Photodiode Surface Layer Segmentation for Dark Current Reduction
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Solution Overview
Problem
Semiconductor devices, particularly CMOS image sensors, face challenges in reducing dark current and white defects during dark time due to surface-related issues, which existing manufacturing methods have not adequately addressed.
Innovation Solution
The semiconductor device incorporates a photodiode with a charge storage layer and a transfer transistor, featuring a surface layer with distinct sub-regions of varying impurity concentrations, where the low-concentration sub-region is closer to the gate electrode than the high-concentration sub-region, to suppress electron generation and enhance charge transfer efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a uniform impurity concentration is used in the surface layer, then the manufacturing process is simple, but dark current and white defects increase due to surface-related issues
Solution Approach 1:
The surface layer is divided into two sub-regions with different impurity concentrations: a first sub-region with lower impurity concentration and a second sub-region with higher impurity concentration. This local differentiation allows the first sub-region to reduce surface-related dark current while the second sub-region maintains adequate charge transfer efficiency, resolving the contradiction between reliability and uniform manufacturing.
2Reliability
If the low-concentration sub-region is positioned closer to the charge storage layer, then surface-related dark current is reduced, but charge transfer efficiency may be compromised
Solution Approach 1:
Different sub-regions are assigned different impurity concentrations based on their functional requirements. The first sub-region closer to the charge storage layer uses lower impurity concentration to minimize surface defects and dark current, while the second sub-region uses higher impurity concentration to ensure adequate charge transfer efficiency, achieving both goals simultaneously.
Solution Approach 2:
The surface layer is segmented into multiple sub-regions with gradient impurity concentrations rather than using a uniform structure. This segmentation allows optimization of each sub-region's properties for its specific functional role, with the lower-concentration region handling surface-related reliability and the higher-concentration region supporting charge transfer precision.
3Manufacturing precision
If high impurity concentration is used throughout the surface layer, then charge transfer efficiency is maintained, but dark current and white defects increase
Solution Approach 1:
Instead of uniformly high impurity concentration, the invention uses locally differentiated concentrations where the first sub-region has lower impurity concentration to suppress dark current generation at the surface, while the second sub-region maintains higher impurity concentration to ensure charge transfer efficiency, thus eliminating the need for uniform high concentration.
Data Source
AI summary
Provided is a semiconductor device with improved performance. The semiconductor device includes a photodiode having a charge storage layer (n-type semiconductor region) and a surface layer (p-type semiconductor region), and a transfer transistor having a gate electrode and a floating diffusion. The surface layer (p-type semiconductor region) of a second conductive type formed over the charge storage layer (n-type semiconductor region) of a first conductive type includes a first sub-region having a low impurity concentration, and a second sub-region having a high impurity concentration. The first sub-region is arranged closer to the floating diffusion than the second sub-region.