Evidence for low-energy ions influencing plasma-assisted atomic layer deposition of SiO₂: Impact on the growth per cycle and wet etch rate

This work provides evidence that plasma-assisted atomic layer deposition (ALD) of SiO₂, a widely applied process and a cornerstone in self-aligned multiple patterning, is strongly influenced by ions even under mild plasma conditions with low-energy ions. In two complementary experimental approaches, the plasma ALD of SiO₂ is investigated with and without the contribution of ions. The first set of experiments is based on microscopic cavity structures, where part of the growth surface is shielded from ions by a suspended membrane. It is observed that a lower growth per cycle (GPC) and a better material quality are obtained when an ion contribution is present. Without any ion contribution, a GPC of 1.45 ± 0.15 Å/cycle and a wet etch rate of 4 ± 1 nm/s (in 30:1 buffered HF) are obtained for a deposition temperature of 200 °C. With an ion contribution, these values decrease, where the magnitude of the decrease appears to be determined by the supplied ion energy dose. For extended ion doses, the GPC decreases to 0.85 ± 0.05 Å/cycle and the wet etch rate to 0.44 ± 0.09 nm/s, approaching the value for a thermal oxide. The important role of ions is confirmed by the second experimental approach, which is based on ion-selective quartz crystal microbalance measurements. By these results, it is demonstrated that ions have a stronger impact on the plasma ALD of SiO₂ than usually considered, providing essential insights for tailoring the film growth.