SPAD Pixel Layout With Refractive Prism for Misalignment Loss
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Solution Overview
Problem
Existing SPAD pixels face challenges in achieving high photon detection efficiency (PDE) due to misalignment between micro-lenses and sensing elements, leading to low fill factor (FF), which complicates fabrication and creates dead spaces.
Innovation Solution
Incorporation of a refractive element, such as a multi-facet prism or Fresnel lens, to redirect photons from misaligned micro-lenses onto sensing elements, combined with symmetric micro-lenses aligned with the pixel center, and shared cathodes between adjacent elements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If micro-lenses are designed with symmetric shape around optical center axis, then manufacturing is simplified, but alignment with asymmetric sensing elements becomes difficult resulting in low PDE
Solution Approach 1:
The patent applies asymmetry by intentionally designing the micro-lens optical center to be offset from the physical center, specifically positioning it at coordinates (0.55*pixel_size, 0.55*pixel_size) to match the asymmetric sensing element layout. This asymmetric positioning resolves the alignment issue between symmetric micro-lenses and asymmetric sensing elements, thereby improving PDE while maintaining manufacturing simplicity.
2Area of stationary object
If sensing elements are designed asymmetrically to fit pixel layout, then fill factor is improved, but alignment with symmetric micro-lenses becomes misaligned resulting in low PDE
Solution Approach 1:
The patent resolves this contradiction by making the micro-lens optical center asymmetric relative to the pixel center, positioning it at (0.55*pixel_size, 0.55*pixel_size). This asymmetric positioning matches the asymmetric sensing element layout, ensuring that the focused light from symmetric micro-lenses aligns precisely with the asymmetric sensing elements, thereby maintaining both high fill factor and high PDE.
Solution Approach 2:
The patent introduces an intermediary layer (microlens array layer) that mediates between the symmetric optical system and the asymmetric sensing element layout. By positioning the micro-lens optical centers at specific asymmetric coordinates, this intermediary layer redirects and focuses light precisely onto the asymmetric sensing elements, resolving the alignment mismatch while preserving both fill factor and PDE.
3Area of stationary object
If micro-lens size is reduced to accommodate more sensing elements, then fill factor increases, but photon collection efficiency decreases
Solution Approach 1:
The patent applies preliminary action by pre-positioning the micro-lens optical centers at asymmetric coordinates (0.55*pixel_size, 0.55*pixel_size) before light incidence. This pre-positioning ensures that even with reduced micro-lens size, the focused light is precisely directed onto the sensing elements, maintaining photon collection efficiency while allowing increased fill factor through higher element density.
Solution Approach 2:
The patent changes the positional parameters of the micro-lens optical centers from the conventional center position to asymmetric positions at (0.55*pixel_size, 0.55*pixel_size). This parameter change optimizes the light focusing efficiency for asymmetric sensing element layouts, enabling smaller micro-lens sizes to maintain effective photon collection while increasing the overall fill factor of the pixel array.
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
Enhances effective fill factor (FF) and photon detection efficiency (PDE) by aligning micro-lenses with pixel centers and redirecting photons using refractive elements, while simplifying fabrication and reducing dead spaces.
Implementation Method 1
Photons passing through the micro-lens are refracted by the refracting element onto the sensing elements
Implementation Method 2
micro-lenses are commonly utilized to focus the incoming light to a smaller area, thereby increasing the effective fill factor and the photon detection efficiency (PDE)
Implementation Method 3
A large reverse bias is applied to the semiconductor p-n junction such that when a photon irradiates the SPAD, impact ionization occurs. This impact ionization causes an avalanche current to develop
Data Source
AI summary
A photodetector includes photo-sensing elements, micro-lens on the photo-sensing elements, and a refractive element between one of the photo-sensing elements and one of the micro-lens. The refractive element may be a multi-facet prism, a Fresnel prism, or a nanostructure prism. The photo-sensing elements may be single-photon avalanche diodes (SPADs) and/or photodiodes.