Tri-Layer Gate Metal Stack for Nanosheet Transistor Threshold Voltage Control
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Solution Overview
Problem
Nanosheet-based transistors face challenges in maintaining consistent threshold voltage due to device-to-device variability in gate metal thickness and the sensitivity of threshold voltage to the thickness of the gate metal material, leading to significant variations in performance.
Innovation Solution
A tri-layer gate metal stack is used, comprising an inner nitride layer, a doped transition metal layer, and an outer nitride layer, which allows for the decoupling of threshold voltage from the thickness of the aluminum doped transition metal carbide layer, enabling precise control of the effective work function and minimizing device-to-device variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-layer gate metal structure is used, then the device structure is simple, but the threshold voltage control is poor due to sensitivity to gate metal thickness variations
Solution Approach 1:
The gate metal is divided into multiple layers (first gate metal layer, second gate metal layer, and third gate metal layer) with different materials and functions. The first layer provides baseline work function, the second layer (aluminum-doped transition metal carbide) provides tunable work function with thickness independence, and the third layer provides additional control. This segmentation allows threshold voltage control without sensitivity to total gate metal thickness variations.
Solution Approach 2:
The patent uses composite gate metal structure combining different materials: transition metal (Ti, Ta, W), transition metal carbide (TiC, TaC, WC), and aluminum-doped transition metal carbide. Each material contributes different electrical properties, allowing the composite structure to achieve desired threshold voltage while being insensitive to thickness variations in the aluminum-doped layer.
2Productivity
If the gate metal thickness is reduced to increase device density, then device density increases, but threshold voltage variations increase due to thickness control difficulties
Solution Approach 1:
The patent changes the work function parameter by introducing aluminum doping into the transition metal carbide layer. By controlling the aluminum doping concentration rather than the total gate metal thickness, the threshold voltage can be precisely controlled even when the gate metal layers are thin (below 3 nm), thus maintaining both high device density and threshold voltage consistency.
Solution Approach 2:
The aluminum-doped transition metal carbide layer acts as an intermediary between the first gate metal layer and the second gate metal layer. This intermediate layer with tunable work function decouples the threshold voltage control from the total gate metal thickness, allowing thin gate structures to achieve reliable threshold voltage control.
3Adaptability or versatility
If aluminum doping concentration is increased to adjust work function, then effective work function changes, but device-to-device variability increases
Solution Approach 1:
The gate metal is segmented into three layers where the aluminum doping is confined to the second gate metal layer. This segmentation isolates the doping variations to a specific layer, preventing propagation of variability across the entire gate structure and reducing device-to-device variations while maintaining work function tunability.
Solution Approach 2:
The composite structure of transition metal carbide with aluminum doping provides a material system where the work function can be tuned through controlled aluminum incorporation. The specific composition (transition metal carbide base with aluminum dopant) provides both tunability and reduced variability compared to uniform doping throughout the entire gate metal structure.
Data Source
AI summary
Embodiments are directed to a method of forming a semiconductor device and resulting structures for controlling a threshold voltage on a nanosheet-based transistor. A nanosheet stack is formed over a substrate. The nanosheet stack includes a first nanosheet vertically stacked over a second nanosheet. A tri-layer gate metal stack is formed on each nanosheet. The tri-layer gate metal stack includes an inner nitride layer formed on a surface of each nanosheet, a doped transition metal layer formed on each inner nitride layer, and an outer nitride layer formed on each doped transition metal layer.