Transparent conductive films for touchscreens: Combining transparency, sheet resistance and flexibility
(M2 PressWIRE Via Acquire Media NewsEdge) With the success of smartphones, tablet PCs and e-readers, touchscreens have become a ubiquitous feature of mobile devices. The new TDK FLECLEAR ITO films offer the flexibility, transparency and slidability needed to design high-performance touchscreens and panels for a wide range of applications.
Once used primarily in stationary terminals such as ticket vending machines at stations, ATMs at banks, or POS registers at convenience stores, touchscreens can now be found almost everywhere, in smartphones, navigation systems, digital cameras, and other mobile devices. Their benefits for designers and manufacturers are clear: No hardware keyboard or keypad is needed, more space is available for a larger display, and the user interface can be configured more flexibly. The leading touchscreen technologies currently available for the high-end mobile devices have one thing in common: their use of transparent electrode materials such as ITO (indium tin oxide) coated films to detect where the screen is being touched.
Resistive and capacitive touchscreen technologies have emerged as the most widespread methods of sensing touch on display surfaces, each with its own set of advantages. Resistive touchscreen panels consist of two thin, transparent, electrically resistive layers with a thin space between them. When the film surface is pressed with a finger or stylus, it comes into contact with the electrode film of the plate in the lower portion and starts the flow of an electric current. The change in voltage is then recognized, detecting the touch position. Its high sensitivity to pressure permits writing with a stylus, and because of its simple design, it is a relatively low-cost solution.
These types of touchscreens, however, cannot support the multi-touch operations common in high-end devices. For these reasons, resistive touchscreens are widely used in small and medium-sized devices such as navigation systems and game machines.
Capacitive touchscreens, by contrast, are the predominant technology for more sophisticated user interfaces. This is because projection capacitive touchscreens (PCT), in particular, have the great advantage that they support the broad array of advanced multi-touch operations offered on smartphones and tablet PCs such as enlarging, reducing the display or rotating images with two fingers. The PCT employs two transparent electrode layers -- one oriented in the X-axis direction and the other in the Y-axis direction -- to form a grid pattern of electrodes (Figure 1). The electrostatic capacitance can be measured very precisely at every individual point or intersection on the grid when the panel surface is touched.
Figure 1: Basic design of projection capacitive touchscreens
The PCT employs two transparent electrode layers -- one oriented in the X-axis direction and the other in the Y-axis direction -- to form a grid pattern of electrodes. The electrostatic capacitance can be measured very precisely at every individual point or intersection on the grid when the panel surface is touched.
High-performance transparent conductive films
Films with a coating of the transparent conductive material ITO represent the state of the art for the transparent electrode layers used in the touchscreens. ITO contains 5 to 10 percent tin oxide and features a transparency equal to that of glass in the visible light spectrum and a conductivity about the same as iron. TDK has developed and offers three kinds of ITO film that are designed for different touchscreen applications and manufacturer needs:
- FLECLEAR produced with sputtering for high-end touchscreen displays with best optical and electrical performance
- FLECLEAR produced with wet-coating for temperature-sensitive and flexible base materials
- FLECLEAR film rolls for the transfer ITO coatings to substrates by the customer
Ideal ITO films for smartphones
The films used in the touchscreens of smartphones and tablet PCs, however, must combine two mutually contradictory properties: maximum total light transmittance and minimum sheet resistance. The dilemma is that the sheet resistance increases if the ITO layer is made thinner to increase total light transmittance, while total light transmittance decreases if the ITO layer is made thicker to lower the sheet resistance.
To solve this problem, TDK coats the films using a physical vapor deposition technology known as sputtering, and in the process optimizes the density of ITO crystals deposited on the PET film (Figure 2). The result is a low sheet resistance of 150 /sq and a high total light transmittance of 92 percent.
Figure 2: Basic principle of ITO layer film formation by the sputtering process
Sputtering is a physical vapor deposition process that takes place in a vacuum device. When the ITO target is bombarded by energetic ions, the particles of the target material are ejected and fall down (sputter) onto the substrate, where they form a thin-film.
TDK FLECLEAR films also feature optical adjustment layers between the ITO layer and base material film in order to optimize the antireflection properties and adjust the color (Figure 3). While the ITO layer is transparent, it has a slightly yellow tint. The role of the optical adjustment layers is to make it colorless and transparent. The desired tone is achieved by adjusting the thickness of these layers.
With these features TDK FLECLEAR films with sputtered ITO coatings offer the excellent performance needed in touchscreens with advanced user interfaces.
Figure 3: The structure of TDK FLECLEAR ITO film produced with sputtering
Because the ITO layer is slightly yellowish, optical adjustment layers are needed to make it colorless and transparent and adjust it to the intended tone. The optical adjustment layer also increases transmittance with antireflection effects.
FLECLEAR films for flexible materials
While the TDK FLECLEAR ITO films produced with the sputtering process are optimized for high-end applications in the touchscreens of smartphones and tablet PCs, the process is not ideal for all materials or applications. Due to the high temperatures involved in sputtering, some materials such as thick plastic substrates and materials with low heat resistance and dimensional stability are difficult to coat. Moreover, a sputtered ITO layer is too brittle for flexible materials.
Figure 4: Highly flexible TDK FLECLEAR ITO films
The adhesive layer between the ITO layer and the base material reduces the stress on the ITO layer, providing these films with outstanding flexibility.
TDK solved this problem by adopting the wet-coating process that leverages the company's know-how in the manufacture of magnetic recording media. Wet-coating ensures a uniform application of ITO particulates on the base film, but does not involve the high process temperatures of sputtering. The wet-coated TDK FLECLEAR films are designed with a 3-layer structure that includes the ITO particulate layer, an adhesive layer and the base material (Figure 4). The adhesive layer reduces the stress on the ITO layer, providing these films with outstanding flexibility, as the product name FLECLEAR implies. Thus, wet-coated TDK FLECLEAR films feature a very stable resistance change ratio under bending. When wrapped around a rod with a diameter of 10 mm, the sheet resistance increases by only 5 percent.
Another major feature of FLECLEAR is that customers can use it to apply an ITO layer directly and easily on various kinds of base materials. FLECLEAR is available in the form of release film rolls to enable coating on the customer side. Color transparent conductive layers that can be colored in many ways are available as pigments and can be added to the adhesive layers. Thanks to their unique 3-layer structure, the wet-coated FLECLEAR film exhibits excellent slidability -- an important feature for applications that involve frequent writing with a stylus.
TDK's FLECLEAR ITO transparent conductive film clearly meets the needs of the fast-growing touchscreen market. TDK's portfolio of technologies used for the production of the films is applicable to not only ITO, but in the future to other transparent conductive materials as well.
Table: Key data of TDK FLECLEAR transparent conductive film FLECLEAR products
Sputtered ITO films Wet-coated ITO films Types Film rolls
Film sheets Film rolls
Release film rolls
(for ITO layer application by customer) Initial sheet resistance [ /sq] 150 600 Total light transmittance [%] 92 88 Haze [%] 0.5 1.0 Resistance change under bending [%] - 5
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