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From laboratory to factory: Why is direct bonding of silicon wafers a key breakthrough in the manufacturing process?
With the rapid development of science and technology, silicon chip manufacturing technology has also ushered in new challenges and opportunities. In this process, silicon wafer direct bonding technology, as an important process breakthrough, is gradually showing its potential value.
What is direct bonding?
Direct bonding, also known as fusion bonding, is a wafer bonding process that does not rely on any intermediate layer. This technology is based on chemical bonds between each other, requiring the surfaces of the two materials to be sufficiently clean, flat and smooth. If the surface is not properly prepared, unbonded areas, so-called “voids” or interface bubbles, may appear.
The direct bonding process includes wafer pretreatment, pre-bonding at room temperature, and annealing at high temperature.
History of direct silicon wafer bonding
The adhesion effect on smooth solid surfaces was first proposed by Desaguliers as early as 1734. After a long period of experimentation and research, the successful direct bonding technology of silicon wafers was first reported in 1986, making the entire process more mature.
Conventional direct bonding technology
Direct bonding mainly refers to the bonding process of silicon materials, and is divided into two categories: hydrophilic and hydrophobic according to the surface chemical structure. The surface condition of a silicon wafer can be determined by measuring the contact angle of a water droplet. The contact angle for hydrophilic surfaces is less than 5°, while for hydrophobic surfaces it is greater than 90°.
The bonding of hydrophilic silicon wafers needs to be performed after continuous cleaning to ensure that the surface is free of any impurities.
Bonding process of hydrophilic silicon wafer
Wafer pretreatment
Before bonding, both wafers must be free of any particle, organic or ionic contamination. Common cleaning processes include dry cleaning (such as plasma treatment) or using chemical wet treatments. Industry standard cleaning procedures such as SC cleaning can effectively remove organic matter and metal ions.
Room temperature pre-bonding
Before actual contact, the wafers must be aligned. The moment the atoms come into contact, bonding will begin immediately if the surfaces are smooth enough. In this process, water molecules coat the surface, allowing chemical reactions to occur.
High temperature annealing
After room temperature pre-bonding, the wafer needs to be annealed to improve the bonding strength. The heat energy applied during this process causes more chemical bonds to react with each other, forming a more stable bonding structure, thereby enhancing the bond strength.
The selection of annealing temperature is critical; too high or too low may affect the overall performance of the chip.
Bonding process of hydrophobic silicon wafers
Wafer pretreatment
Hydrophobic surfaces often require removal of the native oxide layer to enhance carbon-fluorine bond formation. Therefore, rehydrophilization of the surface must be avoided to ensure successful bonding.
Room temperature pre-bonding and high temperature annealing
The bonding of hydrophobic wafers relies on van der Waals forces at room temperature, and gradually forms more stable covalent bonds at high temperatures. This process also requires controlling the proper atmosphere and temperature to avoid interference from moisture.
Prospects of low temperature direct bonding research
Although direct bonding technology has good flexibility in processing a variety of materials, the mismatch in thermal expansion coefficients caused by high annealing temperatures remains a major challenge. To improve this problem, researchers are focusing on developing low-temperature bonding technology.Low-temperature bonding technology will show more potential in the manufacture of multi-wafer microstructures, with applications including accelerometers and micropumps.
As direct silicon wafer bonding technology advances, we can't help but wonder: How will this technology be more widely used in different electronic manufacturing fields and the development of new materials in the future?
