Stacked Image Sensor Pixel Cell with Selectable Shutter Modes
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Solution Overview
Problem
Stacked CMOS image sensors face challenges in achieving both high dynamic range and low power consumption, particularly in capturing images with both bright and dark areas, due to insufficient light absorption and spatial distortion issues in rolling shutter mode, and increased complexity in global shutter mode.
Innovation Solution
The implementation of a pixel cell with a photodiode, transfer transistor, reset transistor, dynamic range enhancement capacitor, and capacitor control transistor within a first semiconductor chip, allowing for optionally selectable rolling shutter and global shutter readout modes through computer programmable digital register settings, and in-pixel correlated double sampling in the global shutter path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pixel size is decreased to increase resolution, then the number of pixels per unit area increases, but the light absorption depth becomes insufficient especially for long-wavelength light
Solution Approach 1:
The patent transitions from planar light absorption to three-dimensional light absorption by forming a light guide structure that extends vertically through the substrate. Light enters through the back surface and is guided along the light guide to reach the photodiode, effectively utilizing the third dimension (depth) to compensate for reduced horizontal light absorption path in miniaturized pixels.
Solution Approach 2:
The light guide acts as an intermediary structure between the incident light and the photodiode. It captures light entering the back surface and directs it to the photodiode, improving the coupling efficiency and ensuring that light reaches the photosensitive element even when the direct absorption path is insufficient due to small pixel size.
2Device complexity
If rolling shutter mode is used to simplify the readout circuitry, then device complexity is reduced, but spatial distortion occurs in the captured image
Solution Approach 1:
The patent implements a dual-mode readout system that can dynamically switch between rolling shutter mode and global shutter mode with transfer. This allows the system to adapt to different application requirements, selecting the appropriate mode based on whether spatial accuracy or circuit simplicity is the higher priority for the current imaging scenario.
Solution Approach 2:
The pixel circuit is designed to perform multiple functions: it can operate in rolling shutter mode for simple applications, or switch to global shutter mode with charge transfer capability for applications requiring high spatial accuracy. This multi-functionality allows a single circuit design to serve diverse imaging needs without requiring separate dedicated circuits for each mode.
3Manufacturing precision
If global shutter mode is implemented to eliminate spatial distortion, then image accuracy is improved, but device complexity and power consumption increase
Solution Approach 1:
The patent separates the global shutter functionality into distinct operational phases: charge accumulation in the photodiode, charge transfer to a storage region, and subsequent readout. This segmentation allows the circuit to achieve global shutter performance without requiring all components to be simultaneously active, thereby reducing the effective complexity and power consumption compared to a fully simultaneous global shutter implementation.
4Reliability
If the substrate thickness is increased to improve light absorption for red light, then light absorption improves, but the pixel area available for other components decreases
Solution Approach 1:
The light guide structure utilizes the vertical dimension to enhance light absorption. By guiding light through the substrate thickness to the photodiode, the system achieves effective light absorption without requiring an increase in the horizontal pixel area. This allows the substrate to maintain its thickness for adequate light absorption while preserving pixel area for other circuit components.
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
This solution enables reduced pixel array size, lower manufacturing costs, and improved dynamic range while allowing for flexible readout modes, reducing spatial distortion and enhancing signal quality across varying light conditions.
Implementation Method 1
The photodiode, transfer transistor, reset transistor, dynamic range enhancement capacitor, capacitor control transistor, and source follower amplifier are disposed within a first substrate of a first semiconductor chip for accumulating an image charge in response to light incident upon the photodiode
Data Source
AI summary
A pixel cell has a photodiode, a transfer transistor, a reset transistor, a dynamic range enhancement capacitor, a capacitor control transistor, an amplifier transistor in a source follower configuration and a rolling shutter row select transistor and a readout circuit block. The photodiode, a transfer transistor, a reset transistor, dynamic range enhancement capacitor, capacitor control transistor, amplifier transistor and rolling shutter row select transistor are disposed within a first substrate of a first semiconductor chip for accumulating an image charge in response to light incident upon the photodiode. The readout circuit block may be partially disposed within a second substrate of a second semiconductor chip and partially disposed within the first substrate wherein the readout circuit block comprises optionally selectable rolling shutter and global shutter readout modes through the use of computer programmable digital register settings. The global shutter readout mode provides in-pixel correlated double sampling.