Material characterization of high aspect ratio silicon nitride diaphragms

Micromachined silicon nitride (Si3 + Na4 diaphragms were recently found to withstand differential pressures in excess of 600 psi (4.1 MPA), despite their very high length-to-thickness aspect ratios (l/t = 500). The square diaphragms are only 0.8 μm (0.00003 inch) thick, yet they span a relatively large are of 160,000 μ2. The diaphragms were made of low-stress silicon nitride deposited on silicon wafers using a Low Pressure Chemical Vapor Deposition (LPCVD) process. The substrate silicon was subsequently masked and anisotropically etched from beneath the thin film in square windows 400 μm on a side. The diaphragms were subjected to burst pressure testing, where they were found to exhibit much higher burst strengths that initially expected, some as high as 1305 psi (9.0 MPa). In this paper the diaphragms; mechanical properties are described as a function of material composition, diaphragm, size and processing parameters. These empirical results will also be compared with the predictions from a solid-modeling theoretical analysis