Highly Selective Photoresist Ashing by Addition of Ammonia to Plasma Containing Carbon Tetrafluoride

A highly selective photoresist ashing process performed at low temperature using a downflow plusma consisting of a carbon tetrafluoride/oxygen (CF₄/O₂) gas mixture was developed for the fabrication of thin film transistor liquid crystal displays (TFTLCDs). Although the ashing rate was increased by using the CF₄/O₂ gas mixture plasma, the etching selectivity for underlying amorphous silicon (a-Si:H) films containing hydrogen decreased. The etching rate of a-Si:H films was decreased by the addition of ammonia (NH₃). Since the etching rate of a-Si:H films decreased to zero at NH₃ flow rates higher than 15 standard cubic centimeters per minute, an infinitely high etching selectivity for the photoresist films was achieved at room temperature. On the basis of the surface analysis results for a-Si:H films, a mechanism for the high etching selectivity of the photoresist films was proposed. Reaction products that were formed on a-Si:H films by the addition of NH₃ gas to CF₄/O₂ gas mixture plasma obstructed the etching of a-Si:H films by fluorine (F) radicals, resulting in the high selectivity, It was found that the NH₃, gas that was added to CF₄/O₂ gas mixture plasma reacted with a-Si:H. resulting in the formation of a protective reaction product which is considered to be an ammonium salt such as (NH₄)2 SiF₆