## 5 Conclusions  
We reported on the thermal stability (up to 700 \({}^{\circ}\)C) of Ag[Ge]/AlN multilayers in different atmospheres and fabricated with and without RF-bias application during deposition. Distinct morphologies and structural evolutions were measured depending on the initial stress state controlled by the bias application. The main conclusions of this investigation are:  
* RF-bias application created disorder and roughness in the as deposited samples (both in planar and cross-sectional morphology) as well as a compressive state in Ag. The presence of Ge was found to smooth the Ag/AlN interface which appeared flatter and more regular in comparison with the Ag/AlN system.
* Wrinkling and cracking were observed at temperatures \(\geq\)400 \({}^{\circ}\)C in air and Ar depending on the initial stress state of the system: compressive for bias samples leading to wrinkling and tensile for non-bias samples leading to cracking. The presence of Ge was determined to change the internal stress state in the Ag layers from compressive to tensile in non-bias samples in the as-deposited state.
* The Ag surface migration is observed prominently in air at 700 \({}^{\circ}\)C for both, bias and non-bias samples, mostly through the previously formed cracks (at 400 \({}^{\circ}\)C) due to stress relaxation. However, the Ag amount on the surface was found to be considerably lower in comparison with Ag/AlN systems annealed at 400 \({}^{\circ}\)C.
* The presence of Ge was proved to hinder the Ag mobility and diffusion at high temperatures in air. Ge accumulation was measured close to Ag particle on the surface.
* Minimal Ag surface migration is observed for all the samples annealed in Ar at any temperatures. In particular, for non-bias samples, the high temperature treatment in Ar atmosphere is found to create a network of cracks extending all over the sample surface. These samples preserve the multilayer periodic ordered structure and keep a high internal tensile stress state up to 700 \({}^{\circ}\)C.  
The present study demonstrates the feasibility to reverse the stress state of as-deposited NML structures (from tensile to compressive) by changing the substrate bias and/or by interfacial modification and consequently affect the thermal evolution and stability. This paves the way for advanced experimental strategies to tailor directional mass outflow in nanoconfined filler systems for advanced nano-joining applications.  
Conceptualization, L.P.H.J., C.C., E.K. and J.J.-R.; Methodology, M.C.; Validation, J.J.-R., L.P.H.J. and C.C.; Formal Analysis, M.C.; Investigation, E.K.; Data Curation, C.C. and E.K.; Writing--Original Draft Preparation, C.C.; Writing--Review and Editing, C.C., E.K., L.P.H.J., J.J.-R.; Supervision, L.P.H.J.; Project Administration, J.J.-R.; Funding Acquisition, J.J.-R.  
This research was funded by EXMONAN Grant No. EU FP7-PEOPLE-2013- IRSES Project EXMONAN.  
The authors acknowledge [PERSON] and [PERSON] for the help in SEM characterization. The authors acknowledge the financial support of EU FP7-PEOPLE-2013-IRSES Project EXMONAN--Experimental investigation and modelling of nanoscale solid state reactions with high technological impact.  
The authors declare no conflict of interest.