Modern FIB-Based Specimen Preparation for APT 195
Figure 1. The first FIB prepared specimen. Note: The entire image width of the image is ~250 mm (image courtesy G. Smith, University of Oxford).
Figure 2. The first reported description of an annular milling process to prepare an APT specimen (reprinted from (Larson et al., 1998a) with permission).
used nearly 20 years later. These sharpening techniques, combined with the later-developed site-specific lift-out methods (Miller et al., 2005, 2007; Thompson et al., 2007), provide the community with the basic tools for FIB-based specimen preparation used today. In this review, we will highlight some of themore recent
developments in FIB-based specimen preparation with a focus on correlative methods, three-dimensional (3D)-site- specific and targeting techniques (especially as related to analysis of semiconductor devices), as well as methods used to prepare and extract samples with more open-space topologies including NWs and NPs. The primary aim is to provide the reader an overview, together with references to key advances in specimen preparation, and also highlight a few of the more sophisticated methods that represent the state-of-the art in FIB-based specimen preparation. Note that the authors have made no effort to be complete
in listing all sample preparation variations that have been reported in the literature. The steps and procedures for modern, standard lift-out and specimen sharpening have been described in detail elsewhere and will not be reproduced here except where necessary to provide a basic skeleton for later discussions. For additional details on the variety of methods that have been developed in this area, the reader is referred to several recent books (Gault et al., 2012; D. J. Larson et al., 2013c; M. K. Miller & R. G. Forbes, 2014).
STANDARD LIFT-OUTMETHOD
As the modern lift-out process is frequently referred to in this review, the process is briefly summarized here and
illustrated in Figure 3. First, the region for extraction is identified and, in this case, marked and protected with ion-deposited Pt (Fig. 3a). Regions around the specific region of interest (ROI) are then removed (regions 1, 2, and 3 in Fig. 3b), including the region underneath, by milling regions 1 and 2 at an undercutting angle (in this case 30° relative to the surface normal). Next, a micromanipulator is attached with ion-deposited Pt (left arrow Fig. 3c) and the entire wedge of material is removed by cutting it free with the ion beam (right arrow Fig. 3c). At this point, additional manipulations or processing steps may be performed on the wedge to either remove material (referred to as deprocessing) or reshape and reorient the wedge for analysis along a direction of the analyst’s choosing. The available options will be discussed later in this review. Once these steps are complete, pieces of the wedge are transferred to individual carrier tips (Thompson et al., 2005) as illustrated in Figure 3e. Depending on the application, a single wedge might provide many equivalent samples for sharpening and analysis. Finally, annular milling is used to sharpen the specimen. This is typically accomplished with a 30 kV gallium-ion beam, but alternative ion sources do exist (Estivill et al., 2016). The fine details of the specimen shaping process has been discussed in many publications as well, and againwe refer the reader to themost recent books for reference (Gault et al., 2012; Larson et al., 2013c; Miller & Forbes, 2014).
ANALYSIS ORIENTATION
The ability to manipulate the analysis direction was an important advance for FIB-based specimen preparation. It allows the user to manipulate analysis yield, spatial resolu-
tion, and the size of the ROI. For example, analyzing thin films parallel to the film interface orientation (cross-section) (Lawrence et al., 2008), serves to both increase the volume of theROI (the film interfaces) (Prosa et al., 2011), and improve the analysis yield by changing the orientation of the applied stress relative to the interface(s) (Eaton&Bayuzick, 1978). In order to reorient the analysis direction, wedges extracted by lift-out need to be transferred from the micromanipulator to a device that can accurately rotate the sample. One such device is the axial rotation manipulator (ARM; Lawrence et al., 2008) shown in Figure 4. Once a wedge of material is transferred to the ARM, the user can reorient the wedge to perform multiple operations (e.g., deprocessing, shaping, capping) before selecting the analysis orientation and transferring the material back to the micromanipulator for attachment to carrier tips for subsequent sharpening and APT analysis. Most lift-out samples are prepared so that the surface of
the original sample is the first part of the tip to be analyzed. However, preparation of the samples so that they are orien- ted in a way that allows analysis to start from within the bulk and proceed toward the original sample surface (backside) (Prosa et al., 2009) can serve multiple purposes. Should structurally weak or otherwise problematic materials or
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