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Flexible image sensors move nearer to market

18/07/2013

A new prototype demonstrate the potential of conformable plastic-based image sensors.

Flexible image sensor
A recently unveiled proof-of-concept device indicate the strides being made towards flexible plastic-based image sensors, and the potential advantages they may have over their inorganic counterparts.

The device developed by Plastic Logic of the UK and France's ISORG, was on show at the LOPE-C 2013 printed electronics conference held in Germany in June 2013.
On the threshold

The Anglo-French sensor is said to be the first conformable organic image sensor of its type, potentially allowing more favorable power-to-weight trade-offs and novel optical design parameters for any systems with a digital imaging element.

"This is the first time an organic thin-cell transistor backplane has been combined with a organic photodiode layer which is printed over the plastic substrate," confirmed Ian Reid of Plastic Logic to Optics.org. "Plastic Logic manufactured the plastic organic thin-film transistor backplane, which was then combined with ISORG's organic photodetector (OPD) technology. Some integration work was involved, but both processes are fundamentally mature and established by the respective companies."

The result was a flexible sensor with a 4 x 4 cm active area; a pitch of 375 microns, with 175 microns pixel size and 200 microns spacing; and a 94 x 95 pixel grid.

"This is a proof-of-concept demonstrator and was designed as such, so we did not push the manufacturing process or the sensor performance to its absolute limit," noted Reid. "The pixel resolution, which is determined by the backplane, equates to 60 pixels per inch (ppi); not spectacular, but chosen to simplify the testing and characterization task at this stage."

Although the developers have set no target resolution for any final product, the high-resolution processing developed by Plastic Logic for the manufacture of backplanes in electrophoretic displays has comfortably achieved densities of 150 to 225 ppi, so steps towards that figure seem readily foreseeable.

Both the backplane and the OPD layer are manufactured using printing operations at ambient temperature, potentially leading to substantial cost-efficiencies compared to the manufacturing processes involved in traditional inorganic components. A printing operation could also enable distributed sensor arrays over larger areas than can be achieved by silicon-based processes, avoiding the limits imposed by wafer size and fabrication technology.

The flexible nature of the sensors should bring its own advantages, lending itself to non-planar sensor arrays following curved or distorted paths, and opening up an extra design dimension for engineers. A further advantage could be the greater spectral range commonly shown by OPD components, and Reid envisages imaging systems able to employ the same component type for near-IR and visible sensing.

"We have known for some time that our flexible plastic technology is usable not just in backplanes for driving displays, but for other types of applications too," he said. "Having industrialized a process successfully for driving electrophoretic displays, it's natural to look at where we can take the technology. We are on the threshold of something very exciting in the flexible electronics area, and the design community is starting to think about what they can do with it."

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Tim Hayes is a contributing editor at Optics.org