Dual Work Function CMOS Devices Using Carbide Electrodes
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Solution Overview
Problem
The challenge in semiconductor manufacturing is to reduce transistor gate dielectric thickness to increase packing densities while mitigating the insulative effect of the polysilicon depletion region and achieving different work functions for PMOS and NMOS transistors, which is not efficiently addressed by single work function metal gates.
Innovation Solution
The method involves forming metal carbide or metal layers with different work functions by selective masking and altering the metal carbide or metal layers with oxygen and/or nitrogen, allowing for the formation of NMOS and PMOS transistors with reduced gate dielectric thickness and efficient CMOS fabrication.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If polysilicon gates are used to achieve different work functions through substitutional doping, then dual work function requirements for PMOS and NMOS are met, but gate depletion region causes insulative effect that worsens as gate dielectric thickness is reduced
Solution Approach 1:
The gate electrode is segmented into two distinct metal layers: a first metal layer (e.g., tungsten, molybdenum, niobium) providing a first work function for PMOS transistors, and a second metal layer (e.g., titanium nitride, tantalum nitride, tungsten nitride) providing a second work function for NMOS transistors. This segmentation allows each metal layer to independently fulfill its work function requirement without the depletion region problems associated with polysilicon gates.
Solution Approach 2:
The gate electrode employs a composite structure combining two different metal materials, each selected for its specific work function characteristics. The first metal material has a work function suitable for PMOS devices (typically higher work function), while the second metal material has a work function suitable for NMOS devices (typically lower work function). This composite approach eliminates the need for substitutional doping and avoids gate depletion entirely.
2Productivity
If gate dielectric thickness is reduced to increase packing density, then transistor size is reduced and more transistors can be packed, but the depletion region's insulative effect is magnified significantly
Solution Approach 1:
The invention extracts and eliminates the depletion region problem by replacing polysilicon gate material with metal materials. By taking out the polysilicon component that causes depletion, the gate electrode maintains purely conductive behavior even when gate dielectric thickness is reduced to enable high packing density. This allows aggressive scaling of gate dielectric thickness without the compounding insulative effect that would occur with polysilicon.
3Ease of manufacture
If single work function metal gates are used, then manufacturing is simplified, but dual work function requirements for PMOS and NMOS transistors cannot be met
Solution Approach 1:
The gate electrode structure implements local quality by having different metal materials in different regions or layers, each optimized for specific transistor types. The first metal layer is optimized for PMOS work function requirements while the second metal layer is optimized for NMOS work function requirements. This local differentiation allows dual work function capability while maintaining a relatively simple two-layer fabrication process.
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 enables the concurrent formation of metal gate transistors with different work functions, reducing feature sizes and increasing packing densities by effectively managing the depletion region and work function requirements for PMOS and NMOS transistors.
Implementation Method 1
altering the metal carbide or metal layers with oxygen and/or nitrogen
Data Source
AI summary
Concurrently forming different metal gate transistors having respective work functions is disclosed. In one example, a metal carbide, which has a relatively low work function, is formed over a semiconductor substrate. Oxygen and/or nitrogen are then added to the metal carbide in a second region to establish a second work function in the second region, where the metal carbide itself establishes a first work function in a first region. One or more first metal gate transistor types are then formed in the first region and one or more second metal gate transistor types are formed in the second region.