced. The blanket reaction mechanism was also employed in a feature scale simulator to investigate high aspect ratio (HAR) trench topography. Results from the feature scale model showed that strong sidewall passivation leads to tapered profiles in HAR SiO2 etching. The incident ion energy and the ratio of the passivating neutral to ion fluxes largely determine the degree of the taper or bowing. Profile control can be obtained by regulating this ratio, with one such method being argon dilution.