Stacked-Chip SPAD Sensor Corner Contact Layout for High Fill Factor
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Solution Overview
Problem
Conventional single-photon avalanche diode (SPAD) sensors face a low fill factor due to the large size of guard rings and support circuitry, which becomes a significant issue as pixel sizes decrease, impacting the efficiency of image sensors.
Innovation Solution
A stacked chip photon sensing system is implemented where highly doped contact regions are only placed in the corner regions of the SPADs, reducing the overall dimensions and allowing for increased fill factor, with support circuitry on a separate chip to optimize the SPAD array's performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If guard rings are made large to protect SPADs from high voltage breakdown, then reliability is improved, but area occupied increases causing fill factor to decrease
Solution Approach 1:
The patent transitions from a planar guard ring structure to a three-dimensional stacked architecture where the SPAD sensor layer is separated from the support circuitry layer. This vertical stacking allows the guard rings to be smaller in the sensor layer while maintaining protection functionality, as the separation distance in the vertical dimension provides additional electrical isolation and reduces the lateral footprint of guard rings.
Solution Approach 2:
The patent divides the SPAD sensor system into separate functional layers: the sensor layer containing SPADs and guard rings, and the support circuitry layer containing readout circuits. This segmentation allows each layer to be optimized independently, enabling smaller guard rings in the sensor layer while maintaining reliable protection through the layered structure.
2Device complexity
If support circuitry is integrated on the same chip as SPADs, then device complexity is reduced, but area occupied by circuitry increases causing fill factor to decrease
Solution Approach 1:
The patent uses vertical stacking to separate the sensor function from the readout function into different layers. The SPADs are located in the first layer while the support circuitry is located in the second layer, connected via through-silicon vias. This three-dimensional integration maintains functional integration while eliminating lateral area conflicts, allowing the active sensing area to occupy nearly the entire pixel area.
3Productivity
If pixel size is reduced to increase resolution, then productivity is improved, but the relative size of guard rings and circuitry increases causing fill factor to decrease
Solution Approach 1:
The stacked architecture moves the conflict from the lateral dimension to the vertical dimension. By stacking the sensor layer and readout layer vertically, the patent enables pixel dimensions to be reduced for higher resolution while the guard rings and circuitry occupy separate vertical spaces, maintaining adequate spacing without consuming lateral pixel area.
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 the fill factor of the SPAD imaging array by minimizing the area occupied by guard rings and support circuitry, enabling more efficient photon detection and improved performance in smaller pixel sizes.
Implementation Method 1
The SPAD regions produce an output pulse when struck by a photon
Implementation Method 2
a single photo-generated carrier can trigger an avalanche multiplication process that causes current at the output of the photon detection cell to reach its final value quickly
Data Source
AI summary
A photon detection device includes a single photon avalanche diode (SPAD) disposed in a semiconductor layer. A guard ring structure is disposed in the semiconductor layer surrounding the SPAD to isolate the SPAD. A well region is disposed in the semiconductor layer surrounding the guard ring structure and disposed along an outside perimeter of the photon detection device. A contact region is disposed in the well region only in a corner region of the outside perimeter such that there is no contact region disposed along side regions of the outside perimeter. A distance between an inside edge of the guard ring structure and the contact region in the corner region of the outside perimeter is greater than a distance between the inside edge of the guard ring structure and the side regions of the outside perimeter such that an electric field distribution is uniform around the photon detection device.