Innovative laser processing of inkjet-printed layers

The integration of electronic components on flexible substrates over large areas remains challenging although is a necessary step to boost the application potential of flexible electronics. We present scalable technological approaches of industrial relevance to enable printable and flexible high-performance metallic interconnects on thin (100-200 µm) and ultrathin (<10 µm) polymer substrates as well as on thin paper (170 µm) and flexible glass substrates (100 µm). It is based on combining conventional inkjet printing with a novel post-processing approach based on a micro-optically designed one-dimensional diode laser source (diode laser array) having a line-shaped beam profile. The continuous wave operation of the diode lasers along with a continuous substrate movement ensures true compatibility of the process with inline printing (R2R compatibility). High selectivity (narrow spectrum of diode lasers) and high post-processing speed (typically in the range of 5-20 m/min, yielding dwell time in the range of milliseconds), allow high-quality metallic structures without damaging thermally sensitive substrates. The key is simultaneous adjustment of the ink properties and the laser post-processing parameters in order to realize metallic interconnects with defined morphology enabling their low electrical resistivity and high flexibility. The work was focused on realization and characterization of Ag structures on ultrathin PET foils with a thickness down to 2.5 µm using proprietary IKTS nano-Ag ink. The properties and structure of these interconnects are compared to those prepared on a range of thin (100-200 µm) polymer (PET, PI), paper and thin glass substrates. As-printed 30 mm long Ag lines after drying were already electrically conducting with electrical resistivity values by a factor of ~7 (paper) to ~14 (PET, PI, thin glass) higher than those of the bulk material. Subsequent millisecond laser processing enabled silver contacts with a low electrical resistivity (~3x of bulk) on flexible substrates with thicknesses down to 2.5 µm. The contacts 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 developed interconnects were validated by realizing electrical contacts for large area flexible arrays of magnetic field sensors. The similar approach using inkjet printing of proprietary IKTS Au and Cu nano-inks followed by the laser post-processing in air also yielded metallic structures on thin polymer foils with electrical resistivity by a factor ~10 higher compared to the values of corresponding bulk materials. Detailed characterization of the structure and mechanical properties of these interconnects is subject of ongoing work.