Demystifying Touchscreen Technology

In the August 25th post on LG's upcoming flagship phone with the quad-core Snapdragon S4 Pro processor, it was mentioned that one of the important features of this phone is a new type of touchscreen technology:

Perhaps more importantly, this will also be the first display from LG to have a new laminated touchscreen, called the G2 Touch Hybrid Display. LG says its "unibody" design let the company shave 30 percent off the screen's thickness. This sounds exactly like the "in-cell" touchscreens LG began mass-producing earlier this month, and that are widely expected to be in Apple's upcoming iPhone.

Unfortunately, the above statement is not accurate. G2 and in-cell are two opposing approaches to constructing touchscreens, and there is an on-going battle in the mobile industry about which of those two ways will become prevalent in the future. Allow me to explain.

First, we need to look at how multi-touch capacitive touchscreens work, and how they were built up to this point.

A capacitive touchscreen is made up of nearly-transparent wires (or electrodes) deposited on a layer of glass or film. They are nearly transparent because they are made of a material called ITO (indium tin oxide). The electrodes are arranged in rows and columns so that they overlap. Each time they overlap, they form a capacitor. A touchscreen controller chip is wired into this overlapping matrix of rows and columns. When your finger passes over one of those tiny capacitors, it disturbs the electrical field of the capacitor - and that disturbance in the force (ha!) is what the touchscreen controller senses.

To form a capacitor, the electrodes have to be separated by some small distance. As a result, the row electrodes were placed on the underside of the glass, the column electrodes were placed on the top side of the glass.

OK so now we have a thin piece of piece of glass, with ITO wires on either side. To build the complete touchscreen, this piece of glass is sandwiched between two things: the display on bottom, and the "cover lens" on the top. The cover lens is usually made from everybody's favorite brand of strengthened glass - Gorilla glass. This is called a "glass/glass" (or "GG") stack - two pieces of glass on top of the display. This is the type of stack used, for example, in the iPad. To reduce the effects of noise from the display on touchscreen performance, the touchscreen glass is attached to the display by a gasket, so that there is an air gap between the display and touchscreen electrodes (electric fields do not propagate as well through air as through other materials). Here's an illustration of those three layers:

Atmel_captive_touchscreen_controller_solution_medium

via www.displayplus.net

If you were to look at this sandwich of 3 layers (cover lens, touchscreen glass, display) you will notice that there's a lot of redundancy. It seems, at first glance, that the middle touchscreen glass is serving no purpose but to host the touchscreen electrodes, and it adds thickness to the phone or tablet for apparently no good reason. At the same time, there is already a piece of glass on top (the Gorilla glass) and lots of glass in the display below - so can one of those host the touchscreen electrodes instead?

The answer is yes. In fact, either of those two other layers is perfectly suited to host the touchscreen electrodes. When the touchscreen electrodes are placed on the cover lens, this is called a "G2" touchscreen (G2 means one piece of glass with both touchscreen rows and columns deposited on it). When the touchscreen electrodes are placed inside the display itself, it is called an "in-cell" touchscreen.

Here's a picture illustrating the difference between the old-style GG stack on the left, and a G2 stack on the right. The difference in thickness is immediately apparent.

34f1_medium

via www.edn-europe.com

By going with either G2 or in-cell, OEMs can make their devices thinner, since that intermediate piece of touchscreen glass is no longer needed. It also makes the devices lighter. And because the cover glass itself is becoming thinner and thinner (e.g. Gorilla Glass II, which is only 0.55mm thick vs. original Gorilla Glass at 1.0mm), we are in for some really thin phones and tablets in the future. OEMs are going the additional step of eliminating the air gap between the display the cover lens, by fully laminating them to each other with "optically clear adhesive" (fancy word for really expensive glue). The result is even more thinness, and better optical performance. The iPhone 4/4S and the HTC One X are both examples of devices with fully laminated touchscreens.

G2 and in-cell are not without their challenges though. In G2 touchscreens for example, depositing the ITO row and column electrodes on fully-strengthened Gorilla Glass is technically challenging. The reason has to do with the processing steps it takes to turn ordinary glass into strengthened Gorilla Glass, and where to interject the additional processing step of depositing the ITO rows and columns on the glass.

For in-cell, embedding the touchscreen electrodes inside the display places them in a very hostile electrical noise environment. Display noise was already a serious challenge for touchscreen controllers when the electrodes were outside the display, and now with them inside the display it's that much more difficult. A potential consequence of using in-cell technology is that touchscreens may become less responsive and less sensitive - but you can be assured that there are hundreds of electrical engineers world-wide working hard to make sure that these problems go away in the future.

Because both G2 and in-cell have their own challenges, it is not clear which one will prevail long-term in the market. There is also a lot of money riding on which one wins. The display vendors like Samsung Mobile Display, LG Display, and Sharp are counting on in-cell winning out, since they can capture more value (read: charge more money) for their displays that integrate touch functionality. At the same time, touchscreen companies like TPK and Wintek that traditionally supplied OEMs like Apple and HTC with touch sensors are doing what they can to keep discrete touchscreens alive through the adoption of G2, before the display vendors integrate their product inside the display and cut them out of the business completely. With both sides fighting for touchscreen supremacy, the real winner will be the consumers.

So...now you know more than you ever needed to about touchscreens. One thing is clear: this upcoming LG flagship phone will be ridiculously thin, and really, really fast. I'm looking forward to getting my hands on it.