Backside Illuminated CMOS Image Sensor with Wavelength-Selective Trenches
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Solution Overview
Problem
The existing CMOS image sensors face challenges in increasing pixel density without reducing light sensitivity, as smaller pixel sizes lead to decreased light incidence, and the backside illumination architecture complicates the fabrication process with multi-step processing and thickness control issues.
Innovation Solution
A CMOS image sensor design with pixel regions under the front surface, multi-layered metal interconnections over the pixels, trenches of varying depths in the backside substrate for different wavelengths, and a glass covering to enhance light transmission, eliminating the need for color filters and simplifying the thinning process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pixel size is reduced to increase pixel density, then the number of pixels increases, but the light sensitivity deteriorates due to decreased light incidence
Solution Approach 1:
The patent inverts the traditional front-side illumination approach by implementing backside illumination. Light is incident on the backside of the substrate rather than the front, allowing light to reach photodiodes without passing through metal interconnections and insulating layers that would block or attenuate light. This inversion resolves the contradiction by maintaining large photodiode areas for high light sensitivity while enabling high pixel density through efficient light collection from the backside.
Solution Approach 2:
The patent transitions from two-dimensional planar light incidence to three-dimensional light path optimization. By illuminating from the backside and allowing light to travel through the substrate thickness to reach photodiodes, the design effectively utilizes the third dimension (substrate depth) to improve light collection efficiency. This enables larger photodiode areas to be maintained even as pixel density increases, resolving the contradiction between pixel density and light sensitivity.
2Reliability
If backside illumination architecture is implemented to improve light sensitivity, then light transmission improves, but the fabrication process complexity increases due to multi-step processing and thickness control
Solution Approach 1:
The patent segments the substrate processing into distinct functional zones: a first region with through-type trenches providing electrical connection to the backside, and a second region with non-through trenches for light incidence. This segmentation allows different fabrication processes to be applied to different regions, simplifying the overall fabrication by avoiding the need for uniform complex processing across the entire substrate while still achieving backside illumination functionality.
Solution Approach 2:
The patent extracts and removes metal interconnections and insulating layers from the light path by forming trenches that eliminate these obstructive elements in the second region. This extraction of harmful components (metal layers blocking light) simplifies the fabrication process compared to attempting to modify existing front-side structures, as it directly removes obstacles rather than requiring complex multi-step modifications to the illumination path.
3Area of stationary object
If metal interconnections are formed over photodiodes in backside illumination, then pixel area increases, but fabrication complexity increases due to additional processing steps
Solution Approach 1:
The patent inverts the spatial arrangement by forming metal interconnections only in the first region (away from the light incident area) rather than covering the entire pixel area. This inversion allows the second region to maintain large open areas for light incidence and photodiode functionality without metal obstruction, while still providing necessary electrical connections in the first region. This resolves the contradiction by achieving both large pixel area and electrical connectivity without requiring complex full-coverage interconnection structures.
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 improves light sensitivity, simplifies the manufacturing process, and allows for uniform light incidence on photodiodes, reducing fabrication complexity and costs by eliminating the need for chemical etching and super contact processes.
Implementation Method 1
a plurality of trenches formed in a backside of the substrate, wherein the trenches have different depths for each wavelength of light
Implementation Method 2
light is transmitted through the backside of the silicon substrate 21, and reaches the photodiode
Data Source
AI summary
A CMOS image sensor includes a plurality of pixel regions formed under a front surface of a substrate, and having photodiodes separated from each other by a field oxide, a multi-layered metal interconnection formed over the pixel regions of the front of the substrate, a bump connected to an uppermost metal interconnection of the multi-layered metal interconnection, a plurality of trenches formed in a backside of the substrate, wherein the trenches have different depths for each wavelength of light, and correspond to the respective pixel regions, and a glass covering the backside of the substrate.