Computational characterization of the area asymmetry effect in capacitively coupled plasmas

Capacitively coupled plasmas (CCP) are widely used in the semiconductor industry for high aspect ratio etch applications. Spatial uniformity of critical plasma discharge characteristics such as charged species and radical densities and ion energy and angular distribution us important for achieving optimum on-wafer results. Designing plasma etch reactors often involves optimizing the ground to power area ratio to balance the charged particle fluxes leading to the formation of a DC self-bias. In a symmetrical reactor the presence or absence of blocking capacitor will not affect the plasma uniformity. However, in reactors with asymmetric electrode configurations, we expect that the plasma non uniformity will be high in the absence of a blocking capacitor. The addition of blocking capacitor is expected to improve plasma uniformity.

In this study, we computationally investigate the impact of reactor asymmetry on the value of DC bias in a multi-frequency CCP reactor operating in low-pressure conditions. The effect of ground to powered area ratio on DC bias and its scaling to power will be presented. The role of power, chemistry, and secondary electron emission effects on plasma uniformity and DC bias will be discussed. The relevance of DC bias in controlling mean ion energy in high aspect ratio applications will be covered.