Wafer bonding is used to join irreversible two wafers together

• Bonding has to be leakproof
• types of bonding:
  • Fusion bonding

    • first the wafer is immersed in acid to create hydrophilic surfaces with O-H bonds

    • then the surfaces are put in contact and hydrogen bonds are created, without pressure

    • at the end, a high temperature treatment (800oC) is given and the bonds become permanent bonds (water is desorbed and strong Si-O bonds are created)

    • surfaces such as Si/Si, SiO₂/SiO₂, Si/Si₃N₄, etc. can be bonded

  • Anodic bonding - in this case temperature + bias are used to form a strong bond between glass and silicon

    • The two wafers are placed on a heater and a bias is applied between them (positive at silicon, negative at the Pyrex/glass wafer)
    • Na+ ions are atracted and because they are mobile, they travel through the the glass wafer to the electrode, both wafers become conductive and the electric field is concentrated at the high-resistance area at the interface between the wafers
    • Electrostatic attractive forces pull the wafers together creating a strong contact, together with the temperature (400oC), creates chemical bonds between glass and Si (oxygen ions drift to the silicon, creating strong Si-O bonds)
    • Surfaces must be very clean and flat
    • advantage: lower temperature is needed, thus this method can be used with wafers patterned with metal, for example

  • eutectic bonding

    • one Si wafer has a layer of gold on top

    • when the two wafers are put in contact and tempereature is raised until eutectic temperature, Au will diffuse in Si, creating a strong alloy at the interface

• Applications of bonding: creating a sealed cavity for a capacitive pressure sensor, for example: