Trench Isolation Structure for CMOS Image Sensor Passivation
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Solution Overview
Problem
CMOS image sensors face challenges in achieving optimal passivation of the substrate, leading to subpar white pixel performance due to insufficient negative charge from high-k dielectric layers, resulting in lower flat band voltage and higher interface trap density.
Innovation Solution
Incorporating a trench isolation structure with a first negatively charged isolation layer, composed of aluminum oxide with a thickness of about 100 angstroms or more, and omitting hafnium oxide, along with hydrogen atoms at the interface to reduce interface trap density, enhances substrate passivation and improves white pixel performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional trench isolation structures are used, then manufacturing process is simple, but white pixel performance is poor due to insufficient passivation
Solution Approach 1:
The trench isolation structure is divided into multiple distinct layers: a first isolation layer (aluminum oxide) with large thickness and negative charge for passivation, and a second isolation layer (silicon dioxide) for filling the trench. This segmentation allows each layer to perform its specific function optimally, improving white pixel performance while maintaining manufacturing feasibility through sequential deposition processes.
Solution Approach 2:
Different regions of the trench isolation structure have different material compositions and properties. The first isolation layer uses aluminum oxide with specific electrical characteristics (large thickness, negative charge) to address passivation needs at the interface, while the second layer uses silicon dioxide for its insulating properties to complete the trench filling. This local differentiation of material properties optimizes overall performance.
2Reliability
If hafnium oxide is included in the isolation structure, then material options are increased, but passivation effectiveness decreases due to positive charge
Solution Approach 1:
The patent deliberately excludes hafnium oxide from the trench isolation structure, extracting this material option from consideration. By removing hafnium oxide (which has positive charge) from the isolation layers, the structure avoids the harmful effect of reduced passivation effectiveness, while still achieving the necessary electrical characteristics through the combination of aluminum oxide and silicon dioxide layers.
Solution Approach 2:
The patent changes the material parameters of the isolation structure by selecting aluminum oxide with large thickness and negative charge for the first layer, and silicon dioxide for the second layer. This parameter selection (material composition, thickness, charge characteristics) optimizes the passivation effectiveness by ensuring the overall structure has sufficient negative charge to counteract interface traps, while maintaining versatility in material selection through the use of two different dielectric materials.
3Reliability
If interface trap density is high, then manufacturing is easier, but flat band voltage is affected and performance decreases
Solution Approach 1:
The first isolation layer of aluminum oxide is deposited with large thickness before the second isolation layer is added. This preliminary action of creating a thick negatively charged layer first establishes strong passivation at the substrate interface, reducing interface trap density and improving flat band voltage before the trench filling is completed. This sequential approach ensures that passivation is established early in the manufacturing process.
Solution Approach 2:
The trench isolation structure uses a composite material system combining aluminum oxide and silicon dioxide in specific layers. The aluminum oxide layer provides strong passivation with negative charge to reduce interface traps, while the silicon dioxide layer provides additional insulation and completes the trench filling. This composite approach achieves both improved flat band voltage and manufacturability by using materials that can be deposited using standard semiconductor manufacturing techniques.
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 solution effectively increases the flat band voltage, reduces undesired electrons, and improves white pixel performance by inducing a large positive charge in the substrate and reducing interface trap density, thereby enhancing the overall image sensor performance.
Implementation Method 1
improves a passivation of the substrate and a white pixel performance of the image sensor
Implementation Method 2
the first isolation layer is negatively charged... a large positive charge may be induced in the substrate
Implementation Method 3
Hydrogen (e.g., H2) atoms are arranged along an interface between the substrate and the first isolation layer
Implementation Method 4
reduce trap density at an interface between the substrate and the first isolation layer
Data Source
AI summary
The present disclosure relates to an image sensor comprising a substrate. A photodetector is in the substrate. A trench is in the substrate and is defined by sidewalls and an upper surface of the substrate. A first isolation layer extends along the sidewalls and the upper surface of the substrate that define the trench. The first isolation layer comprises a first dielectric material. A second isolation layer is over the first isolation layer. The second isolation layer lines the first isolation layer. The second isolation layer comprises a second dielectric material. A third isolation layer is over the second isolation layer. The third isolation layer fills the trench and lines the second isolation layer. The third isolation layer comprises a third material. A ratio of a first thickness of the first isolation layer to a second thickness of the second isolation layer is about 0.17 to 0.38.