Al–NiO energetic composites as heat source for joining silicon wafer

Abstract

Abstract Silicon wafers with the thickness of approximately 250 μm were joined successfully using Al/NiO nano-thermite composites and with an aluminum foil interlayer (thickness of 30 μm), due to energy production from the exothermic reaction between Al (40 nm) and NiO (50 nm) powders. This reaction heat was subsequently transferred across the silicon wafer and reached the Si–Al–Si interface. This joining method reduces the possibility of introducing unwanted impurity and residual from the thermite reaction and subsequently improves the joint quality. Experimental data shows, adding Al micro-powder (5 μm, 60 % by mass) or Ni micro-powder (1 μm, 30 % by mass) into the nano-thermite composite was necessary and effective in joining the silicon wafer. The micro-hardness of the joined zone was measured as 129.3 ± 15.5 HV and 42.3 ± 4.0 HV for the Al and Ni micro-powder modified nano-thermite composites, respectively. The localized yield strength data confirmed the Al micro-powder produced a higher yield strength (350.6 ± 17.8 MPa) than the Ni micro-powder (327.1 ± 55.0 MPa). Energy production from the nano-thermite composites with different mixing ratios with the additive was characterized by Differential Scanning Calorimetry (DSC), and the apparent activation energy of the respective thermite reaction were calculated to investigate the effects of reaction kinetics to the joining process. The reduced joint quality produced by the Ni micro-powder modified Al/NiO nano-thermite composite was due to the low energy release from its thermite reaction (1.06 kJ/g), which was not sufficient to melt the silicon wafer. Moreover, the large activation energy of this thermite reaction (696.09 kJ/mol) hindered the joining process. In contrast, the Al micro-powder modified Al/NiO nano-thermite composite, via a two-step energy release process, produced sufficient energy (1.89 kJ/g) which led to a superior joint quality. In the first step, a pilot reaction (corresponding to the activation energy of 332.96 kJ/mol) between Al (40 nm) and NiO (50 nm) nanoparticles was dominant. In the following step, the reaction between Al micro-powder (5 μm) and NiO (50 nm) nanoparticles became significant and contributed greatly to successful joining.