ALLPCB
Overview
With demand for larger and lower-cost touch panels, and because ITO films are unsuitable for flexible displays and have intrinsic limitations in conductivity and transmittance that are difficult to overcome, many manufacturers have begun researching ITO alternatives, including silver nanowires, metal meshes, carbon nanotubes, and graphene. Among these, silver nanowires and metal meshes are the most mature.
What is metal mesh
A metal mesh uses metals such as silver or copper, or their oxides, patterned on plastic films like PET to form a conductive grid. Its theoretical minimum sheet resistance can reach 0.1 ohm/sq, and it provides electromagnetic shielding to reduce signal interference. However, the patterned sensing lines tend to be relatively wide—especially above 5 μm—causing obvious Moiré interference patterns and making it suitable mainly for displays viewed at greater distances.
Manufacturers and fabrication methods
Early developers included Japanese companies Fujifilm and Gunze, both of which began producing metal mesh films in 2009 and subsequently supplied them for use in touch panels. Fujifilm and Gunze supplied sensors and even modules directly to help downstream manufacturers reduce technical barriers to entry. The U.S. company Atmel offered solutions spanning touch ICs to transparent conductive films, but its later development and recurring process issues in manufacturing affected shipments. As a result, mainland Chinese manufacturers sought to strengthen both R&D and production capabilities to obtain more complete solutions.
There are three main fabrication approaches. The first prints metal ink directly using screen printing. The second deposits a continuous metal layer on PET film, then uses photolithography and development to wash away excess material and form a mesh. The third is similar to the second, but uses silver halide, which is chemically reduced to silver. Fujifilm, which historically produced silver-halide film, used the third method and became a leading supplier of metal mesh films. Other companies, such as Gunze and mainland Chinese manufacturers, typically use the first method, while Atmel used the second. Some companies have developed other techniques, for example the Japanese company Achilles, but these have not yet been commercialized.
Adoption and prospects
In the first half of 2013 several devices adopted metal mesh: ASUS's 7-inch tablet computer and mainland Chinese manufacturer Lenovo's notebooks were early adopters. Mainland Chinese manufacturer Huawei and Japanese company NEC also introduced smartphones with metal mesh, particularly NEC which used metal vapor deposition techniques. In 2013 there were also devices using film touch processes such as GFF or MetalMesh. Although the technology remained at an early stage in 2014, it showed potential to replace ITO in film-type touch panels for medium and large-size products.
Key challenges
Although MetalMesh attracted attention because many companies invested in it and it was relatively inexpensive, it had not yet been mass-produced. As SuperC_Touch general manager Li Xiangyu pointed out, problems include insufficient optical transmittance: forming fine patterned lines requires removing 95%–99% of the touch-sensing coating area, which reduces touch signal strength by 20–100x and is difficult for current touch ICs to support. Also, to be visually imperceptible the metal line width must be under 5 μm, requiring photolithography or precision printing, both of which are costly. Ensuring sub-5 μm metal lines do not break, resolving metal reflectivity, and preventing material oxidation therefore present significant challenges for metal mesh technology.
