Back-Side Illuminated Image Sensor Transparent Electrode Dark Current Reduction
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Solution Overview
Problem
Back-side illuminated image sensors suffer from dark currents due to spontaneous electron-hole pair generation at crystal structure defects, particularly at interfaces between the substrate and insulating regions, leading to malfunctions and reduced sensitivity, especially in blue or ultraviolet wavelength ranges, and existing doping solutions alter sensor performance and transistor behavior.
Innovation Solution
A back-side illuminated image sensor with a transparent conductive electrode covering the entire rear surface, insulated from the substrate, and conductive regions extending perpendicularly from the front surface, where the conductive regions and electrode are biased to a different voltage than the substrate, reducing dark currents and enhancing sensitivity by preventing electron trapping at the substrate-insulating layer interface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If heavily-doped strips are provided at the interfaces between substrate and insulating regions to decrease dark currents, then dark currents are reduced, but sensor sensitivity is altered especially in blue or ultraviolet wavelength ranges
Solution Approach 1:
The patent applies local quality by providing heavily-doped strips only at specific locations (interfaces between substrate and insulating regions) where dark currents are generated, while keeping the rest of the substrate lightly-doped to maintain sensitivity. This localized doping approach targets the problem area without affecting the overall sensor performance.
Solution Approach 2:
The patent changes the doping parameter (doping concentration) from uniform to non-uniform distribution. Heavily-doped strips are created at critical interfaces to suppress dark currents, while the bulk substrate maintains a lighter doping level to preserve sensitivity in blue and ultraviolet wavelength ranges.
2Reliability
If heavily-doped strips are provided at the periphery of substrate portions to decrease dark currents, then dark currents are reduced, but transistor threshold voltage is inappropriately modified
Solution Approach 1:
The patent confines the heavy doping to narrow strips located precisely at the interfaces between the substrate and insulating regions. This localized approach ensures that the heavily-doped areas are restricted to where dark current suppression is needed, while the periphery of substrate portions containing transistors remains lightly-doped, thereby preserving transistor threshold voltage characteristics.
Solution Approach 2:
The patent segments the doping distribution into distinct regions: heavily-doped strips at insulating region interfaces and lightly-doped areas in transistor periphery regions. This segmentation allows independent optimization of dark current suppression and transistor performance in different spatial zones.
3Reliability
If implantation is provided on the rear surface side of the sensor to decrease dark currents, then dark currents are reduced, but manufacturing complexity and technical issues increase
Solution Approach 1:
Instead of performing implantation from the rear surface side of the substrate, the patent inverts the approach by providing heavily-doped strips through the front surface side. This inversion simplifies the manufacturing process by avoiding the technical issues associated with rear surface implantation, such as alignment difficulties and damage to the delicate rear surface structures.
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
Significantly decreases dark currents and improves sensor sensitivity across various wavelengths by preventing electron-hole pair generation at the substrate surfaces, while avoiding the need for heavily-doped regions on the rear surface, thus maintaining optimal performance without altering transistor behavior.
Implementation Method 1
preventing electron trapping at the substrate-insulating layer interface
Implementation Method 2
dark currents are due to the spontaneous random generation of electron-hole pairs at the level of certain defects of the crystal structure of the substrate
Data Source
AI summary
A back-side illuminated image sensor formed from a thinned semiconductor substrate, wherein: a transparent conductive electrode, insulated from the substrate by an insulating layer, extends over the entire rear surface of the substrate; and conductive regions, insulated from the substrate by an insulating coating, extend perpendicularly from the front surface of the substrate to the electrode.