A highly selective photoresist ashing process performed at low temperature using a downflow plasma consisting of a carbon tetrafluoride/oxygen (CF4/O-2) gas mixture was developed for the fabrication of thin film transistor liquid crystal displays (TFT-LCDs). Although the ashing rate was increased by using the CF4/O-2 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 NK, 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 NH, gas to CF4/O-2 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 NH, gas that was added to CF4/O-2 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. (C) 2001 The Electrochemical Society. All rights reserved.