Multi-Layer Metal Gate Formation for FinFET Work Function Tuning
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Solution Overview
Problem
MOS devices with polysilicon gate electrodes experience carrier depletion, known as the poly depletion effect, which increases the effective gate dielectric thickness and makes it difficult to create an inversion layer at the semiconductor surface, affecting the performance of NMOS and PMOS devices.
Innovation Solution
The formation of metal gate electrodes with multiple layers, where the work function is adjusted for each type of transistor, allowing for independent tuning of the gate electrode's work function to match the specific requirements of NMOS and PMOS devices, thereby reducing the poly depletion effect and improving performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If polysilicon gate electrodes are used with doping operations, then the work function can be adjusted to the band-edge of silicon, but carrier depletion effect occurs which increases effective gate dielectric thickness
Solution Approach 1:
The patent changes the material parameter from polysilicon to metal, fundamentally altering the electrical properties to eliminate carrier depletion. The metal gate electrode provides a fixed work function that does not suffer from poly depletion effects, directly resolving the contradiction between work function adjustment and inversion layer creation reliability
Solution Approach 2:
The patent employs composite metal gate structures with multiple layers (e.g., TiN, TaN, W, Mo) to achieve both the desired work function and eliminate poly depletion. The composite structure allows tuning of electrical characteristics while maintaining the benefits of metal gates over polysilicon
2Reliability
If metal gate electrodes with multiple layers are formed, then the poly depletion effect is reduced and inversion layer creation is improved, but the device structure becomes more complex
Solution Approach 1:
The patent applies different metal materials to different regions or layers of the gate electrode structure, with each layer optimized for specific functions (adhesion, work function tuning, depletion reduction). This local optimization achieves the desired electrical performance while managing structural complexity through targeted material selection
Solution Approach 2:
The metal gate electrode structure serves multiple functions simultaneously: providing the necessary work function for threshold voltage control, eliminating poly depletion effects, and enabling reliable inversion layer creation. This multi-functionality justifies the increased structural complexity by delivering multiple benefits from a single component
Data Source
AI summary
A structure and a method of forming are provided. A first work function layer is formed over a first fin and terminates closer to the first fin than an adjacent second fin. A second work function layer is formed over the first work function layer and terminates closer to the second fin than the adjacent second fin. A third work function layer is formed over the first work function layer and the second fin. A conductive layer is formed over the third work function layer.