ICMCTF 2025 Session MC2-1-TuA: Mechanical Properties and Adhesion I

Atomic layer deposition (ALD) holds enormous potential to design interfaces, due to the unique way in which a material is built in an atomic layer-by-layer fashion. When combined with other thin film techniques, such as magnetron sputtering (PVD), without breaking vacuum, the layer-by-layer nature of ALD can be harvested to design (sub)nanoscale interface architectures. An interesting area for this combined deposition are metal-polymer interfaces, where thin amorphous interlayers (IL, 5 nm thick) between metal film and polymer substrate favour strong and stable interfaces [1-3]. Until now, interlayer formation is governed by the film/substrate chemistry and deposition method, preventing high interface quality for the majority of material combinations and fabrication routes. Since ultrathin ALD layers uniquely resemble the reported interlayer in structure and chemistry, interlayer formation can, for the first time, be mimicked artificially to clarify the role of these structures in thin film delamination.

Through a combined ALD/PVD setup, we fabricate and study Al thin films (150 nm) with different ALD interlayer thicknesses (Al₂O₃ + H, 0.12 - 25 nm) on a polyimide substrate. Mechanical properties are measured via uni- and equi-biaxial tensile loading [4] with in-situ X-ray diffraction and electrical resistivity measurements from the evolution of Al film stress, width of the Al diffraction peak and electrical resistivity as a function of IL thickness and applied strain. Adhesion energy between metal film and polymer substrate is calculated using the tensile induced delamination method.

In our study, differences in the system's mechanical behaviour (yield strength, crack onset strain) are found to be driven by the microstructure of the metallic Al layer (film thickness and grain size), while the crack propagation (electrical failure strain) and adhesive performance (buckle density) is dominated by the interface structure. Significant embrittlement and fracture is only observed for thick interlayers (>= 25 nm).

[1] Putz, B. et al., Adv. Eng. Mater. (2022), 2200951

[2] Putz, B. et al. Surf. Coat. Technol. 332, 368–375 (2017)

[3] S. Oh. Et al., Scripta Materialia 65 (2011) 456–459

Residual stress has been a long-standing problem in thin film deposition, and it is critical to the adhesion and physical properties of their applications. In previous works, we have studied the mechanisms and used modeling to explain the stress evolution in the post-coalescence stage (typically 50 nm ~ 400 nm) and steady state (>400 nm or when the stress does not change significantly). The early stage of growth (<50 nm) is not as well studied as the others and yet this state is important to the origin of the thin film stress. Here we present a model to explain the behavior of residual stress in the early stage with the assumption that the deposited particles form hemisphere islands on the substrate. The model is applied to analyze stress measurements of e-beam evaporated Ag and Ni from the wafer curvature measurements. The results suggest that the end of the coalescence stage may not be sufficient to explain the occurrence of the tensile peak in the early state. Rather, the balance between tensile and compressive stress mechanisms as the grain boundary is formed between islands needs to be considered.

In the electronics and semiconductor industries, there is a growing demand for precise characterization of adhesion properties in soft metallic multilayers on fragile substrates, such as semiconductor wafers and glass. Consequently, nondestructive testing methods have become essential to prevent damage to these sensitive substrates during testing. Conductive metallic layers including gold (Au), platinum (Pt), copper (Cu), and silver (Ag) are critical for microchip pathways; however, their ductility poses challenges for adhesion testing on brittle substrates. Traditional nanoscratch methods, which use sphero-conical indenters, rapidly traverse these soft layers and exert significant stress on the fragile substrates. This often results in substrate failure rather than yielding valuable insights into the interfacial adhesion of the layers.

In this study, we introduce a novel scratch testing method specifically designed for soft metallic layers or multilayers on fragile substrates. This approach employs a micro wedge blade indenter, rather than the conventional spheroconical indenter, along with a two-axis tilt stage sample holder, enhancing precision and reducing substrate damage to yield more reliable adhesion measurements. This method is particularly suited for ductile metallic coatings deposited via PVD, CVD, and ALD, providing a robust solution for accurately assessing the adhesion properties of soft metallic coatings on sensitive substrates.

Keywords: Ductile coatings; Fragile substrates; Adhesion testing; Wedge blade indenter; Nanoscratch testing.

The MoN films are gathering increasing attentions for their high-performance characteristics, making the production of high-quality MoN films an important research focus. In this study, Mo-N thin films are coated using radio frequency magnetron sputtering, RFMS, and high intensity power impulse magnetron sputtering, HiPIMS, techniques. The input power and Ar/N₂ ratio are adjusted from 150 to 200W and 15/5 to 18/2 sccm/sccm to control the microstructure. The duty cycle of the HiPIMS from 4 to 10% is also manipulated to trigger higher peak power density and current. A columnar structure feature was observed across all thin films. Nevertheless, the phase of the Mo-N changes under different parameters. Through RFMS, as the Ar/N₂ ratio was raised from 15/5 to 18/2 sccm at 150 W input power, a significant evolution of major Mo₂N to MoN phase was observed. With higher peak current and power density through HiPIMS deposition, a multiple phase feature with decreased grain size of Mo-N phases were discovered. The microhardness, elastic modulus, wear resistance and indentation cracking behavior were investigated. The correlation between microstructure evolution and the mechanical properties were also discussed.

Keywords:RefractoryThin film coatingsMoNHipims

Cold spray is a solid-state additive manufacturing process that produces coatings and standalone parts by accelerating micron-sized metallic particles to supersonic velocities. Upon impact, the particles and substrate undergo plastic deformation, surface oxide layers get disrupted, and direct particle-substrate contact is achieved to attain metallurgical bonding. Our recent works take advantage of the Laser-Induced Particle Impact Test (LIPIT) to produce single microparticle impacts under carefully controlled conditions, providing a unit-process understanding of cold spray physics. Each launched particle is well-characterized: its size, morphology, microstructure, velocity, and in-flight behavior are known. We then analyze impact sites using focused ion beam-scanning electron microscopy (FIB-SEM) to study multiple aspects of the impact event: bonding at particle-substrate and particle-particle interfaces, deformation at high strain rates, and microstructural evolution. The major advantage of this approach is that it is tomographic, providing direct 3D observations of the interfaces, as well as quantitative measurements of the bonded area and microstructural changes around the impact site.

This talk will review several observations that 3D tomography of the impact sites reveals about structure development in cold spray. First, we observe generally non-symmetrical bonding at the particle-substrate interface and conclude that bonding takes place top-down; regions experiencing high strain bond first. These insights conform to a model for particle-substrate bonding through oxide-layer rarefication and provide guidelines for how to optimize processing parameters to produce well-bonded cold spray coatings. Second, our microstructural observations reveal limiting conditions for the development of recrystallization structures. Such information speaks to the development and dissipation of adiabatic heat upon impact.

The ongoing trend toward miniaturization in device components across key technologies demands the synthesis of high-performance nanostructured films with exceptional combination mechanical properties such as high yield strength and plasticity which, however, are mutually exclusive. In order to overcome such trade off, it is crucial to control the atomic composition and the microstructure, going beyond currently nanoengineering design approaches for thin films. One main limitation arises from conventional thin film deposition techniques (sputtering) with limited possibility to fabricate novel microstructures such as with ultrafine grains or nanoscale laminates alternating layers of different compositions and phases with intrinsic dimensions on the order of a few nanometers. Such features could induce mechanical size effects, influencing deformation mechanisms and enabling highly tunable and enhanced mechanical properties.

Here, I will show the potential of Pulsed Laser Deposition (PLD) as a novel technique to synthetize advanced metallic thin films, reporting the fabrication of a variety of microstructures with tailored composition and nanoscale features including compact, nanogranular and crystal/glass ultrafine nanolaminates and focusing on the deformation behavior and mechanical properties.

First, I will focus on the on the fabrication of thin film metallic glasses with different composition ZrCu, ZrCuAl (with also O addition) and controlled microstructure, compact and nanogranular [1]. The mechanical characterization with optoacoustic techniques, nanoindentation and in situ SEM micropillar compression reports large and tailored mechanical properties, above sputter-deposited counterparts, reaching ultimate yield strength (>4 GPa) and ductility (>15 %) for ZrCuAl/O films. Then, I will show the fabrication of ultrafine glass/crystal (ZrCu/Al) nanolaminates with high and tunable density of interfaces (nanolayer thickness <5 nm), reporting shear bands blocking and homogenous deformation, in combination with large plasticity (> 10%) and yield strength (>3.4 GPa) [2].

Lastly, I will focus on the PLD synthesis of CoCrCuFeNi crystalline high entropy alloys showing unique microstructure and ultrafine grains (≈10 nm), triggering Hall-Petch strengthening resulting in high hardness (≈10.5 GPa) and yield strength (1.9 GPa) significantly above sputter-deposited counterparts, while retaining large plastic deformability (30%) [3].

[1] M. Ghidelli et al., Acta Mater., 213, 116955, 2021.

[2] F. Bignoli et al., ACS Appl. Mater. Interfaces, 16, 27, 35686–96, 2024.