Abstract
The corrosion characteristics of metals (Al, Ti, Cr, Fe, Co, Ni, Cu, Mo, Ta, and W), stainless steels (SUS304 and 316), and a Ni-based alloy (Alloy 600) in a downstream reactor of CF4-11%O2 plasma have been studied. The mass change and penetration depth of the specimens were measured after corrosion tests at temperatures in the range of 298 to 573K. The surfaces of the specimens were examined using Auger electron spectroscopy (AES) with Ar+ion sputtering. It was found that metals such as Ti, Cr, Mo, Ta, and W suffered from corrosion leading to a decrease in mass, and their corrosion rates increased with rising test temperature. At 573K, Fe and the stainless steels showed an increase in mass, while Al, Co, Ni, Cu, and Alloy 600 no change in mass. The results of AES analysis exhibited that the corroded surface of Cr, Mo and Ta was covered with a thin film of parent metal oxyfluorides, and that of W with a thin oxide film. Thin films of metal fluorides were formed on Co, Ni and Cu. Thicker films consisting of metal fluorides and oxyfluorides existed on Fe and stainless steels. In the downstream environment of CF4/O2 plasmas, metal fluorides are thermodynamically favored for all the metals examined. The vapor pressures of MoF6, TaF5 and WF6 in the temperature range of 298 to 573K are high enough to evaporate when they are formed. Therefore, the corrosion of metals and alloys in the downstream environment of CF4/O2 plasmas is primarily controlled by the rates of reactions with fluorine atoms, which are generated in the plasmas, and the vapor pressures of metal fluorides formed by the reactions.
