Graded Phosphorus Doping in NFET Source-Drain Regions
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Solution Overview
Problem
The abrupt phosphorous (P) dopant concentration transition from the channel to the extension region in n-type field-effect transistors (NFETs) leads to low breakdown voltage (BV) and high substrate current (Isub), which are not adequately addressed by traditional implant condition tuning methods.
Innovation Solution
The formation of multiple-step epi-SiP or epi-SiC/epi-SiP source/drain (S/D) structures with consecutive epi-Si layers doped at increasing dosages of P, achieving a graded P dopant concentration distribution from the channel to the extension region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a single epi-SiP layer with high P concentration is used to boost DC performance, then DC performance is improved, but breakdown voltage decreases and substrate current increases
Solution Approach 1:
The single epi-SiP layer is segmented into multiple epi-Si layers with progressively increasing P concentrations. This segmentation allows the device to achieve both high DC performance (through overall high doping) and high breakdown voltage (through gradual concentration transition) by dividing the doping function across multiple layers.
Solution Approach 2:
Different regions of the S/D structure are assigned different P concentrations locally. The lower epi-Si layers have lower P concentrations to ensure high breakdown voltage and low substrate current, while the upper epi-Si layers have higher P concentrations to provide strong DC performance. This local quality differentiation resolves the contradiction between DC performance and reliability.
2Reliability
If implant conditions are tuned to reduce doping profile steepness, then breakdown voltage improves, but DC performance degrades
Solution Approach 1:
Instead of uniformly reducing doping steepness through implant condition tuning, the patent changes the parameter of doping concentration distribution by using multiple epi layers with progressively increasing P concentrations. This allows the doping profile to be gradual (improving breakdown voltage) while still achieving high overall doping (maintaining DC performance).
Solution Approach 2:
The S/D structure uses a composite of multiple epi-Si layers doped at different P concentrations rather than a single uniform material. This composite structure enables simultaneous optimization of both breakdown voltage (through the gradual transition from lower to upper layers) and DC performance (through the cumulative high doping effect).
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 approach enhances breakdown voltage and reduces substrate current, improving the overall performance of NFET devices by creating a more gradual dopant transition.
Implementation Method 1
performing in-situ doping of P for each epi-Si layer, wherein consecutive epi-Si layers are doped at increasing dosages of P
Data Source
AI summary
A method of forming NFET S/D structures with multiple layers, with consecutive epi-SiP layers being doped at increasing dosages of P and the resulting device are provided. Embodiments include forming multiple epi-Si layers in each S/D cavity of a NFET; and performing in-situ doping of P for each epi-Si layer, wherein consecutive epi-Si layers are doped at increasing dosages of P.