Uranium dioxide reaction in CF4/O2 RF plasma

Research on the fluorination reaction of UO2 in CF4/O2 RF plasma is carried out at temperatures of up to 370 C under total pressure of 0.3 Torr. The reaction rates are investigated as functions of CF4/O2 ratio, plasma power, substrate temperature, and exposure time to the plasma. It is found that there exists an optimum CF4/O2 ratio of around four for the efficient etching regardless of RF power and substrate temperature. According to the mass spectrometry it is revealed that the major reaction product is uranium hexa-fluoride UF6. Some minor species such as UF4 and UF5 are probably generated parasitically. The highest etching reaction rate at 370 C under 150 W exceeds 1000 monolayers/min, which is equivalent to 0.4 μm/min. Based on the experimental findings, dominant overall reaction of uranium dioxide in CF4/O2 plasma is determined: UO2+3/2CF4+3/8O2=UF6+3/2CO2-x, where CO2-x represents the undetermined mix of CO2 and CO. The overall reaction follows a linear kinetics and is thus rate-determined by the surface reaction between the uranium atom in UO2F2 intermediates on the surface and incoming fluorine atoms or fluorine containing radicals. The activation energy of this reaction is derived as 12.1 kJ/mole. (orig.)