One of the key factors that dictate whether or not the molecules of a thin film form a perfect crystalline layer is the compatibility of the deposited substance with the substrate. More specifically, the surface of the substrate must be comprised of a perfect crystal similar to the one molecules will form on top of it. For instance, a hexagonal crystal on a substrate’s surface could not produce a thin film with a square crystal because the crystalline structure of the wafer pulls the incoming molecules into its own order during the deposition process. Though proper structural compatibility helps facilitate the formation of a thin, even layer of molecules on the substrate, it does not guarantee it. Indeed, it is quite common during the process of cooling down for the molecules of the crystalline sheet to crumple up, thus ruining the smooth, featureless surface required of useable thin films. An example of molecule crinkling can be seen in the image above, where a thin film of iron oxide on a magnesium oxide substrate appears heavily ridged and striated, rather than flawless like a well-made mirror.