Figure 2a: On a flat wafer surface, every location (A,B,C,D) is fully exposed to the chamber (opening angle equal to 180) and receives the full flux of ions in all directions. Figure 2b: In the bottom of pits and trenches (E, F), the angular range within the line of sight of the chamber is reduced. The etch rate can be scaled as the integral of the distribution within the angular range (shaded area under the curve between the vertical lines)