Sintering of Inkjet-printed Cu-nanoparticle Ink in Ambient Conditions Using a Continuous Wave 808 nm Diode Laser
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Sintering of Inkjet-printed Cu-nanoparticle Ink in Ambient Conditions Using a Continuous Wave 808 nm Diode Laser Authors: Halonen, E.1, Koskinen, S.1, Leino, I.1, Heljo, P.1, Mäntysalo, M.1 Affiliations: 1Tampere University of Technology, Department of Electronics, Korkeakoulunkatu 3, 33720, Tampere, Finland ABSTRACT In this paper, we focused on sintering of inkjet-printed copper nanoparticle ink structures using a continuous wave 808nm diode laser. Laser sintering in printed electronics is a rapid sintering method which enables localized sintering. Sintering of Cu inks is usually done in nitrogen atmosphere but the novelty of this study is that successful sintering of Cu ink was done under ambient conditions. The used ink consists of copper nanoparticles covered with a dispersion agent. Photonic sintering is needed to speed up the sintering process to prevent oxidation during sintering. Electrical and mechanical performance of the printed structures was analyzed. Resistivity of 10-12 µΩcm with good repeatability as well as excellent adhesion, were achieved. INTRODUCTION The first nanoparticle inks developed for printable electronics have been gold and silver based inks. These inks provide good conductivity but the bulk metal price is high. In a large scale manufacturing the ink prices and bulk price of the metal become significant. To attract the interest of manufacturers, ink developers have been recently developing copper nanoparticle inks. Some inks are available commercially and some can be acquired for research use as they are still in a development phase. Along with cost-related issues there are also other challenges with silver based nanoparticle inks, which drive the need for copper nanoparticle inks. For example, silver nanoparticle inks are not compatible with soldering processes due to silver leeching [1] and require conductive adhesives for component interconnections. [2] The use of copper nanoparticle inks might bridge the gap between inkjet-printed circuits and soldering processes. Currently only a few types of nanoparticle copper inks are available commercially or for research use. One type of copper nanoparticle ink consists of copper nanoparticles covered with a dispersion agent, which prevents the particles from oxidizing and agglomerating. In printed electronics and in the case of metal nanoparticles particularly, heat exposing is needed to evaporate solvents, decompose dispergents and finally form conductive paths through metal particles. Term sintering actually refers to the latest phase of this process where the dispersing agent has been burnt off and physical connections between particles begin to form. [3] Furthermore, some fundamental studies on sintering of metal nanoparticles have been made during the past years. [4-6] Heat to the ink can be supplied with e.g., oven [7], laser [8], or pulsed light. [9] The sintering time for copper nanoparticle inks needs to be very short due to copper nanoparticle oxidation. The viable solutions to processing inks of this sort are photonic sintering meth
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