Anomalous voltage trends in electronegative capacitively coupled plasmas

Summary form only given. Larger gas densities and lower loss rates at higher operating pressures typically lead to increased plasma density and consequently ion fluxes to the biased electrode. As a result, conventional theory dictates that increasing the operating pressure decreases the bias voltage required to deposit a given power. However, this approach assumes that the plasma density increases uniformly and/or the plasma distribution does not vary with the increase in pressure. In reality, the RF bias voltage is dependent on the local characteristics of the plasma in the vicinity of the wafer which changes based on a multitude of factors. In this paper, results of wafer-based measurements of the RF and self-generated DC bias voltages in dual-frequency capacitively coupled electronegative plasmas will be discussed. Measurements were performed for varying 60 MHz (source) and 13 MHz (bias) powers in Ar/CF₄/CHF₃ plasma over a pressure range of 25 mT to 400 mT sustained in a 5 cm gap capacitively coupled system. In particular, we found that the RF and DC voltage trends agree well with the conventional theory with increase in pressure when high frequency (source) power is low. However, for high source power operations, the RF and DC voltages vary non monotonically with pressure. A 2/3 dimensional plasma equipment model (CRTRS) was used to understand the experimental observations. CRTRS solves the continuity equation for all species coupled with the full momentum equation for ions and the drift-diffusion approximation for electrons. These equations, combined with the Poisson´s equation, which governs the electrostatic fields, are solved implicitly in time. We found that the inclusion of the external circuit in the plasma simulations was critical to accurately predict the anomalous RF and DC voltage trends when using high source powers. Ionization sources are confined in the vicinity of the 60 MHz electrode and the increased collision frequency a- higher pressure leads to depletion of the plasma density locally around the 13 MHz electrode. Consequently, the bias voltage increases despite the increase in pressure. As the pressure is increased further, the increase in gas density is sufficient to overcome the local depletion observed at intermediate pressures which then decreases the bias voltage.