Printed copper connections on flexible substrates

Abstract : This paper presents our first results on printed copper connections on flexible organic substrates obtained in a single-step process. Many progresses have been made in the field of conducting inks and printing systems and potential applications are within reach. Nowadays the most common conducting inks involve silver nanoparticles. However this solution presents some drawbacks like the high cost of these inks due to the base metal and to the synthesis and also their tendency to clog nozzles in industrial inkjet printheads. For these reasons we have started a program on copper inks for the formation of conducting, homogeneous and adherent connections on polyethylene terephthalate films in view of applications in the field of flexible electronics. The main challenge is to print copper patterns on PET with an industrial process that would present a high time to readiness level. Multiple technical obstacles are addressed: i)- The ink needs to be nanoparticle free: we have chosen a metal organic decomposition ink (MOD) where copper salts are combined with ligands and additives, ii) it must be compatible with an inkjet system in terms of viscosity, rheology and iii) the printed copper must be conducting and adherent. This means that reduction of copper ions must be achieved using a post-printing treatment harmless to the substrate. Intense Pulsed Light was chosen. Printed patterns were characterized by various means: macroscopic electrical behaviour by 4 probe measurements, microscopic electrical behaviour by conducting probe Atomic Force Microscopy (AFM). Morphological and chemical analyses were also performed. It is shown that in optimised conditions good conductivity can be achieved. Much work is still needed to fully understand the mechanisms involved and realize electronics circuits.