A 4.0 μm Stacked Digital Pixel Sensor Operating in a Dual Quantization Mode for High Dynamic Range
IEEE Transactions on Electron Devices
It is anticipated that ubiquitous computer vision (CV) and artificial intelligence (AI) applications used on mobile devices will grow significantly. Such applications require battery-powered, always-on mobile devices to support indoor/outdoor, day/night usages. A global shutter (GS), stacked digital pixel sensor (DPS) is a promising candidate to meet such requirements because of its potential for ultralow-power, ultrahigh dynamic range (HDR), and a small form factor. This article presents a prototype 4.0- μm stacked DPS operating in its dual quantization (2Q) to realize HDR. The 4.0- μm DPS pixel is formed with two layers, a backside illuminated pinned photodiode (PD) pixel on the top layer and an in-pixel analog-to-digital conversion (ADC) circuit with 9-bit static random access memory (SRAM) on the bottom layer. A Cu-to-Cu hybrid bonding (HB) technology is used to connect the two layers via pixel-level interconnect. In the 2Q scheme, a time-stamp (TS) quantization and a linear ADC are performed sequentially in the same frame, which extends the dynamic range (DR) with a small number of ADC bits of 9. The DPS with a 1024 × 832 pixel array has achieved a single-exposure ultra HDR of 107 dB in a single frame. The nonlinear conversion characteristic of the TS mode provides an equivalent full well capacity (FWC) of 2000ke-, while the noise floor in the linear ADC mode is 8.3e-.