Chemical Vapor Deposition
In the PVD method described above, it is not easy to deposit a material on the side walls as shown in fig 3.13. This is especially true if the depth is very high and the opening si small. If the ratio of diameter (or width) to the depth, known as aspect ratio, is very high,  PVD cannot give a good side wall coverage (Figure 3.14). However, CVD can be used to get a good side wall coverage (Figure 3.15).

Figure 3.13. Schematic of wafer and sidewall before deposition

Figure 3.14. Illustration of a poorly deposited film

Figure 3.15 a . Illustration of a good film during deposition

Figure 3.15b. Illustration of a good film at the end of deposition