SPAD Metal Layer Stack Aperture Attenuation
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Solution Overview
Problem
Single photon avalanche diodes (SPADs) face limitations in attenuation and linear range due to design constraints, particularly under high ambient light conditions, where the maximum achievable attenuation is restricted by metal layer design rules, leading to non-linear response and increased current consumption.
Innovation Solution
The use of a stack of two or more metal layers with strategically arranged apertures to create an effective aperture that blocks incident photons except along the optical axis, allowing for extended attenuation and improved linear range, while being insensitive to alignment errors and technology constraints.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single metal layer with aperture is used for attenuation, then the structure is simple and easy to manufacture, but the achievable attenuation is limited by metal design rules
Solution Approach 1:
The single metal layer is segmented into multiple metal layers (first metal layer, second metal layer, etc.), each contributing to the overall attenuation. This segmentation allows each layer to be designed with larger aperture dimensions that comply with metal design rules, while the combined effect achieves the required high attenuation ratio.
Solution Approach 2:
The solution transitions from a two-dimensional single-layer aperture to a three-dimensional multi-layer aperture stack. By adding the vertical dimension with multiple metal layers at different heights, the effective aperture area is reduced without requiring any single layer to have impossibly small dimensions, thus satisfying both manufacturing constraints and attenuation requirements.
2Manufacturing precision
If a small aperture is used to achieve high attenuation, then the linear range is extended, but the aperture size may violate metal to metal spacing rules and metal enclosed area rules
Solution Approach 1:
The aperture structure is segmented across multiple metal layers, allowing each individual layer to have larger aperture dimensions that comply with minimum metal width, metal to metal spacing, and metal enclosed area rules. The cumulative effect of multiple layers achieves the required high attenuation ratio without any single layer violating design rules.
Solution Approach 2:
The apertures in different metal layers are nested or aligned with each other, creating a progressively smaller effective aperture area. Each metal layer's aperture is positioned to overlap or align with the layer below, effectively reducing the total open area while keeping individual aperture dimensions within manufacturing constraints.
3Device complexity
If a single metal layer is used, then the device complexity is low, but the alignment sensitivity is high and misalignment impacts attenuation performance
Solution Approach 1:
The cover structure is segmented into multiple metal layers, which distributes the alignment requirements across multiple components. This segmentation increases the system's tolerance to misalignment because the cumulative aperture effect is less sensitive to small positional variations compared to a single critical aperture.
Solution Approach 2:
The multi-layer aperture structure provides a built-in cushioning effect against alignment errors. The redundant layers ensure that even if some layers are slightly misaligned, the overall attenuation performance remains within acceptable ranges, effectively cushioning against the impact of manufacturing tolerances.
4Use of energy by moving object
If the SPAD is left non-attenuated, then the current consumption is low and the response is linear at low light levels, but the SPAD cannot operate under high ambient light conditions
Solution Approach 1:
The SPAD system is made dynamic by providing multiple attenuation configurations (different combinations of metal layers that can be selectively activated). This allows the system to adapt its attenuation level in real-time based on ambient light conditions, transitioning from low attenuation at low light levels to high attenuation under high ambient light, thereby maintaining linear response and appropriate current consumption across varying conditions.
Solution Approach 2:
The multi-layer metal structure provides multi-functionality by enabling the same SPAD to operate effectively across a wide range of lighting conditions. By selectively activating different metal layer combinations, the system can achieve various attenuation ratios, making it universally applicable from low-light to high-ambient-light environments without requiring multiple separate devices.
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 approach extends the achievable attenuation to smaller values, enhances the linear range of SPADs, and allows for different levels of attenuation, reducing the impact of misalignment and design rule limitations, resulting in improved performance under varying lighting conditions.
Implementation Method 1
The metal layers are arranged in the stack with respect to an optical axis such as to open an effective aperture along the optical axis... The cover shields the active area from incident photons
Implementation Method 2
The cover shields the active area from incident photons. The cover comprises a stack of at least the first and a second metal layer
Implementation Method 3
a single photon may generate a charge carrier in a depletion layer of the SPAD that, in turn, triggers an avalanche current due to an impact ionization mechanism
Implementation Method 4
A single photon avalanche diode, SPAD for short, is a solid-state photodetector based on a pn-junction biased beyond its breakdown region
Implementation Method 5
a single photon may generate a charge carrier in a depletion layer of the SPAD
Data Source
AI summary
A single photon avalanche diode, SPAD, comprises an active area which is arranged to generate a photon triggered avalanche current. A cover is arranged on or above the active area. The cover shields the active area from incident photons. The cover comprises a stack of at least the first and a second metal layer. At least one of the metal layers, e.g. the first metal layer, comprises an aperture. The metal layers are arranged in the stack with respect to an optical axis such as to open an effective aperture along the optical axis. By way of the effective aperture a portion of the active area is exposed to incident photons being incident along the optical axis. The effective aperture is smaller than the aperture arranged in the first metal layer.