In an effort to balance the need to manage noise injection, reduce sensor thickness, reduce phone weight, reduce system costs, and increase user experiences, manufacturers have invented many different sensor types. Figure 3 shows a table of some of the latest techniques.
Figure 3: Latest Sensor Manufacturing Techniques and Variations
While it is clear from Figure 3 that there are many different variations on the market, each with their own benefits, more recent trends find vendors attempting to do away with several layers of material by three primary technologies: 1) Sensor on Lens (G1M), 2) On-cell technology, 2) In-cell technology.
Sensor on Lens (Single Layer Multitouch): The industry is calling this “One Glass”, “Direct Pattern Window”, “Sensor on Lens” or “G1M” (for 1 Glass Layer, Multitouch). In development of a Sensor on Lens solution, the sheet glass is first strengthened (hardened through chemical treatment), has ITO deposited and etched, and is then cut into smaller pieces for use in products. Finally, an advanced post process technology is applied to strengthen the glass edge to prevent edge-break susceptibility. Alternatively, some manufacturers cut larger glass into smaller pieces, chemically strengthen them prior to ITO deposition, and then apply the ITO processing. One of the critical issues for creating a low cost Sensor on Lens, however, is the ability to get sufficient multi-touch performance from a single printing layer of ITO. Cypress Semiconductor’s SLIM (Single Layer Independent Multitouch) technology, for example, is a unique combination of ITO patterning and Touch Controller algorithms that allow a sensor to be built with a single ITO layer that does not require vias (bridges or jumpers). As a result, multiple steps of photoresist patterning and removal can be eliminated and multiple layers of glass, adhesive, and acrylic can be removed. All of this makes for a simple, cost and weight efficient sensor.
In fact, as a result of process step removal, the approach can cut the cost of a multitouch panel in half.
On Cell: Some companies are pursuing solutions that begin to eliminate the upper sensor layer all together by designing the ITO Transmit and Receive layers directly into the LCD. These technologies are known as On-cell and In-Cell. The basic promise of On-cell is to eliminate the entire separate sensor development. In On-Cell specifically, the ITO sense layers are patterned directly underneath the top polarizer layer in an LCD. LCD vendors will, in effect, form the touchscreen layers for Transmit and Receive directly into the LCD glass substrate. Because many glass sensors are between 1.0-1.5mm thick, this technology can eliminate substantial thickness and weight in a mobile product.
In Cell: The moment one of the Transmit or Receive layers is patterned below the color filter, the industry refers to this technology as In cell. For in-cell, the promise is for the ITO pattern to be manufactured simultaneously with the VCOM layer of an LCD. This pushes the touch sensing channels deep into LCD. The benefit of this technology is efficiency in manufacturing, elimination of sensor costs and manufacturing steps. The drawback, however, is in the amount of noise created for the touch controller. Because the ITO patterning is so close to the voltage layer in the LCD, the parasitic capacitance to the touch controller can be many times higher than in a traditional sensor construction. Additionally, because the LCD shares the transmit ITO lines on the same layer as the VCOM signaling, it is imperative that the touch controller device perform its scan during the period when the LCD is not being driven. This requires multiplexing these signals and can result in less time available to perform touch scanning. Because of the increase in noise interference and the complexities of scan timing, in-cell remains an evolving technology.
Figure 4: Most Advanced Touchscreens: Sensor on Lens, On-Cell, and In-Cell Cross Section
The mobile phone and tablet space continues to demand ever thinner, lighter, and cheaper touch panels, and there are dozens of different methodologies chasing the best possible solution. Today’s forerunner technologies, Sensor on Lens (Single Layer Multitouch), On-cell and In-cell offer legitimate near-term answers. Despite their promise, however, they each still possess significant commercial and technical barriers before the technologies push into more mainstream products.
It will require dedicated work between panel manufacturers, OEMs, and strategic sourcing organizations to begin to eliminate manufacturing steps in an economical way. Having a design community aware of the deep technical issues with integration in touchscreens is also an important step in creating products that are both cost effective and deliver a great user experience. So, while many designers and products have had to stay on the sidelines of the touch revolution due to cost and supply chain constraints, perhaps some of the more recent innovations to make touch panels cheaper, lighter, and thinner will give some new product ideas the opportunity to make it from a white board concept into the hands of consumers.
About the Author
Trevor Davis is a Cypress veteran who has worked within several different product groups at Cypress. He has most recently led the World Wide Business Development organization for Cypress’s TrueTouch products and is now the Regional Sales Manager for the Rocky Mountain region in the USA. Before TrueTouch, Trevor served in several different Product Marketing and Business Development leadership roles for Cypress Programmable System on Chip (PSOC), USB products, Network Search Engines, CPLDs, and Software products.
Trevor received his undergraduate degree from the United States Air Force Academy and also holds his Masters in Business Administration. Trevor has worked in high technology positions for the military, nonprofit, and commercial sectors. Trevor lives in Seattle, WA and can be reached at firstname.lastname@example.org