Metal Gate Work Function Control via Oxygen Diffusion Barrier
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Solution Overview
Problem
Advanced integrated circuits face challenges in attaining desired effective work functions for NMOS and PMOS transistors with replacement metal gates without increasing fabrication cost and complexity.
Innovation Solution
A process is developed to form gate work function metal layers in PMOS transistors with effective work functions above 4.85 eV and in NMOS transistors with effective work functions below 4.25 eV, involving oxidation of work function metal layers, use of an oxygen diffusion blocking layer, and a getter anneal to adjust the work functions, followed by forming a metal replacement gate material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If oxidation process is applied to work function metal layers to increase effective work function, then PMOS gate work function is improved to above 4.85 eV, but NMOS gate work function also increases which is undesirable
Solution Approach 1:
The patent applies different oxygen exposure conditions to different gate regions: PMOS gates are exposed to oxygen to increase their work function above 4.85 eV, while NMOS gates are protected from oxygen exposure to maintain their work function below 4.25 eV. This is achieved through selective masking or sequential processing steps that allow localized modification of gate properties without affecting other devices.
Solution Approach 2:
The fabrication process is divided into separate stages: first, a common work function metal layer is deposited for both NMOS and PMOS; then, selective oxidation is applied only to PMOS gates through patterned masking or selective area processing; finally, NMOS gates are processed separately to achieve their target work function. This segmentation allows independent optimization of each device type.
2Manufacturing precision
If separate processing steps are used for NMOS and PMOS gates to achieve different work functions, then work function control is improved, but fabrication complexity increases
Solution Approach 1:
The patent combines common processing steps (such as initial metal layer deposition and annealing) that are shared by both NMOS and PMOS gates, while introducing selective steps only where needed. This merging of common operations reduces overall process complexity compared to completely separate processing flows for each device type.
Solution Approach 2:
The work function metal layer is deposited and initially processed with a preliminary anneal to establish a baseline work function before device-specific modifications. This preliminary action creates a uniform starting point that simplifies subsequent selective processing, as both device types begin from the same controlled state.
3Manufacturing precision
If oxygen is added to work function metal layers to increase effective work function, then desired PMOS work function is achieved, but oxygen diffusion to NMOS gates must be prevented
Solution Approach 1:
The patent introduces masking layers or barrier structures as intermediaries that selectively block oxygen diffusion to NMOS gates while allowing oxygen access to PMOS gates. These intermediary elements control the diffusion path of oxygen, enabling the desired selective oxidation without direct harmful exposure to the wrong device regions.
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 process effectively adjusts the work functions of metal gates in NMOS and PMOS transistors, enhancing on-state current densities while maintaining cost and complexity efficiency.
Implementation Method 1
An oxygen diffusion blocking layer is formed over the PMOS gate
Implementation Method 2
A getter anneal extracts the oxygen from the NMOS work function layers into the getter material
Implementation Method 3
The work function metal layers are oxidized to increase their effective work functions to the desired PMOS range
Data Source
AI summary
A process is disclosed of forming metal replacement gates for NMOS and PMOS transistors with oxygen in the PMOS metal gates and metal atom enrichment in the NMOS gates such that the PMOS gates have effective work functions above 4.85 eV and the NMOS gates have effective work functions below 4.25 eV. Metal work function layers in both the NMOS and PMOS gates are oxidized to increase their effective work functions to the desired PMOS range. An oxygen diffusion blocking layer is formed over the PMOS gate and an oxygen getter is formed over the NMOS gates. A getter anneal extracts the oxygen from the NMOS work function layers and adds metal atom enrichment to the NMOS work function layers, reducing their effective work functions to the desired NMOS range. Processes and materials for the metal work function layers, the oxidation process and oxygen gettering are disclosed.