Low-temperature rough-surface wafer bonding with aluminum nitride ceramics implemented by capillary and oxidation actions

Aluminum nitride (AlN) is a ceramic material with excellent dielectric and thermal properties, and it is used in the microelectron and energy fields. Despite polishing, the roughness (rms >10 nm) of AlN wafers fabricated by sintering fails to meet the surface requirements (rms <0.5 nm) for direct wafer bonding. However, rough-surface wafer bonding of paired AlN and AlN can be achieved with the following procedure: (1) rough surfaces are activated by oxygen plasma to introduce strong capillary action, (2) reactions of AlN and bonding species (OH⁻) are promoted by clamping during storage and annealing and (3) low-temperature annealing (<150 °C) joins the two surfaces of AlN/AlN by synthesizing Al₂O₃ at the bonding interface. TEM and XPS results show that a transition layer exists between bonding surfaces wherein the concentration of nitrogen decreases while that of oxygen significantly increases.