Investigating Elastic Anisotropy of 4H-SiC Using Ultra-High Q Bulk Acoustic Wave Resonators

Hexagonal 4H-silicon carbide (4H-SiC) is a transversely isotropic substrate garnering interest for precision MEMS devices such as resonant gyroscopes. This paper investigates the elastic anisotropy of 4H-SiC by utilizing capacitive bulk acoustic wave (BAW) resonators with ultra-high mechanical quality factors (Q) enabled by phononic crystals. We directly measure the value of C₆₆ using Lamé mode resonators for the first time and numerically fit the values of C₁₁ and C₁₂ using BAW elliptical modes in center-supported solid disk resonators. We compare (00 01) 4H-SiC to (111) Si, another in-plane isotropic material and validate (0 001) 4H-SiC's superior robustness to fabrication and design variations. Measurement of in-plane BAW elliptical modes in multiple disk resonators with as-born frequency splits as low as 3 ppm reveal (00 01) 4H-SiC's transverse isotropy across process corners. Lamé mode resonators display a temperature coefficient of frequency (TCF) three times lower compared to its Si counterpart. Finally, this paper provides a modified set of elastic constants for 4H-SiC with a view towards monocrystalline SiC MEMS devices.