High-Speed Thin-Film Transistors on Flexible Substrates
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Solution Overview
Problem
Current flexible electronics using organic semiconductors, amorphous silicon, and poly-crystal silicon face limitations in achieving high-speed and low-power performance due to low electron mobility and high-temperature processing requirements, which are not compatible with low-cost, large-area applications on flexible substrates like plastic.
Innovation Solution
The method involves high-temperature processing steps to form source, drain, and gate channel regions in a thin single-crystal silicon layer before transferring it to a flexible polymer substrate, followed by low-temperature processing for forming a gate stack and electrodes, using dopant diffusion, ion implantation, and annealing, and employing SiO as a gate dielectric to achieve high-frequency operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If high-temperature processing is used to form source, drain, and gate channel regions in single-crystal silicon, then electron mobility and device speed are improved, but processing temperature requirements increase making it incompatible with low-cost flexible substrates
Solution Approach 1:
The fabrication process is divided into two distinct temperature stages: (1) high-temperature processing (above 700°C) for forming source, drain, and gate channel regions in the single-crystal silicon layer while it remains on the rigid substrate, and (2) low-temperature processing for subsequent steps after transferring to the flexible substrate. This segmentation allows high-temperature steps to be performed only when necessary, compatible with the rigid substrate, while the final flexible device operates at low temperatures.
Solution Approach 2:
All high-temperature processing steps are performed in advance while the single-crystal silicon layer is still supported by the rigid substrate that can withstand high temperatures. The flexible substrate is introduced only after these preliminary high-temperature steps are complete, eliminating the need for the flexible substrate to endure high temperatures.
2Ease of manufacture
If organic semiconductors, amorphous silicon, or poly-crystal silicon are used for flexible electronics, then low-temperature processing and ease of fabrication are improved, but electron mobility and high-speed performance deteriorate
Solution Approach 1:
A rigid substrate acts as an intermediary during the fabrication process, enabling high-temperature processing of single-crystal silicon to achieve high electron mobility. The rigid substrate temporarily supports the single-crystal silicon layer during high-temperature steps, then the layer is transferred to the flexible substrate. This intermediary approach allows the final flexible device to achieve performance typically associated with rigid substrates.
Solution Approach 2:
The material phase is changed from polycrystalline or amorphous silicon to single-crystal silicon, fundamentally altering the electron mobility parameter. Single-crystal silicon provides superior electron mobility compared to polycrystalline or amorphous materials, enabling high-speed operation while maintaining flexibility through the transfer process.
3Speed
If single-crystal silicon is used to achieve high electron mobility and RF performance, then device speed and frequency operation are improved, but fabrication complexity and processing difficulty increase
Solution Approach 1:
The fabrication process is segmented into modular stages: (1) forming the single-crystal silicon layer on a rigid substrate, (2) performing high-temperature processing to create source, drain, and gate channel regions, (3) transferring the processed layer to a flexible substrate, and (4) completing low-temperature fabrication steps. This segmentation makes the complex process more manageable and suitable for large-area fabrication.
Solution Approach 2:
The rigid substrate serves as a fabrication intermediary that simplifies high-temperature processing by providing mechanical support and thermal stability during critical steps. This intermediary approach reduces fabrication complexity compared to attempting to process single-crystal silicon directly on flexible substrates, while still achieving the desired flexible final product.
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 the fabrication of high-speed TFTs capable of operating at frequencies up to 10 GHz, with low parasitic resistances and gate threshold voltages, suitable for applications like surveillance systems and digital signage, while being cost-effective and compatible with commercial silicon CMOS processes.
Implementation Method 1
These high-temperature processing steps include dopant (e.g., boron, phosphor and/or arsenic) diffusion
Implementation Method 2
These high-temperature processing steps include dopant (e.g., boron, phosphor and/or arsenic) diffusion, ion implantation and annealing steps
Implementation Method 3
These high-temperature processing steps include dopant (e.g., boron, phosphor and/or arsenic) diffusion, ion implantation and annealing steps
Data Source
AI summary
This invention provides methods for fabricating high speed TFTs from silicon-on-insulator and bulk single crystal semiconductor substrates, such as Si(100) and Si(110) substrates. The TFTs may be designed to have a maximum frequency of oscillation of 3 GHz, or better.