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Fundamental Science of Thin Films

Research Program

Our research program, which features both experiment and theory, focuses on the fundamental materials science and nanoscience of thin films. It is aimed at increasing the atomistic understanding of vapor phase deposition, ion-surface interactions, and phase transitions in advanced materials. We explore the thermodynamics and kinetic pathways controlling phase and nanostructure formation. 

The group probes into the nature of epitaxial layers, textured thin films, and nanoscale materials. Model systems include transition metal nitride, carbide, oxide, and boride; wide-band-gap nitride semiconductors; nanocomposites; superlattices; fullerene-like compounds; and nm-scale metallic multilayers. 

Several reactive deposition techniques are available including:

Magnetically-unbalanced magnetron sputtering

High-power pulsed magnetron sputtering (HIPIMS)

Hybrid high-power/direct current magnetron sputtering (HIPIMS/DCMS)

Our materials science laboratory specializes in developing methods for plasma characterization, analytical and high-resolution electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy, nanoindentation, SEM and TEM in-situ nanoindentation, nano- and micro-tribology, nanopiezoelectrical direct and converse characterization, ab initio calculations including our Synthetic Growth Concept, and multi-billion time-step molecular dynamics simulations.

Group Members

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Grzegorz Greczynski, Associate Professor (group leader)

Lars Hultman, Professor, Head of Thin Film Physics Division, Leave of absence SSF

Joe Greene, Adjunct Professor

Ivan Petrov, Adjunct Professor

Valeriu Chirita, Associate Professor

Björn Alling, Associate Professor

Jens Jensen, Associate Professor

Fredrik Eriksson, Associate Professor

Martin Magnuson, Associate Professor

Olof Tengstrand, Postdoc

Konstantinos Bakoglidis, PhD student

Daniel Edström, PhD student

David Engberg, PhD student

Annop Ektarawong, PhD student

Christopher Tholander, PhD student

Research Highlights

Venting temperature determines surface chemistry of magnetron sputtered TiN films

Schematic representation of the effects of venting temperature on the surface composition of TiN/Si(001) films.

Surface properties of refractory ceramic transition metal (TM) nitride thin films grown by magnetron sputtering are essential for resistance towards oxidation necessary in all modern applications. In this work, typically neglected factors including exposure to residual process gases following the growth and the venting temperature Tv, each affecting the surface chemistry, are addressed. We employ the previously developed Al-cap technique  to separate the effects of residual gas exposure in the high-vacuum (HV) environment during the post-deposition phase from those introduced during the following venting sequence and air exposure. With the help of x-ray photoelectron spectroscopy (XPS) analyses performed on a series of TiN samples as a function of Tv we find that majority of surface reaction products, including TiO2, TiOxNy, and N2 previously detected after prolonged annealing experiments, form shortly after vent, provided that Tv is sufficiently high. This has implications for all sorts of practical studies where the surface composition of TM layers is assumed to be fixed once the same growth protocol is used. We show both, that this is definitely not the case for the TiN model materials system, and that the venting temperature is a key experimental variable and should therefore routinely be recorded and reported together with other processing conditions for such experiments and production. It is reasonable to assume that findings reported here are also relevant for other transition metal nitrides as well as for compounds that form reaction products with gases contained in the atmosphere. The surface characterization by XPS is also affected since the reference core level spectra obtained from as-received films exhibit large dependence on Tv. In addition, the ability to obtain clean oxide-free surfaces by in-situ sputter etching decreases with increasing venting temperature.

Related Publications:

G. Greczynski, S. Mráz, L. Hultman, J.M. Schneider, „Venting temperature determines surface chemistry of magnetron sputtered TiN films“, Applied Physics Letters 108 (2016), 041603

G. Greczynski, I. Petrov, J.E. Greene, and L. Hultman “Al capping layers for non-destructive x-ray photoelectron spectroscopy analyses of transition-metal nitride thin films”, J. Vac. Sci. Technol. A 33 (2015) 05E101-1

Diffusion studies in transition metal nitride (TMN) films

Transition metal nitrides are widely applied as diffusion barrier layers in microelectronic devices. The continued miniaturization of such devices not only poses new challenges to material systems design but also puts high demands on characterization techniques. In our studies we combine two high resolution state-of-the-art methods, namely transmission electron microscopy (TEM) and atom probe tomography (APT), to characterize model TMN diffusion barrier layers down to the atomic scale in order to correlate the microstructure with the performance of the barrier layers.

The example shows a three-dimensional APT reconstruction of a polycrystalline TiN barrier layer after an annealing treatment. Beginning non-uniform grain boundary diffusion of Cu atoms from the Cu metallization through the TiN film is observable. This information can ultimately be used to accurately determine diffusion coefficients in different materials and thus provides a tool for the quantification of diffusion barrier performance.  

Related Publication:

M. Mühlbacher, F. Mendez-Martin, B. Sartory, N. Schalk, J. Keckes, J. Lu, L. Hultman, C. Mitterer, Copper diffusion into single-crystalline TiN studied by transmission electron microscopy and atom probe tomography, Thin Solid Films. 574 (2015) 103–109.

A novel multilayer TiAlCN/CNx films on CoCrMo biomedical alloys

A novel multilayer of TiAlCN/CNx films have been designed and deposited onto CoCrMo biomedical alloy by reactive magnetron sputtering. Our results show that the coating improves the mechanical properties of the uncoated substrate, providing higher hardness and elastic recovery. Potentiodynamic polarization tests also indicate that the coating offers an increased protection against pitting and corrosion in simulated body fluid.

Related Publication:

B.  Alemón , M.  Flores , C.  Canto , E.  Andrade , O.G.  de Lucio , M.F.  Rocha , E.  Broitman, Ion beam analysis, corrosion resistance and nanomechanical properties of TiAlCN/CNx multilayer grown by reactive magnetron sputtering, Nuclear Instruments and Methods in Physics Research Section B 331 (2014) 134-139. http://dx.doi.org/10.1016/j.nimb.2013.12.040, fulltext

Structure and bonding in amorphous iron carbide thin films

Micrographs (a1–4), SAED patterns (b1–4) and PDF patterns (c1–4) of Fe1-xCx thin film samples with carbon content ranging from 20.8 to 71.8 at%.

The structure, chemical bonding, and electrical properties of magnetron sputtered thin films of amorphous iron carbides Fe1-xCx (0.21≤x≤0.72) has been investigated. X-ray, electron diffraction and transmission electron microscopy reveal that the Fe1−xCx  films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy, and a carbon-rich matrix. As show by pair distribution function analysis, the close-range order is similar to those of crystalline Fe3C carbides in all films with additional graphene-like structures at high carbon content. X-ray absorption spectra exhibit an increasing number of unoccupied 3d states and a decreasing number of C 2p states as a function of carbon content. The spectral changes in X-ray absorption signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size-dependent carbide/matrix interfaces. The findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides through size control of amorphous domain structures.

Related Publication:

Andrej Furlan, Ulf Jansson, Jun Lu, Lars Hultman and Martin Magnuson;
J. Phys. Cond. Mat. 27, 045002 (2015), fulltext

Crystallization Characteristics and Chemical Bonding Properties of Nickel Carbide Thin Film Nanocomposites

The amorphous structure, crystallization characteristics, chemical bonding, and electrical properties of magnetron sputtered nickel carbide Ni1-xCx nanocomposites have been investigated for a wide range of carbon contents. The crystallinity of the films was found to be strongly dependent on the carbon content and additional graphene-like structures are found at high carbon content. X-ray diffraction and X-ray absorption studies reveal carbon-containing hcp-Ni (hcp-NiCy phase), instead of the expected rhombohedral-Ni3C phase. The X-ray absorption spectra exhibit an increasing number of unoccupied Ni 3d states and a decreasing number of C 2p states as a function of carbon content. The spectral changes in X-ray absorption signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size-dependent carbide/matrix interfaces. These findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides through the control of amorphous domain structures. It is also shown that the resistivity is not only governed by the amount of carbon, but increases by more than a factor of two when the samples transform from crystalline to amorphous.

Related Publication:

Andrej Furlan, Jun Lu, Lars Hultman, Ulf Jansson and Martin Magnuson;
J. Phys. Cond. Mat. 26, 415501 (2014)

Novel strategy for low-temperature growth of dense, hard, and stress-free refractory ceramic thin films

Growth of fully-dense refractory thin films by means of physical vapor deposition (PVD) requires elevated temperatures Ts to ensure sufficient adatom mobilities. Films grown with no external heating are underdense, as demonstrated by the open voids visible in cross-sectional transmission electron microscopy images and by x-ray reflectivity results; thus, the layers exhibit low nanoindentation hardness and elastic modulus values. Ion bombardment of the growing film surface is often used to enhance densification; however, the required ion energies typically extract a steep price in the form of residual rare-gas-ion-induced compressive stress.

We recently proposed a PVD strategy for the growth of dense, hard, and stress-free refractory thin films at low temperatures; that is, with no external heating. We used TiN as a model ceramic materials system and employed hybrid high-power pulsed and dc magnetron co-sputtering (HIPIMS and DCMS) in Ar/N2 mixtures to grow dilute Ti1‑xTaxN alloys on Si(001) substrates. The Ta target driven by HIPIMS serves as a pulsed source of energetic Ta+/Ta2+ metal-ions, characterized by in-situ mass and energy spectroscopy, while the Ti target operates in DCMS mode (Ta-HIPIMS/Ti-DCMS) providing a continuous flux of metal atoms to sustain a high deposition rate. Substrate bias Vs is applied in synchronous with the Ta-ion portion of each HIPIMS pulse in order to provide film densification by heavy-ion irradiation (mTa = 180.95 amu vs. mTi = 47.88 amu) while minimizing Ar+ bombardment and subsequent trapping in interstitial sites. Since Ta is a film constituent, primarily residing on cation sublattice sites, film stress remains low.

As no external heating is used the substrate temperature Ts during deposition does not exceed 120 °C. As a result of heavy-metal-ion bombardment, the inter- and intracolumnar porosity, typical of refractory ceramic thin film growth at low Ts is eliminated due to effective near-surface atomic mixing as evident by XTEM (see Figure). With the bias voltage Vs = 160 V synchronized to the Ta+ portions of the HIPIMS pulses, film hardness and elastic modulus are 330 and 200% higher than corresponding values for reference DCMS TiN layers (cf. Figure), while the residual stress remains low (-0.7 GPa).

Related Publication

G. Greczynski, J. Lu, I. Petrov, J.E. Greene, S. Bolz, W. Kölker, Ch. Schiffers, O. Lemmer and L. Hultman, “A Novel Strategy for Low-Temperature, High-Rate Growth of Dense, Hard, and Stress-free Refractory Ceramic Thin Films”,  J. Vac. Sci. Technol. A 32 (2014) 041515, fulltext

Atom probe tomography studies of TiSiN films grown by reactive cathodic arc method

This project concerns understanding, and ultimately controlling, the growth of hard ceramic coatings in order to tailor properties such as hardness, wear resistance and thermal stability. To do this, we grow films using reactive cathodic arc deposition and analyze samples using atom probe tomography, electron microscopy and X-ray diffraction. Our study of TiSiN involves developing certain aspects of atom probe tomography in order to successfully apply the technique to the material system in question. The mass spectrum overlap of 28Si and 14N makes them impossible to differentiate using time-of-flight techniques such as atom probe tomography. By growing films using pure 15N, mass spectrum overlaps can largely be avoided and 3D compositional information can be collected. The image above shows how the Si distribution can be resolved on the nanometer scale in a cross-section perpendicular to the film growth direction of a (Ti0.92Si0.08)15N atom probe tomography sample.

Selection of metal ion irradiation for controlling Ti1-xAlxN alloy growth via hybrid HIPIMS/magnetron co-sputtering

We have employed a hybrid HIPIMS/DCMS co-sputtering, with one elemental target (Ti or Al) powered by HIPIMS while the other by DCMS, to grow metastable Ti1-xAlxN alloys with compositions 0.4 ≤ x ≤ 0.74. We discovered that film properties, hardness, elastic modulus, and residual stress, dramatically depend upon which target is powered by HIPIMS (see Fig.  2). Layers grown in the Al-HIPIMS/Ti-DCMS mode have a kinetic solid-solubility limit of x = 0.64 and exhibit high hardness (H up to 30 GPa) due to solid-solution hardening with essentially no intrinsic residual stress, all of which are difficult to achieve by either DCMS alone or by cathodic arc deposition. Ion mixing is facilitated by the enhanced momentum transfer from metal ions during HIPIMS vs. primarily gas ions during DCMS. In sharp contrast, with Ti-HIPIMS/Al-DCMS, two-phase Ti1-xAlxN films are obtained at all compositions. The layers exhibit low hardness in the range 18-19 GPa with high compressive stress, up to -2.7 GPa. The strong difference in film properties is primarily due to the presence of an intense flux of doubly-ionized Ti2+ ions that give rise to the creation of the residual defects and high compressive stress. The defects serve as nucleation centers for the formation of wurtzite-structure AlN precipitates, with decreased film hardness.

Related Publications:

G. Greczynski, J. Lu, M. Johansson, J. Jensen, I. Petrov, J.E. Greene, and L. Hultman, “Role of Tin+ and Aln+ ion irradiation (n = 1, 2) during Ti1-xAlxN alloy film growth in a hybrid HIPIMS/magnetron mode”, Surf. Coat. Technol. 206 (2012) 4202, fulltext

G. Greczynski, J. Lu, M. Johansson, J. Jensen, I. Petrov, J.E. Greene, and L. Hultman “Selection of metal ion irradiation for controlling Ti1-xAlxN alloy growth via hybrid HIPIMS/magnetron co-sputtering”, Vacuum 86 (2012) 1036, fulltext

G. Greczynski, J. Lu, J. Jensen, I. Petrov, J.E. Greene, S. Bolz, W. Kölker, Ch. Schiffers, O. Lemmer and L. Hultman, “Strain-free, single-phase metastable Ti0.38Al0.62N alloys with high hardness: metal-ion energy vs. momentum effects during film growth by hybrid high-power pulsed/dc magnetron cosputtering”, Thin Solid Films 556 (2014) 87

G. Greczynski, J. Lu, J. Jensen, S. Bolz, W. Kölker, Ch. Schiffers, O. Lemmer, J.E. Greene, and L. Hultman, “A Review of Metal-ion-flux-controlled Growth of Metastable TiAlN by HIPIMS/DCMS Co-sputtering”, Surf. Coat. Technol. 257 (2014) 15

Metal vs. Rare-gas Ion Irradiation during Ti1-xAlxN Film Growth by Hybrid HIPIMS/DCMS Co-sputtering using Synchronized Pulsed Substrate Bias

Metastable NaCl-structure Ti1-xAlxN is employed as a model system to probe the effects of metal vs. rare-gas ion irradiation during film growth using reactive high-power pulsed magnetron sputtering (HIPIMS) of Al and dc magnetron sputtering (DCMS) of Ti. The alloy film composition is chosen to be x = 0.61, near the kinetic solubility limit at the growth temperature of 500 °C. Three sets of experiments are carried out: a -60 V substrate bias is applied either continuously, in synchronous with the full HIPIMS pulse, or in synchronous only with the metal-rich-plasma portion of the HIPIMS pulse. Alloy films grown under continuous dc bias exhibit a thickness-invariant small-grain, two-phase nanostructure (wurtzite AlN and cubic Ti1‑xAlxN) with random orientation, due primarily to intense Ar+ irradiation leading to Ar incorporation (0.2 at%), high compressive stress (-4.6 GPa), and material loss by resputtering. Synchronizing the bias with the full HIPIMS pulse results in films which exhibit much lower stress levels (-1.8 GPa) with no measureable Ar incorporation, larger grains elongated in the growth direction, a very small volume fraction of wurtzite AlN, and random orientation. By synchronizing the bias with the metal-plasma phase of the HIPIMS pulses, energetic Ar+ ion bombardment is greatly reduced in favor of irradiation predominantly by Al+ ions. The resulting films are single phase with a dense competitive columnar structure, strong 111 orientation, no measureable trapped Ar concentration, and even lower stress (‑0.9 GPa). Thus, switching from Ar+ to Al+ bombardment, while maintaining the same integrated incident ion/metal ratio, eliminates phase separation, minimizes renucleation during growth, and reduces the high concentration of residual point defects which give rise to compressive stress.

Related Publication:

G. Greczynski, J. Lu, J. Jensen, I. Petrov, J.E. Greene, S. Bolz, W. Kölker, Ch. Schiffers, O. Lemmer and L. Hultman, “Metal vs. Rare-gas Ion Irradiation during Ti1xAlxN Film Growth by Hybrid HIPIMS/DCMS Co-sputtering using Synchronized Pulsed Substrate Bias”, J. Vac. Sci. Technol. A 30 (2012) 061504, fulltext


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Last updated: 06/14/16