Background

Advancements in automobile technology and ever-growing need for communication have made radio communication systems a necessary part of our day-to-day life. Capacity increase in connectivity and sensing demands for vehicles including GPS, entertainment, satellite radio, and systems for collision avoidance [20, 21] are pushing the communication limits further. Expansion in connectivity and communication promised by 5G technology, which will greatly increase vehicle communications, both vehicle to overhead or terrestrial transceivers and vehicle-to-vehicle [22-24] will further widen these boundaries. With growing demand for radio frequency communication, antenna design and fabrication have gained new momentum. While monopole and shark fin antennas have been dominant [20], the increasing need, along with aesthetics and cost, is driving other alternatives. One such alternative is transparent flexible antenna.

Transparent flexible antennas could be unobtrusively mounted to the windows of a vehicle, matching the curve of the glass and located where terrestrial and satellite signals can be received. The substrates used for antenna fabrication therefore need to be high transparent and have an index of refraction that matches with other glass to provide an uninterrupted view through vehicle windows. From a fabrication point of view, these substrates should have good surface quality and high thermal and chemical stability. Corning® Willow® Glass studied in this chapter provides aforementioned qualities, thus providing an excellent substrate for transparent antenna fabrication [13, 17-19]. Furthermore, low cost and high volume that are essential in automotive industry drive the selection of R2R manufacturing.

R2R has been utilized in industries such as document printing and can be applied for the fabrication of antennas and structures for RFID to reduce cost and increase throughput [10,11,25]. R2R manufacturing allows provisions to use glass rolls instead of using glass sheets, which are difficult to handle [16, 19]. At Center for Advanced Microelectronics Manufacturing (CAMM) at Binghamton University, we have worked extensively on R2R processing of Willow Glass [26, 27]. In this chapter we do not fully develop R2R manufacturing. Alternatively, we use 100-gm thick glass in 100-mm diameter wafers in initial tests and 150 mm x 150 mm sheets for the antennas and develop processes in small scale that are fully compatible with R2R manufacturing. Here, we will discuss the use of low-cost etched or semi-additive copper and weak acid etchants compatible with R2R fabrication to make conductive meshes and transparent antennas. Transparent antennas, both rigid and flexible, have been demonstrated using meshed conductors. Refs. [28] and [29] demonstrated transparent antennas using freestanding wire mesh screens patterned and attached to a dielectric (Lexan polycarbonate or polydimethylsiloxane) to form patch antennas. Guan et al. [30] have demonstrated the use of mesh conductors for the fabrication of single-layer antennas by utilizing printed meshes of silver paste with lines as small as

2.5 pm. Furthermore, Hong et al. [31] and Lee and Jung [32] used copper meshes to create microstrip patch antennas and receivers for wireless power transfer, with lines approximately 20-pm wide. This work demonstrates that the initial prototyping on flexible glass can be done with conventional lithographic tools and plating rigs, allowing processes to be developed in wafer and sheet scale that are compatible with R2R systems.

Indium Tin Oxide Transparent Antenna on Flexible Glass

Fabrication

The materials used in this study primarily consisted of Corning® Willow® Glass wafers of 100-pm thickness as the substrate, sputtered indium tin oxide (ITO) as a conductor layer, and an Al-SiO, adhesion layer between ITO and glass as a barrier layer. In most cases, the flexible glass wafers were attached to a polyimide wafer-on-web (web) and these thin-film coatings were applied using our R2R GVE sputtering system.

Design

The goal was to design a single-sided and single-layer antenna that could be directly connected to the co-planer waveguide (CPW) of a GPPO connector. Additionally, the antenna performance had to be optimized for 100-pm-thick glass. Losses in the ITO could be minimized if a small antenna size was used. With these requirements in the building and the work done in [33, 34], a single-sided CPW-fed antenna design serves as the basis for further design and optimization. Optimizations include the additional design frequency of

  • 5.8 GHz and changing the initial design from a split-ring type antenna to a patch antenna to minimize resistive losses and improve performance. Antenna design and simulations were carried out using ANSYS HFSS and CST Microwave Studio software. This allowed for tuning and optimization of the antenna, including compensating for differences in the sheet resistance of the ITO as shown in Figure 12.2. Optimized antenna designs for 2.4 and
  • 5.8 GHz are shown in Figure 12.3.

FIGURE 12.2

Measured S parameters for different ITO sheet resistances. Note that both the magnitude and location of the radiation peak are affected by this value.

FIGURE 12.3

Schematic antenna designs and dimensions for (a) 2.4 GHz antenna and (b) 5.8 GHz antenna. Note that the 5.8 GHz design is smaller in size as indicated on the drawing.

 
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