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aluminum nitride wafer is an excellent material for the production of LED light emitting diodes (LEDs). It can withstand high temperatures, has good electrical insulation properties and is non-toxic. Currently, it is being used in smartphones and other electronic devices that require superior thermal conductivity. However, there are still challenges in the fabrication of this material.
To overcome the challenge of bonding AlN to a rough surface, we have developed an innovative technique that employs capillary action. This technique allows for low-temperature wafer bonding even when the surfaces are textured like Velcro. We have demonstrated that the capillary force is able to overcome the high roughness of the bonding interface by creating a layer of water in the area of the voids and valleys on the mating surface. In the STEM-high-angle annular dark-field micrograph of the bonded sample, the dark layer is clearly visible and its thickness follows the topography of the mating surface.
XPS measurements of the bonding interface showed that the water contact angle was enhanced by the surface activation. This resulted in the substitution of oxygen by fluorine at the surface region, reduction of tightly bound carbon such as carboxylic acids and an overall decrease in the surface roughness. This is in contrast to typical aggressive wet cleaning protocols that attack the grain boundaries of AlN, resulting in poor bonding. The tensile strength of the bonded samples was also measured. It was found that the bonded sample had higher tensile strength than unbonded substrates.