AVS 45th International Symposium
    Plasma Science and Technology Division Tuesday Sessions
       Session PS1-TuM

Paper PS1-TuM2
Fluorocarbon Film Composition and Reactor Effluent from Pulsed PECVD of Difluoromethane, 1,1,2,2-Tetrafluoroethane, and Hexafluoropropylene Oxide

Tuesday, November 3, 1998, 8:40 am, Room 314/315

Session: Pulsed Plasmas
Presenter: C.B. Labelle, Massachusetts Institute of Technology
Authors: C.B. Labelle, Massachusetts Institute of Technology
K.K. Gleason, Massachusetts Institute of Technology
Correspondent: Click to Email
Low dielectric constant (< 2.5) fluorocarbon thin films were deposited from three precursors with relatively low global warming potentials: difluoromethane (CH@sub 2@F@sub 2@), 1,1,2,2-tetrafluoroethane (C@sub 2@H@sub 2@F@sub 4@), and hexafluoropropylene oxide (HFPO, C@sub 3@F@sub 6@O). For each feed gas, reactor effluent and film composition were determined as a function of pulsed plasma excitation on and off timing cycles. Fourier Transform Infrared Spectroscopy (FTIR) confirms only partial decomposition of the feed gas occurs in pulsed PECVD, and only a relatively small number of additional gas-phase effluent species are produced in significant concentration. A minimum reaction set has been proposed for each precursor to account for the major effluent species. Most notable for CH@sub 2@F@sub 2@ and C@sub 2@H@sub 2@F@sub 4@ pulsed plasmas is the competition between CF@sub 2@-producing reactions and HF elimination reactions. HFPO pulsed plasmas, due their lack of H, do not have this competition, but the presence of oxygen in the precursor is significant. Support for these reaction sets is also found from a comparison of film composition. In each case, the CF@sub x@ distribution, as well as the presence or absence of hydrogen in the film, can be explained by the proposed reaction sets. Most notably, the role of the competition between CF@sub 2@-producing reactions and HF elimination reactions is reflected in the CF@sub 2@ vs. quaternary carbon concentrations as determined by carbon-1s x-ray photoelectron spectroscopy. Precursors with dominant CF@sub 2@-producing reactions lead to films with larger CF@sub 2@ concentrations, whereas precursors with dominant HF elimination reactions lead to films with larger quaternary carbon concentrations. In the case of the HFPO films, the high CF@sub 2@ fractions are easily explained by the dominant decomposition of HFPO into CF@sub 2@ and trifluoroacetyl fluoride (CFOCF@sub 3@). More significantly, however, the absence of significant oxygen in the films can be traced to the formation of several very stable oxygen species in the pulsed plasma which exit the reactor without being further broken apart and incorporated into the film. Finally, the global warming impact of the pulsed plasma effluents, and thus, of each film deposition process, has been determined.