# Flexible Antennas: A Review
## 3 Fabrication Techniques for Flexible Antennas
### Chemical Etching  
Chemical etching, often accompanied by photolithography, emerged in the 1960s as a branch-out of the Printed Circuit Board (PCB) industry is the process of fabricating metallic patterns using photoresist and etchants to mill out a selected area corrosively. For fabricating complex designs with high resolution accurately, it is the best choice among all other fabrication techniques [115]. Organic polymers are suitable for photoresists as their chemical characteristics change when they are exposed to ultraviolet light. Current practice in the photolithography based antenna and RF circuits industry relies mainly on positive resists since they present higher resolution than negative resists. A multilayer type of flexible monopole antenna was designed and fabricated on a transparent polyimide substrate for application in wearable glasses in an earlier article [116]. A 100-nm-thick indium-zinc-tin oxide (IZTO)/Ag/IZTO (IAI) is a transparent (81.1%) conducting oxide electrode, which was used as the conductors of antennas and ground planes of the wearable glasses. Physical Vapor Deposition (PVD) process is employed to fabricate this multilayer type of flexible antennas. The fabrication processes are shown in Figure 8.  
Figure 7: Examples of D printed antennas. (**a**) Square patch antenna on NinjaFlex substrate (dimensions in mm A = 65, B = 55, L = 35.8, w = 3, g = 1, t = 7) [107], (**b**) brush-painted wearable antenna on a 3-D printed substrate [112], (**c**) button-shaped 3-D radio-frequency identification (RFID) tag antenna [111], and (**d**) 3-D printed flexible inverted-F antenna (IFA) [113].  
This work shows the feasibility of wearable, flexible antennas for optical and electrical applications using the photolithography process. A practical 5.8 GHz antenna for the wireless operation was fabricated on a flexible glass by photolithography using Shipley S1813 photoresist and a Heidelberg Instruments \(\upmu\)PG 101 pattern generator. [117]. A 100 nm thick aluminum-doped \(SiO_{2}\) layer served as a buffer layer on the glass before 650 nm thick sputter deposition of the indium-tin oxide (ITO) layer. After lithography, the ITO was etched using an oxalic acid etch was annealed in a rapid thermal annealing system in nitrogen at 500 \({}^{\circ}\)C for 5 min with a ramp rate of 5 C/s. Loops and grid configurations are possible using this methodology. A small size (2.5 cm \(\times\) 5 cm) epidermal RFID antenna was fabricated using four different fabrication systems, including photolithography [118]. The process is shown in Figure 8. Here, Au is used as the antenna conductor with a Ti/W adhesion layer on the polyimide substrate. Though high-resolution throughput has been possible using microfabrication, high costs associated with cleanroom facilities, photo masks, photolithography chemicals, and human resources will prevent the epidermal antenna from being inexpensive and disposable. A process flow using photolithography and sputtering was studied for high-quality Cu antenna fabrication on the PET substrate without any damage to the supporting layer [119]. To conclude, complex and fine detailed antennas can be fabricated using photolithography. However, the lengthy process, along with the involvement of dangerous chemicals, high-end expensive cleanroom equipment, photomasks, and chemicals with human resources, limit its application in fabricating flexible antennas.