Effects of the focus ring on the ion kinetics at the wafer edge in capacitively coupled plasma reactors

Abstract

The fabrication process of modern microelectronic devices faces a significant challenge regarding the uniformity of wafer processing during plasma etching. Particularly, nonuniformity is prominent at the wafer edge due to varying electrical properties, leading to sheath bending and distorted ion trajectories. To address this issue, a wafer terminating structure known as a focus ring is employed to modify the sheath structure near the edge of the wafer and ensure uniform ion fluxes. However, the focus ring is subject to erosion caused by the plasma, making it crucial to minimize the ion energy bombarding the focus ring. In light of this, this paper investigates the impact of parameters such as the wafer-focus ring gap, focus ring height, and dielectric constant of the focus ring on the ion angle onto the wafer and the ion energy onto the focus ring. To conduct the analysis, a 2D3V particle-in-cell/Monte Carlo collision model is utilized. The study reveals the existence of horizontal electric fields with opposite directions at the wafer edge and the inner edge of the focus ring. Optimizing the ion angle onto the wafer edge can be achieved by adjusting the material and geometry of the focus ring. Furthermore, reducing the ion energy at the focus ring can be accomplished by increasing the height or decreasing the dielectric constant of the focus ring.