Nitride MISFET Gate Stack for Threshold Voltage Stability
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Solution Overview
Problem
There is room for improvement in semiconductor devices using nitride semiconductors, particularly in achieving enhanced characteristics and manufacturing methods for such devices, especially regarding the normally-off operation and threshold voltage stability.
Innovation Solution
The semiconductor device employs a stacked structure of gate insulating films with different electronegativities, using an oxide film of a first metal and a second metal with lower electronegativity, and a gate electrode composed of a nitride film and a conductive metal layer to prevent oxygen diffusion and maintain polarization, ensuring a positive threshold voltage and reduced variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single-layer gate insulating film is used, then the device structure is simple, but the threshold voltage control and oxygen diffusion prevention are insufficient
Solution Approach 1:
The gate insulating film is divided into two distinct layers: a first gate insulating film (Al2O3) in contact with the nitride semiconductor layer, and a second gate insulating film (SiO2) with higher electronegativity on top. This segmentation allows each layer to perform its specific function - the first layer prevents oxygen diffusion to the interface, while the second layer provides electrical control - thereby resolving the contradiction between reliability improvement and structural complexity.
Solution Approach 2:
The patent employs a composite gate insulating film structure combining two different oxide materials (Al2O3 and SiO2) with different electronegativities. This composite structure leverages the complementary properties of each material - Al2O3 for oxygen barrier functionality and SiO2 for electrical insulation and threshold voltage control - achieving enhanced reliability while managing the complexity through systematic material selection.
2Reliability
If oxygen diffusion is not prevented, then the manufacturing process is simple, but the threshold voltage becomes negative and unstable
Solution Approach 1:
The first gate insulating film (Al2O3) is formed in advance as an oxygen diffusion barrier layer before the gate electrode is constructed. This preliminary action prevents oxygen from reaching the nitride semiconductor layer interface during subsequent manufacturing steps, ensuring that the threshold voltage remains positive and stable without requiring complex modifications to the gate electrode structure itself.
Solution Approach 2:
The first gate insulating film acts as an intermediary barrier between the gate electrode structure and the nitride semiconductor layer. It mediates the oxygen diffusion issue by blocking oxygen transport while allowing the gate electrode to maintain its electrical function, thus resolving the contradiction between structural simplicity and threshold voltage stability.
3Reliability
If a gate insulating film with uniform electronegativity is used, then the material selection is simple, but the oxygen diffusion prevention and electrical control are insufficient
Solution Approach 1:
The gate insulating film structure assigns different local qualities to different layers based on their electronegativity characteristics. The first layer (Al2O3) has lower electronegativity and is positioned where oxygen diffusion prevention is most critical (at the interface with nitride semiconductor). The second layer (SiO2) has higher electronegativity and provides electrical control. This local differentiation of material properties resolves the contradiction between oxygen barrier performance and electrical control functionality.
Solution Approach 2:
The patent changes the electronegativity parameter across the gate insulating film thickness by selecting materials with progressively different electronegativities. This parameter variation creates an optimized gradient structure where each layer's electronegativity is tailored to its specific functional requirement - lower electronegativity for oxygen barrier performance and higher electronegativity for electrical insulation and threshold voltage control.
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 the semiconductor device's characteristics by achieving a positive threshold voltage and reducing oxygen diffusion, thereby improving the normally-off operation and stability of the device.
Implementation Method 1
the second gate insulating film is an oxide film containing a second metal, in which electronegativity of the second metal is smaller than that of the first metal or silicon
Data Source
AI summary
The characteristics of a semiconductor device are enhanced. In a semiconductor device (MISFET) having a gate electrode GE formed on a nitride semiconductor layer CH via a gate insulating film GI, the gate insulating film GI is configured to have a first gate insulating film (oxide film of a first metal) GIa formed on the nitride semiconductor layer CH and a second gate insulating film (oxide film of a second metal) GIb. And, the second metal (e.g., Hf) has lower electronegativity than the first metal (e.g., Al). By thus making the electronegativity of the second metal lower than the electronegativity of the first metal, a threshold voltage (Vth) can be shifted in a positive direction. Moreover, the gate electrode GE is configured to have a first gate electrode (nitride film of a third metal) GEa formed on the second gate insulating film GIb and a second gate electrode (fourth metal) GEb. This prevents the diffusion of oxygen to the gate insulating film GI, and variations in the threshold voltage (Vth) can be reduced.