CMOS Image Sensor P-Type Isolation Layer Crosstalk Reduction
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Solution Overview
Problem
The miniaturization of image sensors increases the likelihood and magnitude of optical and electrical crosstalk between neighboring photosensitive elements, leading to reduced image quality and increased dark current.
Innovation Solution
The use of a highly resistive N-doped semiconductor layer with specific dopant concentrations and biased electrodes to reduce electrical crosstalk and dark current, while maintaining the full well capacity and reducing blooming effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the distance between neighboring photosensitive elements is decreased to increase resolution, then higher resolution is achieved, but optical and electrical crosstalk between photosensitive elements increases
Solution Approach 1:
A P-type isolation layer is introduced as an intermediary structure between neighboring photosensitive elements (N-type photodiodes). This isolation layer acts as a barrier that blocks electrical crosstalk and reduces optical interference, allowing pixels to be placed closer together while maintaining signal integrity and reducing dark current.
Solution Approach 2:
The patent applies different doping types and resistivity characteristics to different regions: N-type high-resistivity material for the photodiode active region to maximize photoelectron generation, and P-type material for the isolation layer to provide electrical isolation. This local differentiation of material properties enables simultaneous optimization of sensitivity and isolation.
2Use of energy by moving object
If miniaturization is pursued to reduce sensor size, then lower power consumption and increased integration are achieved, but crosstalk between photosensitive elements increases
Solution Approach 1:
The P-type isolation layer serves as a mediator that enables miniaturization by providing electrical isolation between closely spaced pixels. This allows the sensor to achieve higher integration density and lower power consumption per pixel while preventing crosstalk that would otherwise increase with reduced pitch.
3Measurement precision
If the concentration of dopant in photosensitive elements is increased to improve sensitivity, then photoelectron generation is enhanced, but dark current increases
Solution Approach 1:
The patent employs N-type high-resistivity material in the photodiode region to maximize photoelectron generation while maintaining low dark current through high material purity and optimized doping concentration. The P-type isolation layer provides additional electrical isolation to prevent dark current leakage between pixels, allowing each pixel to operate with optimal doping levels.
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 minimizes electrical crosstalk between adjacent pixels, enhances image quality, and reduces dark current, thereby improving the overall performance of image sensors.
Implementation Method 1
forming a P-type isolation layer between two adjacent N-type photodiodes
Implementation Method 2
the P-type isolation layer... effectively minimizes electrical crosstalk between adjacent pixels
Implementation Method 3
Each photosensitive element absorbs a portion of the incident image light. Each photosensitive element included in the image sensor, such as photodiodes, generates image charges upon absorption of the image light
Data Source
AI summary
An imaging sensor pixel comprises a highly resistive N− doped semiconductor layer with a front side and a back side. At the front side, there are at least a light sensing region, a transfer gate adjacent to the light sensing region and a P-well region. The P-well region surrounds the light sensing region and the transfer gate region, and comprises at least a floating diffusion region and a first electrode outside of the floating diffusion region, wherein a first negative voltage is applied to the first electrode. The transfer gate couples between the light sensing region and the floating diffusion region. At the back side, there is a back side P+ doped layer comprising a second electrode formed on the back side P+ doped layer, wherein a second negative voltage is applied to the second electrode. The second negative voltage is more negative than the first negative voltage.