A. Michel

Nanoscaled composite TiN/Cu multilayer thin films deposited by dual ion beam sputtering: growth and structural characterisation

Abstract We present a detailed structural characterisation of TiN/Cu multilayers, with bilayer period ranging from 5 to 20 nm, deposited by dual ion beam sputtering on Si(001) substrates. Low-angle X-ray diffraction scans exhibit Bragg reflections up to the 15th order, indicative of a well-defined periodicity and non-cumulative roughness. High-angle X-ray diffraction shows that the multilayers adopt a (001) TiN/(001) Cu texture and are polycrystalline in the plane, the mosaic spread for the TiN and Cu grains being of ∼6° and more than 9°, respectively. High Resolution Transmission Electron Microscopy (HRTEM) observations confirm the cube on cube epitaxial growth, but show the presence of several interfacial and growth defects, introduced to relieve the huge misfit (15.9%) between the two lattices. The HRTEM images also reveal facetted islands growth morphology of Cu, leading to lateral roughness at the TiN/Cu interface as well as fluctuations in the interplanar spacings, which could explain the lack of superlattice reflections in the high-angle X-ray diffraction.

Stress field in sputtered thin films: Ion irradiation as a tool to induce relaxation and investigate the origin of growth stress

The stress state of sputtered Mo thin films was studied, and a detailed analysis of elastic strains, using x-ray diffraction and the “sin2 Ψ method,” was performed. The evolution of the lattice parameter under ion irradiation showed that the usual assumption of a biaxial stress state is not adequate to determine the true stress-free lattice parameter a0 of the film. An original stress model, including a hydrostatic component linked to volume distortions induced by point defects, is required. This model, which describes a triaxial stress field, allows a reliable determination of a0. Furthermore, ion irradiation was shown to be a powerful method for stress relaxation, providing a stress-free lattice parameter solely linked to chemical effects.

Volmer-Weber growth stages of polycrystalline metal films probed by in situ and real-time optical diagnostics

The Volmer-Weber growth of high-mobility metal films is associated with the development of a complex compressive-tensile-compressive stress behavior as the film deposition proceeds through nucleation of islands, coalescence, and formation of a continuous layer. The tensile force maximum has been attributed to the end of the islands coalescence stage, based on ex situ morphological observations. However, microstructural rearrangements are likely to occur in such films during post-deposition, somewhat biasing interpretations solely based on ex situ analysis. Here, by combining two simultaneous in situ and real-time optical sensing techniques, based on surface differential reflectance spectroscopy (SDRS) and change in wafer curvature probed by multibeam optical stress sensor (MOSS), we provide direct evidence that film continuity does coincide with tensile stress maximum during sputter deposition of a series of metal (Ag, Au, and Pd) films on amorphous SiOx. Stress relaxation after growth interruption was testified from MOSS, whose magnitude scaled with adatom mobility, while no change in SDRS signal could be revealed, ruling out possible changes of the surface roughness at the micron scale.

Track formation in amorphous Fe0.55Zr0.45 alloys irradiated by MeV C60 ions: Influence of intrinsic stress on induced surface deformations

Abstract Amorphous Fe0.55Zr0.45 films, having thickness of 400 nm, were grown on silicon substrates by co-deposition using ion beam sputtering. Limited surface roughness makes this system particularly suitable for fine-scale scanning force microscopy analysis and nano-indentation. The samples were irradiated with MeV C60 clusters, and the surface morphology of single impacts was found to have a “doughnut” shape, i.e. hillocks having a central crater. Quantitative evaluation of the deformation was achieved by measuring their height and diameter. When C60 projectiles deviate from normal incidence, a tail emerges along the direction of the incident beam. The height of the hillock and length of the tail are increasing with the incidence angle, and the magnitude of the deformation indicates that the damage mainly occurs due to a radial coherent mass transport outwards from the track core by a compression shockwave-like mechanism. The residual compressive in-plane stress, ∼−0.4 GPa for the as-deposited films, was found to notably influence the C60 induced plastic deformations. Indeed, stress relaxation results in a marked decrease in height combined with a significant widening of the surface features. This “flat” surface morphology is attributed to an enhanced radial efficiency of the pressure pulse, owing to a significant reduction of the hardness of the amorphous film after stress relaxation. The overall picture outlined from our observations suggests that the surface damage induced by single MeV C60 ions possibly is the signature of plastic deformation induced at large distances by an energetic radial pressure pulse. This unsteady shockwave allows the energy transfer outwards from the localised region along the ion path that experiences a sudden transient heating.

A load-lock compatible system for in situ electrical resistivity measurements during thin film growth.

An experimental setup designed for in situ electrical resistance measurement during thin film growth is described. The custom-built sample holder with a four-point probe arrangement can be loaded into a high-vacuum magnetron sputter-deposition chamber through a load-lock transfer system, allowing measurements on series of samples without venting the main chamber. Electrical contact is ensured with circular copper tracks inserted in a Teflon plate on a mounting holder station inside the deposition chamber. This configuration creates the possibility to measure thickness-dependent electrical resistance changes with sub-monolayer resolution and is compatible with use of sample rotation during growth. Examples are presented for metallic films with high adatom mobility growing in a Volmer-Weber mode (Ag and Pd) as well as for refractory metal (Mo) with low adatom mobility. Evidence for an amorphous-to-crystalline phase transition at a film thickness of 2.6 nm is reported during growth of Mo on an amorphous Si underlayer, supporting previous findings based on in situ wafer curvature measurements.

Epitaxial growth of Cu(001) thin films onto Si(001) using a single-step HiPIMS process

We report on a new route to grow epitaxial copper (Cu) ultra-thin films (up to 150 nm thick) at ambient temperature on Si(001) wafers covered with native oxide without any prior chemical etching or plasma cleaning of the substrate. It consists of a single-step deposition process using high power impulse magnetron sputtering (HiPIMS) and substrate biasing. For a direct current (DC) substrate bias voltage of −130 V, Cu/Si heteroepitaxial growth is achieved by HiPIMS following the Cu(001) [100]//Si(001) [110] orientation, while under the same average deposition conditions, but using conventional DC magnetron sputtering, polycrystalline Cu films with [111] preferred orientation are deposited. In addition, the intrinsic stress has been measured in situ during growth by real-time monitoring of the wafer curvature. For this particular HiPIMS case, the stress is slightly compressive (−0.1 GPa), but almost fully relaxes after growth is terminated. As a result of epitaxy, the Cu surface morphology exhibits a regular pattern consisting of square-shaped mounds with a lateral size of typically 150 nm. For all samples, X-ray diffraction pole figures and scanning/transmission electron microscopy reveal the formation of extensive twinning of the Cu {111} planes.

Direct Observation of the Thickness-Induced Crystallization and Stress Build-Up during Sputter-Deposition of Nanoscale Silicide Films

The kinetics of phase transitions during formation of small-scale systems are essential for many applications. However, their experimental observation remains challenging, making it difficult to elucidate the underlying fundamental mechanisms. Here, we combine in situ and real-time synchrotron X-ray diffraction (XRD) and X-ray reflectivity (XRR) experiments with substrate curvature measurements during deposition of nanoscale Mo and Mo1-xSix films on amorphous Si (a-Si). The simultaneous measurements provide direct evidence of a spontaneous, thickness-dependent amorphous-to-crystalline (a-c) phase transition, associated with tensile stress build-up and surface roughening. This phase transformation is thermodynamically driven, the metastable amorphous layer being initially stabilized by the contributions of surface and interface energies. A quantitative analysis of the XRD data, complemented by simulations of the transformation kinetics, unveils an interface-controlled crystallization process. This a-c phase transition is also dominating the stress evolution. While stress build-up can significantly limit the performance of devices based on nanostructures and thin films, it can also trigger the formation of these structures. The simultaneous in situ access to the stress signal itself, and to its microstructural origins during structure formation, opens new design routes for tailoring nanoscale devices.

Anisotropic strain-stress state and intermixing in epitaxial Mo(110)/Ni(111) multilayers: An x-ray diffraction study

The present study deals with the analysis of elastic strains and stresses in high-quality heteroepitaxial Mo/Ni superlattices with periods Λ lying in the range 4.8–27.6 nm. The strain-stress state in this lattice-mismatched system grown under energetic deposition conditions (ion beam sputtering) is rather complex, resulting from three contributions: (i) intrinsic (growth) stress due to atomic peening, (ii) coherency stresses of opposite sign in the two elemental layers due to the observed Nishiyama–Wassermann epitaxial relationship Ni[11¯0](111)∥Mo[001](110), and (iii) interfacial mixing. The measurement of the lattice parameters of Mo and Ni sublayers in various crystallographic directions was performed by x-ray diffraction, using the sin2 ψ method adapted for epitaxial layers. A large anisotropy of elastic strain and associated in-plane coherency stresses is revealed in the Mo sublayers, while for Ni sublayers no such behavior could be detected due to the superimposition of growth variants with threefol...

Stress and Intermixing in Epitaxial Ni(111)/Mo(110) Superlattices

The stress state and intermixing in epitaxial Ni/Mo multilayers grown on (11 2 0) sapphire substrates are investigated using X-ray Diffraction (XRD). Two deposition techniques were used, namely ion beam sputtering (IBS) and magnetron sputtering (MS), to vary the energy of the deposited species. In both cases, high-quality superlattices with a Nishiyama-Wasserman epitaxial relationship Ni [110] (111) // Mo [001] (110) were obtained. The residual stress state appears rather complex, resulting from two contributions: a growth-stress whose magnitude and sign depend on growth conditions and coherency stresses of opposite signs in the two elemental sublayers (tensile for Ni and compressive for Mo). Post-growth ion irradiation at low fluences was used to induce structural changes in a controlled way. For the case of IBS, it resulted in partial stress relaxation, as the growth stress could be almost fully relaxed, while the coherency stresses remained unchanged. For the case of MS, a distinct behavior was found: a stress increase of the tensile component of Mo-sublayers was observed, while a stress reduction of the compressive component was noticed. We attribute this phenomenon to ion irradiation induced intermixing. For the Ni sublayers, this intermixing leads to a stress relaxation. The modeling of the stress evolution during ion irradiation was performed using a triaxial stress analysis which enabled us to determine the ‘stress-free and defect-free lattice parameter’, solely linked to chemical effect.

Perpendicular magnetic anisotropy and spin reorientation transition in L10 FePt films

We investigated the thickness and composition dependence of perpendicular magnetic anisotropy (PMA) in L10 Fe1−xPtx (x = 0.4, 0.5, and 0.55) films. The FePt films with different thicknesses of 35 and 70 A were grown at the substrate temperature Ts = 300 °C by molecular beam epitaxy coevaporation technique. A (001)-oriented epitaxial L10 FePt film was grown on the thin (001)-oriented fcc Pt layer, while a poorly crystallized FePt film was formed on the (111)-textured Pt layer. Our results showed that, at a fixed thickness of 70 A, the PMA of FePt alloy films is enhanced as Pt content increases from 40% to 55%.