A method and apparatus for concentrating plasma on a substrate surface during processing

A chuck assembly (10) for supporting a substrate (14) during processing in a plasma chamber, the substrate having a backside and a top side, the chuck assembly including a platform (12) having an upper portion (15) which has a substrate support surface (18) for receiving and supporting the substrate during processing, the support surface having an area that is substantially equal to the area of the backside of the substrate that is to be placed thereon; and an annular dielectric ring (30) around the upper portion of the platform, the annular ring having a central bore (34) that conforms to and is slightly larger than the outer perimeter of the upper portion (15) of the platform (12), the dielectric ring being made of a dielectric material having a dielectric constant that is less than the dielectric constant of the substrate.