conjugate impedance matching
Radio Frequency Identification (RFID) represents the automatic manner of contactless identification via radiofrequency communication channel. RFID transponder is a tag device attachable on the object of identification. Transponder can respond by sending a content of its embedded memory by modulated backscatter communication to interrogator (reader). In contrast to active RFID transponder, a passive transponder has no battery, instead it gets all the needed energy from the carrier signal of the reader.
Both antenna and semiconductor chip represents the essentially integrated parts of the tag and requires emergent consideration. Particularly, the variant processes of bonding the chip to the contact pads of antenna could drastically change the input impedance of the chip as it seen by antenna.
An objective of this paper is to characterize the connection of the chip into the radiator of antenna in order to ensure the inner conjugate matching of passive transponders for RFID systems.
A severely strong change of the input impedance of the semiconductor chip after flip-chip bonding into the antenna for passive RFID transponder has been found. In contrast to traditional wire bonding process, which normally resulted in the small inductive serial parasitic, the mass production flip-chip bonding process brings the strong shunt capacitance owing to essential redistribution of the electromagnetic field over the chip topology near the contact pad of the rectifier. It has been established that in order to ensure the inner conjugate matching of the rectenna for RFID transponders the corresponding change of the input impedance of the chip should be taken into account.
An original way to characterise the transponder chip as it seen by antenna adequately has been proposed and implemented for a given bonding process into the radiator of transponder antenna. A proposed two-port scheme allows the de-embedding of the intrinsic chip impedance with reduced measurement uncertainties. It has been found that in case of transponder chip fabricated by Samsung Electronics Co., Ltd. the small-signal-mode of operation holds true up to 0,5 mW of received power.