Challenges of display integrated with touch

The market demand for projected capacitive touchscreens continues to soar, and this technology has become, in just a few years' time, the de-facto user interface for smartphones. Over the next couple of years, we will see this growth trend continue, with the technology further trickling down to lower-end feature phones. The industry has developed a well-established touch supply chain for this technology, including discrete sensor vendors patterning indium tin oxide (ITO) films or other transparent conductor material, touch controller IC suppliers, cover lens glass vendors, and display module vendors (LCD or OLED based). The various components of a touch module assembly are then integrated by a combination of contract manufacturers (CMs) for smartphone OEMs, sensor or cover lens vendor subcontractors, or display vendors. Touch module integration involves attaching the flex cable with the touch controller IC to the sensor, laminating the sensor assembly to the display module, and attaching the cover lens.

Display vendors, anxious to capture more value, have allocated significant R&D budgets over the past few years to integrate the touch function inside their LCD module. Display vendors already use ITO deposition for the interconnection of pixel elements across the display. Each pixel requires a thin-film transistor (TFT), with a source, gate, and drain that need to be driven with the appropriate signals to hold pixel data values. With display integrated touch, the discrete sensor vendor is effectively bypassed, simplifying the supply chain.

Early efforts for display-integrated touch focused on optical "sensor-in-pixel" technology, which expanded the circuitry inside the TFT LCD cell to create an optical touch-sensitive cell. These efforts have been largely abandoned and replaced by projected capacitive touch in recent years. Projected capacitance is the same mainstream technology used in the traditional discrete, or "on-stack", sensors. The "cell" is now considered in a broader sense as the area between the top colour filter glass and bottom TFT array glass, the two enclosures between which the liquid crystal material is contained. As long as the touch function is contained between both substrates, the industry considers the display as an "in-cell" design. This means the touch layers can still be discrete layers, either separate from or shared with the layers driving the display, as long as they are contained within the LCD module. The industry has effectively turned to a more practical approach, with the first phones implementing this type of in-cell touch in production this year.

Figure 1 shows in more detail the comparison between the traditional "on-stack" sensor, and the display-integrated variants resulting in "on-cell" and "in-cell" touch panels. The on-stack sensor shows a version with the touch screen cover lens used as the sensor substrate ("sensor-on-lens").

On-cell integration
In their ongoing quest for the thinnest and least expensive touch-enabled display, display vendors seem to consider on-cell as an intermediary step. On-cell integrates the touch layers inside the display but keeps separate layers for touch "receive" (RX) and "transmit" (TX) functions. Both TX and RX sensor layers are using the colour filter (CF) glass substrate instead of a separate glass or PET substrate placed on top of the LCD module. As soon as one of the layers moves below the CF glass, the stackup is called in-cell. For in-cell stackups, there are two possibilities: "one-sided" in-cell if both TX and RX move below CF glass, and "two-sided" or "hybrid" in-cell if only one layer moves below the CF glass. (In practice, the layer below the CF glass will be the TX layer since the RX layer is always closer to the finger touching the screen.)