an apparatus for processing wafer - shaped articles comprises a closed process chamber providing a gas - tight enclosure . a rotary chuck is located within the closed process chamber , and is adapted to hold a wafer shaped article thereon . a lid is secured to an upper part of the closed process chamber . the lid comprises an annular chamber , gas inlets communicating with the annular chamber and opening on a surface of the lid facing outwardly of the closed process chamber , and gas outlets communicating with the annular chamber and opening on a surface of the lid facing inwardly of the closed process chamber .