Reflective Optical Cavity Pixels for Image Sensors
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Solution Overview
Problem
Conventional image pixels in imaging devices suffer from low quantum efficiency and pixel crosstalk due to light passing through the semiconductor substrate only once and being scattered or diffracted into neighboring pixels, which degrades performance, especially in low-light conditions.
Innovation Solution
The implementation of optical cavity image pixels with a reflective cavity formed by vertical, frontside, and backside reflectors surrounding the photodiodes, along with light-guiding structures to enhance light absorption and reduce crosstalk, using CMOS or CCD technology in semiconductor substrates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If light passes through the semiconductor substrate only once in conventional image pixels, then the device complexity is reduced, but the quantum efficiency deteriorates due to light not being fully converted into detectable electric charge
Solution Approach 1:
The optical cavity enables continuous light interaction with the photodiode through multiple reflections. Light that misses the photodiode on the first pass is reflected back by the cavity structure, allowing multiple opportunities for charge generation until the light is fully absorbed or escapes, thereby maximizing quantum efficiency without requiring additional complex components
Solution Approach 2:
The optical cavity acts as an intermediary structure between the incident light and the photodiode. It mediates the light-charge conversion process by trapping and redirecting light within the cavity, increasing the interaction time and probability between photons and the photodiode without requiring direct single-pass optimization
2Reliability
If light passes through the semiconductor substrate only once, then the manufacturing process is simplified, but pixel crosstalk worsens due to light scattering or diffraction into neighboring pixels
Solution Approach 1:
The optical cavity introduces localized optical confinement around each photodiode. By creating a distinct optical environment for each pixel with reflective boundaries, light is kept within its designated pixel region, preventing scattering into neighboring pixels while maintaining the overall simplicity of the pixel array structure
Solution Approach 2:
The optical cavity adds a vertical dimension to light confinement by using top and bottom reflective surfaces. This vertical trapping mechanism complements the lateral pixel arrangement, creating three-dimensional optical confinement that prevents crosstalk without requiring complex lateral isolation structures
3Reliability
If a reflective optical cavity is implemented to increase light absorption, then quantum efficiency is improved, but the device complexity increases due to additional reflective structures
Solution Approach 1:
The optical cavity structure serves multiple functions simultaneously: it reflects light back to the photodiode for enhanced charge generation, provides lateral isolation between pixels to prevent crosstalk, and creates vertical confinement to extend light interaction path. This multi-functionality achieves high quantum efficiency without proportionally increasing device complexity
4Reliability
If conventional single-pass light detection is used, then the imaging speed is maintained, but the sensitivity in low-light conditions deteriorates
Solution Approach 1:
The optical cavity enables continuous light interaction through multiple reflections, giving photons multiple opportunities to generate charge carriers. This continuous interaction process increases the probability of detecting low-intensity light signals without requiring high-speed mechanical or electronic modifications to the imaging system
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 light absorption by the photodiodes and reduces pixel crosstalk, improving the overall performance and efficiency of image sensors, particularly in low-light conditions, by allowing multiple reflections of light within the reflective cavity before detection.
Implementation Method 1
a reflective chamber (cavity) formed from reflective structures in the substrate that at least partially surround the photosensitive elements and reflect light that has passed through the photosensitive element back onto the photosensitive element
Implementation Method 2
Each pixel may include a photosensor such as a photodiode that receives incident photons (light) and converts the photons into electrical charges
Data Source
AI summary
An image sensor may be provided having a pixel array that includes optical cavity image pixels. An optical cavity image pixel may include a photosensitive element in a substrate and a reflective cavity formed from a frontside reflector that is embedded in an intermetal dielectric stack, a backside reflector formed in a dielectric layer above the photosensor that partially covers the photosensor, and sidewall reflectors formed in the substrate between adjacent photosensors using deep trench isolation techniques. Each optical cavity image pixel may also include a light-guide trench above the photosensor that guides light into the reflective cavity for that pixel. Each optical cavity pixel may also include color filter material in the trench. Light that is guided into the reflective cavity by the light-guide trench may experience multiple reflections from the reflectors of the reflective cavity before being absorbed and detected by the photosensor.