Role of chamber dimension in fluorocarbon based deposition and etching of SiO₂ and its effects on gas and surface-phase chemistry

It is well understood that chamber geometry is an influential factor governing plasma processing of materials. Simple models suggest that a large fraction of this influence is due to changes in basic plasma properties, namely, density, temperature, and potential. However, while such factors do play an important role, they only partly describe the observed differences in process results. Therefore, to better elucidate the role of chamber geometry in this work, the authors explore the influence of plasma chemistry and its symbiotic effect on plasma processing by decoupling the plasma density, temperature, and potential from the plasma-surface (wall) interactions. Specifically, a plasma system is used with which the authors can vary the chamber dimension so as to vary the plasma-surface interaction directly. By varying chamber wall diameter, 20–66 cm, and source-platen distance, 4–6 cm, the etch behavior of SiO₂ (or the deposition behavior of fluorocarbon polymer) and the resulting gas-phase chemistry change significantly. Results from in situ spectroscopic ellipsometry show significant differences in etch characteristics, with etch rates as high as 350 nm/min and as low as 75 nm/min for the same self-bias voltage. Fluorocarbon deposition rates are also highly dependent on chamber dimension and vary from no net deposition to deposition rates as high as 225 nm/min. Etch yields, however, remain unaffected by the chamber size variations. From Langmuir probe measurements, it is clear that chamber geometry results in significant shifts in plasma properties such as electron and ion densities. Indeed, such measurements show that on-wafer processes are- - limited at least in part by ion flux for high energy reactive ion etch. However, in situ multipass Fourier transform infrared spectroscopy reveals that the line-averaged COF₂, SiF₄, CF₂, and CF₃ gas-phase densities are also dependent on chamber dimension at high self-bias voltage and also correlate well to the CF_x overlayer stoichiometry under deposition conditions.