The sensitivity of touchscreens

Users who wish to enlarge a digital map on their Apple iPad require neither a mouse nor a keyboard: In order to zoom in on a map section, they simply spread two fingers apart as they drag them across the screen. Computers have never been easier to operate in this regard, which is why more and more displays are now being equipped with a multi-touch feature.

The iPad display is also very impressive in terms of its visual accuracy. In-plane switching technology (IPS) noticeably reduces the color distortion that is common to conventional displays, which means that the image maintains the same level of brilliance even if the user turns the screen to a horizontal position and views it at a very flat angle. This capability is made possible by liquid crystals from Merck.

Touchscreens place great demands on their display function because they are covered by a transparent touch-sensitive coating. In this setup, an additional glass plate reflects the image that the backlight sends to the viewer's eye via the display. This plate is needed to separate the display from the touch-sensitive layer. “Normally, the reflections diminish the brilliance of the image,” says Bernhard Rieger, head of IPS Research at Merck. “We therefore developed liquid crystals that counteract this effect by generating a higher level of color contrast without reducing other quality features such as response times.”
 
This type of progress is making touchscreens commonplace — in cell phones, navigation devices, and PC monitors. According to market researchers from DisplaySearch, manufacturers of these and other devices equipped 29 percent more of them with touchscreens in 2009 than in 2008. Half of these touchscreens utilize resistive technology, which reacts to the pressure of a human finger. By contrast, the so-called capacitive variant, such as that used in the iPad, is used in approximately one-third of the displays. This technology not only recognizes the position of the finger but also the direction in which it is moving at any given moment. At the same time, it does not react to “lifeless” styluses.

Whereas high sensitivity characterizes the touchscreens of today, the future will bring flexible screens and possibly even units that can be rolled up.

This cannot be achieved with conventional silicon technology, however. Up until now, this semiconducting material has been practically indispensable for manufacturing light-emitting diodes (LEDs) for backlighting or transistors for controlling liquid crystals, for example. The problem is that silicon is processed at high temperatures, and it’s also relatively heavy and brittle. This makes the displays that contain it rigid and fragile — and also makes their production extremely expensive.

Organic electronic materials will change all that. These materials comprise either large chains of molecules (polymers) or “small” molecules that, depending on their chemical composition, display insulating, semiconducting, or conducting properties, which means they could replace the silicon now used in the displays. The organic materials can also be dissolved in certain fluids and can thus be printed in the form of ink on flexible foils, layer by layer, at room temperature at a reasonable cost. For this application, Merck developed its new lisicon® materials, which are now ready for market launch. In chemical terms, these materials are based on organic semiconductors, and in terms of their name they form a link to silicon, which lent its name to Silicon Valley.

Merck technology offers innovative alternatives not only in the unit itself but also in the production of the displays. This is the case, for example, with the new isishape® etching pastes, which reduce material consumption, the number of production process steps, and the strain on the environment that is associated with the structuring of conductive layers within the display.
 
To ensure that a screen can recognize touch, the conductive layer made of a material like indium tin oxide (ITO) must be partitioned into insulated sections on the glass plate or film by means of extremely fine lines. This structuring process used to be performed  with the help of lasers or else through procedures requiring the application of resistant protective masks and etching baths.

Both techniques have key disadvantages, however. Lasers require a high investment outlay and operate too slowly when used for planar structuring. Lasers also pose the risk of damage to underlying layers. The protective masks and etching baths, on the other hand, lead to the release, during the subsequent removal and cleaning steps, of several substances that are injurious to health. These substances must be filtered out and separately disposed of in a complicated and expensive procedure during the water purification process.