MOSFET Channel Dopant Deactivation for Threshold Voltage Stability
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Solution Overview
Problem
MOSFET devices face variability issues due to random dopant fluctuation and threshold voltage variations, which are exacerbated by channel profile and gate critical dimension variations, leading to increased device variability.
Innovation Solution
The implementation of selective dopant deactivation in the region underneath the gate of MOSFET devices, achieved through localized carbon implantation or epitaxial layer formation, reduces active dopants in the channel region, thereby reducing threshold voltage roll-off slope and device variability. Additionally, halo implants are used to further improve threshold voltage stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dopants are present in the channel region to enable device operation, then the MOSFET can function, but random dopant fluctuation and threshold voltage variations increase device variability
Solution Approach 1:
The patent extracts harmful dopants from the channel region by forming a sacrificial layer that contains the dopants, then removing this sacrificial layer to deactivate the dopants. This removes the source of random dopant fluctuation while preserving the channel's functional integrity through subsequent epitaxial regrowth.
Solution Approach 2:
The patent performs preliminary dopant deactivation by forming the sacrificial layer and deactivating dopants in the channel region before final device fabrication steps. This preliminary action prevents dopant-related variability from affecting subsequent processing and final device performance.
2Reliability
If dopants are used in the channel region to establish threshold voltage, then the device operates, but threshold voltage roll-off slope increases with gate length variations
Solution Approach 1:
The patent performs preliminary dopant deactivation in the channel region before gate formation, establishing a dopant-free channel that is insensitive to subsequent gate critical dimension variations. This preliminary action decouples threshold voltage from gate length dependencies.
Solution Approach 2:
The patent applies local quality by creating a dopant-free zone specifically in the channel region while maintaining dopants in other regions. This localized modification targets the threshold voltage roll-off issue without affecting other device characteristics.
3Ease of manufacture
If standard doping processes are used to form the channel, then device fabrication is straightforward, but device variability and threshold voltage roll-off cannot be reduced
Solution Approach 1:
The patent segments the channel formation process into distinct stages: initial doping, sacrificial layer formation, dopant deactivation, and epitaxial regrowth. This segmentation allows precise control of dopant distribution while maintaining overall process feasibility through modular fabrication steps.
Solution Approach 2:
The patent introduces a sacrificial layer as an intermediary structure that facilitates dopant deactivation. This intermediary enables the transformation from a doped to an undoped channel through a controlled process that integrates with standard fabrication workflows.
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 selective dopant deactivation and halo implants significantly reduce threshold voltage roll-off slope, leading to improved device stability and reduced variability across different gate lengths, particularly for long channel devices.
Implementation Method 1
The channel region is doped with dopants. The gate dielectric is formed over a portion of the substrate and the channel region. An implant is performed to deactivate dopants in a region underneath the gate.
Implementation Method 2
growing an undoped epitaxial layer over the channel region, wherein growing the undoped epitaxial layer comprises deactivating dopants in the channel region
Data Source
AI summary
A method of fabricating a device on a substrate includes doping a channel region of the device with dopants. The method further includes growing an undoped epitaxial layer over the channel region, wherein growing the undoped epitaxial layer comprises deactivating dopants in the channel region to form a deactivated region. The method further includes forming a gate structure over the deactivated region.