Metal Gate Structure Formation Preventing Bird's Beak Effect
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Solution Overview
Problem
Conventional polysilicon gates in MOS transistors face high resistance, depletion effects, and leakage currents due to scaling issues, requiring the development of optimized work function metal gates that are compatible with both NMOS and PMOS devices, while maintaining precise thickness and composition control to prevent the bird's beak effect in the gate dielectric layer.
Innovation Solution
A method of forming a metal gate structure where a substrate is first coated with a gate dielectric layer, a metal compound layer, and a poly-silicon layer, followed by the formation of mask layers and a patterned photoresist. The photoresist is removed before forming the gate stack, using the mask layers as etching masks to prevent damage and the bird's beak effect, allowing for precise control of the gate stack formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If photoresist is removed after forming the gate stack, then the photoresist-removing process can complete its function, but the gate stack and gate dielectric layer are damaged and bird's beak effect occurs
Solution Approach 1:
The patent applies preliminary action by removing the photoresist before forming the gate stack. The photoresist is removed using oxygen plasma treatment, and the gate stack is formed subsequently using the remaining mask layers as etching masks. This sequence prevents the photoresist removal process from damaging the gate stack and eliminates the bird's beak effect on the gate dielectric layer.
2Temperature
If polysilicon gate is used, then heat resistance is maintained, but resistance is high and working rate is low
Solution Approach 1:
The patent applies parameter changes by transitioning from polysilicon gate material to metal gate material. This material substitution fundamentally changes the electrical parameters, providing lower resistance and higher working rates while maintaining thermal stability through the metal gate structure and optimized gate dielectric layer.
Solution Approach 2:
The patent uses composite materials by combining metal gate material with high-k gate dielectric material. This composite structure leverages the low resistance and high conductivity of metal gates while utilizing the high dielectric constant of the gate dielectric to maintain electrical performance and heat resistance.
3Ease of operation
If polysilicon gate is used, then gate electrode function is provided, but depletion effect increases equivalent thickness and reduces gate capacitance
Solution Approach 1:
The patent applies parameter changes by switching from polysilicon to metal gate material. This change eliminates the depletion effect inherent in polysilicon gates, thereby reducing the equivalent thickness of the gate dielectric layer and increasing gate capacitance while maintaining proper gate electrode control function.
4Length of moving object
If gate dielectric layer thickness is reduced, then scaling is achieved, but leakage current due to tunneling effect increases
Solution Approach 1:
The patent uses composite materials by implementing a gate dielectric layer with high-k material. The high dielectric constant allows the layer to maintain sufficient capacitance and block leakage current even at reduced thickness, enabling continued scaling while preventing tunneling-induced leakage.
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 method protects the gate stack from photoresist-removing process damage, prevents the bird's beak effect, and enables improved electrode characteristics by ensuring precise control over the gate stack formation, enhancing the performance of MOS transistors.
Implementation Method 1
portions of the second mask layer and portions of the first mask layer are removed to form a hard mask by utilizing the patterned photoresist as an etching mask
Implementation Method 2
the gate stack is protected from damages of the photoresist-removing process. The photoresist-removing process does not attack the sidewalls of the gate stack, so a bird's beak effect of the gate dielectric layer is prevent
Data Source
AI summary
A method of forming metal gate structure includes providing a substrate; forming a gate dielectric layer, a material layer and a polysilicon layer stacked on the substrate; forming a first mask layer, a second mask layer and a patterned photoresist on the polysilicon layer; removing portions of the second mask layer and the first mask layer to form a hard mask by utilizing the patterned photoresist as an etching mask; removing the patterned photoresist, and next utilizing the hard mask as an etching mask to remove parts of the polysilicon layer and parts of the material layer. Thus, a gate stack is formed. Since the patterned photoresist is removed before forming the gate stack, the gate stack is protected from damages of the photoresist-removing process. The photoresist-removing process does not attack the sidewalls of the gate stack, so a bird's beak effect of the gate dielectric layer is prevent.