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Plastic Spray Coating Boosts CMOS Sensors

MUNICH, Jan. 23, 2013 — A cost-effective spray coating process that involves an ultrathin film made up of organic compounds has boosted the performance of CMOS sensors.

Image sensors produced using CMOS technology are at the core of every digital camera, and now scientists at Technical University Munich (TUM) have developed a simple technique that improves their performance: electrically conductive plastics.

Organic sensors can be applied to CMOS chips over large and small surfaces as well as to glass or flexible plastic films, new results out of Technical University Munich find. Courtesy of U. Benz/TUM.

The challenge, however, lies in applying the plastic solution to the image sensor’s surface. The team tested spin- and spray-coating methods to apply the solution in its liquid form as precisely and cost-effectively as possible. They looked for a smooth plastic film no more than a few hundred nanometers thick, and discovered that spray coating was the most effective method when using either a simple spray gun or a spray robot.

Organic sensors have already proved their worth in tests, revealing up to three times more sensitivity to light than conventional CMOS sensors, whose electronic components conceal some of the pixels and therefore the photoactive silicon surface. The sensors can be manufactured without the expensive postprocessing step typically required for CMOS sensors, which involves applying microlenses to increase the amount of captured light.

During tests, the organic sensors proved to have up to three times more sensitivity to light than conventional CMOS sensors. Courtesy of A. Heddergott/TUM.

In the TUM method, each part of every pixel, including the electronics, is sprayed with the liquid polymer solution, providing a surface that is 100 percent light sensitive. The sensing surface has low-noise and high-frame-rate properties, the researchers say. In addition, various chemical compounds can be used to capture different parts of the light spectrum; for example, PCBM and P3HT polymers are ideal for detecting visible light, whereas other organic compounds such as squaraine dyes are sensitive to light in the near-infrared region.

“By choosing the right organic compounds, we are able to develop new applications that were too costly up until now,” said professor Paolo Lugli, chairman of nanoelectronics at TUM. “The future uses of organic infrared sensors include driver-assistance systems for night vision, and regular compact and cellphone cameras. Yet, the lack of suitable polymers is the main hurdle.”

The team’s findings were reported in Nature Communications (doi: 10.1038/ncomms2180).  

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