ICMCTF2014 Session TS2-1: Advanced Characterization of Coatings and Thin Films

Cross-sectional synchrotron nanodiffraction allows the volume-averaged characterization of property-depth gradients in hard coatings with submicron resolution [1]. The approach is based on wide angle X-ray diffraction and is performed in transmission geometry along the cross-section of thinned film/substrate lamellas. The small beam size of about 100 nm in diameter and the high brilliance of modern synchrotron sources make it a powerful tool for the advanced thin film characterization. Al-Ti-N is used as a model system to explore the potential of cross-sectional synchrotron nanodiffraction of hard coatings with different architectures and morphologies. The investigated samples comprise a bilayer with different grain sizes, nanograined Al-Ti-N and a combination of monolithic and multilayer film design. Comparative studies for detailed discussions have been conducted by transmission electron microscopy and selected area diffraction.

[1] J. Keckes, M. Bartosik, et al., Scripta Mater. 67 (2012) 748.

Titanium aluminum nitride coatings are objects of many research activities that are focused on the stabilization of the metastable solid solution of fcc-(Ti,Al)N. The (Ti,Al)N coatings are prevalent for high temperature applications such as metal cutting at high cutting speeds or dry cutting, where the thermal stability of fcc-(Ti,Al)N is the limiting factor for the lifetime of the coatings. At high temperatures, the supersaturated fcc-(Ti,Al)N is known to undergo spinodal decomposition into Ti- and Al-rich fcc-(Ti,Al)N, which is followed by the transformation of the Al-rich regions into the thermodynamically stable w-Al(Ti)N, that contains a very low amount of Ti. Although the effect of the lattice strains on the spinodal decomposition is basically known, the experimental studies of the interplay between the local concentration gradients and lattice strains are extremely rare.

In this study, the interplay between the concentration gradients and lattice strains was investigated with the aid of TiN/(Ti,Al)N/AlN multilayer coatings, deposited by simultaneous reactive cathodic arc evaporation of Ti and Al in N₂ atmosphere. The multilayers consisted of a periodic TiN/(Ti,Al)N/AlN motif with the thickness of about 2.7nm. The average [Al]/([Ti]+[Al]) ratio in the multilayers was about 0.3. The in situ high-temperature synchrotron experiments performed at the ROBL BM20 beamline at the ESRF have shown that up to 800°C, the amplitude of the original concentration undulations decreases simultaneously with the decreasing magnitude of the lattice strain. The periodic multilayer structure was preserved up to the maximum annealing temperature of 950°C (hold for 1h). In Ti-rich (Ti,Al)N, a significant decrease of the compressive residual stress occurred at annealing temperature above 850°C as expected for the intermixing of Ti and Al between Ti- and Al-rich layers. During cooling the samples from ≥ 850°C to 100°C, concurrently the decomposition of fcc-(Ti,Al)N into the isostructural Ti- and Al-rich compounds and the transformation of Al-rich (Ti,Al)N into w-Al(Ti)N appeared. The transformation from Al-rich fcc-(Ti,Al)N to w-Al(Ti)N was evidenced by the increase of the multilayer periodicity, which was driven by the unit cell expansion during the phase transition.

The in situ synchrotron experiments combined glancing-angle X-ray diffraction to determine of residual stresses and stress-free lattice parameter and X-ray reflectivity to obtain the thickness of the periodic motif, the modulation of the electron density and/or interface roughness. Analytic transmission electron microscopy was used to characterize the interface quality and to quantify the composition profiles.

Boron nitride (BN) is an interesting material due to its electrical and mechanical properties and close similarities to crystalline carbon. The atoms in BN can be sp² or sp³ hybridized and in the sp² hybridized state (sp²-BN) BN can form hexagonal (h-BN) or rhombohedral (r-BN) lattices, which both bare close structural similarities to graphite. sp²-BN exhibits wide bandgap (~ 6 eV) and can be doped both p- and n-type in addition being chemically and thermally stable. A wide range of applications within electronics and optoelectronics are envisioned for BN. However, BN is the least investigated material among III-N materials, mainly due to the difficulty in deposition of high quality epitaxial thin films. Another difficulty is the determination of the crystalline structure of sp²-BN; h-BN and r-BN differ only in stacking sequence. In-plane lattice constants are the same – 2.504 Å and spacing between basal planes is around 3.333 Å for both crystal structures. This makes it impossible to distinguish between c-axis oriented h-BN and r-BN thin films by using X-ray diffraction (XRD) in Bragg-Brentano geometry and requires atomic resolution investigation by cross-section transmission electron microscopy (TEM)

In previous work, we presented deposition of high quality r‑BN thin films on a‑Al₂O₃ by Chemical Vapor Deposition [1]. Here we present study of the nucleation and film development at the early stages of the growth of sp²-BN thin films on (0001) a-Al₂O₃, (0001) 4H-SiC, (0001) 6H-SiC and (111) 3C-SiC. We have observed in scanning electron microscope that the initial growth starts by formation of triangular features. These features are observed with two orientations rotated 60° with respect to each other. This observation is in-line with our XRD measurements which suggests that the sp²-BN films are epitaxially grown twinned r-BN with 60° rotation between twins. Our TEM results suggest that the growth on a‑Al₂O₃ with an AlN buffer layer starts with around 10 initial basal planes of h‑BN and that the stacking sequence then changes to r-BN. Albeit, it is not possible to observe contribution from h‑BN in XRD. Moreover, SiC substrates are found to be highly suitable for the direct deposition of c-axis oriented crystalline sp²-BN and epitaxy of r-BN is observed on 4H-SiC.

Minatec Nanocharacterization Center, located at Minatec Campus, has a unique cluster of surface analysis investigation tools, with transfer modules allowing multiprobing characterization. Cross-analysis of small objects is supposed to allow a more precise description of the chemical arrangement of the observed structures, though difficulties quickly appear in this approach. Among our instruments, the new Auger nanoprobe opens new perspectives for Auger spectroscopy. Though spatial resolution is still far behind what can be reached with newest microscopy methods, it keeps the convenience and versatility of scruting an amorphous material without any surface modification or sample manipulation. Complementarity with other surface techniques such as XPS or ToF-SIMS is also a key point that must be underlined. Studies will be presented there as illustrative examples, among them the study of Li mechanism in Si negative electrodes [1].

[1] Study of lithiation mechanisms in silicon electrodes by Auger Electron Spectroscopy, J. Mater. Chem. A, 2013, 1, 4956-4965

The microstructure of thin films can be partly controlled by deposition parameters and thermal treatments. Both, however, are usually limited by the deposition method and the substrate properties. This paper focuses on ion irradiation from the keV to the MeV range of thin fcc and bcc metal films. The microstructure can then be tuned by phenomena as preferential sputtering, non-selective and selective grain growth, grain rotation and phase transformation. Even in refractory metals significant grain growth can be achieved at room temperature, which due to substrate limitations is virtually impossible by thermal means. The phenomena are illustrated by analytical models and applications in microelectronics.

We introduce a novel methodology for the in-situ measurement of mechanical stress during thin film growth utilizing a highly sensitive non-contact variation of the classic spherometer. By exploiting the known spherical deformation mode of the substrate the value of the curvature is inferred by measurement of only one point on the substrate’s surface—the sagittal. From the known curvature the stress can be calculated using the well-known Stoney equation. Based on this methodology, a stress sensor has been designed which is simple, highly sensitive, compact, and low cost.

The technique employs the use of a double side polished substrate that offers good specular reflectivity and is isotropic in its mechanical properties, such as <111> oriented crystalline silicon, for example. The measurement of the displacement of the uncoated side during deposition is performed with a high resolution (i.e. 5nm), commercially available, inexpensive, fiber optic sensor which can be used in both high vacuum and high temperature environments (i.e. 10^-7 Torr and 480^oC, respectively). A key attribute of this instrument lies in its potential to achieve sensitivity that rivals other measurement techniques such as the micro cantilever method but, due to the comparatively larger substrate area, offers a more robust and practical alternative for subsequent measurement of other characteristics of the film that might be correlated to film stress.

We present measurement results of nickel films deposited by magnetron sputtering which show good qualitative agreement to the know behavior of the stress in polycrystalline films with deposition rate as previously reported by Thornton and Hoffman.^[1]

¹John A. Thornton, David W. Hoffman, “Internal stresses in titanium, nickel, molybdenum, and tantalum films deposited by cylindrical magnetron sputtering” J. Vacuum Science Technology., Vol. 14, No.1, Jan/Feb. 1977

Proper variation of process parameters during deposition of a-C:H:W allows to produce coatings with tailored properties. In this work a-C:H:W coatings were deposited by unbalanced reactive magnetron sputtering of a WC target using acetylene as reactive gas. The process parameters like reactive gas flow, sputter power and bias voltage were varied according to a central composite experimental design and their influence on coating properties (determined by nanoindentation) like hardness and modulus of elasticity were observed. It turned out that bias voltage and acetylene gas flow affect the hardness of the coatings most. Subsequently, five coatings with predefined mechanical properties were deposited according to the generated model. The mechanical properties were selected in such a way that two coatings exhibit the same i.e. hardness values, using however different process parameters. These 5 coatings were investigated in terms of microstructure, chemical composition, mechanical properties and residual stresses by focused ion beam (FIB), atomic force microscopy, auger electron and Raman spectroscopy and nanoindentations. Residual stresses of these amorphous coatings were determined by means of FIB and digital image correlation (DIC). For this purpose a double slit was milled in the coating which causes the residual stresses to relax. By determining the resulting displacements with DIC the residual stresses can be quantified. It was observed that both the residual stresses and the hardness of the coatings increase with bias voltage whereas with raising acetylene gas flow the hardness decreases by simultaneous increasing residual stresses. The obtained results can be helpful for a tailored coating design considering not only hardness and modulus, but also residual stress level of the a-C:H:W coatings.