Technical and business challenges of display integrated with touch

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 color 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.)

If at least one of the touch layers is below the CF glass, the ITO or metal mesh transparent conductors that form the touch pattern can be shared with a display driver connection. This reduces the number of discrete layers, resulting in a thinner and less expensive module. While there are other options, the industry seems to be settling on hybrid in-cell panels, where 1 ITO layer implements both VCOM and TX conductors, for traditional “vertical alignment” (VA) LCDs. VA LCDs are typically used in smartphones, while tablets use IPS-type LCDs to achieve a wider viewing angle. The VCOM is the reference voltage for all TFT LCDs. The display driver interface (DDI) IC will drive the R, G, or B video amplitude for a pixel to the source of the pixel TFT when its gate is open, and a storage capacitor is formed between the TFT’s drain and this VCOM reference potential. The stored voltage creates an electric field across the liquid crystal material that will then act as a valve to pass the brightness level from the LCD backlight.

In a regular LCD, this VCOM layer is a non-patterned ITO layer that extends across the whole active area of the LCD. The ITO layer can be reused for the TX sensor by patterning the layer into different stripes that represent the rows that will be driven for the touch function. The corresponding RX column sensors are still implemented on a dedicated layer of ITO (or another type of transparent conductor) as shown in Figure 1.