Metal mesh breakthrough enables thinner, more flexible devices

Tanaka Kikinzoku Kogyo, which operates the Tanaka Precious Metals manufacturing business, has announced the development of a single-sided, dual-layer wiring structure for the metal mesh wiring format used in touch sensors. The technology is expected to contribute to higher image quality, thinner devices, increased flexibility and improved durability of smartphone touch panels and other applications.

Currently, high-transparency, projection-capacitive type touch panels with multitouch functionality are commonly used in smartphones. Because of the high transparency, and for considerations of mass production, the touch sensors in projection-capacitive type touch panels generally use transparent electrodes made from indium tin oxide (ITO) etched into a glass substrate. Research is currently being conducted, however, to find replacements for ITO because of difficulties reducing prices in the future and environmental concerns raised by waste liquids generated during the etching process. In addition, ITO has high electrical resistance and is weak when bent, making large and flexible panels difficult. It is therefore not suitable for the future smartphone market.

Accordingly, companies are developing sensors for touch panels using metal meshes, which are already in use in some touch panel displays and PCs. The most common wire width of metal meshes currently in use is 3 to 7 µm, and the wiring portions are within the range of human vision. This poses an issue regarding widespread use in smartphones and other devices that are used at close distances.

Tanaka Kikinzoku Kogyo achieved fine wire formation smaller than 4 µm, which was thought to be difficult. This was accomplished by applying low-temperature sintered silver nano-ink that can be formed into wires on PET film, which is not heat resistant, and fluoropolymer on a PET film or second substrate, causing adsorption and sintering of silver nano-ink on a fluoropolymer surface activated by irradiation with deep ultraviolet light, and using a SuPR-NaP (Surface Photo-Reactive Nanometal Printing) technique. In addition, the company established a manufacturing process for the fine-wire film using a roll-to-roll processing method, making possible bulk printing of metal mesh films with mixed patterns.

Enlarged exterior view of the metal mesh film with a single-sided, dual-layer wiring structure for use in touch sensors.

Touch panels normally have a structure made from two sensor substrates — an X sensor substrate and a Y sensor substrate — but Tanaka Kikinzoku Kogyo developed a technique for forming both the X sensor and Y sensor wiring on a single-layer flexible substrate. The discovery was made by applying the metal mesh wiring technology and forming overlapping silver nano-ink wire circuity on one side of the film (creating a single-sided, dual-layer metal mesh film). As a result, only one sensor substrate is needed — a breakthrough that will contribute to reducing costs as well as improving touch panel image quality and making panels slimmer.

The company also discovered a transparent (conducting) electrode formed by etching ITO on a glass substrate. The team created a structure expected to improve bending strength (increased flexibility) that even metal mesh films cannot withstand, as well as a method of manufacturing this structure.

Tanaka Kikinzoku Kogyo is currently offering samples of metal mesh films with standard specifications (4 µm, single-sided, single-layer structure) and is currently conducting further research and development, with the aim of providing sample shipments of single-sided, dual-layer films in the future. The technology is expected to have application in high-end smartphone touch panels, which are expected to shift to bendable displays in the near future, as well as in the flexible electronic device market.

Image caption: Rendering of a bendable smart device using a single-sided, dual-layer structure mesh film.