High-pressure deflection behavior of laser micromachined bulk 6H-SiC MEMS sensor diaphragms

Single crystalline silicon carbide (SiC) in bulk or thin film form has the potential to be a durable and reliable MEMS pressure sensor in high-temperature and harsh chemical environments. Laser micromachining can fabricate SiC diaphragms for pressure sensors much faster than electrochemical etching, photochemical etching, or deep reactive ion etching without the complicated masking procedures that hinder mass production. In this paper, we present the deflection behavior of laser micromachined 6H-SiC MEMS diaphragms under high pressures and high temperatures. For this purpose, oversized MEMS square diaphragms, 1.5 mm × 1.5 mm and 2.5 mm × 2.5 mm, were intentionally built from bulk 250 μm thick single crystal 6H-SiC wafers using a Q-switched Nd:YAG laser with a wavelength of 1064 nm, pulse repetition rate of 3 kHz, pulse width of 100 ns, and average power of 0.35 W. The diaphragms had an average thickness of 100 μm that included a nearly 50% thick black layer composed of a complex mixture of Si, C, SiC2, Si2C, SiO and SiO2. Pressure-deflection tests up to 425 K revealed that the diaphragms remained hermetically sealed under gas pressure as high as 3.0 MPa (435 psi) and that the black layer had a significant effect on the deflection behavior. An analytical model of bending thin plate under hinged boundary conditions and finite element simulations validated the experimental results.