Wafer Bonding Technique for Packaging of Silicon Wafers

Wafer bonding is a packaging technology used in Silicon wafer fabrication. Wafer bonding methods are used for micro-electromechanical systems (MEMS), microelectronics and optoelectronics technologies. This  ensures a mechanically stable and hermetically sealed Silcon Wafers.

In MEMS devices, it is essential to protect wafer from environmental influences.  Wafers should be safe from sensitive internal structures such as temperature, moisture, high pressure and oxidizing species. Therefore, for the long-term reliability and consistency the encapsulation process, of wafer bonding is very much essential.

Benefits of Wafer Bonding Process:

1. Guard against environmental agents.
2. Maintenance of energy and information flow
3. Helpful for achieving compatibility with the surrounding periphery
4. Helpful in heat dissipation
5. Integration of elements with different technologies

The wafers' diameter range from 100mm Silicon Wafers to 200 mm Silicon Wafer microelectronic devices are packaged using silicon wafer technology. Each type of wafer size requires   specific conditions to achieve strong bonding. The actual bond formed between the si Wafers, is an interaction of conditions and requirements met together.

1. Substrate Surface
2. Bond Temperature
3. Suitable Pressure
4. Applied Force
5. Flatness
6. Smoothness
7. Cleanliness
8. Materials
9. Bonding Environment
10. Substrate Materials

Hence, a particular type wafer bonding technology should be selected with respect to particular substrate wafer and size.  Particular wafer size requires a particular temperature, pressure and gaseous atmosphere. The commonly used and developed bonding methods are as follows:

• Direct Bonding Technique: Direct bonding process anneals wafer together without any intermediate layer. The bonding process is based on chemical bonds between two surfaces. The Silicon Test Wafers surfaces in this approach must be sufficiently clean, flat and smooth.
• Surface Activated Bonding: Surface activated bonding is a low temperature wafer bonding technology. By utilizing this technique, high strength bonding of semiconductor can be obtained.
• Plasma Activated Bonding: It is a derived from the direct bonding technique with hydrophilic surfaces.
• Thermo-Compression Bonding: It is referred to as diffusion bonding or thermo-compression welding.
• Reactive Bonding: In this procedure a highly reactive nano-scale multilayer systems are used as an intermediate layer between the bonding substrates.
• Transient Liquid Phase Diffusion Bonding: This method is applied for bonding metallic and ceramic systems by conventional fusion welding techniques.
• Anodic Bonding: This technique seals glass to silicon without intermediate layer. It is generally utilized for electronics and Micro-Fluidic process.
• Eutectic Bonding: It is also known a eutectic soldering. It is a bonding technique with an intermediate metal layer that can produce an eutectic system.
• Glass Frit Bonding: This technique is also referred as glass soldering. It is used as an encapsulation technology for micro-machined compositions.  

The bonded wafers are characterized to evaluate bonding strength and level of hermeticity in fabrication devices.  Therefore, several different approaches for the wafer bonding are used and described.