CMOS Night Vision Imager Optical Pixel Cavity
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Solution Overview
Problem
Conventional CMOS imagers struggle to match the low light performance of intensifier tubes for night vision applications due to high room temperature dark current and limited near-infrared quantum efficiency, despite their potential for lower cost and smaller size.
Innovation Solution
A CMOS pixel design featuring a photo-sensitive element with a non-resonant optical cavity formed by reflective layers that absorb electromagnetic radiation through multiple passes, reducing bulk dark current and increasing quantum efficiency without increasing silicon volume, achieved by using a combination of reflective layers and a microlens to focus light into a small aperture within the cavity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the absorption region is made thicker to increase quantum efficiency in the near-infrared range, then quantum efficiency is improved, but bulk dark current increases due to increased silicon volume
Solution Approach 1:
The patent introduces a temporal dimension to light absorption by implementing a resonant cavity that traps photons and causes them to bounce multiple times through the absorption region. This allows a thin silicon layer to achieve the same effective absorption path length as a much thicker layer would provide in a single pass, thereby increasing quantum efficiency without increasing bulk silicon volume and avoiding the associated dark current penalty.
Solution Approach 2:
The resonant cavity acts as an intermediary between the incident light and the absorption region. It mediates the interaction by trapping photons and forcing them to repeatedly interact with the thin absorption layer, effectively decoupling the absorption efficiency from the physical thickness of the silicon and preventing bulk dark current generation.
2Device complexity
If conventional CMOS imager design is used, then device complexity is low and manufacturing is simple, but dark current is too high for uncooled night vision operation
Solution Approach 1:
The patent embeds the resonant cavity structure within the existing CMOS pixel architecture. The cavity is formed by depositing reflective layers within the pixel structure, nesting this optical enhancement feature inside the conventional CMOS device without requiring a complete redesign of the underlying technology.
Solution Approach 2:
The invention changes the optical parameters of the CMOS imager by introducing a resonant cavity with specific reflectivity characteristics. This modifies the light-matter interaction parameters to enhance near-infrared absorption while the electrical parameters remain compatible with standard CMOS operation, achieving low dark current without changing the fundamental device architecture.
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 enhances quantum efficiency in the 600-1000 nm range while minimizing bulk dark current, allowing for effective night vision operation without cooling, thus overcoming the limitations of conventional CMOS imagers in low light conditions.
Implementation Method 1
the first reflective layer and the second reflective layer are configured to reflect the electromagnetic radiation substantially toward each other
Implementation Method 2
a photo-sensitive element... When electromagnetic radiation enters the aperture, the first reflective layer and the second reflective layer are configured to reflect the electromagnetic radiation substantially toward each other until substantially absorbed in the cavity
Data Source
AI summary
A pixel design is disclosed. The pixel includes a photo-sensitive element. A first reflective layer substantially overlies the photo-sensitive element. A second reflective layer substantially underlies the photo-sensitive element and forms a cavity with the first reflective layer that is non-resonant with respect to photon absorption. An aperture is formed in either the first reflective layer or the second reflective layer. When electromagnetic radiation enters the aperture, the first reflective layer and the second reflective layer are configured to reflect the electromagnetic radiation substantially toward each other until substantially absorbed in the cavity.