Anodic bonding

Anodic bonding is a wafer bonding process to seal glass to either silicon or metal without introducing an intermediate layer; it is commonly used to seal glass to silicon wafers in electronics and microfluidics. This bonding technique, also known as field assisted bonding or electrostatic sealing, is mostly used for connecting silicon/glass and metal/glass through electric fields. The requirements for anodic bonding are clean and even wafer surfaces and atomic contact between the bonding substrates through a sufficiently powerful electrostatic field. Also necessary is the use of borosilicate glass containing a high concentration of alkali ions. The coefficient of thermal expansion (CTE) of the processed glass needs to be similar to those of the bonding partner.

Anodic bonding can be applied with glass wafers at temperatures of 250 to 400 °C or with sputtered glass at 400 °C. Structured borosilicate glass layers may also be deposited by plasma-assisted e-beam evaporation.

This procedure is mostly used for hermetic encapsulation of micro-mechanical silicon elements. The glass substrate encapsulation protects from environmental influences, e.g. humidity or contamination. Further, other materials are used for anodic bonding with silicon, i.e. low-temperature cofired ceramics (LTCC).

Anodic bonding on silicon substrates is divided into bonding using a thin sheet of glass (a wafer) or a glass layer that is deposited onto the silicon using a technique such as sputtering. The glass wafer is often sodium-containing Borofloat or Pyrex glasses. With an intermediate glass layer, it is also possible to connect two silicon wafers. The glass layers are deposited by sputtering, spin-on of a glass solution or vapor deposition upon the processed silicon wafer. The thickness of these layers range from one to a few micrometers with spin-on glass layers needing 1 µm or less. Hermetic seals of silicon to glass using an aluminum layer with thickness of 50 to 100 nm can reach strengths of 18.0 MPa. This method enables burying electrically isolated conductors in the interface. Bonding of thermally oxidized wafers without a glass layer is also possible.

The procedural steps of anodic bonding are divided into the following:

with a process characterized by the following variables:

The typical bond strength is between 10 and 20 MPa according to pull tests, higher than the fracture strength of glass.

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