Backside Deep Trench Isolation with Metal Grid to Prevent Void Pinch-Off
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Solution Overview
Problem
As pixel sizes in CMOS image sensors decrease, manufacturing becomes increasingly difficult due to challenges in limiting crosstalk between pixels and forming a back side deep trench isolation (BDTI) structure without voids, which affects optical performance.
Innovation Solution
A method involving a second mask and etch process to form narrower BDTI trenches, filling the bottom portion of the first trenches with a lower fill material, and using a substrate-embedded metal grid that penetrates through passivation layers to reduce crosstalk and improve quantum efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel sizes are made smaller to increase resolution, then image sensor performance is improved, but manufacturing difficulty increases and crosstalk between pixels worsens
Solution Approach 1:
The patent divides the isolation structure into multiple segments: a first isolation structure formed in trenches extending from the back surface, and a second isolation structure formed in shallower trenches. This segmentation allows each isolation structure to address specific crosstalk regions, enabling effective pixel isolation even at smaller pixel sizes while maintaining manufacturability through modular processing steps.
2Object-affected harmful factors
If a back side deep trench isolation (BDTI) structure is formed to reduce crosstalk, then pixel isolation is improved, but voids form in the trench structure affecting optical performance
Solution Approach 1:
Instead of forming a single deep trench isolation structure that extends through the entire substrate thickness, the patent applies partial action by creating two isolation structures with different depths. The first isolation structure addresses deeper crosstalk paths, while the second isolation structure addresses shallower paths, collectively providing complete isolation without the void formation problems associated with a single excessive-depth trench.
3Object-affected harmful factors
If a single deep trench isolation structure is formed, then crosstalk reduction is achieved, but the aspect ratio becomes too high causing manufacturing difficulties
Solution Approach 1:
The patent segments the single deep trench isolation into two separate isolation structures formed at different depths. This segmentation reduces the aspect ratio of each individual trench, making them manufacturable with standard processing equipment while collectively providing the crosstalk isolation equivalent to a single deep structure.
4Measurement precision
If narrower BDTI trenches are formed to reduce pixel area, then quantum efficiency is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent uses partial action by forming two isolation structures with different trench widths and depths. The first isolation structure uses narrower trenches to minimize pixel area and maximize quantum efficiency, while the second isolation structure uses wider trenches that are easier to manufacture with standard precision, collectively achieving both high quantum efficiency and manufacturability.
Data Source
AI summary
A process of forming a back side deep trench isolation structure for an image sensing device includes etching first trenches in the back side of a semiconductor substrate, lining the first trenches with dielectric, depositing passivation layers over and within the first trenches, and etching second trenches through the passivation layers into the first trenches, and filling the second trenches to form a substrate-embedded metal grid. Optionally, the bottoms of the first trenches are filled by depositing and etching a lower fill material prior to depositing the passivation layers. The method prevents the passivation layers from pinching off in a way that causes voids within the first trenches. The result is better optical performance such as increased quantum efficiency and reduced crosstalk.