Surface Activated Si-Si Wafer Bonding Using Different Ion Species

Abstract

Surface activated bonding (SAB) on wafer level is enabled by the EVG ComBond® system for irradiation with different ion species. In this process, the native oxide of 200 mm Si wafers is sputter-removed, and an amorphous layer is generated. After ion treatment, the wafers are bonded in ultra-high vacuum (UHV) at room temperature. The impact of Ar, Kr and Xe ion irradiation on the amorphous layer thickness (ALT) was investigated with the goal of optimizing the bonding process. This was i.a. motivated by the fact, that the presence of an amorphous layer reduces the electrical conductivity across the wafer interface [1].The ALT of unbonded wafers was evaluated via spectroscopic ellipsometry (SE) for varying ion species, ion energies and angles of incidence. The results show that heavier ions generate a thinner amorphous layer than light ions (see Fig. 1). This is most likely due to the higher scattering cross section of heavy ions resulting in a lower penetration depth. For each ion type there is a minimum in the ALT as a function of the ion energy which can be attributed to sputter-implantation dynamics. Furthermore, the ALT is decreasing with the angle of incidence as the incoming ions are projected into the material.Transmission electron microscopy (TEM) measurements were conducted on bonded wafer pairs and confirm the trend of heavy ions generating a thinner amorphous layer. Moreover, the TEM measurements show that post-bond annealing at 450°C for 5 h reduces the ALT due to recrystallization near the bonding interface.To ensure mechanical stability for further wafer processing and for the final device, bonding energy measurements were conducted via the Maszara method [2]. Although maximum bonding energy can be achieved for specific parameters with each ion species, Xe irradiated samples tend to have a higher overall bonding energy.[1] P.A. Stolk et al., “Contribution of defects to electronic, structural, and thermodynamic properties of amorphous silicon”, in Journal of Applied Physics, vol. 75, no. 11, pp. 7266-7286 (1994).[2] W.P. Maszara et al., “Bonding of silicon wafers for silicon-on-insulator”, in Journal of Applied Physics, vol. 64, no. 10, pp. 4943-4950 (1988). Figure 1