Buried Light Shield for Backside Illuminated CMOS Image Sensors
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Backside illuminated CMOS image sensors with global shutter functionality face issues due to unwanted charge accumulation in storage regions, leading to image artifacts and inaccurate image representation, primarily caused by light reaching storage regions through gaps between light shields and storage regions.
Innovation Solution
Incorporating a buried light shield between the photodetector and storage region in the image sensor pixel, which can be formed using a light-absorbing or light-blocking material, to prevent unwanted charge accumulation by directing light away from the storage region and ensuring simultaneous charge transfer during global shutter operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a BSI image sensor is used to improve light detection, then light transmission is improved, but unwanted charge accumulation occurs in storage regions
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: photodetector region for light detection and storage region for charge storage, separated by a buried light shield. This segmentation prevents light from reaching the storage region while maintaining efficient light detection in the photodetector region, thereby resolving the contradiction between improved light detection and unwanted charge accumulation.
Solution Approach 2:
A buried light shield is introduced as an intermediary element between the photodetector and storage region. This light shield acts as a mediator that blocks stray light from reaching the storage region, preventing unwanted charge accumulation while allowing the BSI structure to maintain its superior light detection capabilities.
2Illumination intensity
If light shield distance from storage region is increased to reduce blocking, then light transmission improves, but charge accumulation artifacts increase
Solution Approach 1:
Instead of adjusting the lateral distance between the light shield and storage region, the solution moves to a vertical dimension by embedding the light shield within the substrate structure. The buried light shield is positioned at a depth that effectively blocks oblique light paths without requiring lateral adjustment, thus maintaining both good light transmission and preventing charge accumulation artifacts.
Solution Approach 2:
The buried light shield is formed during the substrate processing stage before final pixel assembly, preliminarily establishing the light blocking barrier in the correct position. This preliminary action ensures that the light shield is optimally positioned to block stray light before it can reach the storage region, maintaining image accuracy without requiring post-assembly adjustments.
3Measurement precision
If global shutter is implemented for simultaneous charge transfer, then image accuracy improves, but charge accumulation in storage regions causes artifacts
Solution Approach 1:
The harmful effect of light reaching the storage region is extracted and removed by introducing the buried light shield. This extraction eliminates the source of unwanted charge accumulation, allowing global shutter operation to proceed without artifacts and maintaining high image accuracy throughout the charge transfer process.
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
The buried light shield effectively prevents unwanted charge accumulation in storage regions, enhancing image quality by reducing artifacts and ensuring accurate image capture, as it directs light only to the photodetector and not to the storage region, thereby improving the reliability of image sensors.
Implementation Method 1
a buried light shield is disposed between the photodetector and the storage region
Implementation Method 2
An array of photodetectors accumulates photo-generated charge (e.g., electrons) in response to light striking the photodetectors
Data Source
AI summary
A pixel in an image sensor can include a photodetector and a storage region disposed in one substrate, or a photodetector disposed in one substrate and a storage region in another substrate. A buried light shield is disposed between the photodetector and the storage region. A sense region, such as a floating diffusion, can be adjacent to the storage region, with the buried light shield disposed between the photodetector and the storage and sense regions. When the photodetector and the storage region are disposed in separate substrates, a vertical gate can be formed through the buried light shield and used to initiate the transfer of charge from the photodetector and the storage region. A transfer channel formed adjacent to, or around the vertical gate provides a channel for the charge to transfer from the photodetector to the storage region.