Sputtering and micro-etching are complimentary processes that are often used to reduce touch screen borders and package sizes and ultimately improve touch performance.
“Sputtering” (aka known as suck-and-spit by the inventor) is a process by which an opaque material, usually indium tin oxide (ITO) is put onto glass or plastic, in a vacuum, resulting in a transparent thin film (say, 300 angstroms [number of atoms] thick). This is the starting point for overlay-type touch screens. Today, we also sputter molybednium/aluminum/moly (MAM) over the ITO to make very fine conductive traces (next answer) so that the nutcase designers can have unreasonably tiny borders.
Micro-etching is a method of removing the ITO or MAM in the sub-50 micron line width. On the touch sensor, making the ITO etch lines so tiny that you cannot see them eliminates all shadows, making the image look great and enhances the magic of touch.
Because projected capacitive is a scanned system, there must be a low resistance connection to every ITO row and column, occurring every 6 millimeters or so, and adding up to about 30 electrical signal traces at the edge of the ITO. When the designer has allowed almost no area to make the 30 separate connections, the only way to do it is to use tiny lines micro-etched in the area at the edge of the screen, which is usually hidden by an opaque (black) border.
While micro-etching the transparent ITO is no more expensive than conventional etching, the semi-conductor class equipment is a very expensive capital acquisition which is amortized into the product.
You do not want to have tiny borders on your product unless there is no other option, because this second step is expensive. Normally a projected capacitive screen is made by micro-etching the ITO in the visible touch area. After this etching step, the glass is put back into the sputtering chamber and an opaque layer of MAM is coated over the top of the etched ITO. Then the metallic layer is micro-etched again to create the fine lines at the edge. While the iPhone has micro-etched borders, the iPad does not which helps control the cost.
There is an even more expensive way to make your projected capacitive part known as “SITO”, for single sided ITO, which requires three trips to the sputtering chamber and three trips through the micro-etch line…..but I am not going to say any more so as to not encourage you…….
Dear Astonished Jim in Arkansas,
Touch guy is about to go on vacation, so thank you for an easy question! As I’m sure you know, the iPhone uses projected capacitive technology and one of the main benefits of this technology is that it will “last forever.” Touch Guy did not really mean that you could smash it, or in your case, step on it, and it would still work. But, wow, maybe you can…
So what gives? First, I want you to know that there are several ways to make projected capacitive touch sensors, but all of these methods involve putting the transparent sensing conductors, called ITO (Indium Tin Oxide), behind a protective front layer (I call this layer a lens). The lens on an iPhone is made of a thin layer of glass – you can look in the on/off hole and see how thin this glass is. Behind it is the touch sensor, which is a thicker (0.5mm) layer of glass, and is optically bonded to the thin lens. I say “optically bonded” because there is a continuous glue layer in-between that eliminates the air gap between the touch panel and the lens and is part of the reason the display looks so good.
There are a lot of ways to bond the layers together; the material used is generally known as “OCA” (you guessed it, Optically Clear Adhesive). All automotive windows are two glass layers optically bonded together, and the internal OCA, along with tempering, is what makes the glass “safety glass” with all of its protective properties. When you get a crack in your car window from “that darn rock”, only the outer layer of glass usually cracks. So Jim, that is your answer – when you klutzed the iPhone, you cracked the outer layer of thin glass, but the touch sensing layer remained intact and worked just fine through the broken lens.
But I know you want to know more! Touch International makes all plastic projected capacitive sensors which will not break (think hand-held games for children). They also use chemically strengthened glass for the lens and touch sensor to make it harder to break. There is also another type of cover glass, alumina silicate, which can increase the strength if the lens is likely to get scratched, by, say, the keys in your pocket.
Astonished-Jim-with-the-cracked screen will probably enjoy this story: the iPhone introduction was delayed for months, because the first models of the product had an unbreakable plastic projected capacitive screen. Of course, the big man himself (if you don’t know who this is then tough luck), got the first prototype. By the end of the third week, the junk in his pockets had scratched the lens, making him very, very unhappy. So the lens maker, having invested quite a tidy sum in a plastic lens making facility, had to start over with glass.
We hear that around 25,000 smart phone lenses break each month, so you are not alone. Replacement sensors are available and there you tube tutorials on how to replace them (not easy).
Whew! That was a long answer! If you still want to know more, download the Touch International white paper “Projected Capacitive Touch Screens” – it has a lot of good stuff in it!
And Touch Guy is off! (And planning to opt-out of being X-rayed at the airport)