Elsevier

Surface Science

Volume 600, Issue 11, 1 June 2006, Pages 2275-2287
Surface Science

Low energy ion assisted atomic assembly of metallic superlattices

https://doi.org/10.1016/j.susc.2006.03.018Get rights and content

Abstract

Metallic superlattices with planar, unalloyed (unmixed) interfacial structures are difficult to fabricate by all conventional vapor deposition methods. Molecular dynamics simulations have been used to explore the ways in which inert gas ions can be used to control the atomic assembly of a model Cu/Co metallic super lattice system. High energy, high atomic weight ions are shown to smooth rough interfaces but introduce undesirable intermixing at interfaces. Light ions with very low energies fail to flatten the rough surfaces that are naturally created during deposition at ambient temperature where surface atom mobility is kinetically constrained. The optimum energies for achieving the lowest combination of interfacial roughness and interlayer mixing have been found for each inert gas ion species and the key mechanisms of surface structure reorganization activated by ion impacts have been identified over the range of ion masses and energies studied. Optimum ion energies that maximize the interface structural perfection have been identified.

Introduction

Metallic multilayers with nanoscale layer thicknesses (sometimes referred to as metallic superlattices) exhibit a number of unusual properties [1]. For instance, superlattices consisting of repeated Cr/Sc bilayers with layer thickness in the 5–15 Å range specularly reflect X-rays and can be used as mirrors [2] for X-ray microscopy [3], [4], astronomy [5], [6], lithography [7] and microanalysis [8]. Multilayers consisting of repeated thin (∼50 Å) ferromagnetic layers (such as Co, Fe, Ni and their alloys) separated by thin (10–35 Å) conductive metal layers (such as Cu) exhibit large changes in electrical resistance upon the application of magnetic field [9], [10]. These giant magnetoresistive (GMR) materials have been utilized to construct magnetic sensors [11]. Magnetic tunnel junction multilayers composed of a pair of ferromagnetic layers sandwiching a thin dielectric layer exhibit even larger resistance changes and are being explored for magnetic random access memory (MRAM) [12].

These unusual properties of metallic multilayers are highly sensitive to the atomic scale structure of their interfaces [3], [13], [14], [15], [16], [17]. For each of the applications above, performance is improved if the interfaces separating the different layers are chemically sharp and atomically smooth [2], [15], [16]. Such a structure is invariably metastable and can not therefore be achieved under equilibrium growth conditions. Instead, the kinetic phenomena active during the atomic assembly of the interface are constrained in order to trap a desired structure. The basic approach involves maintaining the growth surface at a low temperature to impede surface kinetics and to then use the momentum, energy or incident angle of the arriving atoms and/or an assisting inert gas ion flux to nudge surface atoms into locations that result in desired structures. Surfactants and surface alloying can also be sometimes exploited to preferentially bias atomic jumps that promote step flow growth leading to planar surfaces [18].

Recent molecular dynamics (MD) simulations have shown that under kinetically constrained growth conditions an increase in an adatom’s translational energy can be used to flatten a growth surface by an impact-induced surface reconstruction mechanism [19], [20], [21]. In this process, the incident energy is transferred to surface atoms resulting in either enhanced surface migration or removal of atoms from loosely bound surface sites. Molecular dynamics simulations have also shown that high incident energy atoms can cause interlayer mixing by an atomic exchange mechanism [19], [20], [21]. It is therefore important to optimize the adatom energy so that it produces the lowest combination of interfacial roughness and interlayer mixing.

Ion beam assisted deposition is also an effective growth approach for modifying surface assembly. Both computational and experimental studies of ion assisted deposition have identified strong ion energy effects upon film structure, stress, density and growth morphology [22], [23], [24]. However, most of these studies have concentrated upon the effects of high energy ions (in the 100—5000 eV range). Unless a glancing incident angle is used, these high energy impacts cause extreme interlayer mixing and it would appear that they are poorly suited for creating the metallic superlattices used for X-ray mirror and magnetoelectronic applications [19], [20], [21].

The use of very low energy ion assistance appears to be a promising approach for manipulating multilayer interfaces. Experiments by Birch et al. have shown that the performance of Cr/Sc X-ray multilayers is improved using 9 eV argon ion assisted deposition [2]. A group at Veeco also investigated the effects of ion assistance on properties of GMR stacks [25]. In their studies, assisting ion fluxes were controlled using radio-frequency bias, gas cluster ion beams and other ion beam treatments. They found that ion energies in the range 10–60 eV significantly improved the GMR ratio of their films. Kools also found that slight increases in kinetic energy (>5 eV) could lead to biased diffusion and the suppression of defect formation [26].

The experiments described above have explored a narrow subset of the many ion energies, ion types and ion incident angles combinations potentially accessible to film growers. These studies also shed little light upon the atomic assembly processes responsible for performance improvements. Computational approaches based upon molecular dynamics have proven useful for exploring the assembly of thin multilayers. The predicted structures have been shown to often be in remarkably good agreement with those obtained by atomic resolution structure characterization methods such as the three-dimensional atomic probe technique [20]. Previous atomistic simulations of low energy ion impacts with small atom clusters have shown that low energy (5–20 eV) ion assisted deposition can reduce interfacial roughness without causing interlayer mixing [27], [28], [29]. However, the precise role of the ion mass and energy upon the atomic scale reconstruction mechanisms were not investigated in these earlier studies.

Roughness and intermixing at interfaces during the vapor deposition of metallic superlattices are controlled by many complicated atomic assembly mechanisms. Under the kinetically constrained growth condition that is used to prevent thermally assisted interdiffusion between layers of different compositions, randomly deposited atoms have insufficient mobility to always migrate to the lowest energy (most coordinated) sites. As a result, a high density of surface terraces form on the surface. Terraces formed on top of other terraces lead to asperities, which can preferentially intercept the incoming vapor flux and therefore grow at an accelerating rate. The adjacent (valley) regions are then flux “shadowed” further increasing the roughness, Fig. 1(a).

The crystal configurations resulting from deposition at low growth temperatures have very large surface areas (and therefore a high surface energy) and contain many (total energy increasing) lattice defects. If kinetically permitted, the system will evolve to a lower energy state where more of the atoms occupy highly coordinated lattice sites. This occurs by atomic jumps from sites of relatively higher energy (low coordination) to sites of lower energy. Fig. 1(b) shows examples of these jumps. A-type jumps allow atoms to diffuse to step edges (ledges) and extend the terrace, and D-type of jumps may also allow atoms on terraces to diffuse through or over ledges to extend the terrace. The E-type of jumps, which fill vacancies in the interior of the crystal, can also occur depending on mobility of atoms around the vacancies. For elemental systems with isotropic surface energies, the lowest energy crystal configuration is a flat surface because it has the smallest surface area. This requires significant surface mobility, which in turn depends on the substrate temperature and the deposition rate. These determine the jump frequency and the time available for an atom to jump on the surface before it is buried (frozen) into a bulk lattice structure.

The energy barriers for atomic jumps are dependent upon the local atomic environment [30]. For instance, jumps on different crystallographic (but flat) surfaces, along a ledge, away from a ledge, down from a terrace or up to a high terrace (see Fig. 1) all have different energy barriers impeding their jumps. In multilayers, the variation of the local composition in the vicinity of the jump path can significantly change the energy barriers [18]. As a result, thermally-activated evolution of a surface is essentially a process where a large number of surface atom jumps occur continuously each with significantly different jump frequencies. During the growth of multilayered systems with nonzero solubility of the constituent metals, the mixing due to the thermally-activated mode of atomic assembly is reduced by depositing the multilayers at a low substrate temperature and a high deposition rate [31]. However, such conditions then result in a rough surface. The ideal atomic assembly environment would enable a high surface mobility with little or no atom transport vertically through the films. Assisting inert gas ions have the potential to facilitate this but the detailed mechanistic insights need to be established.

Here, we utilize a molecular dynamics simulation approach to explore in detail the effects of inert gas ion mass and energy upon atomic assembly mechanisms during low energy ion beam assisted vapor deposition of a metal multilayer system. We focus upon a model Co/Cu system, which has been the subject of extensive experimental study [32], [33] and restrict our study to ions that arrive perpendicular to the surface. We show that these assisting ions can have strong effects on the atomic scale reconstruction processes on a surface. For each ion species (mass) an ion energy range is shown to exist where optimum multilayer interface properties can be achieved. We also show that atomic assembly mechanisms are highly sensitive to the ion mass, resulting in different film structures. We have quantified these phenomena to provide theoretical guidance to developers of practical deposition processes.

Section snippets

Molecular dynamics simulation

In MD simulations, a computational crystal is created by assigning the positions of atoms to an assembly of lattice sites. An interatomic potential is then used to calculate the forces between atoms, and Newton’s equations of motion are used to calculate the velocities and positions of all atoms. This approach dynamically solves for atom vibration around the occupied lattice sites. It also naturally incorporates the energy barriers for the various jump paths and correctly simulates the

Roughness and intermixing at interfaces

A Co-on-Cu interface (rather than Cu-on-Co) was chosen for study because previous work indicated that this surface is more difficult to flatten without mixing [27]. To understand the mechanisms of the ion beam assisted deposition process at multilayer interfaces, Co clusters on a Cu film were used to imitate a rough surface created during the early deposition of cobalt on a copper film. Fig. 2 shows the simulated system. It consisted of 12 uniformly separated Co pyramids on the Cu surface. Each

Roughness

The functional dependence of flattening and mixing upon ion energy and mass can be well captured by a simple two body collision theory. If we assume that the incident ion energy is E0, then the energy transferred to the film, ET, can be approximated from two-body collision theory as ET=4M1M2M1+M2E0sinθ22, where M1 is the mass of the impacting ion, M2 is an effective total mass of surface atoms that interact with the ion, and θ is the ion scattering angle. If we consider only the normal ion

Multilayer growth simulations

Based on the results shown in Fig. 4, growth conditions for producing the multilayer with a low combination of interface roughness and intermixing can be easily determined. We found that for Ar ions, an ion energy of 6 eV produces near optimized film structure. Simulations of the growth of a Cu/Co/Cu multilayer with and without 6 eV ion assistance are shown in Fig. 10(a) and (b). Clearly, without ion assistance, Fig. 10(a), the film contains numerous defects such as voids, rough interfaces, and

Conclusions

Molecular dynamics simulations have been used to study the effects and mechanisms of low energy ion assistance on the interface structures of Cu/Co multilayers. The simulations have revealed that:

  • 1.

    Ion assistance with ion energy of less than 30 eV has very significant effects on interface structures in the Co-on-Cu system. Increasing the ion energy results in a reduction in surface roughness. However, high ion energies/ion masses cause significant intermixing.

  • 2.

    Studies of the atomic assembly

Acknowledgements

We are grateful to the Defense Advanced Research Projects Agency and Office of Naval Research for support of this work under ONR grant N00014-03-C-0288 monitored by Dr. Julie Christodoulou and Carey Schwartz.

References (39)

  • J.Q. Wang et al.

    Mater. Sci. Eng. B

    (2000)
  • S. Honda et al.

    J. Magn. Magn. Mater.

    (1993)
  • W.H. Butler et al.

    J. Magn. Magn. Mater.

    (1995)
  • X.W. Zhou et al.

    Acta Mater.

    (2001)
  • J.K. Hirvonen

    Mater. Sci. Rep.

    (1991)
  • X.W. Zhou et al.

    Surf. Sci.

    (2001)
  • C.L. Liu et al.

    Surf. Sci.

    (1991)
  • X.W. Zhou et al.

    Acta. Mater.

    (1997)
  • M.N. Baibich et al.

    Phys. Rev. Lett.

    (1988)
  • J. Birch et al.

    Vacuum

    (2003)
  • M. Berglund et al.

    J. Microsc.

    (2000)
  • H.M. Hertz et al.
  • A.B.C. Walker et al.

    Science

    (1988)
  • E. Spiller et al.

    Appl. Phys.

    (1993)
  • D.G. Stearns et al.

    Appl. Opt.

    (1993)
  • A. Grudsky et al.
  • J.C.S. Kools

    J. Appl. Phys.

    (1995)
  • H.D. Chopra et al.

    Phys. Rev. B

    (1997)
  • G.A. Prinz

    Science

    (1998)
  • Cited by (0)

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