BSI Image Sensor Sidewall Insulation for Dark Current Reduction
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Solution Overview
Problem
In back side illuminated (BSI) image sensor devices, imperfections at the interface between the metal ground and substrate lead to neutralization of BARC layers, causing dark currents and dark image non-uniformity issues, which impair device quality.
Innovation Solution
A sidewall insulating layer is formed around the metal shield ground trenches to electrically insulate it from the BARC layers, preventing unwanted charge neutralization and enhancing the trapping of defects by electric dipoles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a metal shield ground is formed directly on the substrate to release charges, then charge release function is improved, but interface imperfections cause neutralization of BARC layers leading to dark currents and DINU failures
Solution Approach 1:
An insulating layer is introduced as an intermediary between the metal shield ground and the substrate. This intermediary layer prevents direct contact that causes charge neutralization while still allowing the metal shield ground to perform its charge release function through the substrate, thereby eliminating dark currents and DINU failures without compromising reliability
Solution Approach 2:
The interface structure is segmented into distinct functional layers: the metal shield ground layer, the insulating layer, and the substrate. This segmentation separates the charge release function from the potential harmful direct interface contact, allowing each layer to perform its specific function independently while preventing the neutralization of BARC layers
2Reliability
If BARC layers are used to improve dark current and DINU quality, then charge trapping function is improved, but excess charges on metal shield ground neutralize the BARC layers destroying electric dipoles
Solution Approach 1:
The insulating layer acts as a protective intermediary that prevents excess charges on the metal shield ground from neutralizing the BARC layers. This maintains the electric dipoles in the BARC layers, preserving their charge trapping function and ensuring stable dark current and DINU quality
Solution Approach 2:
The insulating layer is formed in advance during the fabrication process, before the metal shield ground is deposited. This preliminary action ensures that the BARC layers are protected from charge neutralization from the outset, maintaining their compositional stability and charge trapping capability throughout subsequent processing
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 solution effectively reduces dark currents and improves image sensor quality by preventing the neutralization of BARC layers, thereby enhancing the performance and reliability of BSI image sensors.
Implementation Method 1
A sidewall insulating layer is formed around the metal shield ground trenches to electrically insulate it from the BARC layers
Implementation Method 2
When the BARC layers have negative (positive) charges accumulated, they attract positive (negative) charges in the substrate to the BARC/substrate interface to form electric dipoles. And the electric dipoles play the role of a charge barrier, trapping the imperfections or defects such as dangling bonds
Data Source
AI summary
Disclosed is a method of fabricating an image sensor device, such as a BSI image sensor, and so-fabricated image sensor, in which undesired neutralization of charges in BARC layers caused by opposite charges in metal shield grounds is prevented to reduce dark current and enhance device performance. The image sensor comprises a substrate having a plurality of radiation sensors formed adjacent its front surface, a first insulation layer formed over the back surface of the substrate, a BARC layer formed over the first insulation layer, a metal grid disposed over the BARC layer, one or more metal grounds extending from the metal ground into the substrate for grounding purpose, and a sidewall insulating layer disposed between the sidewall of each metal ground and the surrounding BARC layer. The sidewall insulating layer electrically insulates the metal grounds from the surrounding BARC layer.