Chucks for mounting and retaining semiconductor wafers during processing are described, particularly suited for wafer processing involving total immersion of the wafer-chuck structure in a liquid. Chuck structures are disclosed for preventing or hindering processing chemicals from contacting and contaminating large portions of the underside of the wafer undergoing processing, limiting such chemical contact to readily cleaned, relatively small annular regions on the periphery of the wafer. Embodiments include structures with supplemental gas flows on the underside of the wafer as well as the creation of gas/liquid meniscusci to prevent chemical penetration of the wafer's underside. Methods of processing semiconductor wafers employing such chucks are also described.