Image Sensor Potential Barrier Reduces Color Crosstalk
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Solution Overview
Problem
Modern CMOS image sensors face challenges with color crosstalk in small pixel sizes due to red light-generated electrons diffusing into neighboring photodiodes, leading to unwanted color detection and reduced pixel efficiency, especially with thick depletion regions increasing dark current and discontinuity of P+-type pinning layers.
Innovation Solution
Incorporating a deep high energy Boron implantation under blue and green light-receiving pixels to create a potential barrier that directs red light-generated carriers into the 'red' photodiodes, reducing lateral diffusion and crosstalk, while maintaining sufficient epitaxial depth for efficient red light conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If red light photons penetrate deep into the silicon bulk to generate photoelectrons, then red light detection efficiency is improved, but electrons generated in the highly doped P+-type substrate recombine quickly with holes and cannot be collected, reducing red light detection efficiency
Solution Approach 1:
The patent changes the doping concentration parameter by creating a gradient structure where the doping concentration decreases with depth. Specifically, the P-type substrate has a doping concentration that is highest at the surface and decreases deeper into the substrate, allowing electrons generated deep in the substrate to be collected efficiently while maintaining good ohmic contact at the surface
2Productivity
If the depletion region is extended deeper to collect more photoelectrons, then quantum efficiency is improved, but dark current increases due to thermal generation in the highly doped P+-type substrate
Solution Approach 1:
The patent applies parameter changes by creating a doping concentration gradient where the doping level decreases with depth. This allows the depletion region to extend deeper into the substrate to collect more photoelectrons while the lower doping concentration in deeper regions reduces thermal generation of carriers, thereby minimizing dark current
3Productivity
If small pixel size is used to increase sensor resolution, then sensor density is improved, but color crosstalk increases as electrons generated by red light in the substrate diffuse laterally into neighboring photodiodes
Solution Approach 1:
The patent uses parameter changes by implementing a doping concentration gradient that decreases with depth. This creates an electric field that directs laterally diffusing electrons back toward their generation region, reducing color crosstalk between adjacent pixels while maintaining small pixel dimensions for high resolution
Solution Approach 2:
The patent applies asymmetry by creating an asymmetric doping profile where the doping concentration varies with depth rather than being uniform. This asymmetric structure creates directional electric fields that guide electron movement, preventing lateral diffusion into neighboring pixels while maintaining compact pixel geometry
4Reliability
If high doping concentration is used in the P+-type substrate to ensure good ohmic contact, then electrical contact quality is improved, but electrons generated in the substrate recombine quickly with holes, reducing photoelectron collection efficiency
Solution Approach 1:
The patent applies parameter changes by creating a doping concentration gradient where the doping level is highest at the surface for good ohmic contact and decreases with depth to reduce carrier recombination. This gradient structure allows both requirements to be satisfied simultaneously
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 approach effectively reduces color crosstalk and maintains high performance in small pixel CMOS sensors by ensuring red light-generated electrons are collected in the correct photodiodes, enhancing pixel efficiency and minimizing dark current.
Implementation Method 1
Incorporating a deep high energy Boron implantation under blue and green light-receiving pixels to create a potential barrier that directs red light-generated carriers into the 'red' photodiodes
Implementation Method 2
red light photons have low energy and penetrate a region deeper than the above region 104. More specifically, before generating any photoelectrons, the red light photons can penetrate to an interface 105
Data Source
AI summary
An image sensor comprises a substrate of a first conductivity type. First and second pixels are arrayed over the substrate. A potential barrier is formed in a region of the substrate corresponding to the first pixel but not in a region of the substrate corresponding to the second pixel. The second pixel is responsive to a color having a wavelength longer than the color to which the first pixel is responsive. The potential barrier is doped with dopants by a high energy ion implantation dopants or by an ion implantation or diffusion during epitaxial growth of the P-type epitaxial layer.