Traditionally, a focused ion beam (FIB) lift-out sample is considered not proper for atomic-resolution STEM/TEM imaging owing to the surface amorphization caused by the 30kV ion bombardment. Recently, technology has pushed the resolution of Ga ion imaging further such that a lower voltage milling at 2 kV or even down to 500 V is possible, which could minimize the surface damage layer down to 1 nm or less. Here, I will briefly explain how we use a FEI Strata 400 dual beam FIB to prepare TEM samples suitable for atomic-resolution imaging.
1. If your material is an insulator, it is recommended to coat your sample with either gold/palladium or carbon. A significant speedier technique is to use a Sharpie pen to draw a line on your sample. Sharpie works well when you do not have to worry about delamination of your films.
2. After the sample was loaded into the FIB, you need to orient your sample correctly such that the lift-out surface is perpendicular to the zone axis you want to image in the S/TEM. This is easy for bulk crystals as you can always align to the cleaved edges. However, for micron-size crystals, it could be tricky and sometimes hard to achieve a particular orientation you want due to the limitation of the dual-beam geometry.
3 After the FIB lift-out region was identified and correctly oriented, a 20x2 um e-beam deposited Pt protection layer (~5 nm thick) was laid down on top of the desired location. Another 1.5 μm Pt protection layer was deposited by the ion-beam deposition process. After that, two trenches (15x10 um) were milled by the focused ion beam, leaving a slab (1.5 um thick) of the material. This thin section was then cut out by the ion beam and transferred to a copper TEM grid by an Omniprobe.
4. The lift-out section of calcite was sequentially thinned by the focused ion beam at 30, 5 and 2 keV beam energies. This procedure is analogous to mechanical polishing using sandpaper with increasingly finer grits to achieve a fine polished surface. The 1.5 um thick section was first thinned to ~300 nm at 30 keV. The ion thinning was conducted on the top surface first and then the bottom surface by tilting the stage to produce a 2° grazing angle milling condition. Then, the beam energy was lowered to 5 keV (47 pA) to repeat the same thinning process. The continuous thinning of the Pt protection layer, which capped the calcite, was monitored in the SEM spy mode (simultaneous SEM imaging during ion milling). The milling was terminated when the Pt protection layer was less than 100 nm thick. After 5 keV thinning, the beam energy was lowered to 2 keV. The final thinning at 2 keV (28 pA) was finished when the Pt protection layer was barely milled away to expose the top of the sample directly to the ion beam.
Both of the low-keV (5 and 2 keV) milling steps were significant for reducing the thickness of the damaged amorphous layer. With the final milling at 2 keV, the surface damage could be reduced to ~0.5-1.5 nm. This step was critical to the integrity and the resolution that could be obtained for the FIB'ed TEM samples.