Multilayer High-Refractive-Index Waveguide for CMOS Image Sensor Light Collection
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Solution Overview
Problem
Miniaturization of pixels in photoelectric conversion devices leads to decreased light sensitivity due to reduced light receiving area, and existing methods for forming optical waveguides can result in voids and inefficiencies, particularly in CMOS image sensors with multilayer wiring, causing light to be reflected incorrectly or not enter the photoelectric conversion element.
Innovation Solution
A photoelectric conversion device with a multilayer structure featuring high-refractive-index portions that guide light through refractive index differences, where the first high-refractive-index portion is closer to the photoelectric conversion element and has a larger cross-sectional area than the second, allowing effective light reflection and condensation, and the second portion is formed above the first to enhance light entry and reduce voids.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pixels are miniaturized to decrease device size and increase pixel count, then productivity and device integration are improved, but light receiving area decreases and sensitivity deteriorates
Solution Approach 1:
The optical waveguide is divided into multiple layers (first optical waveguide layer and second optical waveguide layer) with different functions. The first layer handles light reflection and condensation, while the second layer facilitates light entry, allowing each segment to optimize for its specific function rather than compromising overall performance
Solution Approach 2:
The patent transitions from a conventional single-layer planar structure to a multilayer three-dimensional structure. The optical waveguide extends in the vertical dimension with multiple layers at different heights, enabling light to be guided and condensed from multiple angles and paths, thereby improving light collection efficiency without increasing pixel footprint area
2Ease of manufacture
If conventional embedding method is used to form optical waveguide, then manufacturing process is simple, but voids are produced in high aspect ratio well-shaped portions
Solution Approach 1:
The optical waveguide formation process is segmented into multiple steps with multiple layers. Instead of attempting to fill a single deep well in one step, the structure is built layer by layer, with each layer being filled separately. This segmentation reduces the aspect ratio of each individual filling operation, preventing void formation while maintaining manufacturing feasibility
Solution Approach 2:
The first optical waveguide layer is formed and filled before forming the second optical waveguide layer. This preliminary action allows the lower layer to be properly filled and cured before adding the upper layer, ensuring complete material placement and eliminating voids that would occur if a single deep structure were attempted
3Device complexity
If single-layer optical waveguide is used, then device structure is simple, but light condensation efficiency is insufficient
Solution Approach 1:
The optical waveguide is segmented into multiple functional layers: the first layer (closer to photoelectric conversion element) with larger cross-sectional area for light reflection and condensation, and the second layer (above the first) with smaller cross-sectional area for light entry. This segmentation allows each layer to be optimized for its specific optical function, improving overall condensation efficiency
Solution Approach 2:
Different regions of the optical waveguide are given different properties: the first optical waveguide layer has a larger cross-sectional area and is positioned to maximize light reflection and condensation, while the second layer has a smaller area optimized for light entry. Each layer's refractive index and geometry are locally optimized for its specific function in the light guiding process
4Reliability
If light is reflected at upper-layer optical waveguide and enters lower-layer optical waveguide, then light guidance is achieved, but reflection conditions are not satisfied and color mixture or noise occurs
Solution Approach 1:
The patent applies different refractive indices to different layers: the first optical waveguide layer has a first refractive index and the second layer has a second refractive index that is higher than the first. This local differentiation of optical properties ensures that light reflection and refraction occur at controlled interfaces, satisfying reflection conditions and preventing unwanted light paths that would cause color mixture or noise
Solution Approach 2:
The refractive index parameter is changed between layers to control light behavior. By making the second layer's refractive index higher than the first layer's, the patent optimizes the optical path for total internal reflection at specific interfaces while preventing light from entering unwanted paths, thereby eliminating color mixture and noise components
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 improves light condensation efficiency, reduces voids, and decreases dark current by optimizing the refractive index layers and their thickness, effectively addressing the sensitivity issues and damage during etching steps.
Implementation Method 1
light is reflected from interfaces due to the refractive index difference between the insulating layer and the high refractive index portion and condensed
Implementation Method 2
a material for embedding uses a material having a high refractive index compared to that of the insulating layer
Implementation Method 3
forming an on-chip microlens on the light receiving portion of the photoelectric conversion device and condensing light to the light receiving portion
Data Source
AI summary
A photoelectric conversion device comprises a high-refractive-index portion at a position close to a photoelectric conversion element therein. And, the high-refractive-index portion has first and second horizontal cross-section surfaces. The first cross-section surface is at a position closer to the photoelectric conversion element rather than the second cross-section surface, and is larger than an area of the second cross-section surface, so as to guide an incident light into the photoelectric conversion element without reflection.