• Image Sensor Technology: Noteworthy Developments in 2017

  • Posted: December 18, 2017
    Author: Ray Fontaine

    We asked Ray Fontaine, Senior Technical Analyst specializing in semiconductor fabrication process analysis and image sensors, for his thoughts on the most interesting developments in image sensor technology in 2017.

    Cameras

    2017 may be remembered as the year that camera and related sensing systems became the undisputed kings of differentiation across smartphone platforms.

    Dual cameras are, of course, wildly popular now in flagship mobile phones, and we are tracking new types of associated autofocus (AF) systems, including: laser-assist, lossless phase detection autofocus (PDAF) in 1.0 µm telephoto camera chips, new types of masked PDAF, etc. Samsung is notable for its preference of a dual photodiode (Dual Pixel) AF system that is successful in its own right, and does not currently require laser-assist AF.

    Samsung 0.9 µm Generation ISOCELL with Tetracell Color Filters  
    Samsung 0.9 µm Generation ISOCELL with Tetracell Color Filters

    Going back a few years, we had expected the emergence of non-Bayer color filter arrays including clear/white channel pixels, but for the most part we are still living in a Bayer RGB ecosystem. A noteworthy twist on Bayer RGB is Samsung’s TetraCell and OmniVision’s 4-Cell strategy for high resolution front-facing cameras. This strategy enables in-pixel binning for greater sensitivity (with lower resolution) for low-lit scenes.

    Finally, as we end 2017 we are happy to announce we have found 0.9 µm generation pixels in mass production! The 24 MP front-facing camera from the Vivo V7+ features a Samsung Tetracell S5K2X7SP with full ISOCELL structure implemented in 0.9 µm pitch pixels with a substantially thicker-than-normal substrate. Increasing the back-illuminated substrate thickness is a trend we see amongst the leaders in small pixel imagers.

    Chip Stacking for Image Sensors

    Chip stacking (image sensor + image signal processor) for image sensors remains an enabling technology for improved camera performance, and this year we documented Sony’s first-generation TSV-based three die stack (now adding a DRAM) in mass production. For two-die stacks, we still primarily see TSV-based chip-to-chip interconnect, although Sony has been using direct bond interconnects (Cu-Cu hybrid bonding, or DBI) since early 2016. We recently saw OmniVision and foundry partner TSMC join the hybrid bonding club and claim the new world record, based on TechInsights’ findings, of 1.8 µm diameter, 3.7 µm pitch DBI pads.

    ChipSelect ACMOS

    Improved Near Infrared Sensitivity

    Moving outside of traditional array imaging, another trend we’ve tracked in 2017 is the continued emergence of cameras with improved near infrared (NIR) sensitivity. We’ve got the STMicroelectronics SOI-based front-illuminated imager from the Apple iPhone X Face ID system in our labs, and expect to document the response from Apples’ competitors in 2018. We’re also analyzing the structures from new process flows in use, such as the pyramid surface diffraction structures on the SmartSens SC5035. Sony has previously announced a similar approach, and we expect a comparable structure in use for OmniVision’s announced Nyxel platform.

    STMicroelectronics IR Camera Chip from Apple iPhone X  
    STMicroelectronics IR Camera Chip from
    Apple iPhone X

    Looking Ahead

    There is, of course, much more happening in the imaging sector than can be conveyed in this summary. We have seen high quality imaging-related papers from IISW, IEDM, ISSCC, and other conferences, so it’s a good indication of continued development of disruptive imaging/sensing platforms. We’ll be tracking these announced and stealth mode technologies expected in 2018 product launches.

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