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…….
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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.
As they say on the London Underground, “Mind the Gap”. The separation between the back of the touch sensor and the front of the LCD is important for two reasons—optical and electrical.
In general, one wants the touch sensor to be as close to the display as practical, so that there is no pronounced parallax error. Parallax occurs when the finger or probe comes in at an angle and is detected “too early”, and thus misses the actual target — such as touching a “z” on the on-screen keyboard, instead of the intended “a”. This is most pronounced in older style infra-red touch technology, because the LED’s are relatively high above the display, but can be a problem with any touch sensor located too far from the display.
But putting the touch panel too close can cause problems if you are able to deflect the touch screen so that it hits the LCD. Your 17” touch panel will probably not have this problem because larger touch screens have a rigid back layer that will not easily bend. Smaller, thinner, touch panels, however, can be pushed into the LCD and a rainbow (moiré pattern) can occur, which is not good for the life of the LCD. In a few cases, for LCD’s without a metal frame, you can actually put the touch screen on the LCD, and the moiré pattern will be annoyingly permanent.
The LCD and backlight can also create enough electrical noise to slow or stop the operation of the sensor, so there needs to be a gap between the LCD and the touch sensor. The required gap size, however, is not a constant. The needed separation varies with the LCD model and touch technology; those lightning-fast tiny switching transistors do emit some electrical noise, believe it or not. For projective capacitive technology, the necessary gap also varies with the IC manufacturer.
So, to mind your gap, a separation (from the surface of the LCD) should be the thickness of the metal frame plus 10/1000’s, which is usually the thickness of the adhesive that holds the touch screen to the LCD. Sometimes, you will be tricked by a particularly noisy LCD and need a larger gap, also handled by better tuning of the electronics, but 40/1000’s (bezel plus adhesive), as a minimum, will usually work.
Do you have a question for Touch Guy? Send him an e-mail at firstname.lastname@example.org.