Paper PS1-WeA9
Characterization of Very High Frequency Capacitively Coupled Plasmas

Wednesday, November 11, 2009, 4:40 pm, Room A1

As semiconductor technology progresses to the 22 nm node, it is becoming increasingly important to fundamentally understand plasma etching processes and apply this understanding to development and improvement of plasma etch equipment. Capacitively coupled plasmas (CCP) have been widely used for dielectric plasma etching. The general trend in recent years has been towards the use of multi-frequency CCPs which include rf sources in the very high frequency (VHF) regime. We characterize one such system in this paper using two/three-dimensional (2/3D) plasma modeling. Modeling results are validated using experimental data for different operating conditions. Plasma simulations have been performed using our in-house 2/3D fluid plasma model. To account for electromagnetic effects at VHF, this model includes the full set of Maxwell equations in their potential formulation. The equations governing the vector potential are solved in the frequency domain after every cycle for multiple harmonics of the driving frequency. Current sources for the vector potential equations are computed using the plasma characteristics from the previous cycle. The coupled set of equations governing the scalar potential and drift-diffusion equations for all charged species are solved implicitly in time. Model validation is performed using radially-resolved electron and ion densities and electron temperature measured with single and double Langmuir probes [1]. Ion density profiles obtained with both probes are generally similar over the range of conditions investigated. Plasma simulations were performed for a wide range of operating conditions [gas pressure (50 – 150 mT), rf power (100 – 1000 W), gases (Ar, O₂, CF₄)] at 60 and 162 MHz with and without a spatially inhomogeneous magnetic field. In agreement with experimental data, we observe that plasma density increases with pressure in Ar while the bulk plasma electron temperature is almost invariant. Plasma density is substantially higher at the higher frequency of 162 MHz. Plasma density is lower in electronegative gases than Ar under identical conditions. Plasma profile changes substantially with application of magnetic field, and the effect of magnetic field is weaker at higher pressures. While electromagnetic effects are strong at 162 MHz, reactor design determines the relative importance of electromagnetic vs. electrostatic effects at 60 MHz.

[1] L. Dorf et al., 2009 AVS Symposium.