Vertical Channel Transfer Transistor for Image Sensor Design
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Solution Overview
Problem
High-performance image sensors require wide-area photodiodes and fast transistors, but existing technologies struggle to optimize the design for efficient charge transfer and reduced dark current while maximizing photodiode area.
Innovation Solution
The image sensor design incorporates a transfer transistor with a vertical channel and pixel transistors with reversed thin film channels, featuring a Recessed Channel Array Transistor (RCAT) structure and Thin Film Transistor (TFT) structure respectively, which allows for wider channel width and reduced channel resistance, enabling faster operation and increased photodiode area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If a conventional horizontal channel transfer transistor is used, then the transistor structure is simple to manufacture, but the photodiode area is reduced and charge transfer speed is limited
Solution Approach 1:
The transfer transistor channel is reconfigured from a horizontal planar structure to a vertical three-dimensional structure. The gate electrode extends vertically through multiple interlayer dielectric layers, creating a vertical channel that allows charge carriers to move vertically between source and drain regions. This dimensional change enables the photodiode to occupy the entire horizontal pixel area while the vertical channel provides efficient charge transfer path without consuming horizontal space.
2Speed
If the transfer transistor channel width is increased to reduce resistance, then the charge transfer speed improves, but the transistor occupies more area reducing photodiode size
Solution Approach 1:
The channel is extended into the vertical dimension, allowing the channel length to increase while the horizontal footprint remains minimal. The gate electrode penetrates through lower and upper interlayer dielectric layers, creating a vertical channel that provides a direct charge transfer path from the photodiode through the transistor to the readout circuitry, reducing resistance without consuming additional horizontal area.
Solution Approach 2:
The transfer transistor is nested vertically within the pixel structure by having the gate electrode pass through multiple stacked interlayer dielectric layers. This nesting approach allows the transistor to be integrated into the vertical stack of the pixel, with the channel forming a vertical path that connects different functional layers without expanding the horizontal pixel area.
3Productivity
If pixel transistors use conventional structures, then manufacturing is straightforward, but channel resistance is high and operation speed is limited
Solution Approach 1:
The conventional transistor structure is inverted by placing the gate electrode at the bottom and forming the channel on top, rather than the traditional configuration. This inverted structure allows the channel to be formed as a thin film layer that can be extensively doped to reduce resistance, while the gate electrode serves as the control element from below. The inversion enables better control over channel properties and reduces fabrication complexity through standardized thin film processes.
Solution Approach 2:
The channel layer is extensively doped to dramatically increase carrier concentration and reduce channel resistance. By changing the doping parameter from lightly-doped to heavily-doped, the channel conductivity is enhanced, enabling faster transistor operation. The thin film channel structure allows for uniform doping throughout the channel thickness, achieving low resistance without increasing channel dimensions.
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 enhances the image sensor's optical sensing capability, increases photodiode area, and reduces dark current through the capacitor structure of the floating diffusion, resulting in improved resolution and performance.
Implementation Method 1
a photodiode formed in a substrate
Data Source
AI summary
Image sensors are provided. The image sensor may include a photodiode formed in a substrate, a lower interlayer dielectric layer formed over the substrate, a drive transistor gate electrode formed over the lower interlayer dielectric layer, and a transfer transistor gate electrode including an upper portion and a lower portion. The upper portion of the transfer transistor gate electrode may be formed over the drive transistor gate electrode. The lower portion of the transfer transistor gate electrode may be formed in a pillar shape and vertically extends from the upper portion of the transfer transistor gate electrode through the drive transistor gate electrode and the lower interlayer dielectric layer into the substrate.