Backside Isolation Structure for CMOS Image Sensor Leakage Control
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Solution Overview
Problem
In CMOS image sensors, the presence of heavily doped regions between well regions and the front surface can lead to the formation of undesired P-N junctions, causing leakage current and issues like dark current or white pixels, which affect the sensor's performance.
Innovation Solution
The solution involves omitting heavily doped regions between well regions and the front surface, using a back side deep trench isolation (BDTI) structure with a conductive cap that extends from the pixel region to the periphery region, and applying an isolation bias to the BDTI structures through conductive plugs and caps, thereby avoiding P-N junctions and enhancing isolation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If heavily doped regions are present between well regions and the front surface, then the isolation between photodetectors is enhanced, but undesired P-N junctions are formed causing leakage current
Solution Approach 1:
The patent removes the heavily doped regions from between the well regions and the front surface, extracting the harmful element that causes P-N junction formation and leakage current while maintaining the isolation function through alternative means
Solution Approach 2:
The patent introduces an intermediary structure (the BDTI structure with conductive cap) that provides isolation between photodetectors without forming P-N junctions, mediating between the need for isolation and the avoidance of leakage current
2Object-generated harmful factors
If heavily doped regions are omitted between well regions and front surface, then leakage current is reduced, but isolation between photodetectors may be compromised
Solution Approach 1:
The BDTI structure with conductive cap serves as an intermediary isolation mechanism that provides photodetector separation without requiring heavily doped regions, thus avoiding P-N junction formation while maintaining isolation reliability
Solution Approach 2:
The patent changes the isolation mechanism from chemical doping (heavily doped regions) to physical/structural isolation (BDTI structure with conductive cap), altering the fundamental parameter of how isolation is achieved
3Reliability
If conventional isolation structures are used, then photodetector isolation is achieved, but the area available for photodetectors is reduced
Solution Approach 1:
The patent transitions from front-side isolation structures to back-side isolation structures, utilizing the depth dimension and back surface of the substrate to achieve isolation without encroaching on the front surface photodetector area
Solution Approach 2:
The isolation function is segmented into separate BDTI structures positioned between individual photodetectors, allowing each photodetector to maintain its full front surface area while still achieving effective isolation through the back-side structures
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 reduces leakage current, improves the area for photodetectors, and enhances the quantum efficiency of the image sensor by eliminating undesired P-N junctions and using metallic BDTI structures as reflectors.
Implementation Method 1
enhances the quantum efficiency of the image sensor by eliminating undesired P-N junctions and using metallic BDTI structures as reflectors
Data Source
AI summary
A semiconductor device includes a substrate having a front side and a back side opposite to each other. A plurality of photodetectors is disposed in the substrate within a pixel region. An isolation structure is disposed within the pixel region and between the photodetectors. The isolation structure includes a back side isolation extending from the back side of the substrate to a position in the substrate. A conductive plug structure is disposed in the substrate within a periphery region. A conductive cap is disposed on the back side of the substrate and extends from the pixel region to the periphery region and electrically connects the back side isolation structure to the conductive plug structure. A conductive contact lands on the conductive plug structure, and is electrically connected to the back side isolation structure through the conductive plug structure and the conductive cap.