Quarter Video Graphics Array Digital Pixel Image Sensing With a Linear and Wide- Dynamic-Range Response by Using Pixel-Wise 3-D Integration

We have developed a quarter video graphics array (QVGA) digital pixel image sensor by using the 3-D integration technology. The pulse-frequency modulation (PFM) analog-to-digital converter (ADC) operates as a digital pixel, which overcomes the signal saturation due to the full well capacity of the photodiode (PD). We have also newly designed a PFM-ADC for pixels with a pinned PD and a floating diffusion to comply with the CMOS image sensor process used to attain high sensitivity and low noise. PDs, comparators, logic circuits, and counters are integrated into two silicon-on-insulator layers by pixel-wise 3-D integration with gold electrodes with a 5 μm diameter, thereby achieving a QVGA resolution for a 20-mm square chip in the 0.18and 0.2-μm process nodes. The developed sensor exhibits both the excellent linearity and a widedynamic range of more than 96 dB. Video images with a high bit depth of 16 bit are also obtained to demonstrate the superior image sensing, capable of capturing the real world at high fidelity.