Dual Work Function Metal Gate CMOS with Selective Capping
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Solution Overview
Problem
Manufacturing semiconductor devices with independently adjustable and stable dual work function metal gates is challenging, as existing methods face issues with dopant interaction, nickel migration, and unsatisfactory mid-gap work functions in CMOS devices.
Innovation Solution
A CMOS device design featuring NMOS transistors with a mid-gap metal gate and a high oxygen affinity cap for low work function, and PMOS transistors with a mid-gap metal gate and a low oxygen affinity cap for high work function, allowing for controlled oxygen concentration in the metal gate stack to achieve desired band edge work functions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If doped polysilicon gates are used to adjust work function, then threshold voltage can be reduced, but device reliability deteriorates due to dopant interaction with gate dielectric and nickel migration
Solution Approach 1:
The patent extracts the problematic dopants from the gate structure by using undoped or lightly-doped polysilicon gates combined with metal silicide layers. This removes the source of dopant interaction with the gate dielectric while maintaining the ability to adjust work function through the metal silicide composition and thickness control.
Solution Approach 2:
The patent employs composite gate structures combining polysilicon with metal silicide layers (such as nickel silicide, cobalt silicide, or tungsten silicide). This composite approach allows independent control of work function through the metal layer while the polysilicon provides structural stability and reduces migration issues.
2Manufacturing precision
If mid-gap metal gates are used, then work function stability is improved, but NMOS and PMOS transistors cannot achieve low threshold voltage due to mid-gap work function
Solution Approach 1:
The patent applies different cap layer materials or thicknesses to different regions of the gate structure for NMOS and PMOS transistors. By selectively modifying the local composition at the gate-dielectric interface, the work function can be independently tuned for each transistor type while maintaining overall structural stability.
Solution Approach 2:
The patent adjusts work function by changing physical parameters such as metal silicide layer thickness, composition ratios, and oxygen concentration. These parameter variations enable continuous tuning of work function values to achieve both stable gates and low threshold voltages for both NMOS and PMOS devices.
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 stable and adjustable work functions for NMOS and PMOS transistors, improving device performance and reliability by controlling oxygen levels at the metal gate-dielectric interface.
Implementation Method 1
NMOS transistor with a metal gate electrode comprising a mid-gap metal with a low work function/high oxygen affinity cap
Implementation Method 2
PMOS transistor with a metal gate electrode comprising a mid gap metal with a high work function/low oxygen affinity cap
Implementation Method 3
controlling the oxygen concentration in the metal gate stack to achieve desired band edge work functions
Data Source
AI summary
A CMOS device having an NMOS transistor with a metal gate electrode comprising a mid-gap metal with a low work function/high oxygen affinity cap and a PMOS transistor with a metal gate electrode comprising a mid gap metal with a high work function/low oxygen affinity cap and method of forming.