Metal Meshes for Sensor Arrays
August 13, 2012, Display Search Emerging Display Technology Conference, Santa Clara, CA—Dan van Ostrand from UniPixel described another alternative to ITO for sensors. The conflicting requirements for transparency and high conductivity are addressed by using copper as the conductor.
They make a metal mesh touch sensor that can be printed in a roll-to-roll production process on to any substrate. The printed lines are as small as 5 microns wide and the pattern is as fine as a 20 micron center-to-center pitch.
The benefit of copper is that it is a very high conductivity material. The larger copper traces can be as little as 0.05 ohms per square, and a 5 micron grid on 600 micron pitch results in a 14 ohm per square resistivity. This low resistance enables very large touch displays, since the signal-noise ration is not reduced by lots of resistance. Currently, their largest sensor array is 400 mm x 600 mm, but the process is scalable to much larger sizes.
They will generate different patterns with various grid spacing to meet resolution and sensitivity requirements. The copper is coated with a special blackening agent to reduce the orange color and also provide some passivation on the copper to minimize oxidization.
The grid materials are flexible and durable, they pass the tape adhesion test where the grid is subjected to a number of contacts with adhesive tape, which is pulled off to remove the grid material. The internal structure is comprised of 10 nm rods which can withstand the flexing of a roll to roll process.
The sensor has higher light transmission than a similar sensor in ITO. The ITO has a transmittance of 87.9 percent, while the copper with 5 micron lines has 92.3 percent. Increasing the line width to 10 micron reduces the transmittance to 89.2 percent, still high than the ITO and much more conductive. The high transmittance also translates to lower color shift and the ability to use the sensor in both reflected and transflective applications.
The lower resistance and capacitance associated with the very fine lines means that the sensor can be scanned faster or with lower power. As the application of the metal is an additive process, the result is less overall processing and higher yields.
