Chapter 30 - 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 and different metals instead of silicon. 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. It 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 known by the trademark “Pyrex”) and Schott Borofloat. 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, then it decreases gradually until it reaches a relatively stable value, when the bonding process is completed. A high quality seal means high bonding strength, low thermal residual stress in the bonded structures, hermeticity of the seal over the product life-time. The thermal residual stress is generated during cooling from the bonding temperature to room temperature and it is caused by the difference between the thermal expansion coefficients of silicon and glass. Many micromechanical devices need a low pressure in their cavities. It can be achieved by anodic bonding in vacuum and use of getters in the sealed cavities. Anodic bonding implies the use of alkali containing materials and it necessitates high electric fields, which usually make it incompatible with microelectronic devices. 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 high quality hermetic seals.