Saturday, September 13, 2014

Apple Proposes Global Shutter BSI Pixel

Apple patent application US20140246568 "Photodiode with different electric potential regions for image sensors" by Chung Chun Wan proposes a vertically stacked fully pinned PDs, whereas the bottom one is used as a storage node (SN) for a GS pixel: "The storage node in global shutter pixels is usually located on the same surface of a semiconductor wafer as the photodiode region, and thus typically needs to be shielded in order to maintain the integrity of the charge stored in the storage node. Also, positioning the storage node on the same surface of a semiconductor wafer as the photodiode reduces the amount of surface area of the photodiode that can be exposed to light, and hence reduces the sensitivity of the pixel." So, here is the proposal:

12 comments:

  1. I just have 2 words for the inventor: red light

    Or, these two: shutter efficiency

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    1. Some guys thought that it was possible to go to the moon by bicycle... Because it was so easy to make an image sensor for iPhone.

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    2. Is this because the red light would penetrate this deep (as would unfiltered infra red). A quick question what if someone used a stacked design to put a ADC at each pixel and increase read speed to such an extent that it effectively was a global shutter without taking up all the die area current high speed deviced do?

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    3. Yes, regarding red light.

      Not sure what you mean, but you could do fully parallel readout and ADC, followed by "slow" digital readout. This starts to look a lot like what was done many decades ago for hybrid infrared focal plane arrays, and by using the Z-plane architecture. This 3D focal-plane image processing work is referenced in my (old) thesis if you care and in some of my earlier publications. There was a lot of work done in the aerospace industry. Stacked sensor technology, with a via for nearly every pixel, has been on everyone's wish list for a generation, just so we can implement these more advanced concepts. Exciting times for sure.

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  2. See above article about patents being invalidated as "obvious".

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  3. Didn't Kodak do something similar for CCD's to prevent smearing?

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    1. What in particular are you referring to? The p-barrier? That has been used for ILT CCDs in Japan for quite a while and probably Kodak used it too. Or are you thinking of some BSI related thing?

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  4. It seems obvious to move the global-shutter stuff to another layer in order to maximize the light-gathering area of the photdiodes. Are there practical issues in doing so?

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  5. All seems out of optics: red, green and blue lights have no difference over the whole patent body.

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  6. What is "Shutter Efficiency" ?

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    1. It is the ratio of the signal generated during the "open shutter" period divided by this signal plus the signal generated (parasitically) during the "closed shutter period, expressed as a percentage. So if the open shutter signal is 100 mV and the total signal is 101 mV, then the shutter efficiency is 100/101 = 0.9901 or 99.01%.

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