CMOS Image Sensor Crosstalk Prevention via Deep Well Drainage
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Solution Overview
Problem
Conventional CMOS image sensors suffer from optical and electrical crosstalk due to the small volume of photoelectric transforming elements and high integration density, leading to low resolution and distorted images, as well as high dark current and noise.
Innovation Solution
A CMOS image sensor is designed with a deep well region in the photodiode guardring region and an N-type substrate to form a drain passageway for thermal electrons, along with an isolation structure that differentiates the depth of the photodiode region from the peripheral circuit region, preventing crosstalk and reducing dark current and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the degree of integration of a pixel is increased to improve resolution, then the resolution is improved, but the sensitivity is decreased due to small volume of photoelectric transforming elements
Solution Approach 1:
The patent extends the photodiode depth into the substrate to increase the photoelectric transforming volume without increasing the planar area. By utilizing the vertical dimension, the photodiode can capture more photons while maintaining high pixel density, thus improving sensitivity without sacrificing resolution.
Solution Approach 2:
The patent implements a nested structure where the photodiode is formed within a well region that is itself formed in the substrate. This nested configuration allows for optimized light collection volume while maintaining compact pixel structure for high integration density.
2Area of stationary object
If semiconductor devices are highly integrated to reduce unit cell area, then the unit cell area is reduced, but crosstalk is frequently generated due to short distance between adjacent pixels
Solution Approach 1:
The patent extracts and removes thermally generated electrons from regions outside the depletion region before they can diffuse into adjacent pixels. By providing dedicated collection paths for these electrons, the patent prevents crosstalk while maintaining narrow pixel spacing for high integration.
Solution Approach 2:
The patent introduces an intermediate well structure that acts as a mediator between adjacent photodiodes. This well region with extended depletion zone serves as a buffer that prevents direct interaction between neighboring pixels, thereby reducing crosstalk while allowing close pixel spacing.
3Productivity
If the gap between pixels is narrowed to increase integration density, then the integration density is increased, but the image sensor malfunctions due to crosstalk
Solution Approach 1:
The patent solves the crosstalk problem by transitioning from a two-dimensional isolation approach to a three-dimensional solution. By extending the depletion region vertically into the substrate, the patent creates effective isolation between pixels without requiring large horizontal gaps, thus enabling high integration density while preventing malfunction.
4Reliability
If an P type substrate is used to achieve good sensitivity, then the sensitivity is improved, but dark current and crosstalk characteristics are inferior
Solution Approach 1:
The patent changes the substrate type from P-type to N-type, which fundamentally alters the electrical characteristics. This parameter change reduces dark current generation and improves the collection efficiency of photo-generated carriers, while the associated well structures address crosstalk issues.
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
The solution effectively minimizes crosstalk, dark current, and noise, enhancing image resolution and sensitivity while allowing for high integration density, suitable for various digital systems.
Implementation Method 1
electrons which are thermally created outside of the depletion region in the semiconductor substrate
Implementation Method 2
The thermal electron is drained through the operational voltage terminal. The hole is drained through a ground terminal.
Implementation Method 3
The optical crosstalk is generated by the refraction of light which is incident to the cell through a micro lens on insulating interlayers having different refractivities or a layer having uneven surface.
Data Source
AI summary
In a method of manufacturing a CMOS image sensor, a P type epitaxial layer is formed on an N type substrate. A deep P+ type well layer is formed in the P type epitaxial layer. An N type deep guardring well is formed in a photodiode guardring region. The N type deep guardring region makes contact with the N type substrate and also be connected with an operational voltage terminal. A triple well is formed in a photodiode region and a peripheral circuit region. The triple well is used for forming a PMOS and an NMOS having different operational voltages. An isolation region is formed in the photodiode region. The isolation region in the photodiode region has a depth different from a depth of an isolation region in the peripheral circuit region.