Combined process for wafer direct bonding by means of the surface activation method

A sequential plasma activation process consisting of oxygen reactive ion etching (RIE) plasma and nitrogen radical activation is proposed for wafer direct bonding at room temperature. The Si wafer surface is activated by oxygen RIE plasma and subsequently exposed to nitrogen radicals. The activated wafers by the two-step process were brought into contact in air followed by keeping them in air for 24 h. The wafers were bonded throughout the whole area and the bonding strength of the interface is as strong as bulk Si without any post-annealing process and wet chemical cleaning steps. XPS study indicates that the silicon surface has thermodynamically unstable characteristics. IR transmission images reveal a considerable amount of water is absorbed in the wafer surfaces during exposure to air after the plasma activation process. The high bonding strength is thought to be due to a diffusion of absorbed water into the wafer surface and a reaction between silicon oxynitride layers on the opposing wafer. TEM images show that an intermediate amorphous layer with thickness of 15 nm is formed across the interface. The bonding is so intimate that no micro-voids are found at the bonding interface. Furthermore, strong bonding of crystalline quartz and fused quartz at room temperature was also obtained by the sequential activation process.