How Wafer Bonding Works
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Anodic bonding
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Anodic bonding is the process of joining a silicon wafer with a glass wafer by heating the components and introducing powerful electric fields to both components. The glass wafer is composed of alkali metal oxides, such as Pyrex. The wafers are cured in an oven up to temperatures as high as 500 degrees Celsius. To achieve a silicon-to-glass interface, a powerful electric field is introduced to the silicon wafer (which is in contact with the glass wafer). After the oxides dissociate, metallic ions bond with the glass wafer. The electric field causes an exchange, in which the glass wafer passes oxygen ions into the silicon wafer. The result is a permanent bond between the silicon and glass wafers.
Eutectic bonding
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In electronic manufacturing applications, vacuum sealing is necessary for some electronic components. Eutectic bonding of wafers is a means of creating the necessary bond for vacuum-sealed components, especially in sensors engineering and manufacturing. A eutectic alloy is formed (gold to silicon; gold to tin; lead to silicon) at a temperature just over the eutectic level and with enough contact force. The silicon wafer is placed in the fixture of the eutectic bonding device. Metal cubes are plated onto a circuit pattern that has been silkscreen-printed onto the silicon wafer. To get the metal cubes onto the wafer, a small lever is moved by the operator of the eutectic bonding device. This lever moves a mechanical arm, at the end of which is a small vacuum head for picking up the metal cubes. The completed wafer bond forms a vacuum tight, solid seal immediately when it cools.
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Fusion bonding
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An oxidized silicon wafer and a silicon donor wafer are used in fusion bonding for creating sensors and semi-conductors. Fusion bonding utilizes heat (1,000 degrees Celsius) from a vertical furnace, and pressure on the edge of the wafer to bond silicon wafers. The top wafer is sliced or trimmed by laser (or dicing machine) to create a product (such as a microprocessor in a computer).
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