ICMCTF2010 Session F3: Application of Ion and Electron Microscopy

There is a wide range of technologically advanced applications, where different material classes such as metals and oxides are combined. The properties of such systems can be strongly affected or even controlled by the occurring interfaces and may not be the same as the properties of their constituent compounds [1,2]. Therefore, interfaces can play a notable role and thus a basic understanding of their microstructure is required to improve the functionality of materials. Since interface phenomena (e.g., segregation, diffusion, formation of interphases) occur in a very narrow area [2], characterization methods with high spatial resolution are essential. Modern transmission electron microscopy (TEM) with a variety of imaging and analytical techniques represents a valuable experimental tool and can be used to study the atomic structure, chemical composition and bonding across interfaces [3]. Such information can be obtained in combination of high-angle annular dark field imaging with X-ray energy dispersive spectroscopy (XEDS) and electron energy-loss spectroscopy (EELS) in scanning TEM (STEM) and this will be demonstrated and discussed for interface studies of various materials systems (e.g., metal – ceramic interfaces, interfaces in biological systems). In particular results obtained on platinum (Pt) films deposited on yttria stabilized zirconia (YSZ) will be shown where the quantitative analysis of XEDS results was performed using the recently developed ζ-factor method [4] in order to detect any irregularities across the interfaces. The local electronic structure and different polymorphic modifications were analyzed by electron energy-loss near edge structures (ELNES) investigations. The measured O-K ELNES at the Pt/YSZ interface were studied in details and compared with the results of ab initio full multiple scattering FEFF [5] modeling. The effects of the TEM specimen preparation can be a limiting factor for the investigations of materials, especially when different materials co-exist or in the case of highly sensitive specimens (e.g., biological specimens). Therefore, several different preparation methods were applied in order to get quality specimens with reduced artifacts.

[1] Z.L. Wang and Z.C. Kang, Functional and Smart Materials, Plenum Press, New York and London, 1998, p. 514

[2] A.P. Sutton and R.W. Ballufi, Interfaces in Crystalline Materials, Clarendon Press, Oxford, 1996, p. 819

[3] M.M. Disko, in M.M. Disko et al. (ed.), Transmission Electron Energy Loss Spectrometry in Materials Science, EMPMD Monograph Series, 1992, p. 271

[4] M. Watanabe and D.B. Williams, J. of Microscopy, 221 (2006) 89

[5] A.L. Ankudinov et al., Phys. Rev., B 58 (1998) 7565

In the last decades ternary and quaternary systems of metal-nitrides for hard coatings have been widely explored. The development of TiAlN coatings as an alternative to the traditional TiN thin films was presented as a solution with better cutting behaviour and resistance to oxidation at high temperatures. The introduction of silicon in this kind of coatings promoting the formation of the nanocomposite nc-TiAlN/a-Si3N4 allowed to increase the hardness to values ≥ 50GPa [1] .Much discussion is going on about the reproducibility of these results, the validation of the mechanical properties of superhard coatings and the influence of impurities and microstructure are important issues [2-4] .

Oxygen has been reported as an impurity playing an important role on the mechanical properties of superhard coatings [4] . Many authors have described the incorporation of contamination oxygen in their films [5-7] . The working pressure and substrate temperature during deposition seems to play an important role on controlling the introduction of oxygen [8,4] and also the microstructure of the films. The aim of this work is to localize oxygen (if it is in bulk or superficial layers) and to carry an exhaustive microstructural characterization in TiAlN and TiAlSiN thin films deposited by magnetron sputtering, using electron microscopy and spectroscopy techniques. Samples with different deposition conditions were investigated. The combination of different characterization techniques such as Scanning Electron Microscopy (SEM-FEG), Transmission Electron Microscopy (TEM), Selected Area electron diffraction (SAED), Electron Energy Loss Spectroscopy (EELS), energy filtered TEM images and X-Ray Photoelectron Spectroscopy are presented as a suitable methodology to characterize the microstructure and chemical composition in nanostrutured coatings.

[1] S. Veprek , Rev. Adv. Mater. Sci. 5 (2003) 6-16

[2] P. Schwaller, et al. , Advanced Engineering Materials 7 (5) (2005) 318-322

[3] S. Veprek, et al. , Surface Coatings and Technology 200 (2006) 3876-3885

[4] S. Veprek, etal, J. Vac. Sci. Technol. B 23(6) (2005)17-21

[5] M.A. Moram, et al., Thin Solid Films 516 (2008) 8569-8572

[6] J. Guillot, et al., Surface and Interface Analysis 34 (2002) 577-582

[7] V. Godinho et al., Eur. Phys. J. Appl. Phys. 43 (2008)333–341

[8] T. Nakano, et al. , Vacuum 83 (2009) 467-469

Al₂O₃ as deposited by high-temperature chemical vapor deposition is widely used for wear and corrosion protection of cemented carbide cutting tools because of its chemical inertness, corrosion resistance and high hardness. Low-temperature deposition by physical vapor deposition usually results in formation of amorphous or metastable Al₂O₃ phases, where only limited information about their thermal stability is available. The aim of this work is to examine the microstructural changes of metastable Al₂O₃ phases formed in sputtered films during annealing.

Al₂O₃ thin films were deposited by a production-scale pulsed direct current magnetron sputtering system. The thermal stability was investigated during vacuum annealing at different temperatures up to 1000°C for 3h and 12h. The morphology of fracture cross-sections was investigated by scanning electron microscopy. Additionally, the microstructure was studied by transmission electron microscopy (TEM) using several techniques such as bright field imaging, selected area diffraction, high-angle annular dark field imaging, high-resolution TEM, and electron energy-loss spectroscopy (EELS). The EELS measurements were performed at an acceleration voltage of 80keV and were used to examine the distribution of alumina polymorphs via detailed analysis of the energy-loss near-edge structure (ELNES) of the Al-L_2/3 and O-K edge. ELNES clearly facilitates the differentiation between different alumina phases due to changes in peak positions and the overall shape.

For the coating deposited under low ion bombardment conditions, diffuse illumination in the central region of the SAD pattern implies the presence of amorphous material, while the diffraction spots on the concentric rings correspond to γ- and/or δ-Al₂O₃ grains. This indicates the co-existence of amorphous and crystalline phases. In contrast, for high-energetic growth conditions clear evidence for γ-Al₂O₃ formation was found. Annealing of the γ-Al₂O₃ structured film at 800°C for 3h results in the irreversible formation of α-Al₂O₃. For the predominantly amorphous film, growth of the γ-Al₂O₃ phase is promoted but no transformation to α-Al₂O₃ was detected, even during annealing at 1000°C for 12h. The obtained results contribute to the understanding of phase-transformations in metastable Al₂O₃ coatings at temperatures typical for cutting processes.

The Cu/MgO interface is a model system for the study of metal ceramic systems used e.g. in microelectronic devices. Due to the difference in lattice parameter of those two fcc crystals with values of a=0.360 nm for Cu and a=0.4212 nm for MgO, there is a large mismatch of ~15% between the two crystals. This mismatch induces the presence of misfit dislocations at the interface in order to accommodate the mismatch strain. Two kinds of dislocation network are usually proposed for this type of interface, a dislocation network consisting of edge dislocation with 1/2 <110> Burgers vector and dislocation line direction along <110> Cu or another one with ½ < 100> Burgers vector and <010> line direction. Depending on the dislocation network present, the bonding of Cu on MgO at the interface will be different.The aim of the present study is to characterize the atomic structure of the interface, and particularly the dislocation network and the chemical bonding between the two crystals. The samples were prepared by magnetron sputtering. Cu exhibits a cube on cube orientation relationship with the MgO substrate, i.e. (001)_Cu//(001)_MgO and [001]_Cu//[001]_MgO. TEM samples were prepared using the Tripod polishing technique and finally Ar⁺ ion milled using a Baltec RES 101 ion milling device. HRTEM was performed with an image-side C_s corrected Jeol 2100F operated at 200kV. In order to reveal the dislocation network, the interfaces were observed along <100> and <110> directions. Along the <100> orientation, some edge dislocations are visible on the Cu side, with a Burgers vector of ½ <100>. Along the <110> orientation, no edge dislocations were observed, but some dislocations are present in the Cu side, with a projected Burgers vector of ½ <110>. This information indicates that the dislocation network consists of edge dislocations with Burgers vector of ½ <100> with a <010> line direction. For this dislocation network it is expected by geometrical considerations that the atomic structure at the interface between Cu and MgO should alternate from Cu on top of Mg to Cu on top of O between the misfit dislocations. On the <110> micrographs, the C_s corrected HRTEM image recorded under a negative C_S value allows to directly discriminate between Mg and O atom columns, which gives information on the atomic structure and thus about the bonding at the interface. The mismatch between the two crystals also induces the presence of uncompensated strain in the Cu side, which was characterized using a Geometrical Phase Analysis of the HRTEM micrographs. This remaining strain creates some distortion of several Cu atomic planes close to the interface.

The TEAM (Transmission Electron Aberration-corrected Microscope) project was driven by the need for improved resolution, sensitivity and precision in the analysis of atomic structure on the nanoscale. After five years of development and construction, the project came to a successful conclusion in the fall of 2009. The TEAM instrument offers half-Angstrom resolution in both scanning probe and broad beam modes of operation and is now available as a user facility at the National Center for Electron Microscopy.

Following an overview of the TEAM project, this talk will describe the instrument’s performance and its initial applications with particular emphasis on atomic-scale analysis of interfaces in materials. The unique technical features of the instrument include a new piezoceramic stage based on AFM technology, a novel high-brightness electron gun, an electron optical corrector for spherical as well as chromatic aberrations and an active pixel sensor for direct electron detection. These new technologies will be explained briefly, and specific examples will be presented to highlight the microscope’s unique capabilities and its capacity to adapt the electron optical environment to the experiment to the problem at hand. By operating between 80 and 300kV, the instrument can be tuned to maximize the experimental signal and minimize the beam damage to the sample. This will be illustrated with observations on Al-Li alloys, Au bicrystals, and atomic layer sheets of graphene and BN.

This presentation will give instances of recent work with the TEAM microscope, ranging from interfaces in metals and alloys to defect structures in metals, oxides and semiconductors. A particular focus will be the atomic structure and properties of an incommensurate grain boundary in gold. We have found that local relaxation of the boundary near the surface leads to a chevron-like defect of whose size and stability is related to the stacking fault energy. By compressing bicrystalline nanopillars of gold in-situ, it was possible to make a direct correlation between atomic interface structure and mechanical properties, providing insights into the effect of defects and surfaces during deformation.

Finally, based on the capabilities of the instrument, some opportunities for its future application in research on interfaces and coatings will be outlined. The National Center for Electron Microscopy is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the U.S. Department of Energy under Contract No. DE-ACO3-76SFOOO98.