[ADS Momentum & HFSS] Spiral inductor: Ports, ground and substrate size.

Hello,

I'm actually designing a spiral inductor (gold) on a InP substrate scalable in a range from 0,1 to 1 nH with the smallest capacitance possible and there are a few questions I would like to ask you.


1 - I did the tutorial of the spiral inductor from HFSS and tried to apply the same method to a InP substrate. As far as I understand I don't need the oxyde anymore since the InP is already isolating. Unfortunately the ground ring path affects the inductance value of my spiral. To deembed this, I thought that I could replace the spiral with a straight rectangular wire, calculate its inductance theoretically and then deembed it to find the added inductance of the ground ring. Is it a valid method?

2 - The return path is not necessary in Momentum, which is not the case for HFSS. Is it true? I read it somewhere but I can't find the source anymore.

3 - Do we see usually a relationship between the width of the substrate and the inductance of a spiral inductor? I was never viewed as a parameter in all the papers I could read about but HFSS is saying otherwise.

Thank you for your help.