Yasuyuki Kawada

Shape Control and Roughness Reduction of SiC Trenches by High-Temperature Annealing

The high-temperature annealing of SiC trenches has been investigated for improving the shape of trenches and the smoothness of trench sidewalls. In a SiH4-added Ar (SiH4/Ar) atmosphere, the transformation of SiC trenches required a pressure of 80 Torr and a temperature of 1700 °C; The inner surface of the trenches became smoother without significant etching, while the sample surface became rougher. From the time dependence of the curvature at the trench upper corner, the authors consider that both surface diffusion and evaporation–condensation contribute to the transformation, as opposed to the annealing in H2 reported by another group where transformation is driven mainly by evaporation–condensation. The present authors did not observe a significant change in trench shape in a H2 atmosphere at annealing temperatures up to 1400 °C, except for a smoothening of sample surfaces. The proposed two-step annealing process, consisting of annealing at 1700 °C in SiH4/Ar followed by annealing at 1400 °C in H2, realized rounded trench corners and smooth surfaces simultaneously without significant etching.

Anisotropic Transformation of 4H-SiC Etching Shapes by High-Temperature Annealing and Its Enhancement by Ion Implantation

The transformation of 4H-SiC etching shapes by high-temperature annealing was investigated. Although the opening of the etching mask was circular, the resulting etched shape was a hexagon, dodecagon, or rounded polygon with more edges, depending on the diameter. A hexagon was transformed into a dodecagon following high-temperature annealing, and a dodecagon was transformed into a rounded polygon.

CVD Filling of Narrow Deep 4H-SiC Trenches in a Quasi-Selective Epitaxial Growth Mode

By mapping the source and HCl flow rates dependent growth rates, the evolving trend of a quasi-selective epitaxial growth (quasi-SEG) that growing very thin epilayer on mesa top and ensuring an extremely low risk of voids defect generation was firstly figured out on a 5-μm 4H-SiC trench. Then, basing on the acquired knowledge, a 25-μm 4H-SiC trench with an aspect ratio up to ~10 was completely filled in the quasi-SEG mode.

Strong impact of slight trench direction misalignment from on deep trench filling epitaxy for SiC super-junction devices

A trench filling epitaxial growth technique using hot-wall chemical vapor deposition with HCl gas has been developed for SiC super-junction (SJ) device fabrication. 2–6 kV class SJ devices require p/n column structures with depths of over 10 µm. However, rapid trench closure before the trench backfilling process is complete makes these structures difficult to realize. Stripe trenches that were intentionally inclined within ±2° on a surface plane towards the direction were formed on an off-angled wafer, and the effects of trench direction misalignment from the off-direction were investigated. Slight trench direction misalignment was found to affect the tilt angle of the mesa top epi-layer strongly. Tilted growth on the mesa top reduced the filling rate at the trench bottom and caused void formation. When a wafer with high orientation-flat accuracy relative to the direction was used, 25-µm-deep trench backfilling was successfully demonstrated.

Effect of Surface Structure on Transformation of 4H-SiC by High-Temperature Annealing

The transformation of SiC etching shapes by high-temperature annealing has been investigated. Without silicon atoms on the surface, transformation of the etching shapes hardly occurred even after annealing in pure Ar at 1700 °C, where transformation should occur without the loss of silicon atoms. When SiH4 was added to Ar, the surface tended to revert to SiC, and the transformation was enhanced with increasing SiH4 addition. Therefore, the presence of silicon atoms is necessary to transform the etching shapes on SiC surfaces.