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, the rotary chuck being adapted to hold a wafer shaped article thereon. A lid is secured to an upper part of the closed process chamber, the lid comprising an upper plate formed from a composite fiber-reinforced material and a lower plate that faces into the process chamber and is formed from a chemically-resistant plastic.