Wafer bondingThere are a number of different methods available for bonding micromachined silicon wafers together, or to other substrates, to form larger more complex devices.A method of bonding silicon to glass that appears to be gaining in popularity is anodic bonding (electrostatic bonding). The silicon wafer and glass substrate are brought together and heated to a high temperature. A large electric field is applied across the join, which causes an extremely strong bond to form between the two materials. Figure 15 shows a glass plate bonded over a channel etched into a silicon wafer (RIE), forming a pipe through which fluid can flow.
 Figure 15. It is also possible to bond silicon wafers directly together using gentle pressure, under water (direct silicon bonding). Other bonding methods include using an adhesive layer, such as a glass, or photoresist. Whilst anodic bonding and direct silicon bonding form very strong joins they suffer from some disadvantages, including the requirement that the surfaces to be joined are very flat and clean. Wafer bonding techniques can potentially be combined with some of the basic micromachined structures to form the valves, pumps, etc, of a microfluid handling system. |