OmniVision Technologies, the world’s largest supplier of CMOS image sensors, have launched their new OmniBSITM architecture, a novel sensor design that adopts a radically different approach to traditional CMOS image sensor technology.
Using backside illumination (BSI), OmniBSI enables OmniVision to continue offering improved image quality while extending its pixel roadmap down to 0.9 micron pixels, which is the key to continued miniaturization of digital imaging technology. OmniVision developed OmniBSI architecture with the support of their long-time foundry and process technology partner, Taiwan Semiconductor Manufacturing Corporation (TSMC).
BSI methodology involves turning the CameraChip sensor upside down so that it collects light through what was previously the backside of the sensor, the silicon substrate. This approach differs from conventional front side illumination (FSI) image sensors, where the amount of light reaching the photosensitive area is limited, in part, by the multiple metal and dielectric layers required to enable the sensor to convert photons into electrons. The FSI approach can block or deflect light from reaching the pixel, ultimately reducing the fill factor and causing additional problems, such as cross talk, between pixels. BSI reverses the arrangement of layers so that the metal and dielectric layers reside below the sensor array, providing the most direct path for light to travel into the pixel. This novel approach optimizes light absorption, enabling OmniVision to build a 1.4 micron BSI pixel that surpasses all the performance metrics of 1.4 micron, and even most 1.75 micron, FSI pixels.
OmniBSI architecture delivers a number of performance improvements over FSI, including increased sensitivity per unit area, improved quantum efficiency and reduced cross talk and photo response non-uniformity, which all lead to significant improvements in image quality. Since light directly strikes the silicon, the fill factor of the image sensor is significantly improved so as to deliver best-in-class low-light sensitivity. A much higher chief ray angle enables shorter lens heights which in turn allows for thinner camera modules, which are ideal for use in the next generation of ultra-thin mobile phones. Finally, BSI technology affords a much larger aperture size, which allows for lower f stops facilitating the development of better performing camera modules with superior camera performance.
“Moving FSI pixel architectures down to 1.4 micron and below, under current design rules, poses some real challenges because metal lines and transistors are driving the aperture of the pixel close to the wavelength of light, its physical limit,” said Howard Rhodes, Vice President of Process Engineering at OmniVision. “To overcome this with traditional FSI pixel technology would require a migration to 65 nm copper process technologies, which would significantly increase the complexity and cost of manufacturing. Because it allows for more than three layers of metal, BSI achieves significant manufacturing benefits without moving to smaller process nodes. This means routing can be simplified and die sizes can be smaller than in FSI sensors, without the need to move to smaller process nodes with all their associated complexities and additional costs.”
“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,” said Dr. Ken Chen, Senior Director, Mainstream Technology Marketing, TSMC. “Combining OmniVision’s imaging expertise with TSMC’s experience in process development, we have delivered a truly advanced technology that defines the future of digital imaging.”
“BSI allows OmniVision to further extend its competitive edge in digital imaging technology, while continuing the use of our production-proven, 0.11 micron process technology. This provides major cost and performance advantages for OmniVision and, ultimately, our customers,” concluded Rhodes.
OmniVision is currently demonstrating an 8Mp, OmniBSI CameraChip sensor, and expect to start sampling the first products before the end of June.
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