Novel focused ion beam in‐situ methods for stressed and cracked TEM sample preparation

Abstract

Conventional Ga + Focus Ion Beam (FIB) dual beam system has proven to be an important and popular tool for Transmission Electron Microscope (TEM) sample preparation [1]. It is especially useful for localized TEM sample preparation. However, it is commonly known that the process can fail due to the internal stress leading to crack. We propose two strategies and their methods are detailed here. One is to release the stress by milling away constraining materials and another is to prethe crack rowthby in‐situ filling method [2]. The two strategies can be broadly applied to most stressed/cracked samples. In the first example, a chemical vapor deposition (CVD) diamond like carbon film with large internal compressive stress has failed in conventional TEM sample preparations. In order to release the internal stress, a ring is cut surrounding the area of interest, the ‘bump’. The cut could be performed by milling at large current (30–65nA) for efficient removal of the surrounding materials. Subsequent removal of the materials on the ‘bump’ is performed at progressively lower current until a 1um lamella remains for a conventional in‐situ lift‐out procedure. The results of TEM analysis showed the amorphous structure and columnar growth of the ‘bump’ area (Fig. 1). To analyze the stress release of the local area using the cutting procedure, digital image correlation (DIC) was employed in this study. A cluster of small dots were deposited on the surface of the ‘bump’ area using carbon gas injection system (GIS) as shown in Fig. 2. After milling the ring pattern to remove the material around the ‘bump’, secondary electron images were taken by electron column of this dual beam system (FEI Quanta3D), before and after the cutting respectively. These images were then imported to the DIC software (Davis) for calculation of the stress/strain release. It was measured by the displacements of the deposited dots. The result shows the stress around the ‘pump’ is released after material removal around the ‘bump’ which led to successful TEM sample preparation. The second example is a hot isostatic pressing (HIP) processed Ti‐SiC composite with internal thermal mechanical stress on the interface area which is however the area of interest. The broad ion beam preparation and conventional FIB preparation all failed due to stress concentration induced cracking at the interface region (Fig. 3). To increase the stability of the lamella, the sample was wedge polished in the parallel direction of the interface. This resulted in both ends of the lamella being held by the matrix material after rough milling process. An in‐situ filling process by e‐beam Pt deposition was then carried out to ‘fill’ the crack and strengthen the interface area. Caution was taken with the cut off step for both ends of the lamella. It is recommended to use parallel milling mode of the FIB in order to keep balance on the cutting speed of both ends so that it can better prevent failure during the cut off. E‐beam Pt deposition at the free end of the lamella may enhance its stability. TEM results have shown the successfully prepared sample with filled crack at the interface and its deformed grains due to thermal mechanical stress induced by the HIP process.