Wafer-Bonding Technologies for Packaging

Wafer bonding is used widely in MEMS fabrication and packaging. It refers to approaches of firmly joining two wafers (or more) to create a stacked and bonded wafer. There are three main kinds of waferbonding processes: direct bonding, field-assisted bonding, and bonding with an intermediate layer. The choice of which is most suitable depends on the particular application and the materials involved. Silicon-to-silicon direct bonding, also called fusion bonding, is a direct silicon-to-silicon bonding technique without the assistance of significant pressure or any intermediate layers or fields. The process requires rather flat and clean surfaces as the bonding interface for the two wafers. The surfaces of silicon wafers are cleaned and rinsed first. Hydroxyl (–OH) groups form on the surfaces of the two wafers with a thin native silicon oxide layer (i.e., hydrophilic surface). Two wafers can be joined together at room temperature, resulting in an immediate weak bond owing to van der Waals forces. Then the bonded wafers are heated to 800 to 1000°C to remove the water molecules and leave behind Si–O–Si bonds at the interface. Owing to perfectly matched thermal-expansion coefficients of the two wafers, silicon direct-bonded wafers have little or no residual stress owing to thermal mismatch. However, silicon direct bonding normally is used as the beginning step in an integrated process because of the rather high annealing temperature and high-quality requirement of the bonding interface in terms of cleanness and flatness.