Printing RFID tags quickly and in high volume results in cost and price structures that facilitate the creation of entirely new markets.
By Henning Rost, Wolfgang Clemens
Applications for silicon-based RFID technology (Radio Frequency Identification) have expanded during the last several years to include consumer-goods supply chains, general logistics, automation, brand protection, and product identification. The general public already has contact with many RFID applications—from non-contact ski passes to company IDs with access control to the timing of sports events.
Printed electronics can expand RFID application areas into price-sensitive mass markets, such as consumer-goods packaging on item level, electronic brand protection, simple identification, electronic ticketing, and many others. One of the long-term goals for RFID producers is to position the technology to replace optical barcodes on goods in supermarkets.
The RFID tag structure
Early efforts in this area grew from simple, organic field-effect transistors (OFETs) developed by means of common cleanroom technology, including photolithography, wet etching, spin coating, and more. Soon, even more complex electronic devices, such as voltage inverters and ring oscillators, were created using organic materials. But what does such an OFET, the basic element of printed electronics described in this article, look like?
An OFET is built from four components (Figure 1) that, except for the substrate, can be printed in the form of electronic ink, as these materials are soluble in common organic solvents. Source and drain electrodes made from a conductive material that lies on top of a flexible substrate carrier that is then covered by a semiconducting polymer followed by a layer of insulating polymer. Completing the structure is a gate electrode that is also deposited in the form of a conductive material. Obviously, the entire stack of a few layers looks relatively simple. That is exactly the reason why such devices can be produced by additive printing processes and end up just a few micrometers thick.
Poly(ethylenterephthalate) (PET) is the substrate material of choice for printed RFID tags. In addition to its electrical properties, it also has the mechanical-properties profile required by high-speed roll-to-roll printing processes. Intrinsically conductive polymers, such as doped polyaniline or the PEDOT/PSS system nanoparticles and metal pastes are suitable materials for electrodes.
The most important layer for integrated circuits—the semiconductor—often consists of conjugated polymers. These plastics are characterized by strongly alternating sequences of single and double bonds. Thus, such plastics possess a delocalized-electron system resulting in semiconductive properties. Associating the term plastics with the attribute semiconductive is still relatively unusual, although polymers showing such properties were described in the early 1970s.
Did you enjoy this article? Click here to subscribe to the magazine.