Flexible high-performance metallic interconnects prepared by innovative diode laser array treatment of inkjet-printed layers
Inkjet printing is a digital technology, offering key advantages in terms of high speed (R2R compatible), low material consumption, and miniaturization of printed features. A thermal post-processing of inkjet-printed structures in furnace approximately for 30 min at temperatures at least 130 °C for silver or >300 °C in case of copper and gold is usually required to remove organic components, sinter the metallic particles, and enable electrical conduction. This is a critical technological step determining, on the one hand, the final morphology and properties of the printed materials, but on the other hand limiting the processing speed, narrowing the range of applications and materials, and increasing the costs of the printed structures. The present work focuses on advanced approach based on high power diode laser array treatment of inkjet printed layers. In this case, the energy of incident light is absorbed selectively by the printed structures leading to their localised heating on a millisecond timescale without damaging the thermally sensitive substrate. The water-based proprietary IKTS metallic nano-ink formulations (silver, gold, and platinum) were used for printing metallic interconnects on various thin (PET, 120 µm, paper, 170 µm) and ultrathin (PET, <10 µm) substrates. All 22 to 30 mm long printed lines were electrically conducting already after drying. In case of printed silver, electrical resistivity values were by a factor of ~7 (paper) to ~14 (PET) higher than those of the bulk material. Subsequent millisecond laser processing enabled silver contacts with a low electrical resistivity (~3x of bulk) even on ultrathin 2.5 µm PET substrates. The interconnects on paper were bendable to a radius as small as 4 mm with the resistivity increase of 1%; a 100 cycle test of bending to a radius of 10 mm led to negligible changes of their resistivity values. The resulting porous morphology of the silver layers appears to be crucial to ensure their high flexibility. The developed interconnects were validated by realizing electrical contacts for large area (75 mm x 200 mm) flexible arrays of magnetic field sensors. In case of gold and platinum inks, this approach also yielded layers with low electrical resistivity, which was in case of gold structures by a factor ~10 higher compared to the values of the bulk material. The effect of laser processing parameters on the electrical properties of interconnects was related to their microstructure modification.