ICMCTF2012 Session TS2-2: Advanced Characterization of Coatings and Thin Films

Mechanical properties of thin films for micro- and nanoelectromechanical systems can be significantly different from those of the material they are produced of. Double-clamed beams were fabricated in order to determine the elastic modulus and the residual stress of AlGaN heterostructures and hexagonal AlN thin films by a laser Doppler vibrometer. For this, FEM simulations were performed showing the beam’s behavior depending on the stimulation at different Eigenmodes and also depending on the torsion modes. Furthermore, the residual stress in the deposited films before structuring was determined by high-resolution X-ray diffraction. The Young’s modulus of the heterostructures was determined by nanoindentation of the as deposited thin film. Additionally, FTIR-ellipsometry was applied in order to determine the residual stress of the thin films for micro- and nanoelectromechanical systems. A good agreement of the results can be stated, which will be presented.

In this work, a new methodology for the simultaneous evaluation of the residual stress and of the Poisson’s ratio in thin coatings is presented.

The key idea is to remove material with a focused ion beam (FIB) in order to induce controlled relaxation strains that can be correlated to the in-plane residual stress and to the Poisson’s ratio of the coating. This is done by adopting a unique novel geometry for the milled trench and by means of digital image correlation (DIC) techniques and ad hoc constitutive models.

The method consists of two sequential steps: (1) FIB milling of two parallel slots and analysis of the consequent relaxation strain that might occur in the central area (along the X-direction which is the direction perpendicular to the slots); (2) milling of two additional slots along a direction perpendicular to the previous one, so to induce a full biaxial stress relief of the central square island. The analysis of the relaxation strain after the second milling step is then performed along the same X-direction. The depth of all trenches and the distance between them are kept equal to the coating’s thickness. The relaxation strain analysis is performed by means of high-resolution in-situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC).

A FE modeling approach was also used to demonstrate that, for a wide range of isotropic materials, the ratio between the two acquired X-strains is a unique simple function of the Poisson’s ratio of the coatings, independently of the elastic modulus. Furthermore, the analysis of the X and Y relaxation strains at each milling step allows the direct evaluation of the residual stress depth profile in the coating.

The model was experimentally validated on a 3.0 µm CAE-PVD Chromium Nitride (CrN) coating.

An equal-biaxial average stress of -5.04 ± 0.75 GPa was detected and found to be in good agreement with that obtained by XRD (adopting the same elastic constants). Moreover, a Poisson’s ratio of 0.23 ± 0.02 was estimated, which is in the range reported in the literature for similar coatings.

The two situations of non-equal biaxial stress and non-isotropic in-plane elastic behavior are analyzed and correcting equations are proposed. Finally, the influence of coating’s texture and microstructure on the reliability of the method is discussed.

The development of new materials for coatings has generally focussed on a limited number of materials with relatively simple crystal structures in which deformation occurs predominantly by the movement of dislocations. However there are crystals with unit cells that are sufficiently large that conventional dislocations are energetically unfavourable. Despite this, such materials are known to be plastic above a ductile-brittle temperature, typically 0.5 – 0.75 of the melting point.

In this paper the low temperature deformation behaviour of single crystals of an orthorhombic Al₁₃Co₄ and a cubic Mg₂Al₃ has been studied at temperatures using micropillar compression. This allows cracking to be suppressed by making the sample sufficiently small, in this case a few microns in diameter. The materials studied had grains of a sufficient size that micropillars could be milled within individual grains, allowing the single crystal flow behaviour in grains of different orientations to be studied. It is shown that there is a very pronounced yield drop in these materials, and that the yield stress appears to be almost independent of temperature, similar to what has been observed in a metallic glasses. In some orientations, the orientations of the slip traces with respect to the pillar orientation were consistent with what would be expected from the slip systems at higher temperatures. However, this was not observed in many cases, suggesting that the glide bands form on or close to planes of maximum shear stress.

In situ microscopy is a powerful method that enables direct visualization of surface morphological, structural, compositional evolution, and often reveals surprising and previously unknown aspects. Since the observations are carried out at the processing conditions (for example, during growth or annealing in vacuum or in a reactive ambient), the phenomena can be quantitatively described with minimal uncertainties in kinetic rate measurements. In this talk, I will showcase the capabilities of in situ variable-temperature scanning tunneling microscopy (VT-STM) using two examples: 1) growth of graphene thin films on SiC(0001) and 2) gas-solid reactions on TiO₂(110).

Barrier properties of thin films with respect to the diffusion of oxygen and water are of crucial importance in assessing their protection performance. Thin SiO_x plasma polymer films have been intensively researched for their possible application as barrier coatings on engineering metals. Their high formability makes them suitable for applications involving forming processes; whereas their superior barrier properties prevent corrosive electrolytes from reaching the film-metal interface. The key step in the development of these films is the simultaneous optimisation of the mechanical and barrier properties to achieve coatings which can hinder the ingress of electrolyte to the interface and at the same time sustain a high resistance to crack formation during forming processes.

This paper focuses on crack initiation of SiO_x plasma polymer films. Various films are deposited in a low pressure microwave plasma to study the effect of process parameters (plasma gas composition, substrate bias and adhesion promoting pre-treatments) on film properties. For the investigation of deformation at various strain values, plasma polymer films were applied on PET-foils and the crack formation was studied in-situ by means of atomic force microscopy (AFM). A custom build AFM-stage was used to apply the desired strain in a perfectly controlled manner. Moreover, the evaluation of water up-take in barrier films was performed on films deposited on gold electrodes by means of discrete polarisation modulation Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) and by collection of cyclic voltammetry data, in atmospheres with controlled humidity and in corrosive electrolytes, respectively. Presented results will demonstrate the correlation between the deposition parameters and crack formation mechanisms. The understanding of the interplay between the plasma parameters and the observed barrier properties will enable the design of high performance thin film coatings.

The authors gratefully acknowledge the support provided by the Deutsche Forschungsgemeinschaft (DFG) within the framework of the SFB-TR 87.

Printed electronics is a developing alternative to conventional photolithography as “green technology”. Direct printing techniques such as inkjet, screen, and gravure printing are adopted to deposit thin films. They are based on an additive manufacturing, which conductive nanoinks or nanopastes are printed on the designated positions, thereby are an innovative and low-cost metallization method. Despite this advantage of the direct printing, there is still limited usage in microwave and millimeter-wave applications. In order to expand the application fields of the printed electronic devices, the sufficient information for the high frequency performance of the directly printed circuits should be investigated. Also, high reliability of the printed electronic devices must be guaranteed for commercialization of feasible radio frequency (RF) applications. The properties of the printed thin film are determined by heat treatment and user environment. Therefore, RF properties of screen-printed silver (Ag) films were characterized with an environmental reliability test. A Ag nanopaste was screen-printed onto a silicon (Si) substrate passivated with SiO₂. The printed films were sintered at 250 ˚C for 30 min, and then were placed in a chamber with 85˚C/85%RH for various durations (100, 300, 500, 1000 hrs). The microstructural evolution and thickness profiles of the sintered conductive tracks were investigated with a field emission scanning electron microscope and 3D nano-scan view, respectively. Oxide layers on the Ag thin films were analyzed with the Auger electron spectroscopy. A network analyzer and Cascade’s probe system in the frequency range of 10 MHz to 20 GHz were employed to measure the S-parameters of the Ag thin films. From the experimental results, the insertion losses at high frequencies increased with increasing the exposure durations to the 85 ºC/85% RH due to the thicker oxide layers. The RF properties of the printed Ag films were affected by the microstructural evolution and the oxide layers, which will be deeply discussed in the conference site and full manuscript.

This paper investigates characteristics and physical mechanism of the resistance random access memory (RRAM) device with TiN/CoSiO_X/Pt structure. The device exhibits excellent characteristic with good endurance of more than 10⁵ times, long retention time of 10⁴ s in 125 ℃ and more stable in resistance switching state. In general, the mechanism is regarded as a redox reaction in the dielectric interface between the TiN electrode and the conductive filament. In this study, both high resistance state (HRS) and low resistance state (LRS) seem to be independent of cell size, so that the formation/rupture of localized conduction filament is preferred as the driving mechanism of the resistance switching in the Pt/CoSiO_X/TiN system. Furthermore, the switching mechanism is investigated by current-voltage (IV) curve fitting to confirmation the filament. Also, the asymmetric phenomenon of the carrier conductor behavior is found at the HRS in high electric filed. The switching behavior in the TiN/WSiO_X/Pt device is regard as tip electric filed by localizing filament between the interface of top electrode and insulator. In addition, the resistance state is relating with the thickness of switching layer between the TiN electrode and the conductive filament.