Silicon microspeaker with out-of-plane displacement

In this paper a new way of increasing the enclosed air volume between the stator and the membrane of an electrostatic loudspeaker is introduced. Instead of using a thicker sacrificial layer, a stress-induced self-raising of the stator is utilized. Corrugation grooves in combination with highly tensile silicon nitride rings are causing a deflection of the stator after the release etch. For a stator diameter of 1 mm an out of plane deflection of up to 59 μm could be measured. On the electrical side, a pull-in voltage between 4 V and 16 V for the membrane and 27 V to 67 V for different stator variants was detected. In the free-field, a sound pressure level of 50 dB SPL at 10 kHz in 10 cm distance was measured for a small array. Variations of design and layout as well as technology parameters were varied to determine the ideal system with regard to maximum deflection, displaced volume and mechanical stability.