Backside Illuminated Image Sensor Pixel Circuit Overlap
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Solution Overview
Problem
Conventional frontside illuminated CMOS image sensors have a reduced fill factor due to pixel circuitry consuming valuable space adjacent to the photodiode region, leading to decreased low light performance.
Innovation Solution
The implementation of a backside illuminated CMOS image sensor with overlapping pixel circuitry, where the circuitry is positioned adjacent to the interlinking diffusion region between the photodiode and the die frontside, allowing for increased pixel circuitry area without obstructing light, and enabling flexible signal routing and reduced noise immunity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If pixel circuitry is positioned adjacent to the photodiode region in frontside illuminated configuration, then the pixel circuitry can be easily routed and connected, but the fill factor is reduced and low light performance deteriorates
Solution Approach 1:
The patent inverts the conventional frontside illuminated configuration by using backside illumination. The photodiode region is positioned to receive light from the backside of the substrate, while the pixel circuitry is positioned on the frontside. This inversion allows the circuitry to overlap with the photodiode region's projection without blocking light, thereby increasing the fill factor while maintaining circuit connectivity.
Solution Approach 2:
The patent utilizes the third dimension (vertical stacking) to resolve the spatial conflict between photodiode and circuitry. By positioning the photodiode region and pixel circuitry in different vertical planes (backside vs. frontside), the design allows horizontal overlap without light obstruction, effectively increasing the fill factor while maintaining circuit functionality.
2Adaptability or versatility
If pixel circuitry area is increased to provide more functionality, then circuit functionality is improved, but the photodiode region area is reduced and light sensitivity deteriorates
Solution Approach 1:
The patent employs vertical stacking to separate the photodiode region (on the backside) from the pixel circuitry (on the frontside). This dimensional separation allows the circuitry to be expanded in the horizontal plane without encroaching on the photodiode area, as both components occupy different vertical zones. Consequently, circuit functionality can be enhanced while maintaining full photodiode area for light sensitivity.
Solution Approach 2:
By inverting the illumination direction to backside illumination, the patent enables the photodiode region to be positioned beneath the pixel circuitry's projection. This inversion allows the circuitry to occupy the frontside space without reducing the effective photodiode area on the backside, thereby improving circuit functionality while preserving light sensitivity.
3Ease of operation
If metal stack is formed over the frontside to redistribute signals, then signal routing is improved, but the pixel circuitry region consumes valuable real estate and fill factor is reduced
Solution Approach 1:
The patent utilizes vertical stacking to position the metal stack and pixel circuitry in different vertical planes. The metal stack is formed on the frontside above the pixel circuitry, allowing signal redistribution without consuming additional horizontal space. This dimensional arrangement enables effective signal routing while maintaining a high fill factor, as the metal layers occupy the vertical space above the circuitry rather than competing for horizontal real estate.
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 configuration enhances light sensitivity, reduces noise, and increases the fill factor by allowing for larger or additional circuit components without compromising the sensor's performance, thereby improving image quality and low light performance.
Implementation Method 1
photoelectrically convert the light incident from a first surface side of the substrate
Data Source
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Figure 3A
AI summary
A backside illuminated ("BSI") imaging sensor pixel (400) includes a photodiode (420) region and pixel circuitry (430) The photodiode region is disposed within a semiconductor die for accumulating an image charge in response to light incident upon a backside of the BSI imaging sensor pixel. The pixel circuitry includes transistor pixel circuitry disposed within the semiconductor die between a frontside of the semiconductor die and the photodiode region. At least a portion of the pixel circuitry overlaps the photodiode region.