# How surface passivation works?

Surface passivation is done to deal with "dangling bonds" (for example, $\mathrm{SiO_2}$ layer on top of a $\mathrm{Si}$ layer). But isn't the new surface will have dangling bonds on its own at the new formed surface?

Surface passivation creates different chemistry on the surface than the chemistry of the bulk solid. Imagine bulk Si with a clean surface (presumably in a vacuum and freshly made). It can be thought of as having "dangling bonds", though this is not necessarily the best way to think about it. Think more generally that the structure on the surface is different to the bulk structure. There may be different elements present and the bonds may be "satisfied". Or they may be strained compared to the bulk material.

Expose the Si to, for example, O2 and the surface layer (your dangling bonds, if you like) will react to give some SiO2 just on the top layer. This new layer may be much less reactive than bulk silicon in normal air, hence can be said to passivate the surface.

If the chemical and physical properties of the new layer are right, the layer will protect the bulk from further reaction: this is the definition of passivation. A similar situation occurs naturally for aluminium, for example, where the Al2O3 layer that forms is both strong and relatively impermeable to air preventing further reaction (aluminium with the layer removed will react so rapidly with oxygen it gets warm and disintegrates). Iron, on the other hand, forms rust which doesn't protect the surface as it is flaky and weaker than the bulk metal. To passivate iron you need to add other elements which encourage the formation of stronger passivating layers (this is how many steels are made not to rust).

Chemical manipulation of the surface can create specific types of passivation that serve a particular purpose rather than relying on the natural reactions of air and the surface. This is common in the semiconductor industry and can also be used to generate the coloured layers on aluminium and titanium products where the goal is aesthetic as much as protective.

• So there would still be "dangling bonds" for the new layer, only less reactive than the previous surface? – Sparkler Mar 22 '15 at 12:54
• @Sparkler IF you insist on thinking in terms of dangling bonds then, yes. But this is a simplistic way of thinking as some surfaces won't have any meaningful equivalent to "dangling bonds" especially when passivated. – matt_black Mar 22 '15 at 19:22
• So the type of required passivation depends on the target environment, right? – Sparkler Mar 22 '15 at 19:25
• @Sparkler yes. different passivation will work in different environments. – matt_black Mar 22 '15 at 19:26
• @Sparkler Think more generally that the structure on the surface is different to the bulk structure. There may be different elements present and the bonds may be "satisfied". Or they may be strained compared to the bulk material. – matt_black Mar 22 '15 at 19:43