Pressure dependence of thin polycrystalline silicon carbide diaphragm resonators

Square poly cry stalline 3C silicon carbide (poly-SiC) thin diaphragms with large aspect ratios (i.e., side length to thickness: L/t ~ 600) are explored as a structural material for micromechanical resonators. The effects of varying pressure on the characteristics of multiple resonant modes are studied. Load-deflection tests reveal a Young´s modulus of E_Y = 344 ± 13 GPa and a residual (built-in) stress of σ₀ = 113 ± 10 MPa. Cyclic loading tests of the diaphragms show no changes in E_Y and σ₀ values after over 30,000 cycles. Resonance measurements of the diaphragms at approximately 5mTorr show peaks with quality (Q) factors of Q´s ≈ 14,000 and 45,000 for the first two flexural resonance modes at 85.5kHz and 137.6kHz. With increasing pressure, the measured resonance frequencies shift higher (i.e., stiffening), by approximately 1.7kHz or 1.9% for the 85.5kHz peak, and 1.8kHz or 1.3% for the 137.6kHz peak. Meanwhile, Q´s are observed to decrease to ~400 at 20Torr for the 85.5kHz resonance and to ~3600 at 2Torr for the 137.6kHz resonance. Simultaneously as the pressure increases, the peak amplitude of the resonance reduces by 97% for the mode at 85.5kHz, and by 93% for the 137.6kHz resonance. These characteristics may be exploited for resonant pressure sensing using poly-SiC.