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 plasma consisting of a carbon tetrafluoride/oxygen (CF4/O- 2) gas mixture was developed for the fabrication of thin film transistor li quid crystal displays (TFT-LCDs). Although the ashing rate was increased by using the CF4/O-2 gas mixture plasma, the etching selectivity for underlyi ng amorphous silicon (a-Si:H) films containing hydrogen decreased. The etch ing rate of a-Si:H films was decreased by the addition of ammonia (NH3). Si nce the etching rate of a-Si:H films decreased to zero at NK, flow rates hi gher than 15 standard cubic centimeters per minute, an infinitely high etch ing 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. Re action 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 fl uorine (F) radicals, resulting in the high selectivity. It was found that t he 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 cons idered to be an ammonium salt such as (NH4)(2)SiF6. (C) 2001 The Electroche mical Society. All rights reserved.