In one aspect, a valve assembly adapted to seal an opening in a chamber is disclosed. Valve assembly includes a housing being adapted for coupling to a chamber surface having the opening therein, the housing including a threshold portion positioned adjacent to the chamber opening, the threshold portion having one or more inlets adapted to supply gas to an interior region of the housing adjacent to the chamber opening; and a sealing surface adapted to selectively (1) seal the opening, and (2) retract from the opening so as not to obstruct substrate passage. Numerous other system aspects are provided, as are methods and computer program products in accordance with these and other aspects.