SPAD Pixel Layout With Central Electrode for Higher Detection Efficiency
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Solution Overview
Problem
Conventional solid-state imaging devices with single-photon avalanche diodes (SPADs) face reduced detection efficiency due to electrodes placed at the end portion, leading to uneven electric fields and increased noise, as they concentrate electric fields in partial regions, causing breakdown states not derived from incident light.
Innovation Solution
A solid-state imaging element with a SPAD having an electrode placed centrally on the light receiving surface, utilizing a microlens with a concavity and possibly a cut or multiple microlenses to collect light away from the electrode region, ensuring an even electric field and improved light collection efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If electrodes are placed at the end portion of the SPAD, then the light receiving surface area is maximized, but the electric field becomes uneven and noise increases
Solution Approach 1:
The patent inverts the conventional electrode placement by positioning the electrode at the central portion of the light receiving surface instead of the end portion. This inversion resolves the contradiction by creating a uniform electric field distribution across the active region while still maintaining effective light detection capability.
2Measurement precision
If a microlens is used to collect light to the central portion of the SPAD, then detection efficiency is improved, but the electrode placement causes electric field concentration
Solution Approach 1:
The patent changes the positional parameter of the electrode from the end portion to the central portion of the light receiving surface. This parameter change simultaneously achieves both goals: the microlens can focus light to the central region where the electrode is located, improving detection efficiency, while the central positioning ensures uniform electric field distribution without concentration at the edges.
3Measurement precision
If the SPAD operates in breakdown state for high sensitivity, then photon detection capability is improved, but the required isolation region reduces the aperture ratio
Solution Approach 1:
The patent segments the pixel structure by placing the electrode and its connection wiring in the central region, separated from the peripheral isolation regions. This segmentation allows the active detection area to be maximized while maintaining the necessary isolation for breakdown operation, thereby improving the aperture ratio without sacrificing sensitivity.
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 detection efficiency by evenly distributing electric fields and collecting incident light to the SPAD's light receiving surface, reducing noise and improving signal output quality.
Implementation Method 1
a light collecting section that causes light from a subject to be collected in the light receiving surface other than a region where the electrode is placed
Implementation Method 2
The SPAD is a photodiode that performs photoelectric conversion in a state where a voltage exceeding the breakdown voltage is applied
Implementation Method 3
an electron avalanche derived from a carrier generated by photoelectric conversion occurs, and the SPAD enters a breakdown state
Data Source
AI summary
To improve detection efficiency in a solid-state imaging element including a SPAD in which an electrode and wiring are placed in a central portion. A solid-state imaging element includes a photodiode and a light collecting section. The photodiode includes a light receiving surface and an electrode placed on the light receiving surface, and that outputs an electrical signal in accordance with light incident on the light receiving surface in a state where a voltage exceeding a breakdown voltage is applied to the electrode. The light collecting section causes light from a subject to be collected in the light receiving surface other than a region where the electrode is placed.