## 2 Materials and Methods  
Reactive joining was performed with four different substrate types: (i) borosilicate glass wafers (Borofloat(r)33, Schott AG, Jena, Germany) of 0.5 mm thickness, (ii) aluminium oxide (Al\({}_{2}\)O\({}_{3}\)) substrate plates (97% Al\({}_{2}\)O\({}_{3}\), Kyocera, Kyoto, Japan) of 1 mm thickness, (iii) silicon (Si) wafers (monitor-grade mono-crystalline \(\langle 100\rangle\)-Si, Okmetic, Vantaa, Finland) of 0.525 mm thickness, and (iv) copper plates (99.95%, oxygen-free high conductivity Cu) of 1 mm thickness. The schematic joining configuration is shown in Figure 1. In the present work, only homogeneous bonds using the same substrate materials were manufactured. The thermal properties of the different substrate types are listed in Table 1. The substrates were metallised by magnetron sputtering with a Ni metallisation layer of 200 nm (Si) to 500 nm (rest) thickness, and with an intermediate Ti-W adhesion layer of 70 nm to 100 nm in case of Si, Al\({}_{2}\)O\({}_{3}\) and borosilicate glass (in case of Cu, no adhesion layer is necessary). For the manufacturing of shear-test specimens, the substrate plates and wafers were cut into smaller plates of 4 mm \(\times\) 4 mm (upper substrate) and 10 mm \(\times\) 20 mm (lower substrate) using a dicing saw (Disco DAD3350). As reactive foils, commercially available Ni(V)-Al RFs (Nanofoils(r); Indium Corporation, Clinton, NY, USA) with a thickness of 60 \(\upmu\)m were used (For commercial production of the reactive foils by sputter deposition, instead of pure, magnetic Ni targets, non-magnetic Ni-V-targets are used in order to facilitate the sputtering process. The intermetallic phase NiAl, which is the predominant product of the self-heating reaction between Ni and Al, has a substantial solubility for V, i.e., vanadium is incorporated into NiAl, here denoted as Ni(V)Al.). The reactive foils are plated with 1 \(\upmu\)m InCuSil(r)-15 (61.5 wt% Ag, 23.5 wt% Cu, 15 wt% Ir; 53.3 at.% Ag, 34.5 at.% Cu, 12.2 at.% In) for oxidation protection. Pure Sn foils (97.5 at.% Sn, Goodfellow) of 10 \(\upmu\)m thickness were used as solder. The Cu substrate plates were later galvanically pre-soldered with 12 \(\upmu\)m (nominally 10 \(\upmu\)m) Sn, because reactive joining with Sn foils proved to be unsuccessful (see Section 3). The reactive foils and solder foils were cut with a laser (pulsed Nd:YAG marking laser, wave length 532 nm) to preforms of 5.5 mm \(\times\) 5 mm (RF), respectively, 5 mm \(\times\) 5 mm (solder foils). The substrates and Sn solder foils were either etched with 10 % HCl and the RFs with 20 % HNO\({}_{3}\) to reduce the amount of surface oxides, rinsed twice in demineralised water and dried with compressed air before assembly, or were only cleaned in an ultrasonic bath with isopropyl alcohol and dried. Both sample cleaning methods lead to the same performance in shear tests, indicating that etching with acids or fluxes is not mandatory. For bonding, a bond pressure of 2.5 MPa to 2.6 MPa was used. The exothermal reaction was initiated at one side of the 4 mm \(\times\) 4 mm bonding area by an electrical discharge using two electrodes placed on the surface of the reactive foil with a distance of approximately 3 mm. The instrumental setups used for reactive joining are shown in Figure 11 (Appendix A). For investigation of the joint quality by optical microscopy (OM) and SEM, cross-sections were prepared by grinding and polishing, as well as by use of a cross-section polisher (Hitachi IM4000) for some SEM samples. OM was performed with a Leica DM6000M optical microscope, SEM-EDS was  
Figure 1: (**a**) schematic cross-section of the initial joining-zone configuration (lower half; symmetric joint), and (**b**) layout of the shear-test samples; with 1: lower substrate, 2: Ti-W adhesion layer (optional), 3: Ni metallisation layer, 4: Sn solder foil or plated layer, 5: Ag-Cu-In protective layer, 6: Ni(V)-Al reactive foil, 7: upper substrate (including metallisation layers).