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The secret of eutectic bonding: How to achieve high-strength connections at low temperatures?
In today's era of rapid technological development, how to achieve high-strength connections at low temperatures has become a key challenge. Eutectic bonding, also known as eutectic bonding, is a wafer bonding technology that uses an intermediate metal layer to form a eutectic system. The characteristic of this technology is that it can realize direct transformation from solid to liquid or from liquid to solid under specific composition and temperature without going through the process of two-phase equilibrium, which greatly reduces the temperature requirements, which is a good foundation for crystallization. The strong connection of circles opens a new door.
The melting temperature of eutectic alloys can often be lower than the melting points of the two pure elements, which is critical for eutectic bonding.
According to research, this technology has been successfully used to transfer epitaxial materials such as GaAs-AlGaAs onto silicon substrates since it was reported by Venkatasubramanian et al. in 1992, and its application in solar cells was further verified in 1994. performance. The advantage of eutectic bonding is the ability to achieve hermetic packaging and electrical interconnection in a single-use process, especially when performed in a low-temperature environment, inducing less stress in the final assembly, making it an ideal solution for electronics plan.
To achieve effective eutectic bonding, several key parameters must be considered, including bonding temperature, duration and tool pressure, each of which has an impact on the final bond strength and reliability.
Overview of eutectic bonding
The basic principle of eutectic bonding is that silicon (Si) and various metals can be alloyed and form a eutectic system. Silicon-gold (Si-Au) and silicon-aluminum (Si-Al) are the most common eutectic formation methods. This bonding procedure is typically applied to silicon or glass wafers coated with Au/Al films.
The correct alloy selection depends on the processing temperature and compatibility of the materials used.
In addition, eutectic bonding has fewer restrictions on the roughness and flatness of the substrate than direct bonding, which makes it more flexible in practical applications. Compared to anodic bonding, high voltages are not required, which is particularly important for electrostatic microelectromechanical systems (MEMS). What is more advantageous is that compared with the bonding process of the organic intermediate layer, eutectic bonding can more effectively promote gas release and improve sealing performance.
Operating procedures
Preprocessing
The key step for successful eutectic bonding is surface preparation. Prior to preparation, oxide layers on the silicon surface act as a diffusion barrier and must be removed to promote a strong bond. Common removal methods include wet chemical etching (such as hydrofluoric acid cleaning), dry chemical etching, and chemical vapor deposition. In some applications, it is also necessary to use hydrogen plasma or fluorine gas such as CF4 to pretreat the surface.
Another way to ensure good adhesion of the eutectic metal to the silicon wafer is to use an adhesion layer. These thin intervening metal layers effectively adhere to the oxide layer and interact with the eutectic metal, thereby promoting bonding to the underlying layer.
Bonding process
When the pre-treatment of the substrate is completed, contact is made immediately to prevent the oxidation layer from forming again. During the bonding process, the substrates are typically exposed to a reduced atmosphere of polar hydrogen and inert gas flow, which helps facilitate metal contact.
The uniformity of heat and pressure in the equipment is critical to the success of the anchoring. When the dopants are brought into contact at the atomic level, they are heated to the eutectic temperature, which promotes intermetallic reactions and is supported using appropriate mechanical pressure.
Cooling process
When the temperature drops below the eutectic point, the material mixture begins to solidify, typically forming a thin film on the silicon substrate. The key lies in correct process parameters to prevent stress-induced cracks during cooling.
Potential uses
With its excellent bond strength, eutectic bonding is particularly suitable for manufacturing pressure sensors or fluidic devices. The manufacturing of micromechanical sensors and actuators can spread electronic or mechanical functions between multiple wafers, opening up new application scenarios.
With the advancement of technology, eutectic bonding is becoming an indispensable part of the field of electronic component manufacturing. In the future, can we truly master this technology and promote the development of more innovations?
