Stacked Image Sensor Pixel Layout for Photodiode Charge Isolation
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Solution Overview
Problem
Current image sensors face challenges in effectively separating and managing electric charges between multiple photodiodes within each pixel, leading to reduced signal-to-noise ratio and conversion gain due to charge transfer between photodiodes.
Innovation Solution
The image sensor design incorporates a pixel array with multiple photodiodes and active regions, where a sub-substrate with an insulating layer is used to block charge transfer between photodiodes, and transistors are strategically placed on both the main and sub-substrates to prevent charge leakage, enhancing charge separation and signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple photodiodes are integrated into each pixel to increase performance, then the light receiving capability is improved, but charge transfer between photodiodes occurs causing reduced signal-to-noise ratio and conversion gain
Solution Approach 1:
The pixel substrate is divided into a main substrate containing first photodiodes and a sub-substrate containing second photodiodes. This segmentation physically separates the photodiodes onto different substrates, preventing charge transfer between them while maintaining the ability to capture multiple wavelengths of light. The pixel separation film further segments the pixel regions on each substrate.
Solution Approach 2:
A pixel separation film is introduced as an intermediary structure between adjacent pixel regions and between the main and sub-substrates. This film acts as a barrier that prevents charge leakage and transfer between photodiodes while allowing the multi-photodiode structure to function. The film is positioned to separate charge paths between different photodiodes.
2Device complexity
If transistors are placed on the same substrate as photodiodes to reduce device complexity, then manufacturing is simplified, but charge leakage between photodiodes increases
Solution Approach 1:
Transistors are segmented and distributed across two separate substrates: some transistors are placed on the main substrate while others are placed on the sub-substrate. This spatial distribution prevents charge leakage paths that would occur if all transistors were on a single substrate, while still maintaining manageable device complexity through the modular two-substrate architecture.
Solution Approach 2:
The device architecture transitions from a single-plane (2D) layout to a multi-layer (3D) stacked configuration. By placing transistors on both the main substrate and the sub-substrate, the design utilizes the vertical dimension to separate charge paths and prevent leakage, effectively adding a new dimension to the device layout to solve the charge isolation problem.
3Reliability
If a sub-substrate is attached to the main substrate to block charge transfer, then charge separation is improved, but device complexity increases
Solution Approach 1:
The overall device is segmented into functional modules distributed across two substrates: photodiodes and their associated transistors are grouped on the same substrate, while pixel separation functions are provided by the pixel separation film. This modular segmentation achieves reliable charge separation while keeping each substrate's internal structure relatively simple and manageable.
Solution Approach 2:
The sub-substrate serves multiple functions simultaneously: it hosts second photodiodes for additional wavelength detection, contains transistors for signal processing, and acts as a physical barrier to prevent charge transfer between substrates. This multi-functionality reduces the need for separate components, thereby limiting the increase in overall device complexity despite the multi-substrate architecture.
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 design improves the signal-to-noise ratio and conversion gain by effectively isolating charges between photodiodes, allowing for higher dynamic range and improved image quality.
Implementation Method 1
a sub-substrate with an insulating layer is used to block charge transfer between photodiodes
Implementation Method 2
Each of the plurality of pixels includes a first photodiode, a second photodiode
Data Source
AI summary
An image sensor includes: a pixel array including a plurality of pixels, wherein each of the pixels includes a first photodiode, a second photodiode, a first transmission gate, a second transmission gate, and a plurality of active regions; and a logic circuit configured to control the pixels. The plurality of active regions include a first active region, a second active region and a third active region. The first active region is disposed adjacent to the first transmission gate. The second active region is disposed adjacent to the second transmission gate. The third active region is electrically connected to the second active region. The first active region and the second active region are disposed on a main substrate including the first and second photodiodes. The third active region is disposed on a sub-substrate attached to the main substrate.