Printable Electronics Hits Display Needs
Printable electronics could reduce the cost of making flexible displays of the kind used in the Kindle and other E-books. At the recent SID 2009 conference, researchers discussed the marriage of printable electronics and low-power displays used in emerging portable devices, which must be light, flexible, low-power, and readable in ambient light.
Michael P.C. Watts, Impattern Solutions, Austin, Texas -- Semiconductor International, 6/24/2009
Printable electronics are becoming integrated with new display technologies, with fundamental impacts on the cost structure of future displays as well as the types of equipment used to make them, speakers said at the recent Society of Information Display (SID) conference in San Antonio, Texas.
Traditional LCDs are transmissive devices, where "aperturing" — the blocking of transmitted light by the thin-film transistor (TFT) and interconnects — can reduce the light output by 30-50%. To keep the TFT small, linewidths and alignment tolerances must be minimized, putting a huge premium on advanced photolithography tools.
Two different printable displays were discussed at SID 2009: active-matrix light-emitting diodes (AMOLEDs) for laptop computers and electrophoretic displays for E-books read under ambient light. AMOLED displays use organic LEDs (OLEDs) as the source of light and an active matrix of control transistors to address each pixel and pass current to the individual diodes in each pixel. As emissive displays, AMOLED displays support a large gamut of colors, a high contrast ratio typical of emissive displays, and are thin and have low power consumption — all ideal attributes for mobile applications.
The most popular electrophoretic displays rely on oppositely charged white and black particles distributed in a fluid capsule. By applying a voltage, the particles can be moved to the surface, changing the display from black to white. They have been key to the success of the Kindle E-book now being viewed by the display community as a breakthrough product.
In both cases, light does not pass through the active matrix so there is no longer a requirement to keep the TFT and interconnect small. As a result, critical dimensions can be large enough to be either inkjet printed or screen printed. Printable electronics need a solution cast or inkjet printable semiconductor. Solution castable organic and metal oxide semiconductors have been reported; the challenge is making a printable semiconductor with good enough electrical properties.
The active matrix for the electrophoretic E-book display is the least demanding on the semiconductor. The display is bistable, so current is needed only when the display switches. Furthermore, the pixels are changed only when a page is turned, so the duty cycle is very low in comparison with a display that shows video. Because the current load is small, concerns about the life of organic semiconductors are largely irrelevant. In many ways , the E-book is the ideal breakthrough application for printable organic semiconductor plastic electronics.
The target for a magazine display is 150 dpi over 8.5 × 11 in., or a 2 Mpixel device with eight levels of grayscale. Plastic Logic Ltd. (Cambridge, UK) and Sony Corp. have reported using organic semiconductors as the active-matrix semiconductor. In the keynote session, Plastic Logic announced plans to ship printed E-books in 2010.
Displays with a printed active matrix for AMOLED were shown by LG Display and Samsung Electronics at SID, and several conference papers addressed the topic. The demands on the semiconductor are much more challenging because the active matrix must supply sufficient current to turn on the OLED, and the display must support moving images at high refresh rates. Both printable organic and metal oxide AMOLEDs were reported at the conference and shown in the exhibit booths.
Speakers said there also is a push to deposit the OLED semiconductor using print technology. DuPont Displays (Wilmington, Del.) reported on a four-layer OLED material set that is all solvent cast and inkjetted. DuPont Displays is teaming with Dainippon Screen (DNS, Kyoto, Japan) to deliver the necessary materials, processes and equipment.
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DuPont Displays is working with DNS on a four-layer OLED material set that is solvent cast and inkjetted. |
Printable displays will have a significant impact on the cost structure of large area displays. Today, the small TFTs in a transmissive LCD are patterned by optical scanners to image 3 µm features with 0.6 µm overlay over 2 m2 Gen 8 glass in 60 sec. These scanners can cost $15M to $20M. The six or more layers of photolithography are the most expensive steps in the process.
The larger features in the TFT for a non-transmissive display can be printed by a number of printing tools that are all a fraction of the cost of an optical stepper. Screen printing and inkjet print equipment are already used for LCDs. Inkjet in particular has been used to manufacture color filters, and clever methods have been developed for controlling drop sizes by controlling the local wetting environment. LCD manufacturing systems have been built using print heads from Diamatix Inc. (Santa Clara, Calif.).
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| A Litrex display-scale inkjet system uses Dimatix print heads. (Source: Litrex) |
Other printing techniques are starting to get traction in LCD manufacturing. LG Display showed a 27 in. diagonal LCD, where the ribs in the color filter were made by UV imprint. It appears that they have developed a roller-on-glass-sheet imprint tool. One paper proposed using a "micro contact printer" to create smaller features. Micro contact is a form of transfer printing onto a rigid substrate using a flexible mold. Semprius Inc. (Durham, N.C.) is commercializing this approach.
Printing an active matrix with small TFTs on a flexible substrate was reported at SID by a team from HP and Arizona State University. They use a single multilevel imprint to create a self-aligned TFT on a stretchable substrate, a process that they said has significant potential. Long term, all the classic paper printing techniques, including flexography, offset and gravure, could be used on a flexible substrate.
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