By Ed Korczynski, Senior Editor
New Way Air Bearings, Aston, PA, has created a new “vacuum chamber stage” demo system which could have application in lithography, ion implant, mask-writing, and metrology processes requiring high precision in deep vacuum. Essentially, instead of putting a precision stage into a vacuum, the vacuum is put into the precision stage — so the chamber volume is little more than the size of the wafer, and all mechanical linkages are outside the chamber.
Methods to keep the mechanization outside of the vacuum chamber in conventional designs include linear and rotary contact seals, rotary Ferro-fluidic seals, and expanding and contracting bellows. Air-bearing structures have also been used to support some sort of a moving member through an aperture in the vacuum chamber wall. However, all such large vacuum chambers tend to have problems with particle generation, component out-gassing, or even simply the large surface areas of chamber walls.
In contrast, this novel small process-chamber design moves with the wafer chuck and is held under vacuum with porous air-bearing technology. Unlike conventional orifice air bearings, porous media controls the airflow across the entire bearing surface through millions of holes in the material, resulting in more consistent air pressure distribution and the ability to create a vacuum down to ~10-8 torr. The very small chamber volume results in minimal pump-down time, requiring relatively smaller and less costly vacuum pumps, and theoretically less expense to build and support systems in manufacturing.
The chamber air bearing surface-pumped grooves bear directly on the plate to which the optics, ion source, or electron source would mount. Since the stage itself would be guided by its topside, running directly on the underside of the base to which the energy source is bolted, the structural loop would be reduced to the thickness of the 2D plate as opposed to that of a large 3D vacuum chamber. Guidance of the stage would be achieved with an annular air bearing separated from the vacuum section of the stage by differentially pumped groves.
This technology could provide strong advantages for immersion lithography, since the differentially pumped vacuum grooves could be used to keep the liquid physically constrained above the wafer during processing. Many reported problems with immersion lithography involve peeling resists, bubbles, and particulates that seem to be induced by the movement of the wafer into and out of the liquid. Liquid flow though the chamber for filtering and/or replenishment can occur through the main “vacuum” ports. — E.K.