BSI Image Sensor Isolation Structure for Crosstalk and Dark Current
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Solution Overview
Problem
Backside illumination (BSI) image sensors face challenges in light-receiving efficiency and dark current characteristics due to insufficient depth of isolation films formed on the rear side of the substrate, leading to crosstalk and blooming phenomena.
Innovation Solution
The formation of isolation films, including a first oxide film that extends along a substrate trench and penetrates the substrate, improves light-receiving efficiency by refracting or reflecting light and preventing charge drift between pixels, while also simplifying the fabrication process by being formed at the same level as other isolation films.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If isolation films are formed on the rear side of the substrate in conventional BSI image sensors, then light-receiving efficiency is improved, but dark current characteristics deteriorate due to insufficient depth of isolation films
Solution Approach 1:
The isolation film structure transitions from a conventional planar configuration to a three-dimensional tapered structure that extends through the substrate thickness. The film width decreases from the rear surface toward the front surface, creating a depth-dependent isolation effect that simultaneously improves light reception at the rear surface while blocking charge drift paths near the front surface.
Solution Approach 2:
The isolation film exhibits spatially varying properties: wider at the rear surface for optimal light interaction and progressively narrower toward the front surface for effective charge isolation. This local variation in film dimensions allows the same structure to fulfill multiple conflicting functions - enhancing light reception where needed while preventing dark current where critical.
2Reliability
If isolation films are formed to sufficient depth to prevent crosstalk and blooming, then dark current characteristics improve, but manufacturing complexity increases
Solution Approach 1:
The isolation film is formed with a controlled width parameter that changes continuously through the substrate thickness. By adjusting the taper angle and penetration depth parameters during fabrication, the film achieves sufficient isolation effect without requiring excessive depth or complex multi-layer structures. The width parameter transitions from a constant value to a depth-dependent variable, optimizing both performance and manufacturability.
3Use of energy by moving object
If isolation films extend deeper into the substrate to refract and reflect light effectively, then light-receiving efficiency improves, but fabrication precision requirements increase
Solution Approach 1:
The tapered isolation film structure is formed as a preliminary feature during the substrate processing sequence, before final device assembly. By establishing the correct film profile early in fabrication, subsequent processing steps can proceed with standard precision requirements rather than needing to accommodate complex deep-structure formation, thereby reducing overall manufacturing precision demands.
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 the light-receiving efficiency of BSI image sensors, reduces dark current, and addresses crosstalk and blooming issues, resulting in improved image sensor performance and productivity.
Implementation Method 1
improves light-receiving efficiency by refracting or reflecting light
Implementation Method 2
improves light-receiving efficiency by refracting or reflecting light
Implementation Method 3
preventing charge drift between pixels
Data Source
AI summary
An image sensor with improved performance, and a method of fabricating the same are provided. The image sensor includes a sensor array region and a pad region, which is disposed outside the sensor array region, the image sensor comprising a first substrate including a first surface, upon which light is incident, and a second surface, which is opposite to the first surface, a first isolation film in the first substrate at the sensor array region, the first isolation film defining a plurality of unit pixels, a second substrate including a third surface, which faces the second surface of the first substrate, and a fourth surface, which is opposite to the third surface, a wiring structure between the second and third surfaces, the wiring structure including an interlayer insulating film and a wiring in the interlayer insulating film, a pad trench in the pad region, the pad trench exposing the wiring through the first substrate, a bonding terminal in the pad trench, the bonding terminal being connected to the wiring, and a second isolation film in the first substrate at the pad region, the second isolation film being adjacent to the pad trench, wherein widths of each of the first and second isolation films decrease in a direction from the second surface to the first surface.