Image Sensor Trench Isolation for Photoelectron Trapping
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Image sensors suffer from photoelectron loss due to trap sites on the interfacial surface between thermal oxide layers and substrates, leading to reduced light sensitivity and fill factor of photodiodes.
Innovation Solution
The method involves forming selective epitaxial growth (SEG) channel stop layers doped with boron within trenches to prevent photoelectrons from being trapped at these sites, using a shallow trench isolation process, and filling the trenches with high-density plasma oxide to enhance the separation of photodiodes from trap sites.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thermal oxide layers are formed on trenches through rounding oxidation process, then inner surfaces of trenches are cured, but trap sites still exist on interfacial surface between thermal oxide layers and substrate causing photoelectron loss
Solution Approach 1:
The harmful interfacial surface between thermal oxide layer and substrate is removed by eliminating the need for rounding oxidation process. The patent forms trenches with vertical walls and directly fills them with isolation material, extracting the problematic thermal oxide layer formation step that created trap sites on curved interfaces.
Solution Approach 2:
Instead of forming rounded trenches and then filling them (which creates interface trap sites), the patent inverts the approach by forming vertical-walled trenches and directly filling them with isolation material, eliminating the thermal oxide layer formation step that caused the harmful effect.
2Reliability
If boron is implanted into sidewall and bottom surfaces of trenches to form NCST impurity regions, then photoelectrons are prevented from being trapped in trap sites, but horizontal portion of NCST impurity regions has thickness of 0.1-0.2 μm reducing photodiode area
Solution Approach 1:
The patent extracts and eliminates the need for NCST impurity regions by removing the source of trap sites (thermal oxide layers on rounded trenches). Since vertical-walled trenches filled with isolation material do not create interface trap sites, the additional boron implantation step and associated horizontal impurity regions are no longer necessary, preserving photodiode area.
3Stability of the object's composition
If rounding oxidation process is performed to form thermal oxide layers on trenches, then inner surfaces are cured, but light characteristic is deteriorated due to reduced photodiode area
Solution Approach 1:
The patent removes the rounding oxidation process entirely, extracting the thermal oxide layer formation step that consumed photodiode area. Vertical-walled trenches provide sufficient structural stability without requiring thermal oxide curing, thus preserving light characteristics and photodiode area.
4Ease of manufacture
If shallow trench isolation process is used to form trenches in substrate, then isolation structure is created, but micro defects result on inner walls of trenches due to damages during etching
Solution Approach 1:
The patent changes the etching parameters to achieve vertical walls instead of rounded walls. By adjusting etching conditions (such as using selective etchants or controlling etch depth), vertical-walled trenches are formed that reduce micro defects on inner walls while maintaining the isolation function, eliminating the need for subsequent rounding oxidation.
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 reduces photoelectron loss and increases the fill factor of photodiodes by minimizing trap sites and improving light sensing efficiency.
Implementation Method 1
forming selective epitaxial growth (SEG) channel stop layers doped with boron within trenches
Implementation Method 2
filling the trenches with high-density plasma oxide
Data Source
AI summary
An image sensor includes a trench formed by a shallow trench isolation (STI) process, a channel stop layer formed over a substrate in the trench, an isolation structure filled in the trench, and a photodiode formed in the substrate adjacent to a sidewall of the trench. In more detail of the image sensor, a trench is formed in a substrate through a STI process, and a channel stop layer is formed over the substrate in the trench. An isolation structure is formed in the trench, and a photodiode is fanned in the substrate adjacent to a sidewall of the trench.