Specifiers should be aware of the underlying material being offered within their chosen touchscreen, as it could result in differences in appearance and, most importantly, performance in the field. Technologies that are effective in one application may suffer from limitations in another.
Why the move away from ITO?
A key driver behind the changes in projected capacitive touch technology is the move to integrate touch into the LCD panel itself, eliminating the need for a separate touchscreen overlay and resulting in thinner, lighter touch enabled devices which are easier to integrate. Optical performance and brightness can also be improved by reducing the distance and layers between the LCD and the user.
However, the processes for manufacturing 'in cell' touchscreens are still evolving. Discrete projected touchscreens overlays using ITO conductors remain the principal technology, at least in smaller, handheld consumer electronic devices. It suffers from drawbacks as the display size increases beyond around 20in, principally as a result of its electrical resistance which hamper its performance and make it an unsuitable material choice for some applications.
So what conductive materials are available for larger sized touchscreens? There are three material technologies at the forefront: copper micro wires; silver metal mesh; and silver nanowires. Three more – carbon nanobuds, conductive polymers and graphene – are all in early stage development and likely to emerge in the coming years. Four key parameters – economics, resistance, visibility and availability – are applicable to the first five of these. Graphene, which is in early stage development, is not yet available commercially.
Initial tooling costs and ongoing material requirements are key issues when considering the cost of touchscreens. Technologies that can be directly written to the substrate materials without a mask require little in the way of tooling and can be produced more cheaply in low volume.
If masks or other tooling is required, then this limits the ability to produce screens of different sizes flexibly in low volume, but has the potential to offer reductions in high volumes on standard sizes.
Copper micro wires score in terms of flexibility. The electrode can be written directly to the substrate, with no lasers, masks/chemicals/etching or tooling needed. Silver nanowires can be customised to a degree, via laser ablation but then additional processing is required to link the conductors at the borders to the controller. Conductive polymers are also applied with screen printing, however, they need to be patterned at either the silkscreen printing stage or after with etching or lasers.
By contrast, silver metal mesh materials are patterned at source, so the size of sensor needs to be specified upfront. This leads to tooling charges which can range from around $10,000 to $20,000 per sensor design depending on screen size. The procedure to deposit the carbon nanobuds is complex using a NanoBud reactor, then a laser patterning process to create electrodes.
Another key factor in manufacturing cost is the number of layers required. Copper micro wires can be insulated, so that the x and y electrodes can be formed in a single layer. The encapsulating insulation also prevents oxidation of the material, which can seriously degrade touchscreen performance when exposed to high heat and humidity in the field. Silver nanowires, metal mesh and conductive polymer sensor constructions generally require two or more layers to insulate the (x and y) conductors, increasing the material content over a single layer design. Carbon NanoBuds are also a two-layer technology. Furthermore, care must be taken to prevent moisture ingress into the materials.
Touchscreen resistance is a key factor in determining sensitivity to touch, or signal to noise ratio (SNR). Higher resistance materials limit the amount of current flowing through the conductors, making it harder to correctly pick out a touch event from surrounding ambient interference (EMI) coming from the display, power source or other surrounding electronics.Clearly, this resistance becomes more of an issue with larger touch screens, especially if features like multi-touch, palm rejection and proximity detection are required.
As indicated above, ITO has generally been limited to smaller touchscreens due to its relatively high resistivity of approximately 100O per square, as a result most touchscreens using this material are smaller than about 22in, beyond which there are significant performance limitations. Silver nanowires have a better resistance than ITO (approximately 30 to 50O per square on PET film substrate). As a result, projected capacitive touch sensors using this technology are scalable up to around 42in (beyond which, again touch performance is hampered). Silver metal mesh has a lower resistance of around 15 to 30O per square, and as a result, can be used in touchscreens up to around 65in in size. Copper micro wires offer the lowest resistance at 5O per square or less, and can be used to create touchscreens of more than 100in. Furthermore, the low resistance provides the best SNR, resulting in touchscreens that can detect touch through thick overlaying glass and even gloves, without the need to drive the control electronics at high voltages or tile the screens using multiple linked controllers.
All discrete overlay projected capacitive touch technologies involve introducing some material element between the user and the screen, which will make some optical difference to the image. With copper micro wire based technologies, the grid of 10µm conductors can be visible, particularly when the display is off. That said, light transmission is excellent and in the range of 90% before any anti reflective treatments are applied. In contrast, silver nanowire and metal mesh technologies enable the creation of less visible conductive tracks (metal mesh in the 5 to 10µm range). However, nanowires and conductive polymer coatings can produce a colour cast or haze over the whole screen, and base light transmission is around 85%.
Availability and longevity
Copper micro wire touch sensors have been in production for nearly 20 years by a handful of specialist manufacturers and are a proven projected capacitive touchscreen technology for large sizes in the harshest environments. Silver metal mesh and nanowire based touch technologies have emerged into the mainstream quickly over the last few years, with many manufacturers installing the necessary printing and laser patterning equipment. The relative newness of these two technologies in the touchscreen world mean that their long term reliability is not yet proven – particularly in relation to how their resistance (and touch performance) changes when exposed to temperature and humidity in challenging applications.
Graphene, a potentially game changing touchscreen material technology, has had promising results released on its strength, transparency and conductivity, but development is still in its relative infancy. Deposited as a one atom thick layer of carbon, it combines a similar low resistance to copper micro wire, with the potential of 'invisible' conductors. However, despite its potential suitability as a material for projected capacitive touchscreens, there are many other applications for this technology, such as water purification, batteries and solar cells, and most developers are focusing their efforts on these areas for the present with touchscreen usage being much lower on the development roadmap.
There remains no such thing as the ‘perfect’ conductive material for projected capacitive touchscreens – and designers should always look for the best combination of performance, optics, durability, scalability and reliability suited to their touchscreen application. The worldwide market for phone and tablet touchscreens dwarfs the commercial AV market – Touch Display Research estimates the ITO replacement market at $13billion by 2023. As a consequence, new touchscreen material developments are focused on this massive market. However, the investment will almost certainly bring benefits to the commercial and industrial market too.
Dr Andrew Morrison is technical director with Zytronic.