WEB Reactive Ni/Al multilayers with 3D Morphologies
Metallic, multilayered thin film systems are well known for their self-propagating reactions with high heat release. Especially binary or ternary aluminide systems are under research, with a variety of metals such as Hf, Ni, Ru, Pd or Pt that are highly miscible in Al (1). Specifically, the influence of the selection of the metals on the ignition temperature, reaction velocity and the layer thickness is under observations. Some parameter, which influence the reaction, are identified as atomic mobility of the metals, microstructure features, internal stress and deposition conditions(2).The achievable temperatures, during the reaction, are enough to melt solder materials. A possible application could be micro- or nano-joining of chips or micromechanical systems.
The bonding strength between microchips can be enhanced significantly with a 3D-structured surface, e.g. with silicon needles (3). With black silicon it is further possible to hinder the propagation of the reaction, which indicates a starting point to control it (4). In this work, we study the influence of such nanostructured surfaces on thermo-physical properties of the self-propagating reaction. It is expected that the nanostructures influence the bilayer morphology, diffusion path and thus, phase formation and residual stress. We present here some experiments, where a 5 µm thick magnetron sputtered Al/Ni multilayer was used as reactive system, with a bilayer thickness of 25 nm. The deposition took place on a Si (111) substrate. To change the morphology of the surface, we carried out a black silicon RIE etching process before the deposition. Needles with a height of 1 µm were produced and then covered with the aluminide multilayer system. Specifically, the difference between the samples before and after the ignition with rapid thermal processing (RTP) are examined and discussed. The layer morphology before and after treatment is investigated using different methods, such as focused ion beam (FIB) or scanning electron microscope (SEM). These data are compared with information about the phase transformation obtained from XRD studies. In addition, differential scanning calorimetry (DSC) was performed to obtain information regarding the ignition temperature and the heat flow released upon annealing, and compared to that of planar morphologies.