New printing technique improves electronic circuit manufacture

Print production (or printed electronics) technology for fine metal wiring that is indispensable for fine electronic circuits has faced several technological difficulties such as in reproducibility due to the contamination of printing plates or other apparatus, in sintering or fusion of metal particles on substrates after printing, in avoiding distortion of plastic substrates by high temperature post-treatment, and in avoiding peeling off the printed wires due to the substrate bending.

The developed technique utilises selective chemisorption phenomenon of silver nanoparticles, included in the silver nanometal ink, on an activated surface produced by ultra-violet irradiation, which is followed by the self-fusion reaction between nanoparticles to afford low resistance silver wiring. This technique enables easy and rapid production of ultra-fine electronic circuits over a large area substrate without using vacuum, with minimum linewidth of 0.8um that strongly adhere to the plastic substrates. A flexible touchscreen sensor produced by this technology is now planned to be in practical use, and is demonstrated this time by an eight inch trial product.

The ‘printed electronics technology’ which takes advantage of printing methods for manufacturing various electronic devices may enable device productions without vacuum and at around room temperature, so that it is expected as a key technology to accelerate prevailing flexible and large area human interface devices and thus to realise computerisation of our entire life scene. Developments of inks and printing techniques are now widely studied, in order to realise print production, in particular, of fine metal wiring that is indispensable to compose fine electronic circuits.

As the ink for metal wiring, most promising is silver nanometal ink that includes silver nanoparticles of 10-100nm (nm is a length of one billionth metre) diameter at high concentration. In order to preserve the ink stability, the nanoparticle surfaces are covered by encapsulating layers. In recent years, manufacturing methods of such silver nanometal inks have been advanced considerably, and the mass production of the inks is now possible. Printing production methods of metal wiring using silver nanometal inks, including screen printing, micro-contact printing and inkjet printing, are also studied.

However, the silver wiring manufactured by the printing so far does not reach the level required for its practical use, in terms of pattern resolution, conductivity, adhesivity to substrate, processing temperature and manufacturing throughput, due to several reasons - the encapsulating layer of the silver nanoparticles causes difficulty in obtaining high quality and low resistivity silver wiring after the printing; the removal process of the encapsulating layer may damage the heat sensitive flexible substrates; or a limitation in the adhering strength of ink droplets on a substrate surface or in the control of droplet volume. It is quite difficult to resolve these problems by improving the existent printing technologies. So it has been strongly required to develop an innovative printing technology that is based on a new printing principle to fully utilise the potential of the silver nanometal ink.

AIST has been promoting the research and development of the printed electronics technology in a wide range of aspects. A part of the research and development has focused on the use of surface modification technologies of substrates on which ink is applied, in order to improve and sophisticate the printing techniques for producing semiconductor layers and ferroelectric layers. During the further studies to improve printing technologies of metal wiring by the surface modification technologies, the researchers found that when they use a specific silver nanometal ink (diameter of silver nanoparticles is about 13nm) that is invented and developed by Professor Masato Kurihara at the academic research institute (taking charge of the faculty of science) in Yamagata University and is now being developed as industrial products by Tanaka Kikinzoku Kogyo K.K, the silver nanoparticles are selectively adsorbed through chemisorption on a certain surface modificated substrate surface, and that the particles exhibit a particle-particle fusion reaction. Based on the analyses of this phenomenon, and also on the study of developing the printing technique based on the analysis, the researchers have finally reached the present achievement.