Low-Thermal-Budget Photonic Processing of Highly Conductive Cu Interconnects Based on CuO Nanoinks: Potential for Flexible Printed Electronics.

In the developing field of printed electronics, nanoparticle based inks such as CuO show great promise as a low-cost alternative to other metal-based counterparts (e.g., silver). In particular, CuO inks significantly eliminate the issue of particle oxidation before and during the sintering process that is prevalent in Cu-based formulations. We report here the scalable and low-thermal-budget photonic fabrication of Cu interconnects employing a roll-to-roll (R2R)-compatible pulse-thermal-processing (PTP) technique that enables phase reduction and subsequent sintering of ink-jet-printed CuO patterns onto flexible polymer templates. Detailed investigations of curing and sintering conditions were performed to understand the impact of PTP system conditions on the electrical performance of the Cu patterns. Specifically, the impact of energy and power of photonic pulses on print conductivity was systematically studied by varying the following key processing parameters: pulse intensity, duration, and sequence. Through optimization of such parameters, highly conductive prints were obtained in <1 s with resistivity values as low as 10 μΩ cm (corresponding to ∼17% of the International Annealed Copper Standard (IACS) conductivity) was achieved. It was also observed that the introduction of an initial ink-drying step in ambient atmosphere, after the printing and before sintering, leads to significant improvements in mechanical integrity and electrical performance of the printed Cu patterns. Moreover, the viability of CuO reactive inks, coupled with the PTP technology and pre-sintering ink-drying protocols, has also been demonstrated for the additive integration of a low-cost Cu temperature sensor onto a flexible polymer substrate.