TiAl Metal Gate Stack Composition for Threshold Voltage Stability
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Solution Overview
Problem
The challenge of tuning threshold voltage in transistor gate structures is exacerbated by scaling, as adjustments to the work function layer thickness are limited due to reduced spacing between transistors, leading to issues like poor gap-fill, void formation, and unpredictable threshold voltage variations, particularly in n-type finFETs.
Innovation Solution
Implementing a gate stack with a TiAl bilayer or trilayer structure, where the TiAl layers have varying Al/Ti ratios, with a Ti-rich layer acting as an oxygen getter to trap oxygen atoms and stabilize the threshold voltage, thereby reducing it by about 50% and increasing saturation current by 15% without adverse effects on p-type transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the work function layer thickness is adjusted to tune threshold voltage, then threshold voltage control is improved, but device spacing increases leading to poor gap-fill and void formation
Solution Approach 1:
The gate structure is segmented into multiple functional layers: a high-k dielectric layer for electrical isolation, a TiAl alloy layer for threshold voltage tuning, and a Ti-rich layer for oxygen gettering. This segmentation allows each layer to perform its specific function independently, enabling threshold voltage control without requiring increased device spacing
Solution Approach 2:
The invention changes the compositional parameter of the TiAl layer by controlling the Al/Ti ratio to create a Ti-rich layer with excess titanium atoms. This parameter change enables the layer to function as an oxygen getter, stabilizing threshold voltage without requiring thickness adjustments that would increase device spacing
2Stability of the object's composition
If the work function layer thickness is increased to stabilize threshold voltage, then threshold voltage stability is improved, but saturation current increases leading to higher power consumption
Solution Approach 1:
The invention converts the harmful effect of oxygen diffusion (which causes threshold voltage instability) into a beneficial function by designing a Ti-rich layer that actively getters oxygen atoms. The oxygen that would otherwise degrade performance is now trapped by the Ti-rich layer, stabilizing threshold voltage without requiring increased layer thickness that would raise power consumption
3Productivity
If scaling is applied to reduce device size, then productivity is improved, but threshold voltage tuning capability deteriorates due to limited adjustment space
Solution Approach 1:
The invention moves the threshold voltage tuning mechanism from the thickness dimension to the compositional dimension. By controlling the Al/Ti ratio in the TiAl layer and creating a Ti-rich layer, threshold voltage can be tuned without changing layer thickness, enabling scaling while maintaining tuning capability
Solution Approach 2:
The gate structure uses composite materials with different functions: high-k dielectric for isolation, TiAl alloy for threshold voltage control, and Ti-rich material for oxygen gettering. This composite approach enables multiple functions to be achieved within a compact structure, supporting both scaling and threshold voltage tuning
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
The TiAl bilayer or trilayer structure effectively stabilizes threshold voltage and enhances performance by trapping oxygen, ensuring consistent transistor operation and improved current capacity without increasing thickness, suitable for future technology nodes.
Implementation Method 1
a TiAl layers with varying Al/Ti ratios, with a Ti-rich layer that acts as an oxygen getter to trap oxygen atoms
Data Source
AI summary
The present disclosure describes a method for the formation of gate stacks having two or more titanium-aluminum (TiAl) layers with different Al concentrations (e.g., different Al/Ti ratios). For example, a gate structure can include a first TiAl layer with a first Al/Ti ratio and a second TiAl layer with a second Al/Ti ratio greater than the first Al/Ti ratio of the first TiAl layer.