Lil background - Silver halide requires lasers
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posted on
Mar 18, 2015 03:56PM
XTouch Metal Mesh Touch Sensors and Diamond Guard Hard Coat Resin (Glass Replacement Technology)
Lil FYI - i'v bolded
In my last blog I discussed the current market for flexible transparent conductively coated films (TCCF). I pointed out in this article that some of the recently developed approaches for making these materials have started to erode ITO/films’ market share in this industry. ITO/Film has been the dominant, if not the only, viable TCCF material available for over 35 years. More than 85% of the ITO/Film produced last year, where ITO/film had almost a 90% of the approximately $1.6 billion of TCCFs (depending on which market study you read), was used to manufacture touch panels (TP). I also mentioned in this article that the growing popularity of larger diagonal size TPs, those with diagonals > 10”, was cutting into ITO/Films sales. This is because these bigger size TPs need much lower sheet resistivities (Rs) of the TCCFs than what currently available ITO/Films can realistically achieve. I also pointed out the continued pricing pressure on the TP market was causing manufacturers to look for cheaper materials. There is little room left in the production of ITO/Films to significantly lower its price because of the relatively high cost of indium and the more expensive vacuum based process needed to make ITO/Films.
Two of the more promising techniques for making alternative TCCFs with suitably lower Rs ranges at potentially lower costs than ITO/Films are; coatings of a layer of randomly deposited of silver nanowires (Ag NW) or micro metallic meshes. This note will focus on micro metallic meshes. Ag NWs will be covered next time. There are also several other methods and/or materials that can be used to make TPs, other than what I am covering here. These will also be discussed in a future blog.
Micro metallic meshes, or just “meshes” as I will refer to them in the rest of this blog, can be made in various ways. It needs to be pointed out that meshes with trace widths greater than 6-7 microns are can be seen relatively easily when one is close to the display. Therefore, only mesh designs with such narrow traces will be considered here. Meshes with wider lines can be used on very large displays where the user stands some distance away from the display monitor or in less expensive and demanding applications such as toys, low cost phones, signature verification devices, etc.
There are several techniques now being used to make these meshes. These are subtractive removal of material, direct printing or embossing with a secondary treatment step(s). In a subtractive process the polymer substrate, which has 100 % of its surface initially covered by a thin conductive layer, is patterned in such a way as to remove most of the coating material. This will leave behind a pattern or mesh of thin connected conductive traces. This patterning can be done by standard photolithographic processes using a photoactive resist, an imaging exposure step, a development step, a chemical etch and then a rinse/wash. Another way to make a subtractive mesh is by using use a laser of the appropriate wavelength to ablate most of the conductive layer away from the film surface. Finally standard photographic methods can be used, where a photoactive emulsion of silver halide is exposed using a fine line mask, or by a visible laser and then developed to create a mesh pattern of micro/nano silver particles.
The subtractive process involves the removal of most of the original material, leaving behind mostly open areas. This is what makes these coatings so transparent, sometimes over 95%. The remaining lines are metallic, leading to relatively low Rs. If the traces are less than 6 microns thick they are “invisible” to a human eye. The removal of most of the coating can make this process somewhat expensive if Ag is used, but relatively cheap if Cu is the coating. But the patterning processes are all well known and the equipment is relatively inexpensive. The other issue with grids made by subtractive processes is that the remaining traces are raised off the surface of the substrate by a relatively significant height. Therefore, they need to be protected from damage either by handling or further processing steps. To expose the whole pattern by laser can be time consuming as well.
In direct printing a layer of conductive material, such as a Cu or Ag loaded conductive ink is printed directly onto the film substrate and then cured. The remaining ink traces can then be sintered to produce a continuous metallic trace instead of one made with connecting nano particles. This will lead to a low resistance trace. Either silkscreen, gravure or flexographic printing methods can be used. Normally the traces are in the 40-20 micron thick range. However, work being done at Clemson University has shown that thinner traces are possible if the coating speed is increased. It is unlikely that direct printing will result in “invisible” traces < 6 microns. One issue with direct printing is that there are a large number of trace patterns used in making touch panels, even within one company’s line of smart phones. This will lead to a large inventory of printing masters that will change often. These direct printed traces will also rise above the surface of the substrate, needing a protective top layer just like the meshes made by subtractive removal required.
A third way to make the mesh based TCCFs is to emboss patterns, either directly in the underlying substrate or into a top polymer layer that has been deposited on the substrate. The embossing patterns can have features < 6 microns, so the subsequent traces will be invisible. If the embossed pattern results in “troughs” in the substrate, then these are filled by over printing them with a conductive based ink or a “catalytic” ink. This ink will then cause Cu to plate our of an electroless Cu plating bath. The resulting traces from either the direct printing method or the catalytic plating process are cured and/or sintered, resulting in embedded low resistive traces below the substrate’s surface. The tops of the embossed pattern can be printed with the catalytic ink as well. These raised traces are then plated up using a similar electroless plating bath as was used with the embedded troughs. As of now, processing speeds and low production yields are two concerns that the industry has with the embossing process. Theoretically, TCCF made by embossing should be very cost effective.
Sometimes when you impose a geometrically repeating pattern on top of another repeating pattern, such as you would get by putting square or rectangular repeating mesh patterns on top of the pixel pattern of LCDs, you create Moiré patterns that can be quite visible to the eye. However, by using specially designed mesh patterns and thin enough traces the Moiré patterns can be eliminated. Almost all micro mesh manufacturers have eliminated this issue with their current products.
I would be remiss in this article if I did not mention that 3M has demonstrated ITO based TPs with diagonals > 40”. I do not know exactly how they do this, but it could be that they are using a custom designed controlled that can handle the small signal to noise ratio and large RC time constant that are generally associated with projective capacity touch panel designs using less conductive traces. Or they could be using analog resistive based TP design. Further speculation on my part about how they achieve these larger size TP designs serves little purpose, so I will not continue.
John Fenn