Microsc. Microanal. 23, 321–328, 2017 doi:10.1017/S1431927616011740
© MICROSCOPY SOCIETY OF AMERICA 2016
Electron Beam-Induced Deposition for Atom Probe Tomography Specimen Capping Layers
David R. Diercks,1,* Brian P. Gorman,1 and Johannes J. L. Mulders2
1Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA 2FEI Electron Optics, 5600 KA Eindhoven, The Netherlands
Abstract: Six precursors were evaluated for use as in situ electron beam-induced deposition capping layers in the preparation of atom probe tomography specimens with a focus on near-surface features where some of the deposition is retained at the specimen apex. Specimens were prepared by deposition of each precursor onto silicon posts and shaped into sub-70-nm radii needles using a focused ion beam. The utility of the depositions was
assessed using several criteria including composition and uniformity, evaporation behavior and evaporation fields, and depth of Ga+ ion penetration. Atom probe analyses through depositions of methyl cyclopentadienyl platinum trimethyl, palladium hexafluoroacetylacetonate, and dimethyl-gold-acetylacetonate [Me2Au(acac)] were all found to result in tip fracture at voltages exceeding 3 kV. Examination of the deposition using Me2Au (acac) plus flowing O2 was inconclusive due to evaporation of surface silicon from below the deposition under all analysis conditions. Dicobalt octacarbonyl [Co2(CO)8] and diiron nonacarbonyl [Fe2(CO)9] depositions were found to be effective as in situ capping materials for the silicon specimens. Their very different evaporation fields [36V/nm for Co2(CO)8 and 21V/nm for Fe2(CO)9] provide options for achieving reasonably close matching of the evaporation field between the capping material and many materials of interest.
Key words: electron beam-induced deposition, atom probe tomography, evaporation field, dicobalt octacarbonyl, diiron nonacarbonyl
INTRODUCTION
The ability to directly create three-dimensional depositions with high spatial resolution using a focused electron beam has facilitated several applications. Electron beam-induced depositions (EBID) from gaseous precursors have been used for repair of photolithography masks (Melngailis & Blauner, 1989), circuit modification (Tao et al., 1991), maskless deposition of nanostructures (Utke et al., 2008), and pro- tection of sample surfaces before ion beam cross-sectioning, such as during making transmission electron microscope (TEM) specimens using a focused ion beam (FIB) instru- ment (Lipp et al., 1996). Indeed, it has now become typical practice to deposit an EBID layer before depositing a thicker ion beam-induced deposition layer to avoid ion beam damage near the top surface of the TEM specimen (Kempshall et al., 2002). This is also the case when making atom probe tomography (APT) specimens by in situ FIB methods (Miller et al., 2005; Thompson et al., 2007). Although the primary purpose of the deposited layer is to protect the specimen surface from ion beam damage during subsequent steps, for APT specimens additional properties may need to be considered. These include (1) imaging contrast between the deposition and specimen, (2) milling rate of the deposition relative to that of the specimen, (3) adhesion of the layer to the specimen surface, (4) potential peak interferences in the mass
*Corresponding author.
ddiercks@mines.edu Received June 7, 2016; accepted September 16, 2016
spectrum, (5) the evaporation behavior and evaporation field of the deposition relative to the specimen, and (6) range of Ga+ ion penetration into the deposition. The benefits of APT analysis are realized to the greatest
extent on features having three-dimensional structures on a size scale of tens of nanometers or less. As such, there are many applications where APT analysis of surface structures can be very informative (Moore et al., 2008; Talbot et al., 2009; Li et al., 2015) and FIB lift-out techniques are the predominant means of extracting these site-specific regions for APT analysis. As mentioned above, in order to protect the surface during the FIB lift-out process a protective layer is required. Further, if the volume of interest is at or very near the surface, then, in order for that volume to remain unda- maged by the ion beam, some of the deposited protective layer will remain at the apex of the final APT specimen. Both in situ and ex situ depositions have been used for
this purpose (Miller et al., 2005; Thompson et al., 2006; Thompson et al., 2007; Mutas et al., 2011). The advantages of in situ EBID are that it can build up the necessary local thickness (~100–500 nm) rapidly and that it does not obscure the search for specific surface features as might be done by a previous ex situ deposition. Along those lines, in situ deposition means that other in situ techniques where prior deposition of a coating would likely interfere with the results, such as electron backscatter diffraction, can first be used to locate a specific feature to target for lift-out. Examples where scanning electron microscopic (SEM) ima- ging or SEM-based techniques can be used for targeting
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