The rise of metal grids/silver nanowires, ITO alternative materials are getting hot
2018-01-02 14:35:00 Editor:MICRON 2
A new generation of indium tin oxide (ITO) replacement materials has become a trend in the past two years. In addition to the emergence of various materials, some touch panel manufacturers have also begun to import these materials, and even related new processes. On the surface, it may only be the use of new materials, but in fact it may cause qualitative changes in the supply chain in the future. The reason why new materials are proposed at this point in time is not only because there are new material competitors who want to join the game, but also because ITO has indeed reached the limit in some applications.
Break through the ITO large-size application challenge Low impedance/flexible materials attract attention
In addition to LCD panels, resistive and projected capacitive touch panels (Touch Module) should be an important application of ITO. Compare the placement relationship between the Touch Sensor and the display panel. Some are placed around the display panel (such as optical or acoustic); and some are required because the sensor is opaque Placed under the display panel (like electromagnetic induction type); In addition, some are placed on the display panel, such as external projected capacitors belong to the latter. Because of this, the projected capacitive sensor (that is, the sensing electrode) must look transparent, otherwise it will affect the visibility of the display panel below. ITO is a sensor material because of its excellent optical transmission and electrical conductivity.
Japan is an important ITO target production area in the world, but the indium (Indium) in ITO materials must rely on the production in mainland China. The production capacity of indium and the remaining storage capacity in the earth have once become the focus of the offensive and defensive war on whether ITO can be used continuously, and it also shows the industry's expectations for new materials; however, there is still no consensus that indium production capacity will appear in the short to medium term. shortage. At the beginning of July 2014, the material price of indium was about RMB 4,970, which was even higher than the price of silver material (RMB 4,295) during the same period. However, in the past year, the price of indium has actually fluctuated and changed significantly. If there is a real shortage, the price should rise all the way, showing that the ups and downs should mean see-saw and adjustment between production capacity and the market.
The transparent electrode of the plug-in projected capacitor must meet the two conditions of light transmission and conductivity at the same time. Conductivity is even more important in the application of larger touch areas; if the sheet resistance is too high, in addition to the increase in the power consumption of the sensing electrode, the control chip may be more laborious to push, which will affect the touch Sensitivity. ITO itself is a very good conductive material, but it must be attached to the film or glass when used as a sensing electrode. Before performing the sensor patterning process, ITO is usually deposited on a thin film (usually a PET material) or glass by magnetron sputtering.
The way to get a lower resistance value is to increase the deposition thickness of the ITO layer, but the increase in the thickness will affect the light transmittance; in addition, the resistance of the film during the sputtering process is poor, so it cannot be larger. Relatively speaking, the resistance of glass is better, and the natural resistance value can also be reduced, but the thickness of glass is poor.
The current mainstream specification of ITO film is about 150 ohms (Ω)/unit area impedance value, which is sufficient for touch areas below 10 inches, but it is difficult to reach the size of a notebook computer or above 20 inches; at the same time, when When the touch area becomes curved or flexible, the fragility of ITO will easily cause the impedance value to rise sharply, and the impedance stability will become very poor.
Therefore, to become the key to the specification of ITO replacement materials is to be able to solve the above problems, including high conductivity, and can be applied to any substrate and any size; in addition to low surface resistance, it must also be able to maintain good Light transmittance; Furthermore, the material must be flexible, and the impedance value and the stability of the sensing electrode can be adapted to the non-planar touch area.
Advanced process blessing The rise of metal grid/nano silver wire
In addition to ITO and other inorganic transparent conductive oxides (Transparent Conductive Oxide, TCO), there are currently about five alternative materials in the touch panel industry: Metal Mesh, Silver Nanowires, Carbon Nanotube, Intrinsically Conductive Polymer and Graphene. The first three are replacement materials that have been actually mass-produced and shipped (Table 1).
However, judging from the selection and layout of touch panel factories, metal grids and nano silver wires have more supporters, especially the former. Both of these materials are metals (silver or copper), and their conductivity is better than ITO, and it is easy to easily reach a surface impedance value of 100 ohms or even below 50 ohms with acceptable light transmittance. Metal is not a transparent material like ITO itself, so it must be meshed or scattered filaments to increase light transmittance. For example, if the material occupies 10% of the area on the substrate surface of a sensor, then There will be 90% of the area for light to directly penetrate the substrate, which means that 90% of the light transmittance can be achieved (if the light transmittance of the substrate is negligible).
Metal mesh and silver nanowires have obvious advantages over ITO in terms of low surface impedance, but more importantly, both support flexible and non-planar touch areas, and there will be no sharp impedance changes. ; And in the application of projected capacitive touch area larger than 30 inches diagonal, the surface impedance of the two is more ideal, especially the metal grid, the grid pattern is consistent, coherent and extensible. Therefore, when forming a larger size sensing pattern, the uniformity of the circuit and pattern is easier to control.
Relatively speaking, the current process of silver nanowires is to first wet coating (Wet Coating) on the film, and the control of uniformity is particularly important. Unlike the continuity of the metal grid, each nano silver wire is a separate individual. The conductivity is achieved through the interleaving and overlapping of the silver wires. If the uniformity of the silver wires is not well distributed, the impedance value will be uniform. The temperature will be affected, or even disconnected.
On the other hand, metal grids also have some shortcomings, especially the problems of light reflection and Moire Effect. At present, the single line width of the metal grid is about 4 microns (μm). If it is too wide, it needs to be blacked on the surface of the grid line to reduce reflection, but this will cause the display panel to be too visually Dim viewing experience; in addition, the reduction of the moiré effect must rely on the graphic design (such as diamond, honeycomb and irregular shapes) and coordinate with the parameters of the display panel (such as Black Matrix and Pixel Pitch).
However, the shortcomings of metal grids and nano silver wires are not problems that cannot be solved in principle. In the future, they will have the opportunity to gradually overcome them through the refinement of the manufacturing process. The reason why touch panel manufacturers choose these replacement materials is that in addition to the superiority of the material itself in key specifications, the manufacturing process is also an important factor.
Taking nano silver wire for example, it can be compatible with existing low-temperature yellow light equipment. Therefore, the investment and depreciation of the equipment will be more cost-effective for the touch panel factory; and in the pre-wet coating process, the material The degree of waste will also be better than the original ITO sputtering method.
As for the metal grid, some manufacturers have proposed the "Additive Process" to replace the "Subtractive Process". Typical examples of the latter are Laser Etching and Yellow Light. The yellow light process is to first deposit materials on the substrate, and then through precise exposure, development, and photoresist removal steps, leaving the desired sensing patterns and traces, and etching away unnecessary materials. The former is like the nano-imprinting process of Oufeiguang and LG Innotek. The sensing image and the circuit are fabricated on the substrate at one time using a roller die, without the need to etch the material.
Other addition processes include the Gravure Printing method proposed by the Industrial Technology Research Institute and the UniPixel Metal Plating method. It will take some time for these new generation processes to completely replace the mature ITO supply chain and yellow light process.
Expanding application/process advantages Emerging materials to replace ITO can be expected
Generally, when judging the introduction opportunity of new materials, the commonly used standard is the material cost, but the cost is often only a result, not a cause. From the perspective of the panel industry, compared to plasma technology, the structure and components of liquid crystal panels are too complicated, and the natural cost should be relatively high; but in terms of the abundance of the supply chain, the investment of many upstream and downstream manufacturers, and the growth of the application end market Due to the timing and other factors, the price of LCD panels has continued to fall, and its impact has not only eliminated plasma panels, but has also tightly suppressed organic light-emitting diodes (AMOLED).
Therefore, the specification advantage of the above-mentioned ITO replacement materials is only a ticket. After the input of touch panel manufacturers, the refinement of the process, and the ability to gain a place in the mainstream application market, the penetration rate will gradually increase; then, There will be more input from upstream and downstream manufacturers, resulting in a continuous decline in costs, making new materials more competitive.
The current supply chain and ecosystem based on ITO are actually quite abundant and stable, and for the largest application market (small and medium-sized mobile phones and tablets), unless the price of indium soars, the throne of ITO is not so Easy to be challenged.
The opportunities for new materials are mainly in two directions: the first is the specification and characteristics, and the second is the investment of the leading manufacturers. The former can be cut in from the application of larger size and non-planar touch area. These two niches are where ITO performs poorly, which can highlight the advantages and value of new materials; the latter is dependent on things like Chen Leading manufacturers such as Hong, OFILM and others have introduced new materials into mainstream applications after process improvement and maturity, providing customers with different choices and gradually building their confidence.