Effect of nonvertical ion bombardment due to edge effects on polymer surface morphology evolution and etching uniformity

Abstract

Maintaining uniform sample etching during a plasma process is a critical requirement for applications in large-scale wafer processing. The interface between the plasma and the sample surface is defined by the plasma sheath, which accelerates ions toward the sample surface. In areas where the plasma sheath is not parallel to the sample surface, such as near the sample edges, the incident ions arrive at shallower, off-normal angles that can result in a greater etch yield relative to other areas of the sample. This phenomenon leads to nonuniform etching, along with characteristic surface morphology evolution. In this work, we utilized a combination of spatial ellipsometry for etching behavior, atomic force microscopy (AFM) for surface morphology evolution, and power spectral density (PSD) analysis to quantify the extent and spatial dependence of the nonuniform etching near the sample edges. The spatial ellipsometry indicated that a region extending for about 1000 μm from the sample edge experiences approximately 10%–15% more thickness loss (∼10–15 nm) relative to areas near the center of the sample under the tested processing conditions. Within this area, the greatest rate of change in the sample thickness occurs within 5 Debye lengths or ∼300 μm from the sample edge. Via AFM analysis, we detected the presence of ripple features that are consistent with directional ion impacts caused by deflection of ions from normal incidence on the surface morphology [Merkulov et al., Appl. Phys. Lett. 80, 4816 (2002)] AFM scans performed in two different sample orientations confirmed that the ripple features are oriented perpendicular to the direction of incident ions and propagate along the direction of the incident ions. Correspondingly, the magnitude of surface roughness decreases as the distance from the sample edge increases. The ripple features were quantified via PSD analysis, which found the presence of a greater population of long-wavelength roughness closer to the sample edge. The findings of this study provide insight into the influence of the plasma sheath distortions near sample edges on the extent of nonuniform sample etching and characteristic surface morphology evolution in plasma etching applications.