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Aluminum Nitride (AlN) is a popular material for electronics, as it offers both high thermal conductivity and electrical insulation. It is also safe for the environment, making it an attractive alternative to materials such as beryllium oxide and boron nitride that pose health risks when machined. AlN is a common choice for high-power electronic applications that require significant heat dissipation, such as power modules and LED packages.
For the purposes of this study, we used a 8-inch polycrystalline AlN wafer that had been sintering. This material had a thermal conductivity of 180 W/mK, a dielectric constant of 8.5, and a coefficient of thermal expansion that closely matches that of Silicon wafers. The surface profile of the wafers was investigated with atomic force microscopy (AFM; Nanoview 1000, FSM-Precision) and X-ray photoelectron spectroscopy (XPS; JEM-2100, JEOL). The AFM results showed that the rms roughness was 16-20 nm 0.5 nm. This poor surface topography would prevent direct wafer bonding under regular conditions.
The XPS analysis revealed that the AlN surface had already been dehydrated to some extent. The reactivity of the bonded substrate surface is likely due to the presence of water molecules that had been absorbed by the nitrides on the opposing surfaces of the AlN/Si interface. The reactivity of the nitrides was further promoted when the bonded substrate was annealed under clamping pressure. The ionized nitrogen in the bonded interface reacts with positive hydrogen ions provided by the absorbed water to form ammonia (NH3). This reaction releases the redundant interfacial water molecules, and the resulting aluminum hydroxide (Al(OH)3) hydrates with the SiO2 on the Si substrate.