Wednesday, January 15, 2020

Brillnics 4um Voltage Domain GS Pixel

MDPI paper "A Stacked Back Side-Illuminated Voltage Domain Global Shutter CMOS Image Sensor with a 4.0 μm Multiple Gain Readout Pixel" by Ken Miyauchi, Kazuya Mori, Toshinori Otaka, Toshiyuki Isozaki, Naoto Yasuda, Alex Tsai, Yusuke Sawai, Hideki Owada, Isao Takayanagi, and Junichi Nakamura from Brillnics is a part of the Special issue on the 2019 International Image Sensor Workshop (IISW2019).

"A backside-illuminated complementary metal-oxide-semiconductor (CMOS) image sensor with 4.0 μm voltage domain global shutter (GS) pixels has been fabricated in a 45 nm/65 nm stacked CMOS process as a proof-of-concept vehicle. The pixel components for the photon-to-voltage conversion are formed on the top substrate (the first layer). Each voltage signal from the first layer pixel is stored in the sample-and-hold capacitors on the bottom substrate (the second layer) via micro-bump interconnection to achieve a voltage domain GS function. The two sets of voltage domain storage capacitor per pixel enable a multiple gain readout to realize single exposure high dynamic range (SEHDR) in the GS operation. As a result, an 80dB SEHDR GS operation without rolling shutter distortions and motion artifacts has been achieved. Additionally, less than −140dB parasitic light sensitivity, small noise floor, high sensitivity and good angular response have been achieved."

9 comments:

  1. Interesting... but I don't know if I trust the table, the dynamic range calculation seems to be black magic.

    Am I missing something?

    Low gain
    @ GS
    DR = 20*log10(40000/30) = 62.5 dB
    @ RS
    DR = 20*log10(40000/13) = 69.8 dB
    High gain
    @ GS
    DR = 20*log10(7000/4) = 64.9 dB
    @ RS
    DR = 20*log10(7000/2.1) = 70.5 dB

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  2. Agree... It seems that the DR calculation is taking the best case of QSat (40000e- low gain) versus the best case of noise (4e-, high gain) which gives us those wonderful 80dB.
    Kind of black marketing magic?
    Anyway I guess this needs further clarification

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    Replies
    1. It is a perfectly fine DR calculation. Both low gain and high gain are captured in 1 image. One gain or the other will be chosen based on light level so yes, 80dB is no black market magic but reality.

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  3. DR number alone does not tell the full story. Just as an example, let's assume a pixel with 1e- FWC and 0.001e- noise. One can boast 60dB DR in that case, but any real-world picture would look like junk.

    This is an exaggerated example, but it should explain that DR is indicative of the sensor real-world performance only when taken in combination with other parameters.

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    Replies
    1. I disagree Vlad. DR should be taken from SNR=1 at lowest light to SNR=1 at saturation (or some other condition if you want linear FW etc.) In your example, one gets a very small DR using this approach. Using the noise floor directly is only a valid shortcut when the read noise is higher than 1e- rms.

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    2. I'd guess there is no common definition. EMVA 1288 defines DR in a way close to yours. However, Brillnics defines it as "DR = Saturation level/ Noise"

      https://www.brillnics.com/technology/high-dynamic-range-image-sensors

      Taking your definition, if we have 2e- read noise, the SNR=1 would be achieved at the signal level of 2.5e-. Then, if FWC is, say, 2,000e-, the DR is 58dB. To my eye, it looks a bit unusual.

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  4. The EMVA standard 1288 requires that the FWC and noises be measured at the same gain setting ...

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    Replies
    1. You are right about EMVA, but the paper does not mention that they calculate/specify the DR according to the EMVA 1288 ....

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    2. Yes, same gain over the full sensor. But if you can choose the gain autonomously per pixel then EMVA is still valid. I think you confuse interscene with intrascene DR definitions...

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