Wafer bonding


Wafer bonding is a packaging technology on wafer-level for the fabrication of microelectromechanical systems, nanoelectromechanical systems, microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation. The wafers' diameter range from 100 mm to 200 mm for MEMS/NEMS and up to 300 mm for the production of microelectronic devices. Smaller wafers were used in the early days of the microelectronics industry, with wafers being just 1 inch in diameter in the 1950s.

Overview

In microelectromechanical systems and nanoelectromechanical systems, the package protects the sensitive internal structures from environmental influences such as temperature, moisture, high pressure and oxidizing species. The long-term stability and reliability of the functional elements depend on the encapsulation process, as does the overall device cost. The package has to fulfill the following requirements:
The commonly used and developed bonding methods are as follows:
The bonding of wafers requires specific environmental conditions which can generally be defined as follows:
  1. substrate surface
  2. * flatness
  3. * smoothness
  4. * cleanliness
  5. bonding environment
  6. * bond temperature
  7. * ambient pressure
  8. * applied force
  9. materials
  10. * substrate materials
  11. * intermediate layer materials
The actual bond is an interaction of all those conditions and requirements. Hence, the applied technology needs to be chosen in respect to the present substrate and defined specification like max. bearable temperature, mechanical pressure or desired gaseous atmosphere.

Evaluation

The bonded wafers are characterized in order to evaluate a technology's yield, bonding strength and level of hermeticity either for fabricated devices or for the purpose of process development. Therefore, several different approaches for the bond characterization have emerged. On the one hand non-destructive optical methods to find cracks or interfacial voids are used beside destructive techniques for the bond strength evaluation, like tensile or shear testing. On the other hand, the unique properties of carefully chosen gases or the pressure depending vibration behavior of micro resonators are exploited for hermeticity testing.