We demonstrate three different ways to incorporate nitrogen at Si-SiO2 interfaces: (i) an O-2/He plasma oxidation of the Si surface followed by an N-2/He plasma nitridation, (ii) an N-2/He plasma nitridation of the Si surface, and (iii) a Si3N4 film deposition on to the Si surface. The two-step interface formation, the O-2/He plasma oxidation followed by the N-2/He plasma nitridation, is shown to yield significantly better interface device properties than the other two approaches. These differences in interface properties are explained by an application of constraint theory based on comparisons of the average bonding coordination of the dielectric layer at the interface with the Si substrate.