ForMikro – Salsa

As part of the BMBF-funded project “ForMikro - Salsa”, the novel ferroelectric ceramic aluminum scandium nitride (AlScN) was established at ISIT.

Ferroelectric materials are used, among other things, as drives for MEMS and in permanent data storage devices. Until now, the lead-containing ceramic lead zirconate titanate (PZT) was the material of choice. PZT exhibits extremely high deformation, but also serious disadvantages, such as its lead content and non-linear behavior. Furthermore, integration on CMOS structures is very difficult due to high deposition temperatures. These disadvantages do not exist with alternative materials such as aluminum nitride (AlN) and AlScN. They are bipolar drivable, high-temperature and long-term stable without requiring high deposition temperatures and have significantly lower electrical losses. However, the achievable deformation is significantly smaller compared to PZT.

This is where the ferroelectric properties of AlScN come into focus. This makes it possible to permanently invert the polarization of the material and thus produce a multilayer stack that couples the performance of several AlScN layers. In this way, the last remaining disadvantage of AlScN can be compensated for.

In cooperation with the Christian-Albrechts-University (CAU), a basic AlScN deposition process was transferred to the ISIT's clean room. This process was then adapted to the tools used on site and further optimized. The process developed in this way allows control over the film stress and almost perfectly oriented layers that can be ferroelectrically inverted on large surfaces. These AlScN films are actively integrated in various other projects.

Based on this stable process, the development and integration of AlScN multilayers was started. Furthermore, the focus was placed on the performance on SOI-like substrates in order to be able to demonstrate the multilayers on MEMS loudspeakers. As part of the project, the first AlScN bilayers with large inverted areas were demonstrated. The first free-standing actuators based on the double layer were also demonstrated as a basis for further applications such as MEMS loudspeakers.