Silicon Nitride Substrate Grain Boundary Control
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Solution Overview
Problem
Existing silicon nitride substrates face challenges in achieving high thermal conductivity and mechanical strength while maintaining thermal shock resistance, particularly in power semiconductor modules where high thermal conductivity is limited by the inclusion of MgSiO3 and RE-containing crystal phases, and bending strength is not sufficiently high.
Innovation Solution
A silicon nitride sintered body with a grain boundary phase composed of amorphous and MgSiN2 crystal phases, where the MgSiN2 crystal phase is precipitated in a controlled amount to enhance thermal conductivity without compromising mechanical strength, and rare earth elements are primarily present in an amorphous form to prevent crystallization-induced weaknesses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the grain boundary phase is made amorphous to increase bending strength, then mechanical strength is improved, but thermal conductivity deteriorates
Solution Approach 1:
The patent applies parameter changes by controlling the crystallization state of the grain boundary phase. Specifically, it transforms the grain boundary phase from completely amorphous to a state with controlled crystallization (30-70% crystallized), containing specific crystal phases (MgSiO3, Mg2SiO4, or MgSiN2) while maintaining amorphous characteristics. This partial crystallization parameter change simultaneously improves thermal conductivity while preserving the bonding strength provided by the amorphous phase, resolving the contradiction between mechanical strength and thermal conductivity.
2Reliability
If 20% or more of the grain boundary phase is crystallized to increase thermal conductivity, then thermal conductivity is improved, but mechanical strength deteriorates
Solution Approach 1:
The patent optimizes the crystallization parameter by controlling it to be within 30-70%, which is higher than the conventional 20% threshold for thermal conductivity improvement but maintained at levels that preserve mechanical strength. This is achieved by selecting specific crystal phases (MgSiO3, Mg2SiO4, or MgSiN2) that form in this crystallization range, thereby simultaneously achieving high thermal conductivity and maintained bending strength.
Solution Approach 2:
The patent applies local quality by creating different phases with different functions: the amorphous phase provides strong bonding between grains (mechanical strength), while the crystalline phases (MgSiO3, Mg2SiO4, or MgSiN2) embedded in the grain boundary provide high thermal conductivity. This local differentiation of phase quality within the grain boundary allows simultaneous optimization of both mechanical strength and thermal conductivity.
3Reliability
If MgSiO3 and RE-containing crystal phases are included to improve thermal conductivity, then thermal conductivity is improved, but bending strength becomes insufficient
Solution Approach 1:
The patent changes the compositional parameter of the grain boundary phase by limiting MgO content to 5-20 mass% and RE2O3 content to 0.1-5 mass%, with a controlled molar ratio of RE2O3/MgO between 0.01-0.5. This compositional control ensures that when MgSiO3 or RE-containing crystal phases form, they do so in quantities and proportions that improve thermal conductivity without excessively compromising bending strength.
Solution Approach 2:
The patent creates a composite grain boundary phase structure combining amorphous matrix with dispersed crystalline phases (MgSiO3, Mg2SiO4, or MgSiN2). This composite structure leverages the high thermal conductivity of the crystal phases while the amorphous matrix maintains the bonding strength, achieving synergistic properties that resolve the contradiction between thermal conductivity and bending strength.
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 a silicon nitride substrate with improved thermal conductivity exceeding 80 W/m·K and bending strength of at least 820 MPa, along with enhanced thermal shock resistance, suitable for high-frequency transistors and power semiconductor modules, preventing crack formation under thermal stress.
Implementation Method 1
the grain boundary phase is made up of an amorphous phase and a MgSiN2 crystal phase
Implementation Method 2
a sintering step of sintering the compacted body in a nitrogen atmosphere at a temperature of 1,800°C to 2,000°C
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
Provided are a silicon nitride substrate made of a silicon nitride sintered body that is high in strength and thermal conductivity, a method of producing the silicon nitride substrate, and a silicon nitride circuit substrate and a semiconductor module that use the silicon nitride substrate. According to the silicon nitride sintered body, in a silicon nitride substrate consisting of crystal grains 11 of β-type silicon nitride and a grain boundary phase containing at least one type of rare earth element (RE), magnesium (Mg) and silicon (Si), the grain boundary phase consists of an amorphous phase 12 and a MgSiN2 crystal phase 13; the X-ray diffraction peak intensity of any crystal plane of a crystal phase containing the rare earth element (RE) is less than 0.0005 times the sum of the diffraction peak intensities of (110), (200), (101), (210), (201), (310), (320) and (002) of the crystal grains of the β-type silicon nitride; and the X-ray diffraction peak intensity of (121) of the MgSiN2 crystal phase 13 is 0.0005 to 0.003 times the sum of the X-ray diffraction peak intensities of (110), (200), (101), (210), (201), (310), (320) and (002) of the crystal grains of the β-type silicon nitride.