Development of a prototype of flexible antennas with a passivation layer for electrical impedance sensors

Background and Objectives: The need for flexible electronics for biomedical and communications applications has completely transformed the field of antenna design and has given rise to the development of flexible antennas. The materials used to create antennas are diverse, but there remains a need to define a design that provides both durability and high performance. Materials and Methods: The test samples were flexible polyimide substrates 500 μm thick, on which layers of Cu and Ti or Ni were formed by vacuum magnetron sputtering. A Robvac VSM300 vacuum setup equipped with three magnetron sources with 50 mm diameter disk targets was used for this purpose. The Cu layer thickness on all samples was 1 μm. The thickness of the Ti and Ni layers was 100 nm. Demetallization was performed using a pulsed laser of a Minimarker 2 setup. The main characteristics of the laser: the wavelength of laser radiation is 1064 μm, the maximum laser pulse energy is 1 mJ, the software and hardware resolutions are 2.5 μm, the scanning device is a 2-axis galvanoscanner. The parameters of the laser demetallization mode were as follows: power 2.6 W, beam speed 1000 mm/s, pulse frequency 99 kHz, pulse duration 8 ns. The demetallization template, which is a flexible loop antenna with the geometry optimized for resonance in the 3.75 GHz range. Two versions of the antenna were manufactured with the conductor line width d equal to 1 mm and 3 mm. Results: It has been found that an additional layer of metal on the surface of the Cu thin-film coating of flexible polyimide substrates improves the quality of demetallization. Without it, areas with defects in the form of an unremoved Cu film short-circuiting individual antenna elements are observed. On samples with an additional passivation metal layer, demetallization occurs exactly according to the template without defects. It has also been found that significantly fewer drops of molten metal remain on samples with a passivation Ni layer. The results of experiments with ready-made antennas have demonstrated a significant sensitivity of the resonance frequency to bending in the range of angles from −60º to +60º, which opens up the possibility of using such antennas as microdisplacement sensors. At the same time, the developed antenna prototypes have a reflection coefficient less than −17 dB in the bending angle range from −100º to +100º. Conclusion: The work has assessed the influence of an additional layer of passivating metal on the quality of laser demetallization (ablation) of a Cu thin-film coating of flexible polyimide substrates. The experimental results obtained allow us to count on the applicability of the developed antennas as micro-displacement sensors. At the same time, the low reflection coefficient of flexible antennas in a wide range of bending angles allows them to be used as the basis of electroimpedance sensors. 

flexible antennas, laser ablation, electroimpedance sensors, copper passivation