Backside Illuminated Image Sensor Trench Isolation
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Solution Overview
Problem
Backside illuminated image sensors face excessive carrier loss and crosstalk due to charge storage regions being distant from the sensor surface, leading to reduced performance.
Innovation Solution
A process involving the formation of backside trenches with field and well isolation implants, antireflective layers, and a temporary carrier wafer to enhance carrier collection and reduce crosstalk, utilizing a silicon-on-insulator or epitaxial wafer with specific doping and layer structures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the silicon substrate is thinned or removed to enable backside illumination, then fill factor and quantum efficiency are improved, but charge storage regions are located far from the backside surface causing excessive carrier loss
Solution Approach 1:
The patent introduces a new spatial dimension by forming raised structures (islands) on the backside surface that extend vertically into the substrate. This dimensional change allows charge storage regions to be positioned closer to the backside illumination surface without compromising substrate integrity, thereby reducing carrier loss while maintaining the benefits of backside illumination.
Solution Approach 2:
The patent embeds charge storage regions within raised structures that are themselves embedded in the substrate. This nested configuration allows the charge storage regions to be positioned optimally close to the backside surface for efficient carrier collection, while the surrounding substrate and isolation structures provide mechanical support and electrical isolation.
2Loss of energy
If charge storage regions are positioned closer to the backside surface to reduce carrier loss, then carrier collection is improved, but crosstalk between adjacent photodiodes increases
Solution Approach 1:
The patent divides the substrate into isolated regions by forming trenches between adjacent raised structures. These trenches are filled with dielectric material to create electrical isolation, effectively segmenting the continuous substrate into discrete, electrically isolated islands. This segmentation prevents crosstalk between adjacent photodiodes while allowing charge storage regions to be positioned close to the backside surface.
Solution Approach 2:
The patent introduces dielectric material as an intermediary substance filling the trenches between raised structures. This intermediary layer provides electrical isolation between adjacent charge storage regions, preventing crosstalk while allowing the raised structures to maintain their optimal positioning for carrier collection.
3Object-generated harmful factors
If isolation trenches are filled with dielectric material to reduce crosstalk, then crosstalk is reduced, but manufacturing complexity increases
Solution Approach 1:
The patent combines multiple fabrication steps into an integrated process flow. The trenches are formed and filled with dielectric material using standard CMOS-compatible processes that are merged with the existing photodiode fabrication sequence. This merging approach reduces manufacturing complexity by utilizing established process modules rather than introducing entirely new fabrication techniques.
Solution Approach 2:
The patent optimizes the depth and width parameters of the trenches to achieve effective isolation with minimal additional processing. By carefully controlling these geometric parameters, the patent achieves crosstalk reduction using shallow trenches that require less complex filling and planarization processes, thereby reducing overall manufacturing complexity.
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 process significantly reduces carrier recombination and crosstalk, improving the image sensor's ability to detect incident light without increasing die size or cost, thus enhancing performance.
Implementation Method 1
A P-type implant is performed through a mask layer to form backside isolation regions
Implementation Method 2
An antireflective coating is formed over the backside surface of the sensor layer
Implementation Method 3
each pixel typically comprises a photodiode... the incident light is no longer impacted by metallization level interconnects and other features of the dielectric layers, and fill factor and quantum efficiency are improved
Data Source
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AI summary
A backside illuminated image sensor comprises a sensor layer implementing a plurality of photosensitive elements (1100) of a pixel array, an oxide (800) layer adjacent a backside surface of the sensor layer, and at least one dielectric layer (1200) adjacent a front side surface of the sensor layer. The sensor layer further comprises a plurality of backside trenches formed in the backside surface of the sensor layer and arranged to provide isolation between respective pairs of the photosensitive elements. The backside trenches have corresponding backside field isolation implant regions formed in the sensor layer, and the resulting structure provides reductions in carrier recombination and crosstalk between adjacent photosensitive elements. The image sensor may be implemented in a digital camera or other type of digital imaging device.