Solid-State Imaging Device Pinning Layer Self-Alignment
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Solution Overview
Problem
Existing solid-state imaging devices face issues with dark currents and afterimages due to misalignment of gate electrodes and differences in threshold voltages among pixel transistors, which are exacerbated by the cancellation of impurities forming the floating diffusion and pinning layers, leading to operational difficulties and increased leakage.
Innovation Solution
A solid-state imaging device is designed with a pinning layer of opposite conductivity type to the floating diffusion, formed shallower than the floating diffusion, and implanted using the gate electrode as a mask, ensuring self-alignment and accurate overlap with the gate electrode, thereby reducing dark currents and leakages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the gate electrodes are formed after the pinning layers and diffusion layers, then the manufacturing process follows conventional sequencing, but the gate electrodes become misaligned causing differences in threshold voltages among transistors
Solution Approach 1:
The pinning layer is formed in advance before the gate electrode, using the gate electrode position as a reference. This preliminary formation ensures that when the gate electrode is subsequently formed, the pinning layer is already positioned correctly, eliminating alignment issues that would occur if the gate electrode were formed first or simultaneously.
Solution Approach 2:
The gate electrode serves a dual function: as the functional component of the transistor and as a mask for forming the pinning layer. By using the gate electrode itself as the positioning reference (mask), the structure achieves self-alignment, eliminating the need for separate alignment processes and ensuring consistent positioning.
2Object-affected harmful factors
If impurities forming the floating diffusion and pinning layers cancel each other, then dark currents and afterimages are reduced, but overlapping the diffusion layers and gate electrodes becomes more difficult
Solution Approach 1:
The gate electrode acts as an intermediary mask that mediates the formation of both the pinning layer and the floating diffusion layer. By using the gate electrode as the masking element for both impurity implantations, precise overlap control is achieved even when the impurities have opposing conductivity types and need to be formed in close proximity.
Solution Approach 2:
The pinning layer is formed in advance before the floating diffusion layer, with both processes using the gate electrode as a mask. This sequential preliminary formation ensures that the shallower pinning layer is established first, providing a foundation for the subsequent floating diffusion layer formation, while maintaining precise spatial relationships through the common mask.
3Object-affected harmful factors
If the amount of implanted impurities in forming the floating diffusion is reduced, then leakages in the floating diffusion are reduced, but overlapping the diffusion layers and gate electrodes becomes more difficult
Solution Approach 1:
The gate electrode serves as an intermediary mask that enables precise overlap control even when low doses of impurities are used. The mask provides a physical reference that ensures accurate positioning of the floating diffusion layer relative to the gate electrode, compensating for the reduced impurity amount that would otherwise make alignment more challenging.
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 reduces dark currents, afterimages, and differences in threshold voltages among transistors, improving image quality by ensuring proper operation and reducing noise in both multilayer and surface-type image sensors.
Implementation Method 1
forming a pinning layer by implanting first impurities into the semiconductor substrate using the gate electrode as a mask; and forming a floating diffusion by implanting second impurities into the semiconductor substrate using the gate electrode as a mask
Implementation Method 2
a photoelectric converter configured to photoelectrically convert incident light to generate signal charge
Data Source
AI summary
A solid-state imaging device includes unit pixels formed on a semiconductor substrate. Each of the unit pixels includes a photoelectric converter, a floating diffusion, a pinning layer, and a pixel transistor. The pixel transistor includes a gate electrode formed on the semiconductor substrate, a source diffusion layer, and a drain diffusion layer. At least one of the source diffusion layer or the drain diffusion layer functions as the floating diffusion. The pinning layer is covered by the floating diffusion at a bottom and a side at a channel of the pixel transistor. A conductivity type of the floating diffusion is opposite to that of the pinning layer.