TSMC inductor design kit vs HFSS simulation

HI guys,

I am simulating a one-turn differential TSMC inductor with HFSS. The Q value from HFSS is dependent on the setup in the simulation. If I use guard ring as return path the Qmax will be lower and f(Qmax) will be higher. If I use a ground ring as return path, the Q max will be higher and f(Qmax) will be lower. Does anyone know how TSMC simulates or measures the differential inductor? Does they use guard ring or ground ring as return path? Thanks!

Wufeng