Electronically Re-Configurable, Non-Volatile, Nano-Ionics-Based RF-Switch on Paper Substrate for Chipless RFID Applications

This article reports the first results of a Nafion® based solid state Non-Volatile 13 electronically reconfigurable RF-Switch integrated to a Co-Planar Waveguide transmission line 14 (CPW) in shunt mode, on flexible paper substrate. The switch is based on a Metal-Insulator-Metal 15 structure formed respectively using Silver-Nafion-Aluminum switching layers. The presented 16 device is fully passive and shows good performance till 3GHz, with an insertion loss less than 3dB 17 in the RF-On state and isolation greater than 15dB in the RF-Off state. Low power DC pulses in the 18 range 10V/0.5mA and -20V/0.15A are used to operate the switch. The device is fabricated in an 19 ambient laboratory condition, without the use of any clean room facilities. A brief discussion on the 20 results and a potential application of this concept in a re-configurable chipless RFID tag is also 21 given in this article. This study is a proof of concept, of fabrication of electronically Re-Configurable 22 and disposable RF electronic switches on low cost and flexible substrates, using a process feasible 23 for mass production. 24


Introduction
In this article for the first time we present the design, development and application of a simple 46 and robust CBRAM based MIM RF-switch on flexible paper substrate. In this experiment, the entire device fabrication was carried out in ambient laboratory conditions, without the use of any clean 48 room facilities. In this study, we aim to prove the feasibility of integrating a MIM switch in 49 electronically reconfigurable RF devices on flexible and low cost substrates like paper, which are 50 seen around in our daily life as packaging materials for goods, transport tickets and visiting cards.

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An RF-Identification tag using the RF-Encoding particle proposed in this paper could be easily 52 fabricated as a sticker, on a paper substrate with adhesive background and could be attached to an 53 object, like a barcode sticker, with far more advanced functionalities in comparison to the optical 54 barcodes, like identification out of the optical-line-of-sight, electronic reconfigurability etc.

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The basic building block of the presented RF-Switch is a nano-ionic MIM switching cell. Fig. 1 57 shows the basic layer structure and working principle of the proposed switching technique. The

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MIM cell is comparable to a parallel plate capacitor in which the electrolyte is replaced by an 59 ion-conductor, which is an electrically insulating material, such as PMMA [7] or doped Chalcogenide 60 glass [4], or Nafion [8], etc. One electrode of the cell is an ion-donor metal like silver or copper, 61 generally called the active electrode and the other is a relatively inert metal like aluminum or gold.

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On the application of an electric field from active to inert electrode, ions from active electrode under 63 the influence of this field grow a filament to inert electrode, through the ion-conductor layer. This

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The presented design is a modified version of the predecessor reported in [8]. The reported 80 version in this article [8] is a 50Ω CPW switch in shunt mode on FR-4 substrate, similar to the one

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The substrate used for the fabrication of the device is 80 grams per sq. meter (GSM) special 95 quality RF-paper developed at the IES lab. This paper is flexible and specially treated to reduce the 96 losses at radiofrequencies and also has an improved surface for better adhesion of metal layers.

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Metal layers are formed on the substrate by thermal vapor deposition of Silver (Active electrode)

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and Aluminum (Inert electrode). Thermal vapor deposition is done with the help of dedicated 99 Nickel masks which carry an engraved aperture of the presented topology in Fig. 3. After formation 100 of the active electrode, the electrolyte layer is formed by spin coating the Nafion solution (supplied 101 by Sigma Aldrich) at a rate of 500RPM for 30 seconds. The formed layer is then air dried on a hot 102 plate at 100 o C for 2 min. Then the inert electrode is formed using thermal vapor deposition.

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Detachable SMA connectors are then attached to the device to feed RF power.

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It is now apparent that the proposed switch could be integrated into most RF-Designs using a 105 three step process. 1. Deposition of active electrode. 2. Deposition of electrolyte/ion-conductor. 3.

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Inert electrode deposition. These steps could be easily carried out in an industrial environment by 107 techniques like flexography [9]. The use of silver metal in the tags could be limited to the switch area 108 only, by adjusting the nickel mask design, so as to reduce further the cost of realization.

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Photograph of the fabricated CPW shunt mode switch is shown in Fig. 4 (b) and the inset shows 110 the microphotograph of MIM switch area revealing the sandwich structure. The thickness of Silver 111 and Aluminum deposit is measured to be 1.3µm and 1µm respectively and that of Nafion deposit is 112 600nm. The measurement is carried out with the help of Dektak-150 mechanical Profilometer. inherent to this polymer in contemplation to materials used classically [4]. Nafion is a sulfonated 115 tetrafluoroethylene based fluoropolymer-copolymer, well known for its fast ion conducting 116 properties and use in proton-exchange-membrane fuel cells, and is stable upto 190 o C [10]. Nafion 117 layer could be easily formed by spin coating and could be air-dried at temperatures around 100 o C.

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From our experience in realization, this material is stable in the ambient air and do not require an 119 additional conformal coating for protection. The maximum temperature observed on the sample 120 during the thermal vapor deposition in around 120 o C.

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Working of the presented CPW shunt mode switch could be explained with reference to Fig. 3.

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MIM switch structure is formed in this device between the 100µm wide shunt line and 300µm wide 123 transmission line sandwiching 600nm layer of Nafion. Shunt line connects the two ground planes on 124 either side of the CPW line. When the MIM switch is Set as shown in Fig. 1, at the switch area labeled 125 in Fig. 3 (b), RF power from Port 1 is short circuited to ground, thus disconnecting it from Port 2.

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This is the RF-Off state of the switch. When the MIM switch is Reset as shown in Fig. 1, the short is 127 removed and Port 1 is connected to Port 2, forming the RF-On state.

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RF response of the switch is given in Fig. 6

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In this experiment the maximum Set resistance value is 10Ω and the minimum Reset resistance 151 value is 40kΩ. This is a good RReset/RSet value. RF response of realization of the CPW shunt mode 152 switch on classical FR-4 substrate [8] is given in Fig. 7 for comparison. This device shows an insertion 153 loss less than 1dB in RF-On state and greater than 16dB in the RF OFF state. From this response it is 154 clear that the realization on paper substrate has acceptable performance. Minor degradation in the 155 On-state insertion loss could be justified by the losses inherent to paper substrate.

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This idea of fabrication of RF-switches is seen to be functional and feasible in several 167 applications in the field of disposable RF-Electronics, in devices such as reconfigurable RFID Tags 168 [11,12], reconfigurable filters and reconfigurable antennas.

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Radio Frequency Identification or RFID is a multi-dimensional ingenious concept that has

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An example of potential advantage of using the proposed switch technology could be explained 182 as follows. Fig. 8 shows the method of frequency shift coding of RF-Encoding Particles (REP) in a 183 chipless RFID tag [10,14]. In this example a shorted dipole resonator is used as the REP. Classically, this is achieved by modifying the dimensions of each REP on the tag as shown in Fig. 8, for this, a 185 unique geometry has to be designed for each REP. But instead, if we introduce the proposed MIM 186 switch technique to this REP as shown in Fig. 8, the same performance could be achieved with a 187 general REP of fixed dimension, by electronically switching the MIM cell.

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If we integrate such REPs depending on how much bit density is required, on a credit card size 190 tag, with contact pad and Set/Reset lines, it forms a general reconfigurable chipless RFID Card. This 191 could be programmed by a dedicated logic device similar to a credit card reader to encode 192 information. And then could be used for identification in wireless mode similar to a conventional 193 chipless RFID tag. An illustration of this idea is depicted in Fig. 9.

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Discussion on the application of this concept for a commercial mass production and use, and 208 the expected convenience is explained in the following paragraphs.

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[17] Reports a 20-bit chipless RFID tag. Let's suppose to use this particular tag for labeling 210 application in which the tag is encoded by using two different dimensions for each resonator. Thus