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The mysterious bonding technology of silicon wafers: Why is direct bonding so critical?
In the current trend of rapid technological development, the manufacturing process of silicon wafers continues to evolve, and direct bonding technology, as one of the key technologies, has received widespread attention in recent years. The core of this technology is that it can directly bond two wafers together without using any intermediate layer, using the principle of chemical bonding, so it is sometimes called "fusion bonding."
During the direct bonding process, the wafer surface needs to be sufficiently clean, flat, and smooth to avoid the formation of voids or interface bubbles.
Basic process of direct bonding
The process of direct bonding can be divided into three main steps: wafer pretreatment, pre-bonding at room temperature and high-temperature annealing. Although this technology is applicable to almost any material, silicon wafers are by far the most mature and widely used material. The application scenarios of this technology cover the manufacturing of silicon oxide wafers, various sensors and actuators, etc.
Basics of chemical bonding
The direct bonding of silicon is based on intermolecular interactions between surfaces, including van der Waals forces and hydrogen bonds. Traditional direct bonding technology usually requires high temperatures, but with the development of technology, many researchers have begun to try to reduce the process temperature to adapt to the thermal expansion coefficients of different materials. For example, the goal is to achieve stable and sealed direct bonding below 450°C, while developing technologies for surface activation such as plasma treatment and chemical mechanical polishing (CMP).
History of direct bonding
As early as 1734, Desaguliers first mentioned the adhesion effect on smooth solid surfaces. His research shows that when the solid surface is smoother, the friction between the two will decrease. When it reaches a certain level of smoothness, the friction will increase again and the solid surfaces will adhere to each other. In 1986, J. B. Lasky and others published the first successful report on direct bonding of silicon, which laid the foundation for the advancement of this technology.
Different types of direct bonding processes
Bonding of hydrophilic silicon wafer
Before bonding hydrophilic silicon wafers, the surface needs to be clean to avoid interference from organic and ionic contaminants. Plasma treatment or UV/ozone cleaning is usually used to meet the pretreatment requirements, and then bonding at room temperature is performed. After contact, a strong Si-O-Si bonding force is formed through the cross-linking reaction between hydrogen and oxygen molecules, thereby achieving operable strength.
Bonding of hydrophobic silicon wafers
For hydrophobic silicon wafers, the surface is treated with chemical solutions such as fluoride to avoid re-hydrophilicity. At this time, bonding mainly relies on van der Waals forces between hydrogen and fluorine atoms. After bonding at room temperature, the chemical bonding is strengthened through an annealing process, ultimately forming a stable Si-Si bonding to achieve an efficient manufacturing process.
The prospect of low-temperature direct bonding
In addition to traditional direct bonding methods, low-temperature direct bonding technology has also gradually received attention, especially when the thermal expansion coefficients of different materials do not match. The research aims to effectively reduce the required annealing temperature on pre-processed wafers and during the bonding of composite materials without causing degradation or changing their properties.
Application cases and future prospects
Direct bonding technology plays a crucial role in the fabrication of microelectromechanical systems (MEMS), such as accelerometers, microvalves, and micropumps. Its flexibility and efficiency make it an important choice for future wafer-level manufacturing.
As the demand for electronic devices continues to increase and technology advances, can direct bonding technology overcome current challenges and become a more widely used solution?
