Combining in situ tensile testing and orientation microscopy in the SEM: A MEMS based setup for studying time dependent deformation of thin films by TKD and STEM

Structures in integrated devices are constantly subjected to residual or thermal stresses during operation. Understanding the relaxation behavior of thin films is therefore critical for improving their reliability. Recently it was shown that Transmission Kikuchi Diffraction (TKD) in the Scanning Electron Microscope (SEM) enables the determination of local crystal orientations with high spatial resolution using standard Electron Backscatter Diffraction (EBSD) instrumentation [1, 2]. Giving access to quantitative information on mechanisms like grain growth, grain rotation and strain gradient evolution, time resolved TKD stands out as a promising technique for the characterization of microstructural changes upon relaxation of thin films. We have implemented a MEMS based tensile device [3] into a custom setup specifically designed for in situ TKD imaging inside the SEM. A scanning TEM detector is used complementarily to access shorter time scales. In this context, a novel technique for the preparation and mounting of freestanding thin film tensile samples is presented, which relies on focused ion beam (FIB) milling and selective, electron-beam-assisted etching of silicon membranes. First stress relaxation results of tests on fcc metallic thin films are shown to demonstrate the capabilities of time resolved TKD

Fracture behavior of metallic thin films as evaluated by bulge-tests and in situ TEM deformation experiments

Metallic thin films generally show a fracture toughness which is considerably lower than that of bulk samples. Although this has been evidenced by several groups, a conclusive understanding of this low fracture toughness is still missing and open questions related with the difficulty of reliably testing very thin films often remain. Bulge testing is a very suitable method allowing reliable investigations of the fracture toughness of thin films by introducing a slit in a freestanding membrane by focused ion beam (FIB) milling. With such tests the fracture toughness of silver and gold films in the thickness range of 100 nm have been determined to be around 2 MPa m1/2 confirming earlier results obtained with other testing techniques on similar metallic thin films. Recent investigations by Preiss et al. [1] gave an explanation for this extremely low fracture toughness based on in-situ observations of the crack tip region by atomic force microscopy (AFM). The AFM scans show stable crack growth mainly along grain boundaries and sliding of grains. Plastic deformation is localized in a very narrow corridor in front of the crack tip and a large plastic zone, as one would typically expect under plane stress, is not observed. We conclude that the spatial confinement of the plastic deformation is the primary reason for the low fracture toughness of metallic thin films. More detailed observations of the deformation mechanisms are of particular interest and are enabled by in situ transmission electron microscopy (TEM). For this a new flexible method for the preparation of thin film samples for in situ mechanical testing in a TEM has been developed [2], which is based on a combination of focused ion beam (FIB) shadow milling and electron-beam-assisted etching with Xenon difluoride precursor gas. Loading of the specimens is performed by a TEM Nanoindenter combined with a Push-to-Pull conversion device. In contrast to existing FIB-based preparation approaches, the area of interest is never exposed to ion beam irradiation and a pristine microstructure is preserved. With this method nanotwinned Cu and Cu-Al thin films were tested in situ in the TEM. Al is an effective element to reduce the stacking fault energy in Cu alloys and leads to increased amount of twinning and detwinning events. The films are tested until final fracture and different deformation mechanism as sliding of grains, twinning and dislocation activity can be correlated with the captured stress-strain curves from the experiment. The fracture behavior of these films will be discussed in the presentation and compared to the bulge-test results. References [1] E. I. Preiß, B. Merle, M. Göken; Understanding the extremely low fracture toughness of freestanding gold thin films by in-situ bulge testing in an AFM; Mat. Sci. Eng. A691 (2017) 2018-2025 [2] J.P. Liebig, M. Göken, G. Richter, M. Mačković, T. Przybilla, E. Spiecker, O.N. Pierron, B. Merle; A flexible method for the preparation of thin film samples for in situ TEM characterization combining shadow-FIB milling and electron-beam-assisted etching; Ultramicroscopy 171 (2016) 82–8

Mechanical testing of twinned copper and copper alloy micropillars

Nanotwinned metals are a promising class of modern materials combining a very high strength and ductility with excellent electrical properties. Their remarkable strength is connected to the high effectiveness of twin boundaries as obstacles to dislocation motion. In order to further characterize these interactions, micropillars containing single coherent twin boundaries with different orientations were compressed with a flat punch and subsequently investigated in the scanning electron microscope. The crystal orientations for compression were selected to activate different slip modes. The aim is to probe the different barrier effects that can act on gliding dislocations. The investigations concentrated on copper and α-brass. The latter is a low stacking-fault energy alloy exhibiting a high density of recrystallization twins. Coherent twin boundaries were selected from an EBSD orientation mapping of the sample and oriented by means of a custom sample holder. FIB-milling at these interfaces yielded micropillar samples containing a single twin boundary. Single crystal reference samples were obtained from the bulk of the grain located on both sides of the twin boundary. The microcompression tests enabled the quantification of the influence of the twin boundary barrier on the strength of each sample. The tests evidenced a strong dependency of the strength of the sample on crystal orientation and stacking-fault energy. The activated glide systems were subsequently identified from slip trace analysis and STEM mapping of lamellas obtained by FIB lift-out from the bulk of the tested micropillars. Please click Additional Files below to see the full abstract

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements