Uranium dioxide reaction in CF4/O-2 RF plasma

Research on the fluorination reaction of UO2 in CF4/O-2 RF plasma is carrie d out at temperatures of up to 370 degrees C under total pressure of 0.3 To rr. The reaction rates are investigated as functions of CF4/O-2 ratio, plas ma power, substrate temperature, and exposure time to the plasma. It is fou nd that there exists an optimum CF4/O-2 ratio of around four for the effici ent etching, regardless of RF power and substrate temperature. According to the mass spectrometry it is revealed that the major reaction product is ur anium hexa-fluoride UF6. Some minor species such as UF4 and UF5 are probabl y generated parasitically. The highest etching reaction rate at 370 degrees C under 150 W exceeds 1000 monolayers/min, which is equivalent to 0.4 mu m /min. Based on the experimental findings, dominant overall reaction of uran ium dioxide in CF4/O-2 plasma is determined: UO2 + 3/2CF(4) + 3/8 O-2 = UF6 + 3/2CO(2-x), where CO2-x represents the undetermined mix of CO2 and CO. T he overall reaction follows a linear kinetics and is thus rate-determined b y the surface reaction between the uranium atom in UO2F2 intermediates on t he surface and incoming fluorine atoms or fluorine containing radicals. The activation energy of this reaction is derived as 12.1 kJ/mol. (C) 1999 Els evier Science B.V. All rights reserved.