Polysilicon TFT Fabrication via Metal Diffusion Control
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Solution Overview
Problem
Existing methods for crystallizing amorphous silicon (a-Si) into polycrystalline silicon (poly-Si) for thin film transistors (TFTs) face challenges such as substrate damage from high-temperature annealing, expensive laser equipment, and contamination from crystallization-inducing metals, which degrade device characteristics and require ineffective gettering processes.
Innovation Solution
A method involving a capping layer and controlled diffusion of crystallization-inducing metals like Ni, Pd, or Ag to form poly-Si, followed by patterning and the formation of additional gettering sites to effectively remove residual metals from channel regions, improving electrical characteristics like leakage current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solid phase crystallization (SPC) process is used to crystallize a-Si layer, then poly-Si layer can be formed, but substrate is damaged due to high temperature annealing for long period
Solution Approach 1:
A capping layer is introduced as an intermediary between the a-Si layer and the crystallization-inducing metal layer. This capping layer controls the diffusion of metal atoms into the a-Si layer, enabling crystallization at lower temperatures while preventing excessive metal contamination that would require aggressive gettering processes which could damage the substrate.
Solution Approach 2:
The invention changes the crystallization parameters by using metal-induced crystallization (MIC) or metal-induced lateral crystallization (MILC) processes that operate at lower temperatures compared to SPC. By controlling the type and amount of crystallization-inducing metal and using a capping layer, the crystallization temperature is reduced, preventing substrate damage.
2Productivity
If excimer laser annealing (ELA) process is used to crystallize a-Si layer, then poly-Si layer can be formed quickly, but expensive laser apparatus is required and crystallized surface is damaged
Solution Approach 1:
The invention replaces expensive laser apparatus with a more economical approach using crystallization-inducing metal layers that can be deposited using standard thin film deposition techniques. The metal layer serves as a temporary crystallization catalyst that is subsequently removed or retained in controlled amounts, providing a cost-effective alternative to ELA while achieving similar crystallization results.
3Temperature
If crystallization-inducing metal is used to crystallize a-Si layer, then crystallization can be performed at lower temperature, but device characteristics are degraded due to metal contamination
Solution Approach 1:
The capping layer acts as a mediator that controls the interaction between crystallization-inducing metal and a-Si layer. It allows sufficient metal diffusion to induce crystallization at low temperatures while limiting excessive metal contamination in the channel region, thereby maintaining device characteristics.
Solution Approach 2:
The invention applies local quality by creating different regions with different metal concentrations. The capping layer ensures that metal is present in sufficient amounts to induce crystallization in the a-Si layer, while the channel region maintains lower metal concentrations to preserve device characteristics. Gettering sites are strategically placed to concentrate metal away from critical regions.
4Reliability
If contact hole gettering site is used to remove crystallization-inducing metal, then some metal can be removed, but gettering is ineffective when distance between channel region and contact hole is increased
Solution Approach 1:
The invention segments the gettering function by creating multiple gettering sites distributed across the semiconductor layer, not relying on a single contact hole. This includes using the contact hole region, the crystallization-inducing metal layer itself, and the capping layer as separate gettering regions, ensuring effective metal removal even when contact holes are positioned far from the channel region.
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 enhances the electrical characteristics of TFTs by effectively removing crystallization-inducing metals, reducing leakage currents, and preventing substrate damage, while maintaining cost-effective and efficient fabrication processes.
Implementation Method 1
controlled diffusion of crystallization-inducing metals like Ni, Pd, or Ag to form poly-Si
Implementation Method 2
crystallized using a crystallization-inducing metal
Implementation Method 3
phase-change the a-Si layer into a poly-Si layer
Implementation Method 4
A crystallization-inducing metal can be effectively gettered from channel regions of the semiconductor layer
Data Source
Figure 1A~1D
Figure 2A
Figure 2B
AI summary
A thin film transistor (TFT), a method of fabricating the same, and an organic light emitting diode (OLED) display device having the TFT. The TFT includes: a substrate; a polycrystalline silicon (poly-Si) semiconductor layer disposed on the substrate, including source, drain, and channel regions, a crystallization-inducing metal, first gettering sites disposed on opposing edges of the semiconductor layer, and a second gettering site spaced apart from the first gettering sites; a gate insulating layer disposed on the semiconductor layer; a gate electrode disposed on the gate insulating layer; an interlayer insulating layer disposed on the gate electrode; and source and drain electrodes disposed on the interlayer insulating layer and electrically connected to the source and drain regions of the semiconductor layer.