Photoresist ashing process using carbon tetrafluoride gas plasma with ammonia gas addition

A low-damage photoresist ashing process was developed for the fabrication of thin-film transistor liquid-crystal displays (TFT-LCDs). This process utilizes a downflow plasma using a carbon tetrafluoride/oxygen (CF4/O-2) gas mixture at room temperature. Although this process simultaneously achieves a high ashing rate and a low etching rate for an underlying amorphous silicon (a-Si:H) film containing hydrogen (H), contact resistance increases. We achieved contact resistances of less than 2 k Omega by the addition of ammonia (NH3) gas into the CF4/O-2 gas mixture plasma. The ratio of reactive fluorine radicals (F) to argon atoms (Ar) decreased with increasing NH3 gas flow rate and became less than 0.7 at the NH3 gas flow rate higher than 15 sccm. Reaction products formed on a-Si:H films by the addition of NH3 gas to the CF4/O-2 gas mixture plasma obstructed the etching of the a-Si:H films by F. On the basis of plasma analysis results for the F-4/O-2/NH3 gas mixture, a possible mechanism for low damage to a-Si:H films was proposed.