Wafer-to-wafer bonding for microstructure formation

Ultrasonic Transducers

“…Since identical materials are bonded together, thermal stress caused by material mismatch is not so rigorous. Normally, fusion bonding requires a high-temperature annealing step to eliminate possible intrinsic void and increase bonding strength [72]. Since most IC metals such as aluminum or gold cannot withstand this high temperature step, fusion bonding is seldom used in MEMS application.…”

“…In this process, wafers are contacted directly, without the assistance of any intermediate layers or external electric field. The bonding force comes from the tendency of smooth surfaces to adhere [72]. To increase the bond strength, after bonding, a thermal annealing step always ensues.…”

“…When a pair of clean, mirror-polished wafers contact with each other, strong bonding takes place [71][72][73]. In contrast to anodic bonding [74][75] or bonding with intermediate layers [77][78][79][80], direct bonding needs neither adhesive agent nor external electrical field.…”

“…During the past decades, many bonding schemes have been developed [7,[71][72][73][74][75][76][77][78][79][80][81][82][83][84], according to their bonding mechanisms, these processes can be classified as direct bonding, anodic bonding, and bonding with intermediate layer.…”

Bonding properties of metals anodically bonded to glass

“…Among the samples, on which the anodic bonding to glass was successfully achieved, were materials having a similar thermal expansion coefficient to the Pyrex-glass, such as Kovar, Alloy 42 and silicon. Since then, with the grow of the microsystems field, this technique has been especially developed for bonding silicon to glass for device assembling and packaging purposes, leaving the use of metals aside for a while [7,8].…”