Void nucleation at a sequentially plasma-activated silicon/silicon bonded interface

Two 4 inch silicon wafers were directly bonded using a sequentially plasma-activated bonding method (i.e. O2 reactive ion etching (RIE) plasma followed by N2 microwave (MW) radicals) at room temperature. The bonded wafers were annealed from 200 to 900 °C in order to explore the nucleation of voids at the interface. The plasma-induced void nucleation was dominated by O2 RIE power over O2 RIE activation time. The thermal-induced void nucleation occurred preferentially at the plasma-induced defect sites. The nucleation of void density was quantitatively determined and explained using high-resolution transmission electron microscopy observations. The electron energy loss spectroscopy results revealed the existence of silicon oxide at the bonded interface. The reduction in bonding strength after annealing at high temperature is correlated to the increase in void density. The contact angle and surface roughness of the sequentially plasma-treated surfaces have been observed to explain the nucleation of voids and the reduction of bonding strength. The plasma-induced defect sites such as nanopores and craters have been identified using an atomic force microscope.