Work Function Metal Thermal Stability via Sacrificial Layer
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Solution Overview
Problem
The challenge in semiconductor devices is to enhance the thermal stability of work function metals to effectively modulate the threshold voltage of transistors without the limitations imposed by subsequent thermal treatments.
Innovation Solution
A method involving the formation of a semiconductor device with a first and second work function layer, where the first work function layer is formed using titanium nitride and the second work function layer includes titanium with species such as aluminum or carbon, both having different work functions, and utilizing a sacrificial layer and compound to achieve thermal stability and threshold voltage modulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a work function metal layer is formed to modulate the threshold voltage of a transistor, then the threshold voltage modulation capability is improved, but the thermal stability deteriorates due to vulnerability to subsequent thermal treatments
Solution Approach 1:
The gate structure is divided into multiple functional layers: a high-k dielectric layer for electrical isolation, an etch stop layer for process control, a sacrificial layer (lanthanum oxide) that is selectively removed, and a work function layer (titanium nitride/titanium) for threshold voltage modulation. This segmentation allows each layer to perform its specific function without interfering with the thermal stability of others.
Solution Approach 2:
The sacrificial layer is formed and positioned in advance before the work function layer. This preliminary action creates a protected configuration where the work function layer can be deposited without direct thermal exposure to the underlying structures, and the sacrificial layer can be removed selectively after the work function layer is in place, preserving thermal stability while enabling threshold voltage modulation.
2Ease of manufacture
If subsequent thermal treatments are applied to the transistor structure, then the manufacturing process can be completed, but the work function metal becomes vulnerable and loses its threshold voltage modulation capability
Solution Approach 1:
The etch stop layer and high-k dielectric layer serve as intermediary protective layers between the work function layer and subsequent thermal processing steps. These layers act as thermal barriers and process protectors, allowing thermal treatments to proceed while shielding the thermally sensitive work function layer from degradation.
Solution Approach 2:
The sacrificial layer (lanthanum oxide) is designed as a temporary, disposable element that is formed, serves its protective function during work function layer deposition, and then selectively removed. This disposable approach allows the work function layer to be created in a protected state without permanent structural modifications that would compromise thermal stability.
3Stability of the object's composition
If the work function metal is made more stable thermally, then the thermal treatment vulnerability is reduced, but the ability to modulate threshold voltage effectively is limited
Solution Approach 1:
Different regions of the gate structure have different material compositions optimized for their specific functions: the high-k dielectric layer provides electrical isolation, the etch stop layer provides process control, and the work function layer (with specific titanium nitride/titanium composition) provides threshold voltage modulation. This local quality optimization allows thermal stability in protective layers while maintaining modulation capability in the work function layer.
Solution Approach 2:
The gate structure employs a composite multi-layer configuration combining materials with different properties: high-k dielectric material for isolation, metal nitride/oxide for etch stopping, rare earth oxide for sacrificial protection, and transition metal nitride/carbide for work function control. This composite structure synergistically achieves both thermal stability and threshold voltage modulation capability.
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 improves the thermal stability of the work function metals, allowing for effective modulation of the threshold voltage without suffering from Fermi-pinning phenomena, thereby enhancing the performance of transistors.
Implementation Method 1
the sacrificial layer pattern is transformed into the sacrificial compound while the first work function layer is formed over the first portion of the etch stop layer
Implementation Method 2
The first work function layer may include titanium nitride, wherein the first work function layer is formed by using titanium tetrachloride (TiCl4)
Implementation Method 3
forming an etch stop layer over the gate dielectric layer
Data Source
AI summary
A method for fabricating a semiconductor device includes: forming a gate dielectric layer over a substrate; forming an etch stop layer over the gate dielectric layer; forming a first work function layer that covers a first portion of the etch stop layer and a sacrificial compound that covers a second portion of the etch stop layer; exposing the second portion of the etch stop layer by removing the sacrificial compound; and forming a second work function layer over the second portion of the etch stop layer and the first work function layer.