Back-Side Illuminated Image Sensor Pixel Design
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Solution Overview
Problem
Conventional image sensors face challenges in maintaining high light guiding efficiency and optical sensitivity due to the diffusion and scattering of incident light by gate structures and metal lines, leading to reduced fill factor and increased optical and electrical crosstalk.
Innovation Solution
The design features a back-side illuminated image sensor with a photoelectric conversion region deeply embedded in the semiconductor substrate, surrounded by a vertically extending isolation region and a transfer gate that transmits charges to a floating diffusion region, preventing light leakage and carrier diffusion, and allowing for a three-dimensional arrangement of photoelectric conversion, floating diffusion, and transistor structures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If gate structures and metal lines are present in the light path, then charge transmission is enabled, but light diffusion and scattering occur reducing light guiding efficiency
Solution Approach 1:
The patent moves the gate structures and metal lines from the optical path plane to a different spatial dimension by implementing back-side illumination. The photoelectric conversion region is positioned at the back surface of the substrate, allowing light to enter without passing through gate structures and metal lines that are located on the front surface, thus eliminating light diffusion and scattering while maintaining charge transmission functionality
Solution Approach 2:
The patent segments the image sensor into distinct functional regions: the front surface contains gate structures and metal lines for charge control and transmission, while the back surface contains the photoelectric conversion region for light detection. This spatial segmentation allows each component to perform its function without interfering with the other, particularly preventing front-surface structures from degrading light guiding efficiency
2Measurement precision
If fill factor is increased to improve light sensitivity, then more photoelectric conversion area is available, but optical and electrical crosstalk increases
Solution Approach 1:
By implementing back-side illumination, the patent allows for increased fill factor as the photoelectric conversion region can occupy a larger portion of the back surface area without being constrained by front-surface structures. The spatial separation between front-surface control structures and back-surface photoelectric regions prevents optical crosstalk, while vertical isolation structures prevent electrical crosstalk between adjacent pixels
Solution Approach 2:
The patent introduces vertical isolation structures (such as deep trench isolation or pinned photodiode structures) that extend from the front surface through the substrate to the back surface. These intermediary structures act as barriers between adjacent photoelectric conversion regions, preventing both optical leakage and electrical carrier diffusion to neighboring pixels, thus eliminating crosstalk while allowing high fill factor
3Loss of energy
If photoelectric conversion region is deeply embedded in substrate, then light guiding efficiency is improved, but manufacturing complexity increases
Solution Approach 1:
The patent inverts the conventional front-side illuminated architecture by implementing back-side illumination. Instead of having the photoelectric conversion region at the front surface where light enters, the conversion region is positioned at the back surface. This inversion allows light to pass directly through the substrate to the photoelectric conversion region without interacting with front-surface structures, improving light guiding efficiency while the substrate itself serves as the light guiding medium, simplifying the overall structure
4Object-generated harmful factors
If isolation region extends vertically beyond photoelectric conversion region, then crosstalk is prevented, but device complexity increases
Solution Approach 1:
The vertically extending isolation structure serves multiple functions simultaneously: it acts as an optical barrier to prevent light leakage between adjacent pixels, an electrical barrier to prevent carrier diffusion to neighboring regions, and a structural element that can be integrated with the substrate formation process. By combining these functions into a single multi-functional structure, the patent prevents crosstalk without proportionally increasing device complexity
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 enhances the signal-to-noise ratio by preventing undesirable optical and electrical crosstalk, maintaining high light guiding efficiency, and increasing the fill factor, thereby improving the overall sensing performance.
Implementation Method 1
A BIS performs photoelectric transformation of incident light passing through a back surface of a semiconductor substrate
Data Source
AI summary
A unit pixel of an image sensor includes a photoelectric conversion region, an isolation region, a floating diffusion region and a transfer gate. The photoelectric conversion region is formed in a semiconductor substrate. The isolation region surrounds the photoelectric conversion region, extends substantially vertically with respect to a first surface of the semiconductor substrate, and crosses the incident side of the photoelectric conversion region so as to block leakage light and diffusion carriers. The floating diffusion region is disposed in the semiconductor substrate above the photoelectric conversion region. The transfer gate is disposed adjacent to the photoelectric conversion region and the floating diffusion region, extends substantially vertically with respect to the first surface of the semiconductor substrate, and transmits the photo-charges from the photoelectric conversion region to the floating diffusion region.