Material-dependent submicrometer particle trapping in capacitively-coupled plasma sheaths in an intermediate collision regime

Understanding submicrometer particle behavior in non-thermal capacitively coupled plasmas (CCPs) is important in the application of CCP reactors in thin-film vapor deposition; nucleated and resuspended particles can deposit on thin films, forming defects. Prior studies of supermicrometer particle behavior in CCP reactors have revealed that particles are trapped in the pre-sheath or sheath regions near electrodes, but have examined in detail neither the trapping of submicrometer particles, nor the influence of particle material properties on trapping. Using laser light scattering (LLS), we examined trapping of submicrometer metal oxide particles (radii in the 211 nm-565 nm range) of 6 distinct material compositions in the pre-sheath/sheath region of a CCP reactor operated at pressures in the 0.5-2.0 Torr range. We specifically focus on trapping near the upper electrode of a horizontally-oriented reactor. In this instance, trapping is brought about by a balance between electrostatic forces and gravitational forces driving particles away from the electrode, with ion drag forces driving particles toward the electrode. LLS measurements reveal that submicrometer particles are trapped near the upper electrode for all particle sizes, types, and operating pressures, with the trapping location at an increased distance away from the electrode with decreased CCP reactor pressure. Interestingly, we find the trapping location shifts slightly farther from the top electrode with increasing material dielectric constant. This suggests that the ion drag force is influenced by particle material properties, though in an unclarified manner. Measured trapping locations are also compared to model predictions where particle charge levels and the ion drag force are calculated using expressions based on ion trajectory calculations in a plasma sheath accounting for ion-neutral collisions. Predicted ion densities required for trapping are a factor of 6-16 higher than calculated at the observed particle trapping locations when applying a dissipative ion-particle encounter model, with more substantial disagreement found when considering a non-dissipative encounter model. In total, our results confirm that submicrometer particle trapping occurs at the upper electrode of CCP reactors, which must be facilitated by a balance largely between electrostatic and gravitational forces opposed by ion drag forces, but suggest future studies will be required to understand how particle material properties affect forces on particles on the plasma volume boundary, and how the ion drag force is sufficiently high to facilitate trapping.

Automated summary

This study used laser light scattering to investigate trapping of submicrometer metal oxide particles near the upper electrode of a CCP reactor. The trapping location moved further from the electrode with decreased reactor pressure, and particle trapping appeared to be influenced by the material dielectric constant. Further research is needed to understand how particle material properties affect forces on particles in plasma and the role of ion drag forces in trapping.