Flexible thin-film NFC tags

Abstract

Thin-film transistor technologies have great potential to become the key technology for leaf-node Internet of Things by utilizing the NFC protocol as a communication medium. The main requirements are manufacturability on flexible substrates at a low cost while maintaining good device performance characteristics, necessary to be compatible with the NFC specifications. Such low-cost flexible NFC tags can be attached to any object with any form factor, connecting this object to the Internet using a smartphone or tablet as an intermediate node. Among all commercial thin-film transistor technologies, metal oxide transistors is a viable technology for this application. The metal-oxide transistors in this work are based on InGaZnO as semiconductor. Since these are unipolar by nature (i.e., they exhibit only n-type transistors), different options to make logic circuits are studied from static and dynamic points of view. The different topologies are diode-load logic, dual-gate diode-load logic, and pseudo-CMOS logic. The static parameters lead to a comparison of soft yield between those circuit topologies, while the transient analysis provides insight on the power consumption and circuit speed. This is indicative for selecting the logic style matching the data rate requirements of the NFC standards. Moreover, metal-oxide NFC circuits that combine 12-bit code generators to the analog front-end of RFID tags are integrated on a PCB board to evaluate performance of a matched and optimized system. The measured data rates of these integrated NFC tags are compatible with the ISO 15693 specifications. Finally, a fully integrated, flexible NFC tag is realized, which comprises the tuning capacitor, rectifier, load modulator, and code generator.