Backside Illuminated Global Shutter Imaging Array with Correction Photodiode
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Solution Overview
Problem
Backside illuminated CMOS imaging sensors face challenges in shielding the floating diffusion node from parasitic photodiode illumination, leading to artifacts due to integrated dark current, especially in global shutter systems, as it is difficult to effectively shield without partially obstructing the main photodiode.
Innovation Solution
The implementation of a CMOS imaging array with a main photodiode and a correction photodiode, where the main photodiode is reset and the correction photodiode measures the charge accumulated between the exposure end and readout time, allowing for the computation of the exposure value by accounting for the correction charge, thereby reducing the impact of parasitic photodiode-generated photoelectrons.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If backside illumination is used to reduce imaging array size, then area efficiency is improved, but shielding of the floating diffusion node becomes difficult leading to parasitic photodiode artifacts
Solution Approach 1:
The pixel sensor is segmented into two separate photodiodes: a main photodiode for capturing the image and a correction photodiode for measuring parasitic photoelectron generation. This segmentation allows independent optimization of each photodiode's function and positioning, enabling the correction photodiode to be placed close to the floating diffusion node for effective artifact correction without obstructing the main photodiode's light path.
Solution Approach 2:
The correction photodiode acts as an intermediary element that measures the parasitic photoelectron generation in the floating diffusion node. By introducing this intermediate measurement mechanism, the system can quantify and subsequently correct the harmful artifacts without needing to physically shield the floating diffusion node, thus resolving the shielding difficulty in backside illumination configurations.
2Adaptability or versatility
If charge is stored on the floating diffusion node during readout in global shutter mode, then global shutter functionality is achieved, but parasitic photoelectrons accumulate causing image artifacts
Solution Approach 1:
The correction photodiode provides feedback information about the parasitic photoelectron generation rate in the floating diffusion node. This feedback is used to compute a correction value that is subtracted from the main photodiode signal, thereby eliminating the artifacts caused by parasitic photoelectron accumulation during the global shutter storage period.
Solution Approach 2:
The parasitic photoelectron generation, which was previously a harmful artifact, is converted into a useful measurement signal by the correction photodiode. By measuring the parasitic photoelectrons generated in the floating diffusion node, the system gains information that can be used to correct and eliminate the artifacts, thus transforming the harmful effect into a beneficial correction mechanism.
3Object-affected harmful factors
If a shield is deposited near the floating diffusion node to block parasitic illumination, then artifact reduction is improved, but the photodiode light path is partially obstructed
Solution Approach 1:
By segmenting the photodetection function into two separate photodiodes, the system eliminates the need for physical shielding near the floating diffusion node. The correction photodiode is positioned to measure parasitic effects without blocking the main photodiode's light path, thus achieving artifact reduction without compromising illumination intensity.
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 minimizes the effects of dark current-related artifacts by reading out the charge in a rolling shutter mode, reducing the storage time of charge on the floating diffusion node and allowing for accurate computation of the exposure value, thus enhancing image quality in backside illuminated systems.
Implementation Method 1
Each pixel sensor includes a photodiode and associated readout circuitry. The photodiode and the active readout circuitry are constructed on the front side of the wafer.
Data Source
AI summary
An imaging array and method for using the same that are adapted for backside illuminated imaging arrays utilizing a global shutter are disclosed. The imaging array includes a plurality of pixel sensors having an ordered array of pixel sensors. Each pixel sensor includes a main photodiode and a correction photodiode. A controller resets all of the main photodiodes at a first time that is the same for all of the pixel sensors, resets all of the correction photodiodes at a second time that is the same for all of the pixel sensors after the first time, and sequentially reads out the pixel sensors. The pixel sensor is read out at a third time that is different for different ones of the pixel sensors. A correction charge is measured that corrects for the different readout times.