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At the recent IEEE International Electron Devices Meeting (IEDM 2012), imec presented the world-first ultra-high frequency Schottky diode based on amorphous Indium-Gallium-Zinc Oxide (IGZO) as semiconductor. This breakthrough achievement will enable the development of thin-film passive UHF RFID tags to replace item-level bar codes.
imec’s research aims to dramatically reduce the cost of the entire RFID by combining UHF operation with a thin-film-based technology. UHF RFID tags have a long reading range (5 to 10 m) and employ small, printed, low-cost antennas. Compared to Silicon, IGZO based technology has the potential to result in a low-cost solution, since IGZO thin film active devices are fabricated using a cheaper, low-temperature process.
This allows the development of chips direct on a plastic foils, such as on the product package. However, IGZO has intrinsically a lower performance than conventional Silicon and other conventional crystalline semiconductors. Therefore, it is a challenge to fabricate ultra-fast active devices based on IGZO.
The diode is the fundamental block in the power supply generator of passive, i.e., battery-less tags. It rectifies the carrier wave captured by the antenna and feeds the power supply on the tag. IGZO is an amorphous semiconductor with gap states that impede the formation of a stable Schottky barrier, irrespective of the metal used. To achieve a stable Schottky barrier, imec developed specific plasma and anneal treatments that alter the chemistry of the Schottky interface.
The resulting IGZO Schottky diodes have a rectification ratio of up to nine orders of magnitude (at +1 V and -1 V), current densities of up to 800 A/cm2at forward bias of 1 V, and a cut-off frequency of 1.8 GHz. When incorporated in a single stage rectifier, the cut-off frequency is 1.1 GHz. The rectifiers are demonstrated to operate at ultra-high frequency (868 MHz) with low losses.
This achievement, together with imec’s recent demonstration of a functioning bidirectional thin-film RFID circuit (at ISSCC2012), are critical research development steps towards the realization of intelligent item-level tagging with broad implementation opportunities in the retail sector.
The research on thin-film UHF RFID technology is supported by the EU FP7-ICT-247798 project ORICLA. Project partners include the project coordinator imec (Belgium), Holst Centre – TNO (The Netherlands), Evonik Industries AG (Germany), and PolyIC (Germany).