I’m kind of new to the touch screen world and am trying to figure out why it is so difficult to get large (over 22”) multi-touch resistive and projected capacitive touch screens. Also, what are the big advantages and performance differences between the competing multi-touch technologies?

Hi Jim:

So you want to know about big ‘uns?

Projected capacitive, our favorite multi-touch technology, has pretty much topped out at 32 inch diagonal sizes. There are bigger ones available, but they have those annoying little wires that nobody likes to see.

In the realm of large format multi-touch, we find that infrared, camera (often called optical) and DST are the most commonly used touch technologies for large sizes (+32”), however IR and DST only support two touches. The mainstream projected capacitive and multi-touch resistive technologies are capable of unlimited touches, but are not as easily scalable (note that unlimited touch is controller-dependent, not sensor-dependent). So if you fancy yourself as Tom Cruise on a big 60 inch display, you will probably have to wait for projected capacitive to make it big.

So, what are we waiting for? We are waiting for transparent low ohm conductors. You will recall that projected capacitive is an X-Y scanned technology, using ITO for that purpose. But, white paper fans amongst you will recall that ITO has a relatively high resistance when compared to something like a copper wire. As the touch sensor gets bigger, the ITO resistance gets higher, and eventually, too high for the electronics to work. So, what is to be done?

TA DA! Debuting now, for your viewing pleasure, are nano-wires (think a Chinese population of angels on the head of a pin) and super-fine line copper webs. Both of these sport the required low resistivity, and only the most critical alien-eye can see them…. we can even coax them into rows and columns. Expect the electronics to grow as well, but, hey, what is an extra multiplexer or two…

So, Mr. Proctor, looks like your wait is over. Technology is getting bigger, not smaller, this time.

For even more info on multi-touch, check out Touch International’s Putting the ‘Touch’ in Multi-Touch White Paper and refer to the graph below.


– Touch Guy

Do you have a question for Touch Guy? Send him an e-mail at [email protected].

Can you please explain what sputtering and micro-etching are and how are they used in touch screen manufacturing to impact touch performance?

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…….

Touch Guy

I’m working on an open source 17″ tablet for the DIY 3D printing community and am in desperate need of a 17″ – 17.1″ USB multi-touch screen overlay with a 16:9 or 16:10 aspect ratio. So far, I am only coming across single touch resistive and IR multi touch panels. We only really need dual touch panels with as good light transparency as possible, as these are graphics editing machines we are making. Projective capacitive panels would be the ideal technology I assume. How can I find such a beast?

So you want a 17 inch projected capacitive touch screen and are having a tough time finding one? You have chosen wisely, because for the printing business, you will want the super sharp image that you get using this optically clear technology.

Even though Touch International makes projected capacitive sensors up to 22 inches, I have to tell you, you are on the cutting edge of wanting the “big guns” for projected capacitive touch screens. Except under special request, all of our projected capacitive panels have to have a perfect image (no shadows), they must do multi-touch (two or more fingers, no ghosting), and they have to have a touch response of less than a 1/10th of a second (among other things). Because of these requirements, we are not always the fastest to market, but we’re sure to deliver some of the best touch screens around.

There are two key reasons that larger-sized projected capacitive is difficult to do. The first is that no-shadow touch screens require very precise manufacturing equipment of the type used to make semi-conductors or TFT displays. Most commonly available are machines that will process projected capacitive sensor sizes up to 15 inches, which is why that size is easy to find. You may know that Touch International has just opened its new China factory, and can process up to 17 inch sensors, so you are in luck (pretty soon). We can (and do) make 19 inch, and larger, projected capacitive panels using other methods, but they break the visible line requirement, so they will not work for you.

The second complication to making big projected capacitive panels is the speed at which the multiple touches are recognized, which is dictated by the touch electronics. As I am sure you know, having read my white papers on the subject, projected capacitive is a scanned system which means that every row and column must be “energized”, and then interpolated to get an overall resolution of 1024 by 1024, which is a lot of work for those little silicon buggers. As the screens get bigger, there are more rows and columns to move into action. This requires multiplexing, or, most often, it takes multiple fancy projected capacitive ASICs working together in a master/slave relationship. A single ASIC, which will handle 17 inch screens, has just hit the market (two touch maximum), so, once again, you are just in time.

So, we may be your source, but you will need to hold your breath a little longer – maybe until Q2.

Touch Guy