# High-Density 3D Printable Chipless RFID Tag with Structure of Passive Slot Rings
## 1 Introduction
[PERSON] also proposed a C-shaped slot nested chipless tag [26]. In order to reduce the interference, the adjacent C-shaped slots are placed on two nested patterns, respectively. The cost is high because of the large area of the tag. Meanwhile, the direction of the excitation electric field has to be consistent with the direction of the C-shaped opening, which limits its application. Three-dimensional (3D) printing is a new emerging technique, which is suitable for the prototype fabrication in a rapid way. With the designed digital model files, a bondable material such as powdered metal or plastic is used to construct the designed object by layer-by-layer printing. It is also called additive manufacturing technique (AM), which can save material and lower the cost. 3D printing technology has been applied in medicine, sensors and electronics [27; 28; 29]. The technology is also used for printing inductors and capacitors in the circuit [30]. 3D printing is a technology with a rapid production of 3D objects directly from digital CAD files. The 3D printing process is a bottom-up technology for 3D object fabrication. No milling or molding is needed anymore. In a 3D printing process, the material or silver paste is deposited in a successive way. Multiple layers of thin film are accumulated on the substrate to build up a solid 3D object. 3D printing process offers many advantages over traditional manufacturing techniques. It is a single step manufacture, with the merits of design freedom, low cost, easy customization and sustainability. For the etching process, a mold is needed to confine the structure. We have used the method to fabricate a prototype of PCB tag [31]. The detail information can be found in the reference. The characteristics are similar to the 3D printing tag. However, its cost is relatively high. Its resolution is lower than that of the 3D printing tag. The substrate that can be used in the etching process is limited by many aspects, including its etching property, its thickness and its composites. It cannot be used on flexible substrate. For the stencil/screen-printing, a mesh is needed for transferring the ink onto a substrate. The areas with the blocking stencil are impermeable to the ink. So, the blocking stencil is necessary. Above all, the 3D printing process shows many merits compared with the traditional process. The designed structure is controlled by the digital file. The layers are defined by software that takes a series of digital cross-sections through CAD software. Descriptions of the slices are then sent to the 3D printer to construct the respective layers. The layers can be constructed in a number of ways depending on the 3D printer being used. In this paper, a nested rectangular slot structure is designed and implemented by the fused deposition modeling (FDM) 3D printing technology. The tag can be printed on either insulating flexible substrates or solid substrates without any observable deterioration on the encoding rules and the spectral characteristics of the tag. The tag adopts a rectangular slot ring nesting structure with increased coding capacity without increasing the area of the tag. It has a compact structure and strong coding capacity.