Buried Light Shielding Structures in Image Sensor Pixels
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Solution Overview
Problem
Conventional image sensors suffer from elevated optical pixel crosstalk and degraded modulation transfer function due to stray light scattering at the silicon oxide/polysilicon and STI/silicon interfaces, leading to unintended light exposure in 'dark' regions between photodiodes.
Innovation Solution
The implementation of buried light shielding structures formed from conductive materials like tungsten, copper, or gold over the substrate between adjacent photodiodes to prevent stray light from entering these regions, thereby reducing crosstalk and improving pixel efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If light guides are formed in the dielectric stack to guide incoming light to photodiodes, then light guidance efficiency is improved, but stray light scattering at silicon oxide/polysilicon and STI/silicon interfaces causes elevated optical pixel crosstalk and degraded modulation transfer function
Solution Approach 1:
A light shield structure formed from conductive material (tungsten, copper, or gold) is introduced as an intermediary element between adjacent photodiodes. This light shield acts as a mediator that blocks stray light scattering at the silicon oxide/polysilicon and STI/silicon interfaces, preventing optical pixel crosstalk while allowing the light guides to function effectively. The light shield structure is formed using standard conductive material deposition processes, making it compatible with existing CMOS fabrication workflows.
2Device complexity
If conventional image sensor structures are used without additional light shielding, then device complexity is kept low, but optical pixel crosstalk increases and modulation transfer function degrades
Solution Approach 1:
The light shield structure is merged with the existing conductive material deposition processes used for forming pixel circuitry. The same conductive materials (tungsten, copper, or gold) and deposition techniques already employed for creating interconnects and transistor gates are utilized to form the light shield structures. This merging approach allows the light shielding function to be added without introducing entirely new fabrication processes, thereby minimizing the increase in device complexity while still improving modulation transfer function.
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 buried light shielding structures effectively redirect stray light back into the intended photodiodes, reducing optical pixel crosstalk and enhancing the modulation transfer function of image sensors.
Implementation Method 1
The buried light shielding structures effectively redirect stray light back into the intended photodiodes
Data Source
AI summary
A front-side illuminated image sensor with an array of image sensor pixels is provided. Each image pixel may include a photodiode, transistor gate structures, shallow trench isolation structures, and other associated pixel circuits formed in a semiconductor substrate. Buried light shielding structures that are opaque to light may be formed over regions of the substrate to prevent the transistor gate structures, shallow trench isolation structures, and the other associated pixel circuits from being exposed to stray light. Buried light shielding structures formed in this way can help reduce optical pixel crosstalk.