ICMCTF2010 Session F1-1: Advances in Characterization of Coatings and Thin Films

In the last years, glow discharge optical emission spectrometry (GDOES) gained more and more importance in the analysis of functional coatings. GDOES thereby represents an interesting alternative to common depth profiling techniques like AES and SIMS, based on its unique combination of high erosion rates and erosion depths, sensitivity, analysis of non-conductive layers and easy quantification even for light elements such as C, N and O. Starting with the fundamentals of GDOES, requirements for high depth resolution and low detection limits regarding parameter optimization and sample quality will be discussed. Furthermore, an overview of new developments in instrument design, aspects for accurate and well resolved thin film analyses and limitations for GDOES measurements as well as quantification will be presented. Results illustrating the high depth resolution, confirmation of stoichiometry, the detection of light elements in coatings as well as contaminations on the surface or in the interface will be demonstrated by measurements of metallurgical coatings and films for industrial applications: Multilayer systems, hard coatings for wear applications, photovoltaic coating systems, implantation profiles and coatings for corrosion resistance. A discussion of the results in comparison to other analysis techniques is presented.

Electro chemical deposition (ECD) is of great importance in particular for long-term and highly durable corrosion protection in various areas such as automotive bodywork, construction industry and several outdoor and off-shore applications under harsh environments. In addition to corrosion protection, other functional properties such as decorative (e.g. color or brightening) and functional (resistance to mechanical use) features are of interest.

Zinc-based coatings represent the huge majority of applications for the protection of steel against corrosion. As number, shape and size of components to be coated are varying, rack loading and bath parameters (hydrodynamics, current density, electrolyte composition and state, pH-value, temperature) have to be considered. In contrast to plasma deposition, the correlation of process parameters and coating properties and the model-based simulation of this interdependence on an industrial scale is not yet satisfactorily understood. This issue is addressed in this work in which zinc electrolytes serve as model system.

Zinc was electro-deposited on steel sheets out of an alkaline non-cyanic bath with different additives. Layer thickness in terms of intra- and inter-sample homogeneity was measured by X-ray fluorescence spectroscopy (XRF). The distribution of layer thickness was related to current density and hydrodynamics and verified by means of global (of the entire electrolyte vessel) and local (in front of the substrates) simulations. The crystal orientation and the morphology were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). As shown, microstructure mainly depends on the current density and the additives (inhibitors, alkali salt, Ni) of the electrolyte bath. According to the crystal orientation, different types of deposit structures (adapted from Fischer) were observed. The mechanical properties such as hardness, modulus and creep as measured by instrumented indentation testing (IIT) are related to the deposit structure and the composition of the zinc layer.

For reference electrolytes in a plating pilot plant, it could be shown that the simulation of the hydrodynamics is strongly correlated to the thickness distribution. In general, the surface of the zinc coating consists of crystal plates with a thickness of less than 50 nm. The roughness increases at lower current densities and for lower amounts of inhibitors whereas the current efficiency decreases at higher current densities and therefore at higher deposition rates.

Energy storage is vital for mobile system (mobiles phones, laptop,…) because it defines the system autonomy. In addition to the main power source (lithium-ion battery), these systems need a small embedded one mainly used to power the internal clock. This source is currently a primary button cell added on the printed circuit board. The aim is to replace this source by a microbattery, that need to be compatible with solder-reflow process. In addition, a microbattery may be relevant for a wide range of applications such as RFID tag and smart packaging, sensors, smartcards…

A microbattery is a monolithic system constituted by more than 10 layers including the active part (both electrodes and the electrolyte), protective layers, current collectors and barriers thin films, with a total thickness not exceeding 15 µm. In order to improve performances of such microbatteries, especially at low temperature, it is essential to optimize electrolyte thin films that ensure the transport of Li ions between the two electrodes. Among all solid electrolyte thin films, LiPON is the most used in lithium microbatteries. Nevertheless, despite its good ionic conductivity (3.10^-6 S/cm), it suffers from a low thermal stability inducing a great increase of its activation energy after solder-reflow process, which is detrimental for the functioning of the microbattery at low temperature.

In order to better understand the role of nitrogen on its ionic conductivity as well as its evolution over thermal treatment, we have prepared various LiPON-type thin films with different compositions by radio-frequency sputtering from a Li₃PO₄ target by varying deposition conditions, in particular nitrogen flow rate. As these thin films are amorphous, we have used the complementarity of XPS, Raman spectroscopy and Nuclear Magnetic Resonance (NMR) to investigate the local structure and its evolution with nitrogen content and after solder reflow treatment. Some correlations have been established between deposition conditions, composition, local structure, ionic conductivity and activation energy. On the basis of these results, we have doped the LiPON thin films and succeeded to greatly enhance its thermal stability as well as its performances at low temperature (patent pending).

In some conditions, impedance spectroscopy measurements achieved on the all-solid-state thin films battery have evidenced a second semi-circle in addition to the one characteristic of the electrolyte, suggesting the formation of an interface between the electrolyte and one electrode, responsible for a capacity fading over cycles. XPS analyses have been achieved to investigate this interface after cycling or thermal treatment.

Ternary Al-Si-N films, as well as pure AlN and SiN_y films, were deposited by DC reactive magnetron co-sputtering from pure Al and Si targets in an Ar/N₂ atmosphere at 200°C and 500°C. Overall composition and bonding states were investigated by X-ray photoelectron spectroscopy (XPS). Photoelectron line positions, broadening and chemical shifts are reported, as well as the nitrogen Auger parameter and its Wagner plot. The results are compared to XRD data that indicate a transition upon increasing silicon content from a single phase Al_1-xSi_xN solid solution to a two-phase composite – Al_1-xSi_xN crystallites surrounded by an amorphous SiN_y phase – at about 6 at.% of Si. Two different compositional regimes were identified from XPS data that are characterized by a distinct change in the evolution of chemical shifts and of the photoelectron line broadening at about 10-15 at.% of Si. This concentration regime repesents the onset of formation of a SiN_y tissue phase thicker than two monolayers on an average. Under these conditions the chemical bonding in the tissue phase is similar to that in bulk SiN_y. The observed changes in the XPS data coincide well with the structural changes in the material at different silicon contents as deduced from XRD and optical measurements, It is shown that a consistent image of the material’s constitution requires the combined use of several different characterization techniques, with XPS just being one of them.

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 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.

The adhesion between the diamond-like carbon coatings (DLC) and the substrate is a limiting parameter for many applications of these coatings. One possibility, how to improve the adhesion, is the use of a buffer layer. As possible candidates for the buffer layers, transition metals forming carbides are discussed. We intended to employ chromium as a buffer between the steel substrate and the tetrahedral bonded amorphous carbon (ta-C) coating not only to improve the adhesion between both counterparts of the system, but also to hinder the diffusion of carbon into the steel substrate. The advantage of chromium is that it forms carbides with narrow homogeneity ranges, which serve like diffusion barriers for a further carbon diffusion.

The main goal of this work was the description of the interface formation between Cr and ta-C during the deposition and after the thermal treatment. For this reason, various Cr/ta-C multilayers with different thicknesses of the Cr layers (10 nm and 20 nm) were deposited on silicon substrates. Furthermore, the film deposition was carried out with different energies of the carbon ions. The Cr/ta-C multilayers were investigated using the small angle X-ray scattering (SAXS), the high resolution transmission electron microscopy (HRTEM) and the electron energy loss spectroscopy (EELS). SAXS yielded information about the electron density and thickness of individual layers and about the interface roughness and morphology. The electron density obtained from the SAXS experiments was employed for the estimation of the sp³/sp² ratio in individual samples, which was verified by EELS. The interface roughness and the morphology of the interfaces were verified by HRTEM.

Electrical properties and switching mechanism of electron-beam evaporated MnO₂ thin films as transition layer of resistive random access memory was investigated in this study. The device with structure Ti/MnO₂/Pt shows reproducible and stable resistive switching behavior traced over 2000 times at room temperature. Transmission electron microscopy analyses are used to confirm the crystalline structure of MnO₂ on Pt bottom electrode. Secondary ion mass spectrometry reveals a change of oxygen distribution in MnO₂ thin film due to material characteristic of variant top electrodes. The current-voltage characteristics of MnO₂ with various top electrodes are presented. We suggest that the interface between MnO₂ and electrodes play an important role on the resistive switching behavior.