ITO alternatives: printed metal mesh is on the rise?
ITO alternatives: printed metal mesh is on the rise?
2017年2月15天Dr Khasha Ghaffarzadeh
The transparent conductive film (TCF) industry has witnessed upheaval in recent years. On the one hand, multiple technologies have been developed as alternatives to the incumbent solution, whilst the incumbent suppliers slashed their prices to protect their share in sluggish markets 1.
Our report Transparent Conductive Films (TCF) 2017-2027: Forecasts, Markets, Technologies provides a detailed assessment of the technology, market and business landscape developments of the TCF industry. We have been closely engaged with the industry over many years, witnessing first-hand the rise and fall of technologies and companies, and correctly predicting the consolidation period of the past few years.
Traditional limitations of printing TCFs
Metal mesh has emerged as a leading commercial alternative to the incumbent. It offers a higher level of performance, particularly for large-area applications, whilst also shortening the value chain. There are however numerous methods that can be employed to manufacture metal mesh TCFs, and not all are equal.
In the early days, printing was hot because it promised rapid additive production using inexpensive printing machines. The development however soon hit a technology limitation in that it could not easily narrow the linewidth of the metal mesh to below 15-20um, meaning that the mesh would remain visible to the naked eye despite high measured transparency levels.
This challenges put printing on the back foot, whilst allowing other metal mesh processes such as those based on photo-patterning to gain traction. This may however be about to change as printing, in various ways, direct or hybrid, is making a comeback into the TCF industry.
The first commercial use of printing was as a hybrid process by the likes of Shenzhen O-Film. In this hybrid approach, a UV resin is first patterned using imprinting, the trenches are then filled using a printing process such as dispensing, and the excess ink is then cleared off. This creates embedded topographically-smooth metal mesh solutions with sub-5um linewidths. This approach was commercialized although we understood that O-Film had some customer issues in 2015. This approach continues to be developed by O-Film and others. An example demonstrating the process is shown below.
(Left): An example of the hybrid process for manufacturing metal mesh. Source: Fujikura Kasei. (Right) examples of O-Film metal mesh TCFs. I took these photos at Mobile World Congress 2014.
Direct sheet-to-sheet screen printing was also developed as a means of making metal mesh. This approach was limited by the traditional shortcomings: here the linewidths were around 24um. This approach is however now being applied to large-area touch tables and touch surfaces for use in, for example, outdoor areas where line visibility is not a concern. The picture below shows an example from Gunze. This product is currently in use in Japan in, for example, gaming panels installed in Akihabara, Tokyo.
Large-area touch tables with screen printed metal mesh with 24um linewidths. Source: Gunze. I took this picture at C-Touch Taiwan 2016
Others are developing gravure offset techniques for printing ultrafine metal mesh structures using silver nanoparticles. One example has been demonstrated by Komura-Tech which has achieved 5um linewidths. This result, shown below, is still in the prototyping stage and the web sizes are limited. As before, the challenge will be in scaling up the process without comprising yield or resolution.
Others such as a major Korean companies had developed similar technology as early as 2012, but gave up because they judged that the cost benefit was not sufficient to dethrone ITO films. This is however a promising step in the right direction.
Note that flexographic printing with larger web sizes is also possible. This technique achieves 10um for the best case, but commercial printing is generally limited to 20um. The printed lines are also thin.
Gravure offset printed fine line metal mesh TCF. (left) microscope image of 5um line metal mesh. (right) plastic surfaces covered with printed Ag metal mesh with 5um linewidth. Source: Komura-Tech. I took the photos at the IDTechEx Show! USA 2016
Gravure offset fine line metal mesh demonstrated by Ishihara Chemical (materials supplier). I tool these photos at Nepcon Japan 2017.
Other novel processes are also being developed to bring printing into the metal mesh TCF business. For example, Toray is now targeting its photo-patterned screen-printable conductive paste at the TCF industry. This ink was initially introduced as a touch screen edge electrode material, enabling the industry to overcome the limitations of standard PTFs (pastes) to reach L/S of 20/20 or lower. This was a success achieving sales on the scale of several (e.g., 2) tonnes per month. Now Toray had demonstrated 3.5um metal mesh patterns using this technology as shown below. This technology is still in the prototyping phase and the cost is still high (we estimate around $40/sqm, far higher than competitors).
(Right) fine linewidth flexible metal mesh made using Toray's photo-patternable Ag conductive paste. (Left) image of medium-sized TCF where I could detect no visible lines. Source: Toray. I took these photos at Nepcon Japan 2017.
Tanaka Metal (recently acquired Metalor) has also developed a novel ink-based metal mesh TCF technology. The substrate is coated with a layer of amorphous fluoropolymer. This is then later patterned using UV irradiation. An image of an activated surface pattern that absorbs silver nanoparticles in then formed. The entire surface is then coated with silver nanoparticle using blade coating. The nanoparticles selectively chemisorb onto the patterned areas, giving rise to the metal mesh structure with ultra-fine linewidths. The Tanaka-estimated price is 200 yen per sqm (approx. $20/sqm). This is still an early stage development and the production process is currently sheet-to-sheet on a 500mm by 500mm substrate.
Tanaka Metal and AIST's silver nanoparticle ink-based approach to metal mesh TCFs. Source: Tanaka Metal. I took this photo based on brochures given to us by the developers.
The use of printing is also being developed to either laydown a catalyst which can then be later thickened using plating, or to deposit photoresists thus effectively replacing photolithography. The latter process is being developed by the likes of Screen Holding or LCY, whereas the former is by the likes of ITRI in Taiwan. The latter process is often envisioned as a stepping-stone towards full direct printing of the metal mesh lines.
Printing is used to deposit photoresist or to lay down plating catalysts. Source: (left) Screen Holdings and (right) ITRI/Kuraki. I took these photos at FineTech Japan 2016.
Yet others, such as Asahi Kasei, are developing printing technologies that can directly roll-to-roll deposit lines as narrow 100nm. This technology is currently under development and the prototyping drums have a 100mm diameter and 50mm width. This technology however may be overshooting the requirements of the TCFs, and might be better suited to other applications.
Of course this has not been all progress on ink-based TCFs. There are many firms that have come with a bang and gone with an eerie silence. For example, ClearJet developed a novel approach to overlapping inkjet-printed conductive rings, and Cima Nanotech developed self-assembled Ag nanoparticles which created a random metal mesh structure. But for each one that leaves the scene another one appears to take its place. For example, we recently learned that XTPL, an early stage polish firm, has developed a printing technology for ultra-fine metal mesh printing.
This is a fast evolving space to watch. To learn more please refer to our report Transparent Conductive Films (TCF) 2017-2027: Forecasts, Markets, Technologies. This report outlines the latest on all technology options for TCFs, developed detailed application- and technology-segmented ten-year market forecasts, and provides interview-based company profiles.
Please do not hesitate to contact me on khasha@IDTechEx.com should you have any questions.
1 Raw indium prices have also falled from $750/ Kg to $225/Kg between 2014-2016, partially accounting for the fall in ITO film prices.