SPAD Image Sensor Trench Isolation and Common Node Grid
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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 need for a guard ring that consumes a large area, thereby reducing the fill factor and making it difficult to shrink pixel size while maintaining performance.
Innovation Solution
The SPAD image sensor design incorporates a trench isolation structure and a common node with a grid structure that adjusts the breakdown region, eliminating the need for a guard ring by compensating with a vertical distance, thus enhancing the fill factor and preventing edge breakdown, allowing for efficient detection of low-intensity radiation without sacrificing area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a guard ring is used to prevent edge breakdown, then reliability is improved, but area is consumed and fill factor is reduced
Solution Approach 1:
The patent removes the guard ring structure from the SPAD device. Instead of using a guard ring to prevent edge breakdown, the invention relies on the intrinsic properties of the depleted region and optimized p-n junction geometry to achieve edge breakdown-free operation, thereby eliminating the area-consuming guard ring structure.
Solution Approach 2:
The patent transitions from a planar guard ring structure to a vertically extended depleted region that spans across the edge of the active area. By extending the depletion zone into the depth dimension rather than relying on lateral guard rings, the design prevents edge breakdown without consuming horizontal pixel area.
2Productivity
If pixel size is reduced to improve resolution, then productivity is improved, but fill factor decreases making it difficult to maintain performance
Solution Approach 1:
By removing the guard ring structure, the patent eliminates the non-active area that previously reduced fill factor. This allows the active sensing area to extend closer to the pixel boundaries, maintaining high fill factor even as pixel dimensions are reduced for higher density arrays.
Solution Approach 2:
The patent optimizes the doping concentrations and junction depths to create a depleted region that efficiently covers the active area without requiring lateral guard rings. This parameter optimization enables smaller pixel sizes while maintaining adequate depletion coverage and fill factor.
3Measurement precision
If reverse bias voltage is increased above breakdown voltage to detect low-intensity radiation, then measurement precision is improved, but edge breakdown occurs causing excessive noise
Solution Approach 1:
The patent removes the guard ring that previously created edge breakdown issues. By eliminating this structure and using optimized junction geometry, the device can operate at high reverse biases above breakdown voltage without experiencing edge breakdown, thus detecting low-intensity radiation with high precision and minimal noise.
Solution Approach 2:
The patent creates a localized optimized depletion region with specific doping profiles that concentrate the electric field in the active sensing area while preventing field concentration at edges. This local quality optimization allows high reverse bias operation with suppressed edge breakdown noise.
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 design enables the SPAD image sensor to maintain high photodetective sensitivity while reducing the pixel area, improving the fill factor and preventing excessive noise, allowing for effective detection of low-intensity radiation without the need for a guard ring.
Implementation Method 1
Avalanche photodiodes (APD) are solid devices that are compatible with traditional CMOS devices. An avalanche process can be triggered when a reverse biased p-n junction receives additional carriers, such as carriers generated by incident radiation.
Implementation Method 2
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.