In this dissertation, the characteristics of hydrogenated amorphous carbon (a-C:H) films was investigated in plasma deposition process, plasma etching process, and after plasma surface modification. The plasma diagnostics was also studied during these processes. a-C:H films were deposited using inductively coupled C2H2 plasma, and the effect of process parameters on the sp2/sp3 hybridization ratio of the films was investigated. A high sp2/sp3 ratio was also observed with high plasma power and low pressure. The sp2/sp3 ratio of the a-C:H films increased with increasing ion density in the C2H2 plasmas generated under various conditions but decreased with the relative density of CH radicals. Ions remove hydrogen atoms from a-C:H films, and CH radicals introduce hydrogen atoms into a-C:H films. The dependence of the etching characteristics of a-C:H films on the sp2/sp3 hybridization ratios was studied for CF4/O2 plasma mixtures. The etch rate of the sp2-rich a-C:H exhibits a linear correlation with the ion density of CF4/O2 plasma, whereas that of the sp3-rich a-C:H exhibits a second-order exponential correlation with O radical density. A combined etch rate model was suggested to explain the etch rates of a-C:H. Ion-enhanced etching is identified as the dominant etching mechanism for the sp2-rich a-C:H, whereas spontaneous chemical etching is the main reaction mechanism for the sp3-rich a-C:H in CF4/O2 plasmas. The wettability of a-C:H films was investigated after CF4 plasma treatment of varied plasma time, source power and bias power in an inductively coupled plasma reactor. The a-C:H was observed to become hydrophilic after a small amount of fluorine incorporation, while more hydrophobic after a large amount of fluorine incorporation. A fluorination model was proposed to estimate the fluorine concentration in fluorinated a-C:H films in terms of absorption rate and desorption rate. A simple method was proposed to estimate the F radical density in CF4 and CF4/O2 plasmas using mass spectrometry. The relative F radical densities were determined by subtracting the F+ signals derived from the CF4 molecules from the total F+ signals measured by mass spectrometry, where the F+ signals derived from the CF4 molecules were estimated from CF3+ signals. The F radical densities were also estimated using optical actinometry, and the values determined by mass spectrometry and optical actinometry exhibited a linear relationship.