Piezoelectric microspeakers built on various diaphragms

Abstract

This paper describes and compares four different types of diaphragm-based piezoelectric microspeakers built on (1) a compressively stressed silicon nitride diaphragm, (2) a parylene diaphragm, (3) a dome-shaped silicon nitride diaphragm, and (4) a PZT bimorph diaphragm. The main innovation in the first device is the usage of a wrinkled diaphragm that supports a flat diaphragm, where piezoelectric actuation happens, and allows a large bending displacement. The second device is to exploit the very low elastic modulus of parylene (a polymer material), and is built on a 1.5 μm thick parylene diaphragm with electrodes and piezoelectric ZnO film. Also described in this paper is an acoustic transducer built on a 1.5 μm thick dome-shaped silicon nitride diaphragm (2 mm in radius, with a circular clamped boundary on a silicon substrate) with electrodes and piezoelectric ZnO film. The dome diaphragm is shown to effectively release residual stress through volumetric change of its shape. Finally, this paper describes a microspeaker (composed of 8 mm square PZT bimorph and bulk-micromachined silicon) that shows flat diaphragm displacement from DC to 8 kHz. A bimorph diaphragm is formed by gluing two 127 μm thick PZT sheets and attaching them to a micromachined silicon substrate.