A highly selective photoresist ashing process for silicon nitride films by addition of trifluoromethane

A highly selective photoresist ashing process was developed for the fabrication of thin-film transistor liquid-crystal displays (TFT-LCDs). This ashing process utilizes downflow plasma consisting of a carbon trifluoromethane/oxygen (CHF₃/O₂) gas mixture at a low temperature. The etching selectivity of photoresist films to silicon nitride (SiN) film increased when using the CHF₃/O₂ gas mixture plasma, as compared to that when using the carbon tetrafluoride/oxygen (CF₄/O₂) gas mixture plasma. At the CHF₃ gas flow rate of 30 sccm, a high etching selectivity ratio of about 1080 for the photoresist films to the SiN films was achieved at room temperature. On the basis of surface analysis results for SiN films and plasma analysis results for the CHF₃/O₂ gas mixture, a mechanism for the high etching selectivity of the photoresist films was proposed. Reaction products that were formed on SiN films by the CHF₃/O₂ gas mixture plasma obstructed the etching of SiN films by fluorine (F) radicals resulting in the high selectivity. It was found that the CHF₃/O₂ gas mixture plasma reacted with SiN, resulting in the formation of a protective reaction product that is considered to be an ammonium salt such as (NH₄)₂SiF₆.