A downstream microwave plasma processing apparatus useful in fabricating an integrated circuit semiconductor device includes a waveguide, a microwave transmitting window perpendicular to a microwave electric field in the waveguide, a plasma generating chamber below the window and a reaction chamber separated from the plasma generating region by a gas-porous microwave shield. The microwave energy is transmitted into the plasma generating chamber through the microwave transmitting window, and generates a plasma which is confined therein by the shield. Radicals of a short-lived reactive gas, generated in the plasma, pass through the shield and impinge onto a workpiece placed in the reaction chamber. Uniform and effective downstream plasma etching or ashing is produced.