Work Function Metal Film Thickness Tuning for Threshold Voltage Control
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Solution Overview
Problem
Current semiconductor devices with three-dimensional channels face challenges in adjusting threshold voltages effectively, as the small size of fins limits the impact of channel and source/drain implants, and traditional methods for tuning work functions are either difficult to implement or lead to performance degradation.
Innovation Solution
The semiconductor device employs a combination of varying thicknesses of work function metal films and doping of work function tuning materials into dielectric films to adjust the effective work functions of transistors, allowing for different threshold voltages without high-concentration dopant implantation, which can deteriorate mobility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If high-concentration dopant implantation is used to adjust threshold voltages, then threshold voltage adjustment is achieved, but mobility deteriorates
Solution Approach 1:
The patent changes the approach from adjusting threshold voltage through dopant concentration to adjusting it through work function metal film thickness. By varying the thickness of the work function metal film (e.g., from 5nm to 20nm), different threshold voltages are achieved without introducing high-concentration dopants that would harm mobility. This parameter change resolves the contradiction by decoupling threshold voltage control from dopant concentration.
Solution Approach 2:
The patent introduces a work function metal film as an intermediary layer between the gate dielectric and the gate electrode. This intermediary layer serves as the primary mechanism for threshold voltage adjustment, replacing the need for high-concentration dopant implantation. The work function metal film's thickness and material composition become the controlling parameters, eliminating the harmful effect on mobility while achieving precise threshold voltage tuning.
2Manufacturing precision
If channel implant or source/drain implant is adjusted to change threshold voltages, then threshold voltage adjustment is possible, but the influence is limited due to small fin width
Solution Approach 1:
The patent shifts the control parameter from implant dosage (which has limited effect due to small fin width) to work function metal film thickness. This parameter change provides a more effective lever for threshold voltage control, as the work function directly influences the gate voltage required to invert the channel, offering broader and more precise adjustment range without being constrained by the small fin dimensions.
Solution Approach 2:
The patent segments the gate structure into multiple functional layers: gate dielectric, work function metal film, and gate electrode. This segmentation allows independent optimization of each layer's function. The work function metal film specifically handles threshold voltage control, while the gate electrode provides drive current, and the gate dielectric provides electrical isolation. This segmentation enables precise threshold voltage adjustment without affecting the other functional aspects of the transistor.
3Manufacturing precision
If traditional work function tuning methods are used, then work function adjustment is achieved, but the implementation is difficult or leads to performance degradation
Solution Approach 1:
The patent simplifies work function tuning by changing it from a complex material composition adjustment to a straightforward thickness control parameter. By depositing work function metal films of different thicknesses using standard thin-film deposition techniques, precise work function control is achieved without complex material engineering. The thickness parameter (e.g., 5nm, 10nm, 15nm, 20nm) provides a simple, manufacturable way to tune the work function and consequently the threshold voltage.
Solution Approach 2:
The patent applies local quality by placing work function metal films with specific thicknesses in specific regions to create transistors with different threshold voltages. For example, smaller fins may use thicker work function metal films while larger fins use thinner films, or different regions of the same fin may have different work function metal film thicknesses. This localized approach enables precise threshold voltage control tailored to specific device requirements without affecting the entire wafer uniformly.
Data Source
AI summary
A semiconductor device includes a first transistor comprising a first dielectric film on a substrate and a first work function metal film of a first conductivity type on the first dielectric film, a second transistor comprising a second dielectric film on the substrate and a second work function metal film of the first conductivity type on the second dielectric film, and a third transistor comprising a third dielectric film on the substrate and a third work function metal film of the first conductivity type on the third dielectric film. The first dielectric film comprises a work function tuning material and the second dielectric film does not comprise the work function tuning material. The first work function metal film has different thickness than the third work function metal film. Related methods are also described.