In-Pixel Microlens Array for Global Shutter Image Sensors
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Solution Overview
Problem
Image sensors with charge storage regions are prone to inaccuracies due to unwanted light exposure during charge readout, leading to incorrect representation of charge generated by photodiodes, particularly in global shutter modes where light incident on the charge storage region corrupts the readout.
Innovation Solution
Incorporating an in-pixel microlens array with varying refractive indices and configurations to redirect incident light away from the charge storage region and towards the photodiode, thereby minimizing light exposure on the charge storage area and improving global shutter efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If a charge storage region is added to store charge before readout, then charge can be held for later readout (global shutter function), but light incident on the charge storage region causes unwanted charge changes that corrupt the readout accuracy
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: a photodiode region for light detection and charge generation, and a separate charge storage region for temporary charge holding. The microlens array is further segmented into multiple microlenses arranged in an array, with each microlens corresponding to a specific pixel location. This segmentation allows the optical path to be controlled independently for each pixel, directing light precisely to the photodiode while preventing light from reaching the charge storage region during the storage period.
Solution Approach 2:
The microlens array acts as an intermediary optical element positioned between the incident light and the pixel structure. Each microlens in the array serves as a mediator that refracts and directs light rays, focusing them onto the photodiode while blocking or redirecting light that would otherwise reach the charge storage region. This intermediary structure enables precise control of light paths without requiring direct mechanical intervention in the charge storage process.
2Device complexity
If light is allowed to reach the charge storage region during charge holding, then the structure remains simple, but the readout becomes inaccurate due to light-induced charge changes
Solution Approach 1:
The microlens array serves multiple functions simultaneously: it focuses incident light onto the photodiode during the integration period, and it blocks or redirects light from reaching the charge storage region during the storage and readout periods. This multi-functionality is achieved by positioning the microlens array directly over the pixel structure and configuring each microlens to create a focused light path that terminates at the photodiode, naturally preventing light from diverging into the charge storage region.
3Measurement precision
If the microlens array is added to redirect light away from the charge storage region, then charge readout accuracy is improved, but the device complexity increases
Solution Approach 1:
The microlens array is nested directly within the pixel structure, positioned above the photodiode and charge storage region. This nested configuration allows the microlens array to be integrated into the existing pixel architecture without requiring separate external optical components. The microlenses are formed in a layer that is part of the pixel structure itself, creating a compact, multi-layered configuration where each element serves its function within a tightly integrated space.
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 microlens array effectively concentrates light onto the photodiode, enhancing the accuracy of charge readout and increasing the global shutter efficiency of image sensors by reducing light-induced errors in charge storage regions.
Implementation Method 1
In-pixel lens arrays with varying refractive indices and configurations are employed to redirect incident light away from the charge storage region
Implementation Method 2
The microlens array effectively concentrates light onto the photodiode, enhancing the accuracy of charge readout
Data Source
AI summary
An image sensor may include an array of pixels. Pixels in the array may include a photodiode that converts incident light into electrical charge and a charge storage region for storing the electrical charge before it is read out from the pixel. Pixels in the array may include a microlens formed over the photodiode that directs light onto the photodiode. Pixels in the array may include an additional array of microlenses between the microlens and the photodiode. The additional array of microlenses may direct light away from the charge storage region to prevent charge stored at the charge storage region from being affected by light that is not incident upon the photodiode. The image sensor may be a backside illuminated image sensor that operates in a global shutter mode.