Room-temperature Wafer Bonding

Bonding technologies play important roles in MEMS fabrication processes, especially for reliability and commercial competitiveness. One of those technologies is wafer direct bonding. Conventional wafer direct bonding utilizes hydrogen bonds between hydroxyl groups on wafer surfaces. It has an advantage of pressure-less bonding; however, high temperature annealing step, required to achieve practical bonding strength, limits its application to MEMS devices. In wafer direct bonding, surface state of the wafers largely influences the bonding. Therefore, several surface treatments have been adapted. Surface activated bonding (SAB) utilizes sputter etching of the bonding surface in vacuum. The sputter etched surface is expected to be in an active state and form strong bonds at lower temperatures. Various materials such as silicon, compound semiconductors and metals can be bonded at room temperature. On the other hand, in case of SAB between oxide wafers, low temperature annealing, typically at 200oC, is effective to achieve strong bonding. This means that a simple model of bond formation between clean surfaces is not enough to explain the bonding process of SAB.