Image Sensor Self-Focusing Pixels with Shifted Diffraction Gratings
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Solution Overview
Problem
Conventional self-focusing pixels in image sensors experience vignetting, leading to degraded sensor sensitivity due to off-centered focusing points for light rays with non-zero average angles of incidence, particularly at the sensor's border.
Innovation Solution
The implementation of self-focusing pixels with two laterally shifted diffraction gratings in multiple metal levels, where the distance between the gratings is optimized to be close to the Talbot length, ensuring that light rays with non-zero angles of incidence converge accurately onto the active area, thereby addressing vignetting and improving sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional self-focusing pixels with single-sided screens are used, then the structure is simple and manufacturing is easy, but light rays with non-zero angles of incidence fail to converge accurately onto the active area, causing vignetting and degraded sensitivity
Solution Approach 1:
The patent applies asymmetry by implementing screens only on specific sides of the photodiode based on the pixel's position in the sensor array. Corner pixels receive screens on two adjacent sides, border pixels on one side, and central pixels receive no screens. This asymmetric configuration optimizes light ray convergence for pixels at different positions, eliminating vignetting effects while maintaining manufacturing simplicity.
Solution Approach 2:
The patent implements local quality by tailoring the screen configuration to the specific location of each pixel within the sensor array. Different pixels receive different screen configurations (corner pixels get two screens, border pixels get one screen, central pixels get no screens), allowing each pixel to be optimized for its local light reception characteristics and angle of incidence requirements.
2Reliability
If screens are added to correct light ray convergence, then sensitivity improves, but the manufacturing process becomes more complex
Solution Approach 1:
The patent merges the screen structure with the existing metal interconnection layers of the sensor. The screens are formed using the same metal deposition and patterning processes already required for the sensor's electrical connections, combining two functions (light ray control and electrical interconnection) into a single integrated structure. This eliminates the need for separate screen manufacturing steps.
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 effectively recenters light rays with non-zero angles of incidence, enhancing the sensitivity and illumination of the image sensor by compensating for the vignetting effect, particularly at the sensor's border, without requiring additional manufacturing steps or equipment.
Implementation Method 1
a first optical grating formed of periodically spaced apart parallel strips... a second optical grating similar to the first grating and shifted with respect to the first grating
Implementation Method 2
a microlens 29 is arranged at the surface of intermediate layer 28, opposite to the substrate portion associated with pixel 11... To concentrate the light intensity received at the surface of pixel 11 towards active area 15
Implementation Method 3
Pixel 11 comprises an active photodiode area 15 formed in the upper part of a portion of a semiconductor substrate 13
Data Source
AI summary
An image sensor is formed by a pixel array including a plurality of pixels. Certain ones of the pixels include, above their active areas, a first optical grating formed of periodically spaced apart parallel strips separated from the active area by a first insulator. Those pixels further include, in another metal level, a second optical grating formed of periodically spaced apart parallel strips separated from the first grating by a second insulator. The second optical grating is laterally shifted with respect to the first grating in a direction orthogonal to a longitudinal direction of the parallel strips.