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07 Feb 2005
Researchers in Japan have devised an image scanner from a sheet of organic electronics that can flex to scan curved objects. James Tyrrell finds out more.
An image scanner that can be rolled up and carried in a pocket could be just around the corner, thanks to Japanese scientists. A team led by Takao Someya and Takayasu Sakurai from the University of Tokyo has developed a scanner from a flexible polymer sheet containing organic electronics.
Unlike conventional scanners, which capture images by moving a linear array of photodetectors over an illuminated page, the Japanese device features no moving parts and operates under ambient light. The thin-film scanner is simply placed over the target object to capture the image below.
Because the sheet scanner can bend and conform neatly to a page, the device could be useful for electronically archiving fragile books and historical documents. Additionally, as the unit requires only ambient illumination, the scanner may help to protect faded and light-sensitive inks. Curved surfaces, such as the label affixed to a bottle of wine, can be accurately and conveniently scanned as the device is able to wrap around the object.
Measuring 0.4 mm thick, the scanner weighs just 1 g and can be carried in a pocket, or simply rolled up and placed in a bag or briefcase. It could lead to a new portable electronic device for capturing images when out of the home or office. At the moment, the scanner's output is black and white, but the Japanese team believes that colour imaging should also be possible by introducing a series of colour filters to the scanner design.
To transfer images from the device to a computer, Someya and his colleagues plan to use existing technologies such as a USB interface. "Besides the sheet scanner, we need a connector, a cable and a silicon LSI chip for the USB interface," Someya told OLE. "Wireless [technology] or mobile phones may also be quite attractive choices to read out digital image data from the scanner."
Previous work on developing flexible pressure sensors for artificial skin and robot applications gave the Tokyo team much practical knowledge of organic semiconductor technology. Inspired, Someya and Sakurai felt that this expertise could be extended to other fields.
"After a long brainstorming [session], we had the idea of a sheet image-scanner," said Someya. "It seemed reasonable to us that the integration of photodetectors with organic transistor active-matrix drivers should be possible."
How the scanner works The image-scanner film contains a 2D array of organic photodetectors that sense black and white tones by detecting the difference in reflected light from bright and dark parts of an image. A matching array of organic transistors provides a means of reading out the image data.
"Instead of a line-by-line mechanical scanning procedure, the signal of the photodiodes is read out by electrically probing the organic transistors," explained Someya. "This avoids the need to use any moveable parts and, as a result, the device is thin, lightweight and mechanically flexible."
For the scanner to work effectively, it is important that the photodiode array responds only to light reflected from the page. To get around the problem, miniature light-shields isolate the back of each photodiode from any ambient lighting incident from above (see figure 1). The sheet also contains a number of transparent apertures that behave as light channels to illuminate the object below.
The team is careful to strike a balance between the number of ambient-light-blocking photodiodes and transparent apertures. There must be a sufficient number of photodiodes to capture useful detail, but at the same time enough gaps in the structure to adequately illuminate the surface. Ideally, any shadows that the light shields may cast on the page should be overwhelmed by the lighting scheme to render a true image of the printed surface.
With an effective imaging area of 4 inch2, the scanner's 5184 sensor cells (each of 700 x 700 μm) currently deliver a scan resolution of 36 dots per inch (dpi).
Simple fabrication The use of carbon-based organic semiconductor technology means that the device benefits from low fabrication costs and suits the manufacture of large-area, flexible plastic substrates. For example, devices can be made at room temperature using printing and roll-to-roll processes.
Manufactured separately on different plastic films, the organic transistor and photodiode matrices are laminated to each other using silver paste. The paste is patterned by an ultrafine printing technique to make the required electrical connections.
Although the circuitry is unable to match the high-speed performance of silicon technology, the design maximizes the operating frequency of the device's organic semiconductor electronics. The researchers claim that their design lowers the typical delay time of the circuit by a factor of five, and reduces power consumption by a factor of seven.
In terms of sensing area and resolution, the team thinks that this is just the beginning and has a number of ideas to ramp up the spec of its current prototype. "To improve the spatial resolution, we need to reduce the size of both the organic transistors and organic photodetectors," said Someya. "Reduction of the device's dimensions is not difficult, but the bottleneck is the size of via holes which realize an electrical connection between the transistors and detectors."
To make via holes, the team currently uses a CO2 laser with an output wavelength of around 10 μm. "To reduce the diameter of via holes, we have a plan to replace the CO2 laser with other shorter-wavelength lasers such as excimer lasers or YAG lasers," revealed Someya. "We believe that [a scanning resolution of] 600 dpi will be feasible in the near future."
Although the reduction in circuit dimensions is certain to benefit spatial resolution, the team must ensure that these smaller photodiodes are still able to deliver a photocurrent density comfortably above the image threshold. To investigate this effect, the researchers prepared a series of photodiodes of various sizes - from 1 mm2 down to 50 μm2 - and measured the photocurrent density under halogen light illumination of 70 mW/cm2. They found that the device dimensions could be reduced to 50 μm2, which is sufficient to give a spatial resolution of 250 dpi with a drop in photocurrent density of only 25%.
Encouraged by this result, the team took their work a step further and ran a series of feasibility experiments using a matrix of organic photodetectors without transistors. The study showed that it should be possible to scale the device to at least 250 dpi or higher and create sheets that are A4 in size. If the researchers can hit upon a way of reducing transistor size, then resolutions exceeding 600 dpi could become feasible.
Commercialization The fundamentals behind the scanner were announced at the IEEE's Electron Devices Meeting in December 2004 in San Francisco, where the work of Someya and his colleagues attracted a lot of attention. The team has patented its idea and is heading towards commercialization. "It will take 3-5 years to get the product to market," estimated Someya. "Before manufacturing the commercial item, we need to solve reliability issues." The researchers are now working hard to lower the operating voltage from 40-80 V to 10 V and improve the stability of the device's organic transistors.
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