In-pixel ADC in 3D technology

Decreasing size of CMOS technology plays a key role in embedding more and more functionalities in CMOS image sensors. By integrating sensing with processing, new types of CMOS imaging systems can be designed and applied in domains such as machine vision, surveillance, or medical imaging. Digital pixel sensors (DPSs) are a good example of smart vision chips including image capturing and processing modules. Indeed, they incorporate, for each pixel, parallel analog-to-digital conversion thus allowing performing such conversion in early stages. Moreover, DPSs offer several benefits such as an improved SNR (Signal Noise Ratio) and a wider dynamic range. In addition, the emergence of new technologies, typically CMOS 3D-IC technology, offers the opportunity to reduce pixel area and improve fill factor, making DPSs suitable for numerous fields of applications. This paper presents a digital pixel sensor (DPS) integrating an in-pixel sigma-delta analog-to-digital converter (ADC). The digital pixel includes a photodiode, a delta-sigma modulation and a digital decimation filter, featuring high-speed conversion, programmable resolutions (up to 10-bit). Based on the CMOS 130 nm 3D-IC Tezzaron technology, an optimized design of the pixel is provided, revealing a high fill-factor of about 90%.