In January BAM, the German Federal Institute for Materials Research and Testing, hosted a one-day workshop to give Nanostrain project partners first hand experience of preparing thin lamellae from the piezoelectric materials for the Nanostrain project. The demonstrations also provided an opportunity for partners to discuss needs and expectations towards specimen preparation for future nanostrain measurements with the different techniques available within the consortium.

BAM production approach is based on a focused ion beam (FIB) technique is used to produce these lamellae with two potentail thin film piezoelectric structures being investigated as potential samples. The first one (system 1) is a standard 20/80 lead zirconate titanate (PZT) thin film on platinized silicon wafer, and the second one (system 2) is a 100 nm thick epitaxially grown PZT film on doped strontium titanate substrate. As revealed by the overview TEM micrographs Figs.1 and 2, the two systems differ considerably.

Whereas system 1 is a polycrystalline film on silicon wafer with various interlayers including a rather thick, presumably thermally grown amorphous SiO2 layer, system 2 was identified as monocrystalline PZT grown epitaxially on strontium titanate wafer.

FIB is a destructive method. System 1 was used to study systematically the impact of Ga+ ion beam irradiation on near surface superficial layer structures of PZT films. This was done by preparing cross-sectional lamellae from areas exposed to different ion doses, different ion beam energies and different beam inclination angles.

The results of the parameter studies on ion implantation into PZT show that under the most unfavourable conditions – i.e. highest ion dose, beam energy and inclination angle – the depth of the affected zone is 26 nm, which is of the same order of magnitude as results published for silicon. On the other hand, under the most favourable conditions – low ion dose, lowest possible accelerating voltage and a glancing beam incidence of 1° – the thickness of the Ga+ implanted layer can be reduced to 2 nm (for 5kV, 5nm for 16kV), comparable to as observed for silicon. This is good news, but it also shows that completely defect-free samples cannot be expected after FIB preparation.

Therefore, the next step is to explore whether lamellae prepared either by the standard and/or by the optimised procedure are suitable for nanostrain measurements. Since at the moment only epitaxially grown PZT films on STO seem to be well adapted to this technique it was decided to start with this system. A first lamella (see Fig.2) was already prepared with the standard procedure and a second lamella will be prepared while taking account of the experiences obtained with system 1. A further idea is to correlate the results of implanted layer thickness with electrical measurements.