Silicon nitride films are preferred for finishing works in micro- and nano-electronic technology. These applications, however, require precision pattering on the field surface that has remained a challenge to achieve. Previously, researchers have identified the dry etching process as an effective solution for the precision problem. Even though the plasma parameters, etching characteristics and treated surface states of plasma-enhanced chemical vapor deposition-formed SiNx films have been previously investigated, it was evident that no comprehensive results could be obtained due to the lack of detailed data on the plasma chemistry.
To this note, Nomin Lim (PhD candidate) and Professor Kwang-Ho Kwon from Korea University together with Professor Alexander Efremov from the State University of Chemistry and Technology explored the etching characteristics and mechanisms of nonstoichiometric plasma-enhanced chemical vapor deposition-formed SiNx thin films in C4F8 + Ar coupled plasma to determine appropriate matching of the gas-phase and heterogenous chemistries. Their main objectives were to determine the influence of the changes in the operating conditions on the SiNx etching kinetics through gas-phase plasma characteristics as well as to identify the gas-phase related parameters for characterizing of the etching process. The work is published in the journal, Thin Solid Films.
Briefly, the SiNx etching rates and the etching selectivity i.e. SiNx/Si and SiNx/SiO2 were all measured as functions of gas pressure, bias power, input power, and C4F8/Ar mixing ratio. Consequently, the plasma diagnostics based on Langmuir probes and zero-dimensional plasma modeling was used to determine the steady-state composition, plasma parameters, and chemistry. Furthermore, the relationship between the measured etching rate and model predicted fluxes of the active species were examined to analyze the etching mechanism of the SiNx.
The authors observed that within the given input process conditions operation range, the SiNx etching kinetics did not only appear similar to Si and SiO2 in the steady-state etching regime but also exhibited the features of the ion-assisted chemical reactions in the neutral-flux-limited mode. Besides, the SiNx etching process was influenced by the fluorocarbon polymer film thickness. Additionally, it was worth noting that the differences in the effective reaction probabilities for SiNx, Si and SiO2 agreed well with the corresponding differences in the fluorocarbon film thickness and reaction thresholds.
According to the authors, changes in the input process parameters of the effective probability including the deposition and etching kinetics of the fluorocarbon polymer films could be characterized by the behavior of the ratio of the fluorocarbon radicals, fluorine atoms and ion energy flux that ideally reflected the fluorocarbon film thickness. The authors believe the importance of the paper in that the developed model adequately described the plasma chemistry by reflecting the relationships between input process parameters and gas-phase composition. This would lead to advancement in silicon-based materials used in micro- and nanotechnology.
Lim, N., Efremov, A., & Kwon, K.-H. (2019). Gas-phase chemistry and etching mechanism of SiNx thin films in C4F8 + Ar inductively coupled plasma. Thin Solid Films, 685, 97-107.