Magnetron-sputtered Thin Films Research Articles

Structure, phase stability and elastic properties in the Ti1–xZrxN thin-film system: Experimental and computational studies

The composition-dependence of the structure and elastic properties of ternary Ti1–xZrxN alloys is systematically investigated by combining thin film growth and ab initio calculations. Single-phase Ti1–xZrxN thin films (0⩽x⩽1) with a rocksalt structure have been deposited using dc reactive magnetron sputtering at Ts=300°C in Ar/N2 plasma discharges. The structure, stress state and phase stability upon thermal annealing were studied by X-ray diffraction (XRD), while the acoustic and elastic properties were measured using Brillouin light spectroscopy, picosecond ultrasonics and nanoindentation. First-principles pseudopotential calculations of the total energy, lattice constants, bulk modulus, and single-crystal elastic constants Cij for several cubic ordered structures of Ti1–xZrxN alloys were also carried out. The positive values of the computed formation energies indicate that the homogeneous Ti1–xZrxN alloys can be only stabilized at high temperatures. However, the magnetron-sputtered thin films were found to retain their as-grown single-phase cubic structure during post-deposition annealing at 850°C for 3h. The calculated equilibrium lattice parameters are in good agreement with the stress-free lattice parameters a0 determined experimentally from XRD using the sin2ψ method: they both exhibit a positive deviation from Vegard-like linear interpolation. The calculated bulk modulus, elastic constants and Poisson’s ratio gradually decrease from TiN to ZrN. These computed values were used to interpret the experimentally derived elastic constants and Young’s modulus as functions of composition.

Analysis of Bulk and Thin Film Model Samples Intended for Investigating the Strain Sensitivity of Niobium-Tin

Bulk samples and thin films were fabricated and characterized to determine their suitability for studying the effect of composition and morphology on strain sensitivity. Heat capacity and resistivity data are used to determine the critical temperature distribution. It is found that all bulk samples contain stoichiometric Nb3Sn regardless of their nominal Nb to Sn ratio. Furthermore, in bulk samples with Cu additions, a bi-modal distribution of stoichiometric and off-stoichiometric Nb-Sn is found. Thus the nominally off-stoichiometric bulk samples require additional homogenization steps to yield homogeneous off-stoichiometric samples. A binary magnetron-sputtered thin film has the intended off-stoichiometric Nb-Sn phase with a mid-point critical temperature of 16.3 K. This type of sample is a suitable candidate for investigating the strain sensitivity of A15 Nb1-βSnβ, with 0.18 <; β <; 0.25. The strain sensitivity of Nb-Sn as a function of composition and morphology is important for an in-depth understanding of the strain sensitivity of composite Nb3Sn wires.

The origin of stresses in magnetron-sputtered thin films with zone T structures

In order to understand the origin of the residual stress state in thin films and its thickness dependence, the structure–stress relation of magnetron-sputtered Cr and CrN layers with thicknesses ranging from 100 nm to 3 μm was investigated in detail. Based on correlations between the layer-thickness-dependent grain size, texture and morphology and the magnitude of the intrinsic and thermal components of the residual stress, a model is proposed that explains the origin of internal stresses of thin polycrystalline films with zone T structures. The model was further extended for the CrN/Cr dual-layer system, where the CrN top layer is epitaxially aligned with the underlying highly (2 0 0)-oriented Cr interlayer. It is shown for the first time that both the intrinsic and thermal stress components are thickness-dependent, which is associated with the layer microstructure.

Real time in situ diagnostics of PVD growth using synchrotron radiation

For specific applications of hard coatings, the microstructure (or nanostructure) has to be tailored for optimum performance. This requires information about the mechanisms of growth and the connection between the microstructure and the deposition parameters. In order to obtain this knowledge, in situ real time growth studies of magnetron-sputtered thin films were carried out. A growth chamber, equipped with two magnetrons and Kapton windows for X-ray diffraction and reflectivity, was mounted on a six-circle goniometer at a synchrotron beam line at ESRF in Grenoble. As an example, X-ray diffraction measurements were carried out in situ during growth of TiN to follow the development of the microstructure. Recrystallization was identified as the mechanism which controlled the development of texture. The driving force for these texture changes arose from minimalization of the sum of the surface energy and the strain energy of the individual grains. As another example, the heteroepitaxial growth of TiN on MgO(001) was studied. Bragg–Brentano as well as grazing-incidence in-plane wide angle scattering was used to establish the pseudomorphic growth of TiN to the underlying MgO(001). Using real-time specular X-ray reflectivity, layer-by-layer growth was observed, with the surface roughening decreasing with an increase in the deposition temperature. The growth of nanocrystalline Au was also investigated. Among the results we found that changes in the (111) fiber texture arose from grain rotations.

Reactive magnetron sputtering of thin films: present status and trends

This paper gives a critical review of the present state of the knowledge in the field of dc reactive magnetron sputtering of compound films. It analyses (i) the hysteresis effect and the methods of its elimination, (ii) problem of stability of reactive sputtering and (iii) deposition of transparent oxides in the transition mode of sputtering. It shows the conditions under which oxides are reactively sputtered with high deposition rates aD oxide achieving up to approximately 77% of that of a pure metal aD Me, i.e. aD oxide/aD Me≈0.77. A special attention is devoted to the elimination of arcing in sputtering of insulating films using pulsed dual magnetron or sputtering of oxides from a substoichiometric target. Also, the ion bombardment of films growing on insulating or unbiased substrates in dc pulsed magnetron sputtering is discussed in detail. As an example, a new possibility to form superhard single-phase films based on solid solutions using dc reactive magnetron sputtering is shown. At the end, future trends in dc reactive magnetron sputtering are outlined.

Oxidation resistance of protective coatings studied by spectroscopic ellipsometry

Spectroscopic ellipsometry is applied to determine the oxidation resistance of TiN and CrN coatings. This technique proves to provide accurate measurements of the oxide layer thickness in a convenient, fast, and nondestructive manner. The magnetron-sputtered thin films were heated in air in the temperature range between 673 and 925 K. The verification with scanning electron microscopy and x-ray reflectometry shows an excellent agreement between these complementary techniques. The rate constant k of the oxidation is directly related to the film morphology in TiN thin films. While understoichiometric films with a dense fine-grained structure exhibit a moderate k of 4 nm2/s at 773 K, overstoichiometric films with a pronounced columnar structure oxidized over seven times faster. The nonstoichiometry in TiNy and CrNy leads to a reduced activation energy for oxidation compared to stoichiometric compounds.

Recent aspects concerning DC reactive magnetron sputtering of thin films: a review

In this paper, attention is paid to DC reactive magnetron sputtering and its implications, such as the hysteresis effect and the instability in the reactive gas pressure, differential poisoning of magnetron cathode, as well as the methods which are used to control the process. These methods include: (a) increasing the pumping speed, which requires considerable additional costs; (b) increasing the target-to-substrate distance, which requires larger vacuum chambers (hence, higher costs) and also results in lower deposition rates; (c) obstructing reactive gas flow to the cathode with the resultant reduction of deposition rate and the need for complicated arrangements; (d) pulsed reactive gas flow, which requires an extensive amount of process optimisation and a continuous monitoring and adjustment of the process parameters; (e) plasma emission monitoring; and (f) voltage control, which are inexpensive and have proved to be powerful techniques for monitoring and controlling the reactive sputtering processes, in real time without disturbing the discharge, for the deposition of high-quality films, reproducibly. In addition, arcing and methods to avoid it are reviewed; this includes, arc initiations and their destructive effects (e.g. driving the process to become unstable, reducing the target lifetime and creating defects in the sputtered films), time required for arcs to occur, and finally, methods of avoiding arcs. The latter includes the use of unipolar or bipolar pulsing techniques at frequencies in the range 10–70 kHz.

Pulsed bias magnetron sputtering of thin films on insulators

It is shown that a pulsed direct current (dc) bias is capable of discharging insulating surfaces and providing a good means of controlling the energy of the ions extracted from the sputter magnetron plasma. Numerical calculations were made for surface potentials of an insulating thin film exposed to a plasma, placed on a conductor with a pulsed dc bias. Surface potentials are calculated as a function of time for pulses of variable frequency and duty. Average ion energy is shown to converge to applied dc biases at higher frequencies. Thin aluminum films were deposited on SiO2 substrates as a function of pulse frequency. Atomic force microscopy scans of the surface show that increasing the frequency of the pulses produces drastic changes in resulting film surface morphology.

Magnetron sputtering of films with controlled texture and grain size

Magnetron sputtering of thin films is a strongly developing field of PVD technologies. This paper reviews the recent progress in sputtering of films with controlled texture and grain size using the ion bombardment of growing films and/or the mixing process, i.e. an incorporation of one or several additional elements into a base material. Special attention is devoted to the formation and properties of nanocomposite coatings with grains smaller than 100 nm. New physical properties of these materials, not attainable in polycrystalline films with grains greater than 100 nm, are outlined. The concept of attaining a new generation of hard nanocomposite coatings with high toughness is also described.

Anisotropic growth of magnetic domains in amorphous FeTb thin films

Magnetic properties of amorphous Fe 71Tb 29 magnetron-sputtered thin films are studied experimentally. All films possess a strong perpendicular anisotropy, with easy axis tilted with respect to the film normal. Using a Kerr microscope the patterns of magnetic domains were observed in these films. The observed domains show a favored growth direction in the film plain. Several mechanisms of this anisotropic growth are discussed. Some results of a micromagnetic simulation of domain growth, carried out in order to elucidate the mechanisms of this domain growth anisotropy, are presented.

Perspective on stresses in magnetron-sputtered thin films

For nearly 10 years, J. A. Thornton and the present author pursued a series of investigations on a curious and initially obscure effect of intrinsic stress reversal in magnetron-sputtered thin films. This study was first undertaken to understand the mechanical failures of decorative chromium when deposited on plastic automotive grilles. The present paper follows the evolution of these investigations in which the more than 1000 samples were mailed in small batches on a round trip between the author’s laboratory in the east and Thornton’s in the west. As the work developed it became clear that the structure-sensitive stress reversal was a phenomenon of broad generality, which reflected subtle changes in the growth microstructures and accompanying properties of magnetron sputtered films. It was postulated that these changes resulted from particle bombardment fluxes controlled by sputtering process parameters. This review recalls various experiments that were conceived to explore mechanisms and map out the stress reversal locus in parameter space.

Infrared Kerr rotation and magnetic properties of amorphous U-Sb films

Magnetron-sputtered thin films of amorphous U-Sb with various U concentrations x have been measured in the infrared photon energy range as low as 0.65 eV using polar Kerr rotation. For x>50 at.% the polar Kerr spectra are dominated by a broad negative peak near 1 eV. For x<50 at.% this peak is absent and the negative Kerr rotation is strongly increasing towards low energies with some indication of a peak at 0.7 eV. Maximum rotation measured is 2.4 degrees at 1 eV in U/sub 51/Sb/sub 49/ at a temperature of 10 K and in a field of 3 T.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

TEM studies of RF magnetron-sputtered thin films

Lead scandium tantalate (PST) thin films sputtered onto sapphire substrates have been studied by using transmission electron microscopy. Samples in transverse section were used to characterize the microstructure of the thin films as a function of distance from the PST-sapphire interface whereas samples in plan section allowed a more detailed, structural investigation of the PST. A liquid nitrogen cold stage was used to induce the paraelectric-ferroelectric phase transitionin situ and to perform heating and cooling experiments on the thin films.

OXIDE SUPERCONDUCTIVITY RESEARCH AT NORTHWESTERN UNIVERSITY

A short review was presented of a wide range of activities involving the high Tc oxide superconductors at the Northwestern University. Calculations of the charge density associated with the d9 shell of the copper atom in the Cu-O planes and the d8 shell of the copper atoms along the Cu-O lines of the unit cell of YBa2Cu3O7 performed by Freeman, Massida and Yu were presented. Results of a detailed study of the pseudoternery phase diagram involving CuO, BaO and YO1.5 were shown which verify the existence of three compounds having the Y:Ba:Cu ratios 123 (the high Tc superconductor), 211 (the so-called green phase) and 132 (a new insulating compound). This work, performed by Mason and coworkers, involved X-ray diffraction, optical spectroscopy, magnetic susceptibility, and electrical transport studies. Transport measurements by Poeppelmeier, Song, Hwu and coworkers on the assembledges YBaSrCu3O7 anf YBa2Cu3O5F2 were reported; no evidence of superconducting behavior at temperatures above 90K was seen in the latter. A detailed study of the magnetic and transport properties of the YBa2−xLaxCu3−xNixO7±δ system was reported. For small Ni concentration (x&lt;0.3) a continuous drop in Tc was observed. For x= 0.4 and 0.5 evidence of two separate transitions, near 75K and 25K, were seen in the resistance (with the resistance going to zero at about 15K); for x=0.7 semiconducting behavior was observed. A continuous drop in the normalized diamagnetic signal was seen with increasing x with the diamagnetic fraction being below 5 % for x=0.7 and above. X-ray diffraction showed a continuous decrease in the orthorhombic fraction and an increase in the tetragonal fraction with increasing x. Results on d.c. magnetron sputtering of thin films on MgO from a stoichiometric 123 target in an argon atmosphere were shown. With proper annealing films with onset temperature of 90K and zero resistance at 68K have been prepared; this work was done by Jin, Lee, Song and coworkers. A brief report on the effect of O-stoichiometry on EPR and susceptibility measurements in the 123 compound performed by Hoffman and coworkers and NMR in the La2−xSrxCuO4 compound by Halpern and coworkers was presented.