A technique of bonding a glass substrate, which contains movable
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.