Digital Pixel Global Shutter CMOS Sensor
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Solution Overview
Problem
CMOS image sensors with global shutter function face challenges in reducing pixel pitch due to the need for multiple transistors in digital pixels, which is a barrier for applications like head-mounted displays and advanced driver-assistance systems in virtual and augmented reality.
Innovation Solution
A solid-state imaging device with digital pixels that include a photoelectric conversion element, a transfer element, a storage element, a reset element, an output buffer, a comparator for AD conversion, and a memory part, allowing for global shutter operation with reduced pixel pitch by performing specific readout and reset processes to optimize charge handling and conversion gains.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If digital pixels with comparator and memory part are arranged in each pixel to enable global shutter function, then global shutter capability is achieved, but pixel pitch cannot be reduced due to the large number of transistors required
Solution Approach 1:
The pixel array is divided into two distinct types: first type pixels with full digital pixel functionality (photoelectric conversion element, transfer element, storage element, comparator, and memory part) that can achieve global shutter, and second type pixels without these components. This segmentation allows the system to achieve global shutter capability where needed while reducing overall pixel pitch by using simpler second type pixels in other regions.
Solution Approach 2:
The reading part is designed to universally handle both first type pixels and second type pixels through a unified readout circuitry. The reading part can selectively read out signals from either pixel type, allowing the system to achieve versatile functionality with a single integrated design rather than requiring separate processing paths.
2Extent of automation
If multiple transistors are mounted in pixels to achieve digital pixel functionality, then AD conversion and memory storage are enabled, but pixel area increases making small pixel pitch difficult
Solution Approach 1:
The pixel array is segmented into first type pixels that contain the full digital pixel structure (photoelectric conversion element, transfer element, storage element, comparator, and memory part) and second type pixels that lack these components. This segmentation enables AD conversion and memory storage functionality in specific regions while keeping other regions compact with smaller pixel areas.
Solution Approach 2:
Different regions of the pixel array are assigned different functional qualities: first type pixels are equipped with complete digital processing capabilities (comparator, memory part) for high-performance applications, while second type pixels are designed with simpler structures for applications where full digital functionality is not required, thereby optimizing overall area utilization.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables the realization of digital pixels with a global shutter function at a small pixel pitch, achieving high sensitivity, broad dynamic range, and high frame rates, suitable for advanced applications like VR and AR.
Implementation Method 1
a photodiode (photoelectric conversion element) and an FD layer (FD) for each pixel
Data Source
AI summary
A reading part, in a first reset period PR1, holds reset transistors in all pixels in a conductive state and executes a first conversion gain reset readout processing HCGRRD, stores an AD conversion code with respect to a first readout reset, signal HCGVRST in a memory part, then, in a transfer period PT1, holds the transfer transistors in all pixels in a conductive state to transfer the accumulated charges in photodiodes PD1 to FD1 to thereby execute a global shutter operation accumulating overflowed charges in storage capacitors CS1. The reading part, when reading each row, executes a first conversion gain signal readout processing, a second conversion gain signal readout processing, and a second conversion gain reset readout processing in order. Due to this, it becomes possible to realize digital pixels provided with a global shutter function at a small pixel pitch.