US Patent 7727366 Balancing pressure to improve a fluid seal

A method and apparatus for fluid sealing a workpiece retained by a workpiece holder are described. A pressure differential can be formed across a fluid seal to counteract fluid attempting to penetrate the fluid seal, which can contaminate the underside of the workpiece. The apparatus can include a ring forming a barrier to fluid entry with the workpiece and a source providing pressure to form the pressure differential. The pressure or the pressure differential can counteract hydroscopic fluid pressure or hydrostatic fluid pressure that is acting to force fluid through the barrier between the ring and the workpiece.