RFID Coil tag is fundamentally an inductor, so take care in define substrate layers, RFID coil tag have not ground plane, just define air layer above and below coil.
Common substrate uses in RFID applications: Epoxy, PET, polymide etc..
common metal: Al and Cu
Yes ADS Momentum can be used successfully for RF-ID Tag design. The simulation of the coil will generally not be resonant but once combined with the chip impedance and any tuning netwrok the results are very good.
To simulate you first have to set up the substrate layers as following:
for example using a PET dielectric and silver cond.
FreeSpace//////// er=1 Boundary Open
PET/////////////// er=2.9 Thickness=50um
FreeSpace_0//// er=1 Boundary Open
The draw the antenna. Set two ports. One of them port type internal and to have 50ohms and the other one set as ground reference associated with port 1.