Microsc. Microanal. 23, 194–209, 2017 doi:10.1017/S1431927616012642
INVITED REVIEW
Modern Focused-Ion-Beam-Based Site-Specific Specimen Preparation for Atom Probe Tomography
Ty J. Prosa,* and David J. Larson Cameca Instruments Inc., 5500 Nobel Drive, Madison, WI 53711, USA
Abstract: Approximately 30 years after the first use of focused ion beam (FIB) instruments to prepare atom probe tomography specimens, this technique has grown to be used by hundreds of researchers around the world. This past decade has seen tremendous advances in atom probe applications, enabled by the continued development of FIB-based specimen preparation methodologies. In this work, we provide a short review of the origin of the FIB method and the standard methods used today for lift-out and sharpening, using the annular millingmethod as applied to atom probe tomography specimens. Key steps for enabling correlative analysis with transmission electron-beam backscatter diffraction, transmission electron microscopy, and atom probe tomography are presented, and strategies for preparing specimens for modern microelectronic device structures are reviewed and discussed in detail. Examples are used for discussion of the steps for each of these methods. We conclude with examples of the challenges presented by complex topologies such as nanowires, nanoparticles, and organic materials.
Key words: focused ion beam, transmission electron microscopy, atom probe tomography, specimen prepara- tion, transmission electron backscatter diffraction
INTRODUCTION
Since the invention of the field ion microscope (Melmed, 1996), and approximately a decade later the atom probe (Panitz et al., 1969), scientists have been interested in preparing needle-shaped specimens that contain certain features of interest for analysis. These features commonly include precipitates, grain boundaries (GBs), phase bound- aries, defects, and others; but now also include semi- conductor devices, nanowires (NWs), nanoparticles (NPs), and more. For many years, the specimen preparation meth- ods used to convert materials into appropriately shaped needles containing such features near the apex were not very accurate, and they often relied on a combination of artisanal persistence (Melmed, 1991), separate microscopic observa- tions [typically transmission electron microscopy (TEM) (Henjered & Norden, 1983)], and quite a bit of luck. The development of focused ion beam (FIB) instruments changed all that. In the early 1980s, Bob Waugh’s expertise with liquid
metal ion sources (Waugh et al., 1984a) enabled him (and his coworkers) to investigate the potential unique capability of a FIB instrument to monitor a field ion specimen while it was being prepared (Smith, 2016). This resulted in the first published report of using a FIB to prepare specimens for atom probe tomography (APT) (Waugh et al., 1984b). The objective of this work was to place GBs within the apex region of a field ion specimen. The FIB was found to be
*Corresponding author.
ty.prosa@
ametek.com Received July 25, 2016; accepted December 7, 2016
exceedingly useful at imaging such boundaries due to elec- tron channeling contrast (Smith, 2016). Figure 1 shows an image of a field ion specimen created using a FIB with a beam of ~50nm diameter using a “sideways on” sputtering approach along with specimen rotation (Waugh et al., 1984b). Although there were no atom probe data associated with this study, it serves as the origination of the use of FIBs to fabricate specimens for APT. Although there were some attempts made over the next
~15 years to use FIBs to prepare field ion specimens (Alexander et al., 1989), limited progress was made. Some speculation emerged that it was time to return to the idea of using the FIB to create specimens (Larson et al., 1998b). Over this period, substantial improvement in FIB hardware occurred, and substantial progress was being made in the use of FIB for creating and thinning specimens for TEM (Overwijk, 1993; Giannuzzi et al., 1997), as well as for shar- pening tips for atomic force microscopy (Vasile et al., 1991). This latter work showed the feasibility of using the cap- abilities of that generation of single-beam FIBs to create sharpened needle structures. Shortly thereafter, Larson et al. (1998a) showed the cap-
© MICROSCOPY SOCIETY OF AMERICA 2017
abilities of the FIB to create field ion specimens froma variety of metallic materials and multilayers using different methods of sharpening, including annularmilling. These investigations also included studies of the effects of ion beam energy and imaging time on Ga-implantation specimen damage (Larson et al., 1999). The annular milling method, first suggested as shown in Figure 2, along with the low-energy modification later reported by Thompson et al. (2006, 2007), still forms the basis for a variety of current sharpening procedures being
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