RFICDUDE
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I suspect that the corner variation of an integrated inductor is mainly due to the variation in parasitic capacitance to the substrate and the interwinding dielectric capacitance. These capacitances impact the self resonant frequency of the inductor, and the effective inductance increases as the frequency approaches the self resonant frequency. It is not that the inherent inductance changes, rather it is that the reactance changes more rapidly because of the second order resonant response (hope that is clear).
The metal spacing and width tolerances mainly impact capacitance. Inductance is fairly insensitive to these variations, but the effective inductance at a given frequency can vary substantially with the capacitance if the self resonant frequency of the inductor is within a couple of octaves of the operating frequency.
This is one reason why VCO inductors tend to be quite small in self inductance (to minimize the impact of parasitic capacitance variation).
I suspect your fast inductor corner corresponds to thick oxide thickness (between substrate and inductor metal) and maybe wide separation between windings.
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