Replacement Gate Stack Work Function Engineering
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Solution Overview
Problem
Conventional semiconductor devices with polysilicon gate electrodes face challenges in achieving high performance due to high gate leakage current and depletion effects, requiring high-k gate dielectrics and metal gate electrodes to optimize work functions for both n-type and p-type field effect transistors, which is difficult to achieve with conventional gate stacks.
Innovation Solution
The introduction of a replacement gate stack with a titanium-oxide-nitride layer between lower and upper titanium nitride layers, and an aluminum layer deposited at a temperature not greater than 420°C, significantly increases the work function of the gate electrode, addressing the need for dual work function metal gates with high electrical conductivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional polysilicon gate electrodes are used, then manufacturing is simpler, but gate leakage current increases and depletion effects occur
Solution Approach 1:
The patent changes the material parameter from polysilicon to metal (titanium nitride), fundamentally altering the electrical properties to eliminate depletion effects and reduce gate leakage current. This material substitution enables the gate electrode to maintain stable electrical characteristics without the degradation issues inherent in polysilicon.
Solution Approach 2:
The patent employs a composite gate stack structure consisting of multiple layers including titanium nitride, aluminum, and titanium-oxide-nitride. This composite structure combines the advantages of different materials: titanium nitride provides high conductivity and stability, aluminum enhances work function control, and the titanium-oxide-nitride layer prevents unwanted reactions, collectively solving the gate leakage problem.
2Reliability
If dual work function metal gates are implemented, then device performance improves, but device complexity increases
Solution Approach 1:
The patent applies local quality by creating different gate stack configurations for n-type and p-type transistors. Each transistor type receives a customized gate stack with specific work function characteristics, allowing optimization of device performance for each type while maintaining a systematic manufacturing approach.
Solution Approach 2:
The gate stack is segmented into distinct functional layers: a base metal layer (titanium nitride) for structural integrity and conductivity, an aluminum layer for work function adjustment, and a titanium-oxide-nitride layer for interface protection. This segmentation allows independent optimization of each layer's properties.
3Productivity
If aluminum layer is deposited at conventional temperatures, then deposition is faster, but reflow of aluminum material occurs
Solution Approach 1:
The patent applies preliminary anti-action by depositing the aluminum layer at a controlled lower temperature that prevents reflow before subsequent processing steps. The titanium-oxide-nitride layer is also deposited beforehand to create a protective interface that further prevents aluminum diffusion and reflow during thermal processing.
4Reliability
If work function is increased for p-type FETs, then transistor performance improves, but gate electrode material requirements become more stringent
Solution Approach 1:
The patent changes the work function parameter of the gate electrode by introducing aluminum and titanium-oxide-nitride layers. These materials have higher work functions that match the requirements for p-type FET operation, enabling improved transistor performance through deliberate material selection and layer design.
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 solution effectively increases the work function of p-type field effect transistors, enhancing their performance by approaching the energy level of the valence band edge of silicon, while also reducing reflow of the aluminum material, thus improving the overall performance of semiconductor structures.
Implementation Method 1
the stack of the lower titanium nitride layer, the titanium-oxide-nitride layer, and the upper titanium nitride layer produces an unexpected result of increasing the work function of the work function metal stack significantly
Implementation Method 2
an aluminum layer deposited at a temperature not greater than 420° C., which is lower than conventional deposition temperatures for aluminum layer and significantly reduces reflow of the deposited aluminum material
Data Source
AI summary
Replacement gate stacks are provided, which increase the work function of the gate electrode of a p-type field effect transistor (PFET). In one embodiment, the work function metal stack includes a titanium-oxide-nitride layer located between a lower titanium nitride layer and an upper titanium nitride layer. The stack of the lower titanium nitride layer, the titanium-oxide-nitride layer, and the upper titanium nitride layer produces the unexpected result of increasing the work function of the work function metal stack significantly. In another embodiment, the work function metal stack includes an aluminum layer deposited at a temperature not greater than 420° C. The aluminum layer deposited at a temperature not greater than 420° C. produces the unexpected result of increasing the work function of the work function metal stack significantly.