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.
One of the big advantages of resistive touch panels is that they are relatively inexpensive to make. Another is that you can use almost anything to create an input signal: finger tip, fingernail, stylus -- just about anything with a smooth tip. (Sharp tips would damage the film layer.)
This technology has a lot of disadvantages, however. First, the analog system is susceptible to drift, so the user may have to recalibrate the touch panel from time to time. (If you owned a PalmPilot or other PDA, you may remember having to occasionally go through the recalibration process on their PalmPilot.) Next, the ITO material used for the conductors is brittle and not well suited for bending. Over time, repeated use can cause the ITO to crack, which disrupts the flow of electricity and can result in a dead spot on the touch screen.
In addition, there needs to be a gap between the two sensor planes that must be bridged in order to make contact between the two. Just about the only material suitable for this gap is air, but this presents some problems of its own.
First, the gap adds to the combined thickness of the display and touch module. As the consumers demand thinner and thinner devices, a single millimeter can be a big deal.
Another problem has to do with the optical properties of the different layers. If you look at a drinking straw in a glass of water, it will look as though it is slightly bent where it enters the water, even though it is straight. This is because light can bend, or "refract," when it makes the transition from one material to another. If the materials have the same index of refraction, the light won't change its path, but if the index of refraction is different, the light will bend.
The space between the plastic and glass layers of a resistive touch panel is filled with air, and the air has a different index of refraction than the other layers, which makes the light bend as it passes from one layer to another. This can create visible artifacts that can impact the display quality.
The air gap is especially a problem when you view the display under high ambient light conditions, such as outdoors in bright sunlight. The outside light passes through the top layer, then bends when it hits the air gap, and can then reflect between the glass and plastic layers before exiting out the front of the display again. This bouncing light can reduce the image's contrast, making the display look washed out and impossible to see.
But probably the biggest problem with resistive panels in consumers' eyes is that they can sense only one touch at a time. If you touch the panel in two places at once, the combined effect will produce one coordinate for the touch point, and that will be different from either of the two actual points. There are ways to create resistive panels that can sense multiple touches at one time, but these can be expensive and complex, such as creating a matrix of separate contact pads on one of the layers.
Projected capacitance
Fortunately, there's a better way. Many mobile devices now rely on a technology known as "projected capacitance," often referred to in the industry as "p-cap" or "pro-cap." According to various sources, resistive touch has rapidly lost market share to pro-cap and is forecast to continue to decline.
Pro-cap is a solid-state technology, which means that it has no moving parts (unlike the resistive touch technology). Instead of being based on electrical resistance, it relies on electrical capacitance.
When you apply an electrical charge to an object, the charge can build up if there is no place for the electrons to flow. This "holding" of electrons is known as "capacitance." You have probably experienced this effect first-hand. When you walk across a carpet in rubber-soled shoes in the winter time, electrons can build up in your body. If you should then reach for a light switch or some other conductive object that does not have a similar built-up charge, those electrons can flow from your body to the object, producing a spark of electricity.
If you apply a charge to a conductor, and then bring another conductor near it, the second conductor will "steal" some of the charge from the first one, just as the light switch did when your finger approached it. If you know what the charge was to start with, you can tell when the amount of the charge has changed. This is the principle behind pro-cap touch screens.
Early capacitance touch technologies required that you actually touch a conductive layer. This approach left the conductor exposed to wear and damage. Today's projective capacitance technology relies on the fact that an electromagnetic field "projects" above the plane of the conductive sensor layer. You can cover the touch module with a sheet of thin glass, for example, and it will still sense when a conductor comes near.
Pro-cap touch screens use two layers of conductors, separated by an insulator (such as a thin sheet of glass, though other insulating layers can be used). The conductors typically are made of transparent ITO, just as with the resistive designs. The conductor layers never have to bend, however, so its brittle nature is not a problem with pro-cap screens.
The conductors in each layer are separate, so that the capacitance of each one can be measured separately. As with a resistive panel, the conductors run at right angles to each other, so that the device can sense an X and a Y position when touched. The difference is that the separate conductors are scanned in rapid sequence, so that all the possible intersections are measured many times per second.
When you touch the screen with your finger, it steals a little of the charge from each layer of conductors at that point. The electrical charge involved is tiny, which is why you don't feel any shock when you touch the screen, but this little change is enough to be measured. Because each conductor is checked separately, it is possible to identify multiple simultaneous touch points.
Pro-cap technology is not without its challenges. The system of conductors is susceptible to electrical noise from electromagnetic interference (EMI). This can be a problem for display devices such as LCD and OLED panels that rely on an active matrix backplane of transistors to rapidly switch the individual subpixels on and off. The touch screen controller must be able to filter out this background noise and figure out which signals are from actual touch points.
The controller is often asked to make other decisions as well. Comparing results from adjacent coordinates can help determine if the touch is hard or soft, or if it is the result of the palm of the hand resting on the screen and thus should be ignored. Some smartphones rely on the touch screen to signal when the phone is being held next to the user's face, so that the screen can be turned off to save power.
All these tasks require significant processing power, which makes the controller more expensive. In addition, the touch screen only works when you apply a conductor; the ball of your finger works, but not your fingernail. Some pro-cap screens will work even if you're wearing thin surgical gloves, but they won't work if you have thick winter gloves on. (The exception is if the gloves themselves are conductive; you can buy gloves with conductors woven into the fingertips so that they can conduct the charge from the screen to your finger.)
In spite of these shortcomings, pro-cap technology has become the dominant choice for mobile devices. And there are improvements on the way that could make them even better.
Can't be too thin or too light
Consumers have made it clear that they want smartphones and other mobile devices to be as thin and lightweight as possible. As a result, design engineers are always looking for technology improvements that let them remove layers and materials from their products. And touch screens are not immune to such scrutiny.
The traditional structure for adding pro-cap touch to a display is to purchase a separate module. You would start with an LCD panel that is made up of two glass layers that contain the liquid crystal material; the top glass sheet is covered with a polarizing layer.
Above that goes the pro-cap touch module, which is made by coating both sides of a glass sheet with a conductor (typically ITO), which is then patterned to create the electrodes. This glass sheet is then laminated to the polarizer layer of the LCD panel described in the previous paragraph.
Finally, a protective cover glass is placed on top of the touch panel so that the top electrodes are not exposed. This cover can also have decorations (such as logos or icons for fixed controls) and be designed to protect the display from damage.
If you've been counting, you'll realize that it all adds up to four different sheets of glass in the stack -- which means that even today's thin smartphones aren't as thin as some might prefer. If manufacturers could eliminate one of these sheets, they'd reduce the space required for glass and the weight of the glass in the display by 25%. Those are savings worth pursuing.
A method that is gaining momentum is called the "one-glass solution" (OGS); it eliminates one of the layers of glass from the traditional pro-cap stack. The basic idea is to replace the touch module glass by a thin layer of insulating material. In general, there are two ways to achieve this.
One approach to OGS is called "sensor on lens." (In this case, the "lens" refers to the cover glass layer.) You deposit an ITO layer on the back of the cover glass and pattern it to create the electrodes. You add a thin insulator layer to the bottom of that, and then deposit a second ITO layer on the back of that, patterning it to create electrodes running at right angles to the first layer. This module then gets laminated onto a standard LCD panel.
The other approach is called "on-cell" pro-cap. (Here the "cell" refers to the LCD display.) A conductive layer of ITO is deposited directly onto the top layer of glass in the LCD panel, and then patterned into electrodes. A thin insulating layer is applied, and then the second ITO layer is patterned with the second layer of electrodes. Finally, the top polarizing layer is applied on top, and the display is completed by adding the cover glass.
This may not make much difference to the end user, but it can make a huge difference for the companies in the supply chain -- including which companies are actually included.
When the touch technology is deposited on the cover glass using the sensor on lens approach, you end up with a separate touch module that can be sold to the LCD display assemblers. This would mean more revenues for the touch technology manufacturers who would supply these modules.
On the other hand, the on-cell alternative means that the LCD panel manufacturers can add these touch layers onto their own panels. The display assemblers would then just have to purchase a simple cover glass to complete the display. The touch module makers would be cut out of the process.
For now, it appears that the sensor on lens approach has an advantage over on-cell solutions. The on-cell approach means that LCD makers would have to make two separate models of each panel: one with touch and one without. This could add cost to an industry that is already running on razor-thin margins. Also, on-cell touch is limited to the size of the LCD panel; sensor on glass modules can be larger than the LCD panel, providing room for the dedicated touch points that are part of many smartphone designs.
LCD vs. OLED
In case you've been wondering where OLED displays fit into all this: An OLED display stack is somewhat different from an LCD stack. It only requires one substrate (glass) layer as opposed to LCD's two, and the OLED material layer is much thinner than the LCD layer. As a result, the finished display can be half as thick as an LCD panel, saving weight and thickness -- which is important in a smartphone design.
(A number of smartphones today use a form of active-matrix OLED display called Super AMOLED; these include several Samsung devices such as theGalaxy S III and the Motorola Droid Razr M).
As a practical matter, glass is still used as the encapsulating layer, so OLEDs generally have two layers of glass. In addition, not all OLEDs are RGB -- some use white emitters instead to try to reduce the differential aging problem, and add a color filter layer to the stack.
In spite of all this, as far as touch screen technologies are concerned, OLEDs are more like LCDs than they are different: Both have active matrix TFT backplanes, and both tend to have a cover glass layer for protection. So essentially the same stack configurations are available to OLED panels.
What's next for touch
No matter which solution wins out, it is clear that pro-cap technology is the best method for touch screens on mobile devices -- at least for the foreseeable future. Still, there are some changes already showing up in touch screen technology.
For example, some panel makers are creating "in-cell" touch panels, where one of the conductive layers actually shares the same layer as the thin film transistors (TFTs) used to switch the display's sub-pixels on and off. (These transistors are fabricated directly on the semiconductor backplane of the display.) This approach not only reduces the electromagnetic noise in the system, but also uses a single integrated controller for both the display and the touch system. This reduces part counts and can make the display component thinner, lighter, more energy efficient and more reliable.
This approach only makes sense for very high volume products, such as a smartphone from a major vendor that is expected to sell millions of units, because the panel will have to be made specifically for that unique model. The first products using "in-cell" touch technology have already appeared on the market, such as the new Apple iPhone 5, but it looks as though it will take years before this approach will become a widespread solution.
