Flexible Polymer Thin-Film Transistor Device Structures And Processes For 13.56 MHz RF Rectifier Circuits
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0937-M08-03
Flexible Polymer Thin-Film Transistor Device Structures And Processes For 13.56 MHz RF Rectifier Circuits Siddharth Mohapatra1, Robert Rotzoll1, Patrick Jenkins1, Viorel Olariu1, Michelle Grigas1, Robert Wenz1, Klaus Dimmler1, and Ananth Dodabalapur1,2 1 OrganicID, 422 E. Vermijo Ave, CO Springs, CO, 80903 2 MER, University of Texas at Austin, Austin, TX, 78758 Introduction Two of the greatest attractions of organic electronics include the ability to fabricate flexible circuits and systems and the ability to utilize inexpensive print-compatible processing techniques for fabrication. This necessitates the development of high performance transistors on flexible substrates. Specifically, our group is involved in developing fully solution processible organic RFID tags for item level tagging. To fabricate organic RFID tags, it is required that some of the sub-circuits such as the rectifiers, operate at the industry specified frequency of 13.56 MHz. To rectify such signals, it is important that transistors have mobilities that are in excess of 0.1cm2/Vsec, Ion/Ioff ratios > 1000, threshold voltages ~ 0 V, low hysteresis, good saturation characteristics and good stability in air under applied bias conditions. Additionally, these properties have to be achieved on 2 -3 µm short channel length transistors, as the speed of a rectifier varies inversely as the square of the channel length. Thiophene based polymer semiconductors have emerged as attractive candidates for fabricating p-channel organic thin film transistors (OTFTs)1,2,3,4. Some of these polymers have been demonstrated to have p-channel mobilities in the range of 0.01 – 0.1 cm2/Vsec in bottom gate geometries on SiO25,6,7,8,9. In this article, we describe our efforts towards the development of a top gate transistor structure with 3µm channel lengths using polymer semiconductors and a spin processible dielectric. These top gate transistor processes are used to fabricate high performance rectifiers that demonstrate rectification with useful efficiency at a frequency of 13.56 MHz. Results and Discussion The transistors developed for the RFID application utilize a top gate, bottom contact structure shown in Figure 1. Processing begins with a 5 mil thick polyethylene-naphthalate or a polyethylene-terepthalate (PEN/PET) polymer substrate upon which a layer of gold is deposited and chemically etched. This layer comprises the source-drain pattern with critical dimensions of 2-3 microns. Appropriate surface treatments are used on the substrate. A polymer semiconductor is spun and annealed and subsequently a polymer dielectric is spun on top of the semiconductor. A patterning step for etching via holes in the dielectric is followed by the deposition of a patterned metal. This second layer of gold acts as the gate electrode and also provides via interconnection through the dielectric. These top gate transistor processes are optimized and used to fabricate RF rectifiers. The target power supply voltage in this application is 20 Volts peak (or 40 Volts peak-peak) at f
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