Touch Technologies
August 13, 2012, Display Search Emerging Display Technology Conference, Santa Clara, CA—Mark Hamblin from Touch Revolution compared and contrasted touch on lens and in-cell technologies. The need for higher performance and lower costs are competing with increased manufacturing complexity in the target technologies.
Capacitive touch interfaces need two layers of ITO to form the sensor array. The move to eliminate the separate sensor glass means that the sensor ITO has to be on some other layer(s). It is possible to design a single-sided array and add jumpers to make up the matrix. After the patterning, the large glass is singulated and tested for opens and shorts. This alternative is appealing to the touch panel manufacturers.
The issues with this approach are in the higher costs for manufacturing, design complexity, a more complex supply chain, and higher thickness and weight than some alternatives. One alternative is to integrate the sensor array with the thin film and color layers and perform the function in cell. Putting the touch layer in the TFT stack up can be troublesome due to the need to optically bond the cover layer to maintain alignment and a good seal from the environment. This bonding makes the display-touch sensor assembly more expensive and means that the whole assembly must be replaced if the cover glass is damaged.
Another possibility is to use a z-stackup approach. The x and y matrix plus jumpers are on the back of the cover lens. This is attached to a mother sheet, a black mask in applied, then the ITO is applied and patterned. The mother sheet is cut into the smaller displays and the edges are polished and strengthened. The specific order of processing steps is malleable as is the hybrid stackup that results.
By applying one axis on the cover and another on a separate layer, the design eliminates the jumpers and other films. The challenges are that this requires a change in the TFT or the color filters to survive the additional processing. If the controller is integrated into the assembly, this increases the design cycle and increases the number of suppliers for that single assembly. In addition, the disparate mechanical, chemical, and electronic capabilities all have unique technology roadmaps, further increasing manufacturing complexity.
Putting the sensor on one of the other existing layers reduces thickness and weight because a glass layer is eliminated. It simplifies the supply chain by moving everything to the display manufacturer, and in theory can reduce costs. The downsides include the need for optical lamination, and high initial tooling costs. The custom nature of this process means that the buyer cannot multi-source the display and loses any possible pricing leverage. Any design changes are very slow, due to the overall complexity. Net, this level of display and sensor will be more expensive.
For touch on lens, the challenges are in cutting and shaping the glass, the need for an additional black mask, and very high potential tht the ITO will be damaged in handling, reducing yields. The positive aspects include lower overall costs, no optical lamination, and the potential for multiple sources. The cons are the low edge strength due the inability to chemically strengthen the edges after the mask is applied, and the whole assembly is unshielded. The individual piece parts need a lot of processing, so the advantages of big glass are lost when the end display is on small glass.
In cell is acceptable for expensive products that need high strength edges. Touch on lens with a mother sheet has lower strength and needs individual piece handling. Glass on glass discrete assemblies are still viable for some applications.
