SPAD Image Sensor Trench Grid for False Avalanche Suppression
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Solution Overview
Problem
Existing image sensors with SPAD photodiodes face challenges in efficiently detecting single photons due to spurious triggering from edge effects, which can lead to false avalanche events, especially when the radiation is not properly managed.
Innovation Solution
The design incorporates a semiconductor substrate with vertically extending trenches forming a grid pattern, featuring a conductive region opaque to radiation and a doped polysilicon layer with varying doping levels to control the electric field intensity, ensuring that only single photogenerated charges trigger avalanches, while the conductive region electrically connects pixels and serves as an optical shield to prevent cross-talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If vertical PN junctions are used to detect single photons, then detection sensitivity is improved, but spurious triggering from edge effects increases
Solution Approach 1:
The patent extracts and removes the harmful edge effects by introducing trenches that laterally delimit the active detection areas. These trenches physically separate the pixel collection areas, preventing edge effects from causing spurious triggering while preserving the vertical PN junction structure for sensitive photon detection.
Solution Approach 2:
The patent introduces an intermediary conductive layer filling the trenches, which serves as both an electrical connection path and an optical shield. This intermediary structure prevents cross-talk between pixels and blocks radiation that could cause false triggering, while maintaining the integrity of the vertical PN junction photodiodes.
2Reliability
If pixels are arranged in a grid pattern with trenches, then cross-talk prevention is improved, but device complexity increases
Solution Approach 1:
The conductive layer filling the trenches performs multiple functions simultaneously: it provides lateral delimitation of pixels, serves as an electrical connection path between pixels, and acts as an optical shield to prevent cross-talk. This multi-functionality reduces the need for additional separate structures, thereby limiting the increase in device complexity.
Solution Approach 2:
The patent merges the functions of pixel isolation, electrical connection, and optical shielding into a single integrated trench structure with a conductive filling layer. This consolidation of multiple functions into one structure achieves reliable pixel isolation without proportionally increasing device complexity.
3Reliability
If conductive region is made opaque to radiation, then cross-talk prevention is improved, but light detection efficiency may be reduced
Solution Approach 1:
The patent segments the device into distinct functional zones: the transparent active detection areas where photons are detected, and the opaque trench regions where the conductive layer blocks radiation. This spatial segmentation ensures that the opaque conductive regions only block stray radiation causing cross-talk, while the transparent detection areas maintain full photon detection efficiency.
Solution Approach 2:
The patent applies different optical properties to different parts of the device: the pixel collection areas maintain transparency for efficient photon detection, while the trench regions contain opaque conductive material to prevent cross-talk. This local differentiation of optical quality ensures that opacity is applied only where needed for isolation, preserving detection efficiency in the active areas.
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 accuracy of single photon detection by minimizing spurious triggering and ensuring that only intended photons generate measurable signals, thereby improving the overall performance of the image sensor.
Implementation Method 1
a conductive region extends in the trenches, said conductive region being in contact with the surface of the first semiconductor layer opposite to the substrate
Implementation Method 2
A SPAD photodiode is essentially formed by a PN junction reverse biased at a voltage greater than its avalanche threshold. When a photogenerated electric charge is injected into the depletion area, if the displacement speed of this charge in the depletion area is sufficiently high, that is, if the electric field in the depletion area is sufficiently intense, the photodiode is capable of avalanching
Implementation Method 3
When a photogenerated electric charge is injected into the depletion area, the photodiode is capable of avalanching. A single photon is thus capable of generating a measurable electric signal
Data Source
AI summary
An image sensor including a plurality of avalanche photodiodes formed inside and on top of a semiconductor substrate of a first conductivity type having a front side and a back side, wherein: trenches vertically extend in the substrate from its front side to its back side, the trenches having, in top view, the shape of a continuous grid laterally delimiting a plurality of substrate islands, each island defining a pixel including a single individually-controllable avalanche photodiode, and including a doped area of collection of an avalanche signal of the pixel photodiode the lateral walls of the trenches are coated with a first semiconductor layer having a conductivity type opposite to that of the collection area, and a conductive region extends in the trenches, the conductive region being in contact with the surface of the first semiconductor layer opposite to the substrate.