OmniVision, TSMC Develop Sensor Technology
David Lammers, News Editor -- Semiconductor International, 5/27/2008 9:13:00 AM
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OmniVision product marketing manager Michael Hepp said the approach was developed with foundry partner Taiwan Semiconductor Manufacturing Co. Ltd. (TSMC, Hsinchu, Taiwan), which will use a fully depreciated 0.11 µm fab and standard tools to manufacture the BSI image sensors.
BSI brings light in through the thinned backside of the silicon substrate to the photosensitive area. With the conventional frontside illumination (FSI) approach, the amount of light reaching the photosensitive area is reduced by the multiple metal and dielectric layers that convert photons into electrons.
BSI has been used in high-end image sensors intended for military applications, where costs are not a barrier. With the pixels in camera phones shrinking in size, Hepp said BSI will support a reduction in the pixel size used in camera modules while improving image quality. The company’s roadmap points to BSI image sensors with a pixel size as small as 0.9 µm on one side. The BSI 1.4 µm pixels are equivalent in performance to 1.7 µm FSI pixels. If an FSI approach was pursued, foundries would be forced to deploy 65 nm copper processes to achieve equivalent performance to the 1.1 µm BSI process technology.
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| BSI allows more light to be absorbed at a wider angle, which supports improved zoom lens technology. BSI supports a shorter path for light to reach the sensor, with no metal layers to block or reflect light. (Source: OmniVision Technologies Inc.) |
Presently, OmniVision is nearly ready to begin sampling BSI sensors with a 1.4 µm pixel size, delivering 8 megapixel image sensors to handset makers in volumes by early next year. Hepp said that in Europe, most high-end phones have camera modules with 3-5 megapixel sensors, while the U.S. market lags in the 2-3 megapixel range.
To date, BSI has been difficult to implement at high yields, he said, adding that other image sensors vendors are also developing BSI technology. BSI brings several advantages. Because light directly strikes the silicon, the fill factor of the image sensor is significantly improved. The approach will support thinner camera modules and improves light sensitivity in low-light situations. It also results in less crosstalk, yielding sharper images, he said.
“With the ability to take in more light at a better CRA [chief ray angle], we can improve the aperture size,” Hepp said. Currently, handset cameras are limited to 2.8 numerical aperture (NA) lenses, but with BSI employed, 2.0 NA lenses are possible.
Ken Chen, senior director of mainstream technology marketing at TSMC, said, “Although backside illumination concepts have been studied for over 20 years, up until now nobody has been able to successfully develop the process for commercial high-volume CMOS sensor manufacturing. Combining OmniVision’s imaging expertise with TSMC’s experience in process development, we have delivered an advanced technology that defines the future of digital imaging.”
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