Re: HFSS Ground problem
Are you working with lossy silicon? Or are you working with low-loss dielectrics?
I usually prefer to use a planar simulator for such structures; they determine the characteristic impedance through port de-embedding techniques that work much better for lossy dielectrics. Unless there is something that really requires full 3D volume meshing, it is a lot more efficient and usually more accurate to use one of the planar EM simulators for such a problem, especially now that most of them will handle thick metal.
--Max