The Fraunhofer FEP project “Piezoelectric thin films for actuating deformable mirrors” was carried out from 1 October 2019 to 30 April 2023 as part of the larger Horizon 2020 initiative “Pilot Integration 3 nm Semiconductor Technology (PIn3S)”. Its aim was to develop sputter‑deposited piezoelectric layers that can be integrated on ultra‑low‑expansion (ULE) glass substrates and provide nanometre‑scale deflections under voltages of ±100 V, with a hysteresis below 1 % and a total thickness below 10 µm. The required deposition rate was set at >0.2 nm s⁻¹, the maximum substrate temperature at 300 °C, and the process had to be scalable to substrates up to one metre in diameter.

The primary material investigated was scandium‑doped aluminium nitride (AlScN). Experiments on the C250 sputter system demonstrated that AlScN films could be deposited at temperatures below 300 °C while maintaining a high deposition rate and low mechanical stress. The films exhibited a piezoelectric coefficient d₃₃ in the range of 20–30 pm V⁻¹, sufficient to achieve the target 10–15 nm deflection. However, the control of the film properties across the substrate was found to be challenging, prompting a shift to alternative piezoelectric materials.

Bismuth ferrite (BiFeO₃, BFO) was selected as a promising alternative. Using a dual‑target sputter setup, the composition of the films was tuned by adjusting the target power ratio. Radial composition gradients similar to those observed for AlScN were also present in BFO, leading to a variation of d₃₃ across the wafer. Measurements with a piezometer revealed d₃₃ values between –6 and –10 pm V⁻¹, with the highest values occurring at a radial distance of 20–30 mm. Deposition at 600 °C produced the largest d₃₃, indicating a strong temperature dependence. Further studies on poling behaviour and hysteresis are still pending.

PZT thin films were investigated in collaboration with the Technical University of Dresden. The LS730S cluster‑rotating sputter system, equipped with four planar magnetrons and individual O₂ mass‑flow controllers, was used to deposit PZT on 6‑inch silicon wafers. The process suffered from inhomogeneous thickness distributions due to the rotating substrate, which were partially mitigated by homogenisation masks. These masks, however, reduced the deposition rate by roughly 50 % and caused sputtered material to accumulate on the masks, leading to frequent cleaning cycles and reduced reproducibility. The reported deposition rate of about 300 nm h⁻¹ was insufficient for the project’s throughput targets.

In addition to the material studies, the project developed multilayer stacks including adhesion layers, electrodes, and seed layers that remained stable at high temperatures and during subsequent piezoelectric deposition. Functional patterns of electrodes and interconnects were successfully fabricated and characterised by the partner Zeiss, confirming the viability of the process flow for deformable mirror fabrication.

The project was coordinated by Fraunhofer FEP and involved close collaboration with Zeiss, the Technical University of Dresden, the Institute of Applied Physics (IFE), and the company scia. Regular bi‑annual meetings ensured alignment of objectives and integration of results. Funding was provided under the European Union Horizon 2020 programme (grant reference 16ESE0380S). The combined effort advanced the understanding of sputter‑deposited piezoelectric films and identified practical routes and remaining challenges for their application in high‑precision deformable mirrors.