CMOS Image Sensor Overflow Drain Segmentation
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Solution Overview
Problem
In CMOS image sensors, the configuration with a photodiode (PD) and overflow drain (OFD) at the back surface of a silicon substrate faces challenges with potential level misalignment and impurity concentration differences, leading to decreased saturation charge and color mixing between pixels.
Innovation Solution
A solid-state imaging device with a charge retention part, an OFD, and a potential barrier, where the OFD includes low and high concentration diffusion layers of the same type, formed at a distance, and a vertical transistor is used to manage charge overflow, preventing potential level changes and color mixing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the OFD is formed as a high concentration diffusion layer close to the potential barrier, then the overflow function is enhanced, but the potential level of the potential barrier changes greatly leading to Qs decrease and color mixing
Solution Approach 1:
The OFD is segmented into two distinct diffusion layers: a first diffusion layer with lower impurity concentration and a second diffusion layer with higher impurity concentration. This segmentation allows each layer to perform its specific function - the first layer maintains stable potential for charge retention, while the second layer provides strong overflow capability - thereby resolving the contradiction between overflow function and potential level stability.
Solution Approach 2:
Different regions of the OFD are assigned different impurity concentrations to achieve different local functions. The first diffusion layer (lower concentration) is positioned where stable potential is needed, while the second diffusion layer (higher concentration) is positioned where strong overflow is needed. This local differentiation resolves the contradiction by optimizing each region for its specific purpose.
2Productivity
If the distance between the OFD and the potential barrier is reduced, then the overflow efficiency is improved, but misalignment or impurity concentration differences cause great changes in potential barrier level
Solution Approach 1:
The OFD is divided into two diffusion layers with different impurity concentrations. The first layer (lower concentration) can be positioned closer to the potential barrier to enable efficient overflow, while the second layer (higher concentration) provides a buffer that stabilizes the potential barrier level, thus resolving the contradiction between overflow efficiency and charge retention reliability.
Solution Approach 2:
The first diffusion layer with lower impurity concentration acts as an intermediary between the potential barrier and the second high-concentration diffusion layer. It mediates the interaction by providing a transition zone that allows efficient charge overflow while preventing the high-concentration second layer from causing excessive potential barrier level changes.
3Device complexity
If a single high concentration OFD is used, then the structure is simple, but Qs decreases and color mixing occurs due to potential level changes
Solution Approach 1:
Instead of using a single high-concentration OFD, the structure is segmented into two diffusion layers with different impurity concentrations. This segmentation maintains structural simplicity while improving manufacturing precision by distributing the functions across two layers, preventing Qs decrease and color mixing.
Solution Approach 2:
The OFD is constructed as a composite structure with two diffusion layers having different impurity concentrations. This composite approach combines the advantages of both low and high concentration regions, achieving stable saturation charge amount while maintaining a relatively simple overall structure.
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 ensures stable charge overflow and prevents saturation charge reduction and color mixing, maintaining image quality by isolating the high concentration OFD from the potential barrier and using a separate route for charge discharge.
Implementation Method 1
a charge is generated in response to incident light through photoelectric conversion by a PD (Photodiode)
Implementation Method 2
a potential barrier that becomes a barrier of the charge that flows from the charge retention part to the OFD
Data Source
AI summary
The present technology relates to a solid-state imaging device and an electronic apparatus that perform a stable overflow from a photodiode and prevent Qs from decreasing and color mixing from occurring. A solid-state imaging device according to an aspect of the present technology includes, at a light receiving surface side of a semiconductor substrate, a charge retention part that generates and retains a charge in response to incident light, an OFD into which the charge saturated at the charge retention part is discharged, and a potential barrier that becomes a barrier of the charge that flows from the charge retention part to the OFD, the OFD including a low concentration OFD and a high concentration OFD having different impurity concentrations of the same type, and the high concentration OFD and the potential barrier being formed at a distance. For example, the present technology is applicable to a CMOS image sensor.