Vt Modulation Layer for Stable Threshold Voltage in NS FETs
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Solution Overview
Problem
As semiconductor devices progress to smaller technology nodes, the complex patterning process for forming metal gate stacks in nanostructure field-effect transistors (NS FETs) becomes challenging, leading to unstable threshold voltage due to reduced separation distances and metal gate boundary diffusion between n-type and p-type FETs, which affects device performance.
Innovation Solution
The implementation of a threshold voltage modulation layer, comprising a capping layer and residual sacrificial layers, wraps around the channel layers of NS FETs, allowing for fine-tuning of the threshold voltage without complex patterning and reducing metal gate boundary diffusion by using the same work-function metal layers for both n-type and p-type FETs, thereby enhancing electron and hole mobilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If separation distances between active device regions are reduced to meet design requirements of smaller technology nodes, then device density and production efficiency are improved, but threshold voltage stability deteriorates due to metal gate boundary diffusion
Solution Approach 1:
A threshold voltage modulation layer is introduced as an intermediary component between the channel layer and the metal gate stack. This modulation layer acts as a mediator that controls the interaction between the metal gate and the channel, preventing direct harmful interaction (boundary diffusion) while enabling controlled threshold voltage adjustment. The modulation layer composition is specifically designed to be different from both the channel layer and the metal gate stack, creating a buffer zone that stabilizes threshold voltage even at reduced separation distances.
Solution Approach 2:
The patent applies different material compositions at different locations within the gate stack structure. The threshold voltage modulation layer has a specific composition that differs from the channel layer and metal gate stack, creating localized functional zones. This local quality variation allows the modulation layer to specifically address threshold voltage control needs at the interface between the metal gate and channel, while other regions maintain their respective functions.
2Manufacturing precision
If complex patterning processes are used to form metal gate stacks with different work function metals for n-type and p-type FETs, then threshold voltage control is improved, but manufacturing complexity and process difficulty increase
Solution Approach 1:
The threshold voltage modulation layer serves multiple functions simultaneously: it acts as a barrier to prevent metal gate boundary diffusion, provides threshold voltage control through its specific composition, and enables the use of the same work function metal layers for both n-type and p-type FETs. This multi-functionality eliminates the need for complex patterning to differentiate metal gate stacks between device types, while still achieving precise threshold voltage control through the modulation layer's material properties.
Solution Approach 2:
The patent controls threshold voltage by changing the composition parameter of the modulation layer rather than by complex spatial patterning. By adjusting the modulation layer's material composition (which differs from both the channel layer and metal gate stack), precise threshold voltage control is achieved through material parameter optimization rather than geometric patterning complexity.
3Ease of manufacture
If same work-function metal layers are used for both n-type and p-type FETs to reduce boundary diffusion, then manufacturing simplicity is improved, but threshold voltage differentiation capability may be compromised
Solution Approach 1:
The threshold voltage modulation layer acts as a mediator that enables the use of identical work function metal layers for both device types while still achieving threshold voltage differentiation. The modulation layer's specific composition (different from both channel layer and metal gate stack) provides the necessary control mechanism, allowing the same metal gate structure to serve different device types with different threshold voltage requirements.
Data Source
AI summary
A semiconductor structure includes a substrate, first channel layers vertically stacked over the substrate in a first region, and second channel layers vertically stacked over the substrate in a second region. The first and second regions have opposite conductivity types. The semiconductor structure also includes a threshold voltage (Vt) modulation layer wrapping around each of the second channel layers in the second region. The first region is free of the Vt modulation layer. The semiconductor structure also includes a gate dielectric layer wrapping around each of the first channel layers and the second channel layers over the Vt modulation layer, and a work function metal layer disposed on the gate dielectric layer and wrapping around each of the first channel layers and the second channel layers.