Nitride MISFET Gate Insulator Shift Threshold Voltage
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Solution Overview
Problem
Current semiconductor devices using nitride semiconductors face challenges in achieving the normally-off characteristic due to issues with trap density and threshold voltage control, particularly with single-layer aluminum oxide gate insulating films which result in negative threshold voltages and increased hysteresis.
Innovation Solution
A semiconductor device with a gate insulating film comprising a laminated structure of oxide films, where the first metal is aluminum and the second metal has lower electronegativity, such as hafnium, is used, with heat treatment to reduce trap density and shift the threshold voltage positively, enabling normally-off operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single-layer aluminum oxide gate insulating film is used, then the manufacturing process is simple, but the threshold voltage becomes negative and hysteresis increases
Solution Approach 1:
The patent applies composite materials by forming a laminated gate insulating film structure consisting of an aluminum oxide film layer and a hafnium oxide film layer. The aluminum oxide layer provides good interface characteristics with the nitride semiconductor, while the hafnium oxide layer with lower electronegativity contributes to positive threshold voltage and reduced hysteresis. This composite structure resolves the contradiction by combining materials with complementary properties to achieve both manufacturability and reliable threshold voltage control.
Solution Approach 2:
The patent employs parameter changes by selecting metals with different electronegativity values for the laminated oxide films. Specifically, the second metal (hafnium) has lower electronegativity than the first metal (aluminum), which fundamentally changes the electrical characteristics of the gate insulating film. This parameter change enables positive threshold voltage and reduced hysteresis while maintaining the laminated structure for manufacturability.
2Reliability
If heat treatment is performed on the oxide film, then trap density is reduced and threshold voltage shifts positively, but the manufacturing process becomes more complex
Solution Approach 1:
The patent applies preliminary action by performing heat treatment on the oxide film after formation to proactively reduce trap density and shift threshold voltage to positive values. This preliminary treatment ensures that the device achieves the desired normally-off characteristic before final assembly and operation, preventing subsequent performance issues and reducing the need for additional corrective processing steps.
Solution Approach 2:
The heat treatment process changes the physical and chemical parameters of the oxide film, including reducing trap density and modifying the threshold voltage to positive values. This parameter change is achieved through controlled thermal processing that activates beneficial effects in the laminated oxide structure while maintaining the overall device architecture, thus managing complexity through targeted parameter optimization.
3Reliability
If a laminated structure of oxide films with different electronegativity is used, then hysteresis is reduced and normally-off operation is achieved, but the manufacturing precision requirements increase
Solution Approach 1:
The patent uses composite materials with a laminated structure of aluminum oxide and hafnium oxide films, where each layer has a specific function. The aluminum oxide layer provides interface quality, while the hafnium oxide layer with lower electronegativity reduces hysteresis and enables normally-off operation. This composite approach achieves reliable hysteresis characteristics while distributing manufacturing precision requirements across multiple layers rather than demanding perfect control of a single complex material.
Solution Approach 2:
The patent applies local quality by assigning different functional properties to different layers of the gate insulating film. Each oxide layer is optimized for its specific role: the aluminum oxide layer for interface characteristics and the hafnium oxide layer for hysteresis reduction and threshold voltage control. This local optimization allows each layer to be manufactured with appropriate precision requirements rather than demanding uniform high precision across the entire structure.
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
The approach effectively improves the semiconductor device characteristics by shifting the threshold voltage positively, achieving the normally-off state and reducing hysteresis, thereby enhancing the device's operational performance.
Implementation Method 1
performing a heat treatment to the oxide film of the second metal
Implementation Method 2
the second metal has electronegativity lower than that of the first metal
Data Source
AI summary
In a semiconductor device (MISFET) having a gate electrode formed over a nitride semiconductor layer with a gate insulating film interposed therebetween, the gate insulating film includes a first gate insulating film (oxide film of first metal) formed on the nitride semiconductor layer and a second gate insulating film (oxide film of second metal). The second metal (for example, Hf) has electronegativity lower than that of the first metal (for example, Al). Since the electronegativity of the second metal is lower than that of the first metal, negative charge is introduced into the oxide film of the first metal due to interfacial polarization, so that the flat-band voltage can be shifted in a positive direction. Accordingly, the threshold voltage which has become negative due to the heat treatment of the oxide film of the first metal can be shifted in the positive direction.