Fang, Feihuang; (2020) High resolution printed flexible copper interconnects. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In the printed electronics industry, conductive copper colloid inks have drawn a great deal of interest as a cost-effective substitute. However, the main challenge with copper is its propensity to oxidise at ambient temperature conditions, which substantially hinders its electrical conductivity. This research aims to print electrically conductive copper interconnects with the highest achievable microscale resolution for developing applications in the subfield of flexible electronics. This thesis is formed by three parts. Firstly, it will focus on the developments of printable copper nanoparticles inks for the direct-write assembly technique. Then, improving the chemical reaction method to enhance the printing resolution becomes the main research area. Finally, exploring printed electronics with copper nanowire inks is going to be tested. Two sets of copper nanoparticles inks have been developed by using chemical reduction methods. Average diameter of 275 nm and 60 nm copper nanoparticles were obtained by using different reducing agents (L-ascorbic acid and hydrazine hydrate). The parameters of the synthesis process are controlled to achieve the optimised nanoparticle size with an uniform shape and distribution. On the other hand, copper nanowires ink is optimised to associate with the printing technique. The conductive printed line has been generated without any conventional heat treatment. As the oxidation issue is the biggest challenge for using copper colloid inks, oxidation prevention has been developed during both synthesis, printing and sintering stages. Oxidation during the ink synthesis is mitigated by double capping agents - polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB). Oxidation during the printing was investigated with different printing patterns. To further minimise the oxidation issue, intense pulse light (IPL) and laser sintering are mentioned in the section of future work. These techniques could provide high energy intensity in milliseconds. The short period is not only useful for reducing the oxidation, but it also prevents the substrate from being damaged. This work successfully demonstrates the ability to print copper ink and achieve very high resolutions (∼10 µm) on both glass and polyamide substrates. Good electrical conductivity has been achieved after 20 layers of repeated printings, which shows electrical resistivity two orders of magnitude lower than bulk copper and both adhesion and bending tests show good durability of printed structures. One layer printing has been achieved by using Cu Nanowries ink. PEDOT:PSS and PVP modifier have been used as capping agents to prevent oxidation as well as improve the adhesion property, and low temperature sintering has been optimised to achieve the best performance of electical conductivity.

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