Photoresist removal using an O2/N2 medium pressure plasma jet with high speed wafer scanning: Unimplanted resist studies

The authors describe a plasma ashing system where a stationary jet of hot, activated gases removes photoresist from a scanning wafer. The jet is created by a reactant stream flowing through a 2.45 GHz surface wave discharge in a 6 mm quartz tube. For O-2/N-2 plasmas in the medium pressure range from 20 to 100 Torr, a luminous plasma jet emerges from the end of the discharge tube that transports both heat and reactive species to the wafer. A single scan results in a Gaussian track profile with a standard deviation of 7 mm for the source-to-substrate distance of 9 mm. A simple model of the ashing process, which assumes a thermally activated ash rate and Gaussian distributions for both power density and reactant flux, unifies the dependence of effective ash rate on the substrate temperature and scan speed at a constant power. The best fit activation energy at 2.5 kW is 0.23 eV, about half of the value found in conventional downstream ashing, implying that diffusion plays a significant role in limiting the ash rate. The peak thermal power density in a 2.5 kW jet at 80 Torr is 160 W/cm(2), resulting in an effective instantaneous ash rate of 2.5 mm/min for a scan speed of 70 cm/s and 200 degrees C chuck temperature. This implies that the time to clear a 1.2 mu m thick resist coating from a 300 mm wafer is 18 s.