Upon completion of the mechanical evaluation, it is necessary to select an appropriate substrate for the touchscreen design. Figure 3 illustrates a typical ITO patterning design for a projected-capacitance touchscreen.

Figure 3. Typical projected-capacitance ITO patterning (red/blue indicate two different layers)
(Click on image to enlarge)

There are two mainstream substrates used for projected-capacitance touchscreens: glass and PET. Assuming device's mechanical design does not drive substrate selection, there are benefits and advantages to both, so choosing the best substrate best fit your device and your marketing strategy is very important. Table 1 illustrates a comparison between the two substrates.

Table 1: Glass and PET Substrate Comparison
(Click on image to enlarge)

Glass substrates are typically used in applications requiring superior optical performance and environmental endurance. In most applications, glass-substrate touch sensors are paired with tempered-glass cover-lens with matching or similar index of refraction. In addition, the cover-lens is usually treated with anti-glare (AG), anti-reflection (AR) and scratch-resistant coatings to further increase optical performance by decreasing the amount of reflectance. Transparency is used to define the amount of light that will pass through a material. Reflectance is measure of the amount of light reflection.

All projected-capacitance touchscreens consists of patterned transparent ITO conductors. Ideally, the reflectance of the ITO patterns should be equal to that of the trace gaps, where there is no ITO patterning. This will ensure that the conductive ITO traces will remain invisible. Glass touch sensors and cover lens can also be chemically tempered to provide drop and impact resistance. Unfortunately, one of the down-sides to using glass projected-capacitance systems is that it is typically more expensive than film.

Film substrates are used primarily for the benefits of thinness, improved lamination yield and its lighter construction. Of course, film touchscreen systems costs significantly less than similar glass substrate solutions. Film substrates are typically paired with relatively inexpensive polymethyl methacrylate (PMMA) acrylic overlays which are prone to surface scratches.

In order to understand the cost differences between film and glass-based touch sensors, it is useful to look at differences in yield of various manufacturing stages. In the manufacture of touch sensors, ITO is sputtered onto a glass/film substrate, resulting in a thin, microscopic deposition of ITO on the substrate surface. A photomask of the required projected-capacitance patterning is created to prepare for the next processing stage.

The photomask is used in a process known as etching to remove areas of unwanted ITO, creating the required ITO patterning. For a dual-substrate projected-capacitance design, the worst-case manufacturing yield is given by the following equation.

(Click on equation to enlarge)

Production experience has shown the following relationships:

(Click on equation to enlarge)

In addition, if the glass touch sensor is to undergo chemical tempering to increase its mechanical strength, it is another potential yield-loss procedure to which film-based solutions are not subjected.

Another very important consideration for projected-capacitance touchscreen design is the user-interface. Capacitive touchscreens are not well-suited for stylus-based applications. Two variants of capacitive stylus are being researched today, conductive-rubber-based stylus and conductive-electret devices, but these have not proven to be promising in delivering a compact, easy to use and cheap stylus.

In addition, capacitive sensing will be affected by the area of contact between the finger and the touchscreen. A fine-tipped stylus will not be able to generate the capacitive signatures of a finger. Capacitive touchscreens are designed mostly for realizing finger gestures. The integration of capacitive touch for web browsing can fully take advantage of accurate finger selection. In websites where there are a lot of link content closely spaced together, it may be difficult to accurately select the desired link.

One approach to interface design is to allow users to sequentially scroll through the link options. While scrolling, each link alternative is magnified for easy touch selection. Once the desired link has been magnified, the user can easily touch the link to access it.

Another approach is to magnify all links in proximity to the finger touch, so that the user can then select his/her desired link from the second set of enlarged links. Consequently, user interface design should take into consideration the inaccuracies associated with finger size, movement and positioning.

The variation in accuracy of a repeating human press is typically 3 to 4 mm, partly due to eye-finger parallax and differences in dexterity. Selection icons/buttons should optimally be greater than 5 mm in diameter. Buttons should also be amply spaced apart so as to improve usability (5 to10 mm at a minimum). If a thumb is intended to operate the button, they should be even larger and spaced even further apart. In addition, some form of visual, aural or haptics feedback should be promptly provided to the user to indicate accurate selection. . Delayed feedback will cause increased user input inaccuracies.

Haptics provides the tactile feedback to finger press events on a solid-state touchscreen. It simulates user feedback using vibration motors (actuators). In mobile devices, the most common actuators are Eccentric Rotating Mass Actuators and Linear Resonant Actuators. Upon a touch event, the surface of the projected-capacitance touchscreen will vibrate to indicate an input has been detected. The strength and duration of the vibration can be adjusted depending on the type of input feedback.

The foundation of a good user-interface design is to keep things simple. Users should be able to perform common tasks within only a few touches of the screen. Not only does this make for a more pleasant user experience, it also lessens the learning curve.

Mechanical evaluation, substrate selection, and user interface design are all very important considerations to a projected-capacitance touchscreen system. Understanding the mechanical constraints will help in substrate selection and ensuring the best possible performance from the touchscreen. Substrate selection is a cost-feature tradeoff among cost, robustness, and optical performance. Keeping the user interface simple and intuitive, with properly sized finger-selection elements will ensure a high degree of usability. Careful consideration of touchscreen design ensures the success of the end product and significantly lowers the development risk.

About the author
Yi Hang Wang is a Product Manager in the CapSense Solution Group at Cypress Semiconductor Corp.. Yi Hang graduated with a BA Sc. Degree in Computer Engineering from the University of Waterloo, Canada.