a polishing apparatus includes a rotatable head assembly including a substrate retainer that incorporates a cavity in the face surface of the substrate retainer for temporarily holding polishing slurry during operation of the polishing apparatus . the cavity resides in the face surface of the substrate retainer at a location adjacent the perimeter surface of the retainer . during operation of the polishing apparatus , slurry flowing along the surface of the polishing pad flows into the cavity where a portion of the slurry is temporarily held . as the head assembly of the polishing apparatus rotates against the polishing pad , slurry continuously flows from the cavity across the polishing pad and is uniformly distributed across the exposed surface of the substrate being polished . an offset in the cavity wall permits used slurry to flow away from the substrate retainer during rotation of the head assembly .