Backside CMOS Pixel Separation Walls for Charge Overflow Isolation
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Solution Overview
Problem
In solid-state image pickup devices, intense light can cause charge saturation and leakage between adjacent pixels, leading to degradation of imaging characteristics due to color mixture and overflow.
Innovation Solution
The implementation of a CMOS image sensor with a pixel separation wall system, comprising front-side and backside trenches, that prevents charge leakage by directing overflowed charges to a floating diffusion area or the power supply electrode, thereby isolating adjacent pixels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a pixel separation wall with front-side and backside trenches is implemented, then charge leakage between adjacent pixels is prevented, but device complexity increases
Solution Approach 1:
The pixel separation wall is divided into two distinct trenches: a front-side trench extending from the front surface and a backside trench extending from the back surface. These two trenches work together to form a complete isolation barrier, segmenting the charge conduction paths between adjacent pixels. This segmentation allows each trench to be optimized independently while achieving complete charge isolation when combined.
Solution Approach 2:
The charge isolation problem is solved by adding a spatial dimension - using both front-side and backside surfaces of the semiconductor substrate. Instead of relying on a single deep trench from one surface, the isolation barrier is constructed by combining trenches from two opposite surfaces, effectively using the third dimension (depth from both sides) to achieve complete charge path blocking.
2Reliability
If the pixel separation wall completely isolates adjacent pixels, then charge leakage is prevented, but manufacturing precision requirements increase
Solution Approach 1:
The front-side and backside trenches do not need to meet perfectly at the center of the pixel separation wall. The isolation effect is achieved as long as the trenches extend sufficiently deep from each surface and overlap or meet within the substrate thickness. This partial action approach provides manufacturing tolerance, as complete isolation is achieved even with some variation in trench depth and positioning.
Solution Approach 2:
The dual-trench structure inherently provides a buffer zone for manufacturing variations. By creating trenches from both surfaces, the design anticipates potential depth control issues and positioning errors, ensuring that even if one trench is shallower than intended, the combined isolation effect still achieves complete charge blocking.
3Reliability
If overflowed charges are directed to the floating diffusion area, then charge leakage between pixels is prevented, but color mixture may still occur
Solution Approach 1:
The pixel separation wall structure extracts and removes the charge overflow path that would otherwise lead to adjacent pixels. By creating deep trenches from both front and back surfaces, the design physically extracts the harmful charge leakage paths and redirects charges to safe discharge locations such as the floating diffusion area or power supply electrode, preventing color mixture.
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 solution effectively prevents charge leakage between adjacent pixels, maintaining image quality by managing overflowed charges and reducing the impact on imaging characteristics.
Implementation Method 1
a photodiode (photoelectric conversion device) that performs photoelectric conversion
Data Source
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AI summary
The present disclosure relates to a solid-state image pickup device and an electronic apparatus that are capable of preventing leakage of charges between adjacent pixels. A plurality of pixels perform photoelectric conversion on light incident from a back surface via different on-chip lenses for each pixel. A pixel separation wall is formed between pixels adjacent to each other, and includes a front-side trench formed from a front surface and a backside trench formed from the back surface. A wiring layer is provided on the front surface. The present disclosure is applicable to, for example, a backside illuminated CMOS image sensor.