Fraunhofer ISIT has been working on the deposition of thin films of the piezoelectric materials aluminum nitride (AlN) and lead zirconate titanate (PZT) for more than 10 years. Currently, sputtering processes with layer thicknesses down to 4 µm for AlN and down to 3 µm for PZT are available. Integration into micromechanical devices is mostly in the form of bending elements (Unimoph), which consist of a passive carrier layer of monocrystalline silicon or polysilicon and the piezoelectric layer on top of it, embedded between two metal electrodes. For actuator applications, PZT is mainly used because its high piezoelectric coefficients allow particularly large deflections and high forces at only low drive voltages. In connection with this, work is currently being done at ISIT primarily on MEMS scanners and speakers.
For sensing applications, AlN is preferred due to its much better signal-to-noise ratio. Currently, it is used at ISIT in ultrasonic transducers, MEMS microphones as well as vibration harvesters. In addition to these established materials, scandium-doped AlN (AlScN) is also being worked on in collaboration with the Christian-Albrecht University of Kiel. Despite the higher piezoelectric coefficients compared to pure AlN, this material retains the advantageous properties (high dielectric strength, IC compatible, low dielectric losses) of AlN and is thus suitable for both sensing and actuating MEMS devices. In the long term, it is expected to completely replace lead-based PZT. Current developments include a highly sensitive magnetoelectric sensor and MEMS scanners based on AlScN.