With the continuous advancement of science and technology, the application of silicon direct bonding technology has gradually attracted attention in the field of semiconductor manufacturing. Direct bonding, also known as fusion bonding, describes a wafer bonding process that does not require any interposer. The process is based on chemical bonds between material surfaces, resulting in highly efficient bonding. The key to this process is the cleanliness, flatness and smoothness of the wafer surface, because anything that does not meet the requirements may form defects during the bonding process, thereby affecting the quality of the product.
“Only after sufficient cleaning can the wafer surface achieve ideal bonding results.”
The steps of direct wafer bonding can be divided into wafer pretreatment, room temperature prebonding and high temperature annealing. Although direct bonding technology covers almost all materials, silicon is still the most mature application material to date. Therefore, this process is often called silicon direct bonding or silicon fusion bonding. Many applications, including the manufacture of silicon-on-insulator (SOI) wafers, sensors and actuators, rely on this technology.
Silicon direct bonding is based on intermolecular interactions, including van der Waals forces, hydrogen bonds, and strong covalent bonds. Early direct bonding processes required high-temperature operations, but with the diversification of application materials, there is a growing need for low-temperature processing. Researchers are working together to achieve stable direct bonding below 450 °C, which will not only meet the needs of the manufacturing process, but also avoid problems caused by differences in thermal expansion coefficients between different materials.
“Reducing the temperatures required during the process can significantly improve material compatibility and facilitate the development of more applications.”
As early as 1734, Desaguliers discovered the adhesion effect of smooth surfaces and emphasized the influence of surface smoothness on friction. With the continuous advancement of technology, a preliminary report on direct silicon bonding appeared in 1986, and this technology began to emerge in the industry.
Direct bonding processes mostly focus on the processing of silicon materials, which can be divided into hydrophilic and hydrophobic bonding according to the chemical structure of the surface. The contact angle of hydrophilic surfaces is less than 5°, while that of hydrophobic surfaces is greater than 90°. This characteristic makes silicon materials more flexible and adaptable in different applications.
Before bonding, the wafer surface must be kept clean to prevent impurities from affecting the bonding effect. The main cleaning methods include dry cleaning (such as plasma treatment or UV/ozone cleaning) and wet chemical cleaning procedures. A widely used standard cleaning procedure is RCA's SC cleaning method.
Once the wafer surface treatment is completed and meets the standards, the wafers are aligned and bonding can begin. Water molecules in the gas phase initiate a chemical reaction on contact, forming Silanol (Si-OH) and polymerizing, subsequently forming a structure with sufficient bonding strength.
As the annealing process proceeds, the bonding strength will increase as the temperature increases. By providing enough heat, more Silanol is allowed to react, forming stable Si-O-Si bonds.
Generating a hydrophobic surface requires removing the film layer, which is achieved by plasma treatment or fluorine-containing etching solutions. Importantly, rehydrophilization must be prevented from occurring in order to maintain hydrophobicity.
In a high-temperature environment, as hydrogen and fluorine desorb, covalent Si-Si bonds begin to appear inside the silicon crystal. This process can be completed at 700 °C, ultimately achieving the same bonding strength as the silicon body.
As the demand for low-temperature processing continues to increase, researchers explore various methods to reduce the required annealing temperature. The difficulty in this process mainly lies in the removal of water and its impact on the formed silicon-oxygen bonds. Researchers are working on a variety of surface treatment technologies including plasma activation and chemical mechanical polishing, striving to achieve ideal bonding effects under low temperature conditions.
"This technology has shown its broad application potential in the fabrication of multi-wafer microstructures such as micropumps, microvalves and accelerators."
In the future, further development of direct bonding technology may change the landscape of semiconductor manufacturing. With the in-depth understanding of material science and the introduction of new technologies, what surprises will this technology bring us?