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Analysis and modeling of gas-phase processes in a CHF3/Ar discharge

: Barz, J.P.; Oehr, C.; Lunk, A.


Plasma Processes and Polymers 8 (2011), No.5, pp.409-423
ISSN: 1612-8850
ISSN: 1612-8869
Journal Article
Fraunhofer IGB ()

The present work deals with the diagnostics of fluorocarbon plasmas by different experimental methods and complementary numerical analysis of the plasma. The plasma diagnostics were performed with non-invasive methods. In this paper, we present results obtained by laser-induced fluorescence and UV absorption measurements. The complementary numerical simulations accounted for the electron-neutral interactions, discharge dynamics, and chemical reactions. Several insights were obtained from the combined experimental and numerical approaches, especially concerning the conclusiveness of the results and previous observations from the literature. As important initial neutral species, CF 3 and H originate from electron collisions with CHF3. The atomic hydrogen is responsible for fluorine abstraction reactions which result in the formation of hydrofluoric acid as well as in unsaturated carbon bonds. It was found from the simulations that in the examined discharges, this reaction is the most important channel for the high-rate production of the CF molecule in the gas phase via CF2 + H ? CF + HF. For this molecule, no anisotropy in the density was found and the production solely occurs in the gas phase. Contrary to that, the production of CF2 via the abstraction of fluorine from CF3 in the gas phase was found to be of less importance for the overall concentration of this molecule. Instead, chemical reactions at the chamber walls which release CF2 were identified experimentally as major sources. According to the simulations, the unsaturated fluorocarbon molecules react to oligomers, such that the net reaction can be roughly described by nCHF3 ? (CF2)n + nHF. A good agreement between the simulated and the experimentally observed densities and rates was obtained, which confirms the interpretation of the ongoing plasma-chemical processes as well as the underlying physical discharge parameters consistently. Fluorocarbon plasmas are applied in several industrial branches. They can be either used for etching in the semiconductor industry or for coating of surfaces with low-k dielectric, hydrophobic and oleophobic polymer films. The net-result -etching or deposition- depends on the fluorine to carbon ratio and thus, on the plasma chemistry. Despite of the successful application of fluorocarbon plasmas in the industry, several details about the chemical kinetics are still a matter of discussion. In particular, this accounts for the kinetics of CF2.