TEM cross-sections. (a) The BAW actuator stack showing the metallic layers of the electrodes and the coupling layer of SiO2 above a PSi single layer. (b) The interface between an etched PSi single layer and the SiO2 coupling layer. The amorphous Si layer was used to seal the PSi layer to avoid any contamination of the production line. (c) Typical interface between PSi and cSi. Scale bars are shown in each micrograph.