Perfect crystal structure can be achieved only by epitaxial growth, where deposits are formed atomic layer by atomic layer. The lattice planes of the deposited film merge seamlessly with those of the substrate on which it is deposited. Even when the crystal lattices of these two materials match in shape, however, they will never be a perfect match in size, owing to differences in atomic spacing between the deposit and the substrate; so, the deposited film will be stretched and distorted to match the substrate. This is one of several possible sources of stress in the growing film, which may force it into compression or into tension, with consequences both for the robustness of the device and for the flatness of the wafer as a whole. A thick compressive film will cause the whole wafer to bow downwards, so it no longer sits flat on the processing surface and becomes much more difficult to handle and work with. Such intrinsic stress becomes problematic at the device scale when the film is patterned, especially in MEMS devices, which twist and distort their components away from the designed shape.
Epitaxial lattice mismatch is not the only way that stress can develop – we have already mentioned that a film can be densified by ion bombardment, and this will also leave it in compression. Stresses also develop if a wafer is heated above room temperature, either during deposition or during a later annealing stage. Although high temperatures may allow material to flow and relieve local stresses, metal layers (with high thermal expansion coefficients) will tend to contract most during cooling, so that they are left under tension (and may crack), whereas insulating layers are left in compression (and may buckle).
It is an irony of the deposition process that methods favouring a dense film with good crystallinity (ion bombardment, hot deposition, annealing) are just those that also introduce stress into the structure.