SPAD Photodiode Trench Grid for Reduced Edge Triggering
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Solution Overview
Problem
Existing image sensors with SPAD photodiodes face challenges in efficiently detecting low-intensity radiation due to spurious triggering from edge effects, which affects the sensitivity and accuracy of single photon detection.
Innovation Solution
The design incorporates a semiconductor substrate with vertically extending trenches forming a grid pattern, featuring a doped polysilicon layer and a conductive region made of metal, such as copper or tungsten, to create individually controllable SPAD photodiodes with a vertical PN junction, where the doping levels and layer structures are optimized to control the electric field intensity, reducing spurious triggering and enhancing sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If vertical PN junctions are used in SPAD photodiodes, then detection sensitivity for low-intensity radiation is improved, but spurious triggering from edge effects increases
Solution Approach 1:
The patent extracts the harmful edge effects from the detection area by introducing trenches that physically separate the active detection regions. The trenches remove the problematic lateral edges where spurious triggering occurs, allowing the vertical PN junctions to maintain their high detection sensitivity without the contaminating edge effects
Solution Approach 2:
The patent introduces an intermediary conductive layer filled in the trenches that serves as both an electrical shield and a structural element. This intermediary layer prevents charge carrier multiplication at the trench edges while maintaining the integrity of the vertical PN junction detection mechanism, thus mediating between the need for sensitive detection and the need to eliminate spurious triggering
2Reliability
If trenches are introduced to reduce edge effects, then spurious triggering is reduced, but device complexity increases
Solution Approach 1:
The conductive layer introduced in the trenches serves multiple functions simultaneously: it acts as an electrical shield to prevent charge carrier multiplication, provides structural support to the trench walls, and can serve as an electrical contact layer. This multi-functionality reduces the need for additional separate components, thereby limiting the increase in device complexity while achieving spurious triggering reduction
3Measurement precision
If conductive regions are added in trenches, then charge carrier multiplication is controlled, but manufacturing complexity increases
Solution Approach 1:
The patent merges the conductive region formation with the existing trench structure by filling the trenches with conductive material during the same fabrication process sequence. The conductive layer is integrated into the trench structure rather than being added as a separate component, combining multiple functions (shielding, structural support, electrical contact) into a single manufacturing step, thus controlling charge carrier multiplication while limiting manufacturing complexity increase
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 enables efficient detection of low-intensity radiation by minimizing spurious triggering and increasing the sensitivity of SPAD photodiodes, allowing for precise single photon detection and improved image sensing capabilities.
Implementation Method 1
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. A single photon is thus capable of generating a measurable electric signal
Implementation Method 2
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.