Stacked Wafer Image Sensor Dynamic Range Extension
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Solution Overview
Problem
Conventional solid state image sensors, particularly in mobile devices, face limitations in dynamic range due to small pixel size and full well capacity, leading to loss of detail in high-contrast scenes and blooming issues, as they struggle to accurately capture both low-light and bright signals simultaneously.
Innovation Solution
The solution involves stacking two wafers, with one wafer containing an array of pixels and the other wafer equipped with counters and readout circuitry, allowing for multiple sampling of each pixel within an exposure period to extend dynamic range without increasing pixel size, thereby preventing saturation and enhancing image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel size is increased to improve full well capacity and dynamic range, then dynamic range is improved, but device area and complexity increase
Solution Approach 1:
The patent transitions from a planar single-wafer architecture to a three-dimensional stacked architecture with two separate wafers. The pixel array is placed on one wafer while readout circuits with counters are placed on another wafer, enabling vertical integration. This dimensional change allows independent optimization of pixel size for dynamic range while maintaining compact overall sensor area through stacking.
Solution Approach 2:
The sensor is divided into two functional segments on separate wafers: a pixel array wafer and a readout circuit wafer. This segmentation allows the pixel array to be optimized for light sensing with appropriate pixel size while the readout circuits are optimized for signal processing with counter elements, resolving the conflict between pixel size requirements and overall sensor area.
2Measurement precision
If multiple counters are added to extend dynamic range, then dynamic range is improved, but device complexity increases
Solution Approach 1:
By moving the counter circuits to a separate second wafer in a stacked configuration, the patent reduces in-pixel complexity while maintaining extended dynamic range functionality. The counters are distributed across the second wafer and connected to corresponding pixels through vertical interconnects, separating the complexity from the pixel array.
Solution Approach 2:
The patent implements a simplified pixel design that can be replicated across the array, with the complex counter functionality copied to corresponding locations on the second wafer. This separation allows standardization of the pixel array while distributing the complexity management to the readout circuit wafer.
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 approach effectively increases the dynamic range of solid state image sensors, preventing saturation and maintaining high resolution and frame rate, even in high-contrast scenes, by allowing multiple charge dumps and extended full well capacity without increasing pixel size, thus improving image capture capabilities.
Implementation Method 1
each of the pixels comprising a photosensor
Data Source
Figure 1A
Figure 1B
Figure 2A
AI summary
A high dynamic range solid state image sensor and camera system are disclosed. In one aspect, the solid state image sensor includes a first wafer including an array of pixels, each of the pixels comprising a photosensor, and a second wafer including an array of readout circuits. Each of the readout circuits is configured to output a readout signal indicative of an amount of light received by a corresponding one of the pixels and each of the readout circuits includes a counter. Each of the counters is configured to increment in response to the corresponding photosensor receiving an amount of light that is greater than a photosensor threshold. Each of the readout circuits is configured to generate the readout signal based on a value stored in the corresponding counter and a remainder stored in the corresponding pixel.