Bimorph Diaphragm Formed by Two PZT Sheets on Micromachined Silicon for Sound Generation

Abstract

This paper describes a microspeaker (composed of a mechanically-polished PZT bimorph diaphragm and bulk-micromachined silicon top cover) that shows flat diaphragm displacement from DC to 16kHz. A bimorph diaphragm is formed by gluing two 127μm thick PZT sheets and attaching them to a micromachined silicon substrate. The front-side pre-deposited nickel electrode was patterned to maximize the stress distribution over a diaphragm with its four edges clamped, when it goes through bending displacement. With un-patterned floating electrode on one side of the diaphragm, the PZT bimorph can generate stresses with opposite signs along the thickness direction without the need of fine alignment among the electrode patterns. It is important to reduce the thickness of the glue layer between the two PZT sheets, in order to maximize the electrical fields inside the PZT sheets, since the glue layer takes some of the applied voltage away. With mechanical polishing and spin-coating of low viscosity resin epoxy, we have been able to reduce the glue layer thickness to about 1 μm with variation less than 0.5 μm from a typical 25μm thickness. The microspeaker with an encapsulating cylindrical package of about 0.5 cc inside volume produces sound pressure level (SPL) of 103.6 ~ 110 dB between 1.3 and 12 kHz when measured 5 mm away from the diaphragm with 190Vpeak-to-peak driving signal. The measured SPL drops as the frequency is reduced below 1 kHz, due to increasing wavelength and thus reduced total acoustic intensity over the measurement area. The diaphragm was measured (with a laser vibrometer) to produce uniform displacement from DC to 16kHz. With 190Vpeak-to-peak sinusoidal input, the displacement at the center of the diaphragm was measured to have 1.86μm displacement at the fundamental resonant frequency of 16.2 kHz.