ICMCTF2015 Session TS2-2: Advanced Characterization of Coatings and Thin Films

The TiN/Cu bilayer system is of high technological importance, with TiN frequently employed as a diffusion barrier and Cu as the metal interconnect in microelectronic devices. This study compares the performance of TiN barrier layers with different microstructures in hindering the in-diffusion of Cu atoms.

Model TiN/Cu bilayers were grown by reactive magnetron sputtering on MgO(001) and Si(001) substrates. Cross-sectional transmission electron microscopy (XTEM) and X-ray reflectivity measurements of the pristine samples showed that variations in the deposition temperature and substrate bias lead to the development of different microstructures in the TiN film, varying from single-crystalline to columnar polycrystalline.

To induce diffusion of Cu atoms into the TiN layer, annealing treatments were carried out in a high-vacuum furnace. The performance of the TiN diffusion barrier was then evaluated by high-resolution XTEM in combination with energy-dispersive X-ray mapping. In addition, three-dimensional reconstructions of the elemental distribution at the Cu-TiN interface were obtained by atom probe tomography.

Single-crystalline TiN grown on MgO(001) at 700 °C with a pulsed substrate bias voltage of 100 V was the most efficient of all investigated sample microstructures in preventing Cu interdiffusion, with a uniform diffusion layer of only 12 nm thickness after annealing at 1000 °C for 12 h. This excellent performance is attributed to the lack of fast diffusion paths such as grain boundaries.

In polycrystalline TiN layers with a dense microstructure deposited on Si(001) at 700 °C with 100 V pulsed substrate bias, indications of beginning grain boundary diffusion of Cu atoms could be found already after annealing at 900 °C for 1 h. With decreasing deposition temperature and substrate bias and thus the evolution of a more open microstructure, the efficiency of the polycrystalline TiN diffusion barrier deteriorated and enhanced Cu diffusivity was observed.

Thin films and micro-scale coatings find increasingly widespread use in applications ranging from superconductors¹ and photovoltaic cells ² to metallurgical surface protection³ and optical assemblies⁴. In order to be successful in such applications, the surface adhesion, fracture toughness, wear-resistance and other mechanical properties of the coatings needs to be characterised and controlled.

As a direct result of the layer-based thin film deposition, high gradients of properties have been observed in terms of the composition⁵ and microstructure⁶. These gradients should be considered in combination with the high magnitude residual stresses induced in thin films⁷. The techniques currently available do not have the resolution necessary to capture the fine scale variation in properties and stress, so that the development of new techniques to resolve such effects at the micro- to nano-scale is required.

Thin film depth resolved residual stress analysis has been performed at the nanoscale using an extension of the new ring-core Focused Ion Beam (FIB) and Digital Image Correlation (DIC) technique⁸. This approach involves incremental FIB milling to stress relieve an ‘island’ of material at the substrate surface. The quantification of strain relief at the surface is accomplished using DIC and the influence of the stresses present at different depths (corresponding to different milling increments) is determined by comparison with Finite Element (FE) modelling. The cross-validation of the results of this approach against a well-established X-ray technique has recently been performed⁹.

Nanoindentation is a well-established and robust technique for precise studies of surfaces¹⁰. The consideration of the work of indentation in thin films over a range of indentation depths¹¹ allows the interpretation of load-displacement data in terms of stiffness and hardness. Critical examination of the work of indentation technique highlights the importance of the indenter shape as well as the interface adhesion¹². Numerical solutions for different indenter shapes and adhesion modes have been presented, resulting in a robust, reliable and accurate approach for depth resolved characterisation of thin films.

The integration of the two experimental techniques described above has the potential to provide the much needed insight into the gradients of mechanical properties and stress in thin film components. This improved understanding will lead to better design methodologies and the advancement of manufacturing techniques.

¹K. S. Il’in, D. Rall, M. Siegel, and A. Semenov, Physica C: Superconductivity 176 (2012).

²S. Y. Myong, Renewable and Sustainable Energy Reviews 37, 90 (2014).

³Y. Jiang, S. Mráz, and J. M. Schneider, Thin Solid Films 538, 1 (2013).

⁴M. Mohamed and M. A. Abdel-Rahim, Materials Science in Semiconductor Processing 27, 288 (2014).

⁵G. He, Y. Zhang, C. Peng, and X. Li, Applied Surface Science 283, 532 (2013).

⁶R. Daniel, J. Keckes, I. Matko, M. Burghammer, and C. Mitterer, Acta Materialia 61, 6255 (2013).

⁷X. Song, K. B. Yeap, J. Zhu, J. Belnoue, M. Sebastiani, E. Bemporad, K. Y. Zeng, and A. M. Korsunsky, Procedia Engineering 10, 2190 (2011).

⁸M. Sebastiani, C. Eberl, E. Bemporad, and G. M. Pharr, Materials Science and Engineering: A 528, 7901 (2011).

⁹E. B. Marcello Gelfi, Brisotto Mariangela, Laura E Depero,; Alexander M Korsunsky, Alexander J Lunt, Marco Sebastiani, Surface and Coatings Technology or Thin Solid Films (2014).

¹⁰C. A. Schuh, Materials Today 9, 32 (2006).

¹¹A. M. Korsunsky and A. Constantinescu, Materials Science and Engineering: A 423, 28 (2006).

¹²A. M. Korsunsky and A. Constantinescu, Thin Solid Films 517, 4835 (2009).

¹³A. Constantinescu, A. M. Korsunsky, O. Pison, and A. Oueslati, International Journal of Solids and Structures 50, 2798 (2013)

The measurement of residual stresses using focused ion beam (FIB) milling and digital image correlation (DIC) method has become a widely used technique for analyzing coating systems. In this method, the internal stresses are relaxed due to material removal by FIB milling and the resulting displacements can then be tracked by digital image correlation (DIC). Also, finite element analysis (FEA) is often used to evaluate the measured deformation and calculate the corresponding residual stress. For this method, the H-Bar geometry (or double slot milling) has been proved to be a simple milling geometry with the advantages of an uniaxial relaxation behavior which can be easily implemented in FEA. However, a detailed study of the influence of bar length on the relaxation strain field has not been carried out, yet.

In general, the milling geometry has a significant influence on the displacement field around the FIB milled region of the investigated material system. Therefore, in this work the influence of the H-Bar dimensions regarding length, width and milling depth in relation to the coating thickness is studied on a chromium nitride (CrN) PVD layer and compared to the finite element analysis.

Thus, for certain geometry parameters a linear relation between relaxed stress and measured strain can be found. Depending on the chosen parameters, a either a plane/strain or a fully uniaxial relaxation behavior can be observed.

The sin²(ψ) method is a widely used X-ray diffraction technique for investigating residual stress in bulk materials. Macroscopic strains are evaluated by measuring the shift of diffraction peaks due to lattice distortion for various ψ orientations of crystallites within the material. In classical methods, the ψ inclinations are achieved by either inclining the X-ray source and detector around the stationary sample or by inclining the sample while keeping the symmetric diffraction geometry fixed, leading in both cases to variable penetration depth. Those methods may therefore show limitations (i) in case of a depth stress gradient (ii) in case of thin coatings due to the small diffracting volume; (iii) in case of multilayer stacking or multiphase layer due to numerous peak overlaps from the different phases. The multiple (hkl) grazing incidence sin²(ψ) method offers several advantages for the characterization of residual stress in coatings and thin films. In this asymmetric diffraction geometry, sample and source are kept fixed while the detector scans the 2θ range and the scattering vector will exhibit a different ψ inclination each time a (hkl) Bragg condition is fulfilled. The grazing incident geometry overcomes some of the previous limitations, since the diffraction signal (i) rises from a fixed and controlled depth and (ii) gets emphasized due to the low angle of incidence.

In this work, residual stresses of multilayer coatings are evaluated by combining grazing incidence sin²(ψ) method and whole powder pattern decomposition. In the latter, peak positions of each phase are constrained by crystalline symmetries which is crucial for overcoming peak overlap issues. Using the software TOPAS, structural parameters (i.e. stress free lattice parameter) can be simultaneously refined with macroscopic strain effects. This approach has been successfully applied to evaluate residual stresses of each constituting materials in various industrial nitride coatings.

Metallic thin films grown by PVD can usually bear stress levels up to several GPa, well in excess of their corresponding yield stress in bulk form. Such excessive stress can lead to premature failure, buckling or delamination. Significant progress has been made in the last decade thanks to the potentiality offered by in situ, real-time stress measurement during deposition, enabling to probe the growth dynamics with sub-monolayer sensitivity. Several quantitative models have been proposed to relate the growth-induced stress to changes in film microstructure, surface morphology, defects incorporation or adsorbed species, especially for high-mobility metals case. On the contrary, the case of low-mobility metals still lacks a reliable modeling.

In this study, our approach is to understand the fundamental phenomena, at the atomic scale, driving stress development during growth of low-mobility sputtered thin films, by combining highly sensitive in-situ stress measurements using Multi beam Optical Stress Sensors (MOSS) with ex-situ structural investigations (XRD, AFM, HRTEM).

As a model system, we studied α-Ta films deposited by magnetron sputtering in Ar atmosphere on crystalline bcc Mo_1-xSi_x [x<0.16] (110) template layers, allowing us to control the lateral grain size D from 40nm to ~1µm. The influence of grain size, growth rate and deposited energy during growth (obtained from Monte Carlo simulations of growth energetics) on stress development was evaluated in real-time using MOSS. Post-deposition 360 keV Kr ions irradiation, at low doses (0.1-2dpa), was used as an effective tool to study stress relaxation. The elastic strain field in as-deposited and irradiated thin films was determined from XRD using the sin²(Ψ) method, adapted for the case of textured layers. Systematic AFM measurements were done on as-grown Ta films to characterize the surface morphology and determine the grain size.

We show for low-mobility metals that the steady-state stress scales linearly with 1/D, highlighting defects incorporation at the grain boundaries. However, comparatively to high-mobility systems, opposite evolutions are evidenced with increasing growth rate, pointing out different mechanisms of defects incorporation and stabilization. Indeed, incorporation at the grain boundaries and the propensity to stabilize defects in-grain during growth are the two main phenomena in competition for the dynamic growth process and associated stress levels. Our results provide new data to extend further Chason’s kinetic model to low-mobility systems. A collaborative work is carried out in this respect with the group of K. Sarakinos (Linköping Univ.) and E. Chason (Brown Univ.).

Via metallization by Cu electrodeposition for interconnection of integrated circuit (IC) chips and printed circuit boards (PCBs) has become increasingly important because it provides high thermal and electrical conductivity and efficient utilization of space in electronic devices. A concave Cu surface is usually created above the via metallization structure, and affects the subsequent stacked-via process. In order to improve the flatness of the surface Cu, PCB industry often prolongs the plating time to a certain period, thereby producing an undesired thick Cu over the board surface. Thinning-surface-Cu method via a chemical etching process is currently adopted in microelectronic industry, and might yield numerous pinholes in the electroplated Cu films, deteriorating the mechanical/electrical reliability of the Cu interconnects. However, the formation mechanism of pinholes upon etching is still unclear to date. It is of great important to probe into the pinhole formation behavior in the electroplated Cu films and its prevention, which is the aim of the present study. The relationship between pinholes and impurities (e.g., C, Cl, and S) within Cu platings were examined by using a scanning electron microscope (SEM), focused ion beam (FIB), and time of flight secondary ion mass spectrometer (TOF-SIMS) in this study . Additionally, effects of isothermal annealing on the redistribution of impurities and the quantity of pinholes were also investigated. The knowledge advances our understanding of how pinholes form and will be helpful in developing its mitigation strategies .

Keywords: Electroplated Cu; pinholes; impurities; annealing; TOF-SIMS

Solder joints with line-bump (Cu-solder) geometry are the most common material configuration in joining two metal parts of different packaging levels. When an electric current is delivered to this line-bump structure, the majority of electron current tends to crowd the entrance point of the solder bump due to a large divergence in the cross-section of the conducting media and a vertical conducting path configured between the on-chip Cu interconnect and solder bump. The materials neighboring the current crowding region (CCR) experience a much higher current stress than other regions of the same solder joint, thereby inducing a significant materials transport away from CCR. This results in serious mechanical/electrical reliability concerns, such as voids formation-propagation along the contact window of the line/bump interface, local solder melting, and a fast depletion of the Cu interconnects. In the present study, we found abnormal electromigration-induced Cu depletion in a typical line-bump joint, where an extremely high Cu depletion did not occur in CCR but in the area with a much lower current density distributed. Electron backscatter diffraction (EBSD) analysis showed that when the Cu interconnect neighbored the Sn grains with c-axes parallel to electron flow, a pronounced depletion of Cu in the cathode side was caused, even though the current density distributed in such area was much lower than CCR. This observation indicated that not only the magnitude of current density but the crystallographic o rientation of Sn played a dominant role in the Cu depletion upon electron current stressing. Finally, a new electromigration failure mechanism in solder joints is proposed in this talk.