Effect of radical species density and ion bombardment during ashing of extreme ultralow-$κ$ interlevel dielectric materials

The significance of ion impact and radical species density on ash-induced modification of an extreme ultralow-$κ$ interlevel dielectric (ILD) material $(κ<2.0)$ in a patterned single damascene structure exposed to $Ar∕O2$ and $Ar∕N2$ dual frequency capacitive discharges is determined by combining plasma diagnostics, modeling of the ion angular distribution function, and material characterization such as angle resolved x-ray photoelectron spectroscopy. Radical species density was determined by optical emission actinometry under the same conditions and in the same reactor in a previous study by the present authors. ILD modification is observed and correlated with changes in the plasma for a range of pressures $(5–60mTorr)$⁠, bias powers $(0–350W)$⁠, and percent Ar in the source gas (0%, 85%). For the $Ar∕O2$ discharge, extensive modification of the ILD sidewall was observed for significant ion scattering conditions, whereas minimal modification of the ILD sidewall was observed under conditions of minimal or no ion scattering. Further, for an identical increase in the O-radical density (∼ an order of magnitude), a different degree of modification was induced at the ILD trench bottom surface depending on whether pressure or percent Ar was used to increase the radical density. The different degrees of modification seemingly correlated with the relative changes in the ion current for increasing pressure or percent Ar. For the $Ar∕N2$ discharge, reduced damage of the ILD sidewall and trench bottom surfaces was observed for increasing pressure (increasing N-radical density) and decreasing ion current to both surfaces. It is, thus, proposed that the mechanism for modification of the porous ILD is dominated by the creation of reactive sites by ion impact under the present conditions. A detailed discussion of the results which support this proposal is presented.