TOKYO -- Researchers at the University of Tokyo Flexible Electronics Research Center have developed a printing technology which enables production of ultrafine silver wiring patterns by patterning using UV irradiation followed by surface coating.

The new SuPR-NaP (for Surface Photo-Reactive Nanometal Printing) technique relies on a surface coating of silver nanometal ink that includes silver nanoparticles at high concentration.

Details of the study were published Apr. 19 in the British online scientific journal Nature Communications.

Printed electronics technology has faced several technological difficulties such as in reproducibility due to the contamination of printing plates or other apparatus; sintering or fusion of metal particles on substrates after printing; avoiding distortion of plastic substrates by high-temperature post-treatment; and avoiding peeling off the printed wires due to substrate bending. The new technique uses selective chemisorption of silver nanoparticles, which are included in the silver nanometal ink, on an activated surface produced by UV irradiation, which is followed by a self-fusion reaction between nanoparticles to afford low-resistance silver wiring. This technique is said to enable easy and rapid production of ultrafine electronic circuits over a large-area substrate without using vacuum, with minimum line widths of 0.8 microns that strongly adhere to plastic substrates. A flexible touch-screen sensor produced by this technology is now planned for practical use and is being demonstrated in an 8" trial version.

The printed electronics technology takes advantage of printing methods for manufacturing various electronic devices may enable device productions without vacuum and at around room temperature. The most promising ink for metal wiring, the researchers say, is a silver nanometal ink that includes silver nanoparticles of 10 to 100nm diameter at high concentration. To preserve ink stability, the nanoparticle surfaces are covered by encapsulating layers.

In recent years, manufacturing methods of such silver nanometal inks have advanced considerably, and 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 has thus far not reached the levels required for 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 existing printing technologies. So an innovative printing technology based on a new printing principle was needed to fully utilize the potential of silver nanometal ink.

New research has focused on use of surface modification technologies of substrates on which ink is applied, in order to improve printing techniques for producing semiconductor layers and ferroelectric layers. During 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) invented and developed by Prof. Masato Kurihara at the academic research institute in Yamagata University and is now in development by Tanaka Kikinzoku Kogyo K.K., the silver nanoparticles are selectively adsorbed through chemisorption on a certain surface-modificated substrate surface, and they 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.

The silver nanometal ink used in the study includes alkylamine encapsulated silver nanoparticles at high concentration with weight ratio of 40% to 60%. It was reported by Yamagata University that when the silver nanometal ink is dried, encapsulating alkylamine molecules whose coordination strength is relatively weak are gradually left, and the aggregation and fusion of silver nanoparticles proceeds even at room temperature. By the use of the silver nanometal ink that exhibits such a peculiar characteristic, the SuPR-NaP technique has been developed.

Carboxyl groups are generated due to the cut of chemical bonds of the polymer by ultraviolet irradiation on the fluoropolymer layer that is used as a surface layer of substrates. When the silver nanoparticles are contacted with the surface, they are directly connected with the carboxyl groups that form stronger bonds than with the encapsulating alkylamine molecules. Many silver nanoparticles are trapped on the surfaces, and then the silver nanoparticles contacted with each other start to fuse. It was found, by the surface-enhanced Raman scattering measurement, that a layer of carboxyl groups is formed at the interface between the silver layer and the activated surface. It is thought that the fusion reaction between the silver nanoparticles would raise the temperature at the silver surfaces, so that the desorption of alkylamine molecules and fusion reaction between the silver nanoparticles proceeds by the avalanche-like process to eventually form a self-fused solid silver layer without voids.

At present, transparent conducting electrodes with use of indium tin oxide (ITO) film are used for touch-screen sensors of smart phones, etc. However, such a sensor uses the hard crystalline oxide film which is easily broken when it is bent. It is also necessary to use vacuum in the production of the electrodes. Therefore, the oxide films have difficulty in terms of flexibility and low-cost production with saving materials. Because of the above reasons, it is now considered to develop transparent conductive electrodes by using network-like metal wiring that is composed of fine metal lines with likely-invisible linewidth of a few micrometres. With use of the developed printing technique, the researchers produced a flexible touch-screen sensor by fabricating silver wiring with a linewidth (2 microns) which is close to the diffraction limit of visible light on a plastic substrate. The obtained touch-screen sensor presents a higher tolerance to bending, and is also better than other transparent conducting electrodes such as ITO, silver nanowires, or graphene, in terms of the transmittance and the sheet resistance. The technology enables easy and rapid production of flexible touch screen sensor sheets without vacuum and at ambient pressure and room temperature and by environment-conscious manufacturing process that can minimize the consumption of the silver nanometal ink.

The technology should be further developed as the technology which enables us to produce touch sensors only by sticking thin plastic films, as the crucial technology in the printed electronics to produce a variety of electronics devices by printing, and also as the new technology to produce patterned metal thin films easily by the applying technique.

Sample shipments of the flexible touch-screen sensor sheet by this technique is now planned to begin next January by Tanaka Kikinzoku Kogyo K. K.

The study is supported partly by the Japan Science and Technology Agency (JST) through Strategic Promotion of Innovative Research and Development Program (S-Innovation), as "Development of Flexible Display based on AM-TFTs Manufactured by Printing Processes with New High-Performance Polymer Semiconductors," and by the same agency through NexTEP Program, as "Touch Screen Sensor Film using Metal Wiring" (Implementing Company, Tanaka Kikinzoku Kogyo K.K.; Representative Researcher: Tatsuo Hasegawa). Acquisition of Yamagata University's intellectual properties related to the silver nanoparticles is supported by JST.