Hybrid LTPS-TAOS TFT Layout for HF-Compatible Display Substrates
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Solution Overview
Problem
The challenge lies in forming TFTs using both Low Temperature Poly-Si (LTPS) and Transparent Amorphous Oxide Semiconductors (TAOS) on the same substrate, as LTPS requires hydrofluoric acid cleaning, which dissolves TAOS, making it impossible to use the same process for both materials.
Innovation Solution
A display device is designed with a substrate where TAOS TFTs are formed with an AlOx sacrificial layer, and through holes are created with specific tapers to prevent hydrofluoric acid from contacting the TAOS, allowing for the formation of both LTPS and TAOS TFTs on the same substrate by using AlOx as an insulating material and a sacrificial layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If LTPS is used for TFT switching in pixels, then high carrier mobility is achieved, but leak current increases and two LTPS transistors must be connected in series
Solution Approach 1:
The patent combines LTPS and TAOS TFTs in a hybrid configuration where LTPS TFTs provide high carrier mobility for drive circuits and TAOS TFTs provide low leak current for pixel switching, merging the advantages of both material systems to resolve the contradiction between speed and reliability
Solution Approach 2:
The patent applies different semiconductor materials to different functional regions: LTPS is used in peripheral drive circuits where high mobility is critical, while TAOS is used in pixel switching TFTs where low leak current is critical, allowing each region to have optimized local properties
2Reliability
If hydrofluoric acid cleaning is applied to LTPS to remove surface oxide, then LTPS TFT performance is improved, but TAOS is dissolved and cannot be used
Solution Approach 1:
The patent segments the substrate into distinct processing regions with different protective layer configurations: LTPS regions have oxide layers removable by HF cleaning, while TAOS regions are protected by etch-resistant sacrificial layers that prevent HF contact, allowing selective processing of different material systems
Solution Approach 2:
The patent introduces etch-resistant sacrificial layers (such as silicon nitride or specific oxide compositions) as intermediary protective structures between the HF cleaning process and the TAOS layer, allowing the harmful HF to pass through or be neutralized before reaching the TAOS, thus protecting it from dissolution
3Reliability
If TAOS is used for pixel switching TFT, then leak current is reduced, but carrier mobility is too low to form drive circuits
Solution Approach 1:
The patent merges LTPS and TAOS TFTs in a hybrid configuration where LTPS TFTs provide high carrier mobility for drive circuits and TAOS TFTs provide low leak current for pixel switching, merging the advantages of both material systems to resolve the contradiction between speed and reliability
Data Source
AI summary
The object of the present invention is to make it possible to form an LTPS TFT and an oxide semiconductor TFT on the same substrate. A display device includes a substrate having a display region in which pixels are formed. The pixel includes a first TFT using an oxide semiconductor 109. An oxide film 110 as an insulating material is formed on the oxide semiconductor 109. A gate electrode 111 is formed on the oxide film 110. A first electrode 115 is connected to a drain of the first TFT via a first through hole formed in the oxide film 110. A second electrode 116 is connected to a source of the first TFT via a second through hole formed in the oxide film 110.