Site-specific preparation of plan-view samples with large field of view for atomic resolution STEM and TEM studies of rapidly solidified multi-phase Al Cu thin films

Abstract

The present work describes application of a method for site-specific plan-view sample preparation for atomic column resolution scanning transmission and transmission electron microscopy (STEM and TEM) from free-standing thin films of multi-phase Al-alloys after laser irradiation induced rapid solidification (RS). The electron-transparent multilayer thin film samples (amorphous Si3N4 substrate plus metal alloy layer) had a total initial thickness of ≈ 130–200 nm. At such large total sample thickness frequently multiple grains of the same or different phases overlap along the electron beam path in the RS microstructures of the alloys. Consequently, accurate atomic resolution images in TEM and STEM mode, and composition sensitive analytical data of the metastable microstructural features presenting in the RS microstructures cannot be obtained with confidence. Using low-energy concentrated Ar ion beam milling, locally thinned regions with thickness of 20–30 nm with large fields of view ≥ (30 μm)2 suitable for high-resolution TEM/STEM studies have been created with micron-scale site-specificity. As example application, atomic scale resolution TEM/STEM imaging has been performed of the banded grain regions of the RS microstructure established in multi-phase AlCu alloy thin films. The sample preparation strategy described here appears to be readily applicable to freestanding thin film specimens in general.