The Effect of Interfacial Ge and RF-Bias on the Microstructure and Stress Evolution upon Annealing of Ag/AlN Multilayers

Abstract

The present study addresses the structural stability and mass outflow of Ag 10 nm/Ge 1 nm/AlN 10 nm nanomultilayers (NMLs) during thermal treatments in different atmospheres (Ar and air). The nanomultilayers were obtained by magnetron sputtering under different deposition conditions (with and without the RF (Radio-Frequency)-bias application). The microstructure of the as-deposited and thermally treated NMLs were analyzed by XRD and SEM techniques, deriving morphology, microstructure and internal stress. Bias application during the deposition is found to create highly disordered interfaces and to have a very strong influence on the morphology and structural evolution with temperature of the nano-multilayers. Complete multilayer degradation is observed for the bias sample when annealed in Ar at 700 ∘C, while the periodic multilayer structure is preserved for the non-bias samples. Structural and morphological changes are observed starting from 400 ∘C, accompanied with Ag surface migration. The highest Ag amount on the surface is detected in air atmosphere for bias and non-bias samples annealed at temperatures as high as 700 ∘C. The presence of Ge is found to strongly hinder the Ag surface migration. Ag outflow is measured to take place only through the network of surface cracks in the AlN barrier formed upon heating. The crack formation and Ag migration are discussed together with the stress relaxation. The present study demonstrates the feasibility to tailor the stress state of as-deposited NML structures and observe different structural evolution depending on the initial conditions. This paves the way for advanced experimental strategies to tailor directional mass outflow in nanoconfined filler systems for advanced nano-joining applications.

Joining Technology Innovations at the Macro, Micro, and Nano Levels

“…Material coating systems, in particular, nanostructured coatings, so-called nanomultilayers (NMLs), show unique properties and offer new opportunities for many applications. Very recently, NML coatings of brazing fillers alternated by a functional barrier (i.e., Ag/AlN, Cu/AlN, Cu/W, Al/AlN, AgCu/AlN, and AlSi/AlN) were designed to serve as low-temperature brazing nanofillers for packaging and assembling miniaturized devices and heat-sensitive components at ever-reduced processing temperature [48][49][50][51][52][53][54]. Fast diffusion and extensive mass transport were observed in these nanomultilayers at temperatures much below the melting temperature of the bulk brazing filler (T > 250 • C), and they were exploited for joining technology applications [48,[50][51][52][53].…”

“…The overall understanding of the effect of these parameters on the NML behavior in various service conditions is crucial for proper NML design. Using the example of Ag/Ge/AlN nanomultilayers, Cancellieri et al [54] showed that the stress state of as-deposited NML structures can be controlled by the substrate bias during the deposition process or by the modification of the layer interfaces through the small addition of a second material. In this way, the thermal evolution and stability of the nanomulitlayers, as well as the directional mass outflow of the brazing filler material, can be tuned.…”

Synthesis, characterisation and thermal behaviour of Cu-based nano-multilayer

“…Another approach is the development of nanomultilayered composite/hybrid joining materials, based on the concept emerged in the last decade [13,23,24]. This type of multilayer consists of a periodic repetition of a few nanometer thick metal (or alloy) layers and chemically inert barriers (usually oxides, nitrides, or refractory metals) [25,26]. The purpose of the construction is to preserve the nanothickness of the metal layers up to their melting point by separating them with the diffusion barriers.…”

Inhibiting the Segregation of Germanium in Silver Nanolayers

“…However, in the nanoscale, poor adhesion of silver to commonly used substrates such as ultra-smooth fused silica or sapphire results in a rough metal-free surface, which increases the scattering losses of the metal film. To tackle this problem, a number of approaches have been proposed, the most prominent being the use of wetting materials exhibiting good adhesion to both silver and the substrate [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The greatest reduction of surface roughness-up to an order of magnitude-is observed when the substrate is wetted with a~2-nm-thick layer of germanium prior to the deposition of silver [2,11].…”

“…Instead of covering the silver layer with a material which exhibits grain boundary diffusion within silver, we introduce another layer below germanium, which is made of gold. Germanium has been proven to segregate within the nanolayers of both silver and gold [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], as Ag and Au exhibit many crucial similarities, like their atomic radius and lattice constant. However, the free enthalpy of the segregation process is influenced by many quantities in which silver and gold differ-e.g., bulk modulus, shear modulus and surface energies [28,42].…”