Solid-State Imaging Device Photodiode Impurity Gradient Self-Alignment
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Solution Overview
Problem
MOS solid-state imaging devices face challenges in miniaturization due to difficulties in forming photodiodes by self-alignment, leading to tradeoffs between afterimage and white dot occurrence, which affect reading characteristics.
Innovation Solution
The formation of photodiodes with different impurity concentrations in the pn junction areas, where a low impurity concentration photodiode is formed close to the transmission gate and a high impurity concentration photodiode is formed outside the side wall, allowing for self-alignment and reduced occurrence of afterimages and white dots.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pixel is miniaturized to increase resolution, then productivity and resolution are improved, but difficulty in forming photodiodes by self-alignment increases
Solution Approach 1:
The patent applies preliminary action by forming the transmission gate electrode first, then using it as a mask to define the photodiode regions through ion implantation. The gate electrode is formed in advance, and subsequent processing steps use its position to automatically define the photodiode structure, enabling self-alignment even in miniaturized pixels. This preliminary formation of the gate electrode simplifies the overall fabrication process.
Solution Approach 2:
The transmission gate electrode serves a dual function: it acts as both the functional transistor component and as a alignment mask for photodiode formation. The gate electrode's own position and dimensions are used to define the photodiode regions, eliminating the need for separate alignment marks or complex multi-step alignment processes. This self-service approach simplifies manufacturing in miniaturized devices.
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 approach enables easy modulation of potential near the transmission gate, inhibits upwelling of charges, and improves reading characteristics, even in miniaturized pixels, by relaxing the electric field and optimizing impurity concentration profiles.
Implementation Method 1
a photodiode 105 serving as a photoelectric conversion unit
Implementation Method 2
forming photodiodes with different impurity concentrations in the pn junction areas
Data Source
AI summary
A solid-state imaging device includes plural photodiodes which are formed in a photodiode area of a unit pixel with no element separating area interposed therebetween and in which impurity concentrations of pn junction areas are different from each other.