SPAD Trench Isolation for Backside Contact and Cross-Talk Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current methods for independent access to front-side illuminated Single-Photon Avalanche Diodes (SPADs) in 3D interconnection structures face challenges such as significant loss of sensitive area and increased optical cross-talk, particularly in small SPADs, with existing solutions like Through Silicon Vias (TSVs) and backside illumination (BSI) having drawbacks.
Innovation Solution
A through-silicon trench isolation scheme is employed, where trenches are used for both electrical and optical isolation, allowing for independent SPAD access from the backside without the need for conductive pillars, maximizing sensitive area and minimizing optical cross-talk by using a highly doped substrate and integrating series quenching resistors, and optionally filling trenches with dielectric or reflective materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If Through Silicon Vias (TSVs) are used for independent SPAD access, then independent front side electrode access is achieved, but sensitive area is significantly lost
Solution Approach 1:
The patent divides the substrate into isolated regions using trenches, with each trench enclosing one or more SPADs. This segmentation allows independent electrical access to each SPAD group through separate contact pads on the backside, eliminating the need for TSVs while preserving sensitive area. The trenches create electrically isolated islands that can be independently controlled and read out.
Solution Approach 2:
The patent moves the contact access from the front side to the back side of the substrate, utilizing the third dimension (depth/thickness) to resolve the conflict between independent access and sensitive area. By providing contact pads on the opposite face of the substrate, the design allows independent SPAD access without occupying front-side sensitive area.
2Ease of operation
If Through Silicon Vias (TSV) approach is used, then independent SPAD contact access is provided, but technological steps increase and manufacturing complexity rises
Solution Approach 1:
The trench isolation structure segments the substrate into independent regions that can be processed and contacted separately. This segmentation simplifies the manufacturing process by allowing standard photolithography and etching steps to create the isolation structures, followed by simple backside contact formation, eliminating the need for complex TSV fabrication sequences.
Solution Approach 2:
Instead of bringing contacts up through the substrate from the backside (TSV approach), the patent inverts the approach by providing contact access directly on the backside through surface contact pads. This inversion eliminates the need for through-substrate via fabrication, reducing manufacturing complexity while achieving the same functional goal of independent access.
3Ease of manufacture
If backside illumination (BSI) is used, then entrance window is placed opposite to structured side, but optical cross-talk increases and detection efficiency is lost
Solution Approach 1:
The patent uses trenches to segment the substrate into isolated regions, preventing optical cross-talk between adjacent SPADs. The trenches act as optical barriers that block stray light from reaching neighboring detectors, while still allowing each SPAD group to be independently accessed from the backside through separate contact pads.
Solution Approach 2:
The patent introduces trenches filled with dielectric or reflective materials as intermediary structures between adjacent SPADs. These intermediary elements serve dual purposes: electrical isolation and optical isolation, blocking cross-talk while allowing independent backside access without requiring full BSI redesign.
4Area of stationary object
If trenches are used for isolation, then optical cross-talk is reduced and sensitive area is maximized, but electrical isolation must be maintained
Solution Approach 1:
The patent designs the trenches to serve multiple functions simultaneously: optical isolation (blocking cross-talk), electrical isolation (separating charge carriers), and structural definition (defining active regions). By making the trenches multi-functional, the design maximizes sensitive area while addressing both optical and electrical isolation requirements through a single structural element.
Solution Approach 2:
The patent employs composite material structures in the trenches, combining dielectric materials for electrical isolation with reflective materials for optical isolation. This composite approach allows the same trench structure to simultaneously address both electrical and optical cross-talk issues, maximizing sensitive area while maintaining proper isolation.
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 enables efficient and simple independent connection of SPADs to a CMOS ASIC with a fill factor greater than 70%, preserving electro-optical performance and reducing series resistance, while avoiding the drawbacks of TSVs and BSI methods.
Implementation Method 1
Single-photon avalanche photodiodes (SPADs) are becoming of wide use in many domains
Implementation Method 2
trenches are used for both electrical and optical isolation
Implementation Method 3
Single-photon avalanche photodiodes (SPADs)
Data Source
AI summary
A semiconductor device, sensor, and array of SPAD cubes are described. One example of the disclosed sensor includes at least one Single Photon Avalanche Diode (SPAD) cube established in a substrate, the at least one SPAD cube including a photosensitive area that is configured to produce an electrical signal in response to light impacting the photosensitive area, where the photosensitive area is positioned at a first side of the at least one SPAD cube, a contact that receives the electrical signal, where the contact is positioned at a second side of the at least one SPAD cube that opposes the first side of the at least one SPAD cube, and at least one trench that spans an entire thickness of the substrate thereby electrically and optically isolating the at least one SPAD cube from adjacent SPAD cubes.