Chapter Thirty Two - Anodic Bonding

Abstract

This chapter explains Anodic Bonding in detail. In its most used form, the anodic bonding process consists of joining silicon with borosilicate glass. Ceramics can also be used instead of glass. The anodic bonding process, known also as “field assisted sealing,” or “electrostatic bonding” was described for the first time by Wallis and Pomerantz in 1968. The anodic bonding is an electrochemical process that relies on the polarization of the alkali-containing glasses. The most used glass materials for anodic bonding are Corning 7740 (commonly mentioned by the trademark “Pyrex”) and Schott Borofloat. Both these types are “old” compositions developed for other applications several decades ago. Monitoring the electrical current through the wafer pair is a method to follow the development of the bonding process. Initially, the bonding current increases sharply due to the increase in the contact area as the wafers are pulled into intimate contact by the electrostatic forces. It is caused by the difference between the thermal expansion coefficients of silicon and glass. The thermal residual stress generated during cooling from the bonding temperature to room temperature is another criterion to evaluate the bond quality. Many micromechanical devices need a low pressure in their cavities. An important issue for the design of the micromechanical structures to be electrostatically bonded is how to avoid the bonding/sticking of the flexible silicon parts to the glass. The anodic bonding is by its physical and chemical nature a method with a high potential resulting in close to perfect hermetic seals.