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 silicon bulk without any post-annealing process and wet chemical cleaning steps. Significant increase in bonding strength was observed by the nitrogen radical treatment after oxygen RIE treatment prior to bonding. XPS study indicates that the silicon surface activated by sequential oxygen RIE and nitrogen radical process has thermodynamically unstable characteristics. IR transmission images reveal considerable amount of absorbing water is absorbed in wafer surfaces during exposure to air after plasma activation process. High bonding strength is thought to be due to a diffusion of absorbing water into wafer surface and a reaction between silicon oxynitride layers on the opposing wafer. TEM images show that an intermediate layer 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.