Effect of Sintering Atmosphere for the Fabrication and Characterization of Flexible RFID Antenna Tags Using Printed Copper Nanoparticulate Patterns

RFID is playing an essential role in the Internet of Things (IoT) technology by enabling contactless communication and information transfer between digital and physical entities. Different research procedures are being developed aiming for the highly precise pattern for metal conductivity, low electrical resistivity, low cost of material, and flexibility of the substrate. Among the electronics materials, copper (Cu) nanoparticle (NP) inks can be the main material for flexible printed electronics because it possesses high electrical conductivity, stability, and low price in addition to anti-ionic migration capability.<br/><br/>Cu NP ink (CP-008, NovaCentrix) was selected for printing and characterizing the conductive patterns. The ink had the following compositional properties: solids content of 88 wt%, a viscosity of 20-40 Pa.S at 50 s^-1, and a density of 3.9 g/ml. Kapton polyimide (PI) sheets with a thickness of 0.102 mm were used as the flexible substrate. Metallic ink comprises metal NPs, capping molecules, additive organic materials, and solvents. Terpineol was included as a solvent in ink, according to the ink specification, but the rest of the composition in ink was not revealed. Removal of organic substances is required to have electrical conductance along with the diffusion of particles resulting in neck growth and grain boundary to obtain electric conductance of the Cu NP patterns. Air, N₂, and five different carboxylic acid vapor groups were used as sintering atmospheres. The sintering temperature varied from 140 to 260 °C, and sintering times varied from 15 to 60 mins. Sintering temperatures were maintained below 260 °C to avoid any deformation and degradation of the substrate, considering the glass transition temperatures of the PI substrates.<br/><br/>Scanning electron microscopy (SEM) was used to analyze the shape and size of the Cu NPs, microstructure, and thickness using cross-section imaging. Then Avizo software was utilized to determine the size distribution and average particle size of the Cu NPs from the SEM images. Using a thermogravimetric analysis (TGA) with an N₂ gas flow rate of 100 ml/min and a temperature ramp of 10 °C/min to achieve 600 °C, the decomposition temperatures and metal weight contents of Cu NP ink were evaluated. The surface roughness of the sintered Cu NP pattern was measured with an atomic force microscope (AFM). Crystallinity and grain sizes of sintered Cu NP patterns were observed using X-ray diffractometry (XRD) with Cu K<i>α </i>(1.5406A) radiation. Electrical characteristics were evaluated using a four-point probe. The hardness of the sintered Cu NP patterns was measured using micro indentation. An adhesion test was conducted following ASTM-D3359-09 standard cross-cut tape test. Folding tests evaluated the flexing or bending capabilities of the conductive lines. The electrical characteristics of the RFID antenna were evaluated in a 13.56 MHz band using a network analyzer.<br/><br/>The electrical performance of the Cu NP patterns was evaluated using RFID antenna fabrication. The antennas were printed and sintered using optimized sintering conditions of Cu NP ink. Three spiral and three-square patterns were designed with different dimensions, such as the number of turns, line width and spacing, inner/outer radii for spiral patterns, and inner/outer diameters for square patterns. Their resonance frequency, return loss (S₁₁), and quality factor (Q-factor) for each antenna were measured using a network analyzer. The Q-factor characterized the capability of the antenna to communicate with an RFID reader and an RFID antenna. 13.56 MHz RFID system was followed due to its excellent performance. Too low or too high a Q factor might result in not being able to read the frequency or narrow peak, along with interference between the RFID reader and RFID antenna. The Q-factors in the range of 5-20 are preferred for RFID antenna tags fabricated in the 13.56 MHz band.