Focused ion beam preparation of microbeams for in situ mechanical analysis of electroplated nanotwinned copper with probe type indenters.

A site-specific xenon plasma focused ion beam preparation technique for microcantilever samples (1-20 µm width and 1:10 aspect ratio) is presented. The novelty of the methodology is the use of a chunk lift-out onto a clean silicon wafer to facilitate easy access of a low-cost probe type indenter which provides bending force measurement. The lift-out method allows sufficient room for the indenter and a line of sight for the electron beam to enable displacement measurement. An electroplated nanotwinned copper (NTC) was cut to a 3 × 3 × 25 µm microbeam and in situ mechanically tested using the developed technique. It demonstrated measured values of Youngs modulus of 78.7 ± 11 GPa and flow stress of 0.80 ± 0.05 GPa, which is within the ranges reported in the literature. LAY DESCRIPTION: In this paper a site specific method is present for making particularly small mechanical tests samples, of the order of 100 the size of a human hair. These small samples can then be used to determine the mechanical properties of the bulk material. Copper with a nano twinned grain structure is used as a test medium. Ion milling was used to cut the sample to shape and a micro probe was used for mechanical testing. Ion milling can cut away very small volumes of material as it accelerates ions at the surface of the sample, atomically machining the sample. Micro probes are a cost-effective small-scale load measurement devices, however, they require a large area for accessing the sample. The indenter requirements are a problem when making you samples with ion milling as ion millers are best at making small cuts. Our aim was to design a cutting strategy which reduces the amount of cutting required while allowing samples to be fabricated anywhere on the sample. We used a chunk lift out technique to remove a piece of material which is then welded to a wafer of silicon this gives sufficient space around the sample for ion milling and testing. The additional space allowed easy access for the probe. A 3 × 3 × 10 µm micro cantilever beam was cut out from copper, this beam was then bent. The force from bending and distance bent was measured and converted into Youngs modulus which is a measure of flexibility. The modulus value measured was comparable to the values reported in other papers.