Aluminum gallium nitride (AlGaN) is a promising material system in the application for deep ultraviolet LEDs, power electronics, and piezoelectric devices. Due to the large polarization electric field along c axis, novel heteropolar devices making use of polarity inversion are expected to be realized. For example, quasi phase-matching wavelength converters and film bulk acoustic resonators have been reported. In the case of polarity inversion by crystal growth, N polar surface tends to be rough owing to the severe growth condition. Recently, polarity inversion by direct wafer bonding is receiving increased attention as an alternative. This approach enables realization of heteropolar films with high crystallinity. Additionally, heterogeneous material integration is possible even if a large lattice mismatch exists. In this paper, we report the direct bonding of 2-inch sputtered AlN wafers with the use of nitrogen plasma activation.
The fabrication starts with the preparation of a pair of 2-inch AlN wafers by sputtering an AlN target onto vicinal c-plane sapphire substrates. The sputtering conditions were an RF power of 700 W, a chamber temperature of 600 °C, and an AlN thickness of 200 nm. The AlN sample for this bonding experiment showed a root mean square surface roughness of 0.6 nm and curvature of 12 km-1. Subsequently, the +c-oriented sputtered AlN wafers were directly bonded by using N2 plasma activation. Plasma irradiation conditions were an RF power of 500W, a chamber pressure of 120 Pa, and an irradiation time of 50 s. Then the wafers were brought into contact for the temporary bonding. With a load application of 300 kgf, the temperature of annealing stage was ramped up from RT to 400 °C through 3 h. Subsequently, the stage temperature was held at 400 °C for 6 h in order to form the strong bond at the AlN/AlN interface. Despite the curvature of AlN, almost whole area of the 2-inch wafer was successfully bonded. Longer annealing time was found to be effective because the wafer did not bond at the near-edge section in the case of an annealing time of an hour. This result suggests the effectiveness of direct bonding approach for heteropolar device application in III-N material system. In the future, the combination of plasma activated bonding and high-crystallinity AlN will be the next target for device applications. We have already reported significant improvement in crystallinity of sputtered AlN by high temperature annealing at 1700 °C. This approach markedly decreased FWHM of X-ray rocking curve in (10-12) from 6031 arcsec to 287 arcsec. If the curvature of 89 km-1 caused by annealing is managed well, novel device applications can be realized.