Back-Illuminated Imaging Sensor Dark Current Reduction
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Solution Overview
Problem
Back-illuminated semiconductor imaging devices face challenges in reducing dark current, which affects their sensitivity and efficiency due to the generation of carriers during periods without light exposure, leading to increased noise and non-uniform shading in images.
Innovation Solution
A back-illuminated imaging device using Semiconductor-on-insulator (SOI) substrates with a doping profile that has a maximum value at a predetermined distance from the insulator layer, decreasing monotonically on both sides, creating a potential barrier to prevent dark current carriers from reaching the front side, and an anti-reflection coating to enhance radiation absorption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a thin semiconductor layer with high electric field is provided to drive carriers quickly to the front side, then carrier collection efficiency is improved, but fabrication complexity increases due to additional backside treatment
Solution Approach 1:
The patent applies parameter changes by modifying the doping concentration profile within the semiconductor layer. By creating a specific gradient where doping concentration varies with depth, the invention generates an internal electric field that drives carriers to the front side without requiring additional backside treatment processes. This resolves the contradiction by achieving high carrier collection efficiency through material parameter optimization rather than process complexity.
2Measurement precision
If p-type or n-type dopant is implanted at the backside to create a doping gradient, then sensitivity is improved by increasing carrier drive to front side, but manufacturing cost increases due to custom processing
Solution Approach 1:
The invention uses parameter changes by establishing a controlled doping gradient during standard epitaxial growth. By adjusting doping concentrations at different stages of layer formation, the desired electric field profile is achieved using conventional semiconductor manufacturing processes. This eliminates the need for expensive custom backside implantation processes while maintaining high device sensitivity.
Solution Approach 2:
The doping gradient is built into the semiconductor layer during the epitaxial growth stage, which occurs before device fabrication. This preliminary action of pre-configuring the doping profile allows subsequent standard processing steps to achieve the desired carrier transport without requiring additional custom doping steps, thereby reducing manufacturing costs.
3Productivity
If doping concentration gradient is created to drive carriers toward front side, then charge collection efficiency is improved, but dark current generation increases
Solution Approach 1:
The patent applies local quality by creating a spatially varying doping concentration profile where different regions have different doping levels optimized for their specific functions. The gradient is designed so that regions closer to the back surface have doping concentrations that minimize dark current generation, while regions closer to the front maintain sufficient doping to drive carriers efficiently. This local optimization resolves the contradiction between charge collection and dark current suppression.
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 solution effectively reduces dark current, stabilizes quantum efficiency over time, and minimizes noise by preventing dark current carriers from penetrating to the front side, while maintaining high sensitivity and efficiency compatible with conventional semiconductor foundry processing.
Implementation Method 1
A general method that is employed to increase the sensitivity of a thinned back-illuminated imager is to implant p-type or n-type dopant at the backside and, with later heat treatments, create a dopant concentration profile which decreases in the direction toward the front side of the thin substrate. In the case of p-type doping, such doping concentration gradient gives rise to an electric field tending to drive light-generated electrons toward the front side.
Implementation Method 2
an anti-reflection coating to enhance radiation absorption
Data Source
AI summary
A method for fabricating a back-illuminated semiconductor imaging device on a semiconductor-on-insulator substrate, and resulting imaging device is disclosed. The device includes an insulator layer; a semiconductor substrate, having an interface with the insulator layer; an epitaxial layer grown on the semiconductor substrate by epitaxial growth; and one or more imaging components in the epitaxial layer in proximity to a face of the epitaxial layer, the face being opposite the interface of the semiconductor substrate and the insulator layer, the imaging components comprising junctions within the epitaxial layer; wherein the semiconductor substrate and the epitaxial layer exhibit a net doping concentration having a maximum value at a predetermined distance from the interface of the insulating layer and the semiconductor substrate and which decreases monotonically on both sides of the profile from the maximum value within a portion of the semiconductor substrate and the epitaxial layer. The doping profile between the interface with the insulation layer and the peak of the doping profile functions as a “dead band” to prevent dark current carriers from penetrating to the front side of the device.