Grass formation and inverse aspect ratio dependent etching in SF6/O2/He plasma etching of silicon trenches: Synergistic control governed by CD effect via SiOxFγ passivation

This study systematically investigated the synergistic regulation mechanism of the SiOxFγ passivation layer on grass formation evolution and the inverse aspect ratio dependent etching (inverse ARDE; ARDE = aspect ratio dependent etching) effect in different critical dimension (CD) silicon trenches under SF6/(SF6 + O2) ratio modulation during SF6/O2/He-based plasma etching. By designing integrated wide (CD = 1.45 μm) and narrow (CD = 0.45 μm) trench patterns and systematically tuning the SF6/(SF6 + O2) ratio (33%–40%) and radio frequency power, the morphology evolution mechanism of silicon trenches—dominated by the SiOxFγ passivation layer’s dynamic equilibrium—was uncovered. Research observations showed that as the SF6/(SF6 + O2) ratio increased, “grass formation” in wide/narrow trenches transitioned regularly: “no grass—grass only in narrow trenches—grass only in wide trenches—no grass.” Concurrently, the etching rate shifted from the inverse ARDE to ARDE. Mechanistic analysis showed that the differential response of different CD trenches to the passivation layer’s “formation-consumption” equilibrium is central to this phenomenon: Narrow trenches, with weaker bottom passivation than wide ones, are more prone to localized passivation consumption and earlier grass formation. Further analysis showed that power enhancement not only intensified ion bombardment to increase microtrench depth and grass height but also promoted gas dissociation and ion deflection, which induced the redeposition of SiOxFγ to form micromasks, extending the grass formation window to higher SF6 concentrations. This study enhances the understanding of how the CD effect regulates the passivation layer to control grass formation and the inverse ARDE effect, laying a theoretical foundation and providing process guidelines for wafer-level selective topography control—such as the in situ fabrication of light-trapping structures or smooth areas.