SPAD Image Sensor Trench Isolation for Edge Breakdown
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Solution Overview
Problem
Existing SPAD image sensors face challenges in achieving high photodetective sensitivity and preventing edge breakdown, which limits their ability to detect low-intensity radiation effectively due to the consumption of a large area by guard rings, making it difficult to shrink pixel size while maintaining performance.
Innovation Solution
The design incorporates a substrate with a common node and trench isolation structure that adjusts the breakdown region, eliminating the need for a guard ring, thereby improving the fill factor and preventing edge breakdown, allowing for efficient detection of low-intensity radiation without sacrificing performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a guard ring is used to prevent edge breakdown, then edge breakdown is prevented, but the pixel area is reduced due to the space occupied by the guard ring
Solution Approach 1:
The patent extracts and removes the guard ring structure from the SPAD device. Instead of using a guard ring to prevent edge breakdown, the invention employs a different approach by forming the breakdown region directly at the periphery of the sensing node through controlled doping, thereby eliminating the need for the guard ring and recovering the pixel area it occupied
Solution Approach 2:
The patent applies local quality by creating a localized high-doping region at the periphery of the sensing node. This localized doping creates a specific breakdown region with controlled electrical properties, allowing edge breakdown prevention through localized structural modification rather than requiring a surrounding guard ring structure
2Measurement precision
If the pixel size is reduced to increase resolution, then image detail is improved, but the photodetective sensitivity is reduced
Solution Approach 1:
The patent enhances photodetective sensitivity in small pixels by creating a localized high-doping region at the periphery of the sensing node. This localized breakdown region maintains efficient photon detection capability even when the overall pixel size is reduced, allowing high resolution without sacrificing sensitivity
Solution Approach 2:
The patent changes the doping parameters locally at the periphery of the sensing node, creating a high-doping region with specific carrier concentration. This parameter change enables the breakdown voltage to be controlled and optimized for small pixel sizes, maintaining photodetective sensitivity despite reduced pixel area
3Reliability
If the breakdown region is optimized for high sensitivity, then photodetective sensitivity is improved, but edge breakdown may occur
Solution Approach 1:
The patent resolves this contradiction by creating a localized high-doping region specifically at the periphery of the sensing node. This local structural modification creates a controlled breakdown region that prevents edge breakdown while maintaining high photodetective sensitivity in the central sensing area
Solution Approach 2:
The patent introduces a peripheral high-doping region as an intermediary structure between the sensing node and the substrate edge. This intermediary breakdown region acts as a protective zone that prevents harmful edge breakdown while allowing the central sensing region to maintain high photodetective sensitivity
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 the sensitivity and reduces noise in SPAD image sensors by optimizing the breakdown region and eliminating the need for a guard ring, allowing for smaller pixel sizes while maintaining high photodetective sensitivity.
Implementation Method 1
An avalanche process can be triggered when a reverse biased p-n junction receives additional carriers, such as carriers generated by incident radiation. For example, in order to detect radiations with low intensities, the p-n junction is biased above its breakdown voltage, thereby allowing a single photon-generated carrier to trigger an avalanche current that can be detected.
Data Source
AI summary
A single photon avalanche diode (SPAD) image sensor is disclosed. The SPAD image sensor includes: a substrate having a front surface and a back surface; wherein the substrate includes a sensing region, and the sensing region includes: a common node heavily doped with dopants of a first conductivity type, the common node being within the substrate and abutting the back surface of the substrate; a sensing node heavily doped with dopants of a second conductivity type opposite to the first conductivity type, the sensing node being within the substrate and abutting the front surface of the substrate; and a first layer doped with dopants of the first conductivity type between the common node and the sensing node.