Plasma etching of high aspect ratio features in SiO2 using Ar/C4F8/O2 mixtures: A computational investigation

Plasma etching of high aspect ratio (HAR) features, typically vias, is a critical step in the fabrication of high capacity memory. With aspect ratios (ARs) exceeding 50 (and approaching 100), maintaining critical dimensions (CDs) while eliminating or diminishing twisting, contact-edge-roughening, and aspect ratio dependent etching (ARDE) becomes challenging. Integrated reactor and feature scale modeling was used to investigate the etching of HAR features in SiO2 with ARs up to 80 using tri-frequency capacitively coupled plasmas sustained in Ar/C4F8/O2 mixtures. In these systems, the fluxes of neutral radicals to the wafer exceed the fluxes of ions by 1–2 orders of magnitude due to lower threshold energies for dissociation compared with ionization. At low ARs (<5), these abundant fluxes of CFx and CxFy radicals to the etch front passivate the oxide to form a complex which is then removed by energetic species (ions and hot neutrals) through chemically enhanced reactive etching, resulting in the formation of gas phase SiFx, COx, and COF. As the etching proceeds into higher ARs, the fractional contribution of physical sputtering to oxide removal increases as the fluxes of energetic species to the etch front surpass those of the conduction constrained CFx and CxFy radicals. The instantaneous etch rate of oxide decreases with increasing aspect ratio (ARDE effect) due to decreased fluxes of energetic species and decreased power delivered by these species to the etch front. As the etch rate of photoresist (PR) is independent of AR, maintaining CDs by avoiding undercut and bowing requires high SiO2-over-PR selectivity, which in turn requires a minimum thickness of the PR at the end of etching. Positive ions with narrow angular distributions typically deposit charge on the bottom of low AR features, producing a maximum in positive electric potential on the bottom of the feature. For high AR features, grazing incidence collisions of ions on sidewalls depositing charge produce electric potentials with maxima on the sidewalls (as opposed to the bottom) of the feature.