Metal Gate Electrode Work Function Control for Low Threshold Voltage
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Solution Overview
Problem
The challenge in semiconductor devices is achieving a low threshold voltage for n-channel type MIS transistors using a metal gate electrode and high-k gate insulating film, as the work function of the metal gate material becomes unstable during the transistor formation process, making it difficult to realize the required effective work function values for adequate performance.
Innovation Solution
A semiconductor device structure is implemented with a metal gate electrode stack that includes a thin under-layer gate electrode of TaCx and a layer containing alkaline earth metal elements or group III elements, which are diffused into the gate insulating film, allowing for a low threshold voltage realization without exposing the gate stack to high temperatures, thus maintaining process simplicity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a metal gate material with low work function is used for n-channel type MIS transistor, then the threshold voltage can be lowered, but the work function becomes unstable during heating steps and shifts to mid-gap value
Solution Approach 1:
The gate electrode is divided into multiple layers: a first metal layer (1.5 nm or less thick) containing low work function material in contact with the gate insulating film, and a second metal layer providing structural stability. This segmentation allows the thin first layer to control work function while the thicker second layer provides stability during heating steps.
Solution Approach 2:
The low work function metal is concentrated in the first metal layer with thickness of 1.5 nm or less that directly contacts the gate insulating film, while the second metal layer provides structural support. This local concentration of functional properties ensures work function control where it matters most (at the interface) while maintaining overall stability.
2Productivity
If the gate insulating film is thinned to achieve higher performance and integration, then device performance improves, but the stability and control of the gate structure becomes more difficult
Solution Approach 1:
The gate electrode uses a composite structure of two different metal layers, where the first layer (1.5 nm or less) provides low work function for high performance, and the second layer provides structural stability. This composite approach allows thin gate insulating films to be used while maintaining gate structure reliability.
3Stability of the object's composition
If a thick metal layer is used to ensure stability during heating, then work function stability improves, but the ability to achieve low threshold voltage decreases
Solution Approach 1:
The gate electrode is segmented into a thin first metal layer (1.5 nm or less) for work function control and a thicker second metal layer for structural stability. This segmentation resolves the contradiction by separating the functions of work function control and structural stability into different layers with appropriate thicknesses.
Solution Approach 2:
The critical parameter is the thickness of the low work function metal layer, which is controlled to be 1.5 nm or less. This parameter change allows the metal to remain stable during heating while still providing sufficient low work function effect to achieve low threshold voltage.
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 effectively lowers the work function of the gate electrode, achieving a threshold voltage suitable for n-channel type MIS transistors with reduced processing complexity and minimal impact on electrical characteristics.
Implementation Method 1
a second metal layer which is diffused in a thickness of 1 monolayer or more and 1.5 nm or less along the second side walls and a second bottom of the second trench via the second gate insulating film
Data Source
AI summary
A semiconductor device includes n- and p-type semiconductor regions separately formed on a substrate, an interlayer insulator formed on the substrate and having first and second trenches formed to reach the n- and p-type regions. There are further included first and second gate insulators formed inside of the first and second trenches, a first metal layer formed inside of the first trench via the first gate insulator, a second metal layer formed in a thickness of 1 monolayer or more and 1.5 nm or less inside of the second trench via the second gate insulator, a third metal layer formed on the second metal layer and containing at least one of a simple substance, a nitride, a carbide and an oxide of at least one metal element of alkaline earth metal elements and group III elements, first and second source/drain regions formed on the n- and p-type regions.