Fabrication of dome-shaped diaphragm with circular clamped boundary on silicon substrate

Abstract

We have successfully fabricated an acoustic transducer built on a 1.5 /spl mu/m thick dome-shaped silicon nitride diaphragm (2,000 /spl mu/m in radius, with a circular clamped boundary on a silicon substrate) with electrodes and piezoelectric ZnO film. The key idea of making such a large 3-D diaphragm is in producing a large spherical etch front (2 mm in radius) in a 2 mm thick silicon substrate by an isotropic silicon etching with a 75 /spl mu/m thick cellophane tape as an etch mask. During the isotropic etching, we observe a self-limiting etching behavior caused by the gas bubbles (etch byproducts) which close the etch window and limit the mass transport of the etchant to the silicon etch surface. This self-limiting etching is advantageously used to control the dome diaphragm dimension. One major advantage of a dome diaphragm is that the diaphragm can effectively release residual stress through volumetric change of its shape, and can easily be wrinkle-free and/or crack-free. The fabricated acoustic transducer has been measured to produce a sound output above 70 dB SPL in 10-200 kHz when the transducer is driven by a 11 V/sub rms/ sinusoidal source.