How it works: The technology of touch screens

Here's a question: What is a technology that you can't see, but is essential to smartphones, tablets and other mobile devices -- and is estimated to generate $16 billion in revenues this year (according to DisplaySearch)? The answer is multitouch touch screens -- which have sparked the explosive growth of the mobile device market.

It was not so long ago that we would tap away on a PalmPilot with a tiny stylus, or exercise our thumbs on a BlackBerry micro-keyboard. Then, in January 2007, along came the , and everything changed. Suddenly, people were wiping their fingers across screens, pinching images and performing other maneuvers that had not previously been part of the interface.

Now we not only take touch input for granted, we expect to be able to use multitouch (using more than one finger on the screen at a time) and gestures as well. What made this touch screen revolution possible, and where is it likely to take us?

Many paths to touch

To begin with, not all touch is created equal. There are many different touch technologies available to design engineers.

According to touch industry expert Geoff Walker of Walker Mobile, there are 18 distinctly different touch technologies available. Some rely on visible or infrared light; some use sound waves and some use force sensors. They all have individual combinations of advantages and disadvantages, including size, accuracy, reliability, durability, number of touches sensed and -- of course -- cost.

As it turns out, two of these technologies dominate the market for transparent touch technology applied to display screens in mobile devices. And the two approaches have very distinct differences. One requires moving parts, while the other is solid state. One relies on electrical resistance to sense touches, while the other relies on electrical capacitance. One is analog and the other is digital. (Analog approaches measure a change in the value of a signal, such as the voltage, while digital technologies rely on the binary choice between the presence and absence of a signal.) Their respective advantages and disadvantages present clearly different experiences to end users.

Resistive touch

The traditional touch screen technology is analog resistive. Electrical resistance refers to how easily electricity can pass through a material. These panels work by detecting how much the resistance to current changes when a point is touched.

This process is accomplished by having two separate layers. Typically, the bottom layer is made of glass and the top layer is a plastic film. When you push down on the film, it makes contact with the glass and completes a circuit.

The glass and plastic film are each covered with a grid of electrical conductors. These can be fine metal wires, but more often they are made of a thin film of transparent conductor material. In most cases, this material is indium tin oxide (ITO). The electrodes on the two layers run at right angles to each other: parallel conductors run in one direction on the glass sheet and at right angles to those on the plastic film.

When you press down on the touch screen, contact is made between the grid on the glass and the grid on the film. The voltage of the circuit is measured, and the X and Y coordinates of the touch position is calculated based on the amount of resistance at the point of contact.

This analog voltage is processed by analog-to-digital converters (ADC) to create a digital signal that the device's controller can use as an input signal from the user.

What's so special about Gorilla Glass?

Many vendors are quick to trumpet the use of Corning's Gorilla Glass in their products. The glass is used as a protective outer layer for many devices, from smartphones to large flat panel televisions. But what makes Gorilla Glass different?

The answer lies in the composition of the glass itself. Most display glass is an alumina silicate formulation, which is made up of aluminum, silicon, and oxygen. The glass also contains sodium ions spread throughout the material. And this is where the difference starts.

The glass is put in a bath of molten potassium at about 400 degrees. The sodium ions are replaced by potassium ions in a process that's a bit like soaking a pickle in salty brine. It's a diminishing process: More of the sodium ions are replaced by potassium at the surface of the glass, and then fewer and fewer are exchanged as you go further into the glass.

Why change from sodium to potassium? Sodium (Na) has an atomic number of 11, while potassium (K) has an atomic number of 19. If you remember your high school chemistry, this indicates that the potassium atoms are significantly larger than the sodium atoms. (The atomic radius of a neutral sodium atom measures out as 180 picometers and potassium at 220 picometers, so potassium measures out as more than 20% larger.)

Imagine that you have a box packed tightly with tennis balls. What would happen if you took out the top layer of tennis balls and replaced them -- one for one -- with larger softballs? The softball layer would be squeezed together much more tightly and it would be harder to get one out.

That's what happens with glass when the potassium ions take the place of the sodium ions. The potassium ions take up more space and create compression in the glass. This makes it more difficult for a crack to start, and even if one does start, it is much less likely to grow through the glass.

The concept of strengthening glass through ion exchange is not new; it has been known since at least the 1960s. And other companies offer glass that has been strengthened by this type of process. Corning's Gorilla brand of strengthened glass has gained considerable market share, however, and has a very visible presence in the marketplace.