The deposition of CTFs (and STFs) requires a directional vapor flux and a sufficiently low substrate temperature, so that the arriving adatoms move only short distances upon initial impingement with the growing film surface. More to the point, as discussed in Chapter 2, for vapor flux normal to the substrate CTFs have their origin in the atomic-level self-shadowing in which the highest features preferentially capture the ballistic vapor flux and shadow the smaller, less fortunate surface features in adjoining columns. Such CTFs have mass densities usually within 10% of the theoretical maximum density [192, 237-€“240]; and the adjoining columns are in intimate contact, separated only by the void network in which the voids are density fluctuations at the atomic level [31]. As I‡ I… decreases, the void network becomes more prominent, the mass density decreases, and the voids increase in size as well as display a broader distribution of sizes [32, 37, 185, 186, 231]. For I‡ I… <30deg , the shadowing distances can be much larger than atomic diameters and depend critically upon the relative heights of adjoining columns, since any adjoining column creates a shadow region where the incoming vapor flux cannot deposit [37, 148].