Hemispherical SPAD Photodiode Layout for High Fill Factor
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Solution Overview
Problem
The miniaturization of electronic devices necessitates smaller single photon avalanche diodes (SPAD) while maintaining a high fill factor and preventing premature triggering, which is challenging due to the critical size of the guard ring and the reduced active region size.
Innovation Solution
A SPAD photodiode design featuring a hemispherical buried region of one conductivity type and a hemispherical core of a different conductivity type, formed using oblique dopant implantation and epitaxial growth, with a guard ring structure that maximizes the active region and reduces the guard ring size, thereby enhancing the fill factor and photon detection probability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the guard ring size is reduced to enable miniaturization of the photodiode, then the device size decreases, but the fill factor deteriorates
Solution Approach 1:
The patent applies spheroidality by forming hemispherical regions (buried region and core) instead of planar structures. The buried region is a first hemispherical region with a second hemispherical core inside it, both having curved surfaces. This spherical geometry allows the photodiode to achieve a compact footprint while maintaining a large active detection area, effectively resolving the contradiction between miniaturization and fill factor preservation.
2Volume of moving object
If the guard ring size is reduced to enable miniaturization, then the device size decreases, but premature triggering increases
Solution Approach 1:
The patent implements nesting by placing a second hemispherical core region inside the first hemispherical buried region. This nested configuration allows the guard ring function to be integrated within the active region structure itself, eliminating the need for a separate peripheral guard ring. The nested regions work together to prevent premature triggering while maintaining a compact overall device size.
3Shape
If oblique implantation is used to form the buried region, then the hemispherical shape is achieved, but the manufacturing complexity increases
Solution Approach 1:
The patent achieves hemispherical shapes through oblique ion implantation at angles between 15° and 30° relative to the substrate surface normal. This specific angular range optimizes the projection of dopant ions to create the desired hemispherical doping profile. The method uses standard semiconductor fabrication equipment and parameters, making the complex shape achievable through conventional manufacturing processes.
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 design increases the fill factor and photon detection probability while reducing the size of the guard ring and Dark Count Rate, allowing for efficient miniaturization of SPADs and accommodating smaller photodiodes with improved performance.
Implementation Method 1
all or part of the first region is formed by an oblique implantation of dopants of the first conductivity type, with respect to the normal to the first face of the substrate, by an angle greater than 20°
Implementation Method 2
the first face of the substrate is topped with an epitaxial layer
Implementation Method 3
A photodiode is a semiconductor device with the ability to capture radiation from the optical domain and transform it into an electrical signal
Implementation Method 4
A SPAD photodiode is a photodiode consisting of a PN junction reverse biased to a voltage above its avalanche voltage. Under these conditions, a single carrier photogenerated in the depletion region can trigger an avalanche created by impact ionization effects
Data Source
AI summary
A photodiode is formed in a semiconductor substrate of a first conductivity type. The photodiode includes a first region having a substantially hemispherical shape and a substantially hemispherical core of a second conductivity type, different from the first conductivity type, within the first region. An epitaxial layer covers the semiconductor substrate and buries the first region and core.