Etching of polyphenylene oxide in a downstream microwave plasma using NF₃, SF₆, O₂ and Ar gas mixtures Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vx021k29n

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  • Plasma etching of polymers is an important process in the fabrication of integrated circuits, printed circuit boards and other emerging technologies. High rates can be obtained using oxygen diluted with a fluorine-containing gas, most commonly CF₄. In this thesis, two less studied fluorinated gases, NF₃ and SF₆ are investigated. Polyphenylene oxide (PPO) is etched in a downstream microwave plasma system using NF₃, SF₆, O₂ and Ar gas mixtures. The effects of gas composition. pressure, substrate temperature, power, etching time and flow rate are experimentally investigated to help elucidate the mechanism of polymer etching. Etch rate maxima in excess of 3 μm/min, twice that observed when etching with CF₄/O₂ mixtures, are achieved by optimizing the process parameters. Additionally, a one-dimensional numerical model based on mass conservation is used to predict the concentrations of active etchant species at the substrate. Surface analysis of selected etched samples using profilometry, X-Ray photoelectron spectroscopy and secondary ion mass spectrometry is reported. The significance of this study can be summarized in the following three features: (1) Optimization of process parameters including feed gas mixtures in achieving higher etch rates, (2) Contribution to the understanding of the effect of physical and chemical processes towards the etching of polymers and (3) The significance, ifany, of non etchant atoms from the feed gases e.g. C, N and S, in the etch process. The etch rate dependence of PPO on NF₃% in the feed gas mixture increased with fluorine content, went through a maximum, and decreased with further increase in fluorine content. The greatest etch rate observed was 3.2 μm/min. The etch rate followed the same trend with SF₆% at high flow rates. The SF₆% at which the maximum was lower than the corresponding NF₃% The etch rate also went through a maximum with pressure at 0.8 ton in NF₃%/O₂ mixtures. The etch rate is independent of substrate temperature at low NF₃% and showed a strong dependence on temperature at higher NF₃%. There was no discemable dependence of substrate temperature in the SF₆ mixtures studied. At low NF₃%, there is a linear dependence of etch rate on flow rate. The etch rate dependence on flow rate followed a saturation curve for at intermediate NF₃% and at all SF₆%. This etch rate dependence was explained using a surface depletion model where the substrate surface is depleted of etchant species at lower flow rates. At high NF₃%, there is no noticable dependence on flow rate, which is explained by surface passivation. The effect of power and the addition of small amounts of SF₆ to NF₃/O₂ mixtures dare also reported The concentrations of reactant and products in NF₃ and SF₆ gas mixtures were simulated for four different El rate constant combinations across a range of feed gas compositions. The behavior at high NF₃ and low O₂ rate constants was different from the other three combinations. The O and F dependence on total flow rate in NF₃ mixtures, exhibited different trends at high NF₃ and low O₂ El rate constants than at the other three rate constant combinations. The former is consistent with the experimental data. However, the model does not take into account depletion of etchants. The dependence of O and F on yo and yF was investigated. The effect of residence time was found to be the controlling factor in wall recombination processes.
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