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Fabrication of Hollow AlAu2 Nanoparticles by Solid State Dewetting and Oxidation of Al on Sapphire Substrate

The Al-Au binary diffusion couple is a classic example of the system exhibiting Kirkendall voiding during interdiffusion. We demonstrate that this effect, which is a major reason for failures of the wire bonds in microelectronics, can be utilized for producing hollow AlAu2 nanoparticles attached to sapphire substrate. To this end, we produced the core-shell Al-Au nanoparticles by performing a solid state dewetting treatment of Al thin film deposited on sapphire substrate, followed by the deposition of thin Au layer on the top of dewetted sample. Annealing of the core-shell nanoparticles in air resulted in outdiffusion of Al from the particles, formation of pores, and growth of the AlAu2 intermetallic phase in the particles. We demonstrated that the driving force for hollowing is the oxidation reaction of the Al atoms at the Au-sapphire interface, leading to the homoepitaxial growth of newly formed alumina at the interface. We developed a kinetic model of hollowing controlled by diffusion of oxygen through the Au thin film, and estimated the solubility of oxygen in solid Au. Our work demonstrates that the core-shell nanoparticles attached to the substrate can be hollowed by the Kirkendall effect in the thin film spatially separated from the particles.


INTRODUCTION
Porous and hollow nanoparticles are considered as promising candidates for a number of nanotechnology-related applications such as drug delivery, catalysis 1 , nanophotonics 2 , energy storage and conversion 3,4 , etc. (see the recent reviews 5,6 for more details). While in the past the synthesis of hollow nanostructures relied primarily on the wet chemistry methods, during the recent decade the methods based on interdiffusion of the components in the solid state are gaining more and more acceptance. A good example of such methods is a nanoscale Kirkendall effect firstly employed by Yin et al. to obtains the hollow Co oxide and chalcogenide nanoparticles 7 . The methods based on diffusion in the solid state offer the advantages of a better control of pores nucleation and of the kinetics of their growth. Since the groundbreaking work 7 was published, the Kirkendall effect-based methods have been extended to a number of other systems to produce hollow nanoparticles and nanotubes 6,[8][9][10][11] .
In most of the published works on synthesis of hollow nanoparticles the main processing steps were performed in colloidal solutions [6][7][8][9][10][11] . However, many applications of hollow nanoparticles in catalysis and nanophotonics require firm attachment of the particles to the substrate made of a different material. In several recent works, the core element of precursor core-shell nanoparticles to be hollowed with the aid of nanoscale Kirkendall effect was produced by solid state dewetting of the thin film deposited on non-wetting substrate 12,13 . Thus the hollow nanoparticles obtained after Kirkendall hollowing were well-dispersed and firmly attached to the substrate by the chemical bonds between the atoms of the film material and the substrate.
However, in these works the substrate played a role of a mere inert support making the hightemperature treatments and sample handling easier. In our recent work on the low-temperature hollowing of the Ag-Au core-shell nanoparticles we demonstrated that the sapphire substrate covered by thin nano-crystalline Au film plays an active role in the hollowing process, draining the Ag atoms from the core of the core-shell nanoparticles by diffusion along the grain boundaries, surfaces and interfaces 14 . We coined the term "Surface diffusion-induced bulk intermixing" to describe the process of formation of several interatomic distances-thick layer of the Au-Ag alloy on the surface of original thin Au film. It is this layer which absorbed the Ag atoms rejected by the growing pore within the nanoparticle. Thus the observed hollowing process was different from the nanoscale Kirkendall effect in which the interdiffusion and reaction processes are confined within the nanoparticles themselves. Involving the substrate and the thin film around the core-shell nanoparticles in hollowing process provides more freedom in pores design. For example, the pore size and the ratio of pore size and external size of the particle could be varied in wide margins which are not limited by the mass conservation requirements within the particle.
In the present work we show that the sapphire substrate can play an active role in the hollowing process of the Al-Au core-shell nanoparticles, by serving as a template for homoepitaxial growth of aluminum oxide formed during reaction of Al atoms diffusing along the film-substrate interface and atmospheric oxygen. We selected the Al-Au system for this study because it is a classical, well-studied system exhibiting intermetallic phases formation and Kirkendall porosity in the bulk state 15 . Most previous studies of nanoparticles hollowing through the nanoscale Kirkendall effect relied on some kind of chemical reaction (oxidation, sulfurization, selenization, etc.), and the relation between the nanoscale and bulk Kirkendall effects in the same system was unclear. In the microelectronics industry, the contact interdiffusion between Au and Al occurs at the interface between the Au wires 16 and Al metallization pads connecting the integrated circuit and the external parts of the device 17 . This binary system exhibits a wide range of intermetallic phases which may form during the device fabrication and service. The Al-Au intermetallic phases exhibit low electrical conductivity which in turn may lead to degradation in the performances of the device and hence act as critical failure mechanism of the wire bond.
Five different intermetallic phases were identified in the equilibrium Au-Al phase diagram. Four of them are stable below the melting temperature of Al, as reported by Vandenberg 18 and Murray 19 . The fifth phase, called the "purple plague" (Al 2 Au) 17,20 , owns its name to the purple-to-black discoloration in the vicinity of the bond as a result of degradation effects. This is the most thermodynamically stable phase in the Au-Al system with the melting temperature of 1333 K, well above the melting point of Al. The Kirkendall voiding during bulk interdiffusion is well-documented in this system, and the layers of micrometer-sized voids significantly weaken the bond strength 15 .
In this work we report the fabrication of porous nanoparticles composed of the intermetallic phase AlAu 2 . We show that the oxidation of Al at the interface between the sapphire substrate and Au thin film is a crucial, rate-controlling step of the hollowing process.

RESULTS AND DISCUSSIONS
The 10 nm thick Al layer was deposited on a (0001)-oriented sapphire substrate followed by an annealing for 18 h at 600 °C in the ultra-high vacuum (2×10 -9 mbar) of the molecular beam epitaxy system. As a result of the annealing, the Al layer agglomerated into isolated, mostly single crystalline particles in the process known as solid state dewetting. This process is driven by the decrease of the total energy of all surfaces and interfaces in the system 21 . The Al particles and the exposed substrate were then coated with 20 nm of Au without breaking the vacuum. The resulted particles exhibited irregular shapes, with an apparent thin oxide layer on the top formed during exposure of the samples to the ambient air (Fig. 1).
The Al-Au core-shell particles were annealed at the temperature of 170 °C for 1 h in three different atmospheres: ambient air, low vacuum (≈5×10 -2 mbar), and in the flow of forming gas.
The annealing in air resulted in the formation of a pore with an irregular tortuous shape and in the transformation of the rest of the particle into a homogeneous Al-Au intermetallic phase, as shown in the cross-sectional transmission electron microscopy (TEM) micrograph and the energy dispersive X-ray spectroscopy (EDS) elemental map in Fig. 2. An increase in the thickness of the oxide layer on the top of the particles was also observed, resulting in an increased roughness of the particle surfaces (Fig. 1). The surface of the Au film between the particles was oxide-free. One can also notice the seams of Au-free exposed substrate surrounding the particles. The annealing of the as-prepared samples in the low vacuum and in forming gas did not result in particles hollowing (see Figs 3 a, b). Semi quantitative compositional analysis of the particles annealed in forming gas demonstrated that while certain intermixing at the Al-Au interface cannot be excluded, the maximum size of the interdiffusion zone is significantly smaller than the height of the particle, and pure Al was still preserved in the particle core (Fig 3   c).
The XRD scan of the samples annealed in air showed the presence of the AlAu 2 intermetallic phase (Fig 1 b,d), together with distinctive peaks of Au and Al 22 . The peak of the AlAu 2 intermetallic was not detected in the as-prepared samples (before the low temperature annealing). Also, the average particle size does not change after the hollowing annealing process. A simple mass balance calculation demonstrates that in the case all Al atoms remain in the hollowed particle, the total amount of Au deposited on the particle surface, and drained from the Au-free seam is not sufficient to form the Au-rich AlAu 2 phase. Therefore, some Al must diffuse out of the particles during annealing (see Supporting Information).
Electron backscattered diffraction (EBSD) mapping revealed a strong out of plane (111) orientation and nearly random in plane orientation of the grains in Au thin film surrounding the particles. The mean grain size in the Au film was 79±3, 96±7 and 130±19 nm for the as deposited sample, and for the samples annealed in air and in forming gas, respectively (Fig. 4).
Thus, the grain growth in the Au film was faster during annealing in the forming gas as compared to annealing in air.
In the sample annealed in air, the energy dispersive X-ray (EDS) maps of the interface between the Au film and sapphire taken in the regions between the particles demonstrated significant interface roughness (Fig 5). On the contrary, this interface in the as-prepared samples was very flat. The high-resolution TEM (HRTEM) observations allowed us to link the interface roughness in the sample annealed in air to the Al-O phase inhomogeneously grown at the interface (see Fig.   5c). While this newly-grown phase exhibited the same out-of-plane lattice periodicity as the underlying sapphire, the difference in contrast between the two indicated higher concentration of defects in the former. The electron energy loss spectroscopy (EELS) scans that were taken from Au film between the particles showed the presence of oxygen in the grain boundaries (distinctive peak at the K edge of the oxygen was observed, and can be seen in Fig. 6). No traces of Al (above the detection limit) were found in the Au layer ( Fig. 7a-b). Also, the bright field TEM of the Au layer demonstrated that the new Al-O phase grows preferentially at the triple lines where the grain boundaries in the Au layer contact the Au-sapphire interface (see Fig. 7c). The particles annealed in the forming gas exhibited neither oxygen at the grain boundaries in Au, nor any significant Au-sapphire interface roughness.
To summarize our experimental findings, we found that annealing of the Al-Au core-shell nanoparticles in air results in simultaneous formation of the pores and of the AlAu 2 intermetallic compound in the particles, accompanied by an increase in roughness of the interface between the inter-particle Au thin film and sapphire substrate. No similar changes occurred in the particles annealed in the oxygen-free atmospheres. To understand the mechanism of particles hollowing, we need to identify the main process driving the formation of the pore. The enthalpies of formation of the AlAu 2 intermetallic and of the Al 2 O 3 oxide are 34.7 and 1674 kJ/mol, respectively, which means that the contribution of the AlAu 2 intermetallic to the driving force of hollowing can be safely neglected 23,24 It is interesting that among all intermetallic phases of the Al-Au system, the AlAu 2 exhibits only the third highest enthalpy of formation, preceded by AlAu and Al 2 Au phases 23 . In this respect it should be noted that the order in which the intermetallic phases form in the interdiffusion zone depends on a number of factors in addition to the phase formation enthalpy, such as the energy barrier for nucleation, the concentration gradient in the interdiffusion zone 25 , or even the geometry of the system 23 . It was shown that the AlAu 2 phase dominates during interdiffusion in thin Al-Au bilayers 23 .
In order to understand the mechanisms of pore formation we developed a model of material transport during annealing of the particles in air which assumes that oxidation of Al is the main reaction driving the hollowing process (Fig. 8). Let us assume that oxygen from annealing ambient can penetrate through the Au film to the Au-sapphire and Au-Al interfaces by the bulk or grain boundary diffusion. On the Au/Al interface, the oxygen penetration will lead to formation of the thin protecting layer of Al 2 O 3 "plugging" 26  We suppose that the oxidation of Al is a fast reaction which does not limit the hollowing process. Thus in the reaction zone at the Au-sapphire interface the oxygen concentration should be close to zero and the Eq. (1) can be simplified: The diffusion of Al atom along Au-sapphire interface can be described by diffusion equation ( In thus equation C Al is a mole fraction of Al atoms on interface, D Al is the diffusion coefficient of Al atoms along the Au-sapphire interface, δ is the effective thickness of interface layer. The last term on the right hand side of Eq. (4) describes the oxidation reaction of Al atoms as appears in Eq. (2).
We will consider steady state solution of Eq. (4) inside a circular reaction zone of radius R m with the following boundary conditions: where R 0 and 0 Al C are the particle radius and the mole fraction of Al in the particle, respectively.
Assuming that R m is roughly equal to the half-distance between the neighboring particles, the second boundary condition (4) implies that there is no exchange of material between them. The steady-state solution of the Eq. (4) with the above boundary conditions is: Then the total diffusion flux of Al atom from the particle can be calculated employing Fick's first law: The volume of the pore is pore Al Al Au Au where ΔN Au is a number of Au atoms absorbed by the particle from the neighboring Au film (due to the reaction with formation AlAu 2 ). Our experimental results indicate that total (external) volume of the particle does not change, which implies Here / Al Al Al NV  and / Au Au Au NV  are initial number atoms Al and Au in the particle. The first term on the right hand side of Eq. (8) is based on the assumption that at the end of the hollowing process the particle contains only the Au 2 Al phase, so that the number of Au atoms is simply twice the number of Al atoms. Combining the Eqs (7) and (8) yields the final expression for the pore volume: with I Al given by the Eq. (6). The Eq. (9) is not applicable at the initial moments of pore formation (short t) prior to full conversion of the particle into the AlAu 2 phase. Thus, the rate of the pore growth is Based on our microstructural observations (see Figs 1, 2) we will adopt the following values of parameters for numerical estimates: R m =1 m, R 0 =375 nm, and R pore =75 nm. Assuming a hemispherical shape of the pore yields:

CONCLUSION
From the results of the present study, the following conclusion can be drawn: 1. We produced the core-shell Al-Au nanoparticles on sapphire substrate by performing the solid state dewetting of thin Al film on sapphire, followed by the deposition of thin Au layer.