Observation of Liquid Movement during PVA Brush Deformation in Post-CMP Cleaning

While contact cleaning with polyvinyl acetal (PVA) brushes in post-chemical mechanical planarization (CMP) processes has the advantage of high cleaning efficiency, it increases the possibility of cross-contamination from the brushes onto the wafers. This study investigated the liquid movement from or into the brushes during its deformation, which significantly influences the removal and reattachment of contaminants. We observed liquid movement due to brush deformation and compared the velocity with theoretical estimation to obtain insights into liquid replacement. First, we measured the liquid velocity around a vertically deformed cylindrical brush. Then, we visualized the liquid flow from three brush deformation types commonly observed in actual cleaning. Experiments with simple uniform deformations demonstrate that the induced velocity was consistent with theoretical estimates, indicating the time-reversible flow around the brush. Contrarily, at local deformation, i.e., edge compression, a significant amount of liquid remained outside the brush after shape recovery, indicating liquid replacement of the liquid inside the brush. Finally, we discuss the relationship between liquid exchange and cleaning or cross-contamination by creating a liquid movement map based on the volume changes observed in the nodule contact path.