The PZT Thin Film Fabrication for PMUT Transducer Using Sputtering Process

Abstract

Piezoelectric micromachining ultrasound transducer (PMUT) is a micromachining-based transducer, which works upon the flexural motion of a thin membrane coupled with a piezoelectric thin film. Lead zirconate titanate (PZT) is the most popular material for PMUT transducers, because of its high piezoelectric constants. The PZT thin film fabricated by sputtering has been reported to have relatively larger piezoelectric constants and better electrical properties than the ones fabricated by other processes. Although the available thickness of PZT film should be in the wide range to give a freedom for designing a good PMUT transducer, only a PZT thin film under 1 has been widely reported by the sputtering process. The reason why it is difficult to fabricate a PZT film thicker than 1 , is the micro-cracks occurring the annealing processes after deposition processes including the sputtering. Moreover, it is more difficult to control the crystal structures of a PZT film when its thickness is getting larger, which result in a lower piezoelectric constants. So the sputtering process and corresponding crystallization condition need to be considerately investigated, to fabricate a relatively thick PZT thin film, without micro crack and good crystal structure.In this thesis, the sputtering process of PZT thin films are studied for fabricating micro-machined-micro-electromechanical system (MEMS) devices such as PMUT. There are various micro-machined electromechanical transducers be developed in VATrans Lab. in Postech, which require from 1-m to 10-m thick PZT film. Therefore, the sputtering technique for fabricating a relatively thick PZT film with excellent piezoelectricity needs to be developed. Moreover, an easy way to patterning the PZT film by sputtering should be also developed for applying it to a MEMS device. For this purpose, the main parameters in sputtering PZT were selected based on the physics reported on the sputtering of PZT and their effects on the deposited PZT films were systematically investigated to find the optimum conditions for sputtering PZT films with a given sputter equipment. In addition, nodule formation, a rarely reported problem in PZT sputtering, was also considered to find the optimum conditions. Obviously the annealing process for crystallization of the deposited PZT thin film should be controlled appropriately to obtain a uniform crack-free PZT thin film that can be used for MEMS devices. Finally, a 4.4-m thick PZT film was successfully fabricated without micro crack. The uniformity across 4 inch wafer is 80%. In addition, a fabrication process for making a PZT film in a PMUT was developed based on the developed PZT sputtering technology. The PMUT fabricated by use of the PZT sputtering was tested and turned out to show the predicted performance by the design that was previously proved by the fabrication techniques based on the Sol-Gel method of PZT thin film.Keywords: PMUT, PZT thin film, sputtering, thick film, stencil mask