CMOS Image Sensor Pixel Isolation for Photocharge Collection
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Solution Overview
Problem
Current CMOS image sensors face limitations in optical characteristics, particularly in the efficient generation and collection of photocharges due to the restricted area defined by device isolation layers, leading to reduced full well capacity and integration density.
Innovation Solution
The implementation of an epitaxial layer with varying doping concentrations and a device isolation layer structure that allows for increased photocharge generation across the entire pixel region, eliminating the need for ion implantation and enhancing integration density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If device isolation layers are used to define pixel regions, then cross-talk between pixels is prevented, but the area available for photocharge generation is reduced
Solution Approach 1:
The device isolation layer is configured to extend only partially through the substrate thickness, creating a three-dimensional isolation structure that does not fully traverse the substrate. This allows photocharges to be collected in regions below the isolation layer, effectively utilizing additional vertical space for photocharge generation while maintaining horizontal isolation between pixels.
Solution Approach 2:
The device isolation layer is segmented to extend only to a first depth from the first surface, rather than continuing through the entire substrate thickness. This segmentation creates distinct isolation zones that prevent cross-talk at the pixel boundaries while leaving the deeper regions available for photocharge collection, thus resolving the area loss problem.
2Quantity of substance
If the pixel region area is increased to improve full well capacity, then photocharge generation capacity is improved, but integration density is reduced
Solution Approach 1:
The invention utilizes the vertical dimension by having the device isolation layer extend only partially through the substrate, enabling photocharge collection in the deeper regions. This allows the horizontal pixel pitch to be reduced for higher integration density while maintaining adequate vertical space for photocharge generation and collection, thus improving both full well capacity and integration density simultaneously.
Solution Approach 2:
The partial-depth device isolation layer creates a nested structure where the isolation function is provided in the upper region while the lower region is available for photocharge collection. This nesting allows multiple functional zones to coexist within the same vertical column, increasing the effective photocharge generation volume without increasing the horizontal footprint.
3Manufacturing precision
If ion implantation is used to create doped regions, then precise doping control is achieved, but fabrication complexity and cost increase
Solution Approach 1:
The invention extracts and eliminates the ion implantation step from the fabrication process. Instead of using ion implantation to create doped regions, the patent relies on the epitaxial growth process to form the semiconductor layers with appropriate doping, thereby simplifying the fabrication process while maintaining the necessary electrical characteristics.
Solution Approach 2:
The epitaxial growth process inherently provides the necessary doping distribution through in-situ doping during layer formation. This self-service approach eliminates the need for separate ion implantation steps, as the doping is automatically incorporated during the epitaxial growth of the semiconductor layers, reducing fabrication complexity while maintaining precision.
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 improves the full well capacity and image quality by utilizing the entire pixel region for photocharge generation, simplifying the fabrication process and reducing costs while preventing cross-talk between pixel regions.
Implementation Method 1
a photodiode formed in the second pixel region and having a second polarity opposite to that of the first polarity to generate photocharges
Data Source
AI summary
A complementary metal-oxide-semiconductor (CMOS) image sensor is provided. The CMOS image sensor may include an epitaxial layer having a first conductivity type and having first and second surfaces, a first device isolation layer extending from the first surface to the second surface to define first and second pixel regions, a well impurity layer of a second conductivity type formed adjacent to the first surface and formed in the epitaxial layer of each of the first and second pixel regions, and a second device isolation layer formed in the well impurity layer in each of the first and second pixel regions to define first and second active portions spaced apart from each other in each of the first and second pixel regions.