Mechanical property evaluation of silicon nitride for insulation substrate

With the transition from silicon (Si) to next-generation materials such as silicon carbide (SiC), power semiconductors are increasingly required to operate under high-voltage and high-temperature conditions. Consequently, the development of insulating substrates with high breakdown voltage, excellent thermal resistance, and superior heat dissipation has become essential. In this study, we investigated the microstructure of silicon nitride (Si₃N₄) and evaluated the mechanical properties of single-layer and double-layer structures using three-point bending and tests. Double-layer specimens were fabricated using three different methods: direct bonding, bonding with adhesive, and simply stacked without adhesive. The bending tests revealed that the directly bonded and single-layer specimens exhibited the highest bending strength, followed by adhesive-bonded specimens, and then unbonded ones. Fatigue tests showed that all specimens endured up to 1×10⁶ cycles without failure under stresses below 70% of the bending strength. However, at stress levels of 71% or higher, both fractured and unfractured specimens were observed. Microstructural observation showed randomly oriented rod-like β-Si₃N₄ grains and irregularly distributed pores. After dielectric breakdown, crack propagation pass was observed under all test conditions, and the correlation of the fatigue damage with the electric breakdown resistance was discussed.