SiC Gate Insulator Nitrogen Diffusion for Mobility
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Solution Overview
Problem
Current methods for forming gate insulating films on silicon carbide (SiC) substrates result in low channel mobility and high anisotropy in MISFETs, making them unsuitable for commercial applications due to defects and poor interface quality between the SiC layer and the gate insulating film.
Innovation Solution
A silicon carbide-oxide layered structure with a V-group element containing oxide layer, such as nitrogen or phosphorus, is formed on a silicon carbide layer, where the V-group element concentration is high in the lower part of the oxide layer, achieving a relative dielectric constant of 3.0 or higher, which improves interface quality and carrier mobility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thermal oxide film is formed at high temperature (1100°C or higher) and annealed, then the gate insulating film quality is improved, but the channel mobility deteriorates to 20 cm²/Vs or lower due to thermal treatment at 950°C or higher
Solution Approach 1:
The gate insulating film is segmented into multiple layers: a first oxide layer formed at high temperature (1100-1300°C) to ensure quality and low interface states, and a second oxide layer formed at lower temperature to preserve channel mobility. This segmentation allows each layer to fulfill different functions, resolving the contradiction between film quality and mobility.
Solution Approach 2:
Different regions of the gate insulating film structure are assigned different qualities: the first oxide layer near the SiC substrate has high quality with low interface states, while the second oxide layer has lower quality but sufficient insulation properties. This local differentiation optimizes both reliability and manufacturing precision.
2Ease of manufacture
If a thermal oxide film is formed on a SiC (0 0 0 1) substrate with poor flatness (steps over surface), then the substrate is suitable for mass production, but the channel mobility becomes 10 cm²/Vs or lower with large anisotropy
Solution Approach 1:
The first oxide layer locally compensates for surface irregularities by forming at high temperature, creating a uniform interface with the SiC substrate. This local quality improvement at the critical substrate interface enables mass production on substrates with inherent flatness variations while maintaining high channel mobility.
Solution Approach 2:
The first oxide layer is formed preliminarily to create a high-quality interface between the gate insulating film and the SiC substrate before forming the second oxide layer. This preliminary action on the substrate surface eliminates the adverse effects of poor flatness, enabling subsequent processing to achieve high channel mobility regardless of initial substrate quality.
3Reliability
If a single oxide layer is formed to achieve high dielectric constant, then the interface state density increases, but if a multi-layer structure is used, the relative dielectric constant can be maintained at 3.0 or higher while reducing interface states
Solution Approach 1:
The gate insulating film is divided into two oxide layers with different formation temperatures and properties. The first oxide layer provides low interface states, while the second oxide layer contributes to the overall dielectric constant. This segmentation achieves both high reliability and sufficient dielectric performance.
Solution Approach 2:
The gate insulating film uses a composite structure of two oxide layers, combining the advantages of high-temperature formed oxide (low interface states) and lower-temperature formed oxide (contribution to dielectric constant). This composite approach achieves both low interface state density and high relative dielectric constant of 3.0 or higher.
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 results in high channel mobility and current drivability in MISFETs, reducing interface state density and enhancing the dielectric constant, thus overcoming the limitations of existing techniques for SiC-based power devices.
Implementation Method 1
a thermal oxide film is formed at a high temperature of 1100° C. or higher in a dry or wet atmosphere
Implementation Method 2
at least a lower part of the V-group element containing oxide layer includes a region where the concentration of the V-group element is high
Data Source
AI summary
A gate insulating film which is an oxide layer mainly made of SiO2 is formed over a silicon carbide substrate by thermal oxidation, and then, a resultant structure is annealed in an inert gas atmosphere in a chamber. Thereafter, the silicon carbide-oxide layered structure is placed in a chamber which has a vacuum pump and exposed to a reduced pressure NO gas atmosphere at a high temperature higher than 1100° C. and lower than 1250° C., whereby nitrogen is diffused in the gate insulating film. As a result, a gate insulating film which is a V-group element containing oxide layer, the lower part of which includes a high nitrogen concentration region, and the relative dielectric constant of which is 3.0 or higher, is obtained. The interface state density of an interface region between the V-group element containing oxide layer and the silicon carbide layer decreases.