| Summary: | 博士 === 中原大學 === 化學工程研究所 === 94 === Fluorinated amorphous carbon (a-C:F) films have attracted much attention in the last several decades due to their chemical stability, low surface energy, low refractive index, good electrical and thermal insulation. Plasma polymerization is one of the most popular techniques for obtaining a-C:F film with several favorable advantages such as a dry process and controlled thickness. The properties of plasma deposited a-C:F film are strongly affected by its chemical structure, which in turn, is strongly dependent on the plasma characteristics. However, few literatures reported the plasma characteristics and process-structure-property relationships of a-C:F film at the same time. In this study, the plasma diagnostics and experimental design methodology were used to characterize the plasma and elucidate the process-structure-property relationships of a-C:F film deposited in 13.56 MHz RF plasma reactor. The dissertator can be divided into four subjects and the results are summarized below.
Firstly, the experimental design methodology was used to investigate the influence of process parameters on the characteristics of fluorocarbon films deposited in CH2F2/CF4 plasma. It was found that the CF4 flow rate and substrate temperature were the most significant factors that affect the deposition rate and refractive index of the deposited film. A higher deposition rate was obtained in films deposited under moderate CF4 feed; lower substrate temperature and higher RF power conditions. The lower refractive index was obtained under higher CF4 feed; lower substrate temperature and moderate RF power conditions. It was also found that the fluorocarbon film with a higher F/C ratio was less cross-linked with a lower refractive index and less thermal stability. Potential reactions were proposed to explain the effect of process parameters on the film characteristics.
Secondly, the plasma diagnostics tools (including OES, Langmuir probe and QMS) were used to investigate the influences of operating parameters on the inductive-coupled CH2F2/CF4 plasma. It was found that CH2F2/CF4 plasma is highly electro-negative. The electron density increases with the increase of RF power, resulting in the enhancement of deposition rate. The dominant ion is CHF2+ for pure CH2F2 plasmas at low RF powers. As the input power is increased, abundant high-carbon ions are produced, C3H3F2+, C3H2F3+ and C3HF4+ ions being the dominant ones. Since the F/C atomic ratio is lower in C3 ions than in CHF2+, the F/C ratio of a-C:F film decreases as the power is increased. When 15% CF4 is added to CH2F2 plasmas, the dominant ionic species shifts to CF3+, and the most abundant C3 ions changes from C3H3F2+ to C3HF4+, which correlates to the experimental results that higher F/C ratio was found in films deposited in CH2F2 plasma with CF4 addition. Although the plasma density is not changed with increasing the CF4 flow rate, the dominant charged species become all perfluorocarbon ions when the feed contains 40% CF4, with the order of CF3+>C3F5+>C4F7+, which results in the decrease of deposition rate.
Thirdly, the aromatic C7F8, cyclic C7F14 and linear C7F16 monomers with Ar or H2 additive gas were used to deposit a-C:F film in a 13.56 MHz RF parallel plate plasma reactor. It was found that the deposition rate decreased with substrate temperature. The monomer contains unsaturated bonds had the highest deposition rate, especially the a-C:F film with F/C ratio higher than 0.8 could be obtained by C7F8/Ar plasma at 400℃ with a deposition rate of 35 nm/min. As the flow rate of hydrogen additive increased, the deposition rate increased at first, reached a maximum and then decreased in C7Fx/H2 process. But too much H2 addition dilutes the concentration of C7Fx monomers. The F/C ratio and refractive index were only slightly influenced by substrate temperature in C7Fx/Ar plasma polymer; however, they were significantly affected by substrate temperature in C7Fx/H2 plasma-deposited film. According to the FTIR and XPS spectra, the film possessed the aromatic-ring structure or the lower F/C ratio had the higher thermal stability. In addition, the TGA data indicated the thermal stability up to a temperature of 450℃ from the film deposited by C7F8 plasma at 400℃.
Finally, the aromatic C7F8 monomer was used to deposit a-C:F film in an RF plasma reactor. While keeping the argon gas inlet position fixed, the inlet position effect of the C7F8 monomer on the deposited a-C:F film characteristics was investigated. It was found that, when C7F8 entered upstream of the plasma, the deposited film possessed no aromatic structure and the deposition rate was low. The F/C ratio of the film was higher than 1.4, and the surface was smooth. For film deposited as the C7F8 entered in the afterglow (the so-called ‘downstream’ process), the deposition rate was quite high and copious amount of the aromatic structure were preserved. The F/C ratio of the film was lower than 1.0, and the surface was rough.
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