Touch is a very hot recently and there are so many techniques developed for it. Although resistive type has the most market share, capacitive type is expected to be winner in the future. Optical type still has small share, and it only can see IR camera type in market, especially all-in-one PC.
But the technology race is not determined yet, and each of these has its superiority and inferiority. For example, now capacitive type occupies small screen size, resistive has low cost, and optical type has large screen.
However,in-cell optical type is very rare. But it has some superiority to complete.
Advantages
- Embedded structure
-It does not need external module or film. - Compatible with TFT LCD
-It can be applied to original TFT manufacture process. - Size
-The only restriction is how big LCD can do. - Multi touch
-It has no restriction to the number of touch, if the processor can track such calculation. - Vision application
-Photosensing may can be used as image sensor if the signal is obviously good.
TFT LCD
The basic TFT LCD structure shows below. The light from backlight would be modulated by LC cell(liquid crystal) between polarizer and analyzer. Thin Film Transistor (TFT) is a transparent circuit to control the direction of LC cell.
In-cell photosensing touch panel
The basic operation could be light pen and finger. Light pen has significantly strong signal, but finger is more weak and need complex algorithm to identify the position. In bright ambient, touch signal is dark area, but a little bright from the reflection of finger in dim ambient.
Because the position of TFT is at rear and behind the LC cell, the light would be lost after passing film and LC. Weak signal would be a problem and explains the fact that iDTI adopted light pen as first step to market.
Photo TFT
The basic principles of photo TFT is the gate voltage form an active channel to let photogenerated electron replenish with n+ to induce photocurrent. The patent of iDTI says the gate is shorted to drain as photo TFT. The embodiment can avoid parasite capacitor ,fast the response, and has the same photocurrent under small photosensitive area. iDTI design this structure to form twice effective active channel in order to extract more current.
According to Planar System's paper, this a-Si photo TFT has peak response in the blue-green range of spectrum. (This is why Planar put photo TFT beside the blue display pixel.)
Circuits
The in-cell optical touch panel besides has not only display circuit but also read out circuit. The read out principle is similar to CMOS image sensor - read the signal of pixel sequentially.
This picture is from the patent iDTI to extract signal of light pen. Left side expresses the photo TFT array for each pixel, and the right side is the readout circuit, consisted of three amplifiers. First amplifier can see as preamp and CDS to purify signal. Then the two amplifier - one for DC and another for AC - in order to get the specific frequency signal from light pen.
Considerations
- Aperture ratio
-Photo TFT would occupy the area of display pixel to decrease aperture ratio and decrease the performance of display. - Signal
-Even the structure of photo TFT can use such small gate channel to sense photon, it still can't work as good as CMOS image sensor yet. For dim light signal, like reflection from finger surface, it is not easy to get significant signal. - Yield
-The final yield of LCD would be influenced by the yield of display pixel and photo pixel both. So the additional process for in-cell photosensing would decrease the original yield of LCD. - MUX
-Two multiplexers would be needed. It can synchronize to each pixel to display and sense at same time. - EMI
-Current for sensing transfered to signal at readout circuit is not stable enough. And more, adjacent circuit channel may be influenced by stray photocurrent. Sharp has reported EMI as a severe problem.
iDTI is a IC design company and such business model is not easy for such built-in in-cell manufacture process. Sharp, LGD, and Samsung are LCD manufacturers and all develop in-cell photosensing touch panel. These manufacture companies has more source to develop it, but for the same reason they also invest on other touch technique.
Microsoft surface2 is very amazing - not only sense the finger touch but also image like QR code, optical character recognition. Surface 2 can be seen as the future application of in-cell photo sensing touch, but it is very expensive now.
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