Multi-patterned Isolation Well for CMOS Image Sensor Crosstalk
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Solution Overview
Problem
In state-of-the-art CMOS image sensors, as pixel pitch decreases into the sub-micron range, lithography misalignment and nonuniform photoresist thickness lead to distortion in the patterned photoresist and implanted isolation wells, resulting in critical performance variations between pixels and sensor chips, affecting full-well capacity and image resolution.
Innovation Solution
Implementing a multi-patterned isolation well formation using two or more photoresist patterning phases and corresponding dopant implantation operations, with each phase's street lines spaced and staggered by an integer multiple of the pixel pitch, to create a composite isolation well structure that reduces photoresist feature distortion and ensures uniform dopant implantation across the pixel array.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If single-mask isolation well patterning is used, then device complexity is reduced, but manufacturing precision deteriorates due to lithography misalignment and photoresist distortion
Solution Approach 1:
The single patterning step is segmented into multiple sequential patterning phases (first phase, second phase, third phase), where each phase creates a portion of the final isolation well pattern. This segmentation allows each individual patterning step to work at relaxed pitch requirements, reducing lithography misalignment and photoresist distortion, thereby improving manufacturing precision while accepting increased process complexity
Solution Approach 2:
The patent transitions from a two-dimensional single-mask approach to a multi-dimensional sequential patterning approach, where patterns are built up through multiple phases with different orientations and pitch requirements. Each phase operates in its own dimensional space with optimized pitch, allowing the final composite pattern to achieve sub-micron precision that would be impossible in a single 2D patterning step
2Measurement precision
If pixel pitch is reduced into sub-micron range, then image sensor resolution is improved, but manufacturing precision deteriorates due to photoresist distortion
Solution Approach 1:
The isolation well patterning is segmented into multiple phases, each creating patterns at relaxed pitch requirements. The first phase creates patterns at one pitch, the second phase at another pitch, and the third phase at a third pitch. This segmentation allows the final sub-micron pattern to be constructed from larger, more uniformly manufacturable features, improving manufacturing precision while enabling higher image resolution
Solution Approach 2:
The patent changes the pitch parameter across different patterning phases, using different pitch values for each phase (first pitch, second pitch, third pitch). This parameter variation allows each phase to operate at optimal manufacturing conditions for its specific pitch requirement, reducing photoresist distortion and improving pattern uniformity while achieving the desired sub-micron resolution
3Reliability
If thick photoresist is applied to prevent dopant penetration, then reliability is improved, but manufacturing precision deteriorates due to photoresist feature distortion
Solution Approach 1:
The single thick photoresist application is segmented into multiple thinner photoresist applications across different phases. Each phase uses a photoresist layer optimized for its specific pitch requirement, avoiding the need for excessively thick photoresist that causes distortion. This segmentation maintains reliable dopant containment while improving pattern uniformity
Solution Approach 2:
The patent employs periodic patterning actions with different pitch requirements in sequence. Each periodic phase uses photoresist optimized for that specific phase's pitch, preventing the accumulation of distortion that would occur with a single thick photoresist layer. This periodic approach maintains both reliability and manufacturing 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 enhances image sensor performance by increasing full-well capacity, reducing pixel-to-pixel cross-talk, and improving the modulation transfer function, leading to more uniform and precise image production with reduced chip-level variation.
Implementation Method 1
p-doped wells typically formed through p-type ion implantation at distributed energies up to 2000 keV or higher
Data Source
AI summary
Pixel isolation wells in a semiconductor image sensor are implemented via two or more photoresist patterning phases and two or more corresponding dopant implantation operations. A distinct photomask is applied in each patterning phase with the isolation-well street lines patterned by each mask spaced from one another by an integer multiple (i.e., 2 or greater) of the pixel pitch, and patterns formed by respective masks being staggered by the pixel pitch.