18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká republika 

3D CHARACTERIZATION OF MATERIAL STRUCTURE USING SCANNING ELECTRON MICROSCOPY AND FOCUSED ION BEAM 3D CHARAKTERIZACE STRUKTURY MATERIÁLU POMOCÍ ŘÁDKOVACÍHO ELEKTRONOVÉHO MIKROSKOPU A FOKUSOVANÉHO IONTOVÉHO SVAZKU Monika HRADILOVÁ a, b, Aleš JÄGER a, Tomáš VYSTAVĚL c, Pavel LEJČEK a a

Department of Metals, Institute of Physics of the ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic, [email protected]

b

Department of Metals and Corrosion Engineering, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic c

FEI Czech Republic s.r.o., Podnikatelska 6, 612 00 Brno, Czech Republic

Abstract The importance of knowledge of the microstructure and its subsequent influence on properties is a basic tenet of material science. However, many commonly used characterization methods (e.g. scanning electron microscopy – SEM) provide only information in two dimensions (2D) and do not allow to obtain depth profile of material. The possibilities of analysis can be enhanced by utilizing focused ion beam (FIB). The FIB instrument allows revealing the structure in the third dimension by controlled and precise milling of the material. In combination with electron beam and suitable detector is possible to describe crystallography (EBSD) or chemical composition (EDS) etc. in three dimensions (3D). Thus, it is feasible to define real size, shape and distribution of microstructure features such as grains, grain boundaries, phases, precipitates and micropores. The SEM and the FIB in one tool is advanced technique for micro(nano)scale analysis of the surfaces and layers as well as for description of bulk ultrafine-grained and nanocrystalline materials. The aim of this contribution is to point out possibilities of high-resolution SEM–FIB instrument in 3D material characterization. For this purpose the selected light metals and their alloys are used. Abstrakt Mnoho metod používaných v materiálovém inženýrství charakterizuje strukturu pouze ve dvou rozměrech (2D), ale v principu neumožňuje analyzovat její hloubkový profil. Díky tomu jsou o struktuře získávány neúplné informace, čímž jsou limitovány i možnosti jejího ovlivnění. Inovaci do materiálové charakterizace přináší fokusovaný iontový svazek (FIB), který umožňuje postupným a definovaným „vymíláním“ materiálu získat jeho hloubkový profil. Při součastném použití iontového a elektronového svazku je v závislosti na použitém detektoru možné určit např. krystalografii (EBSD) nebo chemické složení (EDS) vzorku ve všech třech rozměrech (3D). Podobně lze také charakterizovat skutečnou velikost, tvar a rozložení mikrostrukturních prvků, jako jsou hranice zrn, fáze, mikropóry, apod. Systém kombinující FIB a SEM představuje špičkový nástroj pro 3D analýzu povrchových vrstev stejně jako pro studium objemových ultrajemnozrnných a nanokrystalických materiálů. V příspěvku je popsáno využití zařízení kombinující elektronový a iontový svazek (SEM–FIB) pro 3D materiálovou charakterizaci. Konkrétní příklady aplikací SEM–FIB jsou prezentovány na vybraných lehkých kovech a slitinách hořčíku a hliníku. 1.

INTRODUCTION

Focused ion beam (FIB) systems have been produced commercially primarily for large semiconductor manufacturers [1, 2]. In subsequent years the FIB instrument has become an important technology for a

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18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká republika 

wide array of materials science applications from circuit modification and transmission electron microscopy (TEM) sample preparation to microstructural analysis and prototype nanomachining [2, 3]. The FIB technique is similar to a scanning electron microscope (SEM), except that the beam that is rastered over the sample is an ion beam [3]. The FIB can be operated at low beam currents for imaging or high beam currents for site specific milling [1]. This system can also be exploited for depositing metals and insulators by gas injection system (GIS) [4, 5]. On the other hand electron microscopy (SEM) represents common techniques for description of surface topography by secondary electrons (SE) and backscattered electrons (BSE), crystallographic orientation (electron backscatter diffraction – EBSD), chemical composition (energy dispersive spectroscopy – EDS) etc. However, the SEM based methods provide near-surface information about microstructure [6]. Combination of the SEM and the FIB in one tool represents advanced technique for micro(nano)scale analysis. Application of the focused ion beam (FIB) for three-dimensional (3D) materials characterization resides in precise ion milling through the sample that is followed by characterization capabilities of the SEM. This can be used e.g. to achieve volumetric quantity and distribution of microstructural features and also to reveal their real shape and size in 3D [2, 6, 7]. This contribution gives a fundamental overview to the possibilities of high-resolution SEM-FIB instrument in 3D material characterization. Selected examples of applications are shown to demonstrate the usefulness of the SEM–FIB combination for material analysis. 2.

EXPERIMENTAL DETAILS

High-resolution microscope FEI DualBeam Quanta 3D FEG (Fig.1) was used for 2D and 3D material characterization and analysis. This instrument consists of electron column with field emission gun, ion column containing gallium ion source, and extensive accessories (EDS, EBSD, GIS, etc.). Preparation of the samples depends on the kind of measurement which is applied. 3.

RESULTS AND DISCUSION

3.1

Ion beam imaging

Fig. 1. FEI DualBeam Quanta 3D FEG installed at the Institute of Physics of the AS

The ion beam interacts with the sample resulting in a

CR.

number of various processes such as ion and electron emission, electromagnetic radiation etc. [1, 3, 8, 9]. Most imaging in the FIB is based on detecting the low-energy electrons, often referred to as ion-induced secondary electrons (ISEs) [3]. An image is then obtained by scanning the sample surface, as in the SEM. Ion beams are not as finely focused as electron beams and, partly for this reason, they generally offer lower resolution. However, the contrast mechanisms for ISE generation are different from those for SE generation and can offer complementary information about a sample surface [3, 10]. Fig. 2 shows SE and ISE images of the same area. The ISE imaging typically provides stronger channeling contrast from crystals than the SE imaging. As a result, individual grains are better distinguished in Fig. 2 (b). It should be mentioned that the same detector (Everhart–Thornley detector ; i.e. E-T det) is used to gain the signal from both beams but in case of e-beam also BSE are detected.

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Fig. 2. SE (a) and ISE (b) images show the same area of the pure magnesium after severe plastic deformation with polished and etched surface. (a: HV=20 kV, curr=15 pA, WD=10 mm ; b: HV=30 kV, curr=30 pA, WD=19 mm) 3.2

Cross–section milling and 3D characterarization

One of the main advantages of the SEM–FIB is an ability to reveal the structure in the third dimension [2, 4, 9]. This can be realized by FIB milling of a cross-section and its subsequent imaging by the SEM. It is common to create the cross-section as the “stair” type of trench [9]. Fig. 3 (a) shows an example of the cross-section of non-conductive impurity on pure magnesium and its imaging created immediately after milling by the SEM. For comparison, Fig. 3 (b) shows ion beam imaging of the same area that can be done after appropriate rotation and tilting of the stage with the sample. Note strong contrast between the impurity and Mg for ISE but practically no difference for SE image (see 3.1).

Fig. 3. SE (a) and ISE (b) images show typical cross-section with a shape as the “stair” type of trench. (a: HV=20 kV, curr=15 pA, WD=10 mm, E-T det ; b: HV=30 kV, curr=10 pA, WD=19 mm, E-T det) This process allows to determine thickness of micro(nano)layers as well as to reveal more information about microstructural features etc.

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The creation of the cross-section in area of interest is mostly the easiest possibility of 3D analysis. Next step can include serial sectioning experiments. This resides in gradual FIB milling of the sections (usually from tens to thousands nm in width) followed by the SEM imaging after each cut. When experimental data acquisition is complete, these datasets can be reconstructed and analyzed using image processing and visualization software which gives 3D information Fig. 4. Scheme of sample geometry and beam

of the probed volume [4, 6, 7]. This process is

orientation during SEM–FIB cross-section milling

generally referred as 3D tomography [6, 8]. Fig. 4

experiment.

shows typical sample geometry and beam

orientation for both cross-section milling and above-mentioned SEM–FIB tomography experiments. The beam parameters depend on the size of the features of interest as well as on a number of factors that include chemistry, crystallography, and surface topology [4, 6]. 3D EBSD belongs among the most advanced tomography techniques and it is realized by slightly different way than described above [2, 6, 11]. Fig. 5 illustrates the 3D structure of ultrafine-grained aluminum alloy. Different colours are assigned to various grain orientations. Data for this reconstruction were obtained from 48 slices. The thickness of each slice was 100 nm. The whole process of serial sectioning and EBSD patterning has to be fully automatic because it is time-consuming and last tens of hours.

Fig. 5. 3D EBSD map of ultrafine-grained Al-Mg-Sc alloy reconstructed from 48 slices. The 3D-EBSD tomography and its appropriate visualization can image real grain shape, size and may also reveal numerous unexpected phenomena such as preferred growth of certain boundaries during recrystallization etc.[11]. 4.

CONCLUSION

The contribution gives fundamental overview of the possibilities of high-resolution SEM–FIB instrument in material analysis as well as in 3D material characterization. Selected examples considering ion beam imaging, cross-section milling and 3D topography were demonstrated. From these examples it is clear that SEM-FIB is an effective tool opening new possibilities in complex characterization of material and represents remarkable potential in advanced 3D description of microstructure in terms of site specificity and versatility.

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ACKNOWLEDGMENTS The authors would like to thank to the Academy of Sciences of the Czech Republic (grant KAN300100801 and Institution Research Plan AV0Z10100520) and to the Internal Grant Agency of ICT Prague for financial support. LITERATURE [1] [2]

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