Today’s smart-phone touchscreens offer improved performance compared to their predecessors, but their form factors have remained largely unchanged. In fact, mobile innovation has been relatively stagnant overall. The ideas are there, but current materials such as heavy glass-based touch sensors and inefficient transparent conductive materials like indium tin oxide (ITO) have severely limited smart-phone advances.
According to IDTechEx, ITO accounted for 93% of the transparent conductive material market in 2012. Yet despite its tremendous market share, this ceramic material is far from perfect. First, ITO is expensive, both in its raw material form and to work with. Indium itself is costly, increasing from $300/kg to $800/kg from 2009 to 2012, according to Seeking Alpha.
But even more than the cost of indium itself, the excessive cost of depositing thin films using a vacuum-sputtering process makes the use of this ceramic material prohibitive. ITO also is extremely difficult work to with because of its brittle nature, and it will crack when bent. And, it requires a durable substrate such as glass to achieve high conductivity.
Furthermore, indium is a rare earth material found mostly in China, so its supply is volatile since China has demonstrated a willingness to limit exports and give its industries a competitive advantage.
Taking The Weight Off
One major ongoing trend in mobile innovation is the push to reduce touch sensor thickness to create the sleeker, slimmer phones consumers demand. Device makers have been striving toward lighter, thinner phones for years. But they could only get so far with standard glass-based touch sensors, which include a projected capacitive touch layer sandwiched between two fragile, thick, and heavy glass layers.
Today, device makers are forgoing glass in favor of plastics and other new touch technologies. In-cell and single-layer touch technologies are two exciting touch sensor technologies that that make it possible to eliminate one of the two traditional glass sensor layers entirely, leaving the possibilities for paper-thin smart phones wide open.
In-Cell Touch Technology
To continue its trend of releasing successively thinner generations of smart phones, Apple introduced “in-cell” touch technology in the iPhone 5 (U.S. Patent No. 8,243,027 for a “Touch screen liquid crystal display”). In-cell technology combines the typical two-layer smart-phone touch-enabled display into one piece with touch capacitors located within the LCD itself. Previous iPhones used a touch-sensitive glass panel glued on top of an LCD.
The patent states that in-cell technology can “provide touch-sensing capabilities by allowing the same electrodes used for display updating to also be used for touch sensing.” By having electrodes share duties, driving display and responding to touch input, in-cell technology integrates the LCD and touch sensor into a single display, eliminating a layer of glass.
In addition to enabling a thinner, lighter smart phone, the new technology has the potential to streamline production. In-cell technology drops the number of display components and reduces supply chain complexity. The new production process is more complex, though, and there have been reports of low manufacturing yields. For example, Apple faced production delays as result of moving toward in-cell display technology.
As a result of high production costs, multi-touch displays have traditionally been limited to high-end smart phones, but the demand for mobile-touch capabilities does not end there. Device makers have long considered adding touch multi-touch capabilities to mid-range feature phones, but could not afford to do so, largely because of touch sensor costs. However, single-layer touch sensors may soon make touch capabilities the norm for lower-cost feature phones.
Typical capacitive touch systems require X and Y axis electrodes in separate layers. But recent advances in touch IC controller design make it possible to use a single-layer touchscreen sensor that forgoes one electrode layer while still providing multi-touch capability. Single-layer touch sensors also can be deposited directly onto a phone’s cover glass to create an incredibly thin touchscreen device with potentially even lower costs.
Although single-layer touch sensors cost much less than traditional two-layer touch sensors, their performance is limited and their accuracy, sensitivity, and response time still leave much to be desired.
However, single-layer touch performance can be improved through the use of lower-resistance electrodes. Several companies are evaluating new transparent conductive materials to replace ITO and enable higher-performance single-layer touch sensors.
Also exciting in the mobile market is the quest to develop flexible, unbreakable smart phones. Despite advances like Corning Gorilla Glass, today’s mobile devices are still all too easy to break. Stronger phones that absorb impacts rather than shattering are in high demand.
Samsung has demonstrated several high-resolution flexible display prototypes, including a prototype screen that can be bent forward and backward without any loss of display or image quality. Sony and other companies also are rumored to be working on flexible smart-phone displays.
To enable flexibility and other emerging smart-phone trends, device makers are looking to a new generation of transparent conductive materials. Unlike brittle ITO, several new materials offer the flexibility required to create curved, foldable, and unbreakable screens. Yet in addition to flexibility, effective transparent conductive materials must still deliver high conductivity, excellent optical performance, and, ideally, low material and processing costs. Today’s most viable ITO alternatives include graphene, metal mesh, and silver nanowire technologies.
One potential enabler of mobile touchscreen innovation is graphene, an incredibly thin carbon sheet with impressive strength, transparency, and electrical conductivity. At just one atom thick, graphene is clearly desirable for thin, light mobile devices, while its flexibility makes it an optimal transparent conductive material for the bendable, rollable, and unbreakable designs on the horizon. In fact, the 2010 Nobel Prize in Physics was awarded jointly to Andre Geim and Konstantin Novoselov “for groundbreaking experiments regarding the two-dimensional material graphene.”
Developments in the use of graphene are still in their infancy, and there are still many challenges to overcome. Many hundreds of millions of dollars are now being funneled to develop this wonder material and its potential applications. Yet several key issues are pertinent to touchscreen applications for graphene, including sheet conductivity and the manufacturing process, are yet to be resolved.
Although a perfect sheet of graphene theoretically has a higher conductivity than silver, defect-free layers of graphene are difficult to produce. The presence of ripples and other defects in the graphene layer or growing more than one layer of graphene actually reduces both the electrical and optical properties of the deposited layers. It is also difficult to prevent interactions with the substrate, which can substantially reduce conductivity. Researchers are working hard to understand the mechanisms behind such interactions and are trying to find ways to make better graphene layers.
Cost-effective manufacturability of graphene in large volume is also being intensively researched. At present, an expensive vacuum process is used to create the sheets of graphene, and a scalable low-cost method to make just single layers of graphene has been difficult to find.
Graphene is a very promising material, but it is significantly behind other ITO alternatives in terms of commercialization, though, and it could be several years before it enters the mobile touchscreen market.
Comprising interwoven “grids” of thin copper or silver, roll-to-roll metal mesh technology is another available ITO alternative. Metal mesh’s primary advantages over ITO are its low resistance and its ability to be sold as a pre-patterned film with the circuit pre-defined. Metal mesh is typically made either by printing or through a photographic development process on an optically clear substrate such as polyethylene terephthalate (PET).
The key challenge with metal mesh is the visibility of the metallic grids and optical interaction with the underlying display. To reduce the visibility of the metal mesh, manufacturers are moving to finer metal lines that are just a few microns wide. However, making such narrow lines at high yields has proven to be a serious challenge.
Additionally, the metal mesh generally has a well-defined repeat pattern and interacts with the LCD behind the touchscreen, resulting in very distracting moiré patterns on the screen. To minimize the moiré effect, the metal mesh has to be designed to match with the exact display being used in the consumer device. This complicated process increases the design cycle time and reduces the flexibility of manufacturing and design as any change in the display must be accompanied by a change in the metal mesh.
Metal mesh technologies are being used in a few commercial products for indoor applications and seem poised to grow in this market. But designers will have to improve the optical performance for metal mesh to become viable for mobile applications such as phones and tablets, since devices used in bright sunlight drastically accentuate the pattern visibility issue.
Silver nanowire technology has excellent electrical and optical properties coupled with outstanding flexibility. Single-crystal silver nanowires can be coated onto various substrates, including glass, polycarbonate, and PET film at conductivities much higher than ITO and with better transmission than ITO films. Film-based touch sensors made with silver nanowires are light, thin, and shatterproof. The nanowires also create a flexible transparent conductive layer that is conducive to bendable and curved form factors, or devices with touch capacities wrapped around their edges (see the figure).
Silver nanowire technology also offers both low material and processing costs. This material can be coated at low processing temperatures (<120°C) and is compatible with well-established, high-speed, high-output coating methods such as roll-to-roll slot-die coating. Using durable silver nanowires, OEMs and ODMs can create the next generation of mobile devices that are rollable, foldable, and bendable with high-performance touch interfaces and superior optical performance at price points low enough for the consumer mass market.
Already a leading ITO competitor for large-area touch applications including ultrabooks and all-in-one computers, durable silver nanowires are ideal for emerging mobile technologies. In fact, they’re already used in commercially available mobile products. The popular Japanese Docomo NEC N-07D Medias X smart phone uses a silver nanowire-based film under Corning Gorilla Glass instead of the traditional ITO-based touch sensor. As a result, the phone is incredibly thin (7.8 mm) and light (119 g). It also boasts a highly responsive touch experience and a vibrant HD display.
Finally, the silver nanowire solution is directly patternable. Film and device makers will soon be able to print silver nanowire-based inks in one step, further simplifying the manufacturing process and reducing costs.
As the mobile device industry advances, ITO’s limitations are becoming increasingly evident and device makers are transitioning to a new generation of transparent conductive materials and displays to enable thinner, stronger, and better performing mobile devices. Among ITO alternatives, silver nanowires and metal mesh have seized the largest market share. As a result of their flexibility and low cost points, these two materials also present the greatest opportunity for device makers looking to develop smart phones with flexible, curved, and foldable screens, as well as phones that are completely bezel-free, with touch capabilities wrapped around their edges.
Which of these two materials and techniques will ultimately triumph remains to be seen, but the future is bright for newer and better mobile devices as a result of their commercial availability.