Conference Agenda

This study demonstrates the impact of nano-structured silicon surfaces on the performance of reactive multilayers (RMLs) bonding as a possible substitute for adhesion or solder layer. One of the tested structures are black silicon surfaces fabricated by reactive ion etching (RIE), which were thermally oxidized for enabling a self-propagating reaction. A previous study mentioned the significance of structure density for improving adhesion of reacted material. Hence, the structural density was systematically analysed using top-view scanning electron microscopy (SEM) combined with image processing methodologies, including binarization and the watershed algorithm, providing a robust quantitative framework for assessing surface morphology. In parallel, deep reactive ion etching (DRIE) was employed to produce silicon grass structures of 8 µm height, which proofed to reduce heat loss during reaction, as well as improving mechanical performance of the reacted material. Thus, enabling a self-propagating reaction of deposited reactive material, despite quenching of the same RML composition on bulk silicon. The Al/Ni RMLs had a total thickness of 5 µm, a bilayer thickness 50 nm and a 1:1 atomic ratio between Al:Ni. This approach of high and thin structures enabled the elimination of thermal insulation layers and obviated the need to increase RML thickness by mitigating heat dissipation through the substrate. The unique bending behaviour of the silicon grass structures during reaction effectively reduced stress transfer to the bulk substrate, further enhancing adhesion of the reacted RMLs.

The bonding performance of these two structured surfaces were evaluated using two configurations: (1) silicon-to-silicon bonding, employing 525 µm silicon wafers with a 1 µm thermal SiO₂ layer, metallized with 50 nm Ti, 100 nm Au, and either a 1 µm or 2 µm thick layer of Sn. (2) silicon-to-low-temperature cofired ceramics (LTCC), comprising six layers of DuPont 951 PX tapes with a 15.9 µm AgPd or AgPd/SAC 54 µm thick metallization layer. Bonding tests were conducted under a pressure of 10 MPa applied on a 1 cm² area. The reaction of the RMLs on the structured surfaces were initiated by spark ignition. The results confirmed successful bonding for the silicon-to-silicon configuration, attributed to a successfully established reactive bond between the two chips. However, while initial adhesion was observed in the silicon-to-LTCC bonds, they failed in most cases, indicating a need for further optimization of the interface chemistry and mechanical compatibility. Pull tests performed on the silicon-to-silicon bonds demonstrated high bond strength, underscoring the effectiveness of the structured surfaces in achieving robust and reliable adhesion.

This work establishes a foundational strategy for material reduction by engineering silicon surface structures to enhance reactive bonding processes, presenting implications for applications requiring localized heating, material-efficient bonding solutions. Future studies should focus on identifying more surface structures and their impact on the reaction, as well as including interfacial strategies on the LTCC bonding like laser ablation to improve the performance.