Influence of Different Nano Structure Densities on the Self-Propagating Reaction of Ni/Al Multilayer

Reactive materials have garnered significant interest for bonding applications due to their ability to provide localized and rapid heating, thereby minimizing heat exposure to thermally sensitive components. The reaction parameters of these materials, such as velocity and temperature, are typically tailored through adjustments in layer spacing, total thickness, and atomic ratio of the involved elements. In this study, we consider the surface structures as an additional parameter for reaction tailoring and introduce a new aspect: adhesion of the formed compound without additional intermediate layers on silicon chips with a thermal oxide as heat barrier. Previous experiments involving Ni/Al multilayers on the thermal insulator SiO₂, revealed that the reaction led to substrate damage. The damage inflicted was substantial, as it also caused destruction in the Si layer beneath the thermally produced SiO₂ layer. Additionally, the Ni/Al compound was detaching during the reaction. The layer structure would therefore not be suitable for reactive chip bonding.<br/><br/>To mitigate this issue, we employed a reactive ion etching process to structure the surface, followed by oxidation, to create a thermal insulating layer. This SiO₂ layer not only facilitated self-propagating reactions, but it also additionally leads to an improved material bond. The oxidized nanostructures vary in density and have a height of approximately 1 µm. The nanostructure's densities range between 4 – 27 structures per µm², distributed across three wafer batches. The structure density of the first and second batch amounts to 3.55 - 6.23 structures per µm² and 7.22 - 14.64 structures per µm², respectively. The third batch has a much higher range counting 21.69 - 26.53 structures per µm². There is a process-related variation in structure density between the center and the edge. After separating and cleaning the samples, Ni/Al multilayers with a thickness of 5 µm, bilayer spacing of 50 nm, and an atomic ratio of 50/50 at.% were deposited onto the structured surfaces using direct current magnetron sputtering.<br/><br/>Upon electric impulse triggering, the multilayers underwent self-propagating reactions on all samples. Reaction propagation speed and surface temperature were measured by means of a high-speed camera and high-speed pyrometer. The data have shown that both values distinguish significantly for different nanostructure density. The first two batches showed lower velocities with increasing number of structures. The maximum temperature decreased as well between the first and the second batch. In contrast, the third batch has the highest temperature and propagation velocity as well as the highest structure density. These were results caused by the destruction of the substrate during the reaction, like on a flat SiO₂ surface. Only at the edges, where the structure densities were below 23 structures per µm² the compound stayed on the substrate like in the other two batches.<br/><br/>Since the layer architecture is the same, these differences in reaction propagation and maximum temperature, are attributed to an increased defect density which is a consequence of the growth characteristic of the sputtered layer on the structured surface. Flaking also depends on the structure density. This destructive behavior is mainly attributed to the volume change during the phase transformation. We observed that high structure densities lead to flaking, similar to the reactions on flat SiO₂ surface. In the range between 3 – 15 structures per µm² the layers adhere very well. The surface treatment is therefore proposed as an adhesion promoter for the reactive bonding of silicon chips as well as an additional parameter to tailor the multilayer reaction.