LTPS TFT Sub-threshold Swing via Local Doping
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Solution Overview
Problem
Conventional low temperature polysilicon TFT fabrication processes struggle to achieve high sub-threshold swing for pixel TFTs without degrading the electrical characteristics of driving circuit TFTs, as both regions are fabricated simultaneously, leading to compromised gray scale inversion and contrast ratio in AMOLED displays.
Innovation Solution
The method involves forming a polysilicon layer with dopants of opposite conductivity types in the pixel region through ion implantation and subsequent annealing, allowing for distinct electrical characteristics between driving circuit and pixel regions by increasing sub-threshold swing in the pixel region without affecting the driving circuit region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If ion implantation is performed to increase sub-threshold swing in pixel region TFTs, then gray scale inversion and contrast ratio are improved, but electrical characteristics of driving circuit TFTs are degraded
Solution Approach 1:
The patent applies local quality by performing ion implantation only in the pixel region TFTs while leaving driving circuit TFTs unaffected. A masking layer is used to selectively expose only the pixel region polysilicon layers to ion implantation, creating different electrical characteristics in different regions of the same substrate. This resolves the contradiction by allowing high sub-threshold swing in pixels without degrading driving circuit characteristics.
2Ease of manufacture
If TFTs in pixel and driving circuit regions are fabricated by the same process, then manufacturing simplicity is maintained, but electrical characteristics cannot be optimized for different regions
Solution Approach 1:
The patent maintains ease of manufacture by using a single LTPS fabrication process for both pixel and driving circuit regions, then applies local quality through selective ion implantation in the pixel region. This allows the base process to remain simple and unified while achieving region-specific electrical characteristics through the selective doping step.
Solution Approach 2:
The patent segments the substrate into pixel region and driving circuit region, applying different treatments to each. The masking layer creates spatial segmentation that allows independent optimization of electrical characteristics for each region while maintaining overall process integration.
3Speed
If high carrier mobility is achieved in driving circuit TFTs, then fast response is improved, but sub-threshold swing in pixel TFTs is reduced
Solution Approach 1:
The patent resolves this contradiction by applying local quality through selective ion implantation in the pixel region only. The driving circuit TFTs maintain their high carrier mobility characteristics from the LTPS process, while pixel TFTs receive additional dopants to increase sub-threshold swing, allowing both performance requirements to be met simultaneously in their respective regions.
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 higher sub-threshold swing in pixel TFTs, enhancing gray scale inversion and contrast ratio while maintaining the electrical integrity of driving circuit TFTs, thereby improving display performance without degrading driving circuit characteristics.
Implementation Method 1
The polysilicon layer is annealed to activate the dopants after the masking layer is removed
Implementation Method 2
Ion implantation of a first conductivity type is performed in the polysilicon layer in the pixel region. Ion implantation of a second conductivity type opposite to the first conductivity type is performed in the polysilicon layer in the pixel region
Data Source
AI summary
A system for displaying images. The system comprises a thin film transistor (TFT) device comprising a substrate having a pixel region. An active layer is disposed on the substrate of the pixel region, comprising a channel region, a pair of source/drain regions separated by the channel region. The channel region comprises dopants with a first conductivity type and a second conductivity type opposite to the first conductivity type. A gate structure is disposed on the active layer, comprising a stack of a gate dielectric layer and a gate layer. A method for fabricating a system for displaying images including the TFT device is also disclosed.