Stacked Image Sensor Pixel Cell Selectable Shutter Modes
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Solution Overview
Problem
Stacked CMOS image sensors face challenges with reduced pixel photosensitivity and dynamic range due to miniaturization, particularly in absorbing long-wavelength light, and existing readout modes like rolling shutter and global shutter have limitations such as spatial distortion and increased component requirements.
Innovation Solution
A pixel cell design with a photodiode, transfer transistor, and readout circuit that allows for optionally selectable rolling shutter and global shutter readout modes through computer programmable digital register settings, minimizing off-pixel wiring and incorporating in-pixel correlated double sampling in the global shutter path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If rolling shutter mode is used, then readout is simpler, but spatial distortion occurs in the image
Solution Approach 1:
The patent introduces a programmable readout mode selection mechanism that allows the imaging system to dynamically switch between rolling shutter and global shutter modes based on application requirements. This enables the system to adapt between simplicity and accuracy needs without hardware changes.
2Manufacturing precision
If global shutter mode is used, then spatial distortion is minimized, but more transistors and storage components are required per pixel
Solution Approach 1:
The patent utilizes the third dimension (vertical stacking) by placing storage capacitors and circuit elements on a separate layer above the photodiode array. This layered architecture allows global shutter functionality with additional components without increasing the lateral pixel footprint, thus maintaining high spatial resolution while enabling distortion-free imaging.
3Measurement precision
If pixel size is reduced for miniaturization, then resolution increases, but photosensitivity and dynamic range are lost
Solution Approach 1:
The patent introduces a reflective structure beneath the photodiode that redirects light back through the photosensitive region. This effectively doubles the light absorption path length within the same pixel volume, compensating for reduced photosensitivity caused by miniaturization while maintaining high resolution.
Solution Approach 2:
The patent combines the function of the substrate (which was previously just a mechanical support) with an optical reflection function. By integrating a reflective layer into the substrate structure, the system achieves enhanced light absorption without increasing pixel size or reducing resolution.
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 design reduces pixel array size and manufacturing costs while enabling flexible readout modes, minimizing spatial distortion and optimizing signal-to-noise ratio, thereby improving the dynamic range and efficiency of image capture.
Implementation Method 1
The bottom chip includes an array of light sensitive regions and structures to capture an image... the photodiode, transfer transistor, and reset 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
Data Source
AI summary
A pixel cell has a photodiode, a transfer transistor, a reset transistor, an amplifier transistor and a readout circuit block. The photodiode, transfer transistor, reset transistor and amplifier 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 and converting the image charge to an image signal. The readout circuit block is disposed within a second substrate of a second semiconductor chip and 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.