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/O₂) 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 kΩ by the addition of ammonia (NH₃) gas into the CF₄/O₂ 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/O₂ gas mixture plasma obstructed the etching of the a-Si:H films by F. On the basis of plasma analysis results for the CF₄/O₂/NH₃ gas mixture, a possible mechanism for low damage to a-Si:H films was proposed.