TEM samples of metallic materials can be prepared by mainly two ways: Electrothinning and ion thinning either by ion milling systems or Focused Ion Beam (FIB). In both cases, very thin lamellae can be obtained but residual artefacts are always present on their surfaces. Depending on which information is needed, those artefacts can limit and even prevent us from observing the samples properly. In the general case, electrothinning induces a residual oxide layer, mostly amorphous, that can evolve during TEM observations (Fig.1). On the other side, ions milling and FIB induce amorphous layers, irradiation defects and ions implantations although new FIB systems give the possibility to clean the specimens at low kV.
In this study, we show that ion polishing systems with low acceleration voltage can greatly improve the quality of electrothinned and FIB lamellae. Various cleaning conditions were tested using Precise Ion Polisher (PIPS I from GATAN) and with the new Precise Ion Polisher (PIPS II GATAN2012). Compared to PIPS I, PIPS II provides a better control of the ion beam at an acceleration voltage down to 100V. The sputtering kinetic was measured on various alloys (316L, Ni based alloy 600, oxidized and quenched Zr alloy). To do so, thickness maps were acquired with a FEI TECNAI OSIRIS equipped with an energy filtered imaging system (GIF Quantum from GATAN). Even though macroscopic dusts can be removed after Ar+ cleaning at 500V, thinning and decrease of amorphous layer is only slightly effective in PIPS I. To get a significant thinning rate in PIPS I, an accelerating voltage higher than 1kV has to be used but evidences of irradiation defects were seen on 316L. PIPS II experiments were conducted on various alloys with various ion thinning conditions, on both electrothinned and FIB lamellae. The kinetic rates measured are plotted on Fig.2 showing an effective thinning of lamellae even at 100V. When comparing the lamellae as-electrothinned and as-cut at 30kV by FIB to PIPS II cleaned lamella, a clear decrease of amorphous layer is observed and the quality of the lamellae is greatly improved. To be sure that no irradiation defects are induced by such thinning, we studied PIPS II cleaning on an Au+ pre-implanted 316L. Cross sectioned lamellae were prepared at 30kV in a Helios Nanolab Dual Beam from FEI. The effect of low voltage cleaning on the lamellae is showed on Fig.3. No irradiation defects were seen in non Au+ implanted area. However, a few nanometres thick layer of amorphous is still present on both surfaces. These results show that the thickness and the quality of metallic TEM samples (either prepared by electrothinning or by FIB) can be easily improved by using a complementary thinning/cleaning with low voltage Ar+ ions in PIPS II.
Experiments on 316L received a financial support from the French National Research Agency through the project CoIrrHeSim ANR-11-BS09-006.
To cite this abstract:Laurent LEGRAS, Marie Laure LESCOAT, Stephanie JUBLOT-LECLERC, Aurélie GENTILS; Optimisation of TEM preparation in metallic materials using low voltage ions. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/optimisation-of-tem-preparation-in-metallic-materials-using-low-voltage-ions/. Accessed: April 4, 2020
EMC Abstracts - https://emc-proceedings.com/abstract/optimisation-of-tem-preparation-in-metallic-materials-using-low-voltage-ions/