7.2.2 Step coverage (conformality)
Not all layers require precise thickness control. Sometimes all that matters is that the film coats the entire surface, including vertical walls and (most difficult) the corners at the bottom of deep holes.
We may require an insulating layer of oxide between two conductors.
A protective titanium nitride barrier layer prevents aluminium from diffusing into silicon, and an underlying titanium adhesion layer ensures that the titanium nitride sticks. Similarly, tantalum adhesion layers and tantalum nitride barrier layers are used before depositing copper onto aluminium, and a platinum or palladium layer is used to prevent poisoning by gold.
Thin barriers, or liner layers, may be used to protect the surface during individual process steps, such as protecting an oxide layer against attack by WF6 during tungsten deposition.
For all of these, the layer thicknesses are relatively unimportant, but we must ensure that coverage is complete.
Step coverage is the ratio of the film thickness at its thinnest to that on the open upper face, and only a few techniques produce a fully conformal coating, with step coverage of 1, as shown in Figure 32(a). These are the ones where the film grows outwards from the surface. Figure 32(b) shows the morphology seen when the film is deposited from above. Melting the layer and allowing it to flow under the influence of surface tension can sometimes correct these thickness variations, but only in a few lucky circumstances. It is more likely to make the situation worse.
Instead of coating walls, we may wish to fill up a hole or trench with material. Examples where this might be necessary include:
‘via holes’, filled with metal plugs that make electrical contact to an underlying layer
polysilicon gate electrodes in power transistors, where a voltage controls conduction down the trench wall
insulating gap-fill to prevent conduction across a trench.
Figure 32(c) shows what happens if we attempt such a plug fill, using a technique with poor step coverage. It is greatly preferable not to leave voids, as air gaps are poor insulators, terrible conductors, and mechanically very weak! Techniques that build up a conformal coating may therefore again be required.