Dynamics of Metal Etching and Oxidation in Fluorocarbon/oxygen RF Glow Discharges
Abstract
A variety of in situ diagnostics have been used to determine the dynamic behavior of both the etching and oxidation reactions of tantalum and plutonium in RF glow discharges (plasmas). Numerous aspects of the concerted, heterogeneous reactions involved have been studied with quadrupole mass spectroscopy, emission spectroscopy (actinometry), and fluoroptic thermometry. In addition, direct in situ measurement of reaction rates has been performed with a quartz-crystal microbalance. The chemistry of the CF_4/O _2 discharge and its use in etching both tantalum and plutonium has been studied. Major products from the reaction of CF_4 with O _2 within the plasma include CO _2, CO, COF_2, F _2, and F. Chemical mechanisms are suggested to account for the formation of these products. Measurement of Ta etch rates in CF_4 /O_2 and C_2 F_6/O_2 plasmas as functions of reactor pressure, applied power, temperature, and system residence time reveal numerous chemical effects. The large heat of reaction in the Ta/F etching system, in conjunction with the moderate activation energy and thermally isolated nature of the plasma environment, leads to significant autothermic effects. Fluorocarbon polymer deposition at pressures above 400 mtorr leads to a quenching of the etch reaction in CF_4 /O_2 plasmas. Similarly, the etch behavior of Ta in C_2F _6/O_2 discharges at all pressures mirrors that of Ta in CF_4 /O_2 plasmas at pressures above 400 mtorr. Plutonium etching has also been demonstrated in CF_4/O_2 plasmas. Plutonium oxides etch at rates 5 to 10 times faster than pure Pu metal. The reaction of Pu in the discharge proceeds at least 200 times faster than the reaction of Pu with purely thermal sources of F atoms such as O _2F_2. Such results suggest significant enhancement of the Pu/F reaction by the plasma environment. Lastly, oxidation of Ta in O_2 discharges has been studied as a potential means of low-temperature Ta_2O _5 thin film fabrication. Plasma oxidation proceeds readily at room temperature. Two regimes are observed in the oxidation data: an initial, linear regime where oxide thickness is proportional to plasma exposure time, and a parabolic regime where oxide thickness increases with the square-root of exposure time.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT.......186M
- Keywords:
-
- TANTALUM;
- PLUTONIUM;
- Engineering: Chemical; Physics: Fluid and Plasma; Engineering: Materials Science