Anodic bonding


A technique of bonding a glass substrate, which contains movable ions, and a substrate of silicon, metal, and so on. The substrates are softened by heat, and bonded by the electrostatic attraction of an electrical double layer produced by applying a high voltage across the substrates with the silicon side as the anode.

High precision bonding is achieved due to the bonding process at the substrates' solid state. The bonding strength largely depends on the flatness of the surfaces, although this is not as critical as for silicon fusion bonding. Bonding silicon wafers with materials such as Pyrex glass enables structures with internal cavities, such as capacitive pressure sensors and micropumps, to be fabricated. When bonding two silicon wafers or a silicon wafer and a metal wafer, a thin glass film is formed on the contacting surface of the wafers, or the surface of the silicon wafer is oxidized. The problem with the use of thin films is that at high bonding temperatures, the dielectric breakdown voltage of the films is lowered to the point that sufficient voltage cannot be applied. To reduce the process temperature to room temperature, attempts are being made to form a glass film with a low melting point by sputtering. This solves problems such as the strain and deformation caused by thermal stress, and introduces benefits such as the improvement on precision and the wide choice of materials.


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