SPAD Guard Ring Passivation for Dark Count Reduction
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Solution Overview
Problem
Single photon avalanche diodes (SPADs) with shallow trench isolation (STI) face high dark count rates due to increased deep-level carrier generation centers at the STI interface, which affects their scalability and performance in applications requiring picosecond time resolutions.
Innovation Solution
A semiconductor device with a guard ring and passivation layer surrounding the SPAD, where the passivation layer is formed on the sidewall and bottom surface of the guard ring to reduce the electric field at the edge of the detector, and integrated quenching resistance and read-out electronics are used to limit parasitic capacitances and after-pulses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If shallow trench isolation (STI) is used as a guard ring to improve fill-factor, then device density increases, but dark count rate increases due to deep-level carrier generation centers at the STI interface
Solution Approach 1:
A passivation layer is introduced as an intermediary between the STI guard ring and the active region. This passivation layer mediates the harmful interaction by reducing the electric field at the STI interface, thereby suppressing carrier generation at the STI-active region interface while preserving the high-density benefits of STI-based guard rings
2Area of stationary object
If the active region is placed in direct contact with STI to maximize fill-factor, then device area utilization improves, but dark count rate increases due to electric field enhancement at the interface
Solution Approach 1:
The passivation layer is selectively applied only at the critical STI-active region interface where harmful electric field enhancement occurs. This localized passivation maintains high fill-factor by preserving direct contact elsewhere while specifically addressing the interface region where dark count generation is problematic
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
The solution significantly reduces dark count rates from 1 MHz to 90 kHz, enhances fill-factor, and maintains high sensitivity and timing resolution, suitable for applications like 3D imaging and fluorescence lifetime imaging.
Implementation Method 1
A passivation layer of the first conductivity type may be in contact with the guard ring to reduce an electric field at an edge of the photon avalanche detector
Implementation Method 2
The core of the SPAD includes a p-n junction biased above its breakdown voltage, thus operating in the Geiger mode. In this region of operation, free carriers such as photo generated electron hole pairs, can trigger an avalanche breakdown by impact ionization
Data Source
AI summary
A semiconductor device includes a semiconductor substrate, a photon avalanche detector in the semiconductor substrate. The photon avalanche detector includes an anode of a first conductivity type and a cathode of a second conductivity type. A guard ring is in the semiconductor substrate and at least partially surrounds the photon avalanche detector. A passivation layer of the first conductivity type is in contact with the guard ring to reduce an electric field at an edge of the photon avalanche detector.