Simulation was used to investigate potential distributions around a grid of a Faraday cage and trajectories of ions inside the cage located in a high density CF4 plasma etcher. It was observed that the potential distributions near the edge of the grid openings (or near the grid wires) were disturbed, due to the partial leakage of the plasma through the grid openings whose size was comparable to the sheath thickness. Corresponding trajectories of ions incident through the grid openings near the edge were found to deflect from the surface normal and focus below the grid wires. It is this ion focusing that is responsible for higher etch rates of SiO2 films below the grid wires compared to those below the grid openings at a proper distance between the grid and the substrate surface. When the substrate was located sufficiently far away from the grid plane (8 mm), the ion trajectories overlapped with each other and the etch rates were uniform across the substrate. At the gap of 0.3 mm from the grid plane, however, ion focusing does not play a role due to close proximity to the grid. This resulted in much higher etch rates below the grid openings than those below the grid wires. The etch rates were also measured at various distances between the grid and the substrate surface. The behavior of the simulated distributions of the etch rates showed good agreement with the measured ones.