Usually when piezoelectric membrane actuators are mentioned, one classically thinks of a passive membrane with a piezoelectric actuator on top. In this paper omitting the passive membrane will be suggested. Therefore, the piezoelectric actuator will not be commonly designed as plate capacitor structure, such as that the piezoelectric material sitting between two plate electrodes. Rather will the actuation be caused by structured surface electrodes on only one side of the piezoelectric material, leaving the opposite side free of potential. Several surface electrode designs, starting with interdigital parallel structures, known from surface acoustic wave transducers, spiral and ring structures, ending with star-shaped structures, are tested. The main advantage of this actuation principle is that with the varying electrode design it became possible to also generate an upward movement, i.e., in the direction of electrode side of the membrane. This upward movement has not been achieved with any piezoelectric membrane actuator so far.

This paper introduces for the first time a 100µm thick PZT actuator driven by single sided interdigitated electrodes that achieves a deformation of 15 µm which benefits from the piezoelectric nonlinearity and the reduced stiffness of the substrate. The deformation was at first simulated using a finite element model. The actuator was later fabricated using a standard lithographic process. The characterization followed on using a white light interferometer and a laser Doppler vibrometer.

Using piezoelectric bulk material for manufacturing membrane actuators offers several advantages. Instead of manufacturing e.g. a silicon membrane and then either depositing a piezoelectric thinfilm actuator or mounting a piezo disc or stack to the silicon membrane, it is possible to use the piezoelectric material itself as membrane. Circular lead zirconate titanate (PZT) discs were adapted to silicon surface micromachining technologies. By depositing interdigitated electrode layouts several actuators were structured on one substrate. Those inderdigitated electrode layouts, when actuated, cause an inhomogeneous electric field distribution and thus cause an inhomogeneous mechanical stress distribution within the PZT-substrate. This forces the PZT to deflect in those actuated areas, without the usually needed passive membrane.

This paper will introduce a piezoelectric micro membrane actuator with interdigitated (IDT) surface electrodes for polarization and actuation. The thus achieved polarization and generated electric field are highly inhomogeneous and cause therefore also inhomogeneous strain and stress distributions within the piezoelectric material itself. To equalize the strain and stress the material will deform. This deformation can be manipulated due to electrodes design and clamping of the membrane. The most outstanding achievement with this manipulation is the deflection of the membrane towards the actuated surface and not as is common in piezoelectric membrane actuators towards the passive surface of the device.

Using circular discs of piezoelectric material an easy adaptation to well-known silicon surface micro machining technologies is possible. Thus, several interdigitated electrodes layouts have been positioned on a planar lead-zirconate-titanate disc via lift off processes. Because of an inhomogeneous electric field distribution the actuated areas on the disc conduct a symmetric deformation of the bulk material without any need of a further passive membrane, as is necessary regarding common piezoelectric membrane actuators. The most interesting deformation was shown by star shaped interdigitated electrodes. It was an overall downward movement (i.e. the side opposite the electrodes) of the actuated area and a center movement in the opposite direction about 25% of the downward movement. This phenomenon is further investigated, especially with regard to increasing the upward movement. Therefore, the behavior of the actuated area has been more thoroughly tested, using laser vibrometry, speckle imaging and white light interferometry. Based on the thus salvaged data, those actuators have been investigated especially concerning their deformation behavior in various clamping modes either during polarization and/or during actuation.