Low-energy ion-assisted control of interfacial structures in metallic multilayers

Abstract

A molecular dynamics method has been used to simulate the argon ion-assisted deposition of Cu/Co/Cu multilayers and to explore ion beam assistance strategies that can be used during or after the growth of each layer to control interfacial structures. A low-argon ion energy of 5–10 eV was found to minimize a combination of interfacial roughness and interlayer mixing (alloying) during the ion-assisted deposition of multilayers. However, complete flattening with simultaneous ion assistance could not be achieved without some mixing between the layers when a constant ion energy approach was used. It was found that multilayers with lower interfacial roughness and intermixing could be grown either by modulating the ion energy during the growth of each metal layer or by utilizing ion assistance only after the completion of each layers deposition. In these latter approaches, relatively high-energy ions could be used since the interface is buried and less susceptible to intermixing. The interlayer mixing dependence upon the thickness of the over layer has been determined as a function of ion energy.

Introduction

Metallic thin-film multilayers can possess high strength and hardness [1], [2], [3], [4], [5] and sometimes exhibit unusual electron transport properties such as giant magnetoresistance [6], [7], which is utilized for magnetic field sensing [8] and other spin-based devices [9]. Metal superlattice structures also have tunable high reflectivity that has led to their use in X-ray, soft X-ray, and extreme ultraviolet (EUV) telescopes for solar physics and astronomical research [10], [11], [12]. These applications utilize multilayers that are composed of many metal layers ranging from a few monolayers to several nanometers in thickness.

The roughness and degree of intermixing at the interfaces between the metal layers in multilayer films can greatly affect their properties [13], [14], [15]. While the role of interface structures upon the magnetotransport [6], [7], [13], [14], [15] and optical properties [10], [11] of thin films has been extensively studied, the mechanisms that control interfacial roughness and intermixing during multilayer growth have only recently begun to be addressed. Several experimental studies have sought to understand the kinetic and thermodynamic factors controlling the growth of thin films and multilayers by changing the growth temperature or using plasma-based methods to increase adatom mobility [10], [16], [17], [18]. The best metal superlattices are usually grown at relatively low homologous temperatures to avoid interdiffusion across interfaces. However, these conditions can result in the formation of rough interfaces and other defects [19].

Ion beams [17] and plasmas [10], [20] are widely used to induce inert gas ion impacts with growth surfaces. This results in local energy transfer to surface atoms and enhanced surface atom mobility and enables use of a low epitaxial growth temperature to avoid interlayer diffusion in the subsurface region [21], [22], [23]. However, conventional ion beam-assisted deposition processes utilize high-energy ion fluxes in the 50 eV to 5 keV range during metal deposition. While these have been successfully used to create flat surfaces on thick monolithic films [21], [22], these ion energies exceed the damage threshold energy for multilayers with nanoscopic layer thicknesses and result in significant interlayer mixing [24], [25].

Molecular dynamics (MD) simulations have been used to explore the atomic assembly mechanisms during the vapor deposition of metallic multilayers [26], [27], [28], [29]. The structures predicted by these accelerated deposition rate simulations are surprisingly similar to those observed by three-dimensional atomic probe techniques [30]. The predicted misfit dislocation structures are also similar to those observed by high-resolution transmission electron microscopy [31], [32]. Recent simulations of ion interactions with a model Co-on-Cu bilayer system indicate that low (5–10 eV) energy impacts can be highly effective at flattening small (10 atoms) cobalt clusters on (1 1 1) copper surfaces without inducing interfacial alloying [24], [33], [34]. They also reveal that appropriate combinations of ion species (mass), ion energy, ion fluence, and ion incident angle can be used to selectively activate either atom recoil, atomic exchange, or direct jumping mechanisms to flatten surface asperities [24], [33]. These studies further indicate that nearly perfect multilayer structures can be vapor deposited using ion assistance if the ion energy is maintained between the critical energies for flattening and intermixing. These energies and the energy gap depend on the ion incident angle and ion mass. The widest critical energy gap is achieved for an ion incident angle around 50° from surface normal (which avoids easy atom penetration in crystal channeling directions) using an ion mass comparable to that of the metals [33].

There are many ways, in which a low-energy ion assistance method could be used to control the assembly of a metal multilayer thin film. In the example above, constant energy ion bombardment was used simultaneously with the deposition process. However, the ion energy could also be varied (modulated) during the metals deposition, or the ion bombardment could be applied only after each metal layer had been deposited (sequential assistance). Here, we utilize MD simulations to investigate these three ion assistance approaches for controlling the interfacial structures of a model Cu/Co/Cu trilayer system. Several near-optimal processes for the growth of metallic superlattices with chemically sharp, planar interfaces are identified.