Backside Image Sensor Isolation Structure for Leakage Current Suppression
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Solution Overview
Problem
In backside illuminated image sensors, the reduction in pixel size leads to increased risk of leakage current during charge transfer from the photodiode to the floating diffusion due to reduced inter-regional distances, which can result in charge loss to the ground region and transfer gate.
Innovation Solution
The image sensor employs a multilayer structure with a photodiode in a lower layer and floating diffusion in an upper layer, surrounded by a trench with a gate insulation film that encloses the transfer channel, blocking the shortest path and bypass paths to suppress leakage current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pixel size is reduced to increase integration density, then productivity and device compactness are improved, but leakage current increases due to reduced inter-regional distances between transfer channel and ground region
Solution Approach 1:
An insulation film is introduced as an intermediary element between the transfer channel and the ground region. This insulation film acts as a mediator that blocks the leakage current path without interfering with the normal charge transfer function, thereby resolving the contradiction between reduced pixel size and increased leakage current.
Solution Approach 2:
The space between the transfer channel and ground region is segmented by introducing an insulation film structure. This segmentation divides the continuous conductive path into isolated regions, preventing charge leakage while maintaining the compact pixel structure necessary for high integration density.
2Productivity
If inter-regional distance is reduced to increase integration density, then productivity is improved, but reliability deteriorates due to charge leakage to ground region and transfer gate
Solution Approach 1:
The insulation film serves as a protective intermediary that ensures reliable charge transfer by blocking unintended charge leakage paths. It allows the system to maintain short inter-regional distances for high integration density while preserving charge transfer accuracy through the insulation barrier.
Solution Approach 2:
The insulation film is positioned in advance to block potential leakage current paths before charge transfer occurs. This preliminary protective measure prevents charge leakage to the ground region and transfer gate, ensuring reliable operation even at reduced pixel sizes.
3Reliability
If insulation structure is added to block leakage current, then reliability is improved, but device complexity increases
Solution Approach 1:
The insulation film is applied locally only where needed - specifically between the transfer channel and ground region - rather than throughout the entire device. This localized approach suppresses leakage current at critical points while minimizing the overall structural complexity and material usage.
Solution Approach 2:
The insulation film utilizes the vertical dimension by being formed in a lower layer beneath the transfer channel. This three-dimensional arrangement provides effective leakage current blocking without increasing planar footprint or significantly complicating the device structure, as the insulation function is achieved through layer stacking rather than lateral expansion.
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 configuration effectively reduces leakage current by ensuring isolation between the transfer channel and ground region, preventing charge loss and enhancing charge transfer efficiency.
Implementation Method 1
An upper end of the transfer channel is connected to the floating diffusion. An insulation film blocks at least a shortest path between the transfer channel and the ground region
Data Source
AI summary
An image sensor includes a photodiode, a floating diffusion, a ground region, a transfer channel, and an insulation film. The photodiode is formed in a lower layer of a substrate. The floating diffusion and the ground region are formed in or on top of an upper layer of the substrate. The transfer channel has a lower end connected to the photodiode. An upper end of the transfer channel is connected to the floating diffusion. The insulation film blocks at least a shortest path between the transfer channel and the ground region