Suppressing Nonlinearly-Driven Inhomogeneities in High Frequency CCP’s

Large area, high frequency capacitively coupled plasmas (CCP's) [1-6], widely used for semiconductor and thin film fabrication, require extreme processing uniformity. Large area allows increased production, and higher frequency gives smaller sheath widths and voltages. The latter leads to a decrease in the ion bombarding energy, and higher ion fluxes at the wafer, which are desirable for processing integrated circuits with smaller dimensions. However, at higher frequency and/or larger area, the radial wavelengths of the surface waves in the plasma can become comparable to the reactor radius, leading to standing wave effects and center-high plasma non-uniformities. Center-high non-uniformities are observed in CCP’s even when the driving frequency wavelength is much larger than the system size. This results from nonlinear sheath motion exciting driving frequency harmonics that can be radially near-resonant, enhancing the on-axis power deposition and degrading uniformity (Fig. 1a). We have investigated typical rf-driven asymmetric cylindrical reactors using a triple frequency 2D fluid-analytical simulation [1] and a transmission line (TL) theory incorporating the nonlinear sheath physics [2,3]. The nonlinear TL theory was extended to dual frequency drives [4], and to the full nonlinear sheath dynamics of the symmetric and antisymmetric surface wave modes [5]. Adding a dielectric layer over the substrate electrode was found to improve processing uniformity by increasing the effective (sheath + dielectric) sheath widths and, thereby, the radial wavelengths of the fundamental and nonlinearly-driven harmonic modes [6] (Fig. 1b). This passive intervention is requires no fundamental change to power supplies or processing strategies. This strategy for improving uniformity has the potential to be a low cost and impactful advance to plasma processing.