ICMCTF2012 Session TS2-1: Advanced Characterization of Coatings and Thin Films

The quantitative investigation of the correlation between processing, (spatial) microstructure and properties of coatings and thin films is one of the essential goals of their characterization strategies. The traditional 2D planar section sampling combined with estimations for the spatial situation is powerful but it supplies insufficient information for essential 3D characteristics such as particle volume density and arrangement or connectivity in cases of complex shaped microstructures. Advanced thin films therefore call for an adequate imaging and quantification of the 3D microstructure. Electron tomography is well established but may suffer from a lack of field of view size. Such representative field of view size can be achieved by the help of microstructure tomography based on FIB serial sectioning. This method combines the excellent target preparation possibilities of a focused ion beam (FIB) with all types of SEM contrast including EDX and EBSD. Therefore it enables the exploration of a representative sample volume and the imaging of chemical and structural phenomena with a resolution of a few nanometers. This can be combined locally with a tomography even at the atomic scale using Atom Probe Tomography.

Once the 3D data set is available, their exploitation in 3D image analysis provide detailed quantitative insights into the relation between processing, structure and properties.

The ZrAlN system has recently come under investigation [1] as a system that has an even larger miscibility gap than TiAlN, as well as a large lattice mismatch between ZrN and AlN. Atom probe tomography was performed on magnetron sputtered epitaxial Zr_0.64Al_0.36N thin films grown on MgO(001) substrates kept at 800 °C. A self-organized nanostructure was resolved, consisting of lamellae of Zr(Al)N and Al(Zr)N with a characteristic length scale of ~4 nm in the film plane and extending in the growth direction. In addition, Al was found to segregate to the film/substrate interface, forming a 1 nm thick layer. This was followed by growth of a 4 nm thick Al-depleted zone, self-organization of the Zr(Al)N and Al(Zr)N lamellas during 5 nm film growth, and finally steady-state growth of the lamellar structure.

[1] L. Rogström, L.J.S. Johnson, M.P. Johansson, M. Ahlgren, L. Hultman, and M. Odén

Scripta Materialia 62 (2010) 739-741

The application of FIB/SEM (focused ion beam/scanning electron microscope) instruments is rapidly expanding. This is in part due to the fact that there is a pressing need to understand the structure, composition and physico-chemical properties of modern materials in three dimensions. FIB instruments can unveil sub-surface structural information by creating cross-sections that can easily be imaged or analysed [1]; and serial cross-sectioning expands the instrument’s capabilities to the third dimension.

In the process of serial sectioning, small and thin slices of material are removed in bulk samples by means of FIB milling, creating a series of cross-sections in one direction of the sample. State of the art FIB/SEM instruments provide a focused ion beam with small spot sizes so that slices that are only a few nanometres thick can be removed from the bulk material. Each cross-section can then be imaged and the crystal orientations retrieved by Electron Backscatter Diffraction (EBSD) [2]. The information can then be reconstructed and combined to provide a comprehensive dataset for 3D orientation analysis.

One of the manifold applications is the possibility of characterising plastic deformation in brittle materials due the suppression of cracking in very small bodies. We used 3D EBSD as a way of characterising the three-dimensional (3-D) deformation at high spatial resolution of MgO micropillars compressed ex situ along two crystal directions [3]. We show that for a successful 3D reconstruction of the indexed crystal orientations it is necessary to use the SEM images as reference to correct for offsets. It is also shown that for the reconstruction of compressed and then successively sliced and indexed MgO micropillars, this 3D technique yields information complementary to µ-Laue diffraction or electron microscopy, allowing a correlation of experimental artefacts and the distribution of plasticity.

References

[1] L. A. Gianuzzi et al., Introduction to Focused Ion Beams. Springer, New York, 2005.

[2] S. Zaefferer et al., Mater. Sci. Forum 495-497 (2005) 3.

[3] S. Korte et al., Acta Mater. 59 (2011) 7241.

X-ray photoelectron spectroscopy is the most widely applied of a range of surface analysis techniques. Small area analysis from an area of a few microns and imaging XPS with spatial resolution of a few microns is now common place.¹ The advent of small analysis area XPS heralded an era for XPS depth profiling where a sputter crater is formed by an ion beam (typically Ar) and then XPS analysis formed in the crater. In such a way thin films of up to 1-2 microns in thickness can be analysed. XPS depth profiles can therefore elucidate the chemical composition of a thin film with a depth resolution of a few nanometers . XPS can analyse films and substrate materials which are either insulating or conductive. An important factor in the characterization of thin films is the understanding of the chemistry of the various layers as well as the interfacial chemistry. In order to get a better understanding of this, non-destructive methods (chemically rather than necessarily materially) have been refined such as Angle Resolved XPS and the application of advanced mathematical modelling such as MEMS algorithms have enhanced the information that can be extracted. This technique has become especially powerful for very thin films.

Other recent developments have focussed on improved ion gun design, by lowering the Ar ion energy improved interface resolution is possible. To date, XPS depth profiling has been principally applied to inorganic materials but the recent development of polyatomic ion guns using large carbon based molecules such as Fullerene or Coronene has expanded XPS depth profiling into organic materials².

Examples will be given here which describe the state of the art in XPS depth profiling utilizing both ARXPS and MEMS as well as chemically non-destructive profiling with Polyatomic ion species on both organic and inorganic materials. Examples of nanometer depth resolution and quantitative chemical composition of films of several hundred nanometers in thickness from a range of applications will be given.

1. C.J. Blomfield, Journal of Electron Spectroscopy and Related Phenomena 143 (2005) 41-249

2. G.X. Biddulph, A. M. Piwowar, J.S. Fletcher, N.P. Lockyer, J. C. Vickerman, Anal, Chem, 79, (2007), 7259-7266.