Fluorination reaction of uranium dioxide in CF4/O2/N2 r.f. plasma

Research on the fluorination reaction of UO2 in CF4/O2/N2 r.f. plasma is carried out under 0.3 Torr total gas pressure. The reaction rates are investigated as functions of the CF4/O2/N2 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 the r.f. power and substrate temperature. The reason that the etching reaction rate reaches the maximum at the optimum gas composition is explained by the optical emission spectroscopy (OES) results showing that fluorine atoms are produced most abundantly in the 80%CF4/20%O2 discharge plasma. The highest etching reaction rate at 370 °C under 150 W exceeds 1000 mono-layers/min, which is equivalent to 0.4 μm/min. According to the mass spectrometric analysis, the major reaction product turns out to be uranium hexa-fluoride UF6. Based on the experimental findings, the dominant overall reaction of uranium dioxide in the plasma is determined as UO2+3/2 CF4+3/8 O2=UF6+3/2 (CO or CO2). It is found that this reaction seems to follow a linear kinetics with the activation energy of 12.1 kJ/mol. X-ray photoelectron spectroscopy (XPS) analysis confirms UO2F2 formation on the surface during the reaction and the two step reaction paths: UO2 →UO2F2 →UF6. Experimental results in the CF4/O2/N2 discharge plasma show that the reaction rate enhances with the addition of the third gas in the binary plasma. The ratio of the reaction rate in the ternary to that in the binary plasma saturates twice as much when the volumetric flow rate of the gas reaches 5% flow rate of CF4. It is also found that the reaction rates increase in a linear proportion to the applied plasma power in both the binary and ternary gas discharges and the proportionality in the ternary gas plasma is larger than that in the binary one.