Role of bonding temperature and voltage in silicon-to-glass anodic bonding

In this paper, anodic bonding between 4" Si wafer and glass wafer (Pyrex 7740) has been successfully achieved at low temperature. Two wafers are stacked together after alignment and are put into the bonding chamber. The effects of bonding temperature and voltage are investigated. The bonding temperature ranges from 200°C to 300 °C and the voltage ranges from 200 V to 1000 V. The bonding interfaces are examined with scanning acoustic microscopy (SAM) and scanning electron microscopy (SEM). The chemical bonds formed in the interface are analysed by Raman spectroscopy. High temperature and voltage cause more Na⁺ ions neutralized at the negative electrode, which leads to higher charge density inside the glass wafer. The transition period to the equilibrium state becomes shorter too. At a bonding temperature of 200°C, the unbonded area is 1.5%. At a bonding temperature higher than 250°C, the unbonded area is less than 0.5%. The bubble size decreases with increasing the bonding temperature as well. High voltage has similar effects. The bonding strength is higher than 10 MPa in all conditions. When the bonding temperature is higher than 250°C and the voltage is higher than 400 V, the fracture happens inside the glass instead of at the interface. The bonding strength increases with increasing the bonding temperature and voltage. The anodic bonding mechanisms consist of the oxidation of silicon and the hydrogen bonding between hydroxyl groups. Higher temperature can promote such reactions.