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Now possible: AMOLED displays featuring in-cell touch sensors

20 February 2019 Calvin Hsieh

For display panels in smartphones, the sensor technology that predominates today is in-cell. However, in-cell can be used only on phones with panels utilizing LCD technology. And despite its widespread popularity and application, in-cell cannot be used on phones with AMOLED displays, which deliver superior images to those offered by LCD.

The most popular in-cell touch sensor structure is used on self-sensing in-cell TFT LCDs structured on Vcom blocks and which make use of the LCD Vcom layer as a touch-sensor pattern. Vcom, a portmanteau of the "V" from the word voltage and of the word common, is shorthand for what is known as the common electrode, the first of a pair of electrodes needed to drive voltage to LCD pixels.

Apple, which deploys its own proprietary in-cell touch sensor structure, also makes use of the Vcom layer but adopts mutual sensing as a principle. In Apple's case, its in-cell solution utilizes different sensor pattern designs that look more like many cross sections of transmitters (Tx) and receivers (Rx), unlike the appearance of blocks associated with self-sensing in-cell touch.

In LCDs, Vcom is claimed concurrently by two distinct driver functions: for display TFT driving, which controls and shifts liquid crystals to show grayscale correctly for the color filter; and touch-sensor driving, which is patterned as touch sensor units. It is this twin functionality that makes delivering higher resolution for displays a challenge because more time resources are required to process the display data. Yet in-cell touch technology continues to be an attractive solution to panel makers, boasting better integration on the display's TFT circuitry and integrated circuits.

While LCDs mainly use in-cell, which integrates touch sensors with the display TFT circuitry, the dominant technology for AMOLED displays is on-cell touch, with the touch sensor installed separately on the panel's encapsulation glass substrate. And because on-cell touch sensors are distinct from the TFT substrate, TFT circuitry is not affected. As a result, on-cell does not have the issues presented by in-cell touch. On-cell usually also has better touch sensitivity and performance than in-cell because the on-cell sensor is closer to the finger.

Yet the use of on-cell TFT LCDs in smartphones is declining. Compared with self-sensing in-cell touch, on-cell touch needs an additional process on the color filter glass, increasing manufacturing time and yield risks. Thus, TFT LCD panel makers prefer self-sensing in-cell, not on-cell.

Never possible before
One area that in-cell technology hasn't been able to penetrate is AMOLED displays—until now, that is. Recently, Taiwanese panel maker AUO has been able to deliver a new kind of in-cell touch AMOLED.

AUO's in-cell AMOLED has a touch-sensor pattern process like that adopted by on-cell AMOLED, except that AUO's technology uses the self-sensing pattern. One other major difference is deployment by AUO of additional conductive electrodes to handle traces for blocks and the TFT circuitry for the touch and display-driver integration (TDDI) chip. Its TDDI chip supplier is Raydium.

Embedded touch sensor structures for flexible AMOLED

Common on-cell touch sensor structures adopt a mutual sensing pattern, even though on-cell needs bridges or jumpers for Tx/Rx insulation on the same plane. But what makes AUO's solution unique is that its sensor structure has the blocks on the bottom side of the encapsulation glass, not on the TFT glass. Conduction from the encapsulation to the TFT glass is necessary if the TDDI chip is being adopted.

Technically speaking, in-cell touch cannot produce the same performance as on-cell touch. But AUO seems to have achieved the mass production of 480 x 480 resolution for its smartwatch application, which appears to be acceptable to the company's customers—most likely those who make Fitbit devices. However, its in-cell touch is still not quite ready for smartphone applications just yet given the challenges of implementation on larger display sizes. While smartwatches do not need too much multitouch capability, smartphones do have higher requirements for displays and possess more blocks and traces that need to be taken into consideration.

Trace routing is another critical issue. Mutual sensing capacity generates traces by X+Y (channels), but self-sensing capacities have X*Y. For example, a 6.0-inch smartphone display by mutual sensing likely has 14+28 traces, while self-sensing produces 14*28 traces. If there are too many traces, the in-cell touch sensor structure will likely have difficulty arranging traces going around the edges or in passing through the display active area. If these traces vertically go through the black matrix of the display active area, process and visibility issues become more complicated. As a result, smaller smartwatch displays are more suitable than smartphone displays in adopting the in-cell touch sensor structure.

Self-sensing in-cell TFT LCD does have advantages. It is beneficial in terms of cost and is especially remarkable for photo mask-saving, given that six or seven photo masks are needed to concurrently make display circuitry and touch sensors for an amorphous silicon (a-Si) smartphone display with high definition.

Yet cost is not the major concern with in-cell AMOLED; space is. With in-cell AMOLED, it is possible to save one chip, a flexible printed-circuit cable (FPC), and other passive components, as shown on the illustration below. Saving space is valuable when it comes to smartwatches, especially for round-face types, where circuits are more difficult to arrange than in rectangular types.

Calvin Hsieh is IHS Markit director for touch and user interface
Posted 20 February 2019

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