Multigate Gate Stack Cap for Oxygen-Controlled Work Function Layers
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Solution Overview
Problem
Conventional multigate device fabrication methods face challenges in scaling down device dimensions due to unintentional and undesirable oxidation of gate stack layers, leading to performance degradations such as slower device speed and threshold voltage variations, particularly in static random-access memory (SRAM) devices.
Innovation Solution
A gate stack fabrication method that reduces oxygen vacancies in the work function layer cap by breaking vacuum during the formation of the cap, increasing metal-oxygen bonds and controlling oxidation to mitigate additional oxidation in the work function and gate dielectric layers, thereby improving device performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If gate layers are used to wrap channel layers in multigate devices, then gate control and gate-channel coupling are improved, but the gate opening space is reduced, leaving limited room for fine tuning threshold voltage using multiple work function layers
Solution Approach 1:
The gate stack is segmented into multiple functional layers including a cap layer, work function layer, and gate dielectric layer. This segmentation allows each layer to be optimized independently - the cap layer protects the work function layer while maintaining sufficient gate opening space for threshold voltage tuning through multiple work function layers with different materials and thicknesses.
Solution Approach 2:
The gate structure employs a nested configuration where the cap layer encapsulates the work function layer, which in turn surrounds the gate dielectric layer and channel layers. This nested arrangement maximizes the use of available gate opening space while maintaining proper layer spacing for electrical functionality and threshold voltage control.
2Productivity
If device feature sizes are decreased to scale down IC technology, then device density is increased, but gate stack volume is reduced making it difficult to accommodate multiple work function layers for threshold voltage tuning
Solution Approach 1:
The gate stack employs local quality variations through different material compositions and layer thicknesses at different regions. The cap layer has specific material properties optimized for protection, while the work function layer uses materials with different work functions in different regions to enable local threshold voltage tuning, allowing efficient use of reduced gate stack volume.
Solution Approach 2:
The invention utilizes parameter changes by varying the work function, thickness, and material composition of different gate layers. The cap layer thickness and work function layer composition are adjusted to optimize both the protective function and the threshold voltage tuning capability within the constrained gate stack volume of scaled devices.
3Strength
If oxidation is allowed to occur during gate stack fabrication, then material bonding is improved, but unintentional oxidation of gate stack layers causes performance degradation including slower device speed and threshold voltage variations
Solution Approach 1:
The cap layer serves as an intermediary between the external environment and the sensitive gate stack layers. It provides a protective barrier that prevents unwanted oxidation of the work function and gate dielectric layers while allowing controlled oxidation processes to occur where needed for proper material bonding and interface formation.
Solution Approach 2:
The oxidation process is extracted and controlled to occur only at specific locations and times during fabrication. The cap layer is strategically removed or thinned in certain regions to allow controlled oxidation for bonding, while protecting other regions from unwanted oxidation that would degrade device performance.
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
The method results in faster device speeds and smaller threshold voltage variations, enhancing the performance of multigate devices, especially in SRAM applications by reducing unintended oxidation and performance degradations.
Implementation Method 1
breaking vacuum during the formation of the cap, increasing metal-oxygen bonds
Implementation Method 2
controlling oxidation to mitigate additional oxidation in the work function and gate dielectric layers
Data Source
AI summary
An exemplary method for forming a gate stack of a multigate device includes forming a gate dielectric layer, forming a work function layer over the gate dielectric layer, forming a cap over the work function layer, and forming a gate electrode layer over the cap. Forming the cap includes forming a first portion of a first capping layer over the work function layer, performing an oxygen control treatment, forming a second portion of the first capping layer over the first portion of the first capping layer, and forming a second capping layer over the first capping layer. The oxygen control treatment exposes the first portion of the first capping layer to: oxygen by breaking vacuum, ozonated deionized water, oxygen radicals, an oxygen-containing annealing environment, or a combination thereof. The first capping layer can be a metal nitride layer, and the second capping layer can be a silicon layer.