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Anode bonding is a microelectronics industry method of bonding wafers using heat and an electrostatic field, commonly used for silicon-glass bonding. The technique relies on electrostatic attraction and is used to protect sensitive electronic components. It has limitations on material combinations due to similar coefficients of thermal expansion.
Anode bonding is a method of bonding wafers widely used in the microelectronics industry to seal two surfaces together using a combination of heat and an electrostatic field. This bonding technique is most commonly used to seal a layer of glass to a silicon wafer. Also called field-assisted bonding or electrostatic sealing, it resembles direct bonding in that, unlike most other bonding techniques, it does not normally require an interlayer, but differs in that it relies on the resulting electrostatic attraction between surfaces by the movement of positive ions when a high voltage is applied to the components.
It is possible to use anodic bonding to bond metal to glass and, using a thin intermediate layer of glass, silicon to silicon. However, it is particularly suitable for silicon-glass bonding. Glass must have a high content of alkali metals such as sodium to provide mobile positive ions; a specific type of glass is often used that contains about 3.5% sodium oxide (Na2O).
In the bonding process, the surfaces of the two components are smoothed and thoroughly cleaned to ensure close contact with each other. They are then inserted between two electrodes, heated to 752-932° Fahrenheit (400-500° Celsius), and a potential of a few hundred to a thousand volts is applied, such that the negative electrode, called the cathode, is in contact with the glass, and the positive electrode, the anode, is in contact with the silicon. The positively charged sodium ions in the glass become mobile and move towards the cathode, leaving a positive charge deficit near the silicon wafer boundary, which is then held in place by electrostatic attraction. The negatively charged oxygen ions from the glass migrate towards the anode and react with the silicon as they reach the boundary, forming silicon dioxide (SiO2); the resulting chemical bond seals the two components together.
The technique is used for encapsulating sensitive electronic components to protect them from damage, contamination, moisture and oxidation or other unwanted chemical reactions. Anode bonding is particularly associated with the microelectromechanical systems (MEMS) industry, where it is used to protect devices such as microsensors. The primary benefit of anodic bonding is that it produces a strong, permanent bond without the need for adhesives or excessively high temperatures, as would be required to fuse components together. The main disadvantage of anodic bonding is that the range of materials that can be bonded is limited and there are further limitations on material combinations, because they must have similar coefficients of thermal expansion, i.e. they must expand at similar rates when heated, or the Differential expansion could cause warping and warping.
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