ICMCTF2011 Session TS4-2: Characterization: Linking Synthesis, Microstructure, and Properties

Scaling of ultra-thin films has reached dimensions where traditional analytical approaches start to struggle. Some lack surface sensitivity for separating layers in sputter depth profiling, others suffer from the transient regions at the surface and at interfaces, which are in the same depth range as the thickness of the film itself. The change in sputter rate prevents from establishing an accurate depth scale, while changing sensitivity factors especially in SIMS prevent accurate quantification in these transient regions. Furthermore, a variety of new and complex materials have been introduced such as new gate dielectrics, inducing additional complexity for the quantification.

An emerging technique in this field is high sensitivity high resolution Low Energy Ion Scattering (LEIS). It is the most surface sensitive elemental characterization technique and can provide quantitative information on the first atomic layer. Noble gas ions (He, Ne ...) of low energy (1 - 8 keV) are used as projectiles. The energy of the backscattered ions from elastic collisions depends on the masses of the target atoms and is measured by using electrostatic energy analyzers. In contrast to SIMS, the yields are not affected by the chemical environment. Compared to electron spectroscopic techniques, the surface sensitivity and therefore also the depth resolution are superior.

Ions scattered in deeper layers lose extra energy along the in- and outgoing trajectories. This allows obtaining "non-destructive" depth profiles in the range of 1 - 10 nm ("static mode"). In a dual beam configuration this can be combined with conventional depth profiling using low energy noble gas sputtering ("dynamic mode").

We studied a variety of sample systems by static and dynamic LEIS depth profiling and compared the results with depth profiles acquired by other techniques, especially TOF-SIMS and XPS. The combination of LEIS static and sputter depth profiling allows intrinsic depth scale calibration without using crater depth measurements, and even the continuous measurement of the sputter rate during the profile is possible. This is especially useful for multilayer systems, where the sputter rate changes between the different materials.

Hot dipping is a coating technique pre-eminently used in industry to galvanize machine parts or steel sheet for constructional applications. However, other hot dipping applications have been developed and have a positive effect on specific material properties. For instance, in Fe-Si electrical steels, a Si/Al rich coating is applied to increase the electrical resistivity of the material and consequently, lower the power losses[1][2]. Hot dipped aluminised mild steels have been developed with increased corrosion resistance for high temperature applications by the formation of a dense Al₂O₃ layer [3]. Regardless of the type of steel coated and the intended application, after the interaction between the molten Al and the solid material, three constituents are formed: Fe₂Al₅, FeAl₃ and an Al-rich alloy. The structural morphology, which can negatively affect the wear resistance and the thermal stability [3], also appears to be highly dependent on the chemical composition of the base material [4]. To study thermo-mechanical and compositional effects on the coating behaviour after hot dipping, cold rolling with different reductions was performed in different Fe-Si materials. It was demonstrated that hardness differences between the layers caused crack formation inside the Fe₂Al₅ layer. The present work reports the results obtained on materials that were hot dipped in a hypo-eutectic Al + 1wt.%Si bath. The bath was used to coat Fe-Si steel substrates with variable silicon content with dipping times ranging from 1 to 20 seconds. Before dipping, the samples were heated to 700°C and subsequently immersed in the liquid bath at temperatures of 710°C, 720°C and 740°C. To further evaluate the interactions between Al, Si and Fe, a diffusion annealing treatment at 1000°C was performed. The main diffusing elements during this treatment are Al and Fe, although small variations of the composition are also observed for the alloying element Si. At a certain distance from the surface, voids were observed, which most probably can be related to the Kirkendall effect. A thorough characterization of the formed intermetallics was performed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) and Electron backscattered diffraction (EBSD).

References

[1] J. Barros, J. Schneider and Y. Houbaert, JMMM. 320 (2008) 389.

[2] I. Infante Danzo, K. Verbeken and Y. Houbaert, Defec. Diffu. Forum. 297 (2010) 370.

[3] Wei-Jen Cheng and Chaur-Jeng Wang, Surf. Coat. Technol. 204 (2009) 824.

[4] G. Hameed Awan and Faiz ul Hasan, Mater. Sci. Eng. A 472 (2008) 157.

Here a brief review of state of the art instrumentation and methodology is presented including discussion about features of standard, generalized as well as Mueller-matrix ellipsometry. Many different configurations can be employed such as ex situ, in situ (vacuum, liquids), imaging and real-time spectroscopy. A very important aspect of ellipsometry is modeling strategies and the use of model optical functions is emphasized.

Specific examples of basic thin film characterization are detailed like determination of thickness and refractive index and how color coordinates (L*a*b* and RGB) are derived (niobium oxide on niobium is used as an example). In a second example real-time spectral studies of optical properties of thin films of cellulose and swelling due to humidity are presented. Analysis of non-ideal effects like roughness, grading and thickness nonuniformities are addressed. Modern software includes possibilities to model not only permittivity using ellipsometric data but also permeability and gyrotropic effects, i.e. a full 6x6 constitutive matrix. Such effects can be found in metamaterials and analysis of artificial magnetism in nanosandwiches is presented. Finally some recent studies of complex chiral nanostructures in scarab beetles are used to exemplify the potential of Mueller-matrix ellipsometry.

References

[1] S. Korte and W.J. Clegg, Scripta Mater., 60, 807-810 (2009).

There has been considerable interest in the vapor deposition of metallic thin films in order to improve surface properties [1-4]. In addition, the reactive vapor deposition of multi element or more complex alloys such as austenitic stainless steel was recently reported [5-7]. This work showed that a range of interesting and novel phases can be synthesized. More recently, Pagon et al. [8] reported on the effect of deposition energy on the microstructure and mechanical properties of high speed steel (HSS) films prepared using a filtered cathodic vacuum arc. It was observed that the microstructure was critically dependent on the deposition energy. These previous works showed that it is possible to synthesis a wide range of microstructures in complex alloy thin films, which in turn, can lead to the development of a wide range of properties. Indeed, Leyland and Matthews [9] proposed the concept of exploiting predominantly metallic thin films with a nanograined and/or glassy microstructure for tribological applications where toughness in combination with hardness is required. The present study explores this concept further by investigating the effect of deposition energy and temperature on the evolution of microstructure and properties of reactively (with nitrogen gas) deposited HSS and stainless steel films using a filtered cathodic vacuum arc. The deposited films were characterized in terms of their stoichiometry using x-ray photoelectron spectroscopy, cross-sectional microstructure using transmission electron microscopy, crystal structure using x-ray diffraction and hardness using nanoindentation. These results will be discussed in terms of the evolution of microstructure and the effect on properties.

References

[1] I.I. Beilis, A. Shnaiderman, R.L. Boxman, Surf. Coat. Technol., 203 (5-7) (2008) 501-504.

[2] B.A. Movchan, Demchish.Av, Physics of Metals and Metallography-Ussr, 28 (4) (1969) 83-&.

[3] C. Li, S. Shang, P. Yang, L. Zhang, L. Qi, International Journal of Materials and Structural Integrity, 4 (1) 25-34.

[4] R.E. van de Leest, Corrosion Science, 29 (5) (1989) 507-515.

[5] S.R. Kappaganthu, Y. Sun, J. Cryst. Growth, 267 (2004) 385-393.

[6] S.R. Kappaganthu, Y. Sun, Appl. Phys. A: Mater. Sci. Process., 81 (2005) 737-744.

[7] S.R. Kappaganthu, Y. Sun, Surf. Coat. Technol., 198 (1-3) (2005) 59-63.

[8] A.M. Pagon, J.G. Partridge, P. Hubbard, M.B. Taylor, D.G. McCulloch, E.D. Doyle, K. Latham, J.E. Bradby, K.B. Borisenko, G. Li, Surf. Coat. Technol., 204 (21-22) (2010) 3552-3558.

[9] A. Leyland, A. Matthews, Surf. Coat. Technol., 177-178 (2004) 317-324.

SiC belongs to the special class of ultra-high temperature ceramics that are widely used in structural components such as airframe leading edges and reentry space vehicles due to their ultra-high melting temperatures, excellent high temperature strength, and oxidation resistance. Design and development of new refractory materials with improved properties for these applications requires a detailed atomic-scale understanding of the processes controlling high-temperature thermal and chemical stabilities of these materials. As a first step, we choose SiC(0001) as a model system and investigated its thermal stability in ultra-high vacuum environment.

Using high-temperature (~1400 K), UHV scanning tunneling microscopy (STM), we investigated the thermal stability of single-crystalline SiC(0001). We observe the formation of monolayer and bilayer graphene due to preferential evaporation of Si from the surface. STM images acquired as a function of annealing time indicate that graphene nucleates at the SiC surface step edges and grows inward at the expense of the SiC terraces. From the STM images, we measured time-dependent variation of areal coverages of SiC terraces and graphene. We find that the rate of disappearance of SiC is 3.15 × higher than the rate of growth of graphene. That is, for every layer of graphene formed, 3.15 layers of SiC are consumed. This result is consistent with the expected number of carbon atoms within each SiC layer. Our results provide atomic-scale insights into the factors influencing the thermal stability of SiC surfaces.

In recent years ferromagnetic shape memory alloys (FSMAs) have attracted increasing attention due to their large magnetic field induced strain and the possibility of using these materials as actuators and sensors. The martensitic transformation temperature in these alloys is an important parameter to enhance their applicability. Martensitic transformation temperature can be altered by irradiating thin films of FSMA using high energy heavy ions. In the present study, thin films of Ni-Mn-Sn ferromagnetic shape memory alloy were deposited on Si (100) substrate at substrate temperature of 400^oC by DC magnetron sputtering using target prepared by solid state reaction of Ni, Mn and Sn powders. These films were irradiated by 200 MeV Ag ions at different fluences ranging from 1 x 10¹² to 1 x 10¹⁴ ions/cm². X-ray diffraction (XRD) patterns at room temperature confirm the austenite phase L2₁ crystal structure of the as deposited films. Temperature dependent magnetization measurements reveal that the martensitic transition temperature varies with fluence. Four probe R-T measurements also support the variation of martensitic transformation temperature with fluence as obtained by temperature dependent magnetization data. Atomic force microscopy and scanning electron microscopy were used to observe the surface morphology of irradiated films at different fluences.