_Article_  
The Effect of Interfacial Ge and RF-Bias on the Microstructure and Stress Evolution upon Annealing of Ag/AIN Multilayers  
[PERSON] 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,19,17,19,17,18,19,19,17,19,17,19,17,19,17,19,17,19,17,19,17,19,17,1  
metastable interfacial compounds (which cannot be stabilized as bulk compound). NML systems can be straightforwardly synthesized by Physical Vapor Deposition (PVD) techniques with precise control of e.g., the chemical composition, individual layer thicknesses and modulation periodicity. Tailoring of the NML structure can be achieved through controlled variation of the deposition parameters, like substrate bias, target power and gas pressure. The structural integrity of the NML systems during subsequent processing and/or operation will crucially depend on the residual stresses accumulated during PVD growth, which can be of the order of a few GPa. For example, the thermal degradation of the nanolaminated structure may occur upon heating, as driven by the reduction of both accumulated residual stresses and excess interfacial energy contributions. Moreover, residual stresses in functional thin film systems may affect the adhesion to the parent substrate, alloy miscibility [5; 6], the type and concentration of crystallographic defects, electrical resistivity, optical reflectance, fatigue and corrosion resistance. Notably, residual stresses in thin films can also be tuned on purpose to achieve specific electrical and/or optical properties (including band gap) [7]. A wide variety of NML systems has been successfully fabricated for different purposes by combining alternating nanolayers of a metal, alloy, oxide and/or nitride. For example, very recently, Cu/W, Ag/AlN, Cu/AlN and Ag-Cu/AlN NML [6; 8; 9] coatings have been designed to serve as low temperature brazing nano-fillers for packaging and assembling miniaturized devices and heat-sensitive components at ever-reduced processing temperatures. Comprehensive knowledge on the thermal response of such NML brazing fillers requires profound understanding of their microstructural evolution during thermal, mechanical, chemical and/or electrical loading in different processing environments. The Ag/AlN system represents a typical NML brazing filler system, since it is highly unstable upon thermal treatment. Very fast, massive outflow of the confined Ag metal to the NML surface occurs upon heating in oxygen-rich atmospheres at temperatures as low as 400 \({}^{\circ}\)C, much below the nominal bulk melting temperature of Ag [8]. This outward Ag migration, as accompanied by a relaxation of compressive stresses in the NML, may be exploited for localized low-temperature bonding of micro- and nano-sized components. The present study investigates the effect of the additional tiny amounts of Ge at the Ag/AlN interface in Ag/AlN NMLs (further denoted as Ag[Ge]/AlN NML) on the resulting stress state and structural integrity of the NML system upon heating. Ag-Ge do not react as they are mutually immiscible at room temperature (towards higher temperatures, there is a slight solubility of Ge in Ag), and no intermetallic compounds are formed across all phase diagram compositions [10]. On the one hand, the interfacial Ge may affect the defect concentration and coherence strain at the Ag/AlN interface. On the other hand, the intrinsically very high Ge atomic mobility in contact with (free-electron-like) metals like Ag may drive continuous surface segregation of Ge to the Ag surface during deposition [11]. The presence of such surface-segregated Ge, in turn, impedes the surface diffusivity of Ag adatoms during the deposition step, resulting in strikingly different grain sizes, textures and residual growth stresses [12]. Co-alloying of Ge with Ag thin films for modifying the grain-size was also applied in linear and nonlinear optical thin-film applications [13]. Furthermore, the increase of ohmic losses in Ag/Ge/fused silica sandwic was observed due to Ge segregation towards the Ag film surface and probably also the grain boundaries [14].