Hemispherical SPAD Photodiode Structure for High Fill Factor
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The miniaturization of electronic devices requires smaller photodiodes with improved efficiency, but reducing the size of single photon avalanche diodes (SPAD) while maintaining their functionality is challenging due to the critical role of the guard ring in preventing premature triggering and maximizing the fill factor.
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, which maximizes the active region and reduces the guard ring size, thereby enhancing miniaturization and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the guard ring size is increased to prevent premature triggering, then the reliability is improved, but the fill factor decreases
Solution Approach 1:
The patent transitions from a planar guard ring structure to a three-dimensional configuration where the first doped region extends vertically beneath the core. This vertical extension provides premature triggering prevention in the depth dimension while keeping the lateral footprint minimal, thus maintaining high fill factor.
Solution Approach 2:
The first doped region is nested within the well structure, with the core positioned above it and the second doped region above the core. This nested arrangement allows multiple functional regions to occupy overlapping spatial volumes, enabling the guard ring function to be achieved without lateral expansion.
2Volume of moving object
If the photodiode size is reduced for miniaturization, then the device dimensions are improved, but the photon detection probability decreases
Solution Approach 1:
The patent changes the doping concentration parameters vertically, with the first doped region having a maximum concentration at a depth below the core and decreasing toward the surface. This vertical concentration gradient enhances the electric field in the depletion region, improving photon detection probability despite reduced lateral dimensions.
Solution Approach 2:
The patent creates a composite doped structure combining the first doped region (extending vertically), the undoped or lightly-doped core, and the second doped region (at the surface). This composite structure optimizes both the detection volume and the electric field distribution for enhanced photon detection in miniaturized devices.
3Area of stationary object
If the guard ring size is reduced to increase fill factor, then the area is improved, but the noise increases due to premature triggering
Solution Approach 1:
The patent moves the premature triggering prevention function from the lateral dimension to the vertical dimension by extending the first doped region beneath the core. This allows the lateral guard ring to be minimized for high fill factor while the vertical doped region suppresses noise through enhanced field control in the depletion region.
4Volume of moving object
If the photodiode is miniaturized, then the device complexity is reduced, but the manufacturing precision requirements increase
Solution Approach 1:
The patent uses preliminary ion implantation to create the first doped region with a specific vertical concentration profile before forming the core and second doped region. This preliminary doping action establishes the vertical field control mechanism early in the fabrication process, simplifying subsequent steps and reducing overall manufacturing complexity despite the sophisticated final structure.
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, reduces noise, and accommodates smaller photodiode sizes while maintaining high avalanche voltage, thus addressing the need for efficient miniaturization and 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
Implementation Method 2
the first region and the core are formed using a single mask
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.