Image Sensor Recesses Boost Quantum Efficiency and Reduce Crosstalk
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Solution Overview
Problem
Conventional CMOS image sensors face challenges with low quantum efficiency and increased crosstalk due to reduced pixel dimensions and high reflectivity of semiconductor substrates, leading to decreased optical sensitivity.
Innovation Solution
The image sensor design includes a semiconductor substrate with a photosensitive area featuring recesses and doped regions of different conductivity types, formed through an etching and ion implantation process, which enhances light absorption and reduces crosstalk by guiding light effectively to the photodiodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pixel dimensions are reduced, then the integration density is improved, but the crosstalk between pixels increases and optical sensitivity decreases
Solution Approach 1:
The patent divides the semiconductor substrate into isolated pixel units by forming trenches between adjacent pixels. These trenches physically segment the substrate, preventing optical crosstalk between neighboring pixels while maintaining high integration density. The segmentation is achieved through etching trenches that extend into the substrate and are filled with insulating material.
Solution Approach 2:
The patent applies different structural characteristics to different regions of the pixel array. Specifically, trenches are formed only between pixels, while the pixel regions themselves maintain their photodetector structures. This local differentiation allows high integration density in pixel regions while eliminating crosstalk at pixel boundaries.
2Productivity
If the pixel dimensions are reduced, then the integration density is improved, but the optical sensitivity decreases
Solution Approach 1:
The patent addresses the optical sensitivity issue by extending the light-receiving structure into the vertical dimension. Trenches are formed that extend deep into the substrate, and photodetector structures are positioned at various depths. This three-dimensional arrangement increases the effective light absorption volume without increasing the lateral pixel size, thereby maintaining high optical sensitivity while achieving high integration density.
Solution Approach 2:
The patent implements a nested structure where photodetector elements are positioned within trenches that are themselves nested within the semiconductor substrate. Multiple layers of photodetectors can be stacked vertically within the same lateral footprint, effectively nesting functional elements to increase sensitivity without increasing pixel area.
3Ease of manufacture
If the semiconductor substrate has high reflectivity, then the manufacturing simplicity is maintained, but the light absorption efficiency decreases
Solution Approach 1:
The patent converts the harmful effect of high reflectivity into a beneficial feature by using the reflective substrate to redirect reflected light back into the photodetector structures. The trenches are designed to trap and redirect reflected light, causing it to pass through the photodetector material multiple times, thereby increasing absorption efficiency without requiring anti-reflective coatings or complex substrate modifications.
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 quantum efficiency by increasing the light absorption area and reducing crosstalk, enhancing the overall performance of the image sensor.
Implementation Method 1
The incident light is separated into a combination of light of different wavelengths. For example, the incident light can be separated into a combination of red, blue, and green light. The light of different wavelengths is received by respective optical sensors such as photodiodes and is subsequently transformed into digital signals of different intensities.
Implementation Method 2
an ion implantation process is performed to form a second-conductivity-type doped region on a surface of the first-conductivity-type doped region in the photosensitive area
Data Source
AI summary
An image sensor includes a semiconductor substrate with at least one recess disposed on its surface and in the photosensitive area defined on the surface of the semiconductor substrate, a first-conductivity-type doped region disposed in the semiconductor substrate and in the photosensitive area, and a second-conductivity-type doped region disposed on the surface of the first-conductivity-type doped region and on the surface of the recess. A photosensitive device of the image sensor is formed of the first-conductivity-type doped region and the second-conductivity-type doped region.