Broadband electrostatic transducers: Modeling and experiments

A model for broadband electrostatic transducers capable of generating and detecting ultrasound in air at megahertz frequencies has been developed. This model uses a lumped parameter approximation to describe a transducer with a grooved backplate and a stretched diaphragm. The mechanical stiffness effects included in the model are compressibility of the air gap separating the diaphragm and the backplate, flexure bending stiffness of the diaphragm, and in-plane tension forces applied to the diaphragm. A prototype transducer with backplate grooves 200 μm wide and 3.75 μm deep was constructed using micromachining techniques. Measurements of the electrical admittance and transmit sensitivity were made at various polarization voltages and diaphragm tensions. Model predictions of transducer electrical admittance and transmit sensitivity compare well with experimental data. The resonance frequency is predicted within 50 kHz of the nominal measured value near 500 kHz, and model predictions of transmit sensitivity were within 0–4 dB of measured values.