A concept is presented where a temperature sensitive printed nano structure is integrated into a UHF RFID tag antenna. The printed structure acts as a WORM memory (Write Once Read Many) allowing telling whether a tag has been exposed to excessive temperature since the last read. The 1-bit WORM used in experiments is denned through its resistance where a logical zero equals a high resistance over 2kΩ and a logical one equals a lower resistance, typically less than 50Ω. Setting the bit can be achieved through sintering, which is a process to enhance junctions among nano-particle silver and reduce resistivity by heating. The WORM is designed to modulate the impedance of a passive RFID tag antenna, aiming to change the tag from readable to unreadable or the other way around. The simplest way of modulation is to set the WORM to be parallel with the silicon chip. The tag then works well with the high WORM resistance apparent before sintering and poorly or preferably not working at all after sintered because the chip is short-circuited by the low WORM resistance. A variant of the half wavelength dipole has been used to demonstrate this concept and it should also work with other antenna patterns. Both the WORM memory and the tag antenna are printed on photo paper by an InkJet printer and the silicon chip is connected to the antenna using the electrically conductive ink. Compared to the sensor solutions that integrate the normal battery supported sensors into silicon chips with the aid of A/D converters, the sensor tag presented in this paper is much simpler and allowing easy fabrication. It has long lifetime, a smaller size as well as a lower cost. Since the WORM memory by definition stores the state and can be read out long time after it's been programmed this tag can tell about past events. Potential use include all applications where one wants to detect if a high temperature has occurred since the last time of read and where normal sensors either have too short lifetime or are too expensive.
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