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LCD combines image sensor and touch screen

Sharp has announced that it has developed—and will put into production— an LCD module with built-in touch-screen and scanner functions, providing an alternative to touch-detection systems such as resistive film and capacitive sensing that require an additional structure in front of the LCD “glass”. The screen will act as a contact scanner to input image data to portable products such as PDAs, as an input device for functions such as fingerprint sensing, or as a touch-pad sensor for keyed data entry. Sharp builds the concept on its system-LCD product platform, in which it fabricates drivers and other peripheral circuitry in the same deposited silicon that it uses to construct the transistors of the TFT array itself. This, in turn, is possible because it uses its continuous-grain silicon technology to deposit that semiconductor layer, yielding—the company says—carrier mobilities far in excess of those associated with amorphous silicon, and around halfthose of monocrystalline silicon.

To the conventional TFT array, and at every pixel point on the display, Sharp adds a photodiode. A single-transistor/ single-capacitor cell structure captures charge in proportion to the light falling on the photodiode and reads it out via a dedicated matrix connection. The LCD’s structure shields the photodiode against direct illumination by the module’s backlight. Therefore it “sees” light in two modes: light from the backlight refl ects from any object touching the screen, back on to the diode; or, in a shadow-detection mode, the diode will respond to a touching object obscuring ambient light. The company says it will build touch-screen LCDs that are thinner and clearer, with no extra layers in front of the module, and with fully programmable sensing; you will be able to implement multi-touch, to detect and track more than one point of contact with the screen, and to allow inputs such as re-sizing and rotatingdisplayed windows.

Sharp will build the technique on its Super-Mobile-ASV screen product, a new design of TFT cell that achieves very accurate alignment of the tilt of the liquidcrystal molecules, for a brighter and higher contrast (2000:1) colour image, with8-msec response time.

Peripheral circuitry fabricated on the CG-silicon is in 3.0 and 1.5-micron design rules, according to Mike Brownlow, director of the system-display group at Sharp Laboratories Europe, who says that the next move will be to sub-micron (0.8-micron) geometry. Sharp’s aim is to build a complete system on the glass substrate, with no attached silicon dice. Logic, analogue and power structures are all possible. Power dissipation is a limitation due to the low thermal conductivity of the glass, but not at the power levels necessary to drive LCDs, according to Brownlow, who adds that the company owns IP (intellectual property) that deals with getting heat wayfrom the glass substrate.

The structure will be able to support up to 300 pixels/inch, and the system-LCD platform has a maximum panel size of about 10 in., Brownlow states. An initial integrated- touch panel, now sampling, has a 3.5-in. diagonal measurement, and half-VGA (320x480xRGB) resolution.

Sharp, www.sharpsme.com