Abstract: The invented apparatus is a relatively small-volumed chamber useful for processing a substrate. The apparatus includes a reference member with a substantially flat surface. The apparatus also includes a stage member with a surface that supports the substrate, and that has a gas bearing surrounding this support surface. Gas flows through the bearing are regulated to generate a seal of the substrate from ambient gases upon bringing the gas bearing close to the reference member's flat surface. The seal generated by the gas bearing can also be used to contain process gas in proximity to the substrate. Such process gas can be introduced into and exhausted from the chamber through an inlet and outlet, respectively, defined in the reference member. The apparatus can include a window fixed in the reference member. Patterned light or a particle beam can be directed through the window to the contained substrate to cause selective reactions to occur thereon.