# A 60-GHz Ultra-Thin and Flexible Metasurface for Frequency-Selective Wireless Applications
## 2 Materials and Methods
### Fabrication of 60-GHz Metasurface  
Several conductive inks, pastes and epoxies have been developed and reported for the screen printing of flexible electronics. Intrinsiq UK supplies multiple variants of copper-based nanoparticle pastes and ink depending on variation in the composition percentage, fluid wettability, adhesion due to bonding with the substrate surface, and conductivity which is mainly related to the concentration of  
Figure 2: The geometry of the proposed 60 GHz flexible metasurface in simulation (**a**) simulated unit-cell with the dimensions, (**b**) A 5 \(\times\) 5 unit-cell segment displayed from the infinite metasurface array. the copper particles. The screen printable nanocopper-based paste CP-007 with a solid weight of ~77%, fluid viscosity of 20-35 Pa S at 25 \({}^{\circ}\)C, the density of ~2.6 g/mL is used for printing the metasurface. Another variant to ink CP-005 from Intrinsiq UK was used in Reference [23] with higher density and solid nanoparticle metal content. It has been observed that in this work, using a paste with lesser density, improved wettability and better ink flow has facilitated to achieve the close approximation of actual dimensions by a screen printing process, as the suggested conductive paste flows well at the edges of the mask. While in Reference [23], high density of ink was responsible for the reduced size of the realized pattern, and the shorter length shifted the resonant band to some extent towards higher frequencies. The standard screen of size 200 mm \(\times\) 340 mm, comprising a fine stainless-steel mesh firmly captured within a wooden frame, with a mesh count of 325/inch and a wet print thickness of ~62 um is used. The selected mesh count contains a standard aperture size of ~42 um and a mesh opening ratio of ~39%. The pattern is engraved on the screen with a 12 um thick photo emulsion coating to transfer the paste. A squeeze tilt of 45\({}^{\circ}\) is deployed, and a reasonable level of accuracy has been achieved. The next step involves the curing and sintering of the printed metasurface to achieve the desired level of conductivity, which is also carried out in the manufacturing facilities of the Intrinsiq UK. The heat sintering process is recommended suitable for the conductive paste CP-007 used in this work by the manufacturers, unlike CP-005 ink from Reference [23] where laser sintering was a suitable choice. Laser sintering, though a fast and efficient method, however, involves high risks if a precise control over the intensity of the laser beam and exposure time is not carefully adopted. Heat sintering, on the other hand, has been found quite flexible and adjustable in this regard. The convection oven set at a temperature of 60 \({}^{\circ}\)C is used for drying, while the heat exposure time is 30-60 min. For sintering process, the temperature is increased to 180 \({}^{\circ}\)C for a 1-h duration, and the sample is placed in the convection oven in a reduced atmosphere of argon or nitrogen with a 3% formic acid vapor. The profilometer measurements depict that the layer thickness of the copper pattern is ~7-10 um and the sheet resistance computed by using a 2-point probe test range from 5-7 m\(\Omega\)/sq/25 um over the entire screen-printed metasurface. These values suggest that the computed conductivity of the printed sheet is approximately 1.4 \(\times\) 10\({}^{6}\)-3.3 \(\times\) 10\({}^{6}\) S/m. The screen-printed prototypes of the designed metasurface at 60 GHz are shown in Figure 3a,b on Melinex and Kapton, respectively. The adhesion of the printed patterns is examined by following the manual testing procedures, such as stretching, bending or folding over a certain curvature, as well as destructive testing by scotch tape test. These tests have confirmed the reliability and robustness of the prototypes under strain conditions as no visible cracks or damage is observed afterwards.