From What Materials Are RFID Tags Made?

It depends on the type of transponders to which you are referring. A passive ultrahigh-frequency (UHF) transponder contains a microchip made of silicon. Attached to the chip is an antenna that can be composed of copper, aluminum, silver ink or some other conductive material. The chip and antenna are mounted on a substrate, which can be paper, polyethylene terephthalate (PET) or some other type of plastic. This is usually inserted into a thermal transfer label. - See more at: http://www.rfidjournal.com/blogs/experts/entry?11066#sthash.vNv7vTvM.dpuf