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There are several flammable gases used in semiconductor industries, and some oxidizing gas such as chlorine trifluoride (ClF3) is also used in the same production apparatus. As knowledge of the combustion characteristics of flammable gases with oxidizing gases other than oxygen are lacking, an experimental study to evaluate combustion characteristics was performed. Chlorine trifluoride, the oxidizing gas used in this study was commercialized recently for etching and cleaning purposes because of its powerful oxidizing ability. As flammable gases, tetraethyl orthosilicate (TEOS: Si (OC2H5)4), hydrogen (H2) and ammonia (NH3) were investigated. These gases are toxic, except for hydrogen, and have high reactivity, so these gases were fist diluted with nitrogen separately. The diluted TEOS and ammonia gases were found to ignite spontaneously while being mixed with the diluted ClF3 gas. On the other hand, the diluted hydrogen gas could be mixed with the diluted ClF3 gas without spontaneous ignition. And a H2/ClF3/N2 mixture could be ignited by an electric spark. Therefore, for TEOS and ammonia, "flammable limit" means the limit of spontaneous combustion, and for hydrogen the flammable limit means the usual limit of ignitability by an external ignition source. The flammability diagrams of TEOS/ClF3/N2, NH [3/CW3/N2 and H2/ClF3/N2 mixtures were determined. And reaction kinetics between them were discussed from observed combustion characteristics.