Quarter Video Graphics Array Full-Digital Image Sensing with Wide Dynamic Range and Linear Output Using Pixel-Wise 3D Integration

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We have developed a quarter video graphics array (QVGA) chip for full-digital image sensing by using three-dimensional (3D) integration technology. In-pixel pulse-frequency modulation (PFM) analog-to-digital converter (ADC) gives full-digital imaging pixel, which is known to overcome signal saturation due to the full well capacity of a photodiode (PD). We have also newly designed a PFM-ADC for pixels with a pinned PD and a floating diffusion to achieve high sensitivity and low noise. PDs and ADCs were stacked into two layers by pixel-wise 3D integration with gold electrodes of 5 μm in diameter, thereby enabling QVGA resolution for a 20-mm square chip in 0.18-μm and 0.2-μm process nodes. The developed sensor chip exhibited an excellent linearity and wide dynamic range of more than 96 dB. We also obtained video images with a high bit depth of 16 bit, demonstrating the superior image sensing that can capture the real world at high fidelity.