American Mineralogist, Volume 86, pages 1094–1099, 20010003-004X/01/0009–1094$05.00 1094INTRODUCTIONThe introduction of ion milling in the 1950s for the prepa-ration of mineral specimens for use with the TEM revolution-ized the study of earth materials (Castaing and Labourie 1953;Paulus and Reverchon 1961; Barber 1970). Previous approachesto TEM examination of metals and biological tissues yieldedunsatisfactory results when applied to the brittle oxides thatcomprise the bulk of the Earth and many meteorites. However,a new frontier opened in the microstructural analysis of Earthmaterials once it was shown that most minerals could be thinnedto electron transparency with acceptably low levels of radia-tion damage by ion bombardment at ~6 keV. In conjunctionwith complementary techniques (such as dimpling and tripodpolishing), Ar ion milling has evolved into the standard meansof preparing TEM samples from minerals (Barber 1999).Recently, Giannuzzi et al. (1999, and refs. therein) haveshown that FIB milling may be used to prepare TEM speci-mens for metals, ceramics, composites, semiconductors, andbiological materials. Here, we have extended the viability ofthis technique to a variety of mineral samples. FIB milling of-fers a host of capabilities beyond those exhibited by traditionalAr ion milling, including: (1) sample extraction from extremelysmall volumes of unpolished material; (2) site specificity atthe submicrometer scale; (3) sample imaging by either second-ary ions or electrons during the milling procedure; and (4) rapidprocessing of superhard materials. Our initial results indicatethat FIB milling produces excellent TEM sections for a rangeof mineral structures and compositions, even friable clays.* E-mail: [email protected] ion beam milling: A method of site-specific sample extraction formicroanalysis of Earth and planetary materialsPETER J. HEANEY,1,* EDWARD P. VICENZI,2 LUCILLE A. GIANNUZZI,3 AND KENNETH J.T. LIVI41Department of Geosciences, 309 Deike, Penn State University, University Park, Pennsylvania 16802, U.S.A.2Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution,Washington, D.C. 20560-0119, U.S.A.3Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, Forida 32816, U.S.A.4Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.ABSTRACTArgon ion milling is the conventional means by which mineral sections are thinned to electrontransparency for transmission electron microscope (TEM) analysis, but this technique exhibits sig-nificant shortcomings. In particular, selective thinning and imaging of submicrometer inclusionsduring sample milling are highly problematic. We have achieved successful results using the fo-cused ion beam (FIB) lift-out technique, which utilizes a 30 kV Ga+ ion beam to extract electrontransparent specimens with nanometer scale precision. Using this procedure, we have prepared anumber of Earth materials representing a range of structures and compositions for TEM analysis.We believe that FIB milling will create major new opportunities in the field of Earth and planetarymaterials microanalysis, particularly with respect to ultraprecious mineral and rock samples.Therefore, FIB sample preparation heralds a dramatic advancein the microstructural analysis of mineral surfaces and particu-larly of ultraprecious materials, such as extraterrestrial speci-mens returned to Earth and historically significant gemstones.ALTERNATIVES TO ION THINNINGPrior to the introduction of Ar ion milling, the major meth-ods for preparing samples included electropolishing, ultrami-crotomy, and crushed grain suspension. The last of these is thesimplest and remains appropriate for certain applications. Arock or mineral sample is ground in an agate mortar in an etha-nol solution, and a drop of the suspension then is deposited ona holey carbon film. Upon evaporation of the ethanol, the gridis ready for TEM inspection. The drawbacks to the techniqueare threefold: (1) all textural information is lost when the sampleis crushed; (2) the typically small amount of thin edge limitsthe applicability of high-resolution TEM; and (3) some mate-rials are less susceptible to cleavage than others. Diamond, forexample, will grind an expensive corundum mortar rather thanvice versa.Ultramicrotomy involves the impregnation of a sample in awax or resin block, which then is shaved into thin slices by adiamond knife. Although this technique is standard for bio-logical TEM, it remains a specialized approach in the mineralsciences, suitable for soft clays, cosmic dusts, and bacterial-mineral interfaces (Eberhart and Triki 1972; Noguchi, 1998;Barker and Banfield, 1998; Paquette et al. 1999). The brittlequality of most ceramics leads to a “chattering” of the diamondknife as it sweeps across the specimen, which responds by frag-menting into a series of laminar sections. When the styles andconcentrations of structural defects are an integral part of aHEANEY ET AL.: FOCUSED ION BEAM MILLING 1095mineral study, these artifacts are problematic. Electropolishing,to our knowledge, has never achieved a niche in the prepara-tion of oxide ceramics for TEM. The acid brews required bythis method work well for many metals (Thompson-Russelland Edington 1977), but they amorphize mineral surfaces tooseverely for subsequent crystallographic work.AR ION THINNINGArgon ion milling eliminated many of the problems associ-ated with the metallurgical and biological approaches to samplepreparation, and with the availability of commercially producedion mills in the 1960s, TEM characterization of mineral defectsat the submicrometer scale markedly intensified, as is well docu-mented in Wenk (1976). Nevertheless, Ar ion milling suffers fromcertain limitations. Most significant, the ability to select regionsof interest is constrained to >1 mm2 with conventional ion-thin-ning units. Even state-of-the-art preparation systems utilize beamdiameters of hundreds of micrometers at typical operating volt-ages. Consequently, it is difficult to selectively thin individualfeatures that are micrometer-sized and smaller.In addition, specimens that undergo conventional Ar ionthinning typically require a prior stage of destructive samplepreparation. In most cases, raw specimens are cut and polishedas 30 µm petrographic thin sections from which 3 mm disksare extracted. Moreover, differential
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